Discovery of a primitive damped Lyα absorber near an X-ray-bright galaxy group in the Virgo cluster

We present a new ultraviolet echelle spectrum of PG 1216+069, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, that reveals damped Lyalpha absorption, as well as O I, C II, Si II, and Fe II absorption lines, at z(abs) = 0.00632 near the NGC 4261 galaxy group in the periphery of the Virgo Cluster. The absorber shows no evidence of highly ionized gas, which places constraints on warm-hot'' missing baryons in the X-ray -bright NGC 4261 group. The well-developed damping wings of the Lyalpha line tightly constrain the H I column density; we find log N(H I) = 19.32 +/- 0.03. The metallicity of this subdamped Lyalpha absorber (sub-DLA) is remarkably low, [O/H] = - 1.60(-0.11)(+0.09) , comparable to many analogous high-redshift systems, and the iron abundance indicates that this absorber contains little or no dust. Nitrogen is underabundant: we detect neither N I nor N II, and we show that the absence of nitrogen is not due to ionization effects but rather indicates that [N/O] less than or equal to -0.28 (3 sigma). Despite the proximity of the sight line to the NGC 4261 group, there are no bright galaxies with small impact parameters at the absorption redshift: the nearest known galaxy is a sub-L* galaxy with a projected distance of rho = 86 h(75)(-1) kpc, while the closest L* galaxy is NGC 4260, at rho = 246 h(75)(-1) kpc. The low metallicity and nitrogen underabundance indicate that this low-z sub-DLA is a relatively primitive gas cloud. We consider the nature and origin of the sub-DLA, and we discuss several possibilities. The properties of the sub-DLA are similar to those of the interstellar media in blue compact dwarf galaxies and are also reminiscent of Milky Way high-velocity clouds. 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.