  Spitzer Space Telescope observations of 15 spiral galaxies show numerous
dense cores at 8 microns that are revealed primarily in unsharp mask images.
The cores are generally invisible in optical bands because of extinction, and
they are also indistinct at 8 microns alone because of contamination by more
widespread diffuse emission. Several hundred core positions, magnitudes, and
colors from the four IRAC bands are measured and tabulated for each galaxy. The
larger galaxies, which tend to have longer and more regular spiral arms, often
have their infrared cores aligned along these arms, with additional cores in
spiral arm spurs. Galaxies without regular spirals have their cores in more
irregular spiral-like filaments, with typically only one or two cores per
filament. Nearly every elongated emission feature has 8 micron cores strung out
along its length. The occurrence of dense cores in long and thin filaments is
reminiscent of filamentary star formation in the solar neighborhood, although
on a scale 100 times larger in galaxies. The cores most likely form by
gravitational instabilities and cloud agglomeration in the filaments. The
simultaneous occurrence of several cores with regular spacings in some spiral
arms suggests that in these cases, all of the cores formed at about the same
time and the corresponding filaments are young. Total star formation rates for
the galaxies correlate with the total embedded stellar masses in the cores with
an average ratio corresponding to a possible age between 0.2 Myr and 2 Myr.
This suggests that the identified cores are the earliest phase for most star
formation.
