Directly imaging damped Ly-alpha galaxies at z>2. III: The star formation rates of neutral gas reservoirs at z~2.7

TL;DR

This study uses a novel technique to directly measure star formation rates in damped Ly-alpha systems at z~2.7, revealing that most DLAs are linked to faint, isolated galaxies with low star formation activity.

Contribution

It provides new direct constraints on the star formation rates and host galaxy properties of DLAs, challenging previous assumptions about their association with bright, star-forming galaxies.

Findings

Most DLA hosts have SFR < 0.27 M/yr.

Less than 13% of hosts have SFR > 2 M/yr.

DLAs are likely associated with faint, isolated galaxies.

Abstract

We present results from a survey designed to probe the star formation properties of 32 damped Ly-alpha systems (DLAs) at z~2.7. By using the "double-DLA" technique that eliminates the glare of the bright background quasars, we directly measure the rest-frame FUV flux from DLAs and their neighbouring galaxies. At the position of the absorbing gas, we place stringent constraints on the unobscured star formation rates (SFRs) of DLAs to 2-sigma limits of <0.09-0.27 M/yr, corresponding to SFR surface densities <10^(-2.6)-10^(-1.5) M/yr/kpc^2. The implications of these limits for the star formation law, metal enrichment, and cooling rates of DLAs are examined. By studying the distribution of impact parameters as a function of SFRs for all the galaxies detected around these DLAs, we place new direct constraints on the bright end of the UV luminosity function of DLA hosts. We find that <13% of the hosts have SFR>2 M/yr at impact parameters b < (SFR/(M/yr))^(0.8)+6 kpc, differently from current samples of confirmed DLA galaxies. Our observations also disfavor a scenario in which the majority of DLAs arise from bright LBGs at distances 20 < b < 100 kpc. These new findings corroborate a picture in which DLAs do not originate from highly star forming systems that are coincident with the absorbers, and instead suggest that DLAs are associated with faint, possibly isolated, star-forming galaxies. Potential shortcomings of this scenario and future strategies for further investigation are discussed.