The Heavy‐Element Enrichment of Lyα Clouds in the Virgo Supercluster

Abstract

Using high signal-to-noise ratio echelle spectra of 3C 273 obtained with the Space Telescope Imaging Spectrograph (resolution of 7 km s-1 FWHM), we constrain the metallicities of two Ly? clouds in the vicinity of the Virgo Cluster. We detect C II, Si II, and Si III absorption lines in the Ly? absorber at zabs = 0.00530. Previous observations with the Far Ultraviolet Spectroscopic Explorer have revealed Ly?-Ly? absorption lines at the same redshift, thereby accurately constraining the H I column density. We model the ionization of the gas and derive [C/H] = -1.2, [Si/C] = 0.2 ? 0.1, and log nH = -2.8 ? 0.3. The model implies a small absorber thickness, ~70 pc, and thermal pressure p/k ? 40 cm-3 K. It is most likely that the absorber is pressure confined by an external medium because gravitational confinement would require a very high ratio of dark matter to baryonic matter. Based on a sample of Milky Way sight lines in which carbon and silicon abundances have been reliably measured in the same interstellar cloud (including new measurements presented herein), we argue that it is unlikely that the overabundance of Si relative to C is due to depletion onto dust grains. Instead, this probably indicates that the gas has been predominately enriched by ejecta from Type II supernovae. Such enrichment is most plausibly provided by an unbound galactic wind, given the absence of known galaxies within a projected distance of 100 kpc and the presence of galaxies capable of driving a wind at larger distances (e.g., H I 1225+01). Such processes have been invoked to explain the observed abundances in the hot, X-ray-emitting gas in Virgo. However, the sight line to 3C 273 is more than 10? away from the X-ray emission region. We also constrain the metallicity and physical conditions of the Virgo absorber at zabs = 0.00337 in the spectrum of 3C 273 based on detections of O VI and H I and an upper limit on C IV. If this absorber is collisionally ionized, the O /C limit requires T 105.3 K in the O VI-bearing gas. For either collisional ionization or photoionization, we find that [O/H] -2.0 at zabs = 0.00337.