case western reserve university



Star formation in extreme locales revealed by GALEX

David Thilker (Johns Hopkins University)

I will discuss the implications of recent GALEX UV imaging, which reveals massive star formation in atypical environments including outer spiral disks, intergalactic gas clouds, and early-type galaxies. In particular, I will focus on the Leo Ring and the nearest lenticular galaxy, NGC 404. The Leo Ring, a massive (M_HI ~ 1.8e9 M_sun), 200-kpc-wide structure orbiting the galaxies M105 and NGC3384 with a 4-Gyr period, is a candidate "primordial" intergalactic cloud. Until now it was seen only from HI emission, suggesting the absence of a stellar population. We detected UV light from gaseous substructures of the Leo Ring. These complexes may be dwarf galaxies observed during their formation, but distinguished by their previously reported lack of a dark matter component. In this regard, they resemble tidal dwarf galaxies, although without the enrichment preceding tidal stripping. If structures like the Leo Ring were common in the early Universe, they may have produced a large (yet undetected) population of faint, metal-poor, halo-lacking dwarf galaxies. We have also discovered recent star formation in the outermost portion of NGC 404. Detected FUV-bright sources are concentrated within an extended gaseous structure thought to have been created by a merger event about 1 Gyr ago. In the context of the UV-optical galaxy color-magnitude diagram, the presence of the star forming activity places NGC 404 in the green valley separating the red and blue sequences. The lenticular galaxy is now experiencing a merger-induced, disk-building excursion away from the red sequence toward bluer colors. We conclude the green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to stochastic acquisition of fresh gas.