On the Size and Mass of Photoionized Clouds in Extended Spiral Galaxy Halos

Abstract

The size and mass of two circumgalactic medium (CGM) clouds in the halo (impact parameter = 65 kpc) of a nearby late-type galaxy, MGC-01-04-005 ({cz}=1865 {km} {{{s}}}^-1), are investigated using a close triplet of QSO sight lines (the “LBQS Triplet”). Far ultraviolet spectra obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope find two velocity components in {Ly}α at ∼1830 and 1900 {km} {{{s}}}^-1 in two of these sight lines, requiring minimum transverse cloud sizes of ≥slant 10 kpc. A plausible, but not conclusive, detection of C iv 1548 Å absorption at the higher velocity in the third sight line suggests an even larger lower limit of ≥slant 23 kpc for that cloud. Using various combinations of constraints, including photoionization modeling for one absorber, lower limits on masses of these two clouds of ≳ {10}⁶{M}_⊙ are obtained. Ground-based imaging and long-slit spectroscopy of MCG-01-04-005 obtained at the Apache Point Observatory 3.5 m telescope find it to be a relatively normal late-type galaxy with a current star formation rate (SFR) of ∼ 0.01{M}_⊙ {{yr}}^-1. GALEX photometry finds an SFR only a few times higher over the last 10⁸ years. We conclude that the CGM clouds probed by these spectra are typical because they are at impact parameters of 0.4-0.5 {R}_{vir} from a rather typical, non-starbursting late-type galaxy; thus, these size and mass results should be generic for this class. Therefore, at least some CGM clouds are exceptionally large and massive.