IRAC Instrument Handbook - IRSA - California Institute of Technology

4.11 Extended Source Photometry
IRAC Instrument Handbook
The photometric calibration of IRAC is tied to point sources (calibration stars) measured within a
standard aperture with a radius of 12 arcseconds. This point-source calibration is applied to all IRAC data
products during pipeline processing to put them into units of MJy/sr (1 MJy/sr = 10 17 erg s -1 cm -2 Hz -1 sr -
1 ). This method results in a highly accurate calibration for point sources. However, transferring this
calibration to extended sources requires extra thought. The discrepancy between the (standard) point
source calibration and the extended source calibration arises from the complex scattering of incident light
in the array focal planes. Our best understanding is that there is a truly diffuse scattering that distributes a
portion of the incident flux on a pixel throughout the entire array.
The surface brightness of extended emission in IRAC images will tend to appear BRIGHTER than it
actually is. The reason for this is two-fold. First, photons that would normally scatter out of the PSF
aperture used to measure a point source are instead captured by an extended source. The scattering
depends on the convolution between the IRAC PSF and how the light is distributed across the focal plane,
which is usually quite complex for extended sources (galaxies, ISM and nebulae). Second, photons are
scattered into the aperture from the emission regions outside the aperture. As a thought experiment, one
can imagine a single point source inside an aperture, which is easy to measure. But if four point sources
are placed around it just outside the measurement aperture, each of them scatters light into the aperture,
which leads to an overestimate of the real flux. For the extended source case, we can imagine the same
experiment taken to the limit where all the regions have emitters in them.
For photometry of extended sources, the calculated flux inside an aperture must be scaled by the ratio of
the extended and point source throughputs. The scaling factors (fp/fex) to be used are given in Table 4.7
(the infinite aperture case). Note that these are not really “throughputs," in the sense that they have
anything to do with the number of photons reaching the detector. It is more accurate to think of them as a
special type of an aperture correction. The values in Table 4.7 are for a very extended, red source like the
Zodiacal light.
The most challenging case of extended source photometry is of objects with sizes on arcminute scales,
within apertures of similar size or smaller. Examples might be typical observations of nearby galaxies. In
this case the aperture correction is related both to the aperture size and the underlying surface brightness
distribution of the target. To derive a set of aperture corrections more appropriate to this case, a detailed
analysis of early-type spheroidal galaxies (due to the relative ease of modeling the light profiles of these
stellar-dominated sources), ranging in size from 20 arcseconds to several arcminutes, was carried out. A
summary of the results is given below, including aperture correction curves that may be applied to
photometry of all types of well-resolved galaxies. These extended source aperture corrections are
somewhat larger than the infinite aperture corrections given in Table 4.7.
A commonly encountered problem is that of measuring the total flux of extended objects that are still
smaller than the standard aperture size used for the photometric calibration. For example, the background
galaxies seen in all IRAC images are often slightly extended on size-scales of a few arcseconds. PRFfitting
photometry of such objects will obviously underestimate their fluxes. One methodology for
handling such sources was developed by the SWIRE project; readers are referred to the data release
Calibration 55 Extended Source Photometry