We present a new ultraviolet 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 Fe ii absorption lines at z _ { abs } = 0.00632 near the NGC4261 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 NGC4261 group .
The well-developed damping wings of the Ly \alpha line tightly constrain the H i column density ; we find log N ( H i ) = 19.32 \pm 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-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 ] \leq - 0.28 ( 3 \sigma ) .
Despite the proximity of the sight line to NGC4261 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 \rho = 86 h _ { 75 } ^ { -1 } kpc , while the closest L* galaxy is NGC4260 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 HVCs .
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 .