Regularly spaced 8 micron cores as tracers of the earliest stages of star formation in the spiral arms of nearby galaxies.

Archival Spitzer Space Telescope images of most nearby spiral galaxies show prominent 8 micron emission cores when viewed with an unsharp mask technique. These cores have the IR colors of young star-forming regions, typically a million years old, behind several tens of magnitudes of optical extinction. They are usually invisible in optical images, and yet the sum of their masses divided by their likely age is comparable to the total star formation rate in the galaxy. This suggests they are the earliest stages of star formation. They are often distributed in a regular fashion along the long dust filaments and spurs of spiral arms, suggesting they form by gravitational instabilities in shock-compressed gas.

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