arXiv subjects

Todd M. Tripp

Publications and source records attributed to Todd M. Tripp.

At least 19 recordsLinked to original sources

arXiv2026

X-ray grating spectroscopy as a mission enhancement

We propose to add instruments to any potential future X-ray mission with focussing optics that is considered in NASA's ASTRA framework. Such an instrument is a necessity to study AGN wind outflows and feedback, find the missing baryons, study the intergalactic medium, and analyze abundances and chemical bonds in dust grains throughout the Milky Way. We conclude that those science goals can be achieved with a spectral resolving power > 3000 in the soft X-ray band (about 10-40 Ang) and an effective area a few times larger than current instruments. We describe a possible mission implementation for a soft X-ray grating spectrometer that can be folded in and out or be mounted permanently in the beam. Such an instrument can reach the requirements for a wide variety of host mission properties. A small UV imager and a UV spectrograph can be mounted on the same platform with independent optics. These added instruments vastly enhance the science capabilities of the host mission for a modest cost (100-200 million $) and with weight and power needs that can be easily accommodated in any major mission.

astro-ph.IM↗

arXiv2026

A Measurement of the Thermal and Ionization State of the IGM at $z < 0.5$

We apply a machine-learning-based inference method that exploits the joint Doppler parameter-column density (b-NHI) distribution from Lya forest decomposition to measure the thermal and ionization state of the intergalactic medium (IGM) in four redshift bins spanning z = 0.06 to 0.48, using 82 archival quasar spectra from the Cosmic Origin Spectrograph (COS) on board Hubble Space Telescope (HST). Our results show that the low-z IGM (z < 0.5) is extremely hot and nearly isothermal, with log(T0/K) = 4.45 (+0.08 / -0.12) [T0 = 28183 (+5700 / -6804) K] and gamma = 1.06 (+0.13 / -0.09) at z = 0.1. This temperature lies approx 7sigma (and 7 times) above the canonical prediction (log T0 approx 3.60, i.e. T0 ~ 4000 K, with gamma ~ 1.6 at z = 0), where the IGM is expected to have cooled long after He II reionization. We also measure the hydrogen photoionization rate to be log (GammaHI/s^-1) = -13.70 (+0.10 / -0.08) at z = 0.1, which is about approx 4sigma below the range predicted by current UV-background synthesis models (approx -13.3). To investigate the discrepancy between these high temperatures and theoretical models, we assess the impact of small-scale turbulence. By exploring a parameter grid in turbulent velocity (vtur) and GammaHI, we find that a standard IGM thermal and ionization state combined with unresolved turbulence of vtur simeq 15 km s^-1 can successfully reproduce the observed line widths at z = 0.1. Comparisons with high-resolution Space Telescope Imaging Spectrograph (STIS) expanded data indicate that the observed line widths are unlikely to be caused by instrumental resolution effects. Our findings suggest that either new heating mechanisms or unresolved turbulence are required to explain the unexpectedly broad Lya lines observed in the low-z IGM.

astro-ph.CO↗

arXiv2025

The Warm-Hot Disk-Halo Interface Below the Perseus Spiral Arm

The Milky Way's disk-halo interface mediates energy and mass exchange between the interstellar thin disk and the halo. In the first detailed study of the Perseus arm's disk-halo interface, we combine HST/STIS and COS absorption spectra toward 6 stars and 23 AGNs projected behind a narrow section ($95\degree<l<145\degree$, $-46\degree<b<0\degree$), providing a unique dataset that bridges the disk and its extended vertical structure in these directions. We measure \SII, \SiIV, and \CIV\ absorption, along with \HI\ 21 cm emission, at heights $-70$ pc to $-3.3$ kpc from the mid-plane. The arm's southern vertical structure shows distinct height-dependent behaviors: \HI\ and \SII\ column densities sharply decline with height up to 1.5 kpc, then continue declining at a much shallower rate at greater heights. In contrast, high ion (Si IV and C IV) column densities remain relatively constant throughout the entire height range. In the disk-halo interface, where warm neutral medium dominates, \SII\ and the high ions show similar kinematics, and we find a remarkably uniform \CIV/\SiIV\ ratio (\cIVsiIVav$\,=2.5 \pm 0.5$) within $-0.9$ to $-3.25$ kpc. Both the kinematic correspondence and high-ion ratio are consistent with the high ions probing turbulent mixing layers at the interfaces between warm/cool and hot gas phases. The AGN sightlines reveal minimal circumgalactic medium (CGM) contribution in the low-velocity gas at $|v_{\rm LSR}|< 100$ \kms. The extraplanar absorbing gas may trace material ejected from previous Galactic fountain activity.

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arXiv2025

The Case for High Resolution Spectroscopy in the Ultraviolet

The Astro2020 Decadal Survey declared that the baryon cycle is one of the top-priority science topics for current astrophysics. Space instruments with both high spectral resolution and high throughput in the ultraviolet are required for investigations of low density warm and cold gas present in both the \underline{inner} regions of the baryon cycle (interstellar medium, star-exoplanet interactions, pre-main sequence stars, stellar winds, flows and structures driven by supernovae) and the \underline{outer} regions (outflows of matter and energy from galaxies, circumgalactic media). The Space Telescope Imaging Spectrograph on HST has pioneered such studies, but STIS has its limitations and the lifetime of HST is limited. There is a pressing need for future large instruments with high spectral resolution ($R\approx 100,000$) in the 120--320~nm wavelength band such as the present STIS E140H and E230H capabilities, but with increased throughput to study gas in sight lines to faint sources such as M dwarf stars and circumgalactic clouds. Multi-object spectroscopy at high spectral resolution could enhance observational efficiency. This document describes some of the scientific results obtained with STIS and the new science that an enhanced instrument on a large telescope such as HWO could accomplish. We provide examples of the resolution needed for these science investigations.

astro-ph.IM↗

arXiv2024

A Survey of H I and O VI Absorption Lines in the Outskirts of $z\lesssim0.3$ Galaxy Clusters

The intracluster medium (ICM) in the far outskirts (r $>$ 2-3 R$_{200}$) of galaxy clusters interfaces with the intergalactic medium (IGM) and is theorized to comprise diffuse, multiphase gas. This medium may hold vital clues to clusters' thermodynamic evolution and far-reaching impacts on infalling, future cluster galaxies. The diffuse outskirts of clusters are well-suited for quasar absorption line observations, capable of detecting gas to extremely low column densities. We analyze 18 QSO spectra observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope whose lines of sight trace the gaseous environments of 26 galaxy clusters from within R$_{200}$ to 6 R$_{200}$ in projection. We measure the dN/dz and covering fraction of H I and O VI associated with the foreground clusters as a function of normalized impact parameter. We find the dN/dz for H I is consistent with the IGM field value for all impact parameter bins, with an intriguing slight elevation between 2 and 3 R$_{200}$. The dN/dz for O VI is also consistent with the field value (within 3$σ$) for all impact parameter bins, with potential elevations in dN/dz both within 1-2 R$_{200}$ and beyond 4 R$_{200}$ at $>2σ$. We propose physical scenarios that may give rise to these tentative excesses, such as a buildup of neutral gas at the outer accretion shock front and a signature of the warm-hot IGM. We do not find a systematic excess of potentially associated galaxies near the sightlines where O VI is detected; thus, the detected O VI does not have a clear circumgalactic origin.

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arXiv2024

Fast Outflows and Luminous He II Emission in Dwarf Galaxies with AGN

While stellar processes are believed to be the main source of feedback in dwarf galaxies, the accumulating discoveries of AGN in dwarf galaxies over recent years arouse the interest to also consider AGN feedback in them. Fast, AGN-driven outflows, a major mechanism of AGN feedback, have indeed been discovered in dwarf galaxies and may be powerful enough to provide feedback to their dwarf hosts. In this paper, we search for outflows traced by the blueshifted ultraviolet absorption features in three dwarf galaxies with AGN from the sample examined in our previous ground-based study. We confirm outflows traced by blueshifted absorption features in two objects and tentatively detect an outflow in the third object. In one object where the outflow is clearly detected in multiple species, photoionization modeling suggests that this outflow is located $\sim$0.5 kpc from the AGN, implying a galactic-scale impact. This outflow is much faster and possesses higher kinetic energy outflow rate than starburst-driven outflows in sources with similar star formation rates, and is likely energetic enough to provide negative feedback to its host galaxy as predicted by simulations. Much broader ($\sim$4000 km s$^{-1}$) absorption features are also discovered in this object which may have the same origin as that of broad absorption lines in quasars. Additionally, strong He II $λ$1640 emission is detected in both objects where the transition falls in the wavelength coverage, and is consistent with an AGN origin. In one of these two objects, blueshifted He II emission line is clearly detected, likely tracing a highly-ionized AGN wind.

astro-ph.GA↗

arXiv2024

Measurements of the Thermal and Ionization State of the Intergalactic Medium during the Cosmic Afternoon

We perform the first measurement of the thermal and ionization state of the intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 \lya absorption lines fitted from 12 HST STIS quasar spectra, with a total pathlength of Δz=2.1. We employ the machine-learning-based inference method that uses joint b-N distributions obtained from \lyaf decomposition. Our results show that the HI photoionization rates, Γ, are in good agreement with the recent UV background synthesis models, with \log (Γ/s^{-1})={-11.79}^{0.18}_{-0.15}, -11.98}^{0.09}_{-0.09}, and {-12.32}^{0.10}_{-0.12} at z=1.4, 1.2, and 1 respectively. We obtain the IGM temperature at the mean density, T_0, and the adiabatic index, γ, as [\log (T_0/K), γ]= [{4.13}^{+0.12}_{-0.10}, {1.34}^{+0.10}_{-0.15}], [{3.79}^{+0.11}_{-0.11}, {1.70}^{+0.09}_{-0.09}] and [{4.12}^{+0.15}_{-0.25}, {1.34}^{+0.21}_{-0.26}] at z=1.4, 1.2 and 1 respectively. Our measurements of T_0 at z=1.4 and 1.2 are consistent with the expected trend from z<3 temperature measurements as well as theoretical expectations that, in the absence of any non-standard heating, the IGM should cool down after HeII reionization. Whereas, our T_0 measurements at z=1 show unexpectedly high IGM temperature. However, because of the relatively large uncertainty in these measurements of the order of ΔT_0~5000 K, mostly emanating from the limited redshift path length of available data in these bins, we can not definitively conclude whether the IGM cools down at z<1.5. Lastly, we generate a mock dataset to test the constraining power of future measurement with larger datasets. The results demonstrate that, with redshift pathlength Δz \sim 2 for each redshift bin, three times the current dataset, we can constrain the T_0 of IGM within 1500K. Such precision would be sufficient to conclusively constrain the history of IGM thermal evolution at z < 1.5.

astro-ph.CO↗

arXiv2023

CGM$^2$ $+$ CASBaH: The Mass Dependence of H~I Ly$α$-Galaxy Clustering and the Extent of the CGM

We combine datasets from the CGM$^{2}$ and CASBaH surveys to model a transition point, $R_{\rm cross}$, between circumgalactic and intergalactic media (CGM and IGM, respectively). In total, our data consist of 7244 galaxies at z < 0.5 with precisely measured spectroscopic redshifts, all having impact parameters of 0.01 - 20 comoving Mpc from 28 QSO sightlines with high-resolution UV spectra that cover H I Ly$α$. Our best-fitting model is an exclusionary two-component model that combines a 3D absorber-galaxy cross correlation function with a simple Gaussian profile at inner radii to represent the CGM. By design, this model gives rise to a determination of $R_{\rm cross}$ as a function of galaxy stellar mass, which can be interpreted as the boundary between the CGM and IGM. For galaxies with $10^8 \leq M_{\star}/M_{\odot} \leq 10^{10.5}$, we find that $R_{\rm cross}(M_{\star}) \approx 2 \pm 0.6 R_{\rm vir}$. Additionally, we find excellent agreement between $R_{\rm cross}(M_{\star})$ and the theoretically-determined splashback radius for galaxies in this mass range. Overall, our results favor models of galaxy evolution at z < 0.5 that distribute $T \approx 10^{4}$K gas to distances beyond the virial radius.

astro-ph.GA↗

arXiv2023

The Bimodal Absorption System Imaging Campaign (BASIC) I. A Dual Population of Low-metallicity Absorbers at z $<1$

The bimodal absorption system imaging campaign (BASIC) aims to characterize the galaxy environments of a sample of 36 HI-selected partial Lyman limit systems (pLLSs) and Lyman limit systems (LLSs) in 23 QSO fields at $z \lesssim 1$. These pLLSs/LLSs provide a unique sample of absorbers with unbiased and well-constrained metallicities, allowing us to explore the origins of metal-rich and low-metallicity circumgalactic medium (CGM) at $z<1$. Here we present Keck/KCWI and VLT/MUSE observations of 11 of these QSO fields (19 pLLSs) that we combine with HST/ACS imaging to identify and characterize the absorber-associated galaxies. We find 23 unique absorber-associated galaxies, with an average of one associated galaxy per absorber. For seven absorbers, all with $<10\%$ solar metallicities, we find no associated galaxies with $\log M_\star \gtrsim 9.0$ within $ρ/R_{vir}$ and $|Δv|/v_{esc} \le$ 1.5 with respect to the absorber. We do not find any strong correlations between the metallicities or HI column densities of the gas and most of the galaxy properties, except for the stellar mass of the galaxies: the low-metallicity ([X/H] $\le -1.4$) systems have a probability of $0.39^{+0.16}_{-0.15}$ for having a host galaxy with $\log M_\star \ge 9.0$ within $ρ/R_{vir} \le 1.5$, while the higher metallicity absorbers have a probability of $0.78^{+0.10}_{-0.13}$. This implies metal-enriched pLLSs/LLSs at $z<1$ are typically associated with the CGM of galaxies with $\log M_\star > 9.0$, whereas low-metallicity pLLSs/LLSs are found in more diverse locations, with one population arising in the CGM of galaxies and another more broadly distributed in overdense regions of the universe. Using absorbers not associated with galaxies, we estimate the unweighted geometric mean metallicity of the intergalactic medium to be [X/H] $\lesssim -2.1$ at $z<1$, which is lower than previously estimated.

astro-ph.GA↗

arXiv2022

The CGM$^2$ Survey: Quenching and the Transformation of the Circumgalactic Medium

This study addresses how the incidence rate of strong O VI absorbers in a galaxy's circumgalactic medium (CGM) depends on galaxy mass and, independently, on the amount of star formation in the galaxy. We use HST/COS absorption spectroscopy of quasars to measure O VI absorption within 400 projected kpc and 300 km s$^{-1}$ of 52 $M_{*}\sim 10^{10}$ $M_\odot$ galaxies. The galaxies have redshifts $0.12<z<0.6$, stellar masses $10^{10.1} < M_* < 10^{10.9}$ $M_\odot$, and spectroscopic classifications as star-forming or passive. We compare the incidence rates of high column density O VI absorption ($N_{\rm O\, VI} \geq 10^{14.3}$ cm$^{-2}$) near star-forming and passive galaxies in two narrow stellar mass ranges and, separately, in a matched halo mass range. In all three mass ranges, the O VI covering fraction within 150 kpc is higher around star-forming galaxies than around passive galaxies with greater than $3σ$-equivalent statistical significance. On average, the CGM of $M_*\sim 10^{10}$ $M_\odot$ star-forming galaxies contains more O VI than the CGM of passive galaxies with the same mass. This difference is evidence for a CGM transformation that happens together with galaxy quenching and is not driven primarily by halo mass.

astro-ph.GA↗

arXiv2022

The high-velocity clouds above the disk of the outer Milky Way: misty precipitating gas in a region roiled by stellar streams

The high-velocity clouds (HVCs) in the outer Milky Way at $20^{\circ} < l < 190^{\circ}$ have similar spatial locations, metallicities, and kinematics. Moreover, their locations and kinematics are coincident with several extraplanar stellar streams. The HVC origins may be connected to the stellar streams, either stripped directly from them or precipitated by the aggregate dynamical roiling of the region by the stream progenitors. This paper suggests that these HVCs are "misty" precipitation in the stream wakes based on the following observations. New high-resolution (2.6 km/s) ultraviolet spectroscopy of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud (at 7 km/s resolution) into five components with $T \lesssim 3\times 10^{4}$ K. Photoionization models can explain the low-ionization components but require some depletion of refractory elements by dust, and model degeneracies allow a large range of metallicity. High-ionization absorption lines (SiIV, CIV, and OVI) are kinematically aligned with the lower-ionization lines and cannot be easily explained with photoionization or equilibrium collisional ionization; these lines are best matched by non-equilibrium rapidly cooling models, i.e., condensing/precipitating gas, with high metallicity and a significant amount of HI. Both the low- and high-ionization phases have low ratios of cooling time to freefall time and cooling time to sound-crossing time, which enables fragmentation and precipitation. The H1821+643 results are corroborated by spectroscopy of six other nearby targets that likewise show kinematically correlated low- and high-ionization absorption lines with evidence of dust depletion and rapid cooling.

astro-ph.GA↗

arXiv2022

Galactic Winds across the Gas-Rich Merger Sequence II. Lyman Alpha Emission and Highly Ionized O VI and N V Outflows in Ultraluminous Infrared Galaxies

This paper is the second in a series aimed at examining the gaseous environments of z$\le$0.3 quasars and ultraluminous infrared galaxies (ULIRGs) as a function of AGN/host galaxy properties across the merger sequence. This second paper focuses on the Ly$α$ emission and O VI and N V absorption features, tracers of highly ionized gas outflows, in ULIRGs observed with HST/COS. Ly$α$ emission is detected in 15 out of 19 ULIRGs, and 12 of the 14 clear Ly$α$ detections show emission with blueshifted velocity centroids and/or wings. The equivalent widths of the Ly$α$ emission increase with increasing AGN luminosities and AGN bolometric fractions. The blueshifts of the Ly$α$ emission correlate positively with those of the [O III] emission, where the latter traces the ionized gas outflows. The Ly$α$ escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities, but they do not correlate with nebular line reddening. Among the 12 ULIRGs with good continuum signal-to-noise ratios, O VI and/or N V absorption features are robustly detected in 6 of them, all of which are blueshifted, indicative of outflows. In the combined ULIRG $+$ quasar sample, the outflows are more frequently detected in the X-ray weak or absorbed sources. The absorption equivalent widths, velocities and velocity dispersions of the outflows are also higher in the X-ray weak sources. No other strong correlations are visible between the properties of the outflows and those of the AGN or host galaxies.

astro-ph.GA↗

arXiv2021

Galactic Winds across the Gas-Rich Merger Sequence: I. Highly Ionized N V and O VI Outflows in the QUEST Quasars

This program is part of QUEST (Quasar/ULIRG Evolutionary Study) and seeks to examine the gaseous environments of z < 0.3 quasars and ULIRGs as a function of host galaxy properties and age across the merger sequence from ULIRGs to quasars. This first paper in the series focuses on 33 quasars from the QUEST sample and on the kinematics of the highly ionized gas phase traced by the N V 1238, 1243 A and O VI 1032, 1038 A absorption lines in high-quality Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) data. N V and O VI outflows are present in about 60% of the QUEST quasars and span a broad range of properties, both in terms of equivalent widths (from 20 mA to 25 A) and kinematics (outflow velocities from a few x 100 km/s up to ~10,000 km/s). The rate of incidence and equivalent widths of the highly ionized outflows are higher among X-ray weak or absorbed sources. The weighted outflow velocity dispersions are highest among the X-ray weakest sources. No significant trends are found between the weighted outflow velocities and the properties of the quasars and host galaxies although this may be due to the limited dynamic range of properties of the current sample. These results will be re-examined in an upcoming paper where the sample is expanded to include the QUEST ULIRGs. Finally, a lower limit of ~0.1% on the ratio of time-averaged kinetic power to bolometric luminosity is estimated in the 2-4 objects with blueshifted P V 1117, 1128 absorption features.

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arXiv2021

Metal-enriched halo gas across galaxy overdensities over the last 10 billion years

We present a study of metal-enriched halo gas traced by MgII and CIV absorption at z<2 in the MUSE Analysis of Gas around Galaxies survey and the Quasar Sightline and Galaxy Evolution survey. Using these large and complete galaxy surveys in quasar fields, we study the dependence of the metal distribution on galaxy properties and overdensities, out to physical projected separations of 750 kpc. We find that the cool, low-ionization gas is significantly affected by the environment across the full redshift range probed, with ~2-3 times more prevalent and stronger MgII absorption in higher overdensity group environments and in regions with greater overall stellar mass and star formation rates. Complementary to these results, we have further investigated the more highly ionized gas as traced by CIV absorption, and found that it is likely to be more extended than the MgII gas, with ~2 times higher covering fraction at a given distance. We find that the strength and covering fraction of CIV absorption show less significant dependence on galaxy properties and environment than the MgII absorption, but more massive and star-forming galaxies nevertheless also show ~2 times higher incidence of CIV absorption. The incidence of MgII and CIV absorption within the virial radius shows a tentative increase with redshift, being higher by a factor of ~1.5 and ~4, respectively, at z>1. It is clear from our results that environmental processes have a significant impact on the distribution of metals around galaxies and need to be fully accounted for when analyzing correlations between gaseous haloes and galaxy properties.

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arXiv2021

Thermal Pressures in the Interstellar Medium away from Stellar Environments

Interstellar thermal pressures can be measured using C I absorption lines that probe the pressure-sensitive populations of the fine-structure levels of its ground state. In a survey of C I absorption toward Galactic hot stars, Jenkins & Tripp (2011) found evidence of small amounts ($\sim 0.05\%$) of gas at high pressures ($p/k \gg 10^4{\rm cm^{-3}K}$) mixed with a more general presence of lower pressure material exhibiting a log normal distribution that spanned the range $10^3 \lesssim p/k \lesssim 10^4{\rm cm^{-3}K}$. In this paper, we study Milky Way C I lines in the spectra of extragalactic sources instead of Galactic stars and thus measure the pressures without being influenced by regions where stellar mass loss and H II region expansions could create localized pressure elevations. We find that the distribution of low pressures in the current sample favors slightly higher pressures than the earlier survey, and the fraction of gaseous material at extremely high pressures is about the same as that found earlier. Thus we conclude that the earlier survey was not appreciably influenced by the stellar environments, and the small amounts of high pressure gas indeed exist within the general interstellar medium.

astro-ph.GA↗

arXiv2021

CGM$^{2}$ I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen

We present initial results from the \textit{COS and Gemini Mapping the Circumgalactic Medium} (\mbox{CGMCGM} $\equiv$ CGM$^{2}$) survey. The CGM$^{2}$ survey consists of 1689 galaxies, all with high-quality Gemini GMOS spectra, within 1 Mpc of twenty-two $z \lesssim 1$ quasars, all with S/N$\sim$10 {\emph{HST/COS}} G130M$+$G160M spectra. For 572 of these galaxies having stellar masses $10^{7} M_{\odot} < M_{\star} < 10^{11} M_{\odot}$ and $z \lesssim 0.5$, we show that the \ion{H}{1} covering fraction above a threshold of \NHI$>10^{14} $cm$^{-2}$ is $\gtrsim 0.5$ within 1.5 virial radii ($R_{\rm vir} \sim R_{200m}$). We examine the \ion{H}{1} kinematics and find that the majority of absorption lies within $\pm$ 250 km s$^{-1}$ of the galaxy systemic velocity. We examine \ion{H}{1} covering fractions over a range of impact parameters to infer a characteristic size of the CGM, $R^{14}_{\rm CGM}$, as a function of galaxy mass. $R^{14}_{\rm CGM}$ is the impact parameter at which the probability of observing an absorber with \NHI $>$ 10$^{14}$ cm$^{-2}$ is $>$ 50\%. In this framework, the radial extent of the CGM of $M_{\star} > 10^{9.9} M_{\odot}$ galaxies is $R^{14}_{\rm CGM} = 346^{+57}_{-53}$ kpc or $R^{14}_{\rm CGM} \simeq 1.2R_{\rm vir}$. Intermediate-mass galaxies with $10^{9.2} < M_{\star}/M_{\odot} < 10^{9.9}$ have an extent of $R^{14}_{\rm CGM} = 353^{+64}_{-50}$ kpc or $R^{14}_{\rm CGM} \simeq 2.4R_{\rm vir}$. Low-mass galaxies, $M_{\star} < 10^{9.2} M_{\odot}$, show a smaller physical scale $R^{14}_{\rm CGM} = 177_{-65}^{+70}$ kpc and extend to $R^{14}_{\rm CGM} \simeq 1.6R_{\rm vir}$. Our analysis suggests that using $R_{\rm vir}$ as a proxy for the characteristic radius of the CGM likely underestimates its extent.

astro-ph.GA↗

arXiv2020

The COS Absorption Survey of Baryon Harbors: Unveiling the Physical Conditions of Circumgalactic Gas through Multiphase Bayesian Ionization Modeling

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g., Mg II, S II--S VI, O II--O VI, Ne VIII), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modeling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favor three) distinct ionization phases ranging from $T \approx 10^{4}$ K photoionized gas to warm-hot phases at $T \lesssim 10^{5.8}$ K. For some ions, an apparently single absorption "component" includes contributions from more than one phase, and up to 30% of the H I is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with sub-solar N/$α$, and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model.

astro-ph.GA↗

arXiv2020

Absorption line abundances in the SMC-like galaxy UGC 5282: evidence for ISM dilution from inflows on kiloparsec scales

We present an HST Cosmic Origins Spectrograph (COS) spectrum of the QSO SDSS J095109.12+330745.8 ($z_{em} = 0.645$) whose sightline passes through the SMC-like dwarf galaxy UGC 5282 ($M_B = -16.0$, $cz = 1577$ km s$^{-1}$ ), 1.2 kpc in projection from the central HII region of the galaxy. Damped Ly$α$ (DLA) absorption is detected at the redshift of UGC 5282 with $\log$[$N$(HI) cm$^{-2}\:$] = 20.89$^{+0.12}_{-0.21}\:\:$. Analysis of the accompanying SII, PII and OI metal lines yields a neutral gas metallicity, $Z_{HI}$, of [S/H] $\simeq$ [P/H] $= -0.80\pm0.24$. The metallicity of ionized gas from the central HII region, $Z_{HII}\:$, measured from its emission lines is [O/H]$= -0.37\pm0.10$, a difference of $+0.43\pm 0.26$ from $Z_{HI}\:$. This difference $δ$ is consistent with that seen towards HII regions in other star-forming galaxies and supports the idea that ionized gas near star forming regions shows systematically higher metallicities than exist in the rest of a galaxy's neutral interstellar medium (ISM). The positive values of $δ$ found in UGC 5282 (and the other star forming galaxies) is likely due to infalling low metallicity gas from the intergalactic medium that mixes with the galaxy's ISM on kpc scales. This model is also consistent with broad Ly$α$ emission detected at the bottom of the DLA absorption, offset by $\sim 125$ km s$^{-1}$ from the absorption velocity. Models of galaxy evolution that attempt to replicate population characteristics, such as the mass-metallicity relation, may need to start with a galaxy metallicity represented by $Z_{HI}$ rather than that measured traditionally from $Z_{HII}\:$.

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