arXiv subjects

Jane Charlton

Publications and source records attributed to Jane Charlton.

At least 19 recordsLinked to original sources

arXiv2026

High-S/N Quasar Observations with HST/COS: Deep Fields for Spectroscopy

Hubble is still in prime observing condition for making transformative discoveries in UV astronomy. In this white paper we describe the science case for a deep (S/N>30) UV spectroscopic survey with HST/COS targeting approximately 20 QSOs at 0.5<z<1.5 at good resolution (20 km/s). This survey would capitalize on our current UV capability, produce a legacy dataset enabling community science in many areas of galactic and extragalactic research, and pioneer a path for future UV science with the Habitable Worlds Observatory. Such high-S/N spectra are largely missing from the MAST archives, and would be analogous to the deep Hubble imaging fields (HDF, UDF, Frontier Fields) that have been enormously successful and far-reaching in their science impact. This legacy dataset would enable frontier science programs in several areas, including (1) studies of the CGM and IGM at unparalleled sensitivity, covering a wide range of UV metal lines and reaching very low H I column densities of log N=12.6 and low metallicities near [Z/H]=-2, enabling precision studies of the chemical abundances, ionization, temperature, and baryon and metal budgets of the CGM and IGM; (2) diffuse gas in the Milky Way and Local Group, including high-velocity clouds and gas streams from satellite mergers; (3) AGN outflows, which would be probed in the rest-frame extreme ultraviolet (EUV), covering continuum-generation mechanisms and diagnostics of gas in accretion-disk outflows.

astro-ph.GA↗

arXiv2025

Reporting a Deficit of Intrinsic NV Absorbers in Core-dominated, Radio-loud Quasars

We searched the Hubble Space Telescope Cosmic Origins Spectrograph archive for ultraviolet spectra of 428 AGN to identify intrinsic NV absorption systems. We filtered out Type 2 AGN, blazars, and spectra that do not cover at least part of the velocity window from 5000 km/s blueward to 5000 km/s redward (hereafter, the ``associated'' region) of the NV emission line. This yielded 175 Type 1 quasars, 34 radio-loud, 133 radio-quiet, and eight unconstrained. Our survey uncovered 77 associated NV systems in the spectra of 48 of these low-redshift quasars. We consider the incidence of intrinsic absorbers as a function of quasar properties (optical, radio and X-ray). We find a statistically significant dearth of intrinsic NV systems in the spectra of the 34 radio-loud quasars (6%), compared to 29% of the 133 radio-quiet quasars containing at least one intrinsic system. Assuming intrinsic systems are equally likely to occur in radio-loud and radio-quiet quasars and the orientations of the two subsamples are comparable, there is a 0.1% probability of such a deficit occurring by chance in the radio-loud population. We propose that this deficit of systems is caused by orientation effects. FIRST radio images are available for 14 of the 33 radio-loud quasars. These show that only three of the 14 radio-loud quasars have lobe-dominated morphologies, whereas 11 of the 14 radio-loud quasars have compact radio morphologies, implying that these quasars are face on, and suggesting that clouds that produce NV absorption are rarely found along the polar axis.

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arXiv2025

Kinematic analysis of an Ultra-Strong MgII absorber at z~1.13 linking to Circumgalactic Gas Structures

We present a spectroscopic and imaging analysis of the $z_{gal} \approx 1.1334$ ultra-strong MgII absorption system identified in the $VLT$/UVES spectrum of a background quasar located at $\rho \approx 18$ kpc from a star-forming galaxy. Low ionization metal lines like MgI, FeII, and CaII are also detected for this absorber. The HI lines are outside of the wavelength coverage. The MgII has a rest-frame equivalent width of $W_r(2796) =3.185 +/- 0.032 A^{\circ} $, with the absorption spread across $\Delta v \approx 460$ km~s$^{-1}$ in several components. A component-by-component ionization modeling shows several of these components having solar and higher metallicities. The models also predict a total HI column density of $log[N(HI)/cm^{-2}] \approx 22.5$, consistent with ultra-strong MgII absorbers being sub-Damped Lyman Alpha and Damped Lyman Alpha systems. The absorber is well within the virial radius of the nearest galaxy which has a stellar mass $M_* = 4.7 \times 10^{10}$~M$_\odot$, and a star formation rate of $\approx 8.3$~M$_\odot$~yr$^{-1}$. The absorption is along the projected major axis of the galaxy with a velocity spread that is wider than the galaxy's disk rotation. From the kinematic analysis of the absorber and the galaxy, the origin of the absorption can be attributed to a combination of circumgalactic gas structures, some corotating with the disk and the rest at line-of-sight velocities outside of the disk rotation.

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arXiv2023

Characterizing cool, neutral gas and ionized metals in the outskirts of low-z galaxy clusters

We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z$\approx$0.2 background quasar - foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalog with optically- and SZ-selected cluster catalogs. The median cluster mass of our sample is $\approx 10^{14.2}$ M_sun, with a median impact parameter ($ρ_{cl}$) of $\approx5$ Mpc. We detect significant Lya, marginal CIV, but no OVI absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096$\pm$0.011 A, 0.032$\pm$0.015 A, and <0.009 A (3$σ$) for our sample. The Lya REW shows a declining trend with increasing $ρ_{cl}$ ($ρ_{cl}$ / $R_{500}$) which is well explained by a power-law with a slope of -0.79 (-0.70). The covering fractions (CFs) measured for Lya (21\%), CIV (10\%) and OVI (10\%) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centers or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lya along with the non-detection of Lya signal when the strong absorbers (N(HI) > $10^{13} cm^{-2}$) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.

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arXiv2023

Star Clusters in Tidal Debris

We present results of a Hubble Space Telescope (HST) UBVI-band study of star clusters in tidal tails, using new WFC3 and ACS imaging to complement existing WFPC2 data. We survey 12 tidal tails across seven merging systems, deriving ages and masses for 425 star cluster candidates (SCCs). The stacked mass distribution across all systems follows a power law of the form $dN/dM \propto M^β$, with $β= -2.02 \pm 0.15$, consistent with what is seen in other star forming environments. GALEX and Swift UV imaging provide star formation rates (SFRs) for our tidal tails, which when compared with ages and masses of our SCCs, allows for a determination of the cluster formation efficiency (CFE). We find the CFE increases with increasing SFR surface density, matching the theoretical model. We confirm this fit down at SFR densities lower than previously measured (log $Σ_\text{SFR} \: (\text{M}_\odot \: \text{yr}^{-1} \: \text{kpc}^{-2}) \approx -4.2$), as related to the CFE. We determine the half-light radii for a refined sample of 57 SCCs with our HST WFC3 and ACS imaging, and calculate their dynamical age, finding the majority of them to be gravitationally bound. We also provide evidence of only low-mass ($< 10^4 \: \text{M}_\odot$) cluster formation in our nearest galaxy, NGC 1487, consistent with the theory that this system is a dwarf merger.

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arXiv2023

The Halo21 Absorption Modeling Challenge: Lessons From "Observing" Synthetic Circumgalactic Absorption Spectra

In the Halo21 absorption modeling challenge we generated synthetic absorption spectra of the circumgalactic medium (CGM), and attempted to estimate the metallicity, temperature, and density (Z, T, and nH) of the underlying gas using observational methods. We iteratively generated and analyzed three increasingly-complex data samples: ion column densities of isolated uniform clouds, mock spectra of 1--3 uniform clouds, and mock spectra of high-resolution turbulent mixing zones. We found that the observational estimates were accurate for both uniform cloud samples, with Z, T, and nH retrieved within 0.1 dex of the source value for >90% of absorption systems. In the turbulent-mixing scenario, the mass, temperature, and metallicity of the strongest absorption components were also retrieved with high accuracy. However, the underlying properties of the subdominant components were poorly constrained because the corresponding simulated gas contributed only weakly to the H I absorption profiles. On the other hand, including additional components beyond the dominant ones did improve the fit, consistent with the true existence of complex cloud structures in the source data.

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arXiv2022

Probing the physicochemical properties of the Leo Ring and the Leo I group

We present an absorption line study of the physical and chemical properties of the Leo HI Ring and the Leo I Group as traced by 11 quasar sightlines spread over a 600 kpc X 800 kpc region. Using HST/COS G130/G160 archival observations as constraints, we couple cloud-by-cloud, multiphase, Bayesian ionization modeling with galaxy property information to determine the plausible origin of the absorbing gas along these sightlines. We search for absorption in the range 600 km/s - 1400 km/s consistent with the kinematics of the Leo Ring/Group. We find absorption plausibly associated with the Leo Ring towards five sightlines. Along three other sightlines, we find absorption likely to be associated with individual galaxies, intragroup gas, and/or large-scale filamentary structure. The absorption along these five sightlines is stronger in metal lines than expected from individual galaxies, indicative of multiple contributions, and of the complex kinematics of the region. We also identify three sightlines within a 7-degree X 6-degree field around the Leo Ring, along which we do not find any absorption. We find that the metallicities associated with the Leo Ring are generally high, with values between solar and several times solar. The inferred high metallicities are consistent with the origin of the ring as tidal debris from a major galaxy merger.

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arXiv2021

Using cosmological simulations and synthetic absorption spectra to assess the accuracy of observationally derived CGM metallicities

We used adaptive mesh refinement hydrodynamic cosmological simulations of a $z=1$ Milky Way-type galaxy and a $z=0$ Dwarf galaxy and generated synthetic quasar absorption-line spectra of their circumgalactic medium (CGM). Our goal is to assess whether standard observational spectroscopic analysis methods accurately reproduce intrinsic column densities, metallicities [Si/H], and hydrogen densities $n_{H}$, in simulated absorption-line systems. Without knowledge of the intrinsic simulated properties (blind study), we analysed synthetic COS and HIRES spectra with fixed $S/N=30$ to determine the column densities, metallicity, and $n_{H}$, using Voigt profile fitting combined with Markov-Chain Monte-Carlo single-phase {\sc Cloudy} modelling techniques. To quantify the intrinsic simulated absorbing gas properties, we objectively determined which gas cells along a line of sight (LOS) contribute to detected absorption in the spectra and adopt the unweighted geometric mean of these gas cell properties. For this pilot study, we performed this experiment for five LOS in the Milky-Way galaxy and five LOS in the Dwarf galaxy. We found an average agreement between the "observed" and intrinsic metallicity overestimated within $0.8σ$ or $0.2$ dex for the "Milky-Way"' CGM and overestimated within $1.4σ$ or $0.2$ dex for the Dwarf galaxy CGM. We found that the spectroscopically-derived $n_{H}$ are underestimated within $0.8σ$ or $0.4$ dex of the intrinsic $n_{H}$ for the "Milky-Way" CGM and overestimated within $0.3σ$ or $0.3$ dex for the Dwarf galaxy CGM. The overall agreement suggests that, for single-phase ionisation modelling of systems where there is substantial spread in gas properties, global metallicity measurements from quasar absorption line studies are capturing the average metallicity and ionisation parameters in a given astrophysical environment.

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arXiv2020

Physical Conditions of Five O VI Absorption Systems Towards PG $1522+101$

We present the analysis of five O VI absorbers identified across a redshift path of z $\sim (0.6 - 1.3)$ towards the background quasar PG $1522+101$ with information on five consecutive ionization stages of oxygen from O II to O VI. The combined $HST$ and $Keck$ spectra cover UV, redshifted EUV, and optical transitions from a multitude of ions spanning ionization energies in the range of $\sim (13 - 300)$ eV. Low ionization (C II, O II, Si II, Mg II) and very high ionization species (Ne VIII, Mg X) are non-detections in all the absorbers. Three of the absorbers have coverage of He I, in one of which it is a $> 3 σ$ detection. The kinematic structures of these absorbers are extracted from C IV detected in $HIRES$ spectra. The farthest absorber in our sample also contains the detections of Ne V and Ne VI. Assuming co-spatial absorbing components, the ionization models show the medium to be multiphased with small-scale density-temperature inhomogeneities that are sometimes kinematically unresolved. In two of the absorbers, there is an explicit indication of the presence of a warm gas phase ($T \gtrsim 10^5$ K) traced by O VI. In the remaining absorbers, the column densities of the ions are consistent with a non-uniform photoionized medium. The sub-solar [C/O] relative abundances inferred for the absorbers point at enrichment from massive Type II supernovae. Despite metal enrichment, the inferred wide range for [O/H] $\sim$ [$-2.1, +0.2$] amongst the absorbers along with their anti-correlation with the observed H I suggest poor small-scale mixing of metals with hydrogen in the regions surrounding galaxies and the IGM.

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arXiv2019

The Physical Origins of the Identified and Still Missing Components of the Warm-Hot Intergalactic Medium: Insights from Deep Surveys in the Field of Blazar 1ES1553+113

The relationship between galaxies and the state/chemical enrichment of the warm-hot intergalactic medium (WHIM) expected to dominate the baryon budget at low-z provides sensitive constraints on structure formation and galaxy evolution models. We present a deep redshift survey in the field of 1ES1553+113, a blazar with a unique combination of UV+X-ray spectra for surveys of the circum-/intergalactic medium (CGM/IGM). Nicastro et al. 2018 reported the detection of two O VII WHIM absorbers at $z=0.4339$ and $0.3551$ in its spectrum, suggesting that the WHIM is metal-rich and sufficient to close the missing baryons problem. Our survey indicates that the blazar is a member of a $z=0.433$ group and that the higher-$z$ O VII candidate arises from its intragroup medium. The resulting bias precludes its use in baryon censuses. The $z=0.3551$ candidate occurs in an isolated environment 630 kpc from the nearest galaxy (with stellar mass $\log M_*/M_\odot \approx 9.7$) which we show is unexpected for the WHIM. Finally, we characterize the galactic environments of broad H I Ly$α$ absorbers (Doppler widths of $b=40-80$ \kms; $T\lesssim4\times10^5$ K) which provide metallicity independent WHIM probes. On average, broad Ly$α$, absorbers are ${\approx}2\times$ closer to the nearest luminous ($L>0.25 L_*$) galaxy (700 kpc) than narrow ($b<30$ \kms; $T\lesssim4\times10^5$ K) ones (1300 kpc) but ${\approx}2\times$ further than O\,VI absorbers (350 kpc). These observations suggest that gravitational collapse heats portions of the IGM to form the WHIM but with feedback that does not enrich the IGM far beyond galaxy/group halos to levels currently observable in UV/X-ray metal lines.

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arXiv2018

COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3

We present a sample of 34 weak metal line absorbers at $z< 0.3$ complied via the simultaneous detections ($3σ$) of the SiII$\lambda1260$ and CII$\lambda1334$ absorption lines, with $W_{r}$(SiII)$<0.2$ Å and $W_{r}$(CII)$<0.3$ Å, in archival HST/COS spectra. Our sample increases the number of known low-$z$ "weak absorbers" by a factor of $>5$. The column densities of HI and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of $-3.3 < \log n_{\rm H}/{\rm cm^{-3}} < -2.4$ and $16.0 < \log N_{\rm H}/{\rm cm^{-2}}<20.3$, respectively. The line of sight thicknesses of the absorbers have a wide range of $\sim$1 pc$-$50 kpc with a median value of $\sim$500 pc. The high-ionization OVI absorption, detected in 12/18 cases, always stems from a different gas-phase. Most importantly, 85% (50%) of these absorbers show a metallicity of $\rm [Si/H] > -1.0$ (0.0). The fraction of systems showing high metallicity (i.e., $\rm [Si/H]>-1.0$) in our sample is significantly higher than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM) at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random places with a median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the presence of multiple galaxies in $\sim80$% of the cases, suggesting group environments. The observed $d\mathcal{N}/dz$ of $0.8\pm0.2$ indicates that such metal-enriched, compact, dense structures are ubiquitous in the halos of low-$z$ galaxies that are in groups. We suggest that these are transient structures that are related to outflows and/or stripping of metal-rich gas from galaxies.

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arXiv2017

Discovery of an HI-rich Gas Reservoir in the Outskirts of SZ-effect Selected Clusters

We report on the detection of three strong HI absorbers originating in the outskirts (i.e., impact parameter, $ρ_{\rm cl} \approx (1.6-4.7) r_{500}$) of three massive ($M_{500}\sim3\times10^{14} M_{\odot}$) clusters of galaxies at redshift $z_{\rm cl} \approx 0.46$, in the $Hubble Space Telescope$ Cosmic Origins Spectrograph ($HST$/COS) spectra of 3 background UV-bright quasars. These clusters were discovered by the 2500 deg$^2$ South Pole Telescope Sunyaev$-$Zel'dovich (SZ) effect survey. All three COS spectra show partial Lyman limit absorber with $N(HI) > 10^{16.5} \ \rm cm^{-2}$ near the photometric redshifts ($|Δz/(1+z)| \approx 0.03$) of the clusters. The compound probability of random occurrence of all three absorbers is $<0.02$%, indicating that the absorbers are most likely related to the targeted clusters. We find that the outskirts of these SZ-selected clusters are remarkably rich in cool gas compared to existing observations of other clusters in the literature. The effective Doppler parameters of the Lyman series lines, obtained using single cloud curve-of-growth (COG) analysis, suggest a non-thermal/turbulent velocity of a few $\times10 \ \rm km s^{-1}$ in the absorbing gas. We emphasize the need for uniform galaxy surveys around these fields and for more UV observations of QSO-cluster pairs in general in order to improve the statistics and gain further insights into the unexplored territory of the largest collapsed cosmic structures.

astro-ph.CO↗

arXiv2017

The formation and evolution of star clusters in interacting galaxies

Observations of globular clusters show that they have universal lognormal mass functions with a characteristic peak at $\sim 2\times 10^{5}\, {\rm{M_{\odot}}}$, but the origin of this peaked distribution is highly debated. Here we investigate the formation and evolution of star clusters in interacting galaxies using high-resolution hydrodynamical simulations performed with two different codes in order to mitigate numerical artifacts. We find that massive star clusters in the range of $\sim 10^{5.5} - 10^{7.5}\, {\rm{M_{\odot}}}$ form preferentially in the highly-shocked regions produced by galaxy interactions. The nascent cluster-forming clouds have high gas pressures in the range of $P/k \sim 10^8 - 10^{12}\, \rm{K}\,\rm{cm^{-3}}$, which is $\sim 10^4 - 10^8$ times higher than the typical pressure of the interstellar medium but consistent with recent observations of a pre-super star cluster cloud in the Antennae Galaxies. Furthermore, these massive star clusters have quasi-lognormal initial mass functions with a peak around $\sim 10^{6}\, {\rm{M_{\odot}}}$. The number of clusters declines with time due to destructive processes, but the shape and the peak of the mass functions do not change significantly during the course of galaxy collisions. Our results suggest that gas-rich galaxy mergers may provide a favorable environment for the formation of massive star clusters such as globular clusters, and that the lognormal mass functions and the unique peak may originate from the extreme high-pressure conditions of the birth clouds and may survive the dynamical evolution.

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arXiv2016

A Tale of Two Tails: Exploring Stellar Populations in the Tidal Tails of NGC 3256

We have developed an observing program using deep, multiband imaging to probe the chaotic regions of tidal tails in search of an underlying stellar population, using NGC 3256's 400 Myr twin tidal tails as a case study. These tails have different colours of $u - g = 1.05 \pm 0.07$ and $r - i = 0.13 \pm 0.07$ for NGC 3256W, and $u - g = 1.26 \pm 0.07$ and $r - i = 0.26 \pm 0.07$ for NGC 3256E, indicating different stellar populations. These colours correspond to simple stellar population ages of $288^{+11}_{-54}$ Myr and $841^{+125}_{-157}$ Myr for NGC 3256W and NGC 3256E, respectively, suggesting NGC 3256W's diffuse light is dominated by stars formed after the interaction, while light in NGC 3256E is primarily from stars that originated in the host galaxy. Using a mixed stellar population model, we break our diffuse light into two populations: one at 10 Gyr, representing stars pulled from the host galaxies, and a younger component, whose age is determined by fitting the model to the data. We find similar ages for the young populations of both tails, ($195^{-13}_{+0}$ and $170^{-70}_{+44}$ Myr for NGC 3256W and NGC 3256E, respectively), but a larger percentage of mass in the 10 Gyr population for NGC 3256E ($98^{+1}_{-3}\%$ vs $90^{+5}_{-6}\%$). Additionally, we detect 31 star cluster candidates in NGC 3256W and 19 in NGC 2356E, with median ages of 141 Myr and 91 Myr, respectively. NGC 3256E contains several young (< 10 Myr), low mass objects with strong nebular emission, indicating a small, recent burst of star formation.

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arXiv2016

The Ultraviolet and Infrared Star Formation Rates of Compact Group Galaxies: An Expanded Sample

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended timescales. We expand the census of star formation in compact group galaxies by \citet{tzanavaris10} and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 \micron\ photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFR$_{\mathrm{UV}}$) using the UVOT uvw2photometry. Similarly, we use the MIPS 24 \micron\ photometry to estimate the component of the SFR that is obscured by dust (SFR$_{\mathrm{IR}}$). We find that galaxies which are MIR-active (MIR-"red"), also have bluer UV colours, higher specific star formation rates, and tend to lie in H~{\sc i}-rich groups, while galaxies that are MIR-inactive (MIR-"blue") have redder UV colours, lower specific star formation rates, and tend to lie in H~{\sc i}-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M$_{\odot}$ yr$^{-1}$, indicating this might be the most common value in compact groups. In contrast, the specific star formation rate distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific star formation rate is the best tracer of gas depletion and galaxy evolution in compact groups.

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arXiv2016

On the Covering Fraction Variability in an EUV Mini-BAL Outflow from PG 1206+459

We report on the first detection of extreme-ultraviolet (EUV) absorption variability in the NeVIII 770,780 mini-broad absorption line (mini-BAL) in the spectrum of the quasar (QSO) PG 1206+459. The observed equivalent width (EW) of the NeVIII doublet show a ~4 sigma variation over a timescale of 2.8 months in the QSO's rest-frame. Both members of the NeVIII doublet exhibit non-black saturation, indicating partial coverage of the continuum source. An increase in the NeVIII covering fraction from f_c = 0.59\pm0.05 to 0.72\pm0.03 is observed over the same period. The NeVIII profiles are too highly saturated to be susceptible to changes in the ionization state of the absorbing gas. In fact, we do not observe any significant variation in the EW and/or column density after correcting the spectra for partial coverage. We, thus, propose transverse motions of the absorbing gas being the cause of the observed variability. Using a simple model of a transiting cloud we estimate a transverse speed of ~1800 km/s. For Keplerian motion, this corresponds to a distance between the absorber and the central engine of ~1.3 pc, which places the absorber just outside the BLR region. We further estimate a density of ~5\times10^6 cm^(-3) and a kinetic luminosity of ~10^43 - 10^44 erg/s. Such large kinetic powers suggest that outflows detected via EUV lines are potentially major contributors to AGN feedback.

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arXiv2015

An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature

We present the results of a systematic search for molecular hydrogen (H2) in low redshift ($ 0.05 \lesssim z \lesssim 0.7$) DLAs and sub-DLAs with $N(HI) \gtrsim 10^{19.0}$ cm$^{-2}$, in the archival HST/COS spectra. Our core sample is comprised of 27 systems with a median $\log N(HI) = 19.6$. On the average, our survey is sensitive down to $\log N(H2) = 14.4$ corresponding to a molecular fraction of $\log f_{H2} = -4.9$ at the median $N(HI)$. H2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this $f_{H2}$ limit. The H2 detection rate of $50^{+25}_{-12}$ percent seen in our sample, is a factor of $\gtrsim 2$ higher than that of the high-$z$ sample of Noterdaeme et al. (2008), for systems with $N(H2) > 10^{14.4}$ cm$^{-2}$. In spite of having $N(HI)$ values typically lower by a factor of 10, low-$z$ H2 systems show molecular fractions ($\log f_{H2}=-1.93\pm0.63$) that are comparable to the high-$z$ sample. The rotational excitation temperatures ($T_{01} = 133\pm55$ K), as measured in our low-$z$ sample, are typically consistent with high-$z$ measurements. Simple photoionization models favour a radiation field much weaker than the mean Galactic ISM field for a particle density in the range 10 - 100 cm$^{-3}$. The impact parameters of the identified host-galaxy candidates are in the range $10 \lesssim ρ$ (kpc) $\lesssim 80$. We, therefore, conjecture that the low-$z$ H2 bearing gas is not related to star-forming disks but stems from self-shielded, tidally stripped or ejected disk-material in the extended halo.

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arXiv2014

Halo Mass Dependence of HI and OVI Absorption: Evidence for Differential Kinematics

We studied a sample of 14 galaxies (0.1 < z < 0.7) using HST/WFPC2 imaging and high-resolution HST/COS or HST/STIS quasar spectroscopy of Lya, Lyb, OVI1031, and OVI1037 absorption. The galaxies, having 10.8 < log(M/M_solar) < 12.2, lie within D = 300 kpc of quasar sightlines, probing out to D/R_vir = 3. When the full range of galaxy virial masses and D/R_vir of the sample are examined, 40% of the HI absorbing clouds can be inferred to be escaping their host halo. The fraction of bound clouds decreases as D/R_vir increases such that the escaping fraction is around 15% for D/R_vir < 1, around 45% for 1 < D/R_vir < 2, and around 90% for 2 < D/R_vir < 3. Adopting the median mass log(M/M_solar) = 11.5 to divide the sample into "higher" and "lower" mass galaxies, we find mass dependency for the hot CGM kinematics. To our survey limits, OVI absorption is found in only 40% of the HI clouds in and around lower mass halos as compared to 85% around higher mass halos. For D/R < 1, lower mass halos have an escape fraction of 65%, whereas higher mass halos have an escape fraction of 5%. For 1 < D/R_vir < 2, the escape fractions are 55% and 35% for lower mass and higher mass halos, respectively. For 2 < D/R_vir < 3, the escape fraction for lower mass halos is around 90%. We show that it is highly likely that the absorbing clouds reside within 4 virial radii of their host galaxies and that the kinematics are dominated by outflows. Our finding of "differential kinematics" is consistent with the scenario of "differential wind recycling" proposed by Oppenheimer et al. We discuss the implications for galaxy evolution, the stellar to halo mass function, and the mass metallicity relationship of galaxies.

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