A Deeper Look at Leo IV: Star Formation History and Extended Structure

Abstract

We present MMT/Megacam imaging of the Leo~IV dwarf galaxy in order to investigate its structure and star formation history, and to search for signs of association with the recently discovered Leo~V satellite. Based on parameterized fits, we find that Leo~IV is round, with \(\epsilon < 0.23\) (at the 68\% confidence limit) and a half-light radius of \(r_{h} \simeq 130\) pc. Additionally, we perform a thorough search for extended structures in the plane of the sky and along the line of sight. We derive our surface brightness detection limit by implanting fake structures into our catalog with stellar populations identical to that of Leo~IV. We show that we are sensitive to stream-like structures with surface brightness \(\mu_{r}\lesssim29.6\) mag arcsec\(^{-2}\), and at this limit, we find no stellar bridge between Leo IV (out to a radius of \(\sim\)0.5 kpc) and the recently discovered, nearby satellite Leo V. Using the color magnitude fitting package StarFISH, we determine that Leo~IV is consistent with a single age (\(\sim\)14 Gyr), single metallicity (\([Fe/H]\sim-2.3\)) stellar population, although we can not rule out a significant spread in these value. We derive a luminosity of \(M_{V}=-5.5\pm0.3\). Studying both the spatial distribution and frequency of Leo~IV's 'blue plume' stars reveals evidence for a young (\(\sim\)2 Gyr) stellar population which makes up \(\sim\)2\% of its stellar mass. This sprinkling of star formation, only detectable in this deep study, highlights the need for further imaging of the new Milky Way satellites along with theoretical work on the expected, detailed properties of these possible 'reionization fossils'.