The faint end of the galaxy luminosity function in Abell 1689: a steep red faint end upturn at $z=0.18$

TL;DR

This study reveals a steep upturn in the faint end of the galaxy luminosity function in Abell 1689 at redshift 0.18, indicating an abundance of red, quiescent dwarf galaxies and luminosity segregation in the cluster outskirts.

Contribution

First detection of a steep faint end upturn in the galaxy luminosity function at this redshift, highlighting the prevalence of red dwarf galaxies and luminosity segregation in Abell 1689.

Findings

Steep upturn with alpha ~ -2 in the dwarf galaxy LF.

Red sequence LF shares parameters with total LF, indicating red dwarf dominance.

Luminosity segregation observed with fewer giants in outskirts.

Abstract

We present a deep and wide $I$ luminosity function for galaxies in Abell 1689 ($z=0.183$) from a mosaic of HST WFPC2 images covering $10'$ on the side. The main result of this work is the detection of a steep upturn in the dwarf galaxy LF, with $\alpha \sim -2$. The dwarf to giant ratio appears to increase outwards, but this is because giant galaxies are missing in the cluster outskirts, indicating luminosity segregation. The red sequence LF has the same parameters, within errors, as the total LF, showing that the faint end upturn consists of red quiescent galaxies. We speculate that the upturn is connected to the `filling-in' of the red sequence at $z < 0.4$ and may represent the latest installment of `downsizing' as the least massive galaxies are being quenched at the present epoch.