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Composite image of M33 in three optical filters obtained by the PHATTER survey with the Hubble Space Telescope. Credit: Meredith Durbin, Julianne Dalcanton, and the PHATTER team. 

Galaxies are, fundamentally, amalgams of stars, gas, and dust. The properties of the gas and dust (the interstellar medium, ISM) govern the process of star formation, and can change on short timescales owing to the physics associated with this process. However, after formation, stars are largely indifferent to these processes, meaning that their properties — such as their masses, ages, and compositions — serve as a record of their formation conditions.

 

By using powerful telescopes to resolve galaxies into their composite stars, and comparing these stellar populations to models of stellar evolution, we can learn about the evolution of the star formation and structure in these galaxies over billions of years.

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The Panchromatic Hubble Andromeda Treasury (PHAT) survey used the Hubble Space Telescope (HST) to resolve the Milky Way's neighbor the Messier 31 (M31; the Andromeda Galaxy) into millions of individual stars. Leveraging the common distance of these stars, PHAT provided an unprecedented view of M31's structure and history. The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) program used this same survey design to survey M31's largest satellite galaxy — M33 (the Triangulum). M33 is much lower mass than M31, yet forming stars at a much higher rate, and has a very different structure . Among the numerous science goals of PHATTER, we will learn how M33's environment (i.e. its role as an M31 satellite) may have shaped its structure and star formation. 

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Maps of stellar density for Red Giant Branch (RGB) stars (left) and Main Sequence (MS) stars (right) in M33, from PHATTER. While M33 exhibits a distinctive 'flocculent' spiral structure in its young stellar populations (and ISM), the majority of its stellar mass is orientated in a disk with only two, more subtle, spiral arms. M33's structure may provide clues about its history in the M31 Group, and more generally how disks form and evolve in low-mass galaxies. 

Satellite Galaxies as Fundamental Probes of Galaxy Formation

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Color images (gri filters) of two low-mass satellites (<1 million stars) around the nearby galaxy Messier 94 (M94). As the only two known satellites of M94, its satellite population is completely unique. Adapted from Smercina et al. 2018.

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The cumulative luminosity function of satellites within a projected 150 kpc radius of the 8 best-measured Milky Way-mass systems. M94 is very different from every  other known system. A 'Lonely Giant', its sparse satellite population indicates that there is significant scatter in how low-mass galaxies populate dark matter halos. Taken from Smercina et al. 2021a.

In our current picture of the Universe, galaxies form within much larger halos of dark matter. Rather than being isolated objects, these halos are complex hierarchical systems. As a result, large galaxies like the Milky Way, which live in massive dark matter halos (~1 trillion times the mass of the Sun), are host to many smaller, dwarf 'satellite' galaxies. 

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Though we have indeed discovered 'many' satellite galaxies around the Milky Way (and our neighbor Andromeda/M31), how many should we find, and with which properties? The answer is a significant standing problem for galaxy formation models.

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These small galaxies are important for a number of reasons. While Milky Way-like galaxies have the highest fractions of normal matter, relative to their dark matter, smaller galaxies are increasingly dark matter-dominated. The smallest galaxies' dark matter outweigh their normal, or 'baryonic', matter by factors of >10,000! This means that dwarf galaxies are among the best probes of dark matter in the Universe. The number and properties of satellites around Milky Way-like galaxies also encodes information about the early environments where these big galaxies formed and how they have grown over time. 

 

As dwarf galaxies are very faint, we have just now begun to study the satellites of other nearby galaxies to the depths achieved around the Milky Way. Using wide-field cameras on large telescopes, we can survey the large regions of galactic outskirts where satellites reside. With these new studies has come new insight. For example, we have recently discovered that the nearby galaxy Messier 94 hosts a surprisingly sparse number of satellites — only 2 with more than half a million stars. This 'lonely giant' is truly unusual among nearby galaxies' satellite populations. 

Why Do Galaxies Stop Forming Stars?

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A post-starburst galaxy as seen by HST. The green colors overlaid show molecular gas (traced by CO) observed with ALMA. That the molecular gas remains and is highly compact is an important insight into these galaxies' evolution.

Galaxies can be broadly placed into two categories: (1) those which are actively forming stars, and (2) those whose star formation has essentially ceased. How does this transition occur? 

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Sometimes it occurs extremely rapidly. Mergers between similarly-massive galaxies initially cause large bursts of star formation, but somehow quickly cause a global 'quenching' of star formation. I'm interested in the systems which exist just after this global shut-down event — 'post-starburst' galaxies.  

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Post-starbursts provide unique windows into the processes which work to shut down star formation, and studying them has yielded some surprises. Many simulations have predicted that galactic outflows, driven by either star formation or black hole feedback, likely expels most of the star-forming gas from the center of the merged galaxy. Unexpectedly, our work shows that significant gas remains in these post-starburst galaxies and it is entirely centrally concentrated.

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The most surprising feature of this gas is that it is forming stars <1/10 as efficiently as it should be! This indicates that there must be some source of continual energy injection into the gas, that lowers the efficiency with which it is turned into stars. Understanding this mechanism — whether due to feedback from stars or the central black holes, turbulent motions in the gas, or some combination — is vital to understanding merger-driven galaxy evolution.

Galaxies' Stellar Halos Provide a Record of Their Assembly

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Map of stellar density in the halo of the nearby M81 Group of galaxies, as seen by Subaru HSC. Intensity scales with density, while colors show the average stellar metallicity (blue = 1/100 metallicity of the Sun; green = 1/30 metallicity of the Sun; red = 1/10 metallicity of the Sun). The ongoing gravitational interaction between the 3 main galaxies (M81, M82, and NGC 3077) is clearly visible in the stellar distribution. Orange circles show the approximate tidal radii of M82 and NGC 3077 — material outside is 'unbound' from the satellites and considered part of M81's stellar halo. From Smercina et al. 2020. 

Galaxy mergers are typically not the violent affairs which form post-starburst galaxies. Models predict that galaxies like the Milky Way experience dozens of interactions and mergers with smaller galaxies over their lifetimes. The impacts of these interactions are difficult to infer, especially when the event may have happened  billions of years ago!

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Luckily, these events leave stellar 'debris' in their wakes. This material, much of which exists in the most distant galactic outskirts, forms a galaxy's stellar halo. Like geological strata, stellar halos provide a 'fossil' record of the galaxy's past mergers. Characterizing this fossil record has the potential to provide fundamental insight into galaxy evolution. 

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Stellar halos are incredibly faint — more than 10,000x fainter than the night sky at a dark sky site! — and studying them in detail can only be done with the a select few facilities. I use ground- and space-based facilities, such as Hyper Suprime-Cam on the Subaru telescope and the Hubble Space Telescope, to resolve individual stars in the stellar halos of nearby galaxies. By resolving the stellar halo populations in detail, our goal is to determine  the properties of past merger events and assess how these events drive the evolution of galaxy properties.

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We have recently surveyed the stellar halo of the nearby Milky Way-mass galaxy M81, which is currently undergoing a significant ~1:3 merger with its large satellite M82 (and LMC-like satellite NGC 3077). M81 is in a unique stage where we can measure both its past merger history and its current interaction. Though it experienced a 'quiet' merger history in the past, its current interaction will eventually produce one of the most massive stellar halos currently known — providing crucial insight into the formation of similarly massive halos such as M31. 

Galactic Archaeology with Resolved Stellar Populations

Galactic Archaeology
Satellite Galaxies
Galaxy Quenching
Stellar Halos
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