Observing the Earliest Stages of Star Formation in Galaxies: 8 μm 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 μm cores using an unsharp mask technique; 173, 267, and 60 cores were distinguished, respectively. Color–color distributions suggest a mixture of polycyclic aromatic hydrocarbons 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 radius of ∼18 pc for 5% star formation efficiency. The half-thickness of the disk of 8 μm 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 μm 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 μm cores have optical counterparts.