IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Temperature (K)
Channel 5000 2000 1500 1000 800 600 400 200
1 1.0063 0.9990 0.9959 0.9933 0.9953 1.0068 1.0614 1.5138
2 1.0080 1.0015 0.9983 0.9938 0.9927 0.9961 1.0240 1.2929
3 1.0114 1.0048 1.0012 0.9952 0.9921 0.9907 1.0042 1.1717
4 1.0269 1.0163 1.0112 1.0001 0.9928 0.9839 0.9818 1.1215
Table 4.5 gives the color corrections for the spectrum of the zodiacal light, which is the dominant diffuse
background in the IRAC wavelength range. The first model is the COBE/DIRBE zodiacal light model as
implemented in Spot. The zodiacal light is mostly due to thermal emission from grains at ~ 260 K over
the IRAC wavelength range, except in channel 1 where scattering contributes ~ 50% of the brightness.
The second zodiacal light spectrum in Table 4.5 is the ISOCAM CVF spectrum (5.6−15.9 µm; Reach et
al. 2003, [22]) spliced with the COBE/DIRBE model at shorter wavelengths.
Table 4.5: Color corrections for zodi acal light s pectrum.
Band
COBE/DIRBE model ISOCAM+COBE/DIRBE
Iν ( MJy / sr)
0
K
Iν ( MJy / sr)
0
K
quot
Iν 0
1 0.067 1.0355 0.069 0.40 1.0355 0.42
2 0.24 1.0835 0.26 1.44 1.0835 1.56
3 1.11 1.0518 1.16 6.64 1.0588 7.00
4 5.05 1.0135 5.12 25.9 1.0931 28.4
For sources with complicated spectral shape the color corrections can be significantly different from
unity. The corrections are infinite in the case of a spectrum dominated by narrow lines, because there may
be no flux precisely at the nominal wavelength, which only demonstrates that such sources should be
treated differently from continuum-dominated sources. We calculated one illustrative example which may
prove useful. The ISO SWS spectrum of NGC 7023 is dominated by PAH emission bands and a faint
continuum over the IRAC wavelength range. Table 4.6 shows the color corrections using equation 4.8 and
the ISO spectrum. The large value in channel 1 is due to the presence of the 3.28 µm PAH band, which
dominates the in-band flux relative to the weak continuum at the nominal wavelength of 3.550 µm.
Channel 2 is mostly continuum. Then channel 3 is dominated by a PAH band at 6.2 µm. Channel 4 has
significant PAH band emission throughout, with prominent peaks at 7.7 and 8.6 µm. The values in this
table can be used for comparison to IRAC colors of other sources by anti-color-correction, which gives
the predicted colors for NGC 7023 in the same units as the SSC calibrated data: F F K,
0
0
quot
ν = ν × which
is shown in the last column of Table 4.6. Thus, PAH-dominated sources are expected to have
.
quot
Iν 0
quot
quot
Fν
8µ
m)
/ Fν
( 5.
8µ
m)
= 599 / 237 = 2.
5
(4.13)
0
( 0
Calibration 41 Color Correction