AGN influence on the host galaxy dust temperature?

Our analysis in section 4.2 demonstrates there are no connection between cold dust temperature and the presence of AGN, in agreement with recent result (e.g. Elbaz et al.

– 38 –

Table 2: Confirmed spectroscopic redshift sample Telescope Instrument N^(a) Reference

ESO-VLT VIMOS 49 zCOSMOS

Lilly et al. (2007) Magellan IMACS 26 Trump et al. (2007)

Sloan SDSS 15 Abazajian et al. (2009) Keck II DEIMOS 11 Kartaltepe et al. (2010) MMT Hectospec 1 Prescott et al. (2006)

(a)Number of 70µm/X-ray sources with spectroscpic redshift.

Table 3: Results of the K-S test on the hardness ratio between different catalogs

Catalogs Dmax Significant Hypothesis

level

XMM & Chandra 0.06637 0.0279 Not Reject XMM & 70µm/X-ray 0.1920 0.0026 Reject Chandra & 70µm/X-ray 0.1981 0.0018 Reject

2010). This result could attribute to the spatial scale problem, the cold dust component is so far away from the central energetic source that is insufficient for heating the cold dust component.

Rafferty et al. (2011) used infrared color log(F24µm/F⁷⁰µm), to sample the blackbody temperature profile and concluded that galaxies hosting an AGN present a high dust temperature, which is inconsistent with our temperature fitting result. We reproduced their analysis for our sample, as shown in Figure 11. The 70µm/X-ray galaxies present a higher color index in average than overall 70µm galaxies at all redshifts, a same conclusion to Rafferty et al. (2011). Though the 70µm/X-ray galaxies show higher log(F24µm/F⁷⁰µm), they still agree with the local definition of cold dust (e.g. log(25/60) < 0.2 de Grijp et al.

1985; Sanders et al. 1988b).

To explain the difference in color index, we looked at the evolution of the F24µm/F⁷⁰µm color as a function of the redshift for two typical objects, the star-forming ULIRG Arp220 and the AGN ULIRG Mrk231. We used the galaxy templates provided by Polletta et al.

(2007), as shown in Figure 11. In the case of Arp220, the rest frame 9.7µm absorption line is shifted into the 24µm band at redshift z ∼ 1.5, causing an apparent lower value of the color index. Similarly, at z > 2, the 7.7 and 8.2 µm PAH emission lines are entering the 24µm band, inducing a higher F24µm/F⁷⁰µm color ratio. In the case of Mrk231, the AGN driven power-law continuum produces a more stable color index. Our 70µm/X-ray samples follow the same color index of Mrk231-like galaxies across the redshift range 0 < z < 3, and the remaining parts of our samples follow more a Arp220-like profile. We supported the idea that the F24µm/F⁷⁰µm color is sensitive to hot dust component from nearby region of AGN, on the other hand, for the longer wavelength (e.g. Elbaz et al. (2010) and our temperature fitting work), it would respond to the cold dust temperature from star formation in host galaxy. We summarized that the color of F24µm/F⁷⁰µm and temperature

– 40 –

fitting trace different temperature components of dust, the 24µm is sensitive to warm dust around AGN, therefore we can see the discrepancy of color in X-ray detected 70µm galaxies.

On the other hand, the temperature fitting method and Elbaz et al. (2010) work included the longer wavelength of SED, star formation dominated the cold dust whether the AGN presence or not.

To have a robust measurement of the variation of dust temperature in the host galaxy, we need to explore the longer wavelengths that have less perturbation from emission (e.g.

8.6 µm and 11.2 µm PAH lines) and absorption (e.g. 9.7 µmsilicate line) line features.

For instance, measuring the longer wavelength color (e.g. F70µm/F¹⁶⁰µm) is a more accurate method to estimate the dust temperature (Casey 2012). We compared the value of F70µm/F¹⁶⁰µm between X-ray detected and X-ray undetected 70µm galaxies, for those lacking 160µm measurement 70µm galaxies, we applied upper limit from flux limit as their F160µm monochromatic photometry . The K-S test shows that is 9% draw out of the same population, we cannot rule out they originate from the identically parental sample, this conclusion is consistent with temperature fitting result.

In order to simplify the temperature fitting, the models of temperature usually assume that β is a constant value. However, it depends on which photometric bands are included;

shorter wavelength photometry will induce the lower β value, and probe a systematically higher temperature (Magnelli et al. 2012). Our fitting procedure provides no strong evidence that the cold dust of host galaxy is related to AGN (see section 4.2). Such result is consistent with the latest work from the Herschel observations (Elbaz et al. 2010). Actually, the photometry in the 250µm, 350µm, and 500µm bands provides information on the Rayleigh-Jeans portion of the spectra, and therefore enables a more accurate estimation of the dust temperature of the host galaxy. Such precise measurements show a 2− 3K differences between the dust temperature of a galaxy with AGN and without AGN (Elbaz

Fig. 11.— Distribution of log(F24µm/F⁷⁰µm) color index for 70µm selected sample and 70µm/X-ray sample. Lef t panel : Distribution of the color against redshift for the 70µm galaxies (black crosses) and the 70µm/X-ray sample (red crosses). The green and orange lines indicate the evolution of Mrk231 and Arp220 flux ratio as a function of redshift, computed using the templates from Polletta et al. (2007). Right panel : Histogram of the color index for 70µm-selected galaxies in black and 70µm/X-ray in red, with the red histogram normalized to the peak of black sample for better display (bottom and top axis represent the number of 70µm-selected galaxies and 70µm/X-ray respectively). The 70µm galaxies have a median color index of log(F24µm/F70µm) ∼ −1.25, while for 70µm/X-ray galaxies it is of ∼ −0.99.

Both satisfy the local cold dust definition of log(F24µm/F⁷⁰µm) ≤ −0.7 (de Grijp et al. 1985;

Sanders et al. 1988b).

– 42 –

Fig. 12.— The color index of log(F70µm/F160µm) against redshift for 70µm galaxies. The red color and black color is 70µm/X-ray galaxies and 70µm galaxies without X-ray detection, respectively. The cross symbol indicates the 70µm galaxies have 160µm detection, otherwise, the upper arrow symbol indicates the 160µm photometry of those 70µm galaxies is derived from observation limit. The green and orange lines indicate the evolution of Mrk231 and Arp220 flux ratio as a function of redshift, computed using the templates from Polletta et al. (2007).

et al. 2010). The nondistinctive far-infrared colors imply the physical mechanism drives the AGN hosted galaxies is similar to those star forming galaxies.