A pilot study using deep infrared imaging to constrain the star
  formation history of the XUV stellar populations in NGC 4625
  In a LCDM universe, disk galaxies' outer regions are the last to form.
Characterizing their contents is critical for understanding the ongoing process
of disk formation, but observing outer disk stellar populations is challenging
due to their low surface brightness. We present extremely deep 3.6 micron
observations (Spitzer IRAC) of NGC 4625, a galaxy known for its radially
extended ultraviolet emitting stellar population. We combine the new imaging
with archival UV imaging from the GALEX mission to derive multiwavelength
radial profiles for NGC 4625 and compare them to stellar populations models.
The colors can be explained by the young stellar population that is responsible
for the UV emission and indicate that the current star formation rates in the
outermost disk are recent. Extended star formation in NGC 4625 may have been
initiated by an interaction with neighboring galaxies NGC 4618 and NGC 4625a,
supporting speculation that minor interactions are a common trigger for outer
disk star formation and late stage disk growth.
