arXiv subjects

Charlotte Christensen

Publications and source records attributed to Charlotte Christensen.

At least 19 recordsLinked to original sources

arXiv2026

The Simulated Oxygen Shortage (SOS): Mapping the Missing OVI in Simulated Dwarf Galaxies to Subgrid Physics

Observations reveal extended \OVI\, reservoirs in the circumgalactic medium (CGM) of dwarf galaxies, yet current simulations systematically underpredict \OVI\, column densities. Utilizing two suites run with different simulation codes, the \MM\, simulations (Marvelous Massive Dwarfs and Marvel-ous Dwarfs) and the publicly available FIRE-2 simulations, we explore the role of subgrid models and the resulting CGM phase in shaping \OVI\, production. By comparing observationally derived \OVI\, masses to the mass of oxygen produced over the galaxies' star formation history, we find evidence for an underproduction of oxygen for low-mass simulated galaxies. Despite clear differences in feedback implementation, CGM structure, and metal mixing, we find that \OVI\, in both suites generally self-selects cool/warm ($\rm log\, T\,/K \sim 4.5$), diffuse ($\rm log\,n_{gas}\,/cm^{-3} \sim -5.0 $), and moderately metal-enriched ($\rm log\, Z/Z_{\odot} \sim -1 $) material at large radii from the galaxy. We show that neither the choice of ultraviolet background nor plausible variations in CGM thermal structure can close the gap with observations. Taken together, our results point to a possible underproduction of oxygen in low-mass galaxies. Feedback prescriptions contribute via insufficient metal transport to large radii. Hence, the \OVI\, deficit may motivate an investigation of current modeling choices for supernova yields, star formation, and feedback in low-metallicity environments.

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arXiv2025

Marvelous Metals: Surveying the Circumgalactic Medium of Simulated Dwarf Galaxies

Dwarf galaxies are uniquely sensitive to energetic feedback processes and are known to experience substantial mass and metal loss from their disk. Here, we investigate the circumgalactic medium (CGM) of 64 isolated dwarf galaxies ($6.0<$log(M$_*/M_{\odot}$)$<9.5$) at $z=0$ from the Marvel-ous Dwarfs and Marvelous Massive Dwarfs hydrodynamic simulations. Our galaxies produce column densities broadly consistent with current observations. We investigate these column densities in the context of mass and metal retention rates and the physical properties of the CGM. We find $48\pm11\%$ of all baryons within $R_{200c}$ reside in the CGM, with $\sim70\%$ of CGM mass existing in a warm gas phase, $10^{4.5} 40\%$ towards lower masses. Our findings highlight the CGM (primarily its warm component) as a key reservoir of mass and metals for dwarf galaxies across stellar masses and underscore its importance in understanding the baryon cycle in the low-mass regime. Finally, we provide individual galaxy properties of our full sample and quantify the fraction of ultraviolet observable mass to support future observational programs, particularly those aimed at performing a metal budget around dwarf galaxies.

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arXiv2023

The Role of Mass and Environment on Satellite distributions around Milky Way analogs in the Romulus25 simulation

We study satellite counts and quenched fractions for satellites of Milky Way analogs in Romulus25, a large-volume cosmological hydrodynamic simulation. Depending on the definition of a Milky Way analog, we have between 66 and 97 Milky Way analogs in Romulus25, a 25 Mpc per-side uniform volume simulation. We use these analogs to quantify the effect of environment and host properties on satellite populations. We find that the number of satellites hosted by a Milky Way analog increases predominantly with host stellar mass, while environment, as measured by the distance to a Milky Way-mass or larger halo, may have a notable impact in high isolation. Similarly, we find that the satellite quenched fraction for our analogs also increases with host stellar mass, and potentially in higher-density environments. These results are robust for analogs within 3 Mpc of another Milky Way-mass or larger halo, the environmental parameter space where the bulk of our sample resides. We place these results in the context of observations through comparisons to the Exploration of Local VolumE Satellites and Satellites Around Galactic Analogs surveys. Our results are robust to changes in Milky Way analog selection criteria, including those that mimic observations. Finally, as our samples naturally include Milky Way-Andromeda pairs, we examine quenched fractions in pairs vs isolated systems. We find potential evidence, though not conclusive, that pairs, defined as being within 1 Mpc of another Milky Way-mass or larger halo, may have higher satellite quenched fractions.

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arXiv2023

The Scatter Matters: Circumgalactic Metal Content in the Context of the $M-σ$ Relation

The interaction between supermassive black hole (SMBH) feedback and the circumgalactic medium (CGM) continues to be an open question in galaxy evolution. In our study, we use SPH simulations to explore the impact of SMBH feedback on galactic metal retention and the motion of metals and gas into and through the CGM of L$_{*}$ galaxies. We examine 140 galaxies from the 25 Mpc cosmological volume, Romulus25, with stellar masses between 3 $\times$ 10$^{9}$ - 3 $\times$ 10$^{11}$ M$_{\odot}$. We measure the fraction of metals remaining in the ISM and CGM of each galaxy, and calculate the expected mass of its SMBH based on the $M-σ$ relation. The deviation of each SMBH from its expected mass, $ΔM_{BH}$ is compared to the potential of its host via $σ$. We find that SMBHs with accreted mass above the empirical $M-σ$ relation are about 15\% more effective at removing metals from the ISM than under-massive SMBHs in star forming galaxies. Over-massive SMBHs suppress the overall star formation of their host galaxies and more effectively move metals from the ISM into the CGM. However, we see little evidence for the evacuation of gas from their halos, in contrast with other simulations. Finally, we predict that C IV column densities in the CGM of L$_{*}$ galaxies may depend on host galaxy SMBH mass. Our results show that the scatter in the low mass end of $M-σ$ relation may indicate how effective a SMBH is at the local redistribution of mass in its host galaxy.

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arXiv2021

The Origins of Off-Centre Massive Black Holes in Dwarf Galaxies

Massive black holes often exist within dwarf galaxies, and both simulations and observations have shown that a substantial fraction of these may be off-center with respect to their hosts. We trace the evolution of off-center massive black holes (MBHs) in dwarf galaxies using cosmological hydrodynamical simulations, and show that the reason for off-center locations is mainly due to galaxy-galaxy mergers. We calculate dynamical timescales and show that off-center MBHs are unlikely to sink to their galaxys' centers within a Hubble time, due to the shape of the hosts' potential wells and low stellar densities. These wandering MBHs are unlikely to be detected electromagnetically, nor is there a measurable dynamical effect on the galaxy's stellar population. We conclude that off-center MBHs may be common in dwarfs, especially if the mass of the MBH is small or the stellar mass of the host galaxy is large. However detecting them is extremely challenging, because their accretion luminosities are very low and they do not measurably alter the dynamics of their host galaxies.

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arXiv2021

Not So Heavy Metals: Black Hole Feedback Enriches The Circumgalactic Medium

We examine the effects of SMBH feedback on the CGM using a cosmological hydrodynamic simulation \citep[{\sc Romulus25};][]{Tremmel2017} and a set of four zoom-in `genetically modified' Milky Way-mass galaxies sampling different evolutionary paths. By tracing the distribution of metals in the circumgalactic medium (CGM), we show that \ion{O}{6} is a sensitive indicator of supermassive black hole (SMBH) feedback. First, we calculate the column densities of \ion{O}{6} in simulated Milky Way-mass galaxies and compare them with observations from the COS-Halos Survey. Our simulations show column densities of \ion{O}{6} in the CGM consistent with those of COS-Halos star forming and quenched galaxies. These results contrast with those from previous simulation studies which typically underproduce CGM column densities of \ion{O}{6}. We determine that a galaxy's star formation history and assembly record have little effect on the amount of \ion{O}{6} in its CGM. Instead, column densities of \ion{O}{6} are closely tied to galaxy halo mass and BH growth history. The set of zoom-in, genetically modified Milky Way-mass galaxies indicates that the SMBH drives highly metal-enriched material out into its host galaxy's halo which in turn elevates the column densities of \ion{O}{6} in the CGM.

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arXiv2021

One-Two Quench: A Double Minor Merger Scenario

Using the N-body+Smoothed particle hydrodynamics code, ChaNGa, we identify two merger-driven processes\textemdash disk disruption and supermassive black hole (SMBH) feedback\textemdash which work together to quench L$^*$ galaxies for over 7 Gyr. Specifically, we examine the cessation of star formation in a simulated Milky Way (MW) analog, driven by an interaction with two minor satellites. Both interactions occur within $\sim$100 Myr of each other, and the satellites both have masses 5 to 20 times smaller than that of their MW-like host galaxy. Using the genetic modification process of \cite{Roth2016}, we generate a set of four zoom-in, MW-mass galaxies all of which exhibit unique star formation histories due to small changes to their assembly histories. In two of these four cases, the galaxy is quenched by $z = 1$. Because these are controlled modifications, we are able to isolate the effects of two closely-spaced minor merger events, the relative timing of which determines whether the MW-mass main galaxy quenches. This one-two punch works to: 1. fuel the primary halo's supermassive black hole (SMBH) at its peak accretion rate; and 2. disrupt the cold, gaseous disk of the host galaxy. The end result is that feedback from the SMBH thoroughly and abruptly ends the galaxy's star formation by $z\approx1$. We search for and find a similar quenching event in {\sc Romulus25}, a hydrodynamical $(25\,\mathrm{Mpc})^3$ volume simulation, demonstrating that the mechanism is common enough to occur even in a small sample of MW-mass quenched galaxies at $z=0$.

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arXiv2019

Understanding the circumgalactic medium is critical for understanding galaxy evolution

Galaxies evolve under the influence of gas flows between their interstellar medium and their surrounding gaseous halos known as the circumgalactic medium (CGM). The CGM is a major reservoir of galactic baryons and metals, and plays a key role in the long cycles of accretion, feedback, and recycling of gas that drive star formation. In order to fully understand the physical processes at work within galaxies, it is therefore essential to have a firm understanding of the composition, structure, kinematics, thermodynamics, and evolution of the CGM. In this white paper we outline connections between the CGM and galactic star formation histories, internal kinematics, chemical evolution, quenching, satellite evolution, dark matter halo occupation, and the reionization of the larger-scale intergalactic medium in light of the advances that will be made on these topics in the 2020s. We argue that, in the next decade, fundamental progress on all of these major issues depends critically on improved empirical characterization and theoretical understanding of the CGM. In particular, we discuss how future advances in spatially-resolved CGM observations at high spectral resolution, broader characterization of the CGM across galaxy mass and redshift, and expected breakthroughs in cosmological hydrodynamic simulations will help resolve these major problems in galaxy evolution.

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arXiv2019

Dancing in the Dark: Uncertainty in ultra-faint dwarf galaxy predictions from cosmological simulations

The existence of ultra-faint dwarf (UFD) galaxies highlights the need to push our theoretical understanding of galaxies to extremely low mass. We examine the formation of UFDs by twice running a fully cosmological simulations of dwarf galaxies, but varying star formation. One run uses a temperature-density threshold for star formation, while the other uses an H$_{2}$-based sub-grid star formation model. The total number of dwarf galaxies that forms is different by a factor of 2 between the two runs, but most of these are satellites, leading to a factor of 5 difference in the number of luminous UFD companions around more massive, isolated dwarfs. The first run yields a 47\% chance of finding a satellite around a M$_{halo}$ $\sim 10^{10}$ M$_{\odot}$ host, while the H$_2$ run predicts only a 16\% chance. Metallicity is the primary physical parameter that creates this difference. As metallicity decreases, the formation of H$_2$ is slowed and relegated to higher-density material. Thus, our H$_2$ run is unable to form many (and often, any) stars before reionization removes gas. These results emphasize that predictions for UFD properties made using hydrodynamic simulations, in particular regarding the frequency of satellites around dwarf galaxies, the slope of the stellar mass function at low masses, as well as the properties of ultra-faint galaxies occupying the smallest halos, are extremely sensitive to the subgrid physics of star formation contained within the simulation. However, upcoming discoveries of ultra-faint dwarfs will provide invaluable constraining power on the physics of the first star formation.

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arXiv2019

Implementing Dust Shielding as a Criteria for Star Formation

Star formation is observed to be strongly correlated to dense regions of molecular gas. Although the exact nature of the link between star formation and molecular hydrogen is still unclear, some have suggested that shielding of dense gas by dust grains is the key factor enabling the presence of both. We present a sub-grid model for use in galaxy formation simulations in which star formation is linked explicitly to local dust shielding. We developed and tested our shielding and star formation models using smoothed particle hydrodynamic simulations of solar and sub-solar metallicity isolated Milky Way-mass disk galaxies. We compared our dust shielding-based star formation model to two other star formation recipes that used gas temperature and H$_2$ fraction as star formation criteria. We further followed the evolution of a dwarf galaxy within a cosmological context using both the shielding and H$_2$-based star formation models. We find that the shielding-based model allows for star formation at higher temperatures and lower densities than a model in which star formation is tied directly to H$_2$ abundance, as requiring H$_2$ formation leads the gas to undergo additional gravitational collapse before star formation. However, the resulting galaxies are very similar for both the shielding and H$_2$-based star formation models, and both models reproduce the resolved Kennicutt-Schmidt law. Therefore, both star formation models appear viable in the context of galaxy formation simulations.

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arXiv2018

Multimessenger Signatures of Massive Black Holes in Dwarf Galaxies

Recent discoveries of massive black holes (MBHs) in dwarf galaxies suggest that they may have a more common presence than once thought. Systematic searches are revealing more candidates, but this process could be accelerated by predictions from simulations. We perform a study of several high-resolution, cosmological, zoom-in simulations focusing on dwarf galaxies that host massive black holes at z = 0, with the aim of determining when the black holes are most observable. Larger dwarf galaxies are more likely to host MBHs than those of lower mass. About 50% of the MBHs in dwarfs are not centrally located, but rather are wandering within a few kpc of the galaxy center. The accretion luminosities of MBHs in dwarfs are low throughout cosmic time, rendering them extremely difficult to detect. However, the merger history of these MBHs is optimal for gravitational wave detection by LISA.

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arXiv2018

Sowing black hole seeds: Direct collapse black hole formation with realistic Lyman-Werner radiation in cosmological simulations

We study the birth of supermassive black holes from the direct collapse process and characterize the sites where these black hole seeds form. In the pre-reionization epoch, molecular hydrogen (H$_2$) is an efficient coolant, causing gas to fragment and form Population III stars, but Lyman-Werner radiation can suppress H$_2$ formation and allow gas to collapse directly into a massive black hole. The critical flux required to inhibit H$_2$ formation, $J_{\rm crit}$, is hotly debated, largely due to the uncertainties in the source radiation spectrum, H$_2$ self-shielding, and collisional dissociation rates. Here, we test the power of the direct collapse model in a self-consistent, time-dependant, non-uniform Lyman-Werner radiation field -- the first time such has been done in a cosmological volume -- using an updated version of the SPH+N-body tree code Gasoline with H$_2$ non-equilibrium abundance tracking, H$_2$ cooling, and a modern SPH implementation. We vary $J_{\rm crit} $ from $30$ to $10^3$ in units of $J_{21}$ to study how this parameter impacts the number of seed black holes and the type of galaxies which host them. We focus on black hole formation as a function of environment, halo mass, metallicity, and proximity of the Lyman-Werner source. Massive black hole seeds form more abundantly with lower $J_{\rm crit}$ thresholds, but regardless of $J_{\rm crit}$, these seeds typically form in halos that have recently begun star formation. Our results do not confirm the proposed atomic cooling halo pair scenario; rather black hole seeds predominantly form in low-metallicity pockets of halos which already host star formation.

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arXiv2017

A unified model for age-velocity dispersion relations in Local Group galaxies: Disentangling ISM turbulence and latent dynamical heating

We analyze age-velocity dispersion relations (AVRs) from kinematics of individual stars in eight Local Group galaxies ranging in mass from Carina ($M_{*} \sim 10^{6}$) to M31 ($M_{*} \sim 10^{11}$). Observationally the $σ$ vs. stellar age trends can be interpreted as dynamical heating of the stars by GMCs, bars/spiral arms, or merging subhalos; alternatively the stars could have simply been born out of a more turbulent ISM at high redshift and retain that larger velocity dispersion till present day - consistent with recent IFU studies. To ascertain the dominant mechanism and better understand the impact of instabilities and feedback, we develop models based on observed SFHs of these Local Group galaxies in order to create an evolutionary formalism which describes the ISM velocity dispersion due to a galaxy's evolving gas fraction. These empirical models relax the common assumption that the stars are born from gas which has constant velocity dispersion at all redshifts. Using only the observed SFHs as input, the ISM velocity dispersion and a mid-plane scattering model fits the observed AVRs of low mass galaxies without fine tuning. Higher mass galaxies above $M_{vir} > 10^{11}$ need a larger contribution from latent dynamical heating processes (for example minor mergers), in excess of the ISM model. Using the SFHs we also find that supernovae feedback does not appear to be a dominant driver of the gas velocity dispersion compared to gravitational instabilities - at least for dispersions $σ\gtrsim 25$ km/s. Together our results point to stars being born with a velocity dispersion close to that of the gas at the time of their formation, with latent dynamical heating operating with a galaxy mass-dependent efficiency. These semi-empirical relations may help constrain the efficiency of feedback and its impact on the physics of disk settling in galaxy formation simulations.

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arXiv2017

Going, going, gone dark: Quantifying the scatter in the faintest dwarf galaxies

We predict the stellar mass-halo mass (SMHM) relationship for dwarf galaxies and their satellites residing in halos down to M$_{halo} =$ 10$^7$ M$_{\odot}$ with 10$^4$ M$_{\odot} <$ M$_{star}$($z=0$) $< 10^8$ M$_{\odot}$, and quantify the predicted scatter in the relation at the low mass end, using cosmological simulations. The galaxies were drawn from a cosmological simulation of dwarf galaxies, run with the N-body + SPH code, ChaNGA, at a high resolution of 60 pc. For M$_{halo} > 10^9$ M$_{\odot}$, the simulated SMHM relationship agrees with literature determinations, including exhibiting a small scatter. However, the scatter in the SMHM relation increases dramatically for lower-mass halos. We find that some of this scatter is due to {\em dark dwarfs}, halos devoid of stars. However, even when only considering well-resolved halos that contain a stellar population, the scatter in stellar mass reaches nearly 1 dex for M$_{halo}$($z=0$) 10$^7$ M$_{\odot}$. Much of this scatter is due to including satellites of the dwarf galaxies that have had their halo masses reduced through tidal stripping. The fraction of dark dwarfs (those that contain no stars) increases substantially with decreasing halo mass. When these dark halos are considered, the true scatter in the SMHM at low masses is even larger. At the faintest end of the SMHM relation probed by our simulations, a galaxy cannot be assigned a unique halo mass based solely on its luminosity. We provide a formula to stochastically populate low-mass halos following our results. Our predicted large scatter at low halo masses increases the slope of the resulting stellar mass function on the ultra-faint dwarf galaxy scales currently being probed by such surveys as the Dark Energy Survey or the Hyper-Suprime Cam Subaru Strategic Program, and in the future by the Large Synoptic Survey Telescope.

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arXiv2016

The effects of host galaxy properties on merging compact binaries detectable by LIGO

Cosmological simulations of galaxy formation can produce present-day galaxies with a large range of assembly and star formation histories. A detailed study of the metallicity evolution and star formation history of such simulations can assist in predicting LIGO-detectable compact object binary mergers. Recent simulations of compact binary evolution suggest the compact object merger rate depends sensitively on the progenitor's metallicity. Rare low-metallicity star formation during galaxy assembly can produce more detected compact binaries than typical star formation. Using detailed simulations of galaxy and chemical evolution, we determine how sensitively the compact binary populations of galaxies with similar present-day appearance depend on the details of their assembly. We also demonstrate by concrete example the extent to which dwarf galaxies overabundantly produce compact binary mergers, particularly binary black holes, relative to more massive galaxies. We discuss the implications for transient multimessenger astronomy with compact binary sources.

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arXiv2014

Faint dwarfs as a test of DM models: WDM vs. CDM

We use high resolution Hydro$+$N-Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass ~ 10$^{6-7}$ M$\odot$, total mass 10$^{10}$ M$\odot$ in $Λ$ dominated CDM and 2keV WDM cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1-2 Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the DM model, but proportionally to the SF efficiency, gas outflows lower the central mass density through `dynamical heating', such that all realizations have circular velocities $<$ 20kms at 500$~$pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial color-magnitude diagrams and star formation histories in order to directly compare to available observations. The simulated galaxies formed most of their stars in many $\sim$10 Myr long bursts. The CDM galaxy has a global SFH, HI abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including `baryon physics' in simulations when 1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and 2) differentiating between CDM and non-standard models with different DM or power spectra.

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arXiv2014

Effects of Inclination on Measuring Velocity Dispersion and Implications for Black Holes

The relation of central black hole mass and stellar spheroid velocity dispersion (the M-$σ$ relation) is one of the best-known and tightest correlations linking black holes and their host galaxies. There has been much scrutiny concerning the difficulty of obtaining accurate black hole measurements, and rightly so; however, it has been taken for granted that measurements of velocity dispersion are essentially straightforward. We examine five disk galaxies from cosmological SPH simulations and find that line-of-sight effects due to galaxy orientation can affect the measured $σ$ by 30%, and consequently black hole mass predictions by up to 1.0 dex. Face-on orientations correspond to systematically lower velocity dispersion measurements, while more edge-on orientations give higher velocity dispersions, due to contamination by disk stars when measuring line of sight quantities. We caution observers that the uncertainty of velocity dispersion measurements is at least 20 km/s, and can be much larger for moderate inclinations. This effect may account for some of the scatter in the locally measured M-$σ$ relation, particularly at the low-mass end. We provide a method for correcting observed $σ_{\rm los}$ values for inclination effects based on observable quantities.

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arXiv2014

The effect of models of the interstellar media on the central mass distribution of galaxies

We compare the central mass distribution of galaxies simulated with three different models of the interstellar medium (ISM) with increasing complexity: primordial (H+He) cooling down to 10^4K, additional cooling via metal lines and to lower temperatures, and molecular hydrogen (H_2) with shielding of atomic and molecular hydrogen, in addition to metal line cooling. In order to analyze the effect of these models, we follow the evolution of four field galaxies with V_peak < 120 km/s to a redshift of zero using high-resolution Smoothed Particle Hydrodynamic simulations in a fully cosmological LCDM context. The spiral galaxies produced in simulations with either primordial cooling or H_2 physics have realistic, rising rotation curves. In contrast, the simulations with metal line cooling and otherwise similar feedback and star formation produced galaxies with the peaked rotation curves typical of most previous LCDM simulations of spiral galaxies. The less-massive bulges and non-peaked rotation curves in the galaxies simulated with primordial cooling or H_2 are linked to changes in the angular momentum distribution of the baryons. These galaxies had smaller amounts of low-angular momentum baryons because of increased gas loss from stellar feedback. When there is only primordial cooling, the star forming gas is hotter and the feedback-heated gas cools more slowly than when metal line cooling is included and so requires less energy to be expelled. When H_2 is included, the accompanying shielding produces large amounts of clumpy, cold gas where H_2 forms. Star formation in clumpy gas results in more concentrated supernova feedback and greater efficiency of mass loss. The higher feedback efficiency causes a decrease of low-angular momentum material. (abridged)

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