Ultra-faint Ultraviolet Galaxies at z~2 Behind the Lensing Cluster Abell 1689: the Luminosity Function, Dust Extinction and Star Formation Rate Density

TL;DR

This study uses deep ultraviolet imaging of the galaxy cluster Abell 1689 to detect ultra-faint galaxies at z~2, revealing their luminosity function, dust properties, and star formation rate density, extending knowledge to much fainter galaxies than before.

Contribution

It provides the first detailed measurement of the luminosity function and star formation rate density for ultra-faint galaxies at z~2 using gravitational lensing and deep UV imaging.

Findings

Faint-end slope of luminosity function is -1.74 ± 0.08.

No turnover observed down to MUV = -13.

Star formation rate density at z~2 is 0.148 M/yr/Mpc^3.

Abstract

We identified the z~2 Lyman break galaxies using deep HST ultraviolet (F275W/F336W) imaging of Abell 1689. Because of the imaging depth and the large magnification provided by the cluster, we detect galaxies 100x fainter (-19.5< M_1500 <-13) than previous surveys at this redshift. We are able to calculate the intrinsic sensitivity of the observations as a function of source plane position, allowing determinations of effective volume as a function of luminosity. We fit the faint-end slope of the luminosity function to be alpha = -1.74 +/-0.08, consistent with the values obtained for 2.5 < z < 6. There is no turnover in the luminosity function down to MUV = -13. The trend of increasingly redder UV spectral slopes with luminosity at higher redshifts is observed in our sample, but with redder slopes at all luminosities and average reddening of < E(B - V) >= 0.15. We assume the stars in these galaxies are metal poor (0.2Z_sun) compared to their brighter counterparts (Z_sun), resulting in bluer assumed intrinsic UV slopes and larger derived dust extinction. The total UV luminosity density at z ~ 2 is 4.31x10^26 erg/s/Hz/Mpc^3, more than 70% of which is emitted by galaxies in the luminosity range of our sample. We determine the star formation rate density at z ~ 2 (assuming constant dust extinction correction of 4.2 over all luminosities and a Kroupa IMF) of 0.148 M/yr/Mpc^3, significantly higher than previous determinations because of the additional population of fainter galaxies and the larger dust correction factors.[abridged]