Misc,
Evidence for Cold Accretion: Discovery of Primitive Gas Flowing onto a Galaxy at z~0.274
(2011)cite arxiv:1105.5381 Comment: Submitted to ApJ Letters.
Abstract
We present UV and optical observations from the Cosmic Origins Spectrograph
on the Hubble Space Telescope and Keck of a z = 0.27395 Lyman limit system
(LLS) seen in absorption against the QSO PG1630+377. We detect H I with log
N(HI) = 17.06 0.05 as well as Mg II, C III, Si III, and O VI in this system
while recording upper limits for several other ions. The column densities are
readily explained if this is a multi-phase system, with the intermediate and
low ions arising in a very low metallicity (Mg/H = -1.71\pm0.06) photoionized
gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a
0.3 L* star-forming galaxy projected 37 kpc from the QSO at nearly identical
redshift (z = 0.27406, Delta_v = -26 km/s) with near solar metallicity (O/H =
-0.20\pm0.15). The properties of the gas and star-forming galaxy are consistent
with models of cold-mode accretion streams, where the LLS traces infalling gas
and the O VI arises due to its interaction with the galaxy halo. The gas cannot
trace an outflow due to its very low metallicity. The ionization conditions of
such accreting gas favor intermediate ions such as C III and Si III as
observational tracers of "cold"(~10^4 K) streams which are metal-poor and
predominantly ionized.
