AN EMPIRICAL CHARACTERIZATION OF EXTENDED COOL GAS AROUND GALAXIES USING Mg II ABSORPTION FEATURES

Abstract

We report results from a survey of Mg II absorbers in the spectra of background QSOs that are within close angular distances to a foreground galaxy at z < 0.5, using the Magellan Echellette Spectrograph. We have established a spectroscopic sample of 94 galaxies at a median redshift of z = 0.24 in fields around 70 distant background QSOs (z QSO > 0.6), 71 of which are in an 'isolated' environment with no known companions and located at ρ 120 h −1 kpc from the line of sight of a background QSO. The rest-frame absolute B-band magnitudes span a range from M B − 5 log h = −16.4 to M B − 5 log h = −21.4 and rest-frame B AB − R AB colors range from B AB − R AB ≈ 0 to B AB − R AB ≈ 1.5. Of these 'isolated' galaxies, we find that 47 have corresponding Mg II absorbers in the spectra of background QSOs and rest-frame absorption equivalent width W r (2796) = 0.1 − 2.34 Å, and 24 do not give rise to Mg II absorption to sensitive upper limits. Our analysis shows that (1) W r (2796) declines with increasing distance from 'isolated' galaxies but shows no clear trend in 'group' environments;(2) more luminous galaxies possess more extended Mg II absorbing halos with the gaseous radius scaled by B-band luminosity according to R gas = 75 × (L B /L B * ) (0.35±0.03) h −1 kpc; (3) there is little dependence between the observed absorber strength and galaxy intrinsic colors; and (4) within R gas , we find a mean covering fraction of κ 0.3 ≈ 70% for absorbers of W r (2796) ≥ 0.3 Å and κ 0.1 ≈ 80% for absorbers of W r (2796) ≥ 0.1 Å. The results confirm that extended Mg II absorbing halos are a common and generic feature around ordinary galaxies and that the gaseous radius is a fixed fraction of the dark matter halo radius. The lack of correlation between W r (2796) strength and galaxy colors suggests a lack of physical connection between the origin of extended Mg II halos and recent star formation history of the galaxies. Finally, we discuss the total gas mass in galactic halos as traced by Mg II absorbers. We also compare our results with previous studies. Subject headings: cosmology: observations-intergalactic medium-quasars: absorption lines 1 This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

Mining circumgalactic baryons in the low-redshift universe

“…The distinct boundary between the presence and absence of metal-line absorbers at d ≈ 0.7 R h is seen in all ionic transitions that have been studied, including the observations of Si II, Si III, Si IV, C II, and C IV reported here and those of Mg II absorbers around z = 0.24 galaxies (Chen et al 2010a). It is, however, not clear whether such absorption boundary exists for highly ionized gas probed by the O VI absorption doublet.…”

“…Following the prescription described in Chen et al (2010a), we employ a maximum likelihood analysis to estimate κ for different species. First, the probability that a galaxy inside a dark matter halo of radius R h exhibits an absorber of rest-frame absorption equivalent width Wr W0 at projected distance d is written as…”

Characterizing the chemically enriched circumgalactic medium of ∼38 000 luminous red galaxies in SDSS DR12

“…While strong Mg II absorbers are also found at large distances from QSOs (e.g., Johnson et al 2015), such flat Wr(2796) versus d trend is in stark contrast to known halo gas properties around L * galaxies. These strong absorbers are only found around L * galaxies at d 60 kpc, beyond which the observed Mg II absorbing strength rapidly declines (e.g., Chen et al 2010a).…”

“…We employ a maximum likelihood analysis to compute the best-fit κ Mg II and its associated uncertainties as a function of projected distance d, following the formalism described in Chen et al (2010a). The likelihood of observing an ensemble of galaxies with n showing associated Mg II and m displaying upper limits is…”

“…It has been well-established over the past two decades that typical L * and sub-L * galaxies are surrounded by extended Mg II absorbing gas out to projected distance of d = 50 − 100 kpc with a mean gas covering fraction of κ Mg II 70% (e.g., Bowen et al 1995;Chen et al 2010a) 1 . The presence of chemicallyenriched cool gas at ∼ 100 kpc from star-forming regions can be explained by two competing models: infall (e.g., Mo & Miralda-Escudé 1996;Maller & Bullock 2004) and outflows (e.g., Weiner et al 2009;Murray et al 2011).…”