A Nearby Analog of z ~ 2 Compact Quiescent Galaxies with a Rotating Disk

Abstract

Recent studies have identified a population of compact quiescent galaxies at z ~ 2. These galaxies are very rare today and establishing the existence of a nearby analog could allow us to study its structure in greater detail than is possible at high redshift. Here we present such a local analog, NGC 5845, which has a dynamical mass of M _dyn = 4.3 ± 0.6 × 10¹⁰ M _⊙ and an effective radius of only r _e = 0.45 ± 0.05 kpc. We study the structure and kinematics with HST/WFPC2 data and previously published spatially resolved kinematics. We find that NGC 5845 is similar to compact quiescent galaxies at z ~ 2 in terms of size versus dynamical mass (r _e-M _dyn), effective velocity dispersion versus size (σ_e-r _e), and effective velocity dispersion versus dynamical mass (σ_e-M _dyn). The galaxy has a prominent rotating disk evident in both the photometry and the kinematics: it extends to well beyond >~ 1/3 effective radius and contribute to >~ 1/4 of the total light of the galaxy. Our results lend support to the idea that a fraction of z ~ 2 compact galaxies have prominent disks and positive mass-to-light ratio gradients, although we caution that NGC 5845 may have had a different formation history than the more massive compact quiescent galaxies at z ~ 2.