The Evolution Of The Faint End Of The UV Luminosity Function During The Peak Epoch Of Star Formation (1<z<3)

TL;DR

This study measures the UV luminosity function during the peak epoch of star formation (z=1-3), revealing a steepening faint-end slope with increasing redshift and showing faint galaxies dominate the UV luminosity density.

Contribution

It provides the first robust measurement of the faint-end UV luminosity function evolution during 1<z<3 using deep HST imaging and lensing clusters, extending to very faint magnitudes.

Findings

Faint-end slope steepens from -1.56 to -1.94 between z=1 and z=3.

No turnover observed down to M_UV = -14 AB mag.

Faint galaxies contribute over 55% of the UV luminosity density at 1<z<3.

Abstract

[Abridged] We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at 1<z<3. We use our deep near ultraviolet imaging from WFC3/UVIS on the Hubble Space Telescope (HST) and existing ACS/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACSJ0717 from the Hubble Frontier Field survey and Abell 1689. We use photometric redshifts to identify 780 ultra-faint galaxies with $M_{UV}$<-12.5 AB mag at 1<z<3. From these samples, we identified 5 new, faint, multiply imaged systems in A1689. We compute the rest-frame UV LF and find the best-fit faint-end slopes of $\alpha=-1.56\pm0.04$, $\alpha=-1.72\pm0.04$ and $\alpha=-1.94\pm0.06$ at 1.0<z<1.6, 1.6<z<2.2 and 2.2<z<3.0, respectively. Our results demonstrate that the UV LF becomes steeper from z\sim1.3 to z\sim2.6 with no sign of a turnover down to $M_{UV}=-14$ AB mag. We further derive the UV LFs using the Lyman break "dropout" selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large, and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example), likely dominate. If we restrict our analysis to galaxies and volumes above > 50% completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values ($\alpha=-1.55\pm0.06$, $-1.69\pm0.07$ and $-1.79\pm0.08$ for $z\sim1.3$, 1.9 and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of $-18.5<M_{UV}<-12.5$ covered in this study, produce the majority (55%-60%) of the unobscured UV luminosity density at 1<z<3.