arXiv subjects

A. J. Fox

Publications and source records attributed to A. J. Fox.

At least 19 recordsLinked to original sources

arXiv2026

Connecting the dusty dots: dust depletion and extinction of local interstellar clouds

Investigating the chemical complexity of the interstellar medium (ISM) is key for understanding its physical nature and evolution. In this work, we study parsec-scale interstellar dust clouds in the neutral ISM of the Milky Way using two different probes: dust depletion and dust extinction. We examine their relationship to investigate the distribution of metals and dust in the Solar neighbourhood, and how they are related to the Local Bubble. We use measurements of dust depletion for individual gas clouds along sight lines of sight towards bright O/B stars within 1.1 kpc of the Sun, derived from UV absorption-line spectra. We combine these with parsec-scale 3D dust extinction density maps out to 1.25 kpc. We assume a correlation between dust depletion and dust extinction density, which we use to imply that the absorption components are spatially associated with the peaks in dust extinction density, and to pinpoint the likely locations of the gas clouds in physical space. We identify peaks in the dust extinction curves, and then associate the stronger peaks with the strongest dust depletion components. Independent distance measurements along the line of sight towards one of our targets, theta1 Ori C, validates our results. In our sample, the minimum distance between clouds that have significantly different chemical properties is ~ 100pc, giving an indication on the physical scale on which chemical mixing remains incomplete in the ISM of the Milky Way. For five of the eight targets, we report dust depletion values for gas clouds associated with the Local Bubble. Additionally, we find a velocity gradient that is consistent with the expansion of the Local Bubble, further supporting our methodology. Overall, we show that it is possible to use complementary information from dust depletion and dust extinction to build more detailed maps of ISM metal and dust distributions.

astro-ph.GA↗

arXiv2025

Studying the gaseous outskirts of galaxy groups with coherent Ly alpha absorption patterns

In this study, we explore the properties of diffuse intergalactic gas residing in the outskirts of the four nearby, low-mass galaxy groups NGC1052, NGC5866, NGC4631, and NGC3992 (all at cz<2000 km/s) beyond their group virial radii. Using archival ultraviolet absorption spectra of bright AGN observed with HST/COS, we search for HI Ly a absorption near the groups' recession velocities along 35 sightlines that pass the outer group medium (OGrM) at normalized impact parameters rho/Rvir=1-3. We derive HI column densities of the absorbers and constrain the physical conditions in the gas by using a hydrostatic toy model of the groups' gas environment and assuming photoionization. HI Ly absorption near the groups' recession velocities is detected along 19 sightlines with HI column densities in the range log N(HI)=12.50-14.34, implying a high OGrM detection rate of more than 50 percent. We transform this value into an incidence rate of OGrM absorbers per unit redshift of dN/dz=232pm58.This is 25 percent above the value derived for the general population of Ly a absorbers within z = 0 filaments and more than twice the value for the z = 0 Ly a forest. From the modeling, we obtain lower limits for the gas densities from log n_H=-5.00 to -3.72, comparable to densities found in the overall Ly a forest. Our study unveils a large cross section and overdensity of Ly a absorbers in the outskirts of these four nearby groups. Such an overdensity is in line with a previously proposed scenario, in which AGN feedback lifts gaseous material to large distances beyond the virial radius of groups into the OGrM. However, a larger survey of OGrM absorbers and a comparison with hydrodynamical simulations will be necessary to constrain the cosmological mass density of OGrM absorbers and pinpoint their role in cosmological structure formation and galaxy/group evolution.

astro-ph.GA↗

arXiv2025

Investigating chemical variations between interstellar gas clouds in the Solar neighbourhood

The interstellar medium (ISM) is a fundamental component of the Milky Way. Studying its chemical composition and the level of its chemical diversity gives us insight into the evolution of the Milky Way and the role of gas in the Galactic environment. In this paper, we use a novel simulation technique to model the distribution of total hydrogen between gas components, and therefore derive new constraints on the dust depletion and metallicity. We study individual gas components along the lines of sight towards eight bright O/B stars within 1.1 kpc of the Sun using high-resolution HST/STIS absorption spectra (R sim 114 000). We measure the level of dust depletion for these individual components and find components with higher levels of dust depletion compared to Milky Way sightlines in the literature. We find large ranges in the level of dust depletion among components along lines of sight, up to 1.19 dex. Although it is not possible to directly measure the metallicity of individual components due to the saturated and damped Ly-alpha line, we investigate possible metallicity ranges for individual gas components by exploring many different distributions of the total hydrogen gas between components. We select possible combinations of these gas fractions which produce the minimum metallicity difference between components, and for these cases we determine individual metallicities to accuracies that range between sim 0.1 to 0.4 dex. This work shows that full line-of-sight analyses wash out the level of diversity along lines of sight, and that component-by-component studies give a more in-depth understanding of the chemical intricacies of the interstellar medium.

astro-ph.GA↗

arXiv2021

Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity Clouds

We revisit the reliability of metallicity estimates of high velocity clouds with the help of hydrodynamical simulations. We quantify the effect of accretion and viewing angle on metallicity estimates derived from absorption lines. Model parameters are chosen to provide strong lower limits on cloud contamination by ambient gas. Consistent with previous results, a cloud traveling through a stratified halo is contaminated by ambient material to the point that <10% of its mass in neutral hydrogen consists of original cloud material. Contamination progresses nearly linearly with time, and it increases from head to tail. Therefore, metallicity estimates will depend on the evolutionary state of the cloud, and on position. While metallicities change with time by more than a factor of 10, well beyond observational uncertainties, most lines-of-sight range only within those uncertainties at any given time over all positions. Metallicity estimates vary with the cloud's inclination angle within observational uncertainties. The cloud survives the infall through the halo because ambient gas continuously condenses and cools in the cloud's wake and thus appears in the neutral phase. Therefore, the cloud observed at any fixed time is not a well-defined structure across time, since material gets constantly replaced. The thermal phases of the cloud are largely determined by the ambient pressure. Internal cloud dynamics evolve from drag gradients caused by shear instabilities, to complex patterns due to ram-pressure shielding, leading to a peloton effect, in which initially lagging gas can catch up to and even overtake the head of the cloud.

astro-ph.GA↗

arXiv2019

Tentative Detection of the Circumgalactic Medium of the Isolated Low-Mass Dwarf Galaxy WLM

We report a tentative detection of the circumgalactic medium (CGM) of WLM, an isolated, low-mass (log$M_*/M_\odot\approx7.6$), dwarf irregular galaxy in the Local Group (LG). We analyze an HST/COS archival spectrum of a quasar sightline (PHL2525), which is 45 kpc (0.5 virial radius) from WLM and close to the Magellanic Stream (MS). Along this sightline, two ion absorbers are detected in Si II, Si III, Si IV, C II, and C IV at velocities of $\sim$-220 km s$^{-1}$ (Component v-220) and $\sim$-150 km s$^{-1}$ (Component v-150). To identify their origins, we study the position-velocity alignment of the components with WLM and the nearby MS. Near the Magellanic longitude of PHL2525, the MS-related neutral and ionized gas moves at $\lesssim-190$ km s$^{-1}$, suggesting an MS origin for Component v-220, but not for Component v-150. Because PHL2525 passes near WLM and Component v-150 is close to WLM's systemic velocity ($\sim$-132 km s$^{-1}$), it is likely that Component v-150 arises from the galaxy's CGM. This results in a total Si mass in WLM's CGM of $M_{\rm Si}^{\rm CGM}\sim(0.2-1.0)\times10^5~M_\odot$ using assumption from other COS dwarf studies. Comparing $M_{\rm Si}^{\rm CGM}$ to the total Si mass synthesized in WLM over its lifetime ($\sim$1.3$\times10^5~M_\odot$), we find $\sim$3% is locked in stars, $\sim$6% in the ISM, $\sim$15%-77% in the CGM, and the rest ($\sim$14%-76%) is likely lost beyond the virial radius. Our finding resonates with other COS dwarf galaxy studies and theoretical predictions that low-mass galaxies can easily lose metals into their CGM due to stellar feedback and shallow gravitational potential.

astro-ph.GA↗

arXiv2018

Circumgalactic gas at its extreme: tidal gas streams around the Whale galaxy NGC 4631 explored with HST/COS

We present a detailed analysis of the absorption properties of one of the tidal gas streams around the Whale galaxy NGC4631 in the direction of the quasar 2MASSJ12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong HI Ly alpha absorption in the velocity range +550 to +800 km s^-1 related to gas from a NGC4631 tidal stream known as Spur 2. We measure a column density of log N(HI)=18.68pm0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21cm data. Metal absorption in Spur 2 is detected in the lines of OI, CII, SiII, and SiIII in a complex absorption pattern that reflects the multi-phase nature of the gas. We find that the average neutral gas fraction in Spur 2 towards 2MASSJ12421031+3214268 is only 14 percent. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10^9 M_sun. No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured OI/HI column-density ratio we determine an alpha abundance in Spur 2 of 0.13pm0.07 solar ([alpha/H]=-0.90pm 0.16), which is substantially lower than what is observed in the NGC4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC4631.

astro-ph.GA↗

arXiv2018

New constraints on the nature and origin of the Leading Arm of the Magellanic Stream

We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H_2) in the Leading Arm (region II, LAII) of the Magellanic Stream (MS) towards the Seyfert galaxy NGC 3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multi-phase absorber, delivering important constraints on the nature and origin of LAII. We derive a uniform, moderate alpha abundance in the two main absorber groups at +245 and +190 km s^-1 of alpha/H = 0.30 pm 0.05 solar, a low nitrogen abundance of N/H = 0.05 pm 0.01 solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These alpha, N, and dust abundances in LAII are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H_2 absorption, we determine a high thermal pressure of P/k = 1680 K cm^-3 in LAII, in line with the idea that LAII is located in the inner Milky Way halo at a z-height of <20 kpc where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario, in which LAII stems from the break-up of a metal- and dust-enriched progenitor cloud that was recently (200-500 Myr ago) stripped from the SMC.

astro-ph.GA↗

arXiv2017

Revealing the Ionization Properties of the Magellanic Stream using Optical Emission

The Magellanic Stream, a gaseous tail that trails behind the Magellanic Clouds, could replenish the Milky Way with a tremendous amount of gas if it reaches the Galactic disk before it evaporates into the halo. To determine how the Magellanic Stream's properties change along its length, we have conducted an observational study of the H-alpha emission, along with other optical warm ionized gas tracers, toward 39 sight lines. Using the Wisconsin H-alpha Mapper telescope, we detect H-alpha emission brighter than 30 - 50 mR in 26 of our 39 sight lines. This H-alpha emission extends more than 2-degree away from the HI emission. By comparing H-alpha and [OI] intensities, we find that regions with log NHI = 19.5 - 20.0 are 16 - 67% ionized. Most of the H-alpha intensities along the Magellanic Stream are much higher than expected if the primary ionization source is photoionization from Magellanic Clouds, the Milky Way, and the extragalactic background. We find that the additional contribution from self ionization through a "shock cascade" that results as the Stream plows through the halo might be sufficient to reproduce the underlying level of H-alpha emission along the Stream. In the sparsely sampled region below the South Galactic Pole, there exists a subset of sight lines with uncharacteristically bright emission, which suggest that gas is being ionized further by an additional source that could be a linked to energetic processes associated with the Galactic center.

astro-ph.GA↗

arXiv2017

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way's circumgalactic medium and the Local Group

To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).

astro-ph.GA↗

arXiv2016

An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies

Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening SiIII-selected absorbers and their relation to the circumgalactic medium (CGM) of galaxies at low redshift (z<=0.1), based on the analysis of UV absorption spectra along 303 extragalactic lines of sight obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). Along a total redshift path of Dz=24 we identify 69 intervening SiIII systems that all show associated absorption from other low and high ions. We derive a bias-corrected number density of dN/dz(SiIII)=2.5 for absorbers with column densities log N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section of the CGM around the local galaxy population and find excellent agreement between the model predictions and the observations. We further compare redshifts and positions of the absorbers with that of ~64,000 galaxies using archival galaxy-survey data. For the majority of the absorbers we identify possible host galaxies within 300 km/s of the absorbers and derive impact parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII absorbers and galaxies are highly correlated. Our study indicates that the majority of SiIII-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 per cent. From a detailed ionization model we estimate that diffuse gas in the CGM around galaxies, as traced by SiIII, contains substantially more baryonic mass than their neutral interstellar medium.

astro-ph.CO↗

arXiv2015

Large and small-scale structure of the Intermediate and High Velocity Clouds towards the LMC and SMC

We employ CaII K and NaI D interstellar absorption-line spectroscopy of early-type stars in the Large and Small Magellanic Clouds to investigate the large- and small-scale structure in foreground Intermediate and High Velocity Clouds (I/HVCs). These data include FLAMES-GIRAFFE CaII K observations of 403 stars in four open clusters, plus FEROS or UVES spectra of 156 stars in the LMC and SMC. The FLAMES observations are amongst the most extensive probes to date of CaII structures on 20 arcsec scales From the FLAMES data within a 0.5 degree field-of-view, the CaII K equivalent width in the I/HVC components towards three clusters varies by factors of >10. There are no detections of molecular gas in absorption at intermediate or high velocities, although molecular absorption is present at LMC and Galactic velocities towards some sightlines. The sightlines show variations in EW exceeding a factor 7 in CH+ towards NGC 1761 over scales of less than 10 arcminutes. The FEROS/UVES data show CaII K I/HVC absorption in $\sim$60 per cent of sightlines. No NaI D is found at non-Magellanic HVC velocities aside from a tentative detection towards the star LHA 120-S 93. The range in the CaII/NaI ratio in I/HVCs is from -0.45 to +1.5 dex, similar to previous measurements for I/HVCs. In ten sightlines we find CaII/OI ratios in I/HVC gas ranging from 0.2 to 1.5 dex below the solar value, indicating either dust or ionisation effects. In nine sightlines I/HVC gas is detected in both HI and CaII, and shows similar CaII/HI ratios to typical I/HVCs, and similar velocities, implying that in these sightlines the two elements form part of the same structure.

astro-ph.GA↗

arXiv2015

The large- and small-scale CaII K structure of the Milky Way from observations of Galactic and Magellanic sightlines

Aims: By utilising spectra of early-type stellar probes of known distances in the same region of the sky, the large and small-scale (pc) structure of the Galactic ISM can be investigated. This paper determines the variation in line strength of CaII at 3933.661 A, as a function of probe separation for a sample of stars, including many sightlines in the Magellanic Clouds. Methods: FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic & 4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column density and line width of IS Galactic Ca for a total of 657 stars, of which 443 are Magellanic sightlines. In each cluster there are 43-110 stars observed. Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic early-type stars are presented and combined with data from the literature to study the Ca column density/parallax relationship. Results: For the four Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K EW over transverse scales from 0.05-9 pc is found to vary by factors of 1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in the optically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, the min and max reduced EW for MW gas is found to lie in the range 35-125 mA & 30-160 mA for CaII K and NaI D, respectively. The range is consistent with a simple model of the ISM published by van Loon et al. (2009) consisting of spherical cloudlets of filling factor 0.3, although other geometries are not ruled out. Finally, the derived functional form for parallax and CaII column density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived parallax is 25 per cent lower than predicted by Megier et al. (2009) at a distance of 100 pc and 15% lower at a distance of 200 pc, reflecting inhomogeneity in the CaII distribution in the different sightlines studied.

astro-ph.GA↗

arXiv2015

The First Data Release of the KODIAQ Survey

We present and make publicly available the first data release (DR1) of the Keck Observatory Database of Ionized Absorption toward Quasars (KODIAQ) survey. The KODIAQ survey is aimed at studying galactic and circumgalactic gas in absorption at high-redshift, with a focus on highly-ionized gas traced by OVI, using the HIRES spectrograph on the Keck-I telescope. KODIAQ DR1 consists of a fully-reduced sample of 170 quasars at 0.29 < z_em < 5.29 observed with HIRES at high resolution (36,000 <= R <= 103,000) between 2004 and 2012. DR1 contains 247 spectra available in continuum normalized form, representing a sum total exposure time of ~1.6 megaseconds. These co-added spectra arise from a total of 567 individual exposures of quasars taken from the Keck Observatory Archive (KOA) in raw form and uniformly processed using a HIRES data reduction package made available through the XIDL distribution. DR1 is publicly available to the community, housed as a higher level science product at the KOA. We will provide future data releases that make further QSOs, including those with pre-2004 observations taken with the previous-generation HIRES detectors.

astro-ph.CO↗

arXiv2013

High-resolution absorption spectroscopy of the circumgalactic medium of the Milky Way

In this article we discuss the importance of high-resolution absorption spectroscopy for our understanding of the distribution and physical nature of the gaseous circumgalactic medium (CGM) that surrounds the Milky Way. Observational and theoretical studies indicate a high complexity of the gas kinematics and an extreme multi-phase nature of the CGM in low-redshift galaxies. High-precision absorption-line measurements of the Milky Way's gas environment thus are essential to explore fundamental parameters of circumgalactic gas in the local Universe, such as mass, chemical composition, and spatial distribution. We shortly review important characteristics of the Milky Way's CGM and discuss recent results from our multi-wavelength observations of the Magellanic Stream. Finally, we discuss the potential of studying the warm-hot phase of the Milky Way's CGM by searching for extremely weak [FeX] l6374.5 and [FeIVX] l5302.9 absorption in optical QSO spectra.

astro-ph.GA↗

arXiv2013

The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1

We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM) about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N(H I) selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N(H I) in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z<1 galaxies.

astro-ph.CO↗

arXiv2012

Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310

We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII detected in UVES spectra of GRB 080310, with the afterglow radiation exciting and ionizing the interstellar medium in the host galaxy at a redshift of z=2.42743. To estimate the rest-frame afterglow brightness as a function of time, we use a combination of the optical VRI photometry obtained by the RAPTOR-T telescope array -- which are presented in this paper -- and Swift's X-Ray Telescope observations. Excitation alone, which has been successfully applied for a handful of other GRBs, fails to describe the observed column-density evolution in the case of GRB 080310. Inclusion of ionization is required to explain the column-density decrease of all observed FeII levels (including the ground state 6D9/2) and increase of the FeIII 7S3 level. The large population of ions in this latter level (up to 10% of all FeIII) can only be explained through ionization of FeII, whereby a large fraction of the ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan codes -- initially populate the 7S3 level of FeIII rather than the ground state. This channel for producing a significant FeIII 7S3 level population may be relevant for other objects in which absorption lines from this level -- the UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This provides conclusive evidence for time-variable ionization in the circumburst medium, which to date has not been convincingly detected. However, the best-fit distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar to GRB-absorber distance estimates for GRBs without any evidence for ionization. We find that the presence of time-varying ionization in GRB 080310 is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2) and the low HI column density (log N(HI)=18.7). [abridged]

astro-ph.CO↗

arXiv2012

Rapid-response mode VLT/UVES spectroscopy of super iron-rich gas exposed to GRB 080310. Evidence of ionization in action and episodic star formation in the host

We analyse high-resolution near-UV and optical spectra of the afterglow of GRB 080310, obtained with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT/UVES), to investigate the circumburst environment and the interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLT rapid-response mode (RRM) enabled the observations to start only 13 minutes after the Swift trigger and a series of four exposures to be collected before dawn. A low neutral-hydrogen column-density (log N (HI) = 18.7) is measured at the host-galaxy redshift of z = 2.42743. At this redshift, we also detect a large number of resonance ground-state absorption lines (e.g., CII, MgII, AlII, SiII, CrII, CIV, SiIV), as well as time-varying absorption from the fine-structure levels of FeII. Time-varying absorption from a highly excited FeIII energy level (7S3), giving rise to the so-called UV34 line triplet, is also detected, for the first time in a GRB afterglow. The CrII ground-state and all observed FeII energy levels are found to depopulate with time, whilst the FeIII 7S3 level is increasingly populated. This absorption-line variability is clear evidence of ionization by the GRB, which is for the first time conclusively observed in a GRB afterglow spectrum. We derive ionic column densities at each epoch of observations by fitting absorption lines with a four-component Voigt-profile model. We perform CLOUDY photo-ionization modelling of the expected pre-burst ionic column densities, to estimate that, before the onset of the burst, [C/H] = -1.3 \pm 0.2, [O/H] < -0.8, [Si/H] = -1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 0.2 for the integrated line profile, indicating strong overabundances of iron and chromium. For one of the components, we observe even more extreme ratios of [Si/Fe] \leq -1.47 and [C/Fe] \leq -1.74. [abridged]

astro-ph.CO↗

arXiv2012

High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way

High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found at high galactic latitudes, may play an important role in the evolution of the Milky Way. The extent of this role depends sensitively on their distances and total sky covering factor. We search for HVC absorption in HST high resolution ultraviolet spectra of a carefully selected sample of 133 AGN using a range of atomic species in different ionization stages. This allows us to identify neutral, weakly ionized, or highly ionized HVCs over several decades in HI column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90 km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is essentially complete, i.e., an undetected population of HVCs with extremely low N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm that the predominantly ionized HVCs contain at least as much mass as the traditional HI HVCs and show that large HI HVC complexes have generally ionized envelopes extending far from the HI contours. There are also large regions of the Galactic sky that are covered with ionized high-velocity gas with little HI emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170 km/s drawn from the AGN and stellar samples are similar. This confirms that these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs drops with decreasing vertical height, which is consistent with HVCs being decelerated or disrupted as they fall to the Milky Way disk. The HVCs with |v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and its leading arm at b>0 as well as other large known HI complexes. Therefore there is no evidence in the Local Group that any galaxy shows a population of HVCs extending much farther away than 50 kpc from its host, except possibly for those tracing remnants of galaxy interaction.

astro-ph.GA↗