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 \mu m 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 \sim 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 \sim 18 pc for 5 % star formation efficiency .
The half-thickness of the disk of 8 \mu m cores is 105 pc for NGC 891 and 74 pc for IC 5052 , varying with radius by a factor of \sim 2 .
For NGC 3628 , which is interacting , the half-thickness is 438 pc , but even with this interaction , the 8 \mu m 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 \mu m cores have optical counterparts .