When I was doing my initial fits to the M31 MaNGA spectra I noted two that I initially thought were contaminated by foreground stars, and therefore I masked them to prevent further analysis. One of the two, in MaNGA plateifu 9677-12701 (mangaid 52-8) is a certain foreground star and won’t be discussed further. The other, in plateifu 9678-12703 (mangaid 52-23), turns out to be a luminous red supergiant that’s a genuine resident of M31. This is confirmed by two nearly contemporaneous catalogs of M31 red supergiants: the one by Ren et al. (2021) that I noted previously and Massey et al. (2021), which I stumbled upon more recently.
In fact there are two comparably bright red supergiants in this IFU. One that’s about 9″ north of the masked one probably should have been masked by whatever criteria I used, but it’s likely I failed to notice the fit to the data since I don’t have the patience to look at every spectrum and data fit that pops up. So, here is the spectrum, displayed in (negative) magnitudes with arbitrary zero point. The blue spectrum is the closest match in my SSP library, a 10 Myr old population with the highest metallicity I used (2.5 Z☉. This is one of the theoretical spectra from PyPopstar). This sorta looks right except it’s much too blue. The solution to that is, of course, to add some reddening through dust attenuation.
plateifu 9678-12703 (M31 10 kpc ring)
Spectrum contaminated with red supergiant and closest match SSP model spectrum
The maximum likelihood (non-negative weighted least squares) fit did just that, with only a single stellar contributor and a very high dust attenuation of τV = 3.0. This still doesn’t quite work: the residuals are rather strongly sloped in the blue and the details of the absorption features in the red aren’t quite right.
plateifu 9678-12703 – NNLS fit to spectrum contaminated with red supergiant
I still use a Calzetti attenuation relation in my NNLS fits. The Bayesian fits using Stan have the more flexible attenuation prescription that I described back in this post, and that helped considerably with the continuum as seen in the plot below. The absorption features in the red still aren’t fit well. The model has an even more extreme attenuation estimate with a much “grayer” than Calzetti slope, with τV = 4.38 ± 0.05 and δ = -0.33 ± 0.011see the link above for the meaning of these parameters.
9678-12703 – posterior predictive fit to spectrum
The model star formation history (displayed as a mass growth history below) isn’t completely implausible. The presence of a very luminous evolved star indicates the region is at least some Myr old, and a rapid onset and decline of star formation is typical for star forming regions in mature spirals. The recent episode of star formation added about 7% to the present day stellar mass, while at least 60% was in place by 8 Gyr ago (per the model).
plateifu 9678-12703 (M31 10 kpc ring)
Model mass growth history for a region containing a bright red supergiant
Nevertheless I consider the model results to be highly suspect, mainly based on the very large optical depth. Massey estimates the attenuation for the red supergiant to be AV ≈ 1.18, although this is apparently based on a formula rather than a direct empirical estimate. But another piece of evidence that’s close to a smoking gun is an estimate based on the Balmer decrement of emission. Despite the lack of apparent ionizing sources outside the bright H II regions in the west there is widespread diffuse emission in this region with star-forming like line ratios. The estimated optical depth derived from the Balmer decrement for this region is τV, bd = 1.47 ± 0.23 (1σ), reasonably consistent with the Massey estimate and with the values derived for the rest of the IFU.
How widespread a problem is this? Below I plot the Balmer decrement derived optical depths against the stellar based estimates for all spectra in M31 MaNGA with star forming emission line ratios, about 11% of the entire sample. The 5 most extreme outliers are in this IFU in the regions surrounding the two bright red supergiants (the masked spectrum would also be in this region of the plot). The same 5 regions are also extreme outliers in the SFR vs. stellar mass and SFR vs. Hα plots that I showed early on. So, even though there are many cataloged supergiants in the study region these two appear to be uniquely bright and to have had the largest impact on model results.
M31 MaNGA – Optical depth off attenuation estimated from Balmer decrement vs. model values of τV
Of course there are hotter bright stars in the study area and these could affect results in different and possibly unexpected ways. For example the outermost IFU contains one bright star that GAIA estimates has a surface temperature of 5500 C, which would make it a G supergiant if it’s in M31. I noted in the last post that the model star formation history for that region looks like a post starburst with an age around 800 Myr. This is, I think, several galactic rotation periods, and stars born that long ago should have dispersed by now unless they’re gravitationally bound. There’s no sign of a star cluster there nor is there a cataloged one nearby, so it seems likely to me that the “starburst” is an artifact. As I noted in the last post though the fit to the data is quite good.
These examples illustrate an issue that’s fairly well known, which is that using simple stellar populations as building blocks of low mass stellar systems are potentially affected by so-called “stochastic” effects, which simply means that the distribution of stellar masses can vary randomly from what’s assumed in the SSP models. Specifically, in M31 there are individual stars luminous enough to affect spectra. One possible solution might be to add some stellar spectra to the library. I might give that a try some day.
I’m going away and won’t be writing for a while. I’m hoping to acquire or build a SSP model library based on SDSS MaStar spectra yet this year. This is a much larger collection of stellar spectra than has been previously available and it has the advantage of having the same flux calibration and (approximately) spectral resolution as the SDSS galaxy spectra. I also plan to return to my study of post-starburst galaxies.
On to the final batch, which I don’t think is going to be very interesting.
plateifu 9678-12704 (mangaid 52-22)
If my calculations reported in the last post are correct this is well in the outer disk at a distance of about 15.4 kpc from the nucleus. Other than that there’s nothing much to say about it. There are no cataloged objects of interest in the IFU footprint. There is diffuse emission with a fairly strong gradient decreasing from northwest to southeast, which is basically moving outward in the disk. There may be low level ongoing star formation.
plateifu 9678-12704 (M31 outer)
(L) Hα luminosity density (uncorrected)
(R) SFH summed over IFU footprint
plateifus 9678-6103 and 9678-12702 (mangaid 52-19 and 52-24)
These are both in interarm regions with absolutely no cataloged objects of interest and complete blanks in the Galex false color image. Even diffuse emission is too weak to detect confidently. All regions in both show very low recent star formation with a long period of quiescence.
plateifu 9678-12701 (mangaid 52-25)
Finally, this is the outermost IFU in the program, located an estimated 15.7 kpc from the nucleus and very close to the major axis. Somewhat oddly it’s close to the most vigorously star forming region in this segment of the outer disk but offset by a little more than an IFU width. In fact in the Galex image it appears to be in a sort of notch with few UV bright sources.
MaNGA plateifu 9678-12701 (M31 outer disk)
Cutout from PHAT color image retrieved in Aladin.
No H II regions are cataloged within the IFU footprint but there is diffuse emission throughout with mostly starforming-like line ratios. The region with the highest modeled star formation rate is in the western corner of the IFU, where a number of blue stars can be seen in the PHAT color cutout. There is a single, somewhat isolated bright star near the center of the IFU. The SFH model for that region is in the lower right panel below. The model indicates a rather strong and short burst of star formation a little less than a Gyr ago. How much the model is influenced by the star is hard to say. The fit to the data is actually rather good. The star may be in the foreground: In Gaia DR3 (data retrieved through Aladin) its distance is listed as 2395 pc, which would obviously place it in the Galaxy. But that is based on estimates of its surface temperature and gravity rather than parallax, which is measured as negative and consistent with 0.
MaNGA plateifu 9678-12701 (M31 outer disk)
(TL) Hα luminosity density (uncorrected)
(BL) model star formation rate density
(TR) Star formation history for region with highest SFR density
(BR) SFH for a region with a bright starMaNGA plateifu 9678-12701 (M31 outer disk)
posterior predictive fit to the spectrum of a region with a bright star
After looking through all the data again I have to say I’m puzzled by some of the choices of IFU locations. All but 5 are in or very close to spiral features visible in Galex, but most are offset by as little as an IFU width from regions with more star forming activity. Even plateifu 9678-12703, which is very close to the most active starforming region in the PHAT coverage area, only captures the edge of a series of bright H II regions.
Overall I think the SFH models are successful with some caveats. Areas associated with bright Hα emission are generally showing increasing recent star formation rates reasonably consistent with the level of emission. It’s interesting that there are often nearby regions (separations ~10 pc or so) that have recently peaked but with high 100 Myr averaged SFR. This suggests we can actually see propagation of star formation over short distances and time scales.
A big concern is the effect of sampling small stellar mass regions, and in particular the effect of exceptionally luminous stars on model results. I plan to address this in a follow-up sometime soon.
Before I get into details of the individual IFUs here is the result of a little exercise I did to estimate the deprojected positions of the IFU centers using the canonical values of 77° for the inclination and 38° for the major axis position angle, and applying the coordinate conversions I outlined way back in this post with slight modifications. These are fairly rough estimates since Andromeda’s disk is apparently warped and rather thick, but this may help give some perspective on relative positions in the plane of the galaxy. For reference I’ve drawn semcircles at 6, 10, and 16 kpc, and circled the IFUs that I had placed in the 10 kpc ring. Two of the IFUs — plateifus 9678-6102 and 9678-9102 — now appear to be at or beyond its outer edge at radii of 12.9 and 13.5 kpc, while plateifu 9678-9101 which I had assigned to the outer desk is a bit closer at 12.1 kpc. But, no matter. I will discuss them in the same order as I presented the IFU wide star formation histories several posts ago.
Approximate deprojected coordinates of M31 MaNGA IFUs. Coordinates are in kiloparsecs relative to the galaxy center, with the X axis parallel to the major axis and increasing to the northeast.
plateifu 9677-9102 (mangaid 52-1)
This is already a recurring theme. There is a single cataloged H II region within the IFU footprint that coincides with the highest (uncorrected) Hα luminosity density bin below. The bin with the highest (100 Myr) SFR density is displaced by several parsecs to the northeast. The first region has an increasing star formation rate over the last ~30 Myr, while the second shows a sharp peak and rapid decline over the last 10 Myr. If the models are remotely correct this is clear evidence for propagation of star formation over short distances.
plateifu 9677-9102 (M31 10 kpc ring).
(TL) Hα luminosity density.
(BL) SFR density (100 Myr average)
(TR) SFR history for the region with highest Hα density.
(BR) SFR history for the region with highest SFR density.
plateifu 9677-12702 (mangaid 52-7)
Despite being right in the middle of the 10 kpc ring there’s nothing very interesting in this IFU, with just a single cataloged red supergiant and a small and rather faint H II region that’s at or beyond the edge of the footprint. Only weak diffuse emission is seen in the MaNGA data. Nevertheless there are a few areas with evidence for recent star formation:
plateifu 9677-12702 (M31 10 kpc ring)
(L) map of SFR density
(R) star formation histories for 2 regions with higher than average SFR density.
I recently noticed that one of the imaging products available in Aladin is a color composite assembled from the PHAT F475W and F814W ACS/WFC observations. One thing these are good for is they give you a very rough idea of stellar temperatures. To my eyes at least stars appear orange, white, or blue. Bright blue stars must be young; bright orange ones are evolved (or reddened by dust perhaps) and might be young or old. Notice below that the two areas with relatively high star formation have a sprinkling of bright blue stars, while the bulk of the field contains predominantly orange ones.
The other optical wavelength imaging I look at comes from SDSS. Even though the imaging in this area is incomplete and the processing leaves something to be desired it does have the virtue that Hα is, at low redshift, in the r filter, which forms the green channel in their images. M31 H II regions then are identifiable by their green color. Also, red supergiants look distinctly red since their brightness is still increasing into the near IR.
plateifu 9677-12702 (M31 10 kpc ring)
Cutout of PHAT color composite taken from Aladin
plateifu 9678-6102 (mangaid 52-20)
This lies on the outer edge of the 10 kpc ring, some distance from UV bright sources and H II regions. There is one cataloged planetary nebula and that shows up in my modeling as a region with “AGN” like emission line ratios. One fiber has a much higher modeled specific star formation rate than its surroundings. A map of SSFR and star formation history for that region are shown below. There are bright red and yellow stars in the region which might be (uncatalogued?) red supergiants.
MaNGA plateifu 9678-6102 (M31 10 kpc ring)
(L) map of model specific star formation rate
(R) star formation history for the region with highest SSFR
9678-9102 (mangaid 52-18)
This is the final IFU on the eastern side of the 10 kpc ring. Once again it lies at the outer edge, well away from active star forming regions. There is weak emission throughout, with star forming line ratios through much of the IFU footprint despite the lack of evident star forming regions. There’s some unresolved UV emission in the GALEX color image that roughly corresponds in location to the area of brighter Hα.
The region with the greatest recent star formation has some fairly bright blue and yellow stars in the PHAT color image
plateifu 9678-9102 (M31 10 kpc ring)
(TL) Hα luminosity density
(TR) BPT classiication by [N II]/Hα vs {O III]]/Hβ diagnostic
(BL) 100Myr average specific star formation rate
(BR) model star formation history for the region with highest SSFR
By the way I do check observational quantities in my models against the SDSS product Marvin now and then. Here’s their rendering of the Hα flux:
Qualitatively at least the agreement is excellent. I’d have to check if their fluxes are consistent with my log-luminosities.
plateifu 9678-12703 (mangaid 52-23)
Finally we get to the most interesting IFU in the project, plateifu 9678-12703, which lies very close to the region with the highest recent star formation in the northeastern half of the galaxy. It also appears to coincide in position with one of the regions that Lewis et al. (2015) highlighted (their Figure 2). As can be seen in the Aladin cutouts (from SDSS and PHAT color images) below there are several young stellar objects within and near the IFU footprint: at least two red supergiants (which are a problem); 3 catalogued H II regions, one of which is bright and extended; some OB associations that are centered outside the footprint; and one open star cluster. There are a number of bright blue stars scattered throughout as can be seen in the color PHAT image.
plateifu 9678-12703 (M31 10 kpc ring)
Symbols: yellow circle: H II region
red circle: red supergiant
purple oval: star cluster
blue rhombus: OB association
blue squares: IFU outline
Plotted below are maps of Hα luminosity density and model star formation rate density, along with model star formation histories for two regions. The first is for the bins in the brightest part of the H II region along the western edge of the IFU. The second is for two bins at the position of a cataloged open cluster (Johnson et al. 2016) that’s fairly obvious in the PHAT cutout. The cataloged (log) age of the cluster is 8.4-0.1+0.3 with a mass around 104 M☉. The peak star formation rate in the model history below (bottom right) is at about 500 Myr lookback time with several hundred Myr of enhanced star formation, so this is pretty good agreement.
plateifu 9678-12703 (M31 10 kpc ring)
(TL) Hα luminosity density (uncorrected)
(BL) model star formation rate density
(TR) star formation history for areas with highest Hα luminosity.
(BR) SFH for a region covering a cataloged open cluster
When I did my initial fitting runs on this IFU I noticed one fit that was rather poor which I attributed to a foreground star, and therefore I masked it for subsequent analysis. It turned out though the culprit was not a foreground star but instead a local red supergiant that’s been cataloged (for example) by Ren et al. (2021). Their catalog lists its G band magnitude from Gaia DR2 as 19.1 which makes its absolute magnitude around -5.3, a reasonable value for its presumed spectral type.
This raises an issue that’s fairly well known. Simple stellar population models assume the age zero main sequence is fully occupied according to a well defined initial mass function. This is a fairly innocuous assumption (although the choice of IMF is not) when we’re sampling ~billion solar mass regions, but it’s not so innocuous for cluster size agglomerations, which is what we’re sampling here1the typical binned region has a present day stellar mass around 104 M☉ per my models. The particular problem here is that a single red supergiant is making a significant contribution to the spectrum in the red, and that could be biasing the model SFH in as yet unexplored ways.
The bin at the position of the other bright supergiant in the footprint was analyzed, so lets take a quick look. In the top pane below is the model star formation history, and in the two below the (posterior predictive) fit to the data and the residuals from same. The fit doesn’t look so bad except for a region around 7200 Å, which often seems to be a problem with EMILES spectra.
Superficially the model star formation history looks not implausible, and similar to others I’ve shown. The presence of an evolved star indicates a stellar age in the right general range, as does the relative lack of H II emission. Despite the strength of the burst it adds only about 7% to the present day stellar mass, with as elsewhere the majority of the stellar mass was in place by 8 Gyr ago (per the model, as always).
But, there’s at least one indicator of a problem: the modeled optical depth of attenuation is extraordinarily high at τV ≈ 4.4, compared to the optical depth estimated from the Balmer decrement of τVbd = 1.47 ± 0.23. I plan to discuss this in more detail in a future post, but for now I’m moving on.
plateifu 9678-12703 (M31 10 kpc ring)
model star formation history for a region with high recent SFR
plateifu 9678-9101 (mangaid 52-26)
As mentioned at the top this appears to be in a spur off the 10 kpc ring. There is just one cataloged H II region within the footprint that appears to be compact. Just to the west there is a sprinkling of bright blue stars and an unresolved blob in the Galex color image. The H II region is evident in the map below. Once again the regions with the highest Hα luminosity and highest (100 Myr averaged) star formation rate are slightly offset from each other. The model star formation histories are very similar though.
plateifu 9678-9101 (M31 10 kpc ring)
(TL) Hα luminosity density (uncorrected)
(BL) model star formation rate density
(TR) star formation history for area with highest Hα luminosity.
(BR) SFH for area with highest recent star formation rate
I’m going to stop for now and cover the last 4 outer disk IFUs in (probably) the next post.
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’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.
