Observing the Earliest Stages of Star Formation in Galaxies: 8 micron
  Cores in Three Edge-on Disks
  To study the vertical distribution of the earliest stages of star formation
in galaxies, three edge-on spirals, NGC 891, NGC 3628, and IC 5052 observed by
the Spitzer Space Telescope InfraRed Array Camera (IRAC) were examined for
compact 8 micron cores using an unsharp mask technique; 173, 267, and 60 cores
were distinguished, respectively. Color-color distributions suggest a mixture
of PAHs and highly-extincted photospheric emission from young stars. The
average V-band extinction is ~20 mag, equally divided between foreground and
core. IRAC magnitudes for the clumps are converted to stellar masses assuming
an age of 1 Myr, which is about equal to the ratio of the total core mass to
the star formation rate in each galaxy. The extinction and stellar mass suggest
an intrinsic core diameter of ~18 pc for 5% star formation efficiency. The
half-thickness of the disk of 8 micron cores is 105 pc for NGC 891 and 74 pc
for IC 5052, varying with radius by a factor of ~2. For NGC 3628, which is
interacting, the half-thickness is 438 pc, but even with this interaction, the
8 micron disk is remarkably flat, suggesting vertical stability. Small scale
structures like shingles or spirals are seen in the core positions. Very few of
the 8 micron cores have optical counterparts.
