DISCOVERY OF COLD, PRISTINE GAS POSSIBLY ACCRETING ONTO AN OVERDENSITY OF STAR-FORMING GALAXIES AT REDSHIFTz∼ 1.6

Abstract

We report the discovery of large amounts of cold (T ∼ 10 4 K), chemically young gas in an overdensity of galaxies at redshift z ≈ 1.6 located in the Great Observatories Origins Deep Survey southern field. The gas is identified thanks to the ultra-strong Mg ii λ2800 absorption features it imprints onto the rest-frame UV spectra of galaxies in the background of the overdensity. There is no evidence that the optically thick gas is part of any massive galaxy (i.e., M star > 4 × 10 9 M ), but rather is associated with the overdensity; less massive and fainter galaxies (25.5 mag < z < 27.5 mag) have too large an impact parameter to be causing ultra-strong absorption systems, based on our knowledge of such systems. The lack of corresponding Fe ii absorption features, not detected even in co-added spectra, suggests that the gas is chemically more pristine than the interstellar medium and outflows of star-forming galaxies at similar redshift, including the galaxies of the overdensity itself, and comparable to the most metal-poor stars in the Milky Way halo. A crude estimate of the projected covering factor of the high-columndensity gas (N H 10 20 cm −2 ) based on the observed fraction of galaxies with ultra-strong absorbers is C F ≈ 0.04. A broad, continuum absorption profile extending to the red of the interstellar Mg ii absorption line by 2000 km s −1 is possibly detected in two independent co-added spectra of galaxies belonging to the overdensity, consistent with a large-scale infall motion of the gas onto the overdensity and its galaxies. Overall, these findings provide the first tentative evidence of accretion of cold, chemically young gas onto galaxies at high redshift, possibly feeding their star formation activity. We suggest the fact that the galaxies are members of a large structure, as opposed to field galaxies, might play a significant role in our ability to detect the accreting gas.

Star formation sustained by gas accretion

“…The strong detected OVI seems to arise in interface material surrounding the photoionized clouds responsible for the low ionization absorption. A cold phase has also been found by Giavalisco et al (2011) in an overdensity of galaxies at z = 1.6, and by Kacprzak et al (2012b) along the projected minor axis of a star-forming spiral galaxy at z = 0.7. Low temperature is not necessarily equivalent to cold accretion.…”

GMASS ultradeep spectroscopy of galaxies atz ~ 2

“…We used the GMASS spectra and extensive photometry in the CDFS to perform several studies, including that of quiescent superdense galaxies at z > 1.4 Cappellari et al 2009), the stellar metallicity and outflows of star-forming z ∼ 2 galaxies (Halliday et al 2008;Talia et al 2012), the evolution of the rest-frame colour distribution and dust properties of high redshift galaxies Noll et al 2009), and the properties of galaxies in, and inflow of cold gas into, the galaxy overdensity at z = 1.6 (Kurk et al 2009;Giavalisco et al 2011). The public release of the GMASS spectra will facilitate further studies of the distant galaxies targeted by our survey.…”

“…This dispersion, together with the low (undetected) X-ray emission, classify the structure as a group, rather than a rich cluster, despite the presence of a red sequence of evolved galaxies, which may have formed their stars in a short burst at z = 3. Giavalisco et al (2011) presented the first (tentative) evidence, based on spectra from GMASS and other surveys, of accretion of cold, chemically young gas onto galaxies in this structure at z = 1.6, possibly feeding their star formation activity. Finally, Talia et al (2012) presented evidence for outflowing gas of galaxies at z ∼ 2, with typical velocities of the order of ∼100 km s −1 , as measured in a stack of 74 GMASS spectra of star forming galaxies.…”

The low-metallicity QSO HE 2158 − 0107: a massive galaxy growing by accretion of nearly pristine gas from its environment?

“…Birnboim & Dekel 2003;Kereš et al 2005), which would then light up as an EELR. This process is theoretically predicted, but observational evidence for its existence is still rare with only a few candidate detections (Giavalisco et al 2011;Ribaudo et al 2011), also because the phenomenon is expected to be more common at high redshift than in the local universe. To shed more light on this question, much deeper spectra covering the entire optical window are needed to turn our current limits into robust abundance measurements that can then be directly compared with those of dwarf galaxies.…”