The Color-Dependent Frequency of XUV Disks In Low-Mass E/S0s

¹ Dept. of Physics & Astronomy, University of North Carolina, USA
² Dept. of Physics & Astronomy, Rutgers, State University of New Jersey, USA
³ Spitzer Science Center, California Institute of Technology, USA

e-mail: amoffett@physics.unc.edu

Abstract

We identify a high frequency of Type 1 XUV disks, reflecting recent outer-disk star formation, in a sample of 31 E/S0s with stellar masses primarily below M_∗ ~ 4 × 10¹⁰ M_⊙. Our ~40% identification rate is roughly twice the 20% fraction reported for late-type galaxies. Intriguingly, in the dwarf mass regime (below M_∗ ~ 5 × 10⁹ M_⊙) where gas fractions increase, Type 1 XUV disks occur in ~70% of red-sequence E/S0s but only ~20% of blue-sequence E/S0s, a population recently linked to active disk rebuilding, especially in the dwarf regime. Our statistics are preliminary, but could indicate that Type 1 XUV disks in dwarf E/S0s are primarily related to weak/inefficient outer-disk star formation rather than to star formation capable of driving substantial disk growth.