One simple way to quantify the burstiness of star formation is just to estimate the average star formation rate over large time intervals divided by the average SFR over cosmic time. Of particular interest is the time interval between ~100 Myr and ~1 Gyr since this is roughly the time interval that a post-starburst galaxy is recognizable as such.
Partly because it happens to still be in my active workspace and partly because it’s really interesting I’m going to take another look at SDSS J095343.89-000524.7 (MaNGA mangaid 1-897). This was in the post-starburst ancillary sample, selected from the catalog by Pattarakijwanich et al.
This image from the Subaru HSC-SSP survey1retrieved as a screenshot from the Legacy Survey sky browser. is much deeper than SDSS imaging and clearly shows extended tidal tails and debris, suggesting that these galaxies have been interacting for some time.
SDSS J095343.89-000524.7 (observed as mangaid 1-897).
Image screenshot from Subaru HSC survey.
Moving on to various properties derived from the MaNGA spectroscopy and my SFH models with, still, EMILES based SSP models. First here are maps of stellar mass density and 100 Myr averaged star formation rate density. Note that I rebinned the spectra from two posts ago to try to capture more of the tidal tails while excluding the truly blank regions of sky. There are two clear peaks in the stellar mass density separated by a projected distance of about 11 kpc. The central stellar mass densities are nearly the same at about 108.95 M☉/kpc2 . Interestingly enough the bright white peak in surface brightness appears not to coincide with the western peak in stellar mass density, but is offset by a small amount to the north.
Note also that the highest recent star formation is offset to the north of the apparent western nucleus. I’ll look at that in more detail below.
MaNGA plateifu 10843-9101 (mangaid 1-897). Maps of stellar mass density and star formation rate density.
The ionized gas properties are rather different in the two galaxies. Below are BPT classifications using, as usual for me, just the [O III]/Hβ vs. [N II]/Hα diagnostics and Kauffmann’s classification scheme. Emission line fluxes are generally stronger in the eastern galaxy with mostly star forming line ratios. Note two spectra with “composite” line ratios are near the eastern nucleus and might therefore actually be due to a mix of stellar and AGN ionization.
MaNGA plateifu 10843-9101 (mangaid 1-897). BPT classifications from [O III]/Hβ vs. [N II]/Hα diagnostics
I calculate a few “strong line” gas metallicity estimates from standard literature sources. The one that seems to produce the most consistent estimates is the calibration of Dopita et al. (2016) based on the ratios of [N II 6548]/[S II 6717, 6731] and [N II]/Hα. The eastern galaxy shows a fairly smooth radial gradient while the west is considerably metal enriched in the region with the strongest starburst. The highest metallicity is right at the center of the IFU at the position of the bright white source.
MaNGA mangaid 1-897 (plateifu 10843-9101). Gas phase metallicity 12 + log(O/H) from strong line calibration of Dopita et al. (2016).
Let’s return to the idea I had at the top of the post to look at star formation rates in broad time intervals relative to the mean star formation rate over cosmic time. For this exploratory exercise I used just 4 bins with upper age limits of 0.1, 1.25, 2.25, and (nominally) 14 Gyr. There seems no point being too fastidious about calculating the bin widths: I just used the difference in nominal ages between the endpoints. I did take into account the lookback time to the galaxy, which for this one is about 1 Gyr (z = 0.083), so the final bin has a calculated width of 10.5Gyr. I chose to make the 3rd, intermediate age bin a rather short 1 Gyr wide to look for aging starbursts that might be missed using the typical selection criterion of strong Balmer absorption. In this case there’s no evidence of that: both galaxies seem to have had uneventful histories up until ~1 Gyr ago.
The top row of the plot below is the most interesting: there appear to have been two major bursts of recent star formation, both highly localized to the central region of the western galaxy. If the model estimate of the location of the peak stellar mass density is correct the fiber with the largest star formation excess in the 100 Myr – 1.25Gyr interval is offset just to the north and coincident with the IFU center. The more recent burst is also offset from the older one. There is a hint of recent accelerated star formation over most of both galaxies.
MaNGA plateifu 10843-9101 (mangaid 1-897). Maps of relative average SFR over the designated time intervals.
For the rest of this post I plot model fits to the spectra and star formation histories for the fibers surrounding the two nuclei. These are ordered approximately from north to south and west to east. For reference the IFU center is at (ra, dec) = (148.43291, -0.09018). The model has the peak stellar mass density in the western system at (ra, dec) = (148.4328, -0.09062). The eastern galaxy’s nucleus is at (ra, dec) = (148.4349, -0.09064).
Note below that the plots have different vertical scales. The horizontal scales are the same for both spectra and star formation histories, but at least one SFH plot is slightly misaligned.
Central region – western galaxy
MaNGA mangaid 1-897 — central region of western galaxy – spectra with fits and model star formation histories
Central region – eastern galaxy
MaNGA mangaid 1-897 — central region of eastern galaxy – spectra with fits and model star formation histories
In an earller post I mentioned a MaNGA related paper by Cheng et al. who found nearly 500 systems with post-starburst characteristics that fell in 3 broad categories: centrally concentrated PSB regions, ring-like, and irregularly located. Clearly any galaxy that was selected based on SDSS spectroscopy that’s not a false positive will have a central PSB region, although that of course doesn’t preclude extended post-starburst conditions. This particular galaxy appears to have a remarkably compact post-starburst region.
When time permits again I plan to look at the remaining 40 galaxies in this sample. Unfortunately the larger sample of Cheng et al. appears to have no published catalog.
I haven’t given up on this topic. Just a longer than expected break.
I found two other catalogs of candidate PSBs selected from SDSS spectroscopy. First are the “SPOGs” (Shocked POst-starburst Galaxies” of Alatalo et al. (2016), with the catalog retrieved from VizieR at J/ApJS/224/38/table2. These were selected to have strong emission lines with ratios consistent with shocks as the ionizing mechanism, while also having strong Balmer absorption indicating the presence of a large intermediate age stellar light contribution.
The second was the sample of Pattarakijwanich et al. (2016) retrieved from J/ApJ/833/19/table3. This work used more traditional post-starburst selection criteria although somewhat more relaxed than for example Goto. Together these added 19 galaxies to the sample — 14 SPOGs and 5 from Patta… Together these, along with the Melnick and dePropris sample added about 1000 binned spectra to the sample.
I’m not going to say much about them for now. SDSS thumbnails are below. One thing I note is that a fairly large fraction of these appear to be normal star forming disk galaxies. Most of those, I suspect, are SPOGs. Of course since they were selected from 3″ SDSS spectra it’s entirely possible these galaxies are centrally quenched due to some feedback mechanism.
I’m still thinking about how to quantify “post-starburstiness.” Perhaps something like the stellar mass formed in a time interval like 1.1 – 0.1 Gyr.
I’ve decided to resume SFH modeling despite still not having a fully satisfactory SSP model library. I’m still using the EMILES + PyPopstar hybrid that has served as the stellar input for several years now. The only change I’ve made — strictly for visualization purposes — is to define the stellar ages as representing the middle of each time interval instead of the end. In star formation rate plots this has the effect of smoothing out the SFR a little bit where there are abrupt jumps in time intervals. This has no effect on the modeling process at all.
One of the MaNGA ancillary programs (PI C. Tremonti) observed a sample of 24 candidate post starburst galaxies drawn from 5 different sources (both published and unpublished) with a variety of selection criteria. In addition to these there are 7 PSB’s from the compilation of Melnick and De Propris (2013) in the primary or secondary samples that I added to the sample for a total of 31. I was able to run successful models for 30 data sets, with one having severe calibration issues that I dropped from further analysis. Altogether there were 1,399 binned spectra in the sample with as usual a large range of bins per galaxy: in this case ranging from 6 to 240.
From a diverse set of selection criteria it’s not too surprising that the sample is rather diverse too, with perhaps a few false positives. I’m not sure it makes sense to treat this as a single homogeneous sample, but for now let’s take a look at a few features of the entire data set. I’ll also take a sneak peek at a particularly interesting pair of galaxies.
First, here is a popular absorption line diagnostic, the Lick HδA – Dn(4000 Å) plane. Points are colored by BPT diagnostic determined from the [N II]/Hα and [O III]/Hβ ratios by the usual criteria. The contours are from measurements of a large sample of SDSS galaxies by the MPA-JHU pipeline, which was run on spectra through DR8.
Lick HδA versus 4000Å break strength – MaNGA post starburst sample. Contour lines are for a large SDSS sample with measurements from the MPA-JHU pipeline.
It seems odd that the bulk of the measurements in this sample are displaced from the bulk of the SDSS sample. I wouldn’t completely rule out errors in my measurements but I tested mine against the MPA-JHU measurements a long time ago, and this particular part of the code is unchanged for some time. Anyway, we see a large range of values of these diagnostics, but with relatively few in the passively evolving region at lower right and many in the “green valley.” Almost 1/3 have strong Balmer absorption with HδA > 5Å EW. Many of these also have star forming BPT diagnostics, so it’s not clear that these regions are post starbursts.
Next, here are (100 Myr averaged) star formation rates plotted against stellar mass density, again color coded by BPT diagnostic. The straight line is my calibration of the center of the “star forming main sequence” from some time ago.
Modeled SFR density vs. stellar mass density – MaNGA post starburst sample.
Evidently there are many regions — mostly with star forming emission line ratios — lying near the star forming main sequence, and also a large number in the green valley. Most of those have weak emission lines, AGN, or LINER-like ratios.
Finally, here is a plot of model specific star formation rate against Dn(4000Å). As I’ve written before a number of authors have noted the relation between the 4000Å break strength and stellar age or specific star formation rate and several have used it as a (usually secondary) star formation rate indicator. The straight line is my estimate of the mean relation for spiral galaxies, originally given in this post.
Model specific star formation rate versus 4000Å break strength – MaNGA post starburst sample.
Evidently by these diagnostics this sample has properties that at least overlap with a random selection of normal galaxies. The only thing notably missing are “red and dead” ETGs. However there are good reasons to think that starbursts – and therefore post starbursts – are generally localized regions within galaxies. We need to look at the spatially resolved properties — specifically star formation histories — to see how many genuine post starburst galaxies are in the sample.
I’m going to end for now with one of the more interesting examples in this sample. The western member of this interacting pair has a remarkably bright and white nucleus, which in SDSS imaging indicates a fairly young stellar population.
SDSS J095343.89-000524.7 MaNGA plateifu 10843-9101 (mangaid 1-897)
I slightly altered my usual workflow for this and a few other data sets in this sample. Usually I try to use all spectra and bin to a minimum target SNR (usually 5) for all bins, but since this IFU had a large fraction of blank sky I set the SNR threshold for accepting a bin lower than I otherwise would and left the lowest SNR spectra out of the analysis. Below is a map of the modeled stellar mass density showing the coverage of the analyzed area.
MaNGA plateifu 10843-9101 (mangaid 1-897). Model stellar mass density; analysis coverage
And for now I’ll just show the spectra of the two nuclear regions with posterior predictive fits of the SFH models, along with model star formation histories. The western nucleus has a remarkable K+A like spectrum but with fairly strong emission from a possible AGN. The model star formation history is one of the most unusual I’ve seen. Whether it’s an accurate record of events is of course uncertain.
MaNGA plateifu 10843-9101 (mangaid 1-897). Nuclear spectra with posterior predictive fits and model star formation histories.
I’m going to continue this topic in additional post(s), and perhaps look for a larger sample. A recent paper by Cheng et al. (2024) found nearly 500 galaxies with post starburst properties in MaNGA, but there seems to be no catalog. I’m not sure their selection criteria are easily reproduced.
Three posts ago I did a brief comparison of my usual EMILES based models with the most recent version of Stanway and Eldridge’s BPASS models, which are the first purely theoretical model spectra that seem possibly suitable for full spectrum fitting. I mentioned then that I used a set of models with an age zero upper mass limit of 300 M☉, while most model libraries adopt an upper mass limit of 100 M☉. As is customary in this industry their website contains a number of additional model libraries, including with upper stellar mass limits of 100 M☉ and some with single star evolution only. These alternate sets of models have the same structure as the baseline models, so I just used the same scripts to create R readable data sets with the same ages and subset of metallicities..
Not surprisingly the 1 Myr models with 300 M☉ upper limit are considerably more luminous than either the binary or single star models with 100 M☉ limits, but the latter are still somewhat more luminous than my standard library at the same age. Stars >100 M☉ evolve very rapidly and the difference in model spectra disappears by log(T) = 6.6 (4 Myr).
BPASS solar metallicity age 0 model spectra
To test the differences in model star formation histories I just ran one set of models on the post-starburst galaxy WISEA J080218.38+323207.8 (MaNGA plateifu 10220-3704, mangaid 1-201936). And here’s the comparison:
Model star formation histories for 3 SSP model libraries. MaNGA plateifu 10220-3703 (mangaid 1-201936)
The basic result is there’s no real difference either with the 300 M☉ upper limit models or between the binary and single star evolution models1note that one spectrum in the fifth row above has a rather different model SFH for the binary library. This turned out to be from a convergence failure of the sampler in one chain. Again there’s a sharp downturn in star formation rate at the youngest ages and again there’s that peculiar spike at 1.6 Gyr in all model runs. That feature has serious consequences for the interpretation of the model star formation histories in these post-starburst galaxies. In this case the EMILES based models indicate a strongly centrally concentrated burst that began ~1 Gyr ago and lasted several hundred Myr in the center, while fading away to no significant enhancement outside a few kpc from the center. The BPASS models on the other hand have two distinct bursts near the center that straddle those of EMILES, with a significant amount of mass in a short burst throughout the galaxy. While not necessarily implausible the persistence of the 1.6Gyr spike (and as noted before not just in this galaxy) makes me suspect an artifact of some sort.
As a little bit of an aside this galaxy has one published estimate of a detailed star formation history by French et al. (2018) based on GALEX and SDSS photometry and SDSS spectra (not MaNGA). Their best fit model has two bursts at ~500 Myr and 1.5 Gyr with a total mass contribution of ~20 – 65%. Since the SDSS spectra were 3″ diameter this would be for the central region only. This at least broadly agrees with either the EMILES or BPASS based models. I have roughly 75% of the mass in the burst (EMILES) in the central fiber with somewhat more in the BPASS models, but that drops rapidly.
Well, the quest for an updated SSP library continues. Unfortunately the two likely sources of MaStar based models have yet to publish updates. I’m still considering doing my own. Unfortunately I’m not aware of any open (or for that matter closed) source software for generating SSP model spectra. This seems to be something of a dark art.
After my not so insightful realization recounted last time that my attempt to modify the prior on star formation histories wasn’t actually doing anything I thought a little further about how to specify one. Gaussians are always popular choices for priors, so why not give them a try? For a first cut I added the following lines to the “transformed data” section of the Stan model:
Even though the prior allows values of the stellar contributions that are infeasible given the simplex declaration this causes no technical problems: the models sample without complaint and parameters satisfy all constraints. Execution times are comparable to the original model formulation and convergence diagnostics are OK. But, the model runs had some unexpected features. I did a set of model runs for a single MaNGA galaxy from the post-starburst ancillary sample — mangaid 1-201936 (plateifu 10220-3703). Model star formation histories compared to the original model are shown below for the 58 binned spectra:
MaNGA plateifu 10220-3703 (mangaid 1-201936). Model star formation histories with two different SFH priors.
What’s striking here is that several of the spectra in the low S/N outskirts of the galaxy have nearly constant star formation rates with very little sample variation. In other words the models are basically returning the priors. The cause of this behavior isn’t quite clear. Relatively low signal to noise seems to be necessary, but not sufficient since similarly noisy spectra have essentially the same SFH’s as the original model formulation. It also isn’t due to convergence failure because much longer runs with more adaptation iterations show the same behavior. It is possible perhaps that the posteriors are significantly multimodal and Stan is preferentially falling into one of them. Notable also is that the fits to the data measured by log-likelihood are virtually identical even for the runs with the anomalous SFH’s. At the very least this tells us that uncertainties in quantities derived from the models are considerably larger than within model run variations — of course I have always believed this and said so a number of times.
After trying several variations on this theme that either had none at all or undesirable effects on sampling, and after some additional consideration I think that, given the model parametrization, the uniform on the simplex prior for stellar contributions is actually the one I want. That leaves the question of what, if anything, to do about the abrupt jumps in model star formation rates.
One possibility is simply to redefine the endpoints of the age bins to be, say, halfway between nominal SSP ages instead of at the model ages as is my current practice. In the case of the EMILES library this would mean for example that the 3.75, 4, 4.5 Gyr bins would have widths of 0.25, 0.375, 0.5 Gyr instead of the present 0.25, 0.25, 0.5. This involves no change to the actual model runs at all, so most quantities derived from the models are unchanged.
Another solution is to adopt a library with a more uniform age progression. One with approximately equal increments in log age seems preferred. As yet there have been no published updates to the MaStar based SSP libraries mentioned last time, so I’m waiting for them, while still considering generating my own.
I’m going to briefly return to BPASS based models. After that I’m not sure.
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.
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.
