2002ApJ...581.1039C

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 of ∼10<sup>–4</sup>L*, and an estimated mass of ∼10<sup>7</sup> M<sub>☉</sub>. 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. We estimate this cluster to have a total mass of ∼10<sup>5</sup> M<sub>☉</sub>, to be forming stars at a rate of less than 0.05 yr<sup>–1</sup>, and to have a maximum age of a few million years. 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. From our analysis of the optical spectrum we find the galaxy to have a metallicity Z≃0.06 Z<sub>☉</sub> and to contain a significant amount of internal dust [E(B-V)≃0.28]; both values agree with previous estimates. 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<sup>8</sup> yr) collision between two clumps of stars of subgalactic 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.