arXiv subjects

Mary E. Putman

Publications and source records attributed to Mary E. Putman.

At least 19 recordsLinked to original sources

arXiv2026

The Halo Gas of Local Spiral Galaxies and the Link to Gaseous Satellites

Gaseous satellites orbiting massive host galaxies experience gas stripping and may contribute additional gas to the hosts' circumgalactic medium (CGM). We identify a sample of 21 local spiral galaxies ($<$15 Mpc) and characterize their gaseous satellite populations from existing HI surveys to investigate the connection between host galaxy CGM and satellite gas content. Most of our spiral hosts have $\leq3$ gaseous satellites in their halos with $M_{\rm HI}\gtrsim10^{7}~M_\odot$. Using 26 HST/COS QSO sightlines at impact parameters of 0.1--0.9 $R_{\rm 200c}$, we find that the CGM of $z\sim0$ spiral galaxies show large intrinsic scatters ($\sim1-2$ dex) in ion column densities and harbor a total cool gas mass of $2.5^{+7.5}_{\rm -1.9}\times10^9~(0.3Z_\odot/Z')~M_\odot$, largely consistent with their $z\sim0.2$ counterparts (e.g., COS-Halos). Splitting our sample by the presence of gaseous satellites, we find that galaxy hosts with gaseous satellites have higher detection rates (up to 50\%) in metal ion absorbers, including OI, AlII, CII, SiII, SiIII, and SiIV, possibly because their CGM is more metal enriched or has more ionized gas in the cool phase. However, the CGM column densities show no significant correlation with either the number of gaseous satellites or their total HI masses, suggesting that the contribution from the gaseous satellites to the host CGM is likely small compared to the intrinsic scatters in the CGM profiles, and the profile trends are mainly influenced by sightlines' proximity to the hosts. The CGM detection rate becomes elevated when a sightline is within half the virial radius of a massive gaseous satellite (LMC-like or higher mass), likely due to stripped debris.

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arXiv2026

Too Big to Quench? I. Constraining ISM Stripping of Dwarf Satellites in Milky Way-like Halos

Galaxy environment plays a crucial role in quenching star formation in dwarf galaxies. In Milky Way (MW)-like environments, dwarf satellite quenching is primarily driven by ram pressure stripping (RPS), the direct removal of satellite gas by the host halo gas. Using a suite of 20-pc resolution hydrodynamical wind tunnel simulations, we constrain the satellite mass scale at which the stripping of a dwarf galaxy's interstellar medium (ISM) becomes inefficient in MW-like halos. The simulations include radiative cooling in a multiphase satellite ISM, star formation, and stellar feedback, and vary both satellite masses ($M_{\star}=10^{6.2}, 10^{6.8}, 10^{7.2}\ M_{\odot}$) and host halo gas densities along a first-infall and post-pericentric orbit. We find that the degree of ISM stripping in our dwarf galaxies is consistent with the analytical prediction by McCarthy et al. (2008). Star formation is rapidly quenched when RPS is effective, but can be mildly enhanced or temporarily quenched and subsequently reignited when RPS is incomplete. ISM stripping is efficient for satellites with $M_{\star} \lesssim 10^{7}\ M_{\odot}$ (or $M_{200} \lesssim 10^{10}\ M_{\odot}$) but highly inefficient above this scale. This transitional mass ($M_{\star} \approx 10^{7}\ M_{\odot}$) is 0.5-1 dex lower than that found in observations and cosmological simulations, suggesting that additional mechanisms are needed to quench more massive satellites, such as tidal stripping of the satellite dark matter or RPS from a clumpy gaseous halo.

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arXiv2026

Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG

This paper explores the extent to which the circumgalactic medium (CGM) of Milky Way-like galaxies is located in an extended, ionized, disklike structure. To test this hypothesis, we analyze the spatial and kinematic distributions of different ion species within a sample of MW-like systems in IllustrisTNG. We model commonly observed ions (HI, MgII, SiIV, CIV and OVI) and calculate (1) their angular momentum misalignment from the star-forming disk ($θ$) and (2) the fraction of absorption consistent with galaxy rotation ($f_\mathrm{EWcorot}$). We find that 63% of MgII, 45% of SiIV, 38% of CIV, and 35% of OVI mass along the major axis have kinematics aligned with the galaxy angular momentum axis. We extend this to a mock absorption line survey and quantify $f_\mathrm{EWcorot}$. We find that $f_\mathrm{EWcorot}$(MgII) $\sim80\%$ and $f_\mathrm{EWcorot}$(OVI) $\sim60\%$ at $\sim0.5\ \mathrm{R_{200c}}$, in agreement with recent observational work. We find that in the typical MW analog, there is evidence of cool-warm material in an extended, corotating structure, regardless of whether the angular momentum or observational definition is used. Hence, we expect that the typical MW CGM, especially in the low ions, should be mainly on the plane.

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arXiv2025

The Plane Quasar Survey: An Ionized Extension of the Magellanic Stream on the Northern Side of the Galactic Plane

The Magellanic Stream (MS) is a vast gaseous structure in the Milky Way halo, containing most of its mass in ionized form and tracing the interaction between the Large and Small Magellanic Clouds and the Galaxy. Using HST/COS G160M spectra from the Plane Quasar Survey, we detect C IV absorbers likely associated with the MS, extending to the northern side of the Galactic plane, approximately 60$^\circ$ beyond its previously known ionized extent. These absorbers exhibit position and kinematic alignment and show consistent ionization trends with previously studied MS sight lines. The non-detection of low ions such as Al II and Si II, and the detection of C IV (and Si IV in some sightlines), indicates a highly ionized gas phase. The observed Si IV/C IV column density ratios suggest a gas temperature of $T \sim 10^{5.3}$~K and favor collisional ionization over photoionization. We estimate the newly detected extension increases the previous ionized gas mass of the MS, and its coherent kinematics suggest that it was stripped within the past few hundred Myr and has not yet mixed with the Milky Way halo. The existence of highly-ionized MS gas at a location above the Galactic Plane may constrain the orbital direction of the Magellanic Clouds.

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arXiv2024

The Metallicity Mapping of the Ionized Diffuse Gas at the Milky Way Disk-halo Interface

Metals in the diffuse, ionized gas at the boundary between the Milky Way's interstellar medium (ISM) and circumgalactic medium (CGM), known as the disk-halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain the ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measure of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling to the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between $0.04-3.2\ Z_{\odot}$, implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be $6 - 35$ Myr. We report the detection of an infalling cloud with super-solar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud ($Z < 0.1\ Z_{\odot}$) poses a challenge for Galactic fountain and feedback models.

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arXiv2024

It's a Breeze: The Circumgalactic Medium of a Dwarf Galaxy is Easy to Strip

The circumgalactic medium (CGM) of star-forming dwarf galaxies plays a key role in regulating the galactic baryonic cycle. We investigate how susceptible the CGM of dwarf satellite galaxies is to ram pressure stripping (RPS) in Milky Way-like environments. In a suite of hydrodynamical wind tunnel simulations, we model an intermediate-mass dwarf satellite galaxy ($M_{*} = 10^{7.2}~M_{\odot}$) with a multiphase interstellar medium (ISM; $M_{\rm ISM} = 10^{7.9}~M_{\odot}$) and CGM ($M_{\rm CGM,vir} = 10^{8.5}~M_{\odot}$) along two first-infall orbits to more than 500 Myr past pericenter of a Milky Way-like host. The spatial resolution is $\sim$79 pc in the star-forming ISM and $316-632$ pc in the CGM. Our simulations show that the dwarf satellite CGM removal is fast and effective: more than $95\%$ of the CGM mass is ram-pressure-stripped within a few hundred Myrs, even under a weak ram pressure orbit where the ISM stripping is negligible. The conditions for CGM survival are consistent with the analytical halo gas stripping predictions in McCarthy et al. (2008). We also find that including the satellite CGM does not effectively shield its galaxy, and therefore the ISM stripping rate is unaffected. Our results imply that a dwarf galaxy CGM is unlikely to be detected in satellite galaxies; and that the star formation of gaseous dwarf satellites is likely devoid of replenishment from a CGM.

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arXiv2024

Identifying HI Emission and UV Absorber Associations Near the Magellanic Stream

We present a new technique to identify associations of HI emission in the Magellanic Stream (MS) and ultraviolet (UV) absorbers from 92 QSO sight lines near the MS. We quantify the level of associations of individual HI elements to the main HI body of the Stream using Wasserstein distance-based models, and derive characteristic spatial and kinematic distances of the HI emission in the MS. With the emission-based model, we further develop a comparison metric, which identifies the dominant associations of individual UV absorbers with respective to the MS and nearby galaxies. For ionized gas associated with the MS probed by CII, CIV, SiII, SiIII, SiIV, we find that the ion column densities are generally $\sim$0.5 dex higher than those that are not associated, and that the gas is more ionized toward the tail of the MS as indicated by the spatial trend of the CII/CIV ratios. For nearby galaxies, we identify potential new absorbers associated with the CGM of M33 and NGC300, and affirm the associations of absorbers with IC1613 and WLM. For M31, we find the previously identified gradient in column densities as a function of impact parameter, and that absorbers with higher column densities beyond M31's virial radius are more likely to be associated with the MS. Our analysis of absorbers associated with the Magellanic Clouds reveals the presence of continuous and blended diffuse ionized gas between the Stream and the Clouds. Our technique can be applied to future applications of identifying associations within physically complex gaseous structures.

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arXiv2023

A Comprehensive Investigation of Metals in the Circumgalactic Medium of Nearby Dwarf Galaxies

Dwarf galaxies are found to have lost most of their metals via feedback processes; however, there still lacks consistent assessment on the retention rate of metals in their circumgalactic medium (CGM). Here we investigate the metal content in the CGM of 45 isolated dwarf galaxies with $M_*=10^{6.5-9.5}~M_\odot$ ($M_{\rm 200m}=10^{10.0-11.5}~M_\odot$) using {\it HST}/COS. While H I (Ly$α$) is ubiquitously detected ($89\%$) within the CGM, we find low detection rates ($\approx5\%-22\%$) in C II, C IV, Si II, Si III, and Si IV, largely consistent with literature values. Assuming these ions form in the cool ($T\approx10^4$ K) CGM with photoionization equilibrium, the observed H I and metal column density profiles can be best explained by an empirical model with low gas density and high volume filling factor. For a typical galaxy with $M_{\rm 200m}=10^{10.9}~M_\odot$ (median of the sample), our model predicts a cool gas mass of $M_{\rm CGM,cool}\sim10^{8.4}~M_\odot$, corresponding to $\sim2\%$ of the galaxy's baryonic budget. Assuming a metallicity of $0.3Z_\odot$, we estimate that the dwarf galaxy's cool CGM likely harbors $\sim10\%$ of the metals ever produced, with the rest either in more ionized states in the CGM or transported to the intergalactic medium. We further examine the EAGLE simulation and show that H I and low ions may arise from a dense cool medium, while C IV arises from a diffuse warmer medium. Our work provides the community with a uniform dataset on dwarf galaxies' CGM that combines our recent observations, additional archival data and literature compilation, which can be used to test various theoretical models of dwarf galaxies.

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arXiv2021

The Cosmic Ultraviolet Baryon Survey (CUBS) IV: The Complex Multiphase Circumgalactic Medium as Revealed by Partial Lyman Limit Systems

We present a detailed study of two partial Lyman limit systems (pLLSs) of neutral hydrogen column density $N_\mathrm{H\,I}\approx(1-3)\times10^{16}\,\mathrm{cm}^{-2}$ discovered at $z=0.5$ in the Cosmic Ultraviolet Baryon Survey (CUBS). Available far-ultraviolet spectra from the Hubble Space Telescope Cosmic Origins Spectrograph and optical echelle spectra from MIKE on the Magellan Telescopes enable a comprehensive ionization analysis of diffuse circumgalactic gas based on resolved kinematics and abundance ratios of atomic species spanning five different ionization stages. These data provide unambiguous evidence of kinematically aligned multi-phase gas that masquerades as a single-phase structure and can only be resolved by simultaneous accounting of the full range of observed ionic species. Both systems are resolved into multiple components with inferred $α$-element abundance varying from $[α/\text{H}]\approx\!{-0.8}$ to near solar and densities spanning over two decades from $\log n_\mathrm{H}\mathrm{cm}^{-3}\approx\!-2.2$ to $<-4.3$. Available deep galaxy survey data from the CUBS program taken with VLT/MUSE, Magellan/LDSS3-C and Magellan/IMACS reveal that the $z=0.47$ system is located 55 kpc from a star-forming galaxy with prominent Balmer absorption of stellar mass $M_\star\approx2\times10^{10}M_\odot$, while the $z=0.54$ system resides in an over-dense environment of 11 galaxies within 750 kpc in projected distance, with the most massive being a luminous red galaxy of $M_\star\approx2\times10^{11}M_\odot$ at 375 kpc. The study of these two pLLSs adds to an emerging picture of the complex, multiphase circumgalactic gas that varies in chemical abundances and density on small spatial scales in diverse galaxy environments. The inhomogeneous nature of metal enrichment and density revealed in observations must be taken into account in theoretical models of diffuse halo gas.

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arXiv2021

The Cosmic Ultraviolet Baryon Survey (CUBS) -- III. Physical properties and elemental abundances of Lyman limit systems at $z<1$

(Abridged) We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at $z=0.36-0.6$ discovered within the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS LLSs exhibit multi-component kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states that span several hundred km/s in line-of-sight velocity. Specifically, higher column density components (log N(HI)>16) in all four absorbers comprise dynamically cool gas with $\langle T \rangle =(2\pm1) \times10^4\,$K and modest non-thermal broadening of $5\pm3\,$ km/s. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modeling that takes into account the resolved component structures of HI and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of $160^{+140}_{-50}$ pc. While obtaining robust metallicity constraints for the low-density, highly ionized phase remains challenging due to the uncertain N(HI), we demonstrate that the cool-phase gas in LLSs has a median metallicity of $\mathrm{[α/H]_{1/2}}=-0.7^{+0.1}_{-0.2}$, with a 16-84 percentile range of $\mathrm{[α/H]}=(-1.3,-0.1)$. Furthermore, the wide range of inferred elemental abundance ratios ($\mathrm{[C/α]}$, $\mathrm{[N/α]}$, and $\mathrm{[Fe/α]}$) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the $z<1$ circumgalactic medium.

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arXiv2021

The Cosmic Ultraviolet Baryon Survey (CUBS) II: Discovery of an H$_{2}$-Bearing DLA in the Vicinity of an Early-Type Galaxy at z = 0.576

We report the serendipitous detection of an H$_{2}$-bearing damped Lyman-$α$ absorber at z = 0.576 in the spectrum of the QSO J0111-0316 in the Cosmic Ultraviolet Baryon Survey. Spectroscopic observations from HST-COS in the far-ultraviolet reveal a damped absorber with log[N(HI)/cm^-2] = 20.1 +/- 0.2 and log[N(H$_{2}$)/cm^-2] = 18.97 (-0.06, +0.05). The diffuse molecular gas is found in two velocity components separated by dv = 60 km/s, with >99.9% of the total H$_{2}$ column density concentrated in one component. At a metallicity of $\approx$ 50% of solar, there is evidence for Fe enhancement and dust depletion, with a dust-to-gas ratio $κ_{\text{O}} \approx$ 0.4. A galaxy redshift survey conducted with IMACS and LDSS-3C on Magellan reveals an overdensity of nine galaxies at projected distance d <= 600 proper kpc (pkpc) and line-of-sight velocity offset dv$_{g}$ <= 300 km/s from the absorber. The closest is a massive, early-type galaxy at d = 41 pkpc which contains $\approx$ 70% of the total stellar mass identified at d <= 310 pkpc of the H$_{2}$ absorber. The close proximity of the H$_{2}$-bearing gas to the quiescent galaxy and the Fe-enhanced chemical abundance pattern of the absorber suggest a physical connection, in contrast to a picture in which DLAs are primarily associated with gas-rich dwarfs. This case study illustrates that deep galaxy redshift surveys are needed to gain insight into the diverse environments that host dense and potentially star-forming gas.

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arXiv2021

The Gas Content and Stripping of Local Group Dwarf Galaxies

The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using HI survey data. Within the virial radius of the Milky Way (224 kpc here), 53 of 55 dwarf galaxies are devoid of gas to limits of M$_{\rm HI}<10^4$ M$_\odot$. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically $<10^5$ M$_\odot$). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected HI gas and have HI masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the non-detected dwarf galaxies are within this radius (85$\pm1$ of the 93 non-detected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between $10^{-5}$ and $5 \times 10^{-4}$ cm$^{-3}$ are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role.

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arXiv2020

Characterizing the Circumgalactic Medium of the Lowest-Mass Galaxies: A Case Study of IC 1613

Using 10 sightlines observed with the Hubble Space Telescope/Cosmic Origins Spectrograph, we study the circumgalactic medium (CGM) and outflows of IC1613, which is a low-mass ($M_*\sim10^8~M_\odot$), dwarf irregular galaxy on the outskirts of the Local Group. Among the sightlines, 4 are pointed towards UV-bright stars in IC1613, and the other 6 sightlines are background QSOs at impact parameters from 6 kpc ($<0.1R_{200}$) to 61 kpc ($0.6R_{200}$). We detect a number of Si II, Si III, Si IV, C II, and C IV absorbers, most of which have velocities less than the escape velocity of IC1613 and thus are gravitationally bound. The line strengths of these ion absorbers are consistent with the CGM absorbers detected in dwarf galaxies at low redshifts. Assuming that Si II, Si III, and Si IV comprise nearly 100% of the total silicon, we find 3% ($\sim$8$\times$10$^3~{\rm M_\odot}$), 2% ($\sim$7$\times$10$^3~{\rm M_\odot}$), and 32--42% [$\sim$(1.0--1.3)$\times$10$^5~{\rm M_\odot}$] of the silicon mass in the stars, interstellar medium, and within $0.6R_{200}$ of the CGM of IC1613. We also estimate the metal outflow rate to be ${\rm \dot{M}_{out, Z}\geq1.1\times10^{-5}~M_\odot~yr^{-1}}$ and the instantaneous metal mass loading factor to be $η_{\rm Z}\geq0.004$, which are in broad agreement with available observation and simulation values. This work is the first time a dwarf galaxy of such low mass is probed by a number of both QSO and stellar sightlines, and it shows that the CGM of low-mass gas-rich galaxies can be a large reservoir enriched with metals from past and ongoing outflows.

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arXiv2020

The HI Structure of the Local Volume Dwarf Galaxy Pisces A

Dedicated HI surveys have recently led to a growing category of low-mass galaxies found in the Local Volume. We present synthesis imaging of one such galaxy, Pisces A, a low-mass dwarf originally confirmed via optical imaging and spectroscopy of neutral hydrogen (HI) sources in the Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey. Using HI observations taken with the Karl G. Jansky Very Large Array (JVLA), we characterize the kinematic structure of the gas and connect it to the galaxy's environment and evolutionary history. While the galaxy shows overall ordered rotation, a number of kinematic features indicate a disturbed gas morphology. These features are suggestive of a tumultuous recent history, and represent $\sim 3.5$% of the total baryonic mass. We find a total baryon fraction $f_{\rm bary} = 0.13$ if we include these features. We also quantify the cosmic environment of Pisces A, finding an apparent alignment of the disturbed gas with nearby, large scale filamentary structure at the edge of the Local Void. We consider several scenarios for the origin of the disturbed gas, including gas stripping via ram pressure or galaxy-galaxy interactions, as well as accretion and ram pressure compression. Though we cannot rule out a past interaction with a companion, our observations best support the suggestion that the neutral gas morphology and recent star formation in Pisces A is a direct result of its interactions with the IGM.

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arXiv2020

The Cosmic Ultraviolet Baryon Survey (CUBS) I. Overview and the diverse environments of Lyman limit systems at z<1

We present initial results from the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS is designed to map diffuse baryonic structures at redshift z<~1 using absorption-line spectroscopy of 15 UV-bright QSOs with matching deep galaxy survey data. CUBS QSOs are selected based on their NUV brightness to avoid biases against the presence of intervening Lyman Limit Systems (LLSs) at zabs<1. We report five new LLSs of log N(HI)/cm^-2 >~ 17.2 over a total redshift survey pathlength of dz=9.3, and a number density of n(z)=0.43 (-0.18, +0.26). Considering all absorbers with log N(HI)/cm^-2 > 16.5 leads to n(z)=1.08 (-0.25, +0.31) at z<1. All LLSs exhibit a multi-component structure and associated metal transitions from multiple ionization states such as CII, CIII, MgII, SiII, SiIII, and OVI absorption. Differential chemical enrichment levels as well as ionization states are directly observed across individual components in three LLSs. We present deep galaxy survey data obtained using the VLT-MUSE integral field spectrograph and the Magellan Telescopes, reaching sensitivities necessary for detecting galaxies fainter than 0.1L* at d<~300 physical kpc (pkpc) in all five fields. A diverse range of galaxy properties is seen around these LLSs, from a low-mass dwarf galaxy pair, a co-rotating gaseous halo/disk, a star-forming galaxy, a massive quiescent galaxy, to a galaxy group. The closest galaxies have projected distances ranging from d=15 to 72 pkpc and intrinsic luminosities from ~0.01L* to ~3L*. Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities.

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arXiv2019

Modeling the Baryon Cycle in Low Mass Galaxy Encounters: the Case of NGC 4490 & NGC 4485

Discoveries of low mass galaxy pairs and groups are increasing. Studies indicate that dwarf galaxy pairs are gas rich in the field and exhibit elevated star formation rates, suggestive of interactions. Lacking are dynamical models of observed dwarf galaxy pairs to disentangle the physical processes regulating their baryon cycles. We present new optical data and the first detailed theoretical model of an observed tidal encounter between two isolated low mass galaxies, NGC 4490 & NGC 4485. This system is an isolated analog of the Magellanic Clouds and is surrounded by a ~50 kpc extended HI envelope. We use hybrid $N$-body and test-particle simulations along with a visualization interface $Identikit$ to simultaneously reproduce the observed present-day morphology and kinematics. Our results demonstrate how repeated encounters between two dwarf galaxies can "park" baryons at very large distances, without the aid of environmental effects. Our best match to the data is an 8:1 mass ratio encounter where a one-armed spiral is induced in the NGC 4490-analog, which we postulate explains the nature of diffuse starlight presented in the new optical data. We predict that the pair will fully merge in ~370 Myr, but that the extended tidal features will continue to evolve and return to the merged remnant over ~5 Gyr. This pre-processing of baryons will affect the efficiency of gas stripping if such dwarf pairs are accreted by a massive host. In contrast, in isolated environments this study demonstrates how dwarf-dwarf interactions can create a long-lived supply of gas to the merger remnant.

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arXiv2018

The Frequency of Dwarf Galaxy Multiples at Low Redshift in SDSS vs. Cosmological Expectations

We quantify the frequency of companions of low redshift ($0.013 < z < 0.0252$), dwarf galaxies ($2 \times 10^8$ M$_\odot <$ M$_{*} < 5 \times 10^9$ M$_\odot$) that are isolated from more massive galaxies in SDSS and compare against cosmological expectations using mock observations of the Illustris simulation. Dwarf multiples are defined as 2 or more dwarfs that have angular separations > 55'', projected separations r$_p < 150$ kpc and relative line-of-sight velocities $ΔV_{\rm LOS} < 150$ km/s. While the mock catalogs predict a factor of 2 more isolated dwarfs than observed in SDSS, the mean number of observed companions per dwarf is $N_c \sim 0.04$, in good agreement with Illustris when accounting for SDSS sensitivity limits. Removing these limits in the mock catalogs predicts $N_c\sim 0.06$ for future surveys (LSST, DESI), which will be complete to M$_* = 2\times 10^8$ M$_\odot$. The 3D separations of mock dwarf multiples reveal a contamination fraction of $\sim$40% in observations from projection effects. Most isolated multiples are pairs; triples are rare and it is cosmologically improbable that bound groups of dwarfs with more than 3 members exist within the parameter range probed in this study. We find that $<$1% of LMC-analogs in the field have an SMC-analog companion. The fraction of dwarf "Major Pairs'' (stellar mass ratio $>$1:4) steadily increases with decreasing Primary stellar mass, whereas the cosmological "Major Merger rate'' (per Gyr) has the opposite behaviour. We conclude that cosmological simulations can be reliably used to constrain the fraction of dwarf mergers across cosmic time.

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arXiv2017

Lyman-alpha Absorbers and the Coma Cluster

The spatial and kinematic distribution of warm gas in and around the Coma Cluster is presented through observations of Lyman-alpha absorbers using background QSOs. Updates to the Lyman-alpha absorber distribution found in Yoon et al. (2012) for the Virgo Cluster are also presented. At 0.2-2.0 R_vir of Coma we identify 14 Lyman-alpha absorbers (N_HI = 10^{12.8-15.9} cm^-2) towards 5 sightlines and no Lyman-alpha absorbers along 3 sightlines within 3\sigmav_coma. For both Coma and Virgo, most Lyman-alpha absorbers are found outside the virial radius or beyond 1\sigmav consistent with them largely representing the infalling intergalactic medium. The few exceptions in the central regions can be associated with galaxies. The Lyman-alpha absorbers avoid the hot ICM, consistent with the infalling gas being shock-heated within the cluster. The massive dark matter halos of clusters do not show the increasing column density with decreasing impact parameter relationship found for the smaller mass galaxy halos. In addition, while the covering fraction within R_vir is lower for clusters than galaxies, beyond R_vir the covering fraction is somewhat higher for clusters. The velocity dispersion of the absorbers compared to the galaxies is higher for Coma, consistent with the absorbers tracing additional turbulent gas motions in the cluster outskirts. The results are overall consistent with cosmological simulations, with the covering fraction being high in the observations standing out as the primary discrepancy.

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