arXiv subjects

Anna C. Wright

Publications and source records attributed to Anna C. Wright.

18 recordsLinked to original sources

arXiv2026

The Merger-Driven Formation of Classical Low Surface Brightness Galaxies in Romulus25

We use the Romulus25 cosmological simulation volume to study a large sample of late-type gas-rich galaxies with low central surface brightnesses known as classical low surface brightness (LSB) galaxies and compare them to a mass-matched sample of high surface brightness (HSB) galaxies. We find that classical LSB galaxies make up a substantial fraction of the galaxy population, accounting for ~60% of all central galaxies with 8$\leq$log$_\mathrm{10}$(M$_\star$/M$_\odot$)$\leq$10. In Romulus25, classical LSB galaxies are predominantly formed through major mergers in which the secondary galaxy is co-rotating and aligned with the primary galaxy's gas disk and/or has above average orbital angular momentum at infall. The merger product is a high spin galaxy in which star formation is spread out and inefficient, allowing the galaxy to build up a large supply of relatively unenriched gas. The star formation rates of LSB galaxies are nearly constant over time, leading to stellar populations that are, on average, slightly older and therefore optically redder than those of similar HSB galaxies. However, because LSB galaxies are diffuse and metal-poor, they have very little internal reddening, causing them to appear bluer than HSB galaxies. We also find that, when compared to the bulges of HSB galaxies, the bulges of LSB galaxies are similar in mass, but are lower surface brightness, redder, and more diffuse on average. Despite these differences, classical LSB galaxies are part of the continuum of the galaxy population in Romulus25, constituting one of many evolutionary paths.

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arXiv2026

Figuring Out Gas & Galaxies In Enzo (FOGGIE). XIII. On the Observability of Extended HI Disks and Warps

Atomic Hydrogen (HI) is a useful tracer of gas in and around galaxies, and can be found in extended disk-like structures well beyond a system's optical extent. Here, we investigate the properties of extended HI disks that emerge in six Milky Way-mass galaxies using cosmological zoom-in simulations from the Figuring Out Gas & Galaxies in Enzo (FOGGIE) suite. This paper focuses on the observability of the extended HI in these systems. We find overall agreement with observational constraints on the HI size-mass relation. To facilitate direct comparisons with observations, we present synthetic HI 21 cm emission cubes. By spatially filtering our synthetic cubes to characterize the absence of short baselines in interferometric maps, we find that such observations at 20 Mpc retain ~96%-99%$ of total HI emission on average, but can miss up to ~15% of HI signal outside the central disk due to missing short spacings. This effect is small for more isolated systems, but more significant for more strongly interacting systems, as there is more diffuse signal. This preferentially removes low column density, low velocity dispersion gas in the circumgalactic medium (CGM). The amount of observable material depends strongly on its distribution, distance, and the system's observed orientation, preventing the formulation of a simple correction factor. Therefore, to fully characterize extended disks, their CGMs, and the interfaces between them, including data from large single-dish radio telescopes is likely necessary.

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arXiv2026

The Formation of Dwarf Galaxy Disks

Dwarf galaxies are dark matter-dominated systems that are sensitive to feedback and display a diversity of baryonic morphologies. This makes them excellent probes for understanding dark matter and galaxy evolution. This work investigates the physical processes that influence the sizes of isolated dwarf galaxies using high-resolution cosmological zoom-in simulations of $39$ dwarf galaxies drawn from the Marvelous Massive Dwarfs simulation suite ($7.5 < \log(M_{\star}/M_{\odot}) < 9.1$). Our simulations show that dwarf galaxies initially form as compact galaxies ($R_e < 2$ kpc). However, several of these galaxies ($54\%$) experience periods of gradual size growth at relatively stable sSFR, allowing them to become extended galaxies. We find that the growth of rotation-supported stellar disks is the primary means by which isolated dwarfs become extended in size. These stellar disks are formed by mergers with high orbital angular momentum satellites on high angular momentum (spiraling-in) orbits, which spin up the gas surrounding the central galaxy and contribute $\approx 30 \%$ of the cold gas mass at $z=0$. For these systems, star formation in the angular momentum supported gas and the gradual build up of stars in the disk result in secular size growth.

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arXiv2026

Figuring Out Gas & Galaxies In Enzo (FOGGIE). XIV. The Observability of Emission from Accretion and Feedback in the Circumgalactic Medium with Current and Future Instruments

Observing the circumgalactic medium (CGM) in emission lines from ionized gas enables direct mapping of its spatial and kinematic structure, offering new insight into the gas flows that regulate galaxy evolution. Using the high-resolution Figuring Out Gas & Galaxies In Enzo (FOGGIE) simulations, we generate mock emission-line maps for six Milky Way-mass halos. Different lines (e.g., H$α$, OVI) trace distinct CGM phases and structures, highlighting the importance of observations in multiple species. We quantify the observable CGM mass fraction as a function of instrument spatial resolution and surface brightness sensitivity, finding that sensitivity is the dominant factor limiting detectability across all ions. At fixed sensitivity, higher spatial resolution reveals more structures; at fixed spatial resolution, higher sensitivity recovers a higher percentage of the total mass. We explore CGM kinematics by constructing emissivity-weighted projected velocity maps and comparing line-of-sight velocities between emission lines. OVI shows the largest kinematic deviation from H$α$, while MgII and SiII most closely follow HI velocities. Distinguishing these phases out to 50kpc from the galaxy center requires spectral resolution better than 30km/s for most ion pairs. Additionally, separating inflowing from outflowing gas based on projected kinematics also requires high spectral resolution: at 30km/s, more than 80% of gas above the emission detection threshold can be distinguished kinematically, but this fraction drops to <40% with a resolution of 200km/s. Our results provide predictions for future UV and optical instruments, showing that recovering the multiphase structure and kinematics of circumgalactic emission will require both high sensitivity and fine kinematic resolution.

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arXiv2026

FOGGIE: Figuring Out Gas & Galaxies In Enzo XII. The Formation and Evolution of Extended HI Galactic Disks and Warps with a Dynamic Circumgalactic medium

Atomic hydrogen (HI) is an important component of gas in and around galaxies and forms extended disk-like structures well beyond the extent of starlight. Here we investigate the properties and evolution of extended HI disks that emerge in six Milky Way-mass galaxies using cosmological zoom-in simulations from the Figuring Out Gas & Galaxies in Enzo (FOGGIE) suite. We focus on the formation, evolution, and morphology of extended gaseous disks that emerge in all six systems. We find that median HI column densities drop sharply at the disk edge, with mean column densities outside the disk dominated by dense (N_HI~10^{19} cm^{-2}), clumpy structures. All systems have significant misaligned features (warps or polar rings) at some point in their evolution; however, their frequencies, lifetimes, and origins vary significantly. We find that the morphologies of the FOGGIE disks are correlated with properties of their Circumgalactic Medium (CGM). We place these systems along a continuum based on how populated their CGMs are with HI relative to their central disk. All systems kinematically settle similarly by z=0. The less populated systems tend to form coherently rotating, thin, extended disks while the more populated systems do not. Location on this continuum is independent of disk and halo mass, implying a relation to local environmental factors.

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arXiv2026

Figuring Out Gas & Galaxies In Enzo (FOGGIE) XI: Circumgalactic O VI Emission Traces Clumpy Inflowing Recycled Gas

The circumgalactic medium (CGM) is host to gas flows into and out of galaxies and regulates galaxy growth, but the multiphase, diffuse gas in this region is challenging to observe. We investigate the properties of gas giving rise to O VI emission from the CGM that upcoming missions, such as the Aspera SmallSat, will be able to map in local galaxies. We use the FOGGIE simulations to predict the O VI emission from edge-on galaxies across the redshift range $z=1\rightarrow0$. O VI emission is brightest surrounding small, clumpy structures near the galaxy where the gas density is high. Most of the O VI surface brightness originates from collisionally ionized, $T\sim10^{5.5}$ K, inflowing gas and is not preferentially aligned with the major or minor axis of the galaxy disk. Simulated galaxies with higher halo masses, higher median CGM gas density, and higher star formation rates produce brighter and more widespread O VI emission in their CGM. We show that while O VI emission primarily originates in inflowing gas, turning off outflows in a simulation without star formation feedback eliminates most of the O VI emission. Enrichment from feedback is necessary to mix with the inflowing gas and allow it to glow in O VI. Collectively, our findings point towards a picture where O VI emission traces warm, ionized envelopes of cooler clouds that are accreting onto the galaxy in a metal-enriched galactic fountain. Finally, we show that the detection limit of Aspera is sufficient to detect O VI emission tens of kpc from the galaxy center for $\sim L^\star$ galaxies.

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arXiv2025

FOGGIE X: Characterizing the Small-Scale Structure of the CGM and its Imprint on Observables

One of the main unknowns in galaxy evolution is how gas flows into and out of galaxies in the circumgalactic medium (CGM). Studies observing the CGM in absorption using multiple or extended background objects suggest a high degree of variation on relatively small ($\lesssim 1$ kpc) spatial scales. Similarly, high-resolution simulations generally exhibit small-scale substructure in the gas around galaxies. We examine the small-scale structure of the $z = 1$ CGM using simulations from the FOGGIE (Figuring Out Gas & Galaxies in Enzo) project. We select gaseous substructures ("clumps") by their local overdensity and investigate their physical properties, including temperature, metallicity, and kinematics with respect to the galaxy and the nearby surroundings. FOGGIE resolves clumps down to sphericalized radii $R \sim 0.25$ kpc at $z = 1$. The distribution of clumps peaks at $\sim 10^5$ $\rm M_{\odot}$ and $10^{4}$ K, consistent with relatively condensed, cool gas with a slight preference for inflow-like velocities. Many clumps show internal temperature and density variations, and thus internally varying ionization levels for key diagnostic ions such as HI, MgII, and OVI. The average metallicity in clumps is about a factor 1.5--2$\times$ lower in metallicity than nearby gas, suggesting that the metals are not well-mixed between structured and diffuse CGM, which may have implications for observational metallicity estimations of dense CGM clouds. We estimate the survivability of CGM clumps and find that structures larger than 0.5 kpc are generally long-lived. Finally, we qualitatively compare the simulated cloud properties to Milky Way high-velocity clouds.

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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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arXiv2025

Are Gas-rich Ultra-diffuse Galaxies and Field Dwarfs Distinct?

We explore the differences in gas-rich field Ultra-Diffuse Galaxies (UDGs) and classical dwarf galaxies using an extensive atomic gas (HI) follow-up survey of optically-selected UDG candidates from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalogue. We also compare the SMUDGes-HI observations with two state-of-the-art cosmological hydrodynamical simulations: Numerical Investigation of a Hundred Astrophysical Objects (NIHAO), where UDGs form through a series of bursty star formation episodes and Romulus25, in which UDGs occupy dark matter halos with high spins as a result of major mergers. Although the suggested formation scenarios for UDGs within these simulations are different, the present-day HI masses $M_{HI}$, stellar masses $M_*$, and star formation rates $SFR$ of simulated galaxies are qualitatively and quantitatively consistent with each other and with the observed SMUDGes-HI sample. We find that when controlling for $M_*$, there is a positive correlation between the gas richness $M_{HI}$/$M_*$ and the effective optical radius $R_{eff}$ , and that this trend is not different between the UDG and dwarf populations, within the measured scatter. Taken together, our results suggest that gas-rich, star-forming UDGs and dwarfs are not distinct galaxy populations, either observationally or in simulations.

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arXiv2024

Figuring Out Gas & Galaxies In Enzo (FOGGIE) VIII: Complex and Stochastic Metallicity Gradients at z > 2

Gas-phase metallicity gradients are a crucial element in understanding the chemical evolution of galaxies. We use the FOGGIE simulations to study the metallicity gradients ($\nabla Z$) of six Milky Way-like galaxies throughout their evolution. FOGGIE galaxies generally exhibit steep negative gradients for most of their history, with only a few short-lived instances reaching positive slopes that appear to arise mainly from interactions with other galaxies. FOGGIE concurs with other simulation results but disagrees with the robust observational finding that flat and positive gradients are common at $z>1$. By tracking the metallicity gradient at a rapid cadence of simulation outputs ($\sim 5$--10 Myr), we find that theoretical gradients are highly stochastic: the FOGGIE galaxies spend $\sim 30-50$\% of their time far away from a smoothed trajectory inferred from analytic models or other, less high-cadence simulations. This rapid variation makes instantaneous gradients from observations more difficult to interpret in terms of physical processes. Because of these geometric and stochastic complications, we explore non-parametric methods of quantifying the evolving metallicity distribution at $z > 1$. We investigate how efficiently non-parametric measures of the 2-D metallicity distribution respond to metal production and mixing. Our results suggest that new methods of quantifying and interpreting gas-phase metallicity will be needed to relate trends in upcoming high-$z$ {\it JWST} observations with the underlying physics of gas accretion, expulsion, and recycling in early galaxies.

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arXiv2024

Figuring Out Gas & Galaxies in Enzo (FOGGIE). IX: The Angular Momentum Evolution of Milky Way-like Galaxies and their Circumgalactic Gas

We investigate the co-evolution of the angular momentum of Milky Way-like galaxies, their circumgalactic gas, and their dark matter halos using zoom-in simulations from the Figuring Out Gas & Galaxies in Enzo (FOGGIE) suite. We examine how the magnitude and orientation of the angular momentum varies over time within the halo and between the components of mass. From z~2 to today, and in general across the simulated halos, the specific angular momenta of the central galaxies and the cool gas in their circumgalactic media (T < 10^5 K) increase together. Over that same period, the specific angular momenta of the hot (>10^6 K) and dark components of the halo change minimally. By z~1, the central galaxies have generally lost association with the angular momentum of their full dark matter halo -- both in magnitude and orientation. We find a wide distribution of angular momentum orientations in the halo, varying by up to 180 degrees over small (~tens of kpc) scales and between the different components of mass. The net angular momenta of the galaxies, their circumgalactic gas, and their dark matter halos are generally misaligned with one another at all cosmic times. The present-day orientation of the central galaxies are established at late times (after z=1), after the rates of cosmic accretion and mergers decline and the disks are able to settle and stabilize their orientation.

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arXiv2024

Figuring Out Gas & Galaxies In Enzo (FOGGIE) VII: The (Dis)Assembly of Stellar Halos

Over the next decade, the astronomical community will be commissioning multiple wide-field observatories well-suited for studying stellar halos in both integrated light and resolved stars. In preparation for this, we use five high-resolution cosmological simulations of Milky Way-like galaxies from the FOGGIE suite to explore the properties and components of stellar halos. These simulations are run with high time (5 Myr) and stellar mass (1000 M$_\odot$) resolution to better model the properties and origins of low density regions like stellar halos. We find that the FOGGIE stellar halos have masses, metallicity gradients, and surface brightness profiles that are consistent with observations. In agreement with other simulations, the FOGGIE stellar halos receive 30-40% of their mass from in situ stars. However, this population is more centrally concentrated in the FOGGIE simulations and therefore does not contribute excess light to the halo outskirts. The remaining stars are accreted from 10-50 other galaxies, with the majority of the accreted mass originating in 2-4 galaxies. While the inner halo ($r<50$ kpc) of each FOGGIE galaxy has a large number of contributors, the halo outskirts of three of the five galaxies are primarily made up of stars from only a few contributors. We predict that upcoming wide-field observatories, like the Nancy Grace Roman Space Telescope, will probe stellar halos around Milky Way-like galaxies out to ~100 kpc in integrated light and will be able to distinguish the debris of dwarf galaxies with extended star formation histories from the underlying halo with resolved color-magnitude diagrams.

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arXiv2023

Figuring Out Gas & Galaxies In Enzo (FOGGIE) VI: The Circumgalactic Medium of $L^*$ Galaxies is Supported in an Emergent, Non-Hydrostatic Equilibrium

The circumgalactic medium (CGM) is often assumed to exist in or near hydrostatic equilibrium with the regulation of accretion and the effects of feedback treated as perturbations to a stable balance between gravity and thermal pressure. We investigate global hydrostatic equilibrium in the CGM using four highly-resolved $L^*$ galaxies from the Figuring Out Gas & Galaxies In Enzo (FOGGIE) project. The FOGGIE simulations were specifically targeted at fine spatial and mass resolution in the CGM ($Δx \lesssim 1$ kpc $h^{-1}$ and $M \simeq 200M_\odot$). We develop a new analysis framework that calculates the forces provided by thermal pressure gradients, turbulent pressure gradients, ram pressure gradients of large-scale radial bulk flows, centrifugal rotation, and gravity acting on the gas in the CGM. Thermal and turbulent pressure gradients vary strongly on scales of $\lesssim5$ kpc throughout the CGM. Thermal pressure gradients provide the main supporting force only beyond $\sim 0.25R_{200}$, or $\sim50$ kpc at $z=0$. Within $\sim0.25R_{200}$, turbulent pressure gradients and rotational support provide stronger forces than thermal pressure. More generally, we find that global equilibrium models are neither appropriate nor predictive for the small scales probed by absorption line observations of the CGM. Local conditions generally cannot be derived by assuming a global equilibrium, but an emergent global equilibrium balancing radially inward and outward forces is obtained when averaging over the non-equilibrium local conditions on large scales in space and time. Approximate hydrostatic equilibrium holds only at large distances from galaxies even when averaging out small-scale variations.

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arXiv2022

Snowmass2021 Cosmic Frontier White Paper: Cosmological Simulations for Dark Matter Physics

Over the past several decades, unexpected astronomical discoveries have been fueling a new wave of particle model building and are inspiring the next generation of ever-more-sophisticated simulations to reveal the nature of Dark Matter (DM). This coincides with the advent of new observing facilities coming online, including JWST, the Rubin Observatory, the Nancy Grace Roman Space Telescope, and CMB-S4. The time is now to build a novel simulation program to interpret observations so that we can identify novel signatures of DM microphysics across a large dynamic range of length scales and cosmic time. This white paper identifies the key elements that are needed for such a simulation program. We identify areas of growth on both the particle theory side as well as the simulation algorithm and implementation side, so that we can robustly simulate the cosmic evolution of DM for well-motivated models. We recommend that simulations include a fully calibrated and well-tested treatment of baryonic physics, and that outputs should connect with observations in the space of observables. We identify the tools and methods currently available to make predictions and the path forward for building more of these tools. A strong cosmic DM simulation program is key to translating cosmological observations to robust constraints on DM fundamental physics, and provides a connection to lab-based probes of DM physics.

astro-ph.CO↗

arXiv2021

Star Formation Histories from SEDs and CMDs Agree: Evidence for Synchronized Star Formation in Local Volume Dwarf Galaxies over the Past 3 Gyr

Star Formation Histories (SFHs) reveal physical processes that influence how galaxies form their stellar mass. We compare the SFHs of a sample of 36 nearby (D $\leq$ 4 Mpc) dwarf galaxies from the ACS Nearby Galaxy Survey Treasury (ANGST), inferred from the Color Magnitude Diagrams (CMDs) of individually resolved stars in these galaxies, with those reconstructed by broad-band Spectral Energy Distribution (SED) fitting using the Dense Basis SED fitting code. When comparing individual SFHs, we introduce metrics for evaluating SFH reconstruction techniques. For both the SED and CMD methods, the median normalized SFH of galaxies in the sample shows a period of quiescence at lookback times of 3-6 Gyr followed by rejuvenated star formation over the past 3 Gyr that remains active until the present day. To determine if these represent special epochs of star formation in the D $\leq$ 4 Mpc portion of the Local Volume, we break this ANGST dwarf galaxy sample into subsets based on specific star formation rate and spatial location. Modulo offsets between the methods of about 1 Gyr, all subsets show significant decreases and increases in their median normalized SFHs at the same epochs, and the majority of the individual galaxy SFHs are consistent with these trends. These results motivate further study of potential synchronized star formation quiescence and rejuvenation in the Local Volume as well as development of a hybrid method of SFH reconstruction that combines CMDs and SEDs, which have complementary systematics.

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arXiv2021

The Formation of Isolated Ultra-Diffuse Galaxies in Romulus25

We use the \textsc{Romulus25} cosmological simulation volume to identify the largest-ever simulated sample of {\it field} ultra-diffuse galaxies (UDGs). At $z=0$, we find that isolated UDGs have average star formation rates, colors, and virial masses for their stellar masses and environment. UDGs have moderately elevated HI masses, being 70\% (300\%) more HI-rich than typical isolated dwarf galaxies at luminosities brighter (fainter) than M$_\mathrm{B}$=-14. However, UDGs are consistent with the general isolated dwarf galaxy population and make up $\sim$20\% of all field galaxies with 10$^7$<M$_\star$/M$_\odot$<10$^{9}$. The HI masses, effective radii, and overall appearances of our UDGs are consistent with existing observations of field UDGs, but we predict that many isolated UDGs have been missed by current surveys. Despite their isolation at $z=0$, the UDGs in our sample are the products of major mergers. Mergers are no more common in UDG than non-UDG progenitors, but mergers that create UDGs tend to happen earlier - almost never occurring after $z=1$, produce a temporary boost in spin, and cause star formation to be redistributed to the outskirts of galaxies, resulting in lower central star formation rates. The centers of the galaxies fade as their central stellar populations age, but their global star formation rates are maintained through bursts of star formation at larger radii, producing steeper negative g-r color gradients. This formation channel is unique relative to other proposals for UDG formation in isolated galaxies, demonstrating that UDGs can potentially be formed through multiple mechanisms.

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arXiv2020

The Formation of Ultra-Diffuse Galaxies in the RomulusC Galaxy Cluster Simulation

We study the origins of 122 ultra-diffuse galaxies (UDGs) in the {\sc RomulusC} zoom-in cosmological simulation of a galaxy cluster (M$_{200} = 1.15\times10^{14}$ M$_{\odot}$), one of the only such simulations capable of resolving the evolution and structure of dwarf galaxies (M$_{\star} < 10^9$ M$_{\odot}$). We find broad agreement with observed cluster UDGs and predict that they are not separate from the overall cluster dwarf population. UDGs in cluster environments form primarily from dwarf galaxies that experienced early cluster in-fall and subsequent quenching due to ram pressure. The ensuing dimming of these dwarf galaxies due to passive stellar evolution results in a population of very low surface brightness galaxies that are otherwise typical dwarfs. UDGs and non-UDGs alike are affected by tidal interactions with the cluster potential. Tidal stripping of dark matter, as well as mass loss from stellar evolution, results in the adiabatic expansion of stars, particularly in the lowest mass dwarfs. High mass dwarf galaxies show signatures of tidal heating while low mass dwarfs that survive until $z=0$ typically have not experienced such impulsive interactions. There is little difference between UDGs and non-UDGs in terms of their dark matter halos, stellar morphology, colors, and location within the cluster. In most respects cluster UDG and non-UDGs alike are similar to isolated dwarf galaxies, except for the fact that they are typically quenched.

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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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