Category Archives: Stellar Halos

‘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.

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.

A new faint member of the M81 group – discovering one of the faintest galaxies to be found outside the Local Group

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A deep Subaru Hyper Suprime Cam i-band image of the newly-found dwarf M81 group member d1005+68. The concentric green circles correspond to apertures with 1 and 2× the half-light radius. High confidence member stars are shown in red and lower confidence members shown in blue.

We have discovered a new faint member of the M81 group. With MV~-8, it is one of the faintest ever galaxies found outside the Local Group. In a recently published Letter to the Astrophysical Journal, Adam Smercina and our collaborators discovered this galaxy in deep wide-field imaging from Subaru’s Hyper Suprime Cam by resolving it into individual red giant branch stars. Galaxies at this faintness have extremely low surface brightness and are typically invisible in their diffuse light.

The stars in d1005+68 have brightnesses and colors suggesting a distance of 4+/-0.4 Mpc, consistent with d1005+68 being a group member. It ha a half-light radius of ~200pc, and a metallicity of [Fe/H]~-1.9. These properties are similar to known satellites in the Local Group.

d1005+68 has a projected separation from nearby M81 satellite BK5N of only 5 kpc. As this is well within BK5N’s virial radius, we speculate that d1005+68 may be a satellite of BK5N. If confirmed, this would make d1005+68 one of the first detected satellites-of-a-satellite.

A path to inferring merger histories using stellar halos

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Stellar halos offer a clear path to working out the most dominant satellite to have merged with a galaxy. This is what Richard D’Souza and I claim in a paper just submitted to the Monthly Notices of the Royal Astronomical Society.

Many galaxies have a diffuse halo of stars around them; these stars are thought to be primarily from other satellite galaxies that fall into and merge into the main one (see Bullock et al. 2001 and Bullock & Johnston 2005; see also Bell et al. 2008). Tides tear off the stars from these satellite galaxies and spread them into a diffuse halo. There are uncertainties in this picture; it is possible that stars from the main galaxy (in situ stars) are moved into the diffuse halo by changes in the gravitational potential, and nobody knows which galaxies exactly were tidally shredded to make what halo.

In this paper, we were interested in understanding how the chemical composition (metallicity; a joint probe of how many generations of stars have lived and died in a galaxy, coupled with how well that galaxy keeps its chemical elements rather than spewing them out into intergalactic space) of these stellar halos might be related to the total amount of stars in a stellar halo. Observations (Harmsen et al. 2017) show such a relationship exists, and models predicted such a relationship (Deason et al. 2016), and we wanted to understand how tight such a relationship might be and what factors drive its shape and scatter.

Accreted metallicity of the 6 GHOSTS galaxies in addition to the Milky Way and M31 plotted as a function of their stellar halo mass measured between 10 and 40 kpc. The gray points indicated the accreted stellar metallicity estimated in a wedge along the minor axis for Illustris GHOSTS-like galaxies as a function of the accreted stellar mass measured in an aperture between 10 and 40 kpc. Top: We compare the distribution function of the aperture accreted stellar halo masses of Illustris GHOSTS-like galaxies (grey) with the aperture stellar halo masses of the GHOSTS data (red) measured between 10 and 40 kpc. In contrast, the distribution function of the aperture stellar halo masses (in situ + accreted) of Illustris GHOSTS-like galaxies is not consistent with the observational data.

We use the Illustris hydrodynamical simulations – a powerful theoretical tool for exploring the interplay of physical processes relevant for galaxy formation – to explore this issue. Importantly, if in situ stars are prominent at in halos at the level predicted by Illustris, halos would be too massive and would show too weak a metallicity-mass relation – Illustris dramatically over-predicts in situ stars. Consequently, we focused on accreted stars – stars torn from satellite galaxies through tides. These accreted stars a mass-metallicity relation close to the observed one (see figure, Figure 3 from the paper).

We learned that the metallicity-mass relation of stellar halos exists largely because halos are often dominated by a single disrupted dwarf galaxy – it was massive so drives up the mass of the halo, and it was metal-rich (because large galaxies are more metal rich) and drove up the metallicity of the halo. The metallicity of the halo is a sensitive probe of the extent to which a halo is dominated by the single most massive satellite disruption. This is potentially very important – we should be able to learn about the most massive satellite that merged with the main galaxy by measuring the metallicity of a galaxy’s stellar halo.

We also find that density and metallicity gradients in halos are steeper for earlier accretion/merger times. This conclusion appears to be rather more sensitive to modeling details but nonetheless opens up the intriguing possibility that we can learn about the time of the most important merger by studying the structures of a sample of stellar halos from nearby galaxies.