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

We present deep U, V and I band images of the "ultracompact" blue dwarfgalaxy POX 186 obtained with the Planetary Camera 2 of the Hubble SpaceTelescope. We have also obtained a near-ultraviolet spectrum of the object withthe Space Telescope Imaging Spectrograph, and combine this with a newground-based optical spectrum. The images confirm the galaxy to be extremelysmall, with a maximum extent of only 300 pc, a luminosity ~ 10^-4 L*, and anestimated mass ~ 10^7 M(sun). Its morphology is highly asymmetric, with a tailof material on its western side that may be tidal in origin. The U-band imageshows this tail to be part of a stream of material in which stars have recentlyformed. Most of the star formation in the galaxy is however concentrated in acentral, compact (d ~ 10 - 15 pc) star cluster. The outer regions of the galaxyare significantly redder than the cluster, with V - I colors consistent with apopulation dominated by K and M stars. While these results rule out earlierspeculation that POX 186 is a protogalaxy, its morphology, mass and active starformation suggest that it represents a recent (within ~ 10^8 yr) collisionbetween two clumps of stars of sub-galactic size (~ 100 pc). POX 186 may thusbe a very small dwarf galaxy that, dynamically speaking, is still in theprocess of formation. This interpretation is supported by the fact that itresides in a void, so its morphology cannot be explained as the result of anencounter with a more massive galaxy. Clumps of stars this small may representthe building blocks required by hierarchical models of galaxy formation, andthese results also support the recent "downsizing" picture of galaxy formationin which the least massive objects are the last to form.Comment: accepted for publication in ApJ; 23 pages, 5 figure