arXiv subjects

Sean D. Johnson

Publications and source records attributed to Sean D. Johnson.

69 records · Page 4Linked to original sources

arXiv2019

Ultraviolet Perspectives on Diffuse Gas in the Largest Cosmic Structures

The past decade has seen an explosion of discoveries and new insights into the diffuse gas within galaxies, galaxy clusters, and the filaments composing the Cosmic Web. A new decade will bring fresh opportunities to further this progress towards developing a comprehensive view of the composition, thermal state, and physical processes of diffuse gas in the Universe. Ultraviolet (UV) spectroscopy, probing diffuse 10^4-10^6 K gas at high spectral resolution, is uniquely poised to (1) witness environmental galaxy quenching processes in action, such as strangulation and tidal- and ram-pressure stripping, (2) directly account for the baryon content of galaxy clusters in the cold-warm (T<10^6 K) gas, (3) determine the phase structure and kinematics of gas participating in the equilibrium-regulating exchange of energy at the cores of galaxy clusters, and (4) map cold streams and filaments of the Cosmic Web that feed galaxies and clusters. With a substantial UV undertaking beyond the Hubble Space Telescope, all of the above would be achievable over the entire epoch of galaxy cluster formation. Such capabilities, coupled with already-planned advancements at other wavelengths, will transform extragalactic astronomy by revealing the dominant formation and growth mechanisms of gaseous halos over the mass spectrum, settling the debate between early- and late-time metal enrichment scenarios, and revealing how the ecosystems in which galaxies reside ultimately facilitate their demise.

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arXiv2019

Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances

We present a systematic investigation of the circumgalactic medium (CGM) within projected distances d<160 kpc of luminous red galaxies (LRGs). The sample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar mass M_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions, including the full HI Lyman series and various ionic metal transitions. By comparing the relative abundances of different ions in individually-matched components, we show that cool gas (T~1e4 K) density and metallicity can vary by more than a factor of ten in in an LRG halo. Specifically, metal-poor absorbing components with <1/10 solar metallicity are seen in 50% of the LRG halos, while gas with solar and super-solar metallicity is also common. These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos. We calculate the total surface mass density of cool gas, Σ_cool, by applying the estimated ionization fraction corrections to the observed HI column densities. The radial profile of Σ_cool is best-described by a projected Einasto profile of slope α=1 and scale radius r_s=48 kpc. We find that typical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x1e10 M_sun at d<160 kpc (or as much as 4x1e10 M_sun at d<500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization OVI and low-ionization absorption species exhibit distinct velocity profiles, highlighting their different physical origins. We discuss the implications of our findings for the origin and fate of cool gas in LRG halos.

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arXiv2019

Characterizing circumgalactic gas around massive ellipticals at z~0.4 III. The galactic environment of a chemically-pristine Lyman limit absorber

This paper presents a study of the galactic environment of a chemically-pristine (<0.6% solar metallicity) Lyman Limit system (LLS) discovered along the sightline toward QSO SDSSJ135726.27+043541.4 (zQSO=1.233) at projected distance d=126 physical kpc (pkpc) from a luminous red galaxy (LRG) at z=0.33. Combining deep Hubble Space Telescope images, MUSE integral field spectroscopic data, and wide-field redshift survey data has enabled an unprecedented, ultra-deep view of the environment around this LRG-LLS pair. A total of 12 galaxies, including the LRG, are found at d<~400 pkpc and line-of-sight velocity dv<600 km/s of the LLS, with intrinsic luminosity ranging from 0.001L* to 2L* and a corresponding stellar mass range of Mstar=10^{7-11} Msun. All 12 galaxies contribute to a total mass of Mstar=1.6e11 Msun with ~80% contained in the LRG. The line-of-sight velocity dispersion of these galaxies is found to be σ_group=230 km/s with the center of mass at d_group=118 pkpc and line-of-sight velocity offset of Δv_group=181 km/s from the LLS. Three of these are located at d<~100 pkpc from the LLS, and they are all faint with intrinsic luminosity <0.02 L* and gas phase metallicity of ~10% solar in their interstellar medium. The disparity in the chemical enrichment level between the LLS and the group members suggests that the LLS originates in infalling intergalactic medium and that parts of the intergalactic gas near old and massive galaxies can still remain chemically pristine through the not too distant past.

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arXiv2018

Galaxy and Quasar Fueling Caught in the Act from the Intragroup to the Interstellar Medium

We report the discovery of six spatially extended (10-100 kpc) line-emitting nebulae in the z=0.57 galaxy group hosting PKS0405-123, one of the most luminous quasars at z<1. The discovery is enabled by the Multi Unit Spectroscopic Explorer (MUSE) and provides tantalizing evidence connecting large-scale gas streams with nuclear activity on scales of <10 proper kpc (pkpc). One of the nebulae exhibits a narrow, filamentary morphology extending over 50 pkpc toward the quasar with narrow internal velocity dispersion (50 km/s) and is not associated with any detected galaxies, consistent with a cool intragroup medium (IGrM) filament. Two of the nebulae are 10 pkpc North and South of the quasar with tidal arm like morphologies. These two nebulae, along with a continuum emitting arm extending 60 pkpc from the quasar are signatures of interactions which are expected to redistribute angular momentum in the host interstellar medium (ISM) to facilitate star formation and quasar fueling in the nucleus. The three remaining nebulae are among the largest and most luminous [O III] emitting `blobs' known (1400-2400 pkpc^2) and correspond both kinematically and morphologically with interacting galaxy pairs in the quasar host group, consistent with arising from stripped ISM rather than large-scale quasar outflows. The presence of these large- and small-scale nebulae in the vicinity of a luminous quasar bears significantly on the effect of large-scale environment on galaxy and black hole fueling, providing a natural explanation for the previously known correlation between quasar luminosity and cool circumgalactic medium (CGM).

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arXiv2018

Characterizing Circumgalactic Gas around Massive Ellipticals at z ~ 0.4 I. Initial Results

We present a new Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) absorption-line survey to study halo gas around 16 luminous red galaxies (LRGs) at z=0.21-0.55. The LRGs are selected uniformly with stellar mass Mstar>1e11 Msun and no prior knowledge of the presence/absence of any absorption features. Based on observations of the full Lyman series, we obtain accurate measurements of neutral hydrogen column density N(HI) and find that high-N(HI) gas is common in these massive quiescent halos with a median of = 16.6 at projected distances d<~160 kpc. We measure a mean covering fraction of optically-thick gas with log N(HI)>~17.2 of LLS=0.44^{+0.12}_{-0.11} at d<~160 kpc and LLS=0.71^{+0.11}_{-0.20} at d<~100 kpc. The line-of-sight velocity separations between the HI absorbing gas and LRGs are characterized by a mean and dispersion of =29 km/s and σ_v_{gas-gal}=171 km/s. Combining COS FUV and ground-based echelle spectra provides an expanded spectral coverage for multiple ionic transitions, from low-ionization MgII and SiII, to intermediate ionization SiIII and CIII, and to high-ionization OVI absorption lines. We find that intermediate ions probed by CIII and SiIII are the most prominent UV metal lines in LRG halos with a mean covering fraction of _{0.1}=0.75^{+0.08}_{-0.13} for W(977)>=0.1 Ang at d<160 kpc, comparable to what is seen for CIII in L* and sub-L* star-forming and red galaxies but exceeding MgII or OVI in quiescent halos. The COS-LRG survey shows that massive quiescent halos contain widespread chemically-enriched cool gas and that little distinction between LRG and star-forming halos is found in their HI and CIII content.

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arXiv2018

Does circumgalactic OVI trace low-pressure gas beyond the accretion shock? Clues from HI and low-ion absorption, line kinematics, and dust extinction

Large OVI columns are observed around star-forming, low-redshift ~L* galaxies, with a dependence on impact parameter indicating that most O^5+ particles reside beyond half the halo virial radius (>~100 kpc). In order to constrain the nature of the gas traced by OVI, we analyze additional observables of the outer halo, namely HI to OVI column ratios of 1-10, an absence of low-ion absorption, a mean differential extinction of E(B-V)~10^-3, and a linear relation between OVI column and velocity width. We contrast these observations with two physical scenarios: (1) OVI traces high-pressure (~30 cm^-3 K) collisionally-ionized gas cooling from a virially-shocked phase, and (2) OVI traces low-pressure (<~1 cm^-3 K) gas beyond the accretion shock, where the gas is in ionization and thermal equilibrium with the UV background. We demonstrate that the high-pressure scenario requires multiple gas phases to explain the observations, and a large deposition of energy at >~100 kpc to offset the energy radiated by the cooling gas. In contrast, the low-pressure scenario can explain all considered observations with a single gas phase in thermal equilibrium, provided that the baryon overdensity is comparable to the dark-matter overdensity, and that the gas is enriched to >~Z_sun/3 with an ISM-like dust-to-metal ratio. The low-pressure scenario implies that OVI traces a cool flow with mass flow rate of ~5 M_sun yr^-1, comparable to the star formation rate of the central galaxies. The OVI line widths are consistent with the velocity shear expected within this flow. The low-pressure scenario predicts a bimodality in absorption line ratios at ~100 kpc, due to the pressure jump across the accretion shock.

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arXiv2017

The extent of chemically enriched gas around star-forming dwarf galaxies

Supernova driven winds are often invoked to remove chemically enriched gas from dwarf galaxies to match their low observed metallicities. In such shallow potential wells, outflows may produce massive amounts of enriched halo gas (circum-galactic medium or CGM) and pollute the intergalactic medium (IGM). Here, we present a survey of the CGM and IGM around 18 star-forming field dwarfs with stellar masses of $\log\,M_*/M_\odot\approx8-9$ at $z\approx0.2$. Eight of these have CGM probed by quasar absorption spectra at projected distances, $d$, less than the host virial radius, $R_{\rm h}$. Ten are probed in the surrounding IGM at $d/R_{\rm h}=1-3$. The absorption measurements include neutral hydrogen, the dominant silicon ions for diffuse cool gas ($T\sim10^4$ K; Si II, Si III, and Si IV), moderately ionized carbon (C IV), and highly ionized oxygen (O VI). Metal absorption from the CGM of the dwarfs is less common and $\approx4\times$ weaker compared to massive star-forming galaxies though O VI absorption is still common. None of the dwarfs probed at $d/R_{\rm h}=1-3$ have definitive metal-line detections. Combining the available silicon ions, we estimate that the cool CGM of the dwarfs accounts for only $2-6\%$ of the expected silicon budget from the yields of supernovae associated with past star-formation. The highly ionized O VI accounts for $\approx8\%$ of the oxygen budget. As O VI traces an ion with expected equilibrium ion fractions of $\lesssim 0.2$, the highly ionized CGM may represent a significant metal reservoir even for dwarfs not expected to maintain gravitationally shock heated hot halos.

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arXiv2017

Gauging Metallicity of Diffuse Gas Under An Uncertain Ionizing Radiation Field

Gas metallicity is a key quantity used to determine the physical conditions of gaseous clouds in a wide range of astronomical environments, including interstellar and intergalactic space. In particular, considerable effort in circumgalactic medium (CGM) studies focuses on metallicity measurements, because gas metallicity serves as a critical discriminator for whether the observed heavy ions in the CGM originate in chemically-enriched outflows or in more chemically-pristine gas accreted from the intergalactic medium. However, because the gas is ionized, a necessary first step in determining CGM metallicity is to constrain the ionization state of the gas which, in addition to gas density, depends on the ultraviolet background radiation field (UVB). While it is generally acknowledged that both the intensity and spectral slope of the UVB are uncertain, the impact of an uncertain spectral slope has not been properly addressed in the literature. This Letter shows that adopting a different spectral slope can result in an order of magnitude difference in the inferred CGM metallicity. Specifically, a harder UVB spectrum leads to a higher estimated gas metallicity for a given set of observed ionic column densities . Therefore, such systematic uncertainties must be folded into the error budget for metallicity estimates of ionized gas. An initial study shows that empirical diagnostics are available for discriminating between hard and soft ionizing spectra. Applying these diagnostics helps reduce the systematic uncertainties in CGM metallicity estimates.

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arXiv2016

Magellan LDSS3 emission confirmation of galaxies hosting metal-rich Lyman-alpha absorption systems

Using the Low Dispersion Survey Spectrograph 3 at the Magellan II Clay Telescope, we target {candidate absorption host galaxies} detected in deep optical imaging {(reaching limiting apparent magnitudes of 23.0-26.5 in $g, r, i,$ and $z$ filters) in the fields of three QSOs, each of which shows the presence of high metallicity, high $N_{\rm HI}$ absorption systems in their spectra (Q0826-2230: $z_{abs}$=0.9110, Q1323-0021: $z_{abs}=0.7160$, Q1436-0051: $z_{abs}=0.7377, 0.9281$). We confirm three host galaxies {at redshifts 0.7387, 0.7401, and 0.9286} for two of the Lyman-$α$ absorption systems (one with two galaxies interacting). For these systems, we are able to determine the star formation rates (SFRs); impact parameters (from previous imaging detections); the velocity shift between the absorption and emission redshifts; and, for one system, also the emission metallicity.} Based on previous photometry, we find these galaxies have L$>$L$^{\ast}$. The [O II] SFRs for these galaxies are in the range $11-25$ M$_{\odot}$ yr$^{-1}$ {(uncorrected for dust)}, while the impact parameters lie in the range $35-54$ kpc. {Despite the fact that we have confirmed galaxies at 50 kpc from the QSO, no gradient in metallicity is indicated between the absorption metallicity along the QSO line of sight and the emission line metallicity in the galaxies.} We confirm the anti-correlation between impact parameter and $N_{\rm HI}$ from the literature. We also report the emission redshift of five other galaxies: three at $z_{em}>z_{QSO}$, and two (L$<$L$^{\ast}$) at $z_{em}<z_{QSO}$ not corresponding to any known absorption systems.

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arXiv2015

Characterizing the Chemically-Enriched Circumgalactic Medium of ~38000 Luminous Red Galaxies in SDSS DR12

We report a definitive detection of chemically-enriched cool gas around massive, quiescent galaxies at z~0.4-0.7. The result is based on a survey of 37621 luminous red galaxy (LRG)-QSO pairs in SDSS DR12 with projected distance d<500 kpc. The LRGs are characterized by a predominantly old (age>~1Gyr) stellar population with 13% displaying [OII] emission features and LINER-like spectra. Both passive and [OII]-emitting LRGs share the same stellar mass distribution with a mean of ~11.4 and a dispersion of 0.2 dex. Both LRG populations exhibit associated strong MgII absorbers out to d<500 kpc. The mean gas covering fraction at d<~120 kpc is _MgII > 15% and declines quickly to _MgII ~ 5% at d<~500 kpc. No clear dependence on stellar mass is detected for the observed MgII absorption properties. The observed velocity dispersion of MgII absorbing gas relative to either passive or [OII]-emitting LRGs is merely 60% of what is expected from virial motion in these massive halos. While no apparent azimuthal dependence is seen for _MgII around passive LRGs at all radii, a modest enhancement in _MgII is detected along the major axis of [OII]-emitting LRGs at d<50 kpc. The suppressed velocity dispersion of MgII absorbing gas around both passive and [OII]-emitting LRGs, together with an elevated _MgII along the major axis of [OII]-emitting LRGs at d<50 kpc, provides important insights into the origin of the observed chemically-enriched cool gas in LRG halos. We consider different scenarios and conclude that the observed MgII absorbers around LRGs are best-explained by a combination of cool clouds formed in thermally unstable LRG halos and satellite accretion through filaments.

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arXiv2015

On the origin of excess cool gas in quasar host halos

Previous observations of quasar host halos at z=2 have uncovered large quantities of cool gas that exceed what is found around inactive galaxies of both lower and higher masses. To better understand the source of this excess cool gas, we compiled an exhaustive sample of 195 quasars at z=1 with constraints on chemically enriched, cool gas traced by MgII absorption in background quasar spectra from the Sloan Digital Sky Survey. This quasar sample spans a broad range of luminosities from Lbol=10^44.4 to 10^46.8 erg/s and allows an investigation of whether halo gas properties are connected with quasar properties. We find a strong correlation between luminosity and cool gas covering fraction. In particular, low-luminosity quasars exhibit a mean gas covering fraction comparable to inactive galaxies of similar masses, but more luminous quasars exhibit excess cool gas approaching what is reported previously at z=2. Moreover, 30-40% of the MgII absorption occurs at radial velocities of |v|>300 km/s from the quasar, inconsistent with gas bound to a typical quasar host halo. The large velocity offsets and observed luminosity dependence of the cool gas near quasars can be explained if the gas arises from: (1) neighboring halos correlated through large-scale structure at Mpc scales, (2) feedback from luminous quasars, or (3) debris from the mergers thought to trigger luminous quasars. The first of these scenarios is in tension with the lack of correlation between quasar luminosity and clustering while the latter two make distinct predictions that can be tested with additional observations.

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arXiv2015

On the possible environmental effect in distributing heavy elements beyond individual gaseous halos

We present a study of extended galaxy halo gas through HI and OVI absorption over two decades in projected distance at $z\approx0.2$. The study is based on a sample of $95$ galaxies from a highly complete ($ > 80\%$) survey of faint galaxies ($L > 0.1L_*$) with archival quasar absorption spectra and $53$ galaxies from the literature. A clear anti-correlation is found between HI (OVI) column density and virial radius normalized projected distance, $d/R_{\rm h}$. Strong HI (OVI) absorption systems with column densities greater than $10^{14.0}$ ($10^{13.5}$) cm$^{-2}$ are found for $48$ of $54$ ($36$ of $42$) galaxies at $d < \,R_{\rm h}$ indicating a mean covering fraction of $\langleκ_{\rm HI}\rangle=0.89$ ($\langleκ_{\rm OVI}\rangle=0.86$). OVI absorbers are found at $d\approx R_{\rm h}$, beyond the extent observed for lower ionization species. At $d/R_{\rm h}=1-3$ strong HI (OVI) absorption systems are found for only $7$ of $43$ ($5$ of $34$) galaxies ($\langleκ_{\rm HI}\rangle=0.16$ and $\langleκ_{\rm OVI}\rangle=0.15$). Beyond $d=3\,R_{\rm h}$, the HI and OVI covering fractions decrease to levels consistent with coincidental systems. The high completeness of the galaxy survey enables an investigation of environmental dependence of extended gas properties. Galaxies with nearby neighbors exhibit a modest increase in OVI covering fraction at $d>R_{\rm h}$ compared to isolated galaxies ($κ_{\rm OVI}\approx0.13$ versus $0.04$) but no excess HI absorption. These findings suggest that environmental effects play a role in distributing heavy elements beyond the enriched gaseous halos of individual galaxies. Finally, we find that differential HI and OVI absorption between early- and late-type galaxies continues from $d < R_{\rm h}$ to $d\approx3\,R_{\rm h}$.

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arXiv2013

Discovery of a transparent sightline at ρ < 20 kpc from an interacting pair of galaxies

We report the discovery of a transparent sightline at projected distances of ρ < 20 kpc to an interacting pair of mature galaxies at z = 0.12. The sightline of the UV-bright quasar PG1522+101 at z = 1.328 passes at ρ = 11.5 kpc from the higher-mass galaxy (M_* = 10^10.6 M_Sun) and ρ = 20.4 kpc from the lower-mass one (M_* = 10^10.0 M_Sun). The two galaxies are separated by 9 kpc in projected distance and 30 km/s in line-of-sight velocity. Deep optical images reveal tidal features indicative of close interactions. Despite the small projected distances, the quasar sightline shows little absorption associated with the galaxy pair with a total HI column density it no greater than log N(HI) = 13.65. This limiting HI column density is already two orders-of-magnitude less than what is expected from previous halo gas studies. In addition, we detect no heavy-element absorption features associated with the galaxy pair with 3-σ limits of log N(MgII) < 12.2 and log N(OVI) < 13.7. The probability of seeing such little absorption in a sightline passing at a small projected distance from two non-interacting galaxies is 0.2%. The absence of strong absorbers near the close galaxy pair suggests that the cool gas reservoirs of the galaxies have been significantly depleted by the galaxy interaction. These observations therefore underscore the potential impact of galaxy interactions on the gaseous halos around galaxies.

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arXiv2013

Spatially Resolved Velocity Maps of Halo Gas Around Two Intermediate-redshift Galaxies

Absorption-line spectroscopy of multiply-lensed QSOs near a known foreground galaxy provides a unique opportunity to go beyond the traditional one-dimensional application of QSO probes and establish a crude three-dimensional (3D) map of halo gas around the galaxy that records the line-of-sight velocity field at different locations in the gaseous halo. Two intermediate-redshift galaxies are targeted in the field around the quadruply-lensed QSO HE0435-1223 at redshift z=1.689, and absorption spectroscopy along each of the lensed QSOs is carried out in the vicinities of these galaxies. One galaxy is a typical, star-forming L* galaxy at z=0.4188 and projected distance of rho=50 kpc from the lensing galaxy. The other is a super-L* barred spiral at z=0.7818 and rho=33 kpc. Combining known orientations of the quadruply-lensed QSO to the two foreground galaxies with the observed MgII absorption profiles along individual QSO sightlines has for the first time led to spatially resolved kinematics of tenuous halo gas on scales of 5-10 kpc at z>0.2. A MgII absorber is detected in every sightline observed through the halos of the two galaxies, and the recorded absorber strength is typical of what is seen in previous close QSO--galaxy pair studies. While the multi-sightline study confirms the unity covering fraction of MgII absorbing gas at rho < 50 kpc from star-forming disks, the galaxies also present two contrasting examples of complex halo gas kinematics. Different models, including a rotating disk, collimated outflows, and gaseous streams from either accretion or tidal/ram-pressure stripping, are considered for comparisons with the absorption-line observations, and infalling streams/stripped gas of width >~ 10 kpc are found to best describe the observed gas kinematics across multiple sightlines.

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arXiv2013

Probing the IGM-galaxy connection at z<0.5 II. New insights into the galaxy environments of OVI absorbers in PKS 0405-123

We present new absorption-line analysis and new galaxy survey data obtained for the field around PKS0405-123 at z=0.57. Combining previously known OVI absorbers with new identifications in the higher S/N UV spectra obtained with COS, we have established a sample of 7 OVI absorbers and 12 individual components at z=0.0918-0.495 along the sightline toward PKS0405-123. We complement the available UV absorption spectra with galaxy survey data that reach 100% completeness at projected distances rho<200 kpc of the quasar sightline for galaxies as faint as 0.1L* (0.2L*) out to redshifts of z=0.35 (z=0.5). The high level of completeness achieved at faint magnitudes by our survey reveals that OVI absorbers are closely associated with gas-rich environments containing at least one low-mass, emission-line galaxy. An intriguing exception is a strong OVI system at z=0.183 that does not have a galaxy found at rho<4 Mpc, and our survey rules out the presence of any galaxies of L>0.04L* at rho<250 kpc and any galaxies of L>0.3L* at rho<1 Mpc. We further examine the galactic environments of OVI absorbers and those "Lya-only" absorbers with HI column density log N(HI)>13.6 and no detectable OVI absorption features. The Lya-only absorbers serve as a control sample in seeking the discriminating galactic features that result in the observed OVI absorbing gas at large galactic radii. We find a clear distinction in the radial profiles of mean galaxy surface brightness around different absorbers. Specifically, OVI absorbers are found to reside in regions of higher mean surface brightness at rho<500 kpc (+5 mag Mpc^-2 relative to the background), while only a mild increase in galaxy surface brightness is seen at small rho around Lya-only absorbers (+2 mag Mpc^-2). The additional insights gained from our deep galaxy survey demonstrates the need to probe to low luminosities to better understand absorbing systems.

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