We report the discovery of a low-redshift, sub-damped Ly_ absorber (DLA) in the outer periphery of the Virgo cluster near the X-ray bright NGC4261 galaxy group, in the direction of PG1216+069. We have obtained a new spectrum of PG1216+069 (zQSO = 0.3313) with the E140M echelle mode of the Space Telescope Imaging Spectrograph (STIS), and this spectrum reveals damped Ly_ absorption as well as O I, C II, Si II, and Fe II absorption lines at zabs = 0.00632. Notably, the absorber shows no evidence of highly-ionized gas: Si IV, C IV, and O VI are not signi_cantly detected at this redshift. The lack of highly-ionized gas associated with the absorber places constraints on \warm-hot" missing baryons in the NGC4261 group at _ & 400h1 75 kpc from the group center. The well-developed damping wings of the Ly_ line tightly constrain the H I column density; we _nd log N(H I) = 19.32_0.03. The metallicity of this sub- DLA is remarkably low, [O/H] = 1:60+0:09 0:11, which is comparable to (or less than) the metallicity of many analogous high-redshift systems. The iron abundance indicates that this absorber contains little or no dust. Nitrogen is underabundant; we detect neither N I or N II, and we show that the absence of N I is not due to ionization e_ects but rather indicates a signi_cant underabundance, [N/O] _ 0:28 at the 3_ level. Despite the proximity of the sight line to the Virgo cluster and the NGC4261 group, there are no bright galaxies particularly close to the sight line at the absorption redshift. The nearest known galaxy in projection is a subL_ galaxy with a projected distance _ = 86h1 75 kpc; the closest L_ galaxy is NGC4260 at _ = 246h1 75 kpc. On the basis of the low metallicity and nitrogen underabundance, we conclude that this lowz sub-DLA is a relatively primitive gas cloud. We consider the nature and origin of the sub-DLA, and we _nd several possibilities. The properties of the sub-DLA are similar to those of the interstellar media in blue compact dwarf galaxies, and the sub-DLA characteristics are also reminiscent of the high-velocity clouds seen in the vicinity of the Milky Way. The sub-DLA could also be related to a dwarf spheroidal galaxy, if the absorption arises in gas ejected or stripped from such an object. Finally, the object could simply be a small dark-matter halo, self-enriched by a small amount of internal star formation but mostly undisturbed since its initial formation. In this case, the small halo would likely be an ancient building block of galaxy formation that formed before the epoch of reionization.
- cosmology: observations,
- galaxies: abundances,
- intergalactic medium,
- quasars: absorption lines,
- quasars: individual (PG1216+069)
Available at: http://works.bepress.com/todd_tripp/4/