IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
document for SWIRE, available from the Spitzer documentation website under Legacy projects. Detailed
analysis by SWIRE has indicated that Kron fluxes, with no aperture corrections applied, provide
measurements of small extended sources that agree closely with hand-measured fluxes. Kron fluxes are
provided as one of several flux measures in the popular “SExtractor" software. Note that it is important to
determine that an object actually is extended before using the Kron flux, as it is ill-defined otherwise.
This may be determined by using the stellarity and isophotal area as defined by the SExtractor software.
Selecting limits on these parameters based on their breakdown as a function of signal-to-noise ratio
generally will mimic SExtractor's own “auto" function.
To measure absolute flux on large scales (sizes of order the field of view), consider all the sources of flux
that go into each pixel. The IRAC images are in surface-brightness units. The flux of an extended object
is the integral of the surface brightness over the solid angle of the object. The value of a pixel in an IRAC
BCD is the real sky value plus a contribution from the zodiacal light minus the dark current value at that
pixel. The dark current value is made from observations of a low background region at the north ecliptic
pole and so it contains some small amount of flux of astrophysical origin. The darks have also had an
estimate of zodiacal light subtracted from them before use. The (theoretically) estimated zodiacal light
brightness during an observation is in the BCD header keyword ZODY_EST, and that for the sky dark
observation is listed as SKYDRKZB. While it is possible using the above keywords to recover something
similar to the absolute sky surface brightness, this brightness estimate is still limited by the accuracy of
the underlying model of the zodiacal emission.
In practice, most extended source photometry will usually be performed with respect to a background
region within the image (for example, large aperture photometry of galaxies, nebulae, etc.) and one does
not attempt to measure the absolute sky brightness on large scales (like the zodiacal cloud). The median
value of the pixels located in user-selected background regions is generally a reasonable estimator of the
background.
4.11.1 Best Practices for Extended Sources
Resolved galaxies with apertures centered on the nucleus:
• For sources < 8–9 arcsec in size, treat as point source (small aperture photometry, with local
annular background subtraction)
• For sources > 8–9 arcsec in size, apply extended source aperture corrections (see below).
Emission knots, embedded resolved sources
• If the source is small (compact), treat as point source (small aperture photometry, with local
annular background subtraction)
• If the source is large and fuzzy, use the extended source aperture corrections (see below). Beware
that background structure will introduce large uncertainties (~10%)
Surface Brightness (pixel-to-pixe l measurements)
Calibration 56 Extended Source Photometry