IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
To understand where the IRAC zmag comes from, you can start with the fundamental equation between
magnitudes and flux densities. In one incarnation, it becomes
m - M0 = -2.5xlog(F/F0) (4.16)
Here m is the magnitude of the source you want to measure, M0 is the zero magnitude (= 0), F is the flux
density in Jy of the source you want to measure and F0 is the flux density of a zero magnitude source. For
IRAC channel 1 in cryogenic mission, F0 = 280.9 Jy. Expanded out, this becomes thus
m = -2.5xlog(F) + 2.5xlog(F0) (4.17)
Here 2.5*log(F0) is the same as zmag. Now, since the IRAC images are in units of MJy/sr, we have to do
some manipulation to get the equation to this form. Specifically, the measurable F that we have in IRAC
images is the surface brightness, not the flux density. So therefore the equation becomes
m = -2.5xlog(SB*C) + 2.5xlog(F0) (4.18)
where SB is the measured surface brightness in the image in MJy/sr and C is a conversion factor from
MJy/sr to Jy/pixel. For IRAC channel 1 mosaics with 0.6 arcsec x 0.6 arcsec pixels it equals C =
8.461595 x 10 -6 Jy/pixel/(MJy/sr). Therefore the equation becomes
m = -2.5xlog(SB) + 2.5xlog(F0/C) (4.19)
where zmag now corresponds to the latter term, +2.5xlog(F0/C). Inserting the values of F0 and C
mentioned above, we get zmag = 2.5xlog(280.9/8.461595E-06) = 18.80 mag in channel 1.
Please remember that this is true only for the 0.6 arcsec x 0.6 arcsec pixel scale mosaics. For other pixel
scales you will get a different value. Also, please remember the required corrections (e.g., aperture
correction) that are needed for high accuracy photometry.
Calibration 51 Calculation of IRAC Zmags