arXiv subjects

J. Xavier Prochaska

Publications and source records attributed to J. Xavier Prochaska.

At least 271 records · Page 15Linked to original sources

arXiv2016

The UV-bright Quasar Survey (UVQS): DR1

We present the first data release (DR1) from our UV-bright Quasar Survey (UVQS) for new $z \sim 1$ active galactic nuclei (AGN) across the sky. Using simple GALEX UV and WISE near-IR color selection criteria, we generated a list of 1450 primary candidates with $FUV < 18.5$ mag. We obtained discovery spectra, primarily on 3m-class telescopes, for 1040 of these candidates and confirmed 86% as AGN with redshifts generally at $z>0.5$. Including a small set of observed secondary candidates, we report the discovery of 217 AGN with $FUV < 18$ mag that had no previously reported spectroscopic redshift. These are excellent potential targets for UV spectroscopy before the end of the {\it Hubble Space Telescope} mission. The main data products are publicly released through the Mikulski Archive for Space Telescopes.

astro-ph.GA↗

arXiv2016

The HI Content of the Universe over the Past 10 Gyrs

We use the Hubble Space Telescope (HST) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Ly-alpha absorbers (DLAs) at low redshift (z < 1.6). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path, dz = 123.3 or an absorption path, dX = 229.7. Within this survey path, we identify 4 DLAs, defined as absorbers with HI column density N(HI) >= 10^20.3cm-2, which implies an incidence per absorption length, l(X)= 0.017(+0.014-0.008) at a median survey path redshift of z=0.623. While our estimate of l(X) is lower than earlier estimates at z ~ 0 from HI 21cm emission studies, the results are consistent within the measurement uncertainties. Our dataset is too small to properly sample the N(HI) frequency distribution function f(N(HI),X), but the observed distribution agrees with previous estimates at z > 2. Adopting the z > 2 shape of f(N(HI),X), we infer an HI mass density at z ~ 0.6 of rho_HI = 0.25(+0.20-0.12) x 10^8 Msol Mpc-3. This is significantly lower than previous estimates from targeted DLA surveys with the HST, but consistent with results from low-z HI 21cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyrs.

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arXiv2016

An Ultraviolet Spectrum of the Tidal Disruption Flare ASASSN-14li

We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $Δv = -(250$-400) km s$^{-1}$. Due both to this velocity offset and the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.

astro-ph.HE↗

arXiv2015

MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology

Modern cosmology predicts that a galaxy overdensity is associated to a large reservoir of the intergalactic gas, which can be traced by the Ly$α$ forest absorption. We have undertaken a systematic study of the relation between Coherently Strong intergalactic Ly$α$ Absorption systems (CoSLAs), which have highest optical depth ($τ$) in $τ$ distribution, and mass overdensities on the scales of $\sim$ 10 - 20 $h^{-1}$ comoving Mpc. On such large scales, our cosmological simulations show a strong correlation between the effective optical depth ($τ_{\rm{eff}}$) of the CoSLAs and the 3-D mass overdensities. In moderate signal-to-noise spectra, however, the profiles of CoSLAs can be confused with high column density absorbers. For $z>2.6$, where the corresponding Ly$β$ is redshifted to the optical, we have developed the technique to differentiate between these two alternatives. We have applied this technique to SDSS-III quasar survey at $z = 2.6$ - 3.3, and we present a sample of five CoSLA candidates with $τ_{\rm{eff}}$ on 15 $h^{-1}$ Mpc greater than $4.5\times$ the mean optical depth. At lower redshifts of $z < 2.6$, where the background quasar density is higher, the overdensity can be traced by intergalactic absorption groups using multiple sight lines. Our overdensity searches fully utilize the current and next generation of Ly$α$ forest surveys which cover a survey volume of $> (1\ h^{-1}$ Gpc)$^3$. In addition, systems traced by CoSLAs will build a uniform sample of the most massive overdensities at $z > 2$ to constrain the models of structure formation, and offer a unique laboratory to study the interactions between galaxy overdensities and the intergalactic medium.

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arXiv2015

The Bright Symbiotic Mira EF Aquilae

An incidental spectrum of the poorly studied long period variable EF Aquilae shows [O III] emission indicative of a symbiotic star. Strong GALEX detections in the UV reinforce this classification, providing overt evidence for the presence of the hot subluminous companion. Recent compilations of the photometric behavior strongly suggest that the cool component is a Mira variable. Thus EF Aql appears to be a member of the rare symbiotic Mira subgroup.

astro-ph.SR↗

arXiv2015

The physical properties of z>2 Lyman limit systems: new constraints for feedback and accretion models

We study the physical properties of a homogeneous sample of 157 optically-thick absorption line systems at redshifts ~1.8-4.4, selected from a high-dispersion spectroscopic survey of Lyman limit systems (LLSs). By means of multiple ionisation models and Bayesian techniques, we derive the posterior probability distribution functions for the density, metallicity, temperature, and dust content of the absorbing gas. We find that z>2 LLSs are highly ionised with ionisation parameters between -3 2 are characterised by a broad unimodal distribution over >4 orders of magnitude, with a peak at log Z/Zsun~-2. LLSs are metal poor, significantly less enriched than DLAs, with ~70% of the metallicity PDF below log Z/Zsun<-1.5. The median metallicity of super LLSs with log N(HI)>19 rapidly evolves with redshift, with a ten-fold increase between z~2.1-3.6 (~1.5 Gyr). Based on this sample, we find that LLSs at z=2.5-3.5 account for ~15% of all the metals produced by UV-selected galaxies. The implications for theories of cold gas accretion and metal ejection from galaxies are also discussed.

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arXiv2015

Towards the statistical detection of the warm-hot intergalactic medium in inter-cluster filaments of the cosmic web

Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low-z, a significant fraction of the baryons will be shock-heated to $T \sim 10^{5}-10^{7}$K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable QSO sightline that passes within $Δd = 3$ Mpc from the 7 inter-cluster axes connecting 7 independent cluster-pairs at redshifts $0.1 \le z \le 0.5$. We find tentative excesses of total HI, narrow HI (NLA; Doppler parameters $b<50$ km/s), broad HI (BLA; $b \ge 50$ km/s) and OVI absorption lines within rest-frame velocities of $Δv \lesssim 1000$ km/s from the cluster-pairs redshifts, corresponding to $\sim 2$, $\sim 1.7$, $\sim 6$ and $\sim 4$ times their field expectations, respectively. Although the excess of OVI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find that the covering fractions, $f_c$, of BLAs close to cluster-pairs are $\sim 4-7$ times higher than the random expectation (at the $\sim 2 σ$ c.l.), whereas the $f_c$ of NLAs and OVI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster-pairs may be a signature of the WHIM in inter-cluster filaments. By extending the present analysis to tens of sightlines our experiment offers a promising route to detect the WHIM.

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arXiv2015

A Deep Search For Faint Galaxies Associated With Very Low-redshift C IV Absorbers: II. Program Design, Absorption-line Measurements, and Absorber Statistics

To investigate the evolution of metal-enriched gas over recent cosmic epochs as well as to characterize the diffuse, ionized, metal-enriched circumgalactic medium (CGM), we have conducted a blind survey for C IV absorption systems in 89 QSO sightlines observed with the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS). We have identified 42 absorbers at z < 0.16, comprising the largest uniform blind sample size to date in this redshift range. Our measurements indicate an increasing C IV absorber number density per comoving path length (dN/dX = 7.5 +/- 1.1) and modestly increasing mass density relative to the critical density of the Universe (Omega(C IV) = 10.0 +/- 1.5 x 10^-8 ) from z ~ 1.5 to the present epoch, consistent with predictions from cosmological hydrodynamical simulations. Furthermore, the data support a functional form for the column density distribution function that deviates from a single power-law, also consistent with independent theoretical predictions. As the data also probe heavy element ions in addition to C IV at the same redshifts, we identify, measure, and search for correlations between column densities of these species where components appear aligned in velocity. Among these ion-ion correlations, we find evidence for tight correlations between C II and Si II, C II and Si III, and C IV and Si IV, suggesting that these pairs of species arise in similar ionization conditions. However, the evidence for correlations decreases as the difference in ionization potential increases. Finally, when controlling for observational bias, we find only marginal evidence for a correlation (86.8% likelihood) between the Doppler line width b(C IV) and column density N(C IV).

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arXiv2015

Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a Forest Tomographic Mapping of the COSMOS Field

Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at $z\sim 2.3-3$ within a $11.5'\times13.5'$ area of the COSMOS field ($\sim 1200\,\mathrm{deg}^2$ projected area density or $\sim 2.4\,h^{-1}\,\mathrm{Mpc}$ mean transverse separation), we reconstruct a 3D tomographic map of the foreground Ly$α$ forest absorption at $2.2<z<2.5$ with an effective smoothing scale of $σ_{3d}\approx3.5\,h^{-1}\,\mathrm{Mpc}$ comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased towards regions with enhanced IGM absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at $z=2.45$ associated with a recently-discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be $M_{\rm dm} (z=2.45) = (9\pm4)\times 10^{13}\,h^{-1}\,\mathrm{M_\odot}$, and argue based on this mass and absorption strength that it will form at least one $z\sim0$ galaxy cluster with $M(z=0) = (3\pm 2) \times 10^{14}\,h^{-1}\mathrm{M_\odot}$, although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at $z=2.30$ within a $r\sim 1\,h^{-1}\,\mathrm{Mpc}$ radius and $Δz\sim 0.006$, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Ly$α$ forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters and other features of large-scale structure.

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arXiv2015

Cold gas and a Milky Way-type 2175 Å bump in a metal-rich and highly depleted absorption system

We report the detection of a strong Milky Way-type 2175 Å$ $ extinction bump at $z$ = 2.1166 in the quasar spectrum towards SDSS J121143.42+083349.7 from the Sloan Digital Sky Survey (SDSS) Data Release 10. We conduct follow up observations with the Echelle Spectrograph and Imager (ESI) onboard the Keck-II telescope and the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT. This 2175 Å$ $ absorber is remarkable in that we simultaneously detect neutral carbon (C I), neutral chlorine (Cl I), and carbon monoxide (CO). It also qualifies as a damped Lyman alpha system. The J1211+0833 absorber is found to be metal-rich and has a dust depletion pattern resembling that of the Milky Way disk clouds. We use the column densities of the C I fine structure states and the C II/C I ratio (under the assumption of ionization equilibrium) to derive the temperature and volume density in the absorbing gas. A Cloudy photoionization model is constructed, which utilizes additional atoms/ions to constrain the physical conditions. The inferred physical conditions are consistent with a canonical cold (T $\sim$ 100 K) neutral medium with a high density ($n$(H I) $\sim$ 100 cm$^{-3}$) and a slightly higher pressure than the local interstellar medium. Given the simultaneous presence of C I, CO, and the 2175 Å$ $ bump, combined with the high metallicity, high dust depletion level and overall low ionization state of the gas, the absorber towards J1211+0833 supports the scenario that the presence of the bump requires an evolved stellar population.

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arXiv2015

The chemistry of the most metal-rich damped Lyman $α$ systems at z$\sim2$ II. Context with the Local Group

Using our sample of the most metal-rich damped Lyman $α$ systems (DLAs) at z$\sim2$, and two literature compilations of chemical abundances in 341 DLAs and 2818 stars, we present an analysis of the chemical composition of DLAs in the context of the Local Group. The metal-rich sample of DLAs at z$\sim2$ probes metallicities as high as the Galactic disc and the most metal-rich dwarf spheroidals (dSphs), permitting an analysis of many elements typically observed in DLAs (Fe, Zn, Cr, Mn, Si, and S) in comparison to stellar abundances observed in the Galaxy and its satellites (in particular dSphs). Our main conclusions are: (1) non-solar [Zn/Fe] abundances in metal-poor Galactic stars and in dSphs over the full metallicity range probed by DLAs, suggest that Zn is not a simple proxy for Fe in DLAs and therefore not a suitable indicator of dust depletion. After correcting for dust depletion, the majority of DLAs have subsolar [Zn/Fe] similar to dSphs; (2) at [Fe/H]$\sim-0.5$, a constant [Mn/Fe]$\sim-0.5$ and near-solar [$α$/Fe] (requiring an assumption about dust depletion) are in better agreement with dwarf galaxies than Galactic disc stars; (3) [$α$/Zn] is usually solar or subsolar in DLAs. However, although low ratios of [$α$/Fe] are usually considered more `dwarf-like' than `Milky Way-like', subsolar [Zn/Fe] in Local Group dwarfs leads to supersolar [$α$/Zn] in the dSphs, in contrast with the DLAs. Therefore, whilst DLAs exhibit some similarities with the Local Group dwarf population, there are also notable differences.

astro-ph.GA↗

arXiv2015

Data Reduction with the MIKE Spectrometer

This manuscript describes the design, usage, and data-reduction pipeline developed for the Magellan Inamori Kyocera Echelle (MIKE) spectrometer used with the Magellan telescope at the Las Campanas Observatory. We summarize the basic characteristics of the instrument and discuss observational procedures recommended for calibrating the standard data products. We detail the design and implementation of an IDL based data-reduction pipeline for MIKE data (since generalized to other echelle spectrometers, e.g. Keck/HIRES, VLT/UVES). This includes novel techniques for flat-fielding, wavelength calibration, and the extraction of echelle spectroscopy. Sufficient detail is provided in this manuscript to enable inexperienced observers to understand the strengths and weaknesses of the instrument and software package and an assessment of the related systematics.

astro-ph.IM↗

arXiv2015

The Keck+Magellan Survey for Lyman Limit Absorption III: Sample Definition and Column Density Measurements

We present an absorption-line survey of optically thick gas clouds -- Lyman Limit Systems (LLSs) -- observed at high dispersion with spectrometers on the Keck and Magellan telescopes. We measure column densities of neutral hydrogen NHI and associated metal-line transitions for 157 LLSs at z=1.76-4.39 restricted to 10^17.3 < NHI < 10^20.3. An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing NHI and that the majority of LLSs are highly ionized, confirming previous expectations. The Si^+/H^0 ratio spans nearly four orders-of-magnitude, implying a large dispersion in the gas metallicity. Fewer than 5% of these LLSs have no positive detection of a metal transition; by z~3, nearly all gas that is dense enough to exhibit a very high Lyman limit opacity has previously been polluted by heavy elements. We add new measurements to the small subset of LLS (~5-10) that may have super-solar abundances. High Si^+/Fe^+ ratios suggest an alpha-enhanced medium whereas the Si^+/C^+ ratios do not exhibit the super-solar enhancement inferred previously for the Lya forest.

astro-ph.CO↗

arXiv2015

The Neutral Hydrogen Cosmological Mass Density at z=5

We present the largest homogeneous survey of $z>4.4$ damped Lyman-$α$ systems (DLAs) using the spectra of 163 QSOs that comprise the Giant Gemini GMOS (GGG) survey. With this survey we make the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, $Ω_{\rm HI}$. At such high redshift important systematic uncertainties in the identification of DLAs are produced by strong intergalactic medium absorption and QSO continuum placement. These can cause spurious DLA detections, result in real DLAs being missed, or bias the inferred DLA column density distribution. We correct for these effects using a combination of mock and higher-resolution spectra, and show that for the GGG DLA sample the uncertainties introduced are smaller than the statistical errors on $Ω_{\rm HI}$. We find $Ω_{\rm HI}=0.98^{+0.20}_{-0.18}\times10^{-3}$ at $\langle z\rangle=4.9$, assuming a 20% contribution from lower column density systems below the DLA threshold. By comparing to literature measurements at lower redshifts, we show that $Ω_{\rm HI}$ can be described by the functional form $Ω_{\rm HI}(z)\propto(1+z)^{0.4}$. This gradual decrease from $z=5$ to $0$ is consistent with the bulk of HI gas being a transitory phase fuelling star formation, which is continually replenished by more highly-ionized gas from the intergalactic medium, and from recycled galactic winds.

astro-ph.CO↗

arXiv2015

Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy

We report the serendipitous detection of a 0.2 L$^*$, Lyman-$α$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$ cm$^{-2}$) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalise over uncertainties in the shape and normalisation of the ionizing radiation spectrum, we derive the total hydrogen column density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all the absorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_\odot$. These metallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of $\sim5 M_\odot\,$yr$^{-1}$. Our photoionization model yields extremely small sizes ($<$100-500 pc) for the absorbing clouds, which we argue are typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a $1000$-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.

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arXiv2015

Quasar Quartet Embedded in Giant Nebula Reveals Rare Massive Structure in Distant Universe

All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated by cosmological distances. In a survey for Lyman-alpha emission at redshift z ~ 2, we discovered a physical association of four quasars embedded in a giant nebula. Located within a substantial overdensity of galaxies, this system is probably the progenitor of a massive galaxy cluster. The chance probability of finding a quadruple quasar is estimated to be ~10^-7, implying a physical connection between Lyman-alpha nebulae and the locations of rare protoclusters. Our findings imply that the most massive structures in the distant universe have a tremendous supply (~ 10^11 solar masses) of cool dense (volume density ~1 cm^-3) gas, which is in conflict with current cosmological simulations.

astro-ph.GA↗

arXiv2015

Using machine learning to classify the diffuse interstellar bands

Using over a million and a half extragalactic spectra we study the correlations of the Diffuse Interstellar Bands (DIBs) in the Milky Way. We measure the correlation between DIB strength and dust extinction for 142 DIBs using 24 stacked spectra in the reddening range E(B-V) < 0.2, many more lines than ever studied before. Most of the DIBs do not correlate with dust extinction. However, we find 10 weak and barely studied DIBs with correlations that are higher than 0.7 with dust extinction and confirm the high correlation of additional 5 strong DIBs. Furthermore, we find a pair of DIBs, 5925.9A and 5927.5A which exhibits significant negative correlation with dust extinction, indicating that their carrier may be depleted on dust. We use Machine Learning algorithms to divide the DIBs to spectroscopic families based on 250 stacked spectra. By removing the dust dependency we study how DIBs follow their local environment. We thus obtain 6 groups of weak DIBs, 4 of which are tightly associated with C2 or CN absorption lines.

astro-ph.GA↗

arXiv2015

Deep HeII and CIV Spectroscopy of a Giant Lyman alpha Nebula: Dense Compact Gas Clumps in the Circumgalactic Medium of a z~2 Quasar

The recent discovery by Cantalupo et al. (2014) of the largest (~500 kpc) and luminous Ly-alpha nebula associated with the quasar UM287 (z=2.279) poses a great challenge to our current understanding of the astrophysics of the halos hosting massive z~2 galaxies. Either an enormous reservoir of cool gas is required $M\simeq10^{12}$ $M_{\odot}$, exceeding the expected baryonic mass available, or one must invoke extreme gas clumping factors not present in high-resolution cosmological simulations. However, observations of Ly-alpha emission alone cannot distinguish between these two scenarios. We have obtained the deepest ever spectroscopic integrations in the HeII and CIV lines with the goal of detecting extended line emission, but detect neither line to a 3$σ$ limiting SB $\simeq10^{-18}$ erg/s/cm$^2$/arcsec$^2$. We construct models of the expected emission spectrum in the highly probable scenario that the nebula is powered by photoionization from the central hyper-luminous quasar. The non-detection of HeII implies that the nebular emission arises from a mass $M_{\rm c}\lesssim6.4\times10^{10}$ $M_{\odot}$ of cool gas on ~200 kpc scales, distributed in a population of remarkably dense ($n_{\rm H}\gtrsim3$ cm$^{-3}$) and compact ($R\lesssim20$ pc) clouds, which would clearly be unresolved by current cosmological simulations. Given the large gas motions suggested by the Ly-alpha line ($v\simeq$ 500 km/s), it is unclear how these clouds survive without being disrupted by hydrodynamic instabilities. Our study serves as a benchmark for future deep integrations with current and planned wide-field IFU such as MUSE, KCWI, and KMOS. Our work suggest that a $\simeq$ 10 hr exposure would likely detect ~10 rest-frame UV/optical emission lines, opening up the possibility of conducting detailed photoionization modeling to infer the physical state of gas in the CGM.

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