Mid-IR Color-Color Diagrams
Mid-infrared colors have been used to identify the presence of AGN-heated dust in the spectral energy distributions of galaxies. We organize AGN mid-IR diagnostics according to the instrument/bandpasses used below.
Physical Motivation
Donley et al (2012) provide us with a very useful illustration that shows spectral energy distributions of galaxies with increasing contribution from AGN (from 0% to 95%) for both obscured and unobscured quasar components. In the obscured case, a dust attenuation of AV=2 mag was applied to the QSO template. In both panels of Figure 1 (unobscured and obscured QSO), one can clearly see the excess mid-infrared contribution at 3-10 μm, which can be probed with, e.g., Spitzer/IRAC and WISE observations. In the optically-obscured case, the UV and optical emission is very suppressed and therefore the accretion disk would not be observable directly as a UV excess or bright point source in the optical. Some of these systems may still be picked up through their narrow emission line signatures and/or X-ray emission, while other more extremely buried cases may be elusive at all other bands.
Figure 1. Reproduced from Donley et al. (2012): "Composite SEDs constructed using the QSO1 and M82 templates of Polletta et al. (2008), scaled to give 1-10 μm AGN contributions of 0% (red) to 95% (purple). The final SEDs have been normalized at 1.6 μm. In the lower panel, we apply an extinction of AV = 2 to the QSO1 SED using the Draine (2003) extinction law. The four IRAC bands at z = 0 are shaded. While luminous unobscured and obscured AGNs have very different UV-optical SEDs, luminous AGNs should display a red MIR power-law SED regardless of obscuration."
IRAC/MIPS Color Selections
A few methods were developed based on Spitzer/IRAC photometry.
log(S5.8/S3.6) > -0.1
log(S8.0/S4.5) > -0.2
log(S8.0/S4.5) > 0.8 * log(S5.8/S3.6) + 0.5
From Lacy et al (2007): "The log(S5.8/S3.6) cut was moved by +0.1 compared to [Lacy et al (2004)]. The exact position of this cut is not critical, and it was felt that shifting this would help remove non-AGN contaminants, particularly as the final 5.8 micron fluxes in the XFLS catalog were increased by 10% in the final version of the XFLS catalog relative to the catalog used [by Lacy et al (2004)]."
ch1 = [3.6]
ch2 = [4.5]
ch3 = [5.8]
ch4 = [8.0]
Vega Magnitudes:
(ch3 - ch4) > 0.6
(ch1 - ch2) > 0.2 * (ch3 - ch4) + 0.18
(ch1 - ch2) > 2.5 * (ch3 - ch4) - 3.5
Alonso-Herrero et al (2006) and Donley et al (2007) both fit a power-law between 3.6 and 8 micron and minimized to select galaxies whose IRAC SEDs followed a power law with spectral index < -0.5. Donley et al. (2007) define a good fit with "a cut in the probability of P>0.1", where "P is the probability that a fit to a power-law distribution would yield a value greater than or equal to the observed ; a probability of 0.5 corresponds to a reduced chi2 of 1."
The revised AGN selection criteria from Donley et al. (2012) are defined, for x=log(f5.8/f3.6) and y=log(f8.0/f4.5), as follows:
x > 0.08
y > 0.15
y > (1.12 * x)-0.27
y < (1.12 * x)+0.27
f4.5 > f3.6 && f5.8 > f4.5 && f8.0 > f5.8
KI = Ks and IRAC bands
The KI criteria are: Ks-[4.5]>0 and [4.5]−[8.0]>0
KIM = Ks, IRAC, MIPS bands
The IM criteria are: [8.0]-[24]>-2.9x([4.5]-[8.0])+2.8 and [8.0]-[24]>0.5
See figures below, and the captions from the original article. These authors also derived color vs. redshift tracks and AGN diagnostics, as detailed below.
Figure above: KI Method (Fig. 3 from Messias et al. 2012). Each panel presents a specific group: (a) early/late, (b) starburst, (c) hybrid, and (d) AGNs. The dotted portion of the tracks refers to the 0 < z < 1 redshift range and the solid to 1 < z < 7. Red circles along the lines mark z = 2.5. The light- and dark-gray regions show the photometric scatter due to a magnitude error of, respectively, 0.1 and 0.2 in the bands considered (equivalent to 10% and 20% error in flux, respectively). The criteria are: Ks-[4.5]>0 and[4.5]−[8.0]>0
Figure above: IM portion of the KIM (Fig. 5 from Messias et al. 2012). Each panel presents a specific group: (a) early/late, (b) starburst, (c) hybrid, and (d) AGNs. The dotted portion of the tracks refers to the 0 < z < 1 redshift range and the solid to 1 < z < 7. Red circles along the lines mark z = 2.5. The light- and dark-gray regions show the photometric scatter due to a magnitude error of, respectively, 0.1 and 0.2 in the bands considered (equivalent to 10% and 20% error in flux, respectively). The IM criteria are: [8.0]-[24] > -2.9*([4.5]-[8.0])+2.8 and [8.0]-[24] > 0.5.
5) IRAC color vs. redshift
The advantage of the color-color selections described above is that they require no redshift information. However, when available, the redshift can help break degeneracies as a given SED will make a redshift track that may loop back onto itself within the color-color space. Namely, some high-redshift star-forming galaxies start entering the Lacy and Stern wedges at higher redshifts (especially z>1). In this section, we list some examples of mid-IR color diagnostics that include the redshift information.
5.1) Juneau et al. 2013 (J13)
J13 compared redshift tracks of the IRAC ch1-ch4 color for infrared luminous star-forming galaxies (yellow tracks); dust-free stellar population models (purple tracks) and various AGNs (black lines), including Compton-thick NGC 6240. The have opted to use the most extreme SF galaxy track (IRAS 12112; in cyan) as a dividing line between star-forming galaxies (below) and AGN (above). From the SEDs, this method appears to work well at z>0.5, but may be incomplete with respect to the most extremely obscured cases such as NGC 6240. Also see Figures 2 & 3 from Stern et al 2005 for redshift tracks of various SED types and their IRAC colors.
Figure: Reproduced from J13.
5.2) Messias et al. 2012 (also see Messias et al. 2013)
When secure redshifts are available, the following colors can be used:
Ks-[4.5]>0 at z<1
[4.5]−[8.0]>0 at 1.1<z<2.5
[8.0]−[24] at z > 2.5–3.
In the absence of a secure redshift, the authors recommend using either the "KI" or "KIM" color-color (see above).
WISE Color Selections
"Analysis of Figure 6 suggests that a color cut at W1 – W2 = 0.8 offers an extremely robust AGN sample which is still highly complete. For some uses, a slightly less conservative color cut at W1 – W2 = 0.7 might be preferable, providing a powerful compromise between completeness and reliability for WISE AGN selection."
2) Mateos et al. (2012):
Selection criteria calibrated with the Bright Ultrahard XMM-Newton Survey (BUXS).
y = 0.315 * x
where x ≡ log10(f12um/f4.6um) and y ≡ log10(f4.6um/f3.4um)
The top and bottom boundaries of the wedge are obtained by adding y-axis intercepts of +0.297 and −0.110, respectively. The MIR power-law α=−0.3 bottom-left limit corresponds to: y = −3.172 * x + 0.436
Completeness and Contamination
Any selection method suffers from incompleteness and contamination, and we usually need to make a trade-off between the two. The emphasis on completeness or purity will depend on the details of the scientific question. We can obtain a good indication of the completeness by measuring the recovery rate of known AGNs. Here, we show one such example work by Mateos et al (2012), where the authors used a sample of Bright Ultrahard X-ray AGNs and computed the recovery through WISE colors separating Type 1 (optically unobscured) and Type 2 (optically obscured) subsets from the parent X-ray sample.
Figure: Annotated version of Figure from Mateos et al (2012) of the recovery fraction of AGN as a function of AGN X-ray luminosity for Type 1 (blue) and Type 2 (red) systems. Highlight of the higher success rate at the luminous end, for both type 1s and 2s.
Figure: Annotated version of Figure from Mateos et al (2012) of the recovery fraction of AGN as a function of AGN X-ray luminosity for Type 1 (blue) and Type 2 (red) systems. We highlight the higher success rate for unobscured AGN (type 1, in blue) relative to obscured AGN (type 2, in red).