NGC4656UV: A uv-selected tidal dwarf galaxy candidate

Abstract

We report the discovery of a UV-bright tidal dwarf galaxy (TDG) candidate in the NGC4631/4656 galaxy group, which we designate NGC4656UV. Using survey and archival data spanning from 1.4 GHz to the ultraviolet, we investigate the gas kinematics and stellar properties of this system. The H I morphologies of NGC4656UV and its parent galaxy NGC4656 are extremely disturbed, with significant amounts of counterrotating and extraplanar gas. From UV-FIR photometry, computed using a new method to correct for surface gradients on faint objects, we find that NGC4656UV has no significant dust opacity and a blue spectral energy distribution. We compute a star formation rate of 0.027 M ̇yr-1 from the far-ultraviolet flux and measure a total H I mass of 3.8 × 108 M ̇ for the object. Evolutionary synthesis modeling indicates that NGC4656UV is a low-metallicity system whose only major burst of star formation occurred within the last 260-290 Myr. The age of the stellar population is consistent with a rough timescale for a recent tidal interaction between NGC4656 and NGC4631, although we discuss the true nature of the object - whether it is tidal or pre-existing in origin - in the context of its metallicity being a factor of 10 lower than its parent galaxy. We estimate that NGC4656UV is either marginally bound or unbound. If bound, it contains relatively low amounts of dark matter. The abundance of archival data allows for a deeper investigation into this dynamic system than is currently possible for most TDG candidates. © 2012. The American Astronomical Society. All rights reserved.