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 ( ), 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 at ∼1830 and 1900 in two of these sight lines, requiring minimum transverse cloud sizes of 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 kpc for that cloud. Using various combinations of constraints, including photoionization modeling for one absorber, lower limits on masses of these two clouds of 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 . 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 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.