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.