POX 186: A Dwarf Galaxy in the Process of Formation?

Abstract

We present deep U, V and I band images of the "ultracompact" blue dwarf galaxy POX 186 obtained with the Planetary Camera 2 of the Hubble Space Telescope. We have also obtained a near-ultraviolet spectrum of the object with the Space Telescope Imaging Spectrograph, and combine this with a new ground-based optical spectrum. The images confirm the galaxy to be extremely small, with a maximum extent of only 300 pc, a luminosity ~ 10^-4 L*, and an estimated mass ~ 10^7 M(sun). Its morphology is highly asymmetric, with a tail of material on its western side that may be tidal in origin. The U-band image shows this tail to be part of a stream of material in which stars have recently formed. Most of the star formation in the galaxy is however concentrated in a central, compact (d ~ 10 - 15 pc) star cluster. The outer regions of the galaxy are significantly redder than the cluster, with V - I colors consistent with a population dominated by K and M stars. While these results rule out earlier speculation that POX 186 is a protogalaxy, its morphology, mass and active star formation suggest that it represents a recent (within ~ 10^8 yr) collision between two clumps of stars of sub-galactic size (~ 100 pc). POX 186 may thus be a very small dwarf galaxy that, dynamically speaking, is still in the process of formation. This interpretation is supported by the fact that it resides in a void, so its morphology cannot be explained as the result of an encounter with a more massive galaxy. Clumps of stars this small may represent the building blocks required by hierarchical models of galaxy formation, and these results also support the recent "downsizing" picture of galaxy formation in which the least massive objects are the last to form.