After a fairly long break I want to get back to M31 and MaNGA for one, or perhaps several posts and take a more detailed look at my model results. I still haven’t decided where I’m going to take this investigation. I may examine every IFU or just the ones that I found most interesting, and I’m not sure which of the many quantities that I estimate I’ll discuss. Besides my models I’ve retrieved a number of catalogs of interesting objects using Aladin. These include in particular H II regions (Azimlu et al. 2011), OB associations (Magnier et al. 1993), and red supergiants (Ren et al. 2021). All of these are products of recent or ongoing star formation. There are of course a huge number of catalogs of just about every type of astronomical object found in galaxies, and I may examine some more depending on what interests me.
For orientation here’s a screencap of the Legacy Survey sky browser’s false color GALEX image of the northern half of M31 with the IFU positions overlaid and labelled with MaNGA’s plateifu identifiers. As a reminder these are all located within the PHAT survey footprint and specifically within the region for which star formation histories were estimated by Williams et al. (2017).
Screen capture of Legacy Survey Galex image of M31 with MaNGA IFU overlay
Before getting to individual IFU’s here is one more set of IFU-wide results. The following three graphs are model mass growth histories in units of present day solar mass per kiloparsec2. These are uncorrected for projection effects.
There are a couple interesting points here. There’s a clear stratification of mass density with projected radius, with about a factor 30 decline from the innermost to outermost IFU. This is in fairly good agreement with Williams’ estimate in their Figure 14.
The other thing to note is that all regions had most (> 55%) of their stellar mass in place by 8 Gyr ago and 92-99% in place by 1 Gyr ago. The largest fraction of recent star formation is in the IFU 9678-12703, which is very close to the region with the highest SFR in this half of the galaxy. There is also a trend towards later mass build up with increasing radius, which is completely consistent with the “inside-out” growth paradigm. The outermost IFU, 9678-12701 at about 16kpc radius has formed about 5% of its present day stellar mass in the past Gyr.
As I said in the previous post I don’t see clear evidence for a widespread burst of star formation that’s widely believed to have occurred around 2-4 Gyr ago. A confounding factor in my models is that they invariably show jumps in SFR at times when the interval between SSP model ages change and the two oldest of these occur at 1 and 4 Gyr, so this produces a possibly spurious period of apparently accelerated star formation. I hope to find (or perhaps produce) a set of SSP models with a better age distribution this year.
Growth of stellar mass density – inner disk M31 MaNGA IFU’sGrowth of stellar mass density – M31 MaNGA IFU’s in 10 kpc ringGrowth of stellar mass density – outer disk M31 MaNGA IFU’s
I think I’m going to hit publish now and resume with inner disk IFU’s next time.
I’ll resume my M31 posts soon (I hope), but I wanted to do a short post on the recent Zoogems HST observation of IC 3025 which is a dwarf elliptical in the Virgo cluster that was selected as part of the “post-starburst” galaxy sample. Thanks mostly to its membership in Virgo this galaxy is fairly well studied and even has multiple HST observations. Just for fun I tried to make a false color RGB image from three observations, with two in the IR through F160W and F110W filters, and the blue channel from the Zoogems observation in F475W.
IC 3025
False color composite from HST WFC3 IR images in F160W and F110W filters (proposal ID 11712, PI Blakeslee) and ACS/WFC F475W filter (proposal ID 15445, PI Keel).
This used a program named SWarp (author Bertin) to rescale and align the images and STIFF (also Bertin) to combine them, with some Photoshop work in a mostly futile attempt to get a more pleasing color balance and clean up some of the hot pixels. I don’t know exactly how STIFF maps counts to gray scale levels, but despite the odd color cast this picture may actually give a reasonably accurate rendering of the relative fluxes in each filter. The galaxy as a whole has a g-J color of about 1.3 mag (based on my measurements with APT and NED) and J-H ≈ 0.2 mag. per Jensen et al. (2015), so an orange or even green color in the body of the galaxy is not so unreasonable.
The blue(er) central region is notable and apparently real also. This is one of a distinct class of dwarf early type galaxies with blue centers, given the designation dE(bc) by Lisker et al. (2006). The blue centers are almost certainly due to recent star formation, as I’ll verify below.
There are 3 bright, unresolved clusters near the center with a number of others scattered around the body of the galaxy. By my measurements with the manual Aperture Photometry Tool the brightest of these has a g band (F475W) magnitude of 20.71 and J (110W) of 20.084, or g-J ≈ 0.62. The other two near the galaxy center are slightly fainter and considerably redder: g = 21.5 and 22.6 for the western and eastern flanking clusters, with g-J ≈ 1.2 for both. Jensen et al. (cited above) measure the distance modulus to be m-M = 31.42, which makes the F475W absolute magnitude of the central cluster equal to -10.71. Like the Zoogems target I discussed several months ago this would be quite luminous for a galactic globular cluster but is typical for a dwarf galaxy’s nuclear star cluster (Neumayer, Seth, and Boker 2020). This distance modulus, which corresponds to a luminosity distance of 19.2 Mpc, is considerably larger than the canonical distance to the Virgo cluster of m-M = 31.09 (per Jensen again). This is one of several lines of evidence that the galaxy is currently falling into the cluster.
Like the other galaxies in the Zoogems “post-starburst” sample the SDSS spectrum was incorrectly classified by the SDSS spectro pipeline as coming from a star, but this one has a correct redshift and has been used in science studies (for example in Lisker et al. cited above). From the reported position the fiber center was just west of the brightest central cluster and includes both that one and the cluster just to the west. The spectrum is very much typical of a post-starburst, with deep Balmer absorption and a shallow 4000Å break. I measure HδA = 7.24 ± 0.60Å and Dn4000 = 1.26 ± 0.0141this spectrum was analyzed in the JHU/MPA pipeline with nearly identical values and uncertainties, very similar values to the other two that I posted about last year. Finally, although it’s far from evident on visual inspection, there are firm (4-5 σ) detections of Hα and S[II] 6717, 6730 in emission. No other emission lines were detected.
IC 3025 – SDSS spectrum
I used my usual star formation history modeling code with the metal poor subset of the EMILES SSP library as described here, which produced the estimated star formation and mass growth histories:
IC 3025 – Star formation history and mass growth history modeled from SDSS spectrum
with a very good fit to the data except for a small region around 7500Å (which is often the case with the EMILES library):
IC 3025 – posterior predictive fit to spectrum from SFH model
My results can be compared fairly directly to an analysis by Lisker et al. (cited above), who performed some simple stellar population modeling on SDSS spectra with what appears to be their own unreleased code. They limited their populations to 3 discrete ages with the oldest fixed at 5 Gyr and the mass fractions and ages for the other 2 chosen from a finite set of possible values.
Perhaps surprisingly my results agree rather well with theirs. For VCC 21 (the Virgo Cluster Catalog designation for IC 3025) their best fit had about 9% of the total mass in young and intermediate age populations, with the young population chosen at 9 Myr age and 0.3% of mass and the intermediate population age of 509 Myr.
My models also show three broad periods of star formation with some lulls in between that can conveniently be divided into young, intermediate, and old populations. The youngest SSP models in my metal poor subset are 30 Myr, so of course there can’t be any truly young populations in the model. The peak in recent star formation was at ~70 Myr with a steep decline at the youngest lookback times. Around 1% of the present day stellar mass in the fiber footprint is in stars younger than 100 Myr, with just under 10% under 1 Gyr.
Based on the colors we can infer that the acceleration of star formation that began ~1 Gyr ago was limited to the central region and the presumed nuclear star cluster. The remainder of the galaxy and its cluster system must already have been quiescent by then.
Edit
I mentioned above my SFH models indicated there were firm detections of Hα and the [S II] doublet in emission. Although [N II] wasn’t detected at better than the 1σ level it’s still possible to make a strong line metallicity estimate from the posteriors. I also plot the marginal posterior for Hα luminosity below:
IC 3025
(L) Hα luminosity from SDSS spectrum
(R) log(O/H) estimated from [N II]/Hα and [S II]/Hα
Using Calzetti’s calibration of the Hα – SFR relation this implies a current day star formation rate ~10-4.5 M☉/yr. This should be considered an upper limit since we don’t know the ionizing source. Using Dopita’s calibration of the [N II]/Hα plus [S II]/Hα strong line metallicity estimator the upper limit to 12+log(O/H) is around 8, which is subsolar by almost an order of magnitude.
In this post I’m going to compare IFU wide star formation histories from my models to those of Williams et al. (2017) in the nearest 83″ by 83″ PHAT tile to each MaNGA IFU in the study. I picked the Williams paper for comparison mostly because it’s possible to! They give a complete tabulation of model results for all regions and all 4 sets of isochrones that they used, and these are available through the Vizier service. Specifically I used their Table 2, which provides star formation rate densities summed over all metallicities. Since the SSP model spectra I use are based on BaSTI isochrones I initially compared to their BaSTI based models. One problem with the Williams comparison is the authors had a very wide youngest time bin of 300 Myr, which is where my models should generally have the highest precision (I make no strong claims about accuracy). It would be nice to do a similar comparison to the earlier companion paper on recent star formation by Lewis et al., which gives a much finer grained view of the last ~half Gyr, but unfortunately there is no published tabulation of their results.
At the other end of the timeline the oldest bin is also very wide, from 8 to 14 Gyr lookback time. This isn’t a surprise: the limits for reliable photometry of individual stars were rather shallow, no fainter than m = 28 or Mg ≈ 3.6 according to Lewis. This is brighter than the main sequence turnoff at 8 Gyr, so any information about the truly ancient star formation history is coming from giant branch stars which have very similar evolutionary tracks at old ages1Checked by downloading a few isochrones from the BaSTI website.
At the end of my last post I mentioned the necessity to correct densities for the rather large inclination of M31’s disk. It turns out though that I reproduce Williams’ Table 3 from their Table 2 if the densities are uncorrected. Their tabulated SFR densities are in units of 10-4 M☉arcmin-2/year. One arcminute at their adopted M31 distance is about 0.227 kpc, so to convert to star formation rates per kpc2 the values in table 2 are multiplied by 19.8 × 10-4. From my models I sum the star formation rates over all modeled spectra in each IFU and divide by the total area in fibers, with each fiber covering a projected area of 42.78 pc2. Note that I do not try to analyze a single composite spectrum summed over the entire IFU since dust attenuation is quite patchy.
The graphs below overlay my modeled star formation rate densities on those of Williams in the tile with the nearest center to that of each IFU. The ribbons indicate the nominal 95% credible limits of SFR. These are certainly wildly optimistic. Table 2 of the paper includes uncertainty estimates, which I chose not to include. SFR densities are linearly scaled with different limits for each plot. The time scale is logarithmic. Something in between linear and logarithmic would seem more appropriate since this perhaps gives too much space to very recent times, but I haven’t found a suitable scaling.
These are ordered into three groups by location: the inner disk which is everything inside the 10 kpc ring, the 10 kpc ring, and the outer disk which is everything outside the 10 kpc ring. There’s some ambiguity about the locations of the plateifu’s 9678-9101 and 9678-12704. The first of these is about 12 kpc from the nucleus in what could be either a wide section of the 10 kpc ring or a separate structure. 9678-12704 appears in projection to be just outside the 10 kpc ring but it may be considerably farther out in a segment of spiral arm at ~15 kpc.
Commentary will continue after the graphs. I will discuss the individual IFU’s in more detail in later post(s).
Inner Disk
M31 MaNGA ancillary program – my star formation history models summed over each IFU compared to nearest tile in Williams et al. (2017) with BaSTI isochrones. Inner disk IFU’s.
10 Kpc ring
M31 MaNGA ancillary program – my star formation history models summed over each IFU compared to nearest tile in Williams et al. (2017) with BaSTI isochrones. 10 kpc ring IFU’s.
Outer disk
M31 MaNGA ancillary program – my star formation history models summed over each IFU compared to nearest tile in Williams et al. (2017) with BaSTI isochrones. Outer disk IFU’s.
The first two things I noticed were that star formation in every region declines monotonically from very early times to at least 4 Gyr ago. It also starts out lower than in the PHAT team’s models. Because of this early time mass deficit all of my models have smaller current day stellar mass densities by varying amounts. I don’t really have a pat explanation for this. Some authors have posited a “dazzle effect”2I’m going to discuss this a little further at the end of the post where recent star formation obscures the contribution of old populations. It’s certainly likely that this occurs, but if these Bayesian models are behaving as I hoped this lack of information should manifest as larger uncertainties rather than a systematic bias. Well, my hope could be wrong. On the other hand I don’t see strong evidence in these models for such an effect. From my eyeball analysis I don’t see an obvious correlation between present day star formation and the size of the early time deficit.
Another possibility is a systematic difference in the amount or shape of attenuation between my models and theirs. There is another well known “degeneracy” between stellar age and attenuation in SFH modeling, but I haven’t yet investigated whether this could be occurring here.
The PHAT models have a very long interval from 8 to ~2-3 Gyr ago with very little star formation. Some authors find evidence for a large increase from about 2-4 Gyr which is usually attributed to a merger or perhaps close encounter with M33. This isn’t seen in the BaSTI based models but there is a large more recent burst from about 1-2 Gyr lookback time. My models see neither a cessation of star formation nor a particularly large burst at intermediate ages. As I’ve noted before my models “want” to have smoothly time varying light3and therefore mass contributions and this might make a modest burst at moderately large ages difficult to discern. Another confounding factor arises from the abrupt changes in age intervals (at 0.1, 0.5, 1, and 4 Gyr) which results in the sawtooth pattern in SFR that’s obvious in every plot above.
At ages younger than 1 Gyr there’s generally good agreement about the course of star formation up until the youngest age bin of width 300Myr in the PHAT models. My models have anywhere from slightly to dramatically higher SFR densities averaged over the most recent age bin. I suspect this is because many of the IFU positions were chosen to be in regions with active star formation. In particular the plateifu 9678-12703 (mangaid 52-23) is very close to the region in the 10 kpc ring with the highest density of ongoing star formation in the northern half of the disk.
I plan to discuss the individual IFUs in more detail in a later post. Below the fold are some more graphics: mass growth histories and SFR densities compared to the PADOVA isochrone based models.
One of the ancillary programs (with principal investigator Julianne Dalcanton) in the final MaNGA release targeted 18 fields in the disk of the Andromeda galaxy M31. The targets were selected from within the footprint of the “Panchromatic Hubble Andromeda Treasury,” aka PHAT1not my coinage., also with PI Dalcanton. The initial PHAT survey description was in Dalcanton et al. (2012) and was followed by a lengthy series of papers. Especially relevant for this discussion are two papers describing estimates of the recent and ancient star formation histories of the disk outside the area dominated by bulge light: Lewis et al. (2015), “The Panchromatic Hubble Andromeda Treasury. XI. The Spatially Resolved Recent Star Formation History of M31” and Williams et al. (2017), “PHAT. XIX. The Ancient Star Formation History of the M31 Disk.” For reference here is a mosaic of HST images in the F475W filter with the IFU locations overlaid:
Mosaic of HST F475W images of PHAT study region with M31 MaNGA IFU positions overlaid
Zooming out to show the whole disk here they are overlaid on a false color FUV+NUV image from GALEX, which gives a pretty good picture of where stars are actually forming:
GALEX false color NUV+FUV image of M31 with MaNGA IFU positions overlaid – screencap from Aladin
This data set provides an excellent opportunity to compare my SFH modeling code to a completely different, more direct, method of inferring star formation histories namely counting resolved stars in color magnitude diagrams. I recently completed model runs for all 18 IFU’s with the same Voronoi binning of stacked RSS spectra, the same modeling code and SSP model spectra as I’ve used for a while now.
There’s no redshift listed in the DRP catalog; NED gives a heliocentric redshift of -0.001, but for purposes of calculating intrinsic quantities I need the “Hubble flow” redshift. I adopted a distance of 761 (± 11) kpc or distance modulus of (m-M)0 = 24.407 from Li et al. (2021), which is the most recent and according to the authors most precise determination to date. With my adopted Hubble constant of H0 = 70 km/sec/Mpc this makes the Hubble flow recession velocity 53.27 km/sec or zdist = 0.0001777. The angular scale is 3.69 pc/arc-second. This distance estimate is a few percent smaller than the PHAT team authors and most other recent literature I reviewed, but fortunately most other sources of uncertainty are much larger.
An issue I noticed early on was the modelled values of the optical depth of attenuation were right at 0 for almost all spectra with only a few much larger exceptions. A quick check of the metadata showed that the values adopted for the foreground galactic extinction almost certainly were taken from the SFD dust maps which faithfully capture the intrinsic dust content of M31 albeit at rather low resolution. These hugely overestimate the actual foreground galactic extinction and that has multiple undesirable consequences. So, I assigned a single extinction value of E(B-V) = 0.055 to all IFU’s, consistent with the NED value of AV = 0.17 mag. The preliminary runs were redone with the newly adopted extinction value.
After binning to a minimum mean SNR of 5 there were 2,624 spectra in the 18 IFUs, of which I ran models for 2,621. Three spectra had apparent foreground stars, although one of those might actually be a red supergiant in M31. The fibers are basically sampling star cluster size and stellar mass regions so a single extremely luminous star could potentially affect a spectrum.
I’m only going to show a few summary results for the entire sample in this post. My goal is to do a more detailed quantitative comparison to (at least) the SFH models of Wilson, for which there are extensive results tabulated. There are of course many catalogs of interesting objects within M31, and I plan to look at some of them.
First, here is a plot of the (100 Myr averaged) star formation rate density against stellar mass density, color coded by BPT diagnostic. The solid line is my estimate of the “spatially resolved star forming main sequence” based on a small sample of non-barred spiral galaxies. The dashed line is the estimate of Bluck et al. (2020), which I commented previously appears to mark approximately the location of the green valley at least with regard to my models. A striking feature of this plot is the apparent stratification into at least three distinct groups that can be interpreted as starforming, quiescently evolving, and passively evolving. I suspect this observed stratification is just the result of hand picking a small number of “interesting” regions. Most or perhaps all of the points in the passively evolving group are in the IFU closest to the bulge, while most of those along and above the SFMS lie near the most vigorously star forming regions in the PHAT footprint. Especially noteworthy are 5 outliers that are well above any others in the plot in terms of SFR density. These are all in the same IFU (plateifu 9678-12703) which is located within the largest star forming region in that quadrant of the “10 kpc ring.”
100 Myr average star formation rate density vs. stellar mass density for 2621 binned spectra in M31 disk. Solid and dashed lines are my and Bluck’s central estimates of the “spatially resolved star forming main sequence.
Next are plots of star formation rate density against Hα luminosity density. The left panel is for all spectra color coded by BPT diagnostic, with Hα adjusted by the modeled amount of stellar attenuation. The right panel shows regions with star forming BPT diagnostics only, with Hα corrected by the observed Balmer decrement. The solid line in both panels is Calzetti’s calibration of the Hα – SFR relationship. The relationships plotted here are consistent with what I’ve seen in other MaNGA samples and with published values, which is encouraging.
Star formation rate density vs. Hα luminosity density for 2621 binned spectra in M31 disk. (L) Emission corrected for modeled stellar attenuation. (R) For regions with star forming emission line ratios only: emission corrected from estimated Balmer decrement.
The obvious point of comparison to my models are the detailed star formation histories in the two PHAT papers mentioned at the top. Unfortunately there is no detailed tabulation of model results in the paper by Lewis et al. The paper by Williams et al. has extensive tables, but there are still a few obstacles to detailed comparisons which I will discuss next time.
A few more items from my handwritten notes that I want to get in pixels. I have never previously tried to correct surface densities for inclination in disk galaxies, but for comparison purposes and because of the large inclination of M31’s disk I need to do so here. I adopted an inclination angle of 77°, so a 1″ radius fiber covers a 3.69 x 16.4 pc (semi major and minor axes) elliptical region, or 190 pc2. Densities need to be adjusted downward by a factor 4.45 or -0.648 dex2This adjustment was not made in the plots above. Since these are plots of densities against densities all points would just shift downwards along lines of slope one..
In order to achieve 100% coverage of the IFU footprints the exposures were dithered to three different positions with overlapping fiber positions. Comparing the area in fibers to the area in spaxels in the cubes the overfilling factor averages 0.217 dex or 65%. The total area in all cubes is 10,731 arcsec2, or a deprojected area of 0.65 kpc2. The most distant IFU from the nucleus is at a projected radius of about 16 kpc. A simple extrapolation to the ≈800 kpc2 area of the disk within that radius is probably unsafe.
One final map to anticipate the next post(s). Wilson provides tables of model star formation rates for 16 age bins, 826 regions, and 4 different sources of isochrones including the same BASTI isochrones I use. The complete data set is available through Vizier. In the plot below I created a map of the recent star formation rate density interpolated to nominal 10″ resolution from their Table 2 models with BASTI isochrones. This should be compared to their Figure 16 (they use logarithmic scaling).
Current (300 Myr average) star formation rate density in the PHAT footprint per models of Wilson et al. (2017) with positions of MaNGA IFUs overlaid.
I just have a quick comment about my last two subjects. I mentioned both of them have exceptionally strong Balmer absorption as measured by the Lick index HδA. They also have similar 4000Å break strengths:
IC 0976: Dn4000 = 1.308±0.005, HδA = 8.05±0.31
MCG +07-33-040: Dn4000 = 1.153±0.009, HδA = 8.06±0.41
For context here’s a variation of the same plot I’ve shown several times of the MPA-JHU measurements for a large sample of SDSS galaxy spectra with their locations overlaid:
Dn4000 – Hδ of SDSS spectra of post-starburst galaxies IC 976 and MCG +07-33-040 overlaid on measurements for a large sample of SDSS spectra
Both galaxies have HδA indexes near the upper limits of any measurements in SDSS, and both are clearly in the post-starburst area of the HδA-Dn4000 plane. Depending on your interpretation of the 4000Å break strength index IC 976 could be slightly older or have a slightly lower specific star formation rate, but the difference is small. Using the toy evolutionary models that people often use these two galaxies could easily be at slightly different stages of the same evolutionary trajectory.
In fact though the detailed star formation history models show rather different trends over the last ~Gyr, with recall MCG+07-33-040 having a more extended and more recently terminated period of enhanced star formation than IC 976, while the latter had considerably more stellar mass added by the starburst.
This nicely illustrates a point I raised 3 posts ago, which is that this particular pair of indexes can’t break the “burst age – burst mass” degeneracy. Full spectrum fitting with non-parametric star formation histories potentially can. I’m still not prepared to take these models too literally.
I’m going to try to keep this one short. IC 976 is another post-starburst galaxy that was selected and recently observed by HST for the Zoogems project (proposal ID 15445, PI Keel). I took a shot at creating a color image by combining the ACS observation taken with the F475W filter (approximately equivalent to SDSS g band) with r and z band images from the Legacy Survey. Well that wasn’t too rewarding since this galaxy appears quite featureless.
IC 976 – RGB image created for Legacy Survey r and z band images + HST ACS F475W image from proposal ID 15445, PI W. Keel
Like the galaxy in the previous post the SDSS spectro pipeline misclassified this galaxy’s spectrum as a star with a recession velocity of ≈ 1200 km/sec. Unlike the galaxy in the previous post IC 976 is well known to have a post-starburst nuclear spectrum, and its correct heliocentric redshift of 0.00509 is listed in NED and confirmed with my own redshift estimation code. If that’s its Hubble flow redshift (doubtful) its distance would be about 21.8 Mpc (distance modulus m-M=31.7) and the 3″ SDSS fiber would cover 315 pc.
SDSS spectrum of IC 976 nucleus with best fit template overlay
Once again I ran my SFH modeling code on the SDSS spectrum, using only my metal rich PYPOPSTAR+EMILES ssp library, with results below:
Modeled star formation and mass growth histories of central region of IC 976 from SDSS spectrum 340044889930622976.
Despite the superficially similar spectra1this has a nearly identical HδA index of 8.1 ± 0.3 Å. this model favors an older (peak at 800 Myr lookback time), stronger, and shorter burst than the previous example. The model’s burst strength of ≈ 40 % of the present day stellar mass seems high, but the estimated total stellar mass within the fiber footprint is only ≈108.5 M☉, which is likely a small fraction of the galaxy’s total stellar mass. For a rough estimate of the total mass the SDSS g band Petrosian magnitude is listed as 13.6, making the absolute magnitude -18.1. With a solar g band absolute magnitude of 5.11 the galaxy’s luminosity is ≈ 109.3 L☉, and assuming a stellar mass to luminosity ratio around 1 the mass would therefore be ≈ 2×109 M☉. If the merger added a little over 108 M☉ to the system as implied by this model the mass ratio of the progenitors would be on the order of 20:1.
IC 976 was one of 7 post-starburst galaxies in an IFU based spectroscopic study by Pracy et al. (2012). This galaxy2designated “E+A 6” in the paper. had a very strong negative radial gradient in the Balmer absorption index, as did 5 of the 6 others in the study. They concluded that centrally concentrated starbursts fueled by minor mergers was the most likely cause of their present evolutionary state. The lack of any apparent tidal features in the available imaging of this galaxy likely reflects the age of the merger and mass ratio of the progenitors.
The Hubble Space Telescope “gap filler” program “Gems of the Galaxy Zoos” (proposal ID 15445, PI William Keel) had several prospective targets that I played a small role in selecting, and this recent HST observation was one of them. The actual target was the small disturbed galaxy at top left, which I will refer to as MCG +07-33-040. I don’t know if it was fortuitous that the larger and brighter UGC 10200 was also imaged in the same ACS field, but these are clearly interacting or at least have in the recent past, as is the small system in the upper right, which is identified as a blue compact galaxy with redshift z=0.00556 in Pustilnik et al. (1999). I’m going to focus on the top left galaxy in this post.
Galaxies UGC 10200 (lower right) and MCG +07-33-040 (upper left). HST/ACS, F475W filter. Proposal ID 15445, PI Keel.
What interested me wasn’t the galaxy image so much as its SDSS spectrum, which has three interesting characteristics:
SDSS spectrum of central part of MCG +07-33-040
First, this is a classic post starburst galaxy spectrum with extremely strong Balmer absorption lines1My code measures the Lick index HδA as an exceptionally strong 8.06 ± 0.41 Å. and no obvious evidence of emission. In fact, although this designation isn’t used much anymore, it’s actually a classic “A+K” spectrum which reverses the usual “K+A” terminology to indicate the light is dominated by early type (i.e. young) stars. Second and third, the spectrum was misclassified as coming from a white dwarf star, and the redshift was erroneously estimated as around 0.004 which was the maximum allowed for stars in the SDSS data reduction pipeline. Using a variation of the code that I use to measure redshift offsets I get a robust value of z = 0.006682 ± 9E-06
Template fit to SDSS spectrum of MCG +07-33-040
This is almost exactly the same redshift as its nearby companion UGC 10200 (also in the HST image above), which has a securely determined z = 0.00664
SDSS spectrum of central region of UGC 10200
For the rest of this post I’m going to assume the Hubble flow redshift is the measured one, which with my adopted cosmological parameters2which for the record are H0 = 70 km/sec/Mpc, Ωm = 0.27, Ωλ = 0.73. make the luminosity distance 28.8 Mpc, the distance modulus m-M = 32.3 mag, and the angular scale 138 pc/” or about 7 pc per ACS pixel. The projected distance between the centers of the two bright galaxies in the HST image is about 96″ or 13.2 kpc.
I spent some time last weekend downloading and starting to learn the software Aperture Photometry Tool (APT), which is interactive software for manually performing aperture photometry. Zooming in on the center of the presumed post starburst galaxy I located the reported position of the SDSS fiber as closely as I could. In the screenshot below the aperture radius was set to 30 pixels, the same size as the SDSS spectroscopic fibers. I measured the F475W AB magnitude to be 17.90 ± 0.013 without sky subtraction, which is close enough to the SDSS g band fiberMag estimate of 18.05. The SDSS g band Petrosian magnitude estimate is 15.16, so the fiber contains about 7% of the total galaxy light.
Central region of MCG +07-33-040 with position and size of SDSS fiber overlaid. Screenshot from APT
A striking feature of the HST image is there are many point-like symmetrical objects embedded within the otherwise nearly featureless diffuse light of the galaxy. Within the SDSS fiber footprint I count about 8-10 of these (the range being due to some uncertainty about what to call point-like and symmetrical). In order to get a handle on their contribution to the spectrum I did aperture photometry on them using a 3 pixel radius aperture with median sky subtraction from a 5 to 8 pixel radius annulus. The apparent magnitudes of the 5 brightest objects range from about 22.6 to 23.1. The summed luminosity of those 5 amounts to only 3.5% of the total light in the fiber, so the spectrum is mostly telling us something about the diffuse starlight. Even if one or more of those objects are foreground stars they can’t be a significant source of contamination. Clicking around the blank regions of the HST field I found fewer than one star per SDSS fiber size region, so it’s likely there are few if any foreground stars within the visible extent of the galaxy.
There is plenty of observational and theoretical evidence that massive star clusters are formed in mergers and close encounters of galaxies. As a coincidental example the merger remnant NGC 3921 — which was one of the 4 galaxies in my last post — has over 100 young globular clusters located both in the main body and southern tidal tail (Schweizer et al. 1996; Knierman et al. 2003). The brightest source in this galaxy (near the southern edge of the visible fuzz) has an apparent magnitude of m ≈ +21.7, which for the adopted distance modulus is M ≈ -10.6. With a solar g band absolute magnitude of 5.11 this corresponds to L ≈ 1.9×106 L☉ . The 5 brightest objects within the fiber have absolute magnitudes between about -9.7 and -9.2. These would be quite luminous for galactic globular clusters, but they’re likely to be fairly young and would fade by at least a few magnitudes as they age.
I haven’t tried a more sophisticated analysis of these objects’ sizes, but using the APT radial profile tool the presumed clusters look little different from nearby foreground stars and all that I’ve examined have FWHM diameters around 2-2.5 pixels. A strict upper limit to their sizes is therefore around 14 pc.
Someday I may undertake a complete census and luminosity function of the cluster system in this galaxy, and perhaps also look at the neighboring starburst galaxy UGC 10200. These systems by the way are cataloged as an interacting dwarf galaxy pair by Paudel et al. (2018) with a total stellar mass of log(M*) = 9.5 and a 3:1 mass ratio, which makes the estimated stellar mass of this galaxy just under 109 M☉. The system is very gas rich, with a neutral hydrogen mass estimated (by the same source) of 109.69 M☉. There are actually at least two published HI maps of this system. The one below, from Thomas et al. (2004) shows that atomic hydrogen extends over essentially the entire extent of the Hubble image above, including the target galaxy.
VLA map of HI gas in UGC 10200 system
Next I turn to star formation history models for the post starburst spectrum at the top of the post. This uses the same Stan model code as my MaNGA investigations with some minor pre- and post-processing adjustments. I ran two separate models. One used a metal poor subset of the EMILES SSP libraries with Z ∈ {[-2.27], [-1.26], [-0.25]} with, as usual, Kroupa IMF and BaSTI isochrones. I did not attempt to append younger models, so the youngest age is 30Myr. For completeness I also ran a model with my usual EMILES subset + PYPOPSTAR models and Z ∈ {[-0.66], [-0.25], [+0.06], [+0.40]}. First, here is the modeled star formation history with the metal poor subset. I’ve again used a logarithmic time scale and linear star formation rate scale.
Model star formation history of central region of MCG +07-33-040 using metal poor subset of EMILES SSP library
Next is the metal rich subset:
Model star formation history of central region of MCG +07-33-040 using metal rich subset of EMILES+pypopstar SSP library
Both model runs show a fairly steep ramp up in star formation beginning at about 600Myr lookback time and a steep decline around 50Myr ago. The lingering star formation in the metal rich model might be a manifestation of the infamous “age metallicity degeneracy” since Balmer Hα emission is too low to support this level of star formation. Comparing the mass growth histories exposes a more subtle effect: the metal poor models have a consistently higher mass fraction at any given epoch. Also, the period of accelerated star formation involved a slightly smaller fraction of the present day stellar mass.
Mass growth histories of MCG +07-33-040 using metal poor and metal rich subsets of EMILES SSP library
Both models fit the data well. In terms of mean log-likelihood the metal poor model outperformed the metal rich, but only by about 0.4%. The proper Bayesian way to compare models is through the “evidence,” which is hard to estimate accurately. I suspect the metal poor model would be at least slightly flavored because it has fewer parameters than the metal rich one.
Posterior predictive fit to SDSS spectrum of MCG +07-33-040
The duration of accelerated star formation (about which both models agree) is a little surprising in light of simulations that usually show a fairly short SF burst in the first passage in mergers. But, simulations have only explored a small range of the potential parameter range. Studies of low mass galaxies with extended, massive HI haloes might be of interest.
One more sanity check. Suppose the closest approach between our target and UGC 10200 was 60Myr ago, allowing another 10Myr before (presumably) supernova feedback quenched star formation. Assuming the relative motion is transverse to our line of sight traveling 13.2 kpc in 60Myr implies an average separation speed of ≈215 km/sec. This is a perfectly reasonable value for a galaxy pair or loose group.
Finally for this spectrum, here is a quick look at emission line fluxes. Even though visually not at all obvious several lines were detected at marginal (>2σ) to high (>5σ) confidence. A couple of surprises are the [O I] 6300Å line, which is often only marginally detected even in star forming systems, is a firm (3σ) detection and stronger than the usually more prominent [O III] doublet. Also, the [S II] 6717-6730 doublet is a firm detection while the [N II] doublet is not. What this means is unclear to me. Most of the “strong emission line” metallicity indicators that I have formulae for include [N II] (or [O II] which are out of the wavelength range of these spectra), so it isn’t really possible to make a gas metallicity estimate. It’s a safe guess it’s subsolar though.
line
[Ne III] 3869
Hζ
[Ne III] 3970
Hε
Hδ
Hγ
Hβ
[O III] 4959
[O III] 5007
[O I] 6300
[O I] 6363
[N II] 6548
Hα
[N II] 6584
[S II] 6717
[SII] 6730
mean
17.1
2.3
1.5
1.6
1.9
2.1
7.9
2.4
4.9
8.2
2.8
2.9
39.1
2.5
14.4
14.2
s.d.
6.3
2.0
1.4
1.4
1.6
1.8
3.1
2.0
2.9
2.8
1.9
2.0
2.6
1.8
2.8
2.8
ratio
2.7
1.1
1.1
1.1
1.2
1.2
2.6
1.2
1.7
3.0
1.5
1.5
15.2
1.4
5.2
5.2
Flux measurements for tracked emission lines in spectrum of MCG +07-33-040. Units are 10-17 erg/sec/cm2
There are at least two questions about this galaxy that it would be nice to have answers for. First, since the SDSS fiber only includes about 7% of the luminosity and a similar fraction of the stellar mass we really don’t know if it is recently quenched globally or just where SDSS happened to target. My guess from this HST image is that it is globally quenched because its companion UGC 10200 shows clear evidence of dust lanes and extended star forming regions even in this monochromatic image, while the diffuse light in this galaxy looks relatively featureless. A definitive answer would require IFU spectroscopy though.
A second question is whether the star cluster system is truly young or primordial (or both). This would require color measurements from a return visit by HST using at least one more filter in the red. Estimating a luminosity function is feasible with the existing data, although it would have rather shallow coverage. From my casual clicking around the image it appears to be possible to reach magnitudes a little larger than +24 with reasonable precision.
When this topic first came up on the old Galaxy Zoo talk I thought these might comprise a new and overlooked category of galaxies. In fact though all of the examples I investigated belonged to cataloged galaxies and most of the spectra were of small regions in much larger nearby galaxies. A few galaxies that were in the original Virgo Cluster Catalog and excluded from the EVCC because of lack of redshift confirmation should be added back. There were probably only a few like this one with large errors in redshift estimates and high signal to noise spectra. I haven’t spent enough time with the literature to know if rapidly quenched dwarf galaxies are especially interesting. Maybe they are.
While browsing through the ADS listing of papers that cite Schawinski’s paper that I’ve been discussing for a while I came across this one by Haines et al. with the full title “Testing the modern merger hypothesis via the assembly of massive blue elliptical galaxies in the local Universe”. Besides being on the same theme of searching for post-starburst or “transitional” galaxies in the local universe that I’ve been pursuing for some time the paper was interesting because it made use of IFU based spectroscopic data that predates MaNGA. As it happens 4 of the 12 galaxies have observations in the final MaNGA release, providing an excellent opportunity to compare results from completely independent data sets.
The “modern merger hypothesis” that the authors tested relates to a topic I’ve discussed before, which is that N-body simulations show that strong, centrally concentrated starbursts are a possible outcome of major gas rich galaxy mergers around the time of coalescence. If some feedback process (an AGN or supernovae) rapidly quenches star formation there will ensue a period of time when the galaxy will be recognizable as post-starburst.
In a series of long and rather difficult (and influential judging by the number of citations) Hopkins and collaborators (2006, 2008a, 2008b) have made a case that major gas rich mergers with accompanying starbursts are in fact the major pathway to the formation of modern elliptical galaxies. They claim that their merger hypothesis accounts for a variety of phenomena, including the growth and evolution of supermassive black holes and quasars.
The specific aspect of the merger hypothesis this study tried to address was the prevalence of strong centrally concentrated starbursts in a sample of ellipticals in the process of forming as evidenced by visible disturbances consistent with recent mergers. The main tool they used was a suite of simple star formation history models with exponentially decaying star formation rate with single (also exponentially decaying) bursts on top of varying ages and decay time scales. They used these to predict just two quantities: Balmer absorption line strength measured by the average of the Lick HδA and HγA indexes, and the 4000Å break strength index Dn4000. For reference here is a screen grab of their model trajectories:
Predected trajectories in the Hδ – Dn4000 plane per Haines et al. (2015). Clipped from the electronic journal paper.
This is a pretty standard calculation variations of which have been performed for decades, and this graph looks much like others I have seen in the literature. A fairly basic problem with it though is that position in the Balmer – D4000 plane doesn’t uniquely constrain even the recent stellar evolution. In astronomers’ parlance there is a “degeneracy”1the term refers to a situation in which multiple combinations of some parameters of interest produce effectively equivalent values of some observable(s), or of course the converse. The best known example is the “age-metallicity degeneracy,” which refers to the fact that an old metal poor population looks like a younger metal rich one in several respects such as broad band colors. between burst strength (if any) and burst age. This is a well known problem with the Balmer line strength index that was already recognized by Worthey and Ottaviani (1997), who developed these indexes. Adding a second index in the form of the 4000Å break strength doesn’t break the degeneracy: there are regions of the plane where bursting and non-bursting populations overlap, as can be seen clearly in the graphic above. This is actually a problem for any attempt to identify post-starburst galaxies. After correcting for emission most ordinary starforming galaxies have strong Balmer absorption lines, so using that index alone will certainly produce many false positives. On the other hand selection criteria like those used by Goto and many others before and after — selecting for both strong Balmer absorption and weak emission — will capture only a small interval in post-starburst galaxies’ life cycles.
Hδ line strength vs. 4000Å break index for a large (~380K) sample of SDSS galaxy spectra. Measurements from the MPA-JHU analysis pipeline downloaded from SDSS Skyserver
Let’s get to results. Some basic details of the sample are in the table below. Morphological classifications are from McIntosh et al. (2014) as given in this paper. The abbreviations are SPM: spherical post merger; pE: peculiar Elliptical. The two marked pE/SPM didn’t have a strong consensus among several professional classifiers. I list them in order of my own visual impression of degree of disturbance. I also list redshifts taken from the MaNGA catalog and Petrosian colors.
NED name
NYU ID
mangaid
plateifu
Morph
z
u-r
g-i
NGC 3921
541044
1-617445
10510-6103
SPM
0.019
1.97
0.86
MRK 385
719486
1-604970
8940-6102
pE/SPM
0.028
1.43
0.63
MRK 366
100917
1-603309
7993-1902
pE/SPM
0.027
1.59
0.79
NGC 1149
22318
1-37155
8154-6103
pE
0.029
2.29
1.11
Columns: (1) Common catalog designation (NED name). (2) NYU VAC ID. (3) MaNGA mangaid. (4) MaNGA plateifu. (5) Morphology (see text). (6) redshift from MaNGA DRP catalog. (7-8) Petrosian u-r and g-i colors from NYU VAC via the MaNGA DRP catalog.
The main prediction of the merger with accompanying centrally concentrated starburst hypothesis the paper tests is that the Balmer absorption index should be large and have a negative gradient with radius while the 4000Å break strength should be low with a positive gradient. The authors concluded that only one member of their sample — nyu541044 — clearly falls in the post-starburst region (marked as region 4 in the graph above) of the <Hδ, Hγ> – Dn4000 plane. The two pE/PM galaxies, both of which are in my sample, lie in the starforming region 1. They inferred from this that these galaxies are undergoing at most a weak burst. I’m going to mildly disagree with that conclusion.
Measured values for the specified indexes from Haines et al. (2015). Clipped from the electronic journal paper.
I have calculated the pseudo Lick index HδA and Dn4000 as part of my analysis “pipeline” since I started this hobby. I actually make these measurements in the initial maximum likelihood fitting step since they don’t depend on modeling except for small (usually) emission corrections. I don’t calculate an Hγ index, but its theoretical behavior is similar to Hδ. I’m trying here just to verify the approximate magnitude and radial trends of the chosen indexes. The two IFUs used in the Haines study had larger spatial coverage than these MaNGA observations (but much smaller wavelength coverage, which will become important). Instead of their strategy of binning in annuli I used my usual Voronoi binning strategy with a minimum target S/N. There were some oddities in the NYU estimates of effective radii so I chose to use distances from the IFU center in kpc for these plots. The distances assigned to the multiply binned spectra are the same as Cappelari’s published code produces; for single fiber spectra it’s just the position of the fiber center.
My measurements agree reasonably well with those of Haines et al. All three of the most disturbed galaxies have central Hδ indexes > 5Å with NGC 3921 (plateifu 10510-6103, nyu541044) having a larger central value and steeper gradient in the inner few kpc than the two pE/SPM galaxies. The fourth galaxy shows no obvious trend in either index with radius2The next several plots show trend lines for each galaxy computed by fitting simple loess curves to the data using the default parameters in ggplot2. These, and especially the confidence bands included in the plots, should not be taken seriously!. The central values where the S/N is highest are in good agreement.
Lets turn to the results of star formation history models, which I ran on all 4 data sets. First, here are 100Myr averaged star formation rate density and specific star formation rate versus distance:
Star formation rate density vs. distance from IFU center (kpc) for 4 disturbed early type galaxies.Specific star formation rate density vs. distance from IFU center (kpc) for 4 disturbed early type galaxies.
Three of these galaxies are clearly experiencing centrally concentrated episodes of star formation, and two are at or near starburst levels in specific star formation rate near their centers. As seen below two of these straddle my estimate of the “spatially resolved star forming main sequence” while the one presumed post-starburst galaxy reaches it in the central region.
Star formation rate density versus stellar mass density for 4 disturbed early type galaxies
As I’ve shown several times before there’s a reasonably tight linear relationship between modeled star formation rate and Hα luminosity density. The plot shows Hα luminosity density corrected for modeled stellar redenning, which certainly underestimates attenuation in emission regions. The modeled star formation rates are consistently above the Kennicut relation shown as the straight line as I’ve seen in every sample I’ve looked at.
Star formation rate density vs. Hα luminosity density for 4 disturbed early type galaxies
Finally, lets take a look at detailed star formation histories. Instead of my usual practice of plotting them all in a grid here I just display 2 binned star formation histories. One comprises the innermost 7 bins, which since the fibers are arranged in a hexagonal grid should form a regular hexagon around the IFU center. These range in “radius” from about 0.75 to 1.1 kpc in these four galaxies. The second is for an “annulus” in approximately the outer kpc of each IFU. The extent of the IFU footprints ranges from 3.1 to 5.9 kpc. I calculate these by summing the contributions in each SFH model contributing to the bins, not by running new models for binned spectra. Since the dithered fiber positions overlaps this overestimates the total mass in each bin, but I care about the shape and timing of events rather than the absolute values of star formation rate estimates.
The next 4 plots display the results. Lookback time is logarithmically scaled with the same range and ticks for each SFH. Vertical scales are linear and differ for each graph. The graphs are in the same order as the basic information table above. As I’ve written before these models “want” to have smoothly varying mass per time bin which has the unfortunate effect of producing jumps in the apparent SFR when the bin widths change. In the BaSTI isochrone based SSP models these occur at 100 Myr, 1 Gyr, and 4 Gyr and can sometimes be quite prominent.
With caveats out of the way the one clear post-starburst in the sample had (per the model) a powerful and short starburst at ≈300 Myr lookback time, with a small amount continuing to the present (this can’t be seen at the scale of the graph, but ongoing star formation is ~1 M☉/yr). The total mass contribution from the burst and subsequent star formation is around 15%.
The two apparent ongoing starbursts have later bursts of star formation that are slightly weaker in terms of total mass contribution and peak star formation rate, but still quite significant. All three of the starburst/post-starburst galaxies appear to have had two major waves of late time (last ~2 Gyr or less) star formation. As I’ve written before in merger simulations the progenitors usually complete a few orbits before coalescence, with some enhanced star formation around each perigalactic passage. I hesitate to take these models that literally.
Turning finally to the last and least disturbed galaxy, NGC 1149, despite the bursty appearance of the SFH there’s no evidence for a major starburst in the cosmologically recent past. Whether an older starburst can be detected in this kind of modeling approach needs investigating.
One last set of graphs that may be useful. These show cumulative star formation histories — basically the cumulative sum of mass contributions starting from the oldest time bin. This is similar to a mass growth history which is a popular visualization. In my calculation of the latter the contributions are to the present day stellar mass, so an allowance for mass loss and remnant mass is made3these come from the source of the SSP models and are themselves models. Probably they are somewhat better than guesses. These things are basically black boxes to users.. The graphs are for the central regions only. Note the major virtue of these is that the contributions of major episodes of star formation can be estimated at a glance.
Cumulative star formation histories for central regions of 4 disturbed early type galaxies
To wrap up this part of the post 3 of these galaxies are compatible with the “modern merger hypothesis,” that is they have experienced centrally concentrated but spatially wide spread starbursts. The reason two of them don’t have post-starburst characteristics in the Hδ – D4000 plane is their starbursts are still underway. The current burst of star formation contributes about 5-10% of the mass in the central regions of these two. How much more is available is unknown (at least to me until I get around to finding out if there are HI mass estimates available).
Future plans: I’ve completed model runs on the 24 “post-starburst” galaxies in the MaNGA ancillary program dedicated to them. I may have something to say about them. I also may have something to say about one of the Zoogems targets that I had a small part in selecting.
As I mentioned two posts ago there are 24 of these galaxies in the final MaNGA data release, a remarkable 11% of the full sample. I ran my SFH model code on all of these along with the prerequisite redshift offset routine1I actually completed these some time ago. I just haven’t had time to do much analysis or write about them. SDSS thumbnails of the sample are shown below. As expected none of these have significant spiral structure visible at SDSS resolution, but at least a few are noticeably disturbed.
SDSS thumbnail images of Schawinski et al.’s blue early type galaxies in MaNGA final data release (SDSS DR17)
I’m just going to discuss a few topics in this post. I’ll save a more detailed discussion for when I’ve completed analysis of the ancillary post-starburst sample, which is underway now. First, here are velocity fields calculated for the stacked RSS data, with a signal to noise cutoff of 3, the same as I used for my analysis of rotation curves of disk galaxies. Note in the graphic below the ordering is different from the image thumbnails.
Line of sight velocity fields of Schawinski et al.’s blue early type galaxies in the final MaNGA data release
By my count (based entirely on visual inspection) all but 2 of these exhibit large scale rotation, with perhaps 15 or 16 classifiable as regular rotators with the remainder containing multiple velocity components including a couple with (perhaps) kinematically distinct cores. The preponderance of rotating systems surprised me at first, but according to a review by Cappellari (2016) large scale rotation is predominant at least at lower stellar masses (Schawinski et al. characterized their sample as being “low to intermediate mass” among early type galaxies). The velocity fields indicate that many of these contain stellar disks, perhaps embedded in large bulges. That’s still consistent with classification as “early type galaxies.” Apparently the original Galaxy Zoo classification page used the term “elliptical” as the early type galaxy choice, but in the data release paper by Lintott et al. (2011) there’s a statement that the “elliptical” class should comprise ellipticals, S0’s, and perhaps Sa’s from Hubble’s classification scheme.
Depending on how my effort to do non-parametric line of sight velocity modeling goes I may return to examine the kinematics of this sample in more detail, in particular to look for evidence of gas and stellar kinematic decoupling.
Turning to the recent star formation history this sample runs the gamut from large scale starbursts to passively evolving as seen in the plot of (100 Myr averaged) star formation rate versus stellar mass density for all analyzed binned spectra (of which there were 1525 in the full sample). For reference the straight line is my estimate of the center of the local “spatially resolved star formation main sequence.” This is just a weighted least squares fit to the sample of 20 non-barred spirals with star forming BPT diagnostics that I discussed some time ago. My SFMS relation has the same slope as estimated by Bluck but is offset higher by about 0.7 dex, which probably just reflects the very different methods used to estimate star formation rates. The contour lines are the densest part of the relationship from the passively evolving Coma cluster sample that I also discussed in that post. The majority of the blue etg sample falls in the green valley, consistent with Schawinski et al.’s observation that only about 1/2 of the sample showed evidence for ongoing star formation.
“Spatially resolved” star formation rate density versus stellar mass density for 24 blue early type galaxies in final MaNGA data release. Contour lines are corresponding values for 33 passively evolving Coma cluster galaxies.
Most of the points offset the most on the high side of the SFMS come from just two galaxies: MRK 888, which I’ve discussed in the last few posts, and SDSS J014143.18+134032.8 (this is apparently not in any “classical” catalog). The legacy survey cutout below clearly shows an extended tidal tail that’s a certain sign of a relatively recent merger.
SDSS J014143.18+134032.8, a disturbed, star-bursting blue early type galaxy
I just want to take a quick look at this one: below are maps of the star formation rate density and SSFR as well as scatterplots of the same against distance from the IFU center. As with MRK 888 ongoing star formation is widespread with a peak near the center, a classic case of a merger fueled starburst. In this galaxy star formation peaks in a ring somewhat outside the nucleus. The ring can be seen clearly in the SDSS cutout and must consist of HII regions.
SDSS J014143.18+134032.8 (mangaid 1-41541; plateifu 8095-1902)
Star formation rate density and specific star formation rate – maps and scatterplots against radius in kpc.
Schawinski et al. briefly discuss the possibility that their blue ETG’s could be progenitors of E+A (aka K+A) galaxies. This galaxy and MRK 888 are plausible candidates — if star formation shut off rapidly they would certainly exhibit strong Balmer absorption for a time after emission lines disappeared since they already do. Other members of this sample are already fading towards the red sequence, and if they ever qualified as “post-starburst” it must have been in the past.
I plan to look at star formation histories in more detail after I’ve completed model runs on the MaNGA post-starburst sample.
The first iteration of Galaxy Zoo led to several collections of distinct objects, including a sample of 215 “blue early type galaxies” published in Schawinski et al. (2009)1which inexplicably and consistently says there were 204 objects while the catalog published in Vizier contains 215.I found this an interesting group of galaxies, partly because of a possible link to post-starburst (K+A) galaxies that was discussed in the original paper. The authors discuss at some length the likelihood that these are results of mergers in the cosmologically recent past, with at least one of the progenitors being gas rich. Many (at least 25% and possibly more than half) were found to be currently starforming and the rest likely to have only recently ceased forming stars as inferred from their blue colors.
The ongoing Zoogems program has 12 of Schawinski’s blue ETGs on its target list, of which 6 have been observed so far as of mid-January 2022. Somewhat surprisingly there are 24 in the final MaNGA release, over 11% of the sample!
Taking a look at the 6 with HST observations I would say none of these are typical ellipticals. Five show some degree of spiral structure although in 4 it’s embedded in a more diffuse body. One appears to me to be an S0 with both inner and outer rings — this is in agreement with the one published morphological classification I’ve found. All of the others appear more disky than ellipsoidal to me, although this is just my possibly flawed qualitative judgment. At least two are visibly disturbed. One (CGCG 315-014) is connected to a nearby galaxy with a long tidal tail as seen in the Legacy Survey thumbnail below. Markarian 888, which will be the subject of the rest of this post, has shells that extend well past the main body of the galaxy and prominent, centrally concentrated dust lanes.
CGCG 315-014 Legacy Survey Thumbnail
So far it’s the only Zoogems blue etg target with a MaNGA observation (two others on the target list are in MaNGA but of course there’s no guarantee they will ever be observed). As is often the case the IFU could have been larger — this was observed with a 37 fiber bundle giving 111 dithered spectra in the RSS file.
MRK 888 SDSS thumbnail with MaNGA IFU footprint
As always the first step in analyzing these data is to estimate redshift offsets for each spectrum, and from there we get a velocity field, which in this case shows a rapid rotator with a fairly symmetrical radial velocity pattern.
Mrk 888 (MaNGA plateifu 9894-3703) velocity field
Visual inspection suggests the line of sight velocity distribution is consistent with a rotating thin disk, so I fed the data to my Gaussian process based rotation modeling code, with results summarized below. In fact the model does an excellent job of accounting for the data, with residuals (not shown) from the model fit (top right) in a range of ±15 km/sec. One unusual feature of the velocity field is the rotation velocity turns over at somewhat less than one effective radius. Whether the rotation curve declines smoothly outside the IFU footprint or is kinematically disconnected from the outer parts of the galaxy is of course unknowable at this time.
Gaussian process rotation model results
I also ran my usual star formation history modeling code on the data binned to 97 spectra. First, here are some summary results. The stellar mass density declines roughly exponentially, which is consistent with a disky morphology:
Model estimate of stellar mass density vs. radius
Next are maps of the estimated Hα luminosity density and, on the right, the BPT classification from the [O III]/Hβ vs. [N II]/Hα diagnostic. The contours are elliptical with major axes closely aligned to the rotation axis (the posterior mean for the angle is the dashed line in the velocity field plot above). Again, the emission appears to arise in a disk.
The proper interpretation of the “composite” BPT classification is something I think I’ve written about in the past. It was originally suggested to indicate a mix of AGN and stellar ionization, but here it arises in a thin ring of weak but detectable emission just outside the star forming region. If it’s truly composite it’s likely to arise from a mix of weak star formation and ionization by hot evolved stars. In any case there’s no evidence for an AGN in the optical data.
(L) Hα luminosity density (R) BPT classification from [O III]/Hβ vs {N II]/Hα diagnostic
Next are maps of the modeled (100 Myr average) star formation rate density and specific star formation rate, and in the second row scatter plots of the same estimates against radius in kpc. The trends with radius are somewhat unusual, especially for SSFR which in a normal disk galaxy typically increases with radius even if the highest total star formation rates are centrally concentrated. Highly centrally concentrated star formation in the aftermath of mergers is predicted by some simulations.
(TL) star formation rate density; (TR) specific star formation rate; (bottom row) scatter plots vs. radius
A couple more graphs will round out my discussion of summary model estimates. As I’ve shown several times before there’s a pretty tight linear relationship between modeled SFR density and estimated Hα luminosity density. In this plot Hα is corrected for modeled stellar attenuation, which is expected always to underestimate the attenuation in emission line emitting regions. That, and the fact that Hα emission and the model star formation rate estimates probe order of magnitude different time scales probably account for the systematic offset from the standard calibration given by the straight line.
Model star formation rate density vs. Hα luminosity density corrected for stellar attenuation. Straight line is calibration from Calzetti (2012).
And, once again I show a map of the modeled optical depth of stellar attenuation. The region of highest optical depth nicely tracks the visible dust (the HST image at the top is rotated about 90º from the SDSS image). Outside the dusty region there appears to be a shallow gradient, which might indicate that the nearer side is to the northeast.
Map of modeled optical depth of stellar attenuation
Finally here are plots of the model star formation history for all spectra ordered by distance from the IFU center. In the inner 1.5 kpc or so there’s some recent burstiness with possibly a very recent acceleration of star formation. For reasons I’ve discussed recently I don’t take either the timing or magnitude of bursts of star formation too seriously, but the behavior of the models is consistent with a recent revival of star formation due presumably to a merger, for which there are multiple lines of evidence.
model star fomation histories for all spectra
With 24 of these galaxies and another 31 from the compilation of Melnick and dePropris and the post-starburst ancillary program in the final release of MaNGA these samples satisfy my criteria of being manageably sized for my computing resources while large enough to say something about the groups. So, when time permits I plan to take a look. I already have the data in hand.
