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Figure 1: The IRAC 3.6$\mu$m images of the seven targeted galaxies: NGC6822, M33, NGC300, NGC2403, M81, NGC0247, and NGC7793. The red line on each figure spans $10\farcm$

Figure: The distances (in pixels) to the nearest $3.6\,\micron$ source for each $4.5\,\micron$ source in M33 along the x-axis ($\Delta x$) and y-axis ($\Delta y$) of the images. The blue and red histograms show the distance distributions. The radius of the concentric circles are 0.5pixel (green) and 1pixel (black). In this case, most sources ($>90\%$) have a match within $0.5$pixels, and the density of nearest matches falls rapidly between $0.5-1.0$pixel ($<10\%$ additional matches) while the number of duplicate matches increase ($<0.5\%$ duplicate matches). At larger distances, the distribution essentially flattens. Similiar distributions are observed for the other six galaxies (see Table1).

Figure 3: The differences between aperture and PSF photometry magnitudes as a function of the PSF magnitudes for the $m_{psf}<15$ sources in NGC6822 and M81. For the less crowded case of NGC6822, the two measurements generally agree in all four IRAC bands, with the scatter increasing for the fainter sources. The same is true for the $3.6\,\micron$ and $4.5\,\micron$ images of M81, but at $5.8\,\micron$, the scatter increases rapidly, and for $8.0\,\micron$ the situation worsens further.

Figure 4: The $m_{4.5}$vs. $m_{3.6}-m_{4.5}$ (top-row), $m_{5.8}$vs. $m_{4.5}-m_{5.8}$ (middle-row), and $m_{8.0}$vs. $m_{5.8}-m_{8.0}$ (bottom-row) color magnitude diagrams (CMDs) for the $>3\sigma$ sources in NGC6822, M33, NGC300 and NGC2403. The red lines represent isodensity contours.

Figure 5: Same as Figure4, but for the galaxies M81, NGC247, NGC7793, and (for comparison) a $6$deg$^2$ region of the NOAO Bootes Field from the Spitzer Deep Wide Field Survey (SDWFS, Ashby et al.2009) data.

Figure 6: Apparent magnitude histograms for all the sources in the catalog, with the shaded regions showing the sources in M33. Overall, the source lists become incomplete at (locations marked by the dashed lines) $m_{3.6}\gtrsim 18$, $m_{4.5}\gtrsim 18$, $m_{5.8}\gtrsim 17$ and $m_{8.0}\gtrsim 16$. The M33 source-list is roughly $\sim 0.5$mag shallower than for the other galaxies. The dotted lines show the apparent-magnitude histograms of the SDWFS catalog sources (scaled-up by a factor of $\times 15$ for clarity).

Figure 7: Mid-IR color histograms for all the sources in the catalog, with the shaded histograms showing the distribution of sources with $1\,\sigma$ uncertainty in color $\lesssim 0.2$. The sharp peak of color distribution at $m_{3.6}-m_{4.5}\simeq 0$ is due to the Rayleigh-Jeans tail of the spectra of all normal stars. The longer wavelengths are increasingly dominated by dusty stars, with color distribution peaks at (locations marked by the dashed lines) $m_{3.6}-m_{5.8}=m_{4.5}-m_{5.8}\simeq 1$, $m_{3.6}-m_{8.0}=m_{4.5}-m_{8.0}\simeq 2$, and $m_{5.8}-m_{8.0}\simeq 1$. The dotted lines show the mid-IR color histograms of the SDWFS catalog sources (scaled-up by a factor of $\times 15$ for clarity).