disk galaxies – Extragalactic

2023-09-232023-09-24

MaNGA in M31 – final word for now and some pitfalls

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 τ_V = 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 τ_V = 4.38 ± 0.05 and δ = -0.33 ± 0.01¹.

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 A_V ≈ 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 τ_{V, 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 τ_V

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.

2023-09-09

MaNGA in M31 — details part 4

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 star

MaNGA 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.

2023-09-08

MaNGA in M31 — details part 3

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:

9677-12702_sfr_2bins — 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:

9678-9101 (M31 10 kpc ring) Marvin measurement of Hα flux [link]

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.

9678-12703_cutouts — 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 here¹. 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 τ_V ≈ 4.4, compared to the optical depth estimated from the Balmer decrement of τ_V^bd = 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.

2023-08-172023-08-17

MaNGA in M31 – details part 2 – the inner disk

Instead of trying a systematic investigation I’m just going to go through each IFU and discuss whatever I found interesting, with no particular theme in mind. I still don’t really know what I’m going to find since it’s been a while since I looked at model results. Besides modeling star formation histories for each spectrum I calculate summaries in the form of posterior marginal means, standard deviations, and a few quantiles for a large number of quantities. Some of these are highly model dependent such as 100 Myr averaged star formation rates and specific star formation. Some are only weakly model dependent, such as emission line fluxes¹. One thing I haven’t looked at much is stellar metallicities and especially their evolution in the models. There are always contributions from all metallicity bins at all times in my models, and how to interpret them or whether even to try still puzzles me. I am starting to look more seriously at strong emission line metallicity estimates. The estimator proposed by Dopita et al. (2016) based on [N II], [S II], and Hα seems especially promising since they’re usually detected with reasonable precision in SDSS spectra.

So, the plan is to look at each IFU, working my way outward in the disk in the same order as my second post in this series.

plateifu 9677-12705 (mangaid 52-4)

This is the innermost IFU with a projected distance from the nucleus of 1.9 kpc. According to Walterbos and Kennicutt (1988) the effective radius of the bulge is 2 kpc, so a significant fraction of the light is coming from bulge stars.

What’s most interesting about this IFU is what it lacks, which is any significant star formation. I also saw little spatial variation in model star formation histories, so I’ll simply repeat the IFU wide history compared to the nearest PHAT tile:

Innermost IFU 9677-12705 SFR and mass growth histories compared to models for nearest PHAT region.

This region had the most rapid initial stellar mass growth and conversely the steepest decline in SFR of any of the MaNGA IFU’s, which is completely consistent with the consensus “inside out” growth paradigm.

One other moderately interesting result is that despite the lack of young stars there are detectable emission lines throughout with a mix of “LINER” and composite like line ratios from the [N II]/Hα vs. [O III]/Hβ diagnostic and the classification scheme of Kauffmann with Schawinski’s addition of the LINER/AGN divide. As is well known by now LINER (and presumably “composite” although I haven’t seen literature on the issue) emission can be spatially extended and does not at all necessarily indicate ionization by an AGN. M31 has widespread emission from diffuse ionized gas. About 14% of all binned spectra had line ratios in these categories and “AGN” like, and 90% of the LINER-like spectra are in this IFU. A similar fraction of spectra have star forming emission line ratios, which reflects the patchy nature of star formation in M31.

plateifu 9677-12705 – BPT class per [N II]/Hα vs [O III]/Hβ diagnostic

plateifu 9677-6102 (mangaid 52-3)

There’s little to say about this one. The entire IFU is offset by a small amount from some GALEX UV bright sources and there are no objects in any of the catalogs I’ve loaded within the footprint. The only prominent feature is a very prominent dust lane that covers the southeastern half of the IFU. Oddly, the estimated specific star formation rate tracks the dust rather closely.

plateifu 9677-6102 (M31 inner disk). Specific star formation rate

There’s a clear correlation between SSFR and optical depth of attenuation, and also with the “tilt” of the attenuation relation:

plateifu 9677-6102 (M31 inner disk). Specific star formation rate vs. dust optical depth.

Whether this is meaningful or a modeling artifact I can’t say at this point. I kept my simple single component dust model for these runs even though M31 is known to have both a foreground screen and embedded dust.

plateifu 9677-6103 (mangaid 52-2)

This again is in a nearly featureless area except for a prominent dust lane, with no sources in any catalog I consulted. The entire IFU lacks significant emission and there is no evidence in the models for significant recent star formation. Oddly, there’s a very similar relation between model specific star formation rate and model optical depth:

plateifu 9677-6103 (M31 inner disk). Specific star formation rate vs. dust optical depth.

plateifu 9677-12701 (mangaid 52-8)

This is the closest IFU to the nucleus that lies within a significant spiral structure as seen by GALEX. The thumbnail below shows its position overlaid on the false color GALEX image available within Aladin. The IFU appears to lie in a spur off a spiral arm a little farther out² and a number of discrete UV bright sources are visible within its footprint, which is marked with the irregular set of blue symbols. Also shown are cataloged positions of H II regions (yellow dots), red supergiants (red diamonds), and an OB association (blue square)³.

9677-12701-galexcutout — Thumbnail of plateifu 9677-12701 (M31 inner disk) overlaid on GALEX false color image. Yellow dots: cataloged H II regions. Red dots: cataloged red supergiants. Blue square: OB association.

Let’s look at a couple of maps. The blank area at upper right was masked due to a likely foreground star. The spectra in the chain of blank areas at bottom had Hα partially masked. Units in the Hα luminosity density map are log₁₀ ergs/sec/kpc², uncorrected for attenuation. Units of the SFR density maps are log₁₀ M_☉/yr/kpc².

9677-12701_ha_sfrmaps — plateifu 9677-12701 (M31 inner disk). (L) Uncorrected Hα luminosity density. (R) 100 Myr average SFR density.

To a pretty good approximation regions that are relatively bright in Hα track the UV bright areas and cataloged H II regions. There are two areas that stand out as having much higher than average SFR density. One, at lower left, coincides with a bright H II region. The other one, at center right, has low Hα luminosity but lies right on the cataloged position of a red supergiant. The presence of an evolved star and absence of emission suggests that star formation has recently (in the last ~70 Myr, say) ended in that area. Comparing the model star formation histories the region with little Hα emission does show a sharp drop-off after a peak at 10 Myr lookback time:

9677-12701_sfh_2regions — plateifu 9677-12701 (M31 inner disk) – model star formation histories for 2 star forming regions.

One other thing I’ll just note for now is that regions with the highest star formation rate tend to have neighboring regions with higher than average star formation as well. These seem to occur in clumps or chains a few 10s of parsec in size. I will get, eventually, to some more dramatic examples.

plateifu 9677-9101 (mangaid 52-9)

This and the next IFU are in a spiral segment that some authors call the “6 kpc ring,” but the GALEX false color image shows no very bright UV sources and there are no cataloged young objects within the footprint.

One mildly interesting result is that the modeled 100 Myr SFR density correlates rather strongly with Hα luminosity density, but an order of magnitude higher than predicted from Calzetti’s calibration. All of the emission in this region appears to be from diffuse ionized gas as there are no cataloged discrete sources of emission, and no regions with starforming line ratios. A literal interpretation of this, which might even be true, is that star formation has ceased in the recent past.

9677-9101_ha_sfr — plateifu 9677-9101 (M31 inner disk). Star formation rate density vs. Hα luminosity density.

plateifu 9677-12704 (mangaid 52-5)

This is also in the 6 kpc spiral feature but in an area with no bright UV sources and that appears to be heavily dust obscured in optical images. Since I don’t have anything very interesting to say about this region I’ll just post the modeled star formation history for the region within the IFU footprint with the highest modeled SFR density. This is near the western edge of the IFU and isn’t associated with any cataloged young objects.

plateifu 9677-12704 (M31 inner disk). Star formation rate history for a region within the IFU footprint with the highest modeled recent SFR.

The region with the highest Hα luminosity is near the southwest edge and covers the position of a cataloged planetary nebula. The emission line ratios are inconsistent with a starforming region, falling in Kauffmann’s “AGN” region.

plateifu 9677-12703 (mangaid 52-6)

This and the last IFU are in an inter-line region between the 6 and 10 kpc structures as seen by GALEX, but with lots of diffuse starlight and relatively little dust. Emission lines are weak or undetected throughout, but there is a cataloged H II region near the southern edge. The peak in Hα luminosity density is easily seen in the map below in the bottom left pane. The region with the highest SFR density is displaced by ~10 pc. from the region with highest Hα luminosity. Interestingly, the SFR models show significant differences in recent histories: the region with highest SFR shows a very sharp and short lived peak at ~10 Myr, while the highest Hα luminosity region is still growing in SFR (per the model). Again, I hesitate to take these model histories too literally, especially at the youngest ages, but these are consistent with the fact that ionized gas emission will fade rather rapidly as the most massive stars in a region evolve away from the main sequence.

plateifu 9677-12703 (M31 inner disk). (TL) SFR density (100 Myr average) (BL) Hα luminosity density. (TR) SFR history for the region with highest SFR density. (BR) SFR history for the region with highest SFR Hα luminosity density.

plateifu 9678-12705 (mangaid 52-21)

I don’t have much to say about this one either. It lies in a region that’s almost completely blank in the GALEX imaging, with a rather uniform sprinkle of stars in PHAT and the DSS2 image displayable in Aladin. Ionized gas emission is weak or undetected throughout. For the sake of having a graph to display here is a histogram of the per spectrum mean specific star formation rate (100 Myr average as always) comparing this IFU to the innermost one — plateifu 9677-12705.

9678-12705_ssfr — Distributions of mean specific star formation rate in two MaNGA M31 IFU’s

I hope to finish off M31 in one or at most two more posts. Next up are IFUs that fall in or near the 10 kpc ring, followed by the outer disk.

2023-03-232023-03-23

The MaNGA M31 ancillary program – comparing star formation history models

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 M_g ≈ 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 ages¹

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

10 Kpc ring

sfh_10kpc — 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

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”² 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 light³ 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.

2023-02-28

The MaNGA M31 ancillary program – preliminaries

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 PHAT¹, 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:

m31manga_phathst — 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:

m31manga_galex — 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 z_dist = 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 A_V = 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.”

m31manga_sigmam_sigmasfr — 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.

m31manga_sigmaha_sigmasfr — 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 dex².

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).

m31manga_wilsonsfr — 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.

2022-09-28

A little more on the “burst age – burst mass degeneracy”

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 Hδ_A. They also have similar 4000Å break strengths:

IC 0976: D_n4000 = 1.308±0.005, Hδ_A = 8.05±0.31
MCG +07-33-040: D_n4000 = 1.153±0.009, Hδ_A = 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:

hd_d4000_2psb — D_n4000 – 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 Hδ_A indexes near the upper limits of any measurements in SDSS, and both are clearly in the post-starburst area of the Hδ_A-D_n4000 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.

2022-09-17

IC 976

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.

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.

IC 976 redshift measurement — 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 spectra¹ 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 ≈10^8.5 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 ≈ 10^9.3 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 galaxy² 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.

2022-01-282022-04-02

What fraction of Schawinski’s “Blue early type galaxies” are ellipticals?

The first iteration of Galaxy Zoo led to several collections of distinct objects, including a sample of 215 “blue early type galaxies” published in Schawinski et al. (2009)¹I found this an interesting group of galaxies, partly because of a possible link to post-starburst (K+A) galaxies that was discussed in the original paper. The authors discuss at some length the likelihood that these are results of mergers in the cosmologically recent past, with at least one of the progenitors being gas rich. Many (at least 25% and possibly more than half) were found to be currently starforming and the rest likely to have only recently ceased forming stars as inferred from their blue colors.

The ongoing Zoogems program has 12 of Schawinski’s blue ETGs on its target list, of which 6 have been observed so far as of mid-January 2022. Somewhat surprisingly there are 24 in the final MaNGA release, over 11% of the sample!

Taking a look at the 6 with HST observations I would say none of these are typical ellipticals. Five show some degree of spiral structure although in 4 it’s embedded in a more diffuse body. One appears to me to be an S0 with both inner and outer rings — this is in agreement with the one published morphological classification I’ve found. All of the others appear more disky than ellipsoidal to me, although this is just my possibly flawed qualitative judgment. At least two are visibly disturbed. One (CGCG 315-014) is connected to a nearby galaxy with a long tidal tail as seen in the Legacy Survey thumbnail below. Markarian 888, which will be the subject of the rest of this post, has shells that extend well past the main body of the galaxy and prominent, centrally concentrated dust lanes.

So far it’s the only Zoogems blue etg target with a MaNGA observation (two others on the target list are in MaNGA but of course there’s no guarantee they will ever be observed). As is often the case the IFU could have been larger — this was observed with a 37 fiber bundle giving 111 dithered spectra in the RSS file.

MRK 888 SDSS thumbnail with MaNGA IFU footprint

As always the first step in analyzing these data is to estimate redshift offsets for each spectrum, and from there we get a velocity field, which in this case shows a rapid rotator with a fairly symmetrical radial velocity pattern.

Mrk 888 (MaNGA plateifu 9894-3703) velocity field

Visual inspection suggests the line of sight velocity distribution is consistent with a rotating thin disk, so I fed the data to my Gaussian process based rotation modeling code, with results summarized below. In fact the model does an excellent job of accounting for the data, with residuals (not shown) from the model fit (top right) in a range of ±15 km/sec. One unusual feature of the velocity field is the rotation velocity turns over at somewhat less than one effective radius. Whether the rotation curve declines smoothly outside the IFU footprint or is kinematically disconnected from the outer parts of the galaxy is of course unknowable at this time.

I also ran my usual star formation history modeling code on the data binned to 97 spectra. First, here are some summary results. The stellar mass density declines roughly exponentially, which is consistent with a disky morphology:

Model estimate of stellar mass density vs. radius

Next are maps of the estimated Hα luminosity density and, on the right, the BPT classification from the [O III]/Hβ vs. [N II]/Hα diagnostic. The contours are elliptical with major axes closely aligned to the rotation axis (the posterior mean for the angle is the dashed line in the velocity field plot above). Again, the emission appears to arise in a disk.

The proper interpretation of the “composite” BPT classification is something I think I’ve written about in the past. It was originally suggested to indicate a mix of AGN and stellar ionization, but here it arises in a thin ring of weak but detectable emission just outside the star forming region. If it’s truly composite it’s likely to arise from a mix of weak star formation and ionization by hot evolved stars. In any case there’s no evidence for an AGN in the optical data.

(L) Hα luminosity density (R) BPT classification from [O III]/Hβ vs {N II]/Hα diagnostic

Next are maps of the modeled (100 Myr average) star formation rate density and specific star formation rate, and in the second row scatter plots of the same estimates against radius in kpc. The trends with radius are somewhat unusual, especially for SSFR which in a normal disk galaxy typically increases with radius even if the highest total star formation rates are centrally concentrated. Highly centrally concentrated star formation in the aftermath of mergers is predicted by some simulations.

(TL) star formation rate density; (TR) specific star formation rate; (bottom row) scatter plots vs. radius

A couple more graphs will round out my discussion of summary model estimates. As I’ve shown several times before there’s a pretty tight linear relationship between modeled SFR density and estimated Hα luminosity density. In this plot Hα is corrected for modeled stellar attenuation, which is expected always to underestimate the attenuation in emission line emitting regions. That, and the fact that Hα emission and the model star formation rate estimates probe order of magnitude different time scales probably account for the systematic offset from the standard calibration given by the straight line.

Model star formation rate density vs. Hα luminosity density corrected for *stellar attenuation*. Straight line is calibration from Calzetti (2012).

And, once again I show a map of the modeled optical depth of stellar attenuation. The region of highest optical depth nicely tracks the visible dust (the HST image at the top is rotated about 90º from the SDSS image). Outside the dusty region there appears to be a shallow gradient, which might indicate that the nearer side is to the northeast.

Map of modeled optical depth of stellar attenuation

Finally here are plots of the model star formation history for all spectra ordered by distance from the IFU center. In the inner 1.5 kpc or so there’s some recent burstiness with possibly a very recent acceleration of star formation. For reasons I’ve discussed recently I don’t take either the timing or magnitude of bursts of star formation too seriously, but the behavior of the models is consistent with a recent revival of star formation due presumably to a merger, for which there are multiple lines of evidence.

model star fomation histories for all spectra

With 24 of these galaxies and another 31 from the compilation of Melnick and dePropris and the post-starburst ancillary program in the final release of MaNGA these samples satisfy my criteria of being manageably sized for my computing resources while large enough to say something about the groups. So, when time permits I plan to take a look. I already have the data in hand.

2021-11-242021-12-01

Another look at the PYPOPSTAR SSP model library

After a month off I returned to have another look at Millan-Irigoyen et al.’s high resolution “pypopstar” SSP model spectral libraries. First, I couldn’t find a more suitable subset of the full library than I used last time, so I decided just to try augmenting the existing Emiles based library with some younger spectra from pypopstar. Of course I had already done this with models from the 2013 update of BC03, so the plan was to replace those with a slightly finer grained selection at the young end. That raises the question of which ages to select. The youngest age model in the BaSTI isochrone based library is 30Myr (log T = 7.48), and we’re spoiled for choice of models at younger ages than that: there are 53 between log T = 5 and log T = 7.45, far more than necessary. Looking at the graph below, which just plots model spectra for the solar metallicity bin at decadal time invervals there’s very little spectral evolution between 10⁵ and 10⁶ years with the latter being slightly brighter at all relevant wavelengths. This is no surprise since even the most massive stars have main sequence lifetimes ∼10⁶ years. The model spectra continue to get brighter up to around 10^6.6 years (4 Myr) and then turn around, becoming noticeably fainter and redder by 10⁷ years.

pyspecz02 — pypopstar solar metallicity model spectra in decadal age increments

I decided to take the log T = 6 models as youngest, discarding the sub Myr ones altogether. This is mostly due to the inability to distinguish them and also just for purposes of visualization. I usually use logarithmically scaled lookback time axes in SFH history plots, and selecting a minimum value of 5 results in too much real estate given to very recent times where usually nothing much is happening.

Without giving this a lot of thought I selected just 3 ages to add: log T = 6, 6.51, and 7. The youngest Emiles model is log T = 7.48, so this gives nearly constant increments around 0.5 dex. This choice gives a reasonably smooth transition from the theoretical spectra to empirical ones, except for maybe the lowest metallicity bin. I also chose the “total” spectra including both stellar and emission continuum light in hopes of better modeling the continuum in star forming galaxies. To merge the high resolution pypopstar models into the library I just used a spline fit to interpolate the model spectra onto the same wavelength grid as Miles. This should (I hope) preserve total flux nearly enough. A more refined treatment would also consider that these still have higher resolution than Miles spectra, which are around 2.5 Å. I didn’t take the time. The merged library therefore has 56 time bins times 4 metallicity bins for a total of 224 model spectra. I retained the same rest frame wavelength range (3464.9 – 8864 Å) as the Emiles subset I’ve been using for several years

youngspec_combo — The youngest SSP model spectra for the EMILES library augmented with young pypopstar spectra

The obvious next step is to use this library in some models and see how they compare to Emiles. Paging through my samples of spirals with MaNGA observations I picked, for no really good reason, this one:

MaNGA plateifu 8452-12703 (mangaid 1-148068)

Clearly it has star forming regions in its arms as well as a prominent bar and rather red, possibly passively evolving nucleus. After binning to my usual threshold S/N of 5 there were 122 spectra, which were analyzed in the usual way using both Emiles and Emiles + popstar. And here’s the main result of interest, the model star formation histories for all 122 spectra, ordered by distance from the nucleus.

Model star formation histories for MaNGA plateifu 8452-12703 compared. Red: Emiles + pypopstar Blue: Emiles + BC03

There’s little or no difference in the model star formation histories for the common components of the libraries. The pypopstar components indicate that the star formation rate continues at relatively constant rates up to recent times. The modest differences at the young end don’t necessarily mean anything. I more or less arbitrarily assigned an age of 10Myr to the BC03 model spectra, which were actually taken from 1Myr models. There’s no real way to tell what the actual effective age of those contributors is — if it’s typically younger than 10Myr the SFR in the youngest bin would be correspondingly higher and a little lower in the next age bin.

Given the similarities in the detailed star formation histories it shouldn’t be much of a surprise that summary quantities are quite similar too. To illustrate a few, here are mean values of the stellar mass surface density:

Model mean values of stellar mass density for MaNGA plateifu 8452-12703 compared — Emiles + pypopstar vs Emiles + BC03

the star formation rate surface density (100 Myr average):

Model mean values of SFR density for MaNGA plateifu 8452-12703 compared — Emiles + pypopstar vs Emiles + BC03

The specific SFR:

ssfr_comp — Model mean values of SSFR for MaNGA plateifu 8452-12703 compared — Emiles + pypopstar vs Emiles + BC03

The lines with confidence intervals in these plots are from OLS fits taking no account of nominal uncertainties in either sets of variables, and shouldn’t be used to infer any trends. In any case all differences are very small. Finally, here are histograms of all sample values of SFR density for all spectra. Again, these are nearly identical:

sigma_sfr_dist_comp — Sample distributions of SFR density over all spectra compared — Emiles + pypopstar vs Emiles + BC03

After running multiple sets of models it became apparent that this wasn’t a very stringent test of the usefulness of the proposed library additions because this galaxy has very anemic star formation. In fact it’s one of Masters et al.‘s “passive” red spirals, which I should have recognized. It was also one of the first several dozen galaxies with AGN found in MaNGA, which doesn’t necessarily (but might, along with perhaps the prominent bar) account for the weak star formation. My model runs show “LINER” like emission line ratios in the center, which does point to the presence of a weak AGN.

Briefly now, I picked two more disk galaxies with obvious regions of vigorous star formation and repeated this exercise. To make this short I’m just going to post the star formation histories for all binned spectra.

MaNGA plateifu 8449-3703 (RA 169.299, DEC 23.586)

MaNGA 1-488712, plateifu 8449-3703 — SDSS cutout

Model star formation histories for MaNGA plateifu 8449-3703 compared. Blue: Emiles + pypopstar Red: Emiles + BC03

MaNGA plateifu 8318-9101 (RA 196.086 DEC 45.057)

MaNGA 1-259618, plateifu 8318-9101 — SDSS cutout

Model star formation histories for MaNGA plateifu 8318-9101 compared. Blue: Emiles + pypopstar Red: Emiles + BC03

Spectra in nearby age and metallicity bins are highly corrrelated, which among other things means that adding or subtracting some from the set of “predictors” potentially changes the values inferred for others as well. In these two sets of model runs we do see some differences in the common Emiles portion of the libraries, but they’re quite small and change no qualitative inferences. So my conclusion for now is that adding these theoretical spectra is a reasonable strategy, but one that doesn’t have much apparent impact on model results.

Well that’s probably all for a while. The final MaNGA data release is now promised for December 2021, which should approximately double the number of galaxies and I hope offer some data reduction improvements. There will also be a very large release of stellar spectra that should form the basis for new SSP libraries in the (hopefully) near future.