On the size and mass of photoionized clouds in extended spiral galaxy halos

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 $Ly\alpha$ at ∼1830 and 1900 $km{s}^{-1}$ in two of these sight lines, requiring minimum transverse cloud sizes of $⩾10$ 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 $⩾23$ kpc for that cloud. Using various combinations of constraints, including photoionization modeling for one absorber, lower limits on masses of these two clouds of $\gtrsim {10}^6{M}_{\odot }$ 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 $\sim 0.01M_{\odot }{yr}^{-1}$. 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.