‘Life and Death in Nearby Galaxies’ – our work covered in Sky and Telescope

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Katya Gozman and I attended the AAS 240 meeting in Pasadena, CA, and presented our work with Subaru on resolved stellar halos. Katya’s work shows that M94 has a low mass, low metallicity halo, and that on this basis she concludes that M94’s huge pseudobulge (that contains more or less half of its stars!) wasn’t significantly driven by mergers. I presented work (about to be submitted) about one definite ultra-faint satellite in the M81 group, and 6 lower surface brightness candidates. This is cool, but what was really strange is that instead of gathering around M81 – the big, Milky Way-like Galaxy – they are clustered around a much smaller galaxy (1/10 of the mass) – NGC 3077. Something about M81 suppresses the number of dwarf galaxies around it – stronger tides than we imagined? feedback from M81 itself as it is forming is unexpectedly intense? This is not reproduced by current models of galaxy formation, and has something really important to tell us about how to use satellites for small-scale cosmology.

This was picked up by the press, and Monica Young from Sky and Telescope did a really lovely, brief, to the point article that combines insights from both of our works: ‘Life and Death in Nearby Galaxies‘.

Both the known (red) and candidates (yellow) in the M81 group are distributed in a highly asymmetric way around M81; they are instead clumped near NGC 3077 – a LMC-mass galaxy to the south of the group. It signals that most of the M81 group satellites didn’t belong to M81 (did it destroy them? did they not form effectively to begin with?). Instead, most fell in to the group recently! This will impact our understanding of galaxy formation at low masses – (at least some?) little galaxies have more satellites than big ones?
The definite dwarf has high enough surface brightness to be clearly visible. Most candidates are much more diffuse, and will require HST or JWST follow-up.

Post starburst galaxies – they don’t run out of gas!

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From a NRAO press release on our team’s work.

Post-starburst galaxies were previously thought to scatter all of their gas and dust—the fuel required for creating new stars—in violent bursts of energy, and with extraordinary speed. Now, new data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveals that these galaxies don’t scatter all of their star-forming fuel after all. Instead, after their supposed end, these dormant galaxies hold onto and compress large amounts of highly-concentrated, turbulent gas. But contrary to expectation, they’re not using it to form stars.

Click here to view a press release from NRAO for more details, and two ApJ (1, 2) papers that are led by my close collaborator Dr. Adam Smercina.

Post-starburst galaxies were previously believed to expel all of their molecular gas, a behavior that caused them to stop forming stars. New observations have revealed that these galaxies actually hold onto and condense star-forming fuel near their centers, and then don’t use it to form stars. Here, radio data of PSB 0570.537.52266 overlaid on optical images from the Hubble Space Telescope, show the dense collection of gas near the center of the galaxy. See the press release from NRAO for more details.
Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO/AUI/NSF)

Mergers with large satellites bring in little friends

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Ever since Richard D’Souza and I learned that M31 suffered a large merger a few Gyr ago, we have been very curious about the impacts that this merger could have had on the satellite population. It is clear that the LMC has brought in a number of satellites, and so it’s reasonable to expect that other large mergers (e.g., the Milky Way’s merger with Gaia-Enceladus, or M31’s large merger) might have brought in satellites. If mergers do bring in satellites, are their properties distinctive? This may help us to understand:

  • star formation histories of satellites
  • their satellite orbits, angular momentum, planes
  • their radial distributions

We wanted to build intuition for how these events proceed using high-resolution N-body models – the ELVIS suite of simulations.

In brief, we find that massive accretions (like the LMC merging with the Milky Way, or M31’s large merger) should deliver significant numbers of satellites to galaxies. The fraction varies depending on merger history, spanning 10%-70% of all satellites. These accretions temporarily enhance the number of satellites close to the main Milky Way-like galaxy, can lead to escaping galaxies(!), and often impart angular momentum to a satellite system.

The most important effect that mergers have on satellites is to clump their accretion time distributions. These satellites are typically accreted within a Gyr of the massive satellite, leading to a short-lived surge of satellite accretion. This signature can be observed! By using star formation shut-off time as a proxy for infall time in the Milky Way and M31, we measure a clear signature of satellite infall at the time of the three most important mergers in the Local Group – the mergers of Gaia-Enceladus and the LMC with the Milky Way, and M31’s large merger several Gyr ago. 

This work is published in the Monthly Notices of the Royal Astronomical Society, and was presented at the Summer 2021 AAS meeting. Sky and Telescope picked up the story, and did an excellent write-up, linked here.

  • Illustration of a massive accretion (blue) bringing in both bound satellites (purple) and satellites that it gravitationally influences (yellow).

The current and future stellar halo of M81, as seen by Subaru’s Hyper Suprime Cam

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For a few years, our group has been studying the stellar halos and satellites of nearby Milky Way-mass galaxies, with the goal of understanding the diversity in their merger histories, the effects of those mergers on their host galaxies, and the diversity of their satellite systems. We have been working for years on a beautiful dataset for the M81 group, and I’m really proud to report that Adam Smercina and the rest of the team just had a paper accepted to the Astrophysical Journal where we studied the diffuse outskirts of the M81 group using resolved stars.

We used Subaru’s Hyper Suprime-Cam to resove stellar populations around M81 in its interaction with M82 and NGC 3077, revealing M81’s stellar halo in never-before-seen detail. Using careful star-galaxy separation techniques and artificial star tests, calibrated with HST data in the survey area, we resolved the halo to unprecedented V-band equivalent surface brightnesses of 33 mag/sq. arcsec., and produced the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81’s halo as one of the lowest mass and metal-poorest known, with a stellar mass of ~1.1×10^9 solar masses, and a metallicity [Fe/H] ~ -1.2 – indicating a quiet past merger/accretion history. Our global halo census finds that tidally unbound material from M82 and NGC 3077 is currently adding significantly to M81’s halo, providing an extra 5×10^8 solar masses of material with [Fe/H] ~-0.9. We further show that, in a few Gyr, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe, similar in mass and metallicity to the massive, metal-rich stellar halo of M31. This illustrates an essential feature of stellar halo formation and evolution – it is possible, with one ‘accretion event’ (a merger with M82 and NGC 3077) to leapfrog from a low-mass, anemic halo similar to the MW directly into a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.

An image of the stellar halo of the M81 group.

Density image of RGB stars, with intensity mapped to stellar density, where each ‘channel’ represents stars in three bins of metallicity: [Fe/H] ∼ −0.8 (red), [Fe/H] ∼ −1.2 (green), and [Fe/H] ∼ −1.8 (blue). Each channel was smoothed using first a tophat filter of size ∼20 kpc (to bring out substructure), and then a Gaussian filter of width ∼1 kpc. The interiors of M81, M82 and NGC 3077 have been filled with images from HST (credit: NASA, ESA, and the Hubble Heritage Team) – Fig. 14 from Smercina et al. 2020 (https://arxiv.org/pdf/1910.14672.pdf)

The Andromeda galaxy’s most important merger about 2 billion years ago likely gave rise to M32

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One of the most persistent problems that has bothered me over the years is the difficulty of actually using a stellar halo to determine the properties of the most important merger to have affected a galaxy. There have been a number of problems – convincing ourselves that the parts of the halo we use are accreted in origin, understanding how many satellites make important contributions to the halo, and assembling the required resolved star datasets to allow careful measurement of halo properties. Using GHOSTS in concert with galaxy formation models (mostly Illustris), we realized that by focusing on the outer parts of a stellar halo along the minor axis one is dominated by accreted stars, and that most of those accreted stars in practice come from the one most massive satellite to have been accreted. In this way, we are able to quantify the likely mass and metallicity of the most massive satellite to have merged with a given central galaxy.

Richard D’Souza and I decided to use this method on the exquisite dataset collected over the years by the PAndAS, SPLASH and Brown et al. teams to determine the mass, metallicity and accretion time of the Andromeda Galaxy’s most massive accretion in this paper (or here for the arXiv version). Andromeda has a huge, metal-rich stellar halo, and this alone strongly constrains the range of cosmologically-acceptable merger histories to those involving a massive, metal-rich accretion in the last several Gyr. By comparing the observations with large and metal-rich simulated stellar halos from Illustris, we realized that the inner stellar halo, the giant stellar stream and M32 were all likely to be associated with this large merger event, and that the properties of these three observational features were best reproduced by a single merger with a ~1-3×1010 MSun disk galaxy around 2Gyr ago. This galaxy, the progenitor to M32 (which we imaginatively named M32p – it even has its own Wikipedia entry now, not our doing!), used to be the 3rd largest galaxy in the Local Group before its untimely demise. Interestingly, the inferred merger history is in excellent accord with a completely independent analysis by Hammer et al. which used the properties of the M31 disk in concert with the giant stellar stream to come to the same conclusion of a large, recent merger for M31 – two independent analysis, using completely different constraints, coming to a very similar conclusion.

The Andromeda galaxy shreds the large galaxy M32p, likely giving rise to M32, the inner stellar halo and M31’s giant stellar stream. Credit: Richard D’Souza. Image of M31 courtesy of Wei-Hao Wang. Image of stellar halo of M31 courtesy of AAS/IOP.

This merger history has a range of important implications. Firstly, this lends strong support to an interpretation of M32 as a tidally-stripped core of a previously much larger system, we believe a star-forming disk galaxy. Second, the disk of M31 survived a large minor, or perhaps even a major merger in the last 2Gyr – this has important implications for our understanding of disk thickening and survival. Importantly, he bulge pre-dates this interaction – M31’s large bulge is seemingly unassociated with M31’s largest merger. This lends further support to the notion that galaxy merging and bulge formation are not trivially associated with each other. Finally, this work paves the way for applying this method to other galaxies, allowing us to empirically chart out what the long-term effects of mergers are on galaxies.

This work really captured Richard’s and my imaginations over the last year or so, and was tremendous fun, and it was exciting to see that it captured the imagination of lots of other people, and was widely covered in the blogosphere (astrobites, Discover magazine), amateur astronomical press (e.g., Sky and Telescope, Astronomy), radio (TalkRADIO in London, Stateside on Michigan Radio/NPR) and wider press (e.g., Smithsonian Magazine, CNN, Guardian, Detroit News, UM press release).

 

The Lonely Giant Problem

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In a recently-accepted paper, Adam Smercina, I and our collaborators have discovered that the satellite galaxies of Milky Way-like galaxies vary much more than previously anticipated. We conducted a deep g-band survey of the satellites around the MW-mass central galaxy M94 (NGC 4736), out to 150 kpc in galactocentric radius, with Subaru’s Hyper Suprime-Cam. Despite our expectation of discovering ~10 ‘classical’ satellites (scaling from the MW and 3 other nearby Milky-Way like galaxies), we discovered only two – both with M* < 106 MSun. Via extensive artificial galaxy testing, we have determined that our survey is >85% complete down to a stellar mass limit of ~4×105 MSun. Thus, M94 very likely only has two low-mass satellites within 150 kpc.

In order to schematically illustrate the implications of this finding for galaxy formation, we explore simple halo occupation models for painting galaxies onto dark matter halos and subhalos. If we adopt a ‘standard’ halo occupation model, following an extrapolation of e.g., Behroozi et al. 2013, we expect M94-like systems (with 2 satellites within 150kpc) <0.2% of the time. Furthermore, none of these modeled systems have a most massive satellite less massive than106 MSun.

A significantly more stochastic halo occupation model that increases the scatter in the SMHM relation all the way up to Mpeak > 1010 MSun produces M94-like systems much more frequently, >4% of the time, fits the luminosity function of all Milky-Way like satellite systems better and produces a few systems with low-mass dominant satellites.

We suggest that the considerable variation in the number (and character) of satellites of central galaxies like the Milky Way contains important information about how galaxies might populate dark matter halos, and in particular appears consistent with a framework in which there is large scatter between dark matter subhalo mass and the mass of the satellite galaxy living in that halo.

Satellite stellar mass functions and statistics for M94 and other nearby galaxies and EAGLE halos, assuming two halo occupation models. Left: Satellite mass functions for nearby galaxies: M94 (orange), the MW (blue), M31 (red), M81 (green), and M101 (purple). Also shown are the median (black line) and 50% (dark gray), 90% (gray), and 99% (light gray) confidence intervals for simulated satellite mass functions for MW-mass galaxies taken from the dark matter in the EAGLE hydrodynamical simulation.

Arthur F. Thurnau Professorship

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I’ve been selected to be a 2018 Arthur F. Thurnau Professor. The Arthur F. Thurnau Professorship honors “tenured faculty whose commitment to and investment in undergraduate teaching has had a demonstrable impact on the intellectual development and lives of their students.”

I love teaching, and it’s been my great bounty to be in a department with colleagues who care deeply about teaching and have been willing to share their time, insights and mentorship with me. Working with the University of Michigan’s Center for Research on Learning and Teaching, the oldest teaching center in the country, has been a constant source of inspiration, guidance, and expertise. The students are the reason we do this, and I appreciate their insights and partnership in working to create meaningful learning experiences.

Evidence for maintenance mode AGN feedback at z~1

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Ivana Barišić, Arjen van der Wel and collaborators have charted out the dependence of radio AGN activity on galaxy velocity dispersion (a proxy for black hole mass), stellar mass and star formation using the ultra-deep LEGA-C spectroscopic dataset from VIMOS on the VLT coupled with new radio data from the VLA for the COSMOS field. We find that radio-loud AGN at redshift 1, when the Universe is a little less than half of its present age, occur almost exclusively in old galaxies with high velocity dispersions in excess of 175 km/s, corresponding to black hole masses above 100 million solar masses. At a fixed stellar mass the fraction of z~1 radio-loud AGN is 5 – 10 times higher than in the local universe, suggesting that quiescent, massive galaxies at z~1 switch on as radio AGN on average once every Gyr.

The relationship between star formation activity (colors – red is quiescent, blue is star forming), radio activity (radio AGN are large symbols), velocity dispersion (a proxy for black hole mass), and stellar mass at z~1 from the LEGA-C and a VLA 3GHz surveys in the COSMOS field. As is seen locally, quiescence at z~1 is associated with large black hole mass; furthermore, one sees evidence for radio loud AGN activity in those quiescent systems with large black holes. Such behavior is consistent with models in which radio maintenance-mode AGN feedback is a driver of quiescence in galaxies.

This paper extends to z~1 the evidence associating quiescence with large black hole mass discussed by Terrazas et al. (2016, 2017). Importantly, because of the deep radio data and increased duty cycle of radio AGN activity at z~1, there is a clear association between quiescence and radio activity -not only are galaxies with large black holes quiescent, but also one can directly see the radio AGN operating in a significant fraction of these systems. This paper was accepted for publication in the Astrophysical Journal.

 

Mapping the efficiency of AGN feedback in galaxies

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The M31 galaxy, an example of a partially quiescent galaxy, where we argue that feedback from AGN incompletely suppresses star formation.

AGN feedback efficiency is a gradual function of supermassive black hole mass and stellar mass. This empirical mapping of the effects of AGN feedback from Bryan Terrazas and collaborators (ApJ, in press) both tests galaxy formation models and highlights a population of galaxies with partially suppressed star formation – galaxies like M31 or M81 that we term partially quiescent.

One of the ‘grand challenges’ of galaxy evolution is to better understand the drivers of a quiescence – a lack of star formation in galaxies. Early cosmological simulations predicted that all galaxies in the centers of their dark matter halos (‘central’ galaxies) should have star formation, governed by the cooling rate of gas in halos and regulated by feedback from massive stars and supernovae. Yet, many galaxies – typically those with high central stellar densities, high velocity dispersion, often high stellar mass, and large supermassive black holes – lack star formation. Many physical processes are possibly relevant – e.g., changes in cooling efficiency of halos as a function of mass, changes in star formation efficiency as a function of gravitational potential, the injection of large amounts of energy from supermassive black holes (AGN feedback).

In an earlier paper, we used a sample of central galaxies with directly measured black hole masses to show that quiescent galaxies have a much larger supermassive black hole than star-forming galaxies of the same stellar mass, in excellent accord with galaxy formation models where AGN feedback is responsible for suppressing star formation in galaxies.

Projections of the sSFR–MBH–M∗ data cube with our sample of 91 central galaxies with directly measured black hole masses, stellar masses and specific star formation rates denoted by colored points. (a) sSFR as a function of M∗. The gray data points indicate a sample of local galaxies to show the star forming main sequence. The dashed line indicates the boundary below which the sample is no longer complete; (b) sSFR as a function of MBH; (c) MBH as a function of M∗. Color gradients indicate the values for the axis not shown. The lines at the bottom right of (a) and (c) indicate the errors on M∗. Figure 1 of Terrazas et al. 2017.

In this paper, we asked a crucial follow-up question – is the shut-off of star formation all or nothing, or is partial suppression of star formation possible? We find that star formation rate is a continuous function of supermassive black hole mass. Galaxies with low-mass supermassive black holes (like our Milky Way) have typical star formation rates, galaxies with intermediate mass black holes (like M31 or M81) have somewhat suppressed star formation, whereas galaxies with huge supermassive black holes (like M87) have dramatically suppressed star formation. We call galaxies with intermediate black hole mass partially quiescent (in other works these have been dubbed ‘green valley’ galaxies, as they often have colors intermediate between the blue star forming galaxies and red quiescent galaxies). These galaxies are not uncommon, and given that many of them (like M31 or M81) have long-lived stable star forming disks, we suggest that these are in a near-equilibrium state in which star formation is incompletely suppressed by AGN feedback. Interestingly, no galaxy formation model correctly reproduces the observed trends in detail, offering a new and important test of their AGN feedback implementations.

Brian Devour successfully defends his thesis

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Example SDSS postage stamp images for representative galaxies in Brians sample, placed according to their positions in our parameter space. Top panel shows face-on galaxies, bottom panel shows edge-on galaxies (the upper-leftmost bins are omitted in the bottom panel since there are very few highly flattened galaxies in this region of parameter space). Background contours show the overall galaxy distributions.

Brian Devour has successfully defended his PhD thesis, entitled Dust and its Effects on Measures of Galaxy Properties. Brian uses near-IR and mid-IR survey datasets to select galaxies in an inclination-independent way and then quantifies how their properties change as a function of galaxy inclination as a non-parametric measure of attenuation. In addition to his two completed papers, Devour & Bell (2016; Global dust attenuation in disc galaxies: strong variation with specific star formation and stellar mass, and the importance of sample selection) and Devour & Bell (2017; Revealing strong bias in common measures of galaxy properties using new inclination-independent structures), Brian added two new chapters. One chapter carefully develops, tests and generates error estimates for his new inclination-independent galaxy structure metrics (the metric was introduced in Devour & Bell 2017). The other chapter carries out the first ever census of dust attenuation in disc galaxies as a simultaneous function of stellar mass, specific star formation rate, size and concentration. He finds that attenuation is a strong function of all parameters, but especially specific star formation rate and size. Because specific star formation rate and stellar mass are expected to control the stellar populations (through the star formation history and metallicity of galaxies), he can use the intercept of the color-inclination relation as a function of structural parameters to explore the effects of face-on dust. Simple scaling relation-based models in which dust optical depth scales only with gas density and metallicity, coupled with published radiative transfer simulations, reproduce the observed patterns in relative attenuation and face-on color with startling accuracy.

Congratulations, Brian!