Far IR photometric propagation analysis

Abstract

In order to investigate the feasibility of an all-sky guide star catalogue for an FGS operating in the FIR, we cross-match the sources selected in three test regions from the IRAS PSC and from the Spitzer surveys with both the AllWISE catalogue and Gaia DR3. We find 143 and 318 matched IRAS sources within 20° from the NGP and within 2° from the GC, respectively. In addition, we identify 3607 sources from the Spizer/SEP survey and 5398 sources from the Spitzer/MIPSGAL survey within 1° from the GC. We compare the AllWISE magnitudes W4 (22 um) versus the IRAS [25] and Spitzer/MIPS [24], and we confirm that W4 \simeq [24] \simeq [25] due to the similarity of these three passbands. By means of the color-color plots, G–W1 vs. W2–W3, we are able to identify and classify stars and extragalactic objects (e.g. galaxies and QSO’s) in the fields towards intermediate and high Galactic latitudes. We show evidence that stars without IR excess have null color-indexes, so that W2 \simeq W3 \simeq W4 \simeq [24], as the spectral energy distribution in the MIR and FIR of the stars outside the Galactic plane can be well represented by the Rayleigh–Jeans approximation. Moreover, extragalactic sources show positive color-indexes of about W3–W4 \simeq 2 mag, due to the thermal emission from the dust in the ISM.

The Galactic Center is a much more challenging region because of the stellar crowding, as well as the strong optical extinction associated with a high sky IR background. Actually, the distribution [24] vs. W4 in the GC region shows a larger spread and systematic offset with respect to the corresponding distribution in the SEP region. Similar results are found for the other passbands [24] vs. W3 or [24] vs. W2. These effects may be due to the non-negligible IR extinction towards the Galactic Center and/or by an instrumental systematic bias produced by the high sky background that contaminates the photometric aperture of the lower resolution WISE data.