THE RAPID DECLINE IN METALLICITY OF DAMPED Lyα SYSTEMS AT z ∼ 5

Abstract

We present evidence that the cosmological mean metallicity of neutral atomic hydrogen gas shows a sudden decrease at z > 4.7 down to Z = −2.03 +0.09 −0.11 , which is 6σ deviant from that predicted by a linear fit to the data at lower redshifts. This measurement is made possible by the chemical abundance measurements of 8 new damped Ly-α (DLA) systems at z > 4.7 observed with the Echellette Spectrograph and Imager on the Keck II telescope, doubling the number of measurements at z > 4.7 to 16. Possible explanations for this sudden decrease in metallicity include a change in the physical processes that enrich the neutral gas within disks, or an increase of the covering factor of neutral gas outside disks due to a lower ultra-violet radiation field and higher density at high redshift. The later possibility would result in a new population of presumably lower metallicity DLAs, with an increased contribution to the DLA population at higher redshifts resulting in a reduced mean metallicity. Furthermore, we provide evidence of a possible decrease at z > 4.7 in the comoving metal mass density of DLAs, ρ metals (z) DLA , which is flat out to z ∼ 4.3. Such a decrease is expected, as otherwise most of the metals from star-forming galaxies would reside in DLAs by z ∼ 6. While the metallicity is decreasing at high redshift, the contribution of DLAs to the total metal budget of the universe increases with redshift, with DLAs at z ∼ 4.3 accounting for ∼ 20% as many metals as produced by Lyman break galaxies.

Star formation sustained by gas accretion

“…4). The expected properties coincide with those observed in Lyman limit systems (LLS; N HI > 10 17 cm −2 ) and damped Lyman-α absorbers (DLA; N HI > 10 20 cm −2 ), including inflows of 100 km s −1 and metallicities between 10 −2.5 and 10 −1 Z for z between 5 and 0 (e.g., Tornatore et al 2010;Rafelski et al 2014). The closer to the center of the gravitational potential the denser the gas, and the lower the redshift the more diffuse.…”

“…From 0 < z < 5, the relationship has a slope of −0.2 dex per unit z, a large scatter (±1 dex at a given z), and a lower limit that extends at least to Z /3000 (Rafelski et al 2012(Rafelski et al , 2014Cooke et al 2011b). There are also reports of null detection at high redshift: Z < 10 −4.2 Z at z = 3.4 (Fumagalli et al 2011a, LLS), and Z < 10 −4 Z at z = 7.1 (Simcoe et al 2012, LLS/DLA).…”

The neutral hydrogen cosmological mass density atz= 5

“…Rafelski et al (2014) report a decrease in the metal mass density in DLAs from z = 5 to 4.5, hinting at an abrupt change in the enrichment of H I gas past z = 5. This may be caused by a change in the population of objects containing neutral hydrogen, which could be accompanied by a similarly abrupt evolution in H I .…”

“…The decline in the QSO space density at z > 3 means that relatively few redshift >4.4 QSOs were observed by the SDSS, and those that were typically have too low a signal-to-noise (S/N) to reliably identify DLAs. For example, Rafelski et al (2012Rafelski et al ( , 2014 find a misidentification rate of 26 per cent for DLA candidates from SDSS DR5 at z > 4 and of 97 per cent for candidates from DR9 at z > 4.7. For this reason, smaller DLA surveys have been performed at higher redshift, often using higher resolution spectra to make robust identifications of DLAs.…”

Quasar 2175 Å dust absorbers – II. Correlation analysis and relationship with other absorption line systems

“…In the literature for DLAs, two correlations have been studied extensively. One is the evolution of metallicity as a function of redshift; metallicity gradually increases with decreasing redshift (Prochaska et al 2003;Kulkarni et al 2005;Rafelski et al 2012Rafelski et al , 2014. The second one is the correlation between the metallicity and kinematics (Wolfe & Prochaska 1998;Ledoux et al 2006;Prochaska, Hennawi, & Herbert-Fort 2008;Neeleman et al 2013), which is largely due to the underlying massmetallicity relation that has been well-established in galaxies at high and low redshifts (Tremonti et al 2004;Savaglio et al 2005;Erb et al 2006;Maiolino et al 2008;Møller et al 2013;Neeleman et al 2013).…”