IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
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IRAC Instrument Handbook
Figure C.4: APEX PRF-fitted photometry with a PRF Map vs. aperture photometry in the Se rpe ns
test field. PRF and aperture fluxes have been corrected as described in the text.
C.3.4 The GLIMPSE Test Field
We also analyzed the GLIMPSE AORKEY 9225728 in a similar manner. This produced similarly good
agreement between the aperture and fitted fluxes. In addition, we stacked the residuals of the brighter
sources in an attempt to determine the size of any systematics, and plotted out the ratio of the residuals to
the uncertainties for the inner four pixels closest to the source position. No significant residual could be
found in a stack of 111 sources with channel 1 fluxes between 50 and 100 mJy, corresponding to a limit
of ~0.1% on the size of any systematic residual. Similarly, no significant difference could be found for the
distribution of the ratio of residual to uncertainty between the pixels near to the peak star position and
pixels in the remainder of the image.
C.3.5 Photometry of Moderately-Resolved Sources
Point source fitting is most appropriate for true point sources. The flux of astronomical objects that are
extended will be underestimated by such a procedure. Nearly all fields observed by IRAC have a
substantial population of faint (10s of micro-Jy) background sources, which are in fact galaxies, and in a
typical 100-second exposure these can approach 100 galaxies per IRAC frame at 3.6 microns. Although a
casual visual inspection of the IRAC data would seem to indicate that the majority of these sources are
compact and point-like, in fact treating them as such will lead to substantial errors in photometry, as these
objects are typically resolved on a scale of ~1 arcsecond (e.g., Lacy et al. 2005, [17]).
This issue has been studied in substantial detail in the IRAC Dark Field, which is the dark current
calibration field for IRAC. This is an extremely deep IRAC pointing of approximately 200 square
arcminutes near the north ecliptic pole, and which reaches the confusion limit in all IRAC bands. More
importantly, there is also deep high spatial resolution HST optical imaging over the same field, which can
provide prior information on true source sizes and shapes.
Point source fitting was used to extract photometry for the IRAC Dark Field. An examination of the
point-source subtracted residual images shows clearly that the residuals mimic the HST source
morphology, conclusively demonstrating that IRAC does in fact resolve the majority of the faint galaxies.
This result is strongest at the shorter IRAC wavelengths, where the spatial resolution is higher and the
galaxies may be slightly more extended. This result was hardly unexpected - calculations of expected
galaxy angular sizes assuming a modern cosmology indicated that most galaxies would be marginally
resolved by IRAC almost regardless of distance, modulo changes in galaxy morphology with redshift and
the ability to detect faint extended emission.
Curves of growth were generated for the galaxies, and when used in conjunction with the optical priors,
the amount of error associated with point source fitting was quantified. Sources below a few micro-Jy
start to be affected by confusion issues, so we describe here results for galaxies brighter that this. At 3.6
microns, roughly 50% of all galaxies are demonstrably resolved by IRAC. In 20% of the objects, the use
of point source fitting will underestimate the true flux by a factor of two or more.