Discovery of a Primitive Damped Lyα Absorber near an X‐Ray–bright Galaxy Group in the Virgo Cluster

We present a new UV echelle spectrum of PG1216+069, obtained with HST+STIS,which reveals damped Lya (DLA) absorption as well as O I, C II, Si II, and FeII absorption lines at z(abs) = 0.00632 near the NGC4261 group. The absorbershows no evidence of highly-ionized gas, which places constraints on "warm-hot"missing baryons in the NGC4261 group. The well-developed damping wings of theLya line tightly constrain the H I column density; we find 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 many analogous high-redshiftsystems, and the iron abundance indicates that this absorber contains little orno dust. Nitrogen is underabundant; we detect neither N I or N II, and we showthat this is not due to ionization effects but rather indicates that [N/O] <-0.28 (3sigma). Despite the proximity to NGC4261 group, there are no brightgalaxies close to the sight line at the absorption redshift. The nearest knowngalaxy is a sub-L* galaxy with a projected distance rho = 86 kpc; the closestL* galaxy is NGC4260 at rho = 246 kpc. The low metallicity and [N/O] indicatethat this low-z sub-DLA is a relatively primitive gas cloud. We consider thenature and origin of the sub-DLA, and we find several possibilities. Theproperties of the sub-DLA are similar to those of the interstellar media inblue compact dwarf galaxies and are also reminiscent of Milky Way HVCs. Or, theobject could simply be a small dark-matter halo, self-enriched by a smallamount of internal star formation but mostly undisturbed since its initialformation. In this case, the small halo would likely be an ancient buildingblock of galaxy formation that formed before the epoch of reionization.Comment: Accepted for publication in The Astrophysical Journa