9.5.1.1 IRAS Data

When in doubt (or when starting out), it's always best to go back to actual data. We retrieved IRAS images of the ONC region above. As we move the mouse over the image, we get a readout of pixel position $(x,y)$, sky position $(\alpha,\delta)$, and flux, in MJy/sr. This calibrated flux value, specifically, is the number we need to care about. IRAS observations at 25, 60, and 100 microns are reasonably close to MIPS wavelengths at 24, 70, and 160 microns, and will give us a sense of what to expect. Indeed, the ISM is very bright here! At 100 microns, peak flux values are $\sim 24,000 \; {\rm MJy/sr}$! (Note for users where ISM does not dominate: the ISSA plates have had the Zodiacal light already removed from them. See below and the moving target chapter for more information.) For any given target, not just the ONC, other sources of background flux may be much more significant. So, we next look at other ways of getting a background estimate.