The X-ray to MIR relation

X-ray (LX) and MIR (LMIR) luminosity both have been used as robust indicators of an

intrinsic AGN power. Since the MIR emission originates from re-processed UV emission, it is crucial to study the correlation between the MIR and the X-ray luminosity for understanding the structure of the hot dust surrounding the central SMBH as well as the AGN accretion physics. Several studies have investigated the LX − LMIR relation and found a strong

correlation between X-ray and MIR luminosity (e.g., Lutz et al. 2004; Gandhi et al. 2009; Fiore et al. 2009; Lanzuisi et al. 2009; Lusso et al. 2011; Asmus et al. 2015; Stern 2015). The linear LX− LMIR correlation has been investigated for local Seyfert galaxies by Lutz et al.

(2004) using low angular resolution MIR data, and later by Gandhi et al. (2009) using the spatially resolved high resolution data. Gandhi et al. (2009) found that even the obscured AGNs follow the same correlation as the unobscured AGNs without large offsets or scatter. Using Spitzer, this correlation has been extended towards higher luminosities for luminous

Figure 4.9 Relation of the intrinsic X-ray (L2−10 keV) and the 6µm (L6µm) luminosities for

our sample of AGNs. Stars and circles are Type 1 and Type 2 AGN, respectively. Filled symbols mark Herschel-detected sources while empty symbols are Herschel-undetected ones.

quasars (e.g., Fiore et al. 2009; Lanzuisi et al. 2009; Stern 2015), and shown to be valid even for radio-loud AGNs (e.g., Hardcastle, Evans & Croston 2009). Recently, Stern (2015) have demonstrated a luminosity-dependent LX− LMIR relation for luminous quasars, reporting

the LX− LMIR fit bends at higher luminosities to lower LX-to-LMIR ratios.

I investigate the correlation between the X-ray emission and AGN MIR luminosity over a wide dynamic range in luminosities and redshifts using both Type 1 and Type 2 AGNs in the CCLS. In Figure 4.9, I show the intrinsic 2–10 keV X-ray luminosity (L2−10 keV) against

the uncontaminated MIR (L6µm) luminosity derived from the best-fitting AGN component

of the sample of AGNs. Stars and circles represent Type 1 and Type 2 AGNs, respectively. Filled and empty symbols indicate the Herschel-detected and Herschel-undetected sources.

For comparison, I also show the LX− L6µm relation from Lutz et al. (2004; dotted line),

Gandhi et al. (2009; dash-dotted line), Fiore et al. (2009; solid line) and Stern et al. (2015; dashed curve). Lutz et al. (2004) and Gandhi et al. (2009) presented this relation for local Seyfert galaxies, establishing the correlation at low luminosities, while Fiore et al. (2009) and Stern (2015) investigated this relation for the most luminous quasars, presenting the relation from the Seyfert to the powerful quasar regime. I convert the monochromatic luminosity measured at different wavelengths for these comparison samples (e.g., 5.8µm and 12µm) to L6µm using the AGN template. The LX− L6µm distribution of moderate-

luminosity AGNs is in broad agreement with previous studies (e.g., Lutz et al. 2004; Gandhi et al. 2009; Fiore et al. 2009; Stern 2015).

Some of the observed scatter can be attributed to the fact that this study is extending the previous relation, which was derived for a sample of local Seyfert galaxies, to a sample spanning a much wider range of luminosity and redshift. It is also plausible that in a fraction of these sources, the SED-fitting procedure over- or under-estimates the nuclear contribution, which results in a MIR luminosity. However, how much of this uncertainty is inherently a result of the physical conditions of the AGN and torus cloud, as compared to observational selection effects, remains an important unresolved issue, which is beyond the scope of this work. Nevertheless, the comparison points out that, on average, the MIR luminosity derived from the SED-fitting is a reasonably good measure of the AGN luminosity. Indeed, the assumption that the MIR emission is dominated by the AGN emission because of accretion on to the central black hole rather than star-formation from the host galaxy is plausible for most of the sources. This made the X-ray-to-MIR correlation the tightest among the other multiple wavelength correlations found for AGN and especially intriguing because of its applicability to all different AGN types.

I also show the ratio of the X-ray-to-MIR luminosity with respect to X-ray luminosities in Figure 4.10. Gray filled symbols indicate the individual sources which are detected in Herschel photometry, and gray open symbols represent the Herschel-undetected sources. Black squares indicate mean values in the X-ray luminosity bins for the total sample. Red

Figure 4.10 The ratio between the X-ray and the 6µm luminosities for our sample of AGNs. Gray filled symbols indicate the individual sources which are detected in Herschel photometry, and gray open symbols represent the Herschel-undetected sources. Black squares indicate mean values in the X-ray luminosity bins. Red stars and blue circles represent the Type 1 and Type 2 AGN, respectively. The horizontal dotted line marks the average LX − L6µm ratio of local Seyferts from Lutz et al. (2004).

stars and blue circles represent mean values for the Type 1 and Type 2 AGN, respectively. The horizontal dotted line marks the average LX− L6µm ratio of local Seyfert galaxies

from Lutz et al. (2004). This figure indicates that both Type 1 and Type 2 AGNs closely follow the same correlation, indicating good agreement with the Lutz et al. (2004) relation. Gandhi et al. (2009) also have reported that the obscured and unobscured AGN follow the same correlation. It is interesting because the relation might depend on the structure of obscuring dust torus in the sense that the unobscured (Type 1) AGNs should have higher MIR luminosities compared to the obscured (Type 2) AGNs at the same intrinsic power. This suggests that the observed MIR emission is important for determining the AGN bolometric accretion energetic for both obscured and unobscured AGNs.

One advantage of adopting the rest-frame MIR luminosity as an AGN power estimator is that, contrary to the X-ray luminosity, this quantity was measured homogeneously for all the sources, regardless of obscuration of AGNs. Hence, it can be used to derive the AGN bolometric luminosity for the entire AGN population, for example, optically- and infrared-selected AGNs, including the objects with no X-ray emission. Therefore, I derive the MIR bolometric corrections using the bolometric luminosity of AGNs derived from the intrinsic X-ray luminosity with the luminosity-dependent bolometric correction described in Marconi et al. (2004). The MIR bolometric correction is obtained as follows:

log Lbol = 0.70 × log νLν(6µm) + 14.12 (4.4)

Using the MIR bolometric correction, one can derive the AGN bolometric luminosity of Compton-thick AGNs, which might not be detected in the X-ray band because of the high NH absorption.