arXiv subjects

Charlotte R. Christensen

Publications and source records attributed to Charlotte R. Christensen.

At least 19 recordsLinked to original sources

arXiv2025

Survivors and Zombies: The Quenching and Disruption of Satellites around Milky Way Analogs

It is necessary to understand the full accretion history of the Milky Way in order to contextualize the properties of observed Milky Way satellite galaxies and the stellar halo. This paper compares the dynamical properties and star-formation histories of surviving and disrupted satellites around Milky Way-like galaxies using the DC Justice League suite of very high-resolution cosmological zoom-in simulations of Milky Way analogs and their halo environments. We analyze the full census of galaxies accreted within the past 12 Gyrs, which including both surviving satellites at $z=0$, and dwarf galaxies that disrupted and merged with the host prior to $z=0$. Our simulations successfully reproduce the trends in $M_*$-[Fe/H]-[$α$/Fe] observed in surviving Milky Way satellites and disrupted stellar streams, indicating earlier star-formation for disrupted progenitors. We find the likelihood and timescales for quenching and disruption are strongly correlated with the mass and time of infall. In particular, none of the galaxies accreted more than 12 Gyrs ago survived, and only 20% of all accreted galaxies with $M_*>10^8M_\odot$ survive. Additionally, satellites with highly radial trajectories are more likely to quench and disrupt. Disruption proceeds quickly for $\geq10^6M_\odot$ satellites accreted $10{-}12$ Gyr ago, often on timescales similar to the $\sim300$ Myr snapshot spacing. For high-mass satellites, the disruption timescale is faster than the quenching timescale. As a result, 92% of disrupted galaxies remain star-forming up until disruption. In contrast, Ultra Faint Dwarfs (UFDs) tend to quench prior to accretion, and 94% of UFDs accreted up to 12 Gyr ago survive at $z=0$.

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arXiv2024

Testable predictions of outside-in age gradients in dwarf galaxies of all types

We use a sample of 73 simulated satellite and central dwarf galaxies spanning a stellar mass range of $10^{5.3}-10^{9.1} M_\odot$ to investigate the origin of their stellar age gradients. We find that dwarf galaxies often form their stars "inside-out," i.e., the stars form at successively larger radii over time. However, the oldest stars get reshuffled beyond the star forming radius by fluctuations in the gravitational potential well caused by stellar feedback (the same mechanisms that cause dwarfs to form dark matter cores). The result is that many dwarfs appear to have an "outside-in" age gradient at $z=0$, with younger stellar populations more centrally concentrated. However, for the reshuffled galaxies with the most extended star formation, young stars can form out to the large radii to which the old stars have been reshuffled, erasing the age gradient. We find that major mergers do not play a significant role in setting the age gradients of dwarfs. We find similar age gradient trends in satellites and field dwarfs, suggesting environment plays only a minor role, if any. Finally, we find that the age gradient trends are imprinted on the galaxies at later times, suggesting that the stellar reshuffling dominates after the galaxies have formed 50% of their stellar mass. The later reshuffling is at odds with results from the FIRE-2 simulations. Hence, age gradients offer a test of current star formation and feedback models that can be probed via observations of resolved stellar populations.

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arXiv2024

Bursting with Feedback: The Relationship between Feedback Model and Bursty Star Formation Histories in Dwarf Galaxies

We use high-resolution cosmological simulations to compare the effect of bursty star formation histories on dwarf galaxy structure for two different subgrid supernovae (SNe) feedback models in dwarf galaxies with stellar masses from $5000 <$ M$_*$/M$_\odot$ $< 10^{9}$. Our simulations are run using two distinct supernova feedback models: superbubble and blastwave. We show that both models are capable of producing galaxies that are cored and reproduce observed scaling relations for metallicity, luminosity, mass, and size. We show that continuous bursty star formation and the resulting stellar feedback are able to sustain dark matter cores in the higher dwarf galaxy mass regime, while the majority of ultra-faint and classical dwarfs retain cuspy central dark matter density profiles. We find that both subgrid SN models are able to create bursty star formation histories. We find that effective core formation peaks at M$_*$/M$_\odot$ $\simeq 5 \times 10^{-3}$ for both feedback models. Galaxies simulated with superbubble feedback peak at lower mean burstiness values relative to blastwave feedback, indicating that core formation in the superbubble sample may be less motivated by the burstiness of star formation.

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arXiv2023

Environment Matters: Predicted Differences in the Stellar Mass--Halo Mass Relation and History of Star Formation for Dwarf Galaxies

We are entering an era in which we will be able to detect and characterize hundreds of dwarf galaxies within the Local Volume. It is already known that a strong dichotomy exists in the gas content and star formation properties of field dwarf galaxies versus satellite dwarfs of larger galaxies. In this work, we study the more subtle differences that may be detectable in galaxies as a function of distance from a massive galaxy, such as the Milky Way. We compare smoothed particle hydrodynamic simulations of dwarf galaxies formed in a Local Volume-like environment (several Mpc away from a massive galaxy) to those formed nearer to Milky Way-mass halos. We find that the impact of environment on dwarf galaxies extends even beyond the immediate region surrounding Milky Way-mass halos. Even before being accreted as satellites, dwarf galaxies near a Milky Way-mass halo tend to have higher stellar masses for their halo mass than more isolated galaxies. Dwarf galaxies in high-density environments also tend to grow faster and form their stars earlier. We show observational predictions that demonstrate how these trends manifest in lower quenching rates, higher HI fractions, and bluer colors for more isolated dwarf galaxies.

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arXiv2021

Stellar Migration and Chemical Enrichment in the Milky Way Disc: A Hybrid Model

We develop a hybrid model of galactic chemical evolution that combines a multi-ring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc's abundance structure: (i) the dependence of the [O/Fe]-[Fe/H] distribution on radius $R_\text{gal}$ and midplane distance $|z|$; (ii) the changing shapes of the [O/H] and [Fe/H] distributions with $R_\text{gal}$ and $|z|$; (iii) a broad distribution of [O/Fe] at sub-solar metallicity and changes in the [O/Fe] distribution with $R_\text{gal}$, $|z|$, and [Fe/H]; (iv) a tight correlation between [O/Fe] and stellar age for [O/Fe] $>$ 0.1; (v) a population of young and intermediate-age $α$-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age-[O/H] and age-[Fe/H] relations, with large scatter and a median age of $\sim$4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate $\sim$2 Gyr ago improve agreement with the observed age-[Fe/H] and age-[O/H] relations, but worsen agreement with the observed age-[O/Fe] relation. None of our models predict an [O/Fe] distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice).

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arXiv2021

Quenching timescales of dwarf satellites around Milky Way-mass hosts

Observations of the low-mass satellites in the Local Group have shown high fractions of gas-poor, quiescent galaxies relative to isolated dwarfs, implying that the host halo environment plays an important role in the quenching of dwarf galaxies. In this work, we present measurements of the quenched fractions and quenching timescales of dwarf satellite galaxies in the DC Justice League suite of 4 high-resolution cosmological zoom-in simulations of Milky Way-mass halos. We show that these simulations accurately reproduce the satellite luminosity functions of observed nearby galaxies, as well as the variation in satellite quenched fractions from $M_* \sim 10^5$ solar masses to $10^{10}$ solar masses. We then trace the histories of satellite galaxies back to $z \sim 15$, and find that many satellites with $M_* \sim 10^{6-8}$ solar masses quench within 2 Gyr of infall into the host halo, while others in the same mass range remain star-forming for as long as 5 Gyr. We show that this scatter can be explained by the satellite's gas mass and the ram pressure it feels at infall. Finally, we identify a characteristic stellar mass scale of $10^8$ solar masses above which infalling satellites are largely resistant to rapid environmental quenching.

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arXiv2020

Ultra-faint dwarfs in a Milky Way context: Introducing the Mint Condition DC Justice League Simulations

We present results from the "Mint" resolution DC Justice League suite of Milky Way-like zoom-in cosmological simulations, which extend our study of nearby galaxies down into the ultra-faint dwarf (UFD) regime for the first time. The mass resolution of these simulations is the highest ever published for cosmological Milky Way zoom-in simulations run to $z=0$, with initial star (dark matter) particle masses of 994 (17900) M$_\odot$, and a force resolution of 87 pc. We study the surrounding dwarfs and UFDs, and find the simulations match the observed dynamical properties of galaxies with $-3 < M_V < -19$, and reproduce the scatter seen in the size-luminosity plane for $r_h\gtrsim200$ pc. We predict the vast majority of nearby galaxies will be observable by the Vera Rubin Observatory's co-added Legacy Survey of Space and Time (LSST). We additionally show that faint dwarfs with velocity dispersions $\lesssim5$ km/s result from severe tidal stripping of the host halo. We investigate the quenching of UFDs in a hydrodynamical Milky Way context, and find that the majority of UFDs are quenched prior to interactions with the Milky Way, though some of the quenched UFDs retain their gas until infall. Additionally these simulations yield some unique dwarfs that are the first of their kind to be simulated, e.g., an HI-rich field UFD, a late-forming UFD that has structural properties similar to Crater 2, as well as a compact dwarf satellite that has no dark matter at $z=0$.

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arXiv2020

Inside Out and Upside-Down: The Roles of Gas Cooling and Dynamical Heating in Shaping the Stellar Age-Velocity Relation

Kinematic studies of disk galaxies, using individual stars in the Milky Way or statistical studies of global disk kinematics over time, provide insight into how disks form and evolve. We use a high-resolution, cosmological zoom-simulation of a Milky Way-mass disk galaxy h277 to tie together local disk kinematics and the evolution of the disk over time. The present-day stellar age-velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighborhood measurement in the Milky Way. A crucial element of this success is the simulation's dynamically cold multi-phase ISM, which allows young stars to form with a low velocity dispersion ($σ_{\mathrm{birth}}$$\sim 6 - 8\ \mathrm{km}\, \mathrm{s}^{-1}$) at late times. Older stars are born kinematically hotter (i.e., the disk settles over time in an "upside-down" formation scenario), and are subsequently heated after birth. The disk also grows "inside-out", and many of the older stars in the solar neighborhood at $z=0$ are present because of radial mixing. We demonstrate that the evolution of $σ_{\mathrm{birth}}$ in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disk galaxies from $z\sim 2-3$ to the present-day, in which the disk evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way's AVR with observed kinematics of high $z$ disk galaxies.

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arXiv2020

Roll of the Dice: A Stochastically Sampled IMF Alters the Stellar Content of Simulated Dwarf Galaxies

Cosmological simulations are reaching the resolution necessary to study ultra-faint dwarf galaxies. Observations indicate that in small populations, the stellar initial mass function (IMF) is not fully populated; rather, stars are sampled in a way that can be approximated as coming from an underlying probability density function. To ensure the accuracy of cosmological simulations in the ultra-faint regime, we present an improved treatment of the IMF. We implement a self-consistent, stochastically populated IMF in cosmological hydrodynamic simulations. We test our method using high-resolution simulations of a Milky Way halo, run to $z=6$, yielding a sample of nearly 100 galaxies. We also use an isolated dwarf galaxy to investigate the resulting systematic differences in galaxy properties. We find that a stochastic IMF in simulations makes feedback burstier, strengthening feedback, and quenching star formation earlier in small dwarf galaxies. For galaxies in halos with mass $\lesssim10^{8.5}$ M$_\odot$, a stochastic IMF typically leads to lower stellar mass compared to a continuous IMF, sometimes by more than an order of magnitude. We show that existing methods of ensuring discrete supernovae incorrectly determine the mass of the star particle and its associated feedback. This leads to overcooling of surrounding gas, with at least ${\sim}$10 per cent higher star formation and ${\sim}$30 per cent higher cold gas content. Going forward, to accurately model dwarf galaxies and compare to observations, it will be necessary to incorporate a stochastically populated IMF that samples the full spectrum of stellar masses.

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arXiv2018

Reignition of Star Formation in Dwarf Galaxies

The Local Group hosts a number of star-forming dwarf galaxies that show evidence of periods of little to no star formation. We use a suite of cosmological simulations to study how star formation is reignited in such galaxies. We focus on isolated galaxies at $z=0$ with halo masses between 9.2$\times$10$^8$ M$_\odot$ and 8.4$\times$10$^9$ M$_\odot$, where star formation is typically shut off by reionization or by supernova feedback. Nearly 20% of these simulated galaxies later restart star formation, due to interactions with streams of gas in the intergalactic medium, indicating that this mechanism is relatively common in this mass range and that many isolated dwarfs at $z=0$ may not have been isolated throughout their histories. The source of this gas is not necessarily cosmic filaments. Rather, the dwarfs interact with gas thrown off by nearby galaxy mergers or streams extending from neighboring galaxies. While high ram pressure interactions of this nature lead to stripping, the encounters that reignite star formation are low density and/or low velocity and thus low ram pressure, resulting in compression of the hot gas in the halos of our dwarfs. The gas mass bound up in hot halos can be substantial -- at least an order of magnitude greater than the mass contained in HI. Consequently, we find that dwarfs that have experienced reignition tend to be more HI-rich and have a higher M$_{HI}$/M$_{*}$ ratio at $z=0$ than galaxies with continuous star formation. Using this fact, we identify galaxies in the Local Volume that might have "gappy" star formation histories, and can be studied by the Hubble Space Telescope or the James Webb Space Telescope.

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arXiv2018

Tracing Outflowing Metals in Simulations of Dwarf and Spiral Galaxies

We analyze the metal accumulation in dwarf and spiral galaxies by following the history of metal enrichment and outflows in a suite of twenty high-resolution simulated galaxies. These simulations agree with the observed stellar and gas-phase mass-metallicity relation, an agreement that relies on large fractions of the produced metals escaping into the CGM. For instance, in galaxies with Mvir ~ 1e9.5 -- 1e10 solar masses, we find that about ~ 85% of the available metals are outside of the galactic disk at z = 0, although the fraction decreases to a little less than half in Milky Way-mass galaxies. In many cases, these metals are spread far beyond the virial radius. We analyze the metal deficit within the ISM and stars in the context of previous work tracking the inflow and outflow of baryons. Outflows are prevalent across the entire mass range, as is reaccretion. We find that between 40 and 80% of all metals removed from the galactic disk are later reaccreted. The outflows themselves are metal enriched relative to the ISM by a factor of 0.2 dex because of the correspondence between sites of metal enrichment and outflows. As a result, the metal mass loading factor scales as eta_metals \propto v_circ^-0.91, a somewhat shallower scaling than the total mass loading factor. We analyze the simulated galaxies within the context of analytic chemical evolution models by determining their net metal expulsion efficiencies, which encapsulate the rates of metal loss and reaccretion. We discuss these results in light of the inflow and outflow properties necessary for reproducing the mass-metallicity relation.

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arXiv2018

In-N-Out: the gas cycle from dwarfs to spiral galaxies

We examine the scalings of galactic outflows with halo mass across a suite of twenty high-resolution cosmological zoom galaxy simulations covering halo masses from 10^9.5 - 10^12 M_sun. These simulations self-consistently generate outflows from the available supernova energy in a manner that successfully reproduces key galaxy observables including the stellar mass-halo mass, Tully-Fisher, and mass-metallicity relations. We quantify the importance of ejective feedback to setting the stellar mass relative to the efficiency of gas accretion and star formation. Ejective feedback is increasingly important as galaxy mass decreases; we find an effective mass loading factor that scales as v_circ^(-2.2), with an amplitude and shape that is invariant with redshift. These scalings are consistent with analytic models for energy-driven wind, based solely on the halo potential. Recycling is common: about half the outflow mass across all galaxy masses is later re-accreted. The recycling timescale is typically about 1 Gyr, virtually independent of halo mass. Recycled material is re-accreted farther out in the disk and with typically about 2-3 times more angular momentum. These results elucidate and quantify how the baryon cycle plausibly regulates star formation and alters the angular momentum distribution of disk material across the halo mass range where most of cosmic star formation occurs.

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arXiv2018

Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter for Stellar Halos

The velocity anisotropy parameter, beta, is a measure of the kinematic state of orbits in the stellar halo which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for beta as a function of radius from three suites of simulations, including accretion only and cosmological hydrodynamic simulations. We find that both types of simulations are consistent and predict strong radial anisotropy ( ~0.7) for Galactocentric radii greater than 10 kpc. Previous observations of beta for the MW's stellar halo claim a detection of an isotropic or tangential "dip" at r~20 kpc. Using the N-body+SPH simulations, we investigate the temporal persistence, population origin, and severity of "dips" in beta. We find dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z~1 can result in a present day low (isotropic to tangential) value of beta over a wide range of radii and angular expanse. While all of these mechanisms are plausible drivers for the beta dip observed in the MW, in the simulations, each mechanism has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of beta(r) is required for measuring the mass of the MW halo, we note significant transient dips in beta could cause an overestimate of the halo's mass when using spherical Jeans equation modeling.

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arXiv2017

How to Reconcile the Observed Velocity Function of Galaxies with Theory

Within a Lambda Cold Dark Matter (LCDM) scenario, we use high resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in observed velocity functions. We measure velocities in as similar a way as possible to observations, including generating mock HI data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an "observed" velocity function in agreement with observations, reconciling the large number of low-mass halos expected in a LCDM cosmological model with the low number of observed dwarfs at a given velocity. We then explore the source of the discrepancy between observations and theory, and conclude that the dearth of observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the HI linewidth severely underestimate the maximum halo velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored dark matter density profiles can contribute to the lower observed velocity of galaxies, but only for galaxies in which the velocity is measured interior to the size of the core (~3 kpc).

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arXiv2016

The Role of Baryons in Creating Statistically Significant Planes of Satellites around Milky Way-Mass Galaxies

We investigate whether the inclusion of baryonic physics influences the formation of thin, coherently rotating planes of satellites such as those seen around the Milky Way and Andromeda. For four Milky Way-mass simulations, each run both as dark matter-only and with baryons included, we are able to identify a planar configuration that significantly maximizes the number of plane satellite members. The maximum plane member satellites are consistently different between the dark matter-only and baryonic versions of the same run due to the fact that satellites are both more likely to be destroyed and to infall later in the baryonic runs. Hence, studying satellite planes in dark matter-only simulations is misleading, because they will be composed of different satellite members than those that would exist if baryons were included. Additionally, the destruction of satellites in the baryonic runs leads to less radially concentrated satellite distributions, a result that is critical to making planes that are statistically significant compared to a random distribution. Since all planes pass through the centre of the galaxy, it is much harder to create a plane from a random distribution if the satellites have a low radial concentration. We identify Andromeda's low radial satellite concentration as a key reason why the plane in Andromeda is highly significant. Despite this, when co-rotation is considered, none of the satellite planes identified for the simulated galaxies are as statistically significant as the observed planes around the Milky Way and Andromeda, even in the baryonic runs.

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arXiv2015

Bulge Formation via Mergers in Cosmological Simulations

The latest generation of cosmological simulations are on the verge of being able to resolve the structure of bulges for the first time. Hence, we review the current state of bulge formation in cosmological simulations, and discuss open questions that can be addressed in the near future by simulators, with a particular focus on merger-driven bulge growth. Galaxy mergers have long been assumed to produce classical bulges in disk galaxies. Under this bulge-formation model, though, the high rates of mergers in Cold Dark Matter galaxy formation theory predict many more classical bulges than are observed. Furthermore, simulations of galaxy formation continue to generally produce too massive of bulges. Feedback offers a promising avenue for reducing merger-driven bulge growth by maintaining high gas fractions in galaxies and ejecting low-angular momentum gas driven to the centers of galaxies. After reviewing the results of relevant research that has been published to date, we use cosmological simulations to explore the ability of feedback to reduce or even prevent bulge growth during mergers. In dwarf galaxies, mergers actually reduce the central concentration of galaxies as the induced burst of star formation drives out low angular momentum material. This result shows the potential for feedback to reduce central mass growth. However, we also demonstrate that it is very difficult for current stellar feedback models to reproduce the small bulges observed in more massive disk galaxies like the Milky Way. We argue that feedback models need to be improved, or an additional source of feedback such as AGN is necessary to generate the required outflows.

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arXiv2015

Consequences of bursty star formation on galaxy observables at high redshifts

The star formation histories (SFHs) of dwarf galaxies are thought to be \emph{bursty}, with large -- order of magnitude -- changes in the star formation rate on timescales similar to O-star lifetimes. As a result, the standard interpretations of many galaxy observables (which assume a slowly varying SFH) are often incorrect. Here, we use the SFHs from hydro-dynamical simulations to investigate the effects of bursty SFHs on sample selection and interpretation of observables and make predictions to confirm such SFHs in future surveys. First, because dwarf galaxies' star formation rates change rapidly, the mass-to-light ratio is also changing rapidly in both the ionizing continuum and, to a lesser extent, the non-ionizing UV continuum. Therefore, flux limited surveys are highly biased toward selecting galaxies in the \emph{burst} phase and very deep observations are required to detect all dwarf galaxies at a given stellar mass. Second, we show that a $\log_{10}[νL_ν(1500{\rm Å})/L_{{\rm H}α}]>2.5$ implies a very recent quenching of star formation and can be used as evidence of stellar feedback regulating star formation. Third, we show that the ionizing continuum can be significantly higher than when assuming a constant SFH, which can affect the interpretation of nebular emission line equivalent widths and direct ionizing continuum detections. Finally, we show that a star formation rate estimate based on continuum measurements only (and not on nebular tracers such as the hydrogen Balmer lines) will not trace the rapid changes in star formation and will give the false impression of a star-forming main sequence with low dispersion.

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arXiv2014

The Milky Way Tomography with SDSS. V. Mapping the Dark Matter Halo

We present robust constraints from the Sloan Digital Sky Survey (SDSS) on the shape and distribution of the dark matter halo within the Milky Way (MW). Using the number density distribution and kinematics of SDSS halo stars, we probe the dark matter distribution to heliocentric distances exceeding 10 kpc and galactocentric distances exceeding 20 kpc. Our analysis utilizes Jeans equations to generate two-dimensional acceleration maps throughout the volume; this approach is thoroughly tested on a cosmologically derived N-body+SPH simulation of a MW-like galaxy. We show that the known accelerations (gradients of the gravitational potential) can be successfully recovered in such a realistic system. Leveraging the baryonic gravitational potential derived by Bovy & Rix (2013), we show that the gravitational potential implied by the SDSS observations cannot be explained, assuming Newtonian gravity, by visible matter alone: the gravitational force experienced by stars at galactocentric distances of 20 kpc is as much as three times stronger than what can be attributed to purely visible matter. We also show that the SDSS data provide a strong constraint on the shape of the dark matter halo potential. Within galactocentric distances of 20 kpc, the dark matter halo potential is well described as an oblate halo with axis ratio qDM=0.7+/-0.1; this corresponds to an axis ratio qDM=0.4+/-0.1 for the dark matter density distribution. Because of our precise two-dimensional measurements of the acceleration of the halo stars, we can reject several MOND models as an explanation of the observed behavior.

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