LX vs. LIR relation

The relation between hard X-ray luminosity and total infrared luminosity provides us with another method to determine whether star formation from host galaxy or AGN predominates the overall SED. While color-color selection can be altered by the presence of specific lines (Donley et al. 2008), total infrared luminosity is more robust because it is derived from 8-1000µm continuum (Smail et al. 2011).

In Figure 8 we display the intrinsic rest frame hard X-ray (2− 10keV) luminosity against total (8− 1000µm) infrared luminosity for our 70µm/X-ray galaxies, according to their LIR luminosity, subdivided into three sub-samples: star-forming galaxies, LIRGs, and ULIRGs (same symbols as in Figure 7). The hard X-ray luminosity has been rectified NH

absorption according to HRs and k-correction, totally, there are 116 sources have hard X-ray detection, and 96 out of 116 (91%) have well-defined HRs that we could measured intrinsic rest-frame 2-10keV luminosity (absorption correction and k-correction detail, please see Section 3.5). The X-ray luminosity versus Infrared luminosity plot shows a

-1 0 1 2 3 ([5.8]-[8.0])_vega

-0.5 0.0 0.5 1.0 1.5

([3.6]-[4.5])vega

log(L_IR) < 11.0 LO ^•

11.0 ≤ log(LIR) < 12.0 LO ^•

12.0 ≤ log(LIR)

Arp220 Mrk231

Fig. 7.— [3.6]-[4.5] versus [5.8]-[8.0] color-color diagram of 70µm galaxies with X-ray de-tection. The different LIR-selected galaxy sample follow the description of Figure 1, with blue stars, green squares and red cross indicating star-forming galaxies, LIRGs and ULIRGs respectively. The pink solid continuous line represents the boundaries of the AGN region defined by Stern et al. (2005). Orange dash line and black solid line represent the evolution with redshift from z = 0 to z = 3 of the colors of Arp220 and Mrk231 templates, respec-tively. The S05 criterion are not efficient in separating AGNs and Arp220-like star-forming dominated ULIRGs.

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tight correlation with Spearman’s ρ ∼ 0.88. However, this strong correlation may be a manifestation of an observational bias. To confirm this, we need to estimate the upper and lower limits in the LX vs. LIR plot. In the case of the total infrared luminosity, we slice the whole 70µm catalog (Kartaltepe et al. 2010) into several redshift bins from z = 0.033 to z = 3, and identify the maximum and minimum total infrared luminosity. For the hard X-ray luminosity, we simulate the lower X-ray luminosity in the same redshift bins using the Chandra flux limits, and measure the upper X-ray luminosity directly from the XMM catalog (Brusa et al. 2010). The results are displayed as gray regions in Figure 8. These observational limits and their evolution with redshift indeed explain the tight correlation between the infrared and X-ray luminosities.

By studying X-ray properties of star-forming galaxies, Ranalli et al. (2003)

demonstrated that hard X-ray luminosity is also proportional to star formation rate (SFR).

In fact, the most important contributors of X-ray emission in such galaxies are the high mass X-ray binaries (HMXBs). Franceschini et al. (2003) explored the X-ray properties of a sample of ULIRGs; most of them are without AGN signature. The conclusion of their work is that the X-ray luminosity and spectral shape of some ULIRGs are dominated by hot thermal plasma and X-ray binaries, originated in recent starburst region.

On the other hand, because of the presence of the parsec-scale dust “torus” surrounding the accretion disk of the central SMBHs, AGNs emit light in the infrared wavelength (Gandhi et al. 2009). Indeed, the ultraviolet and optical light emit by the central accretion disk is absorbed by the dust and reemit in infrared. Gandhi et al. (2009) observed the core of nearby AGNs with unprecedented high spatial resolutions in both mid-IR and X-ray wavelength, demonstrated a strong correlation between 12.3µm and hard X-ray luminosities without the star formation perturbation from host galaxy. Although observed frame in 12.3µm would influenced by PAH 11.3µm line in Gandhi et al. (2009) samples, the

Fig. 8.— Absorption-corrected rest frame 2-10keV luminosity versus total infrared luminos-ity of 70µm selected galaxies with X-ray detection. Symbols are the same than in Figure 7.

The X-ray luminosity error bars are derived from the estimated flux errors and the total infrared luminosity error bars are derived from the 1σ probability distribution of the χ²SED fitting. Gray regions represent the areas covered by maximum and minimum luminosities in different redshifts bins drawn from the full X-ray selected and and 70µm selected samples.

These observational limitations are inducing the apparent strong correlation between LXand LIR observed in in COSMOS field. Black dot line and black solid line represent the LX vs LIR relations for pure AGN and and star-forming galaxy samples, respectively. Dash-dot line and dash line represents 1 σ dispersion of AGN and star-forming equations (Mullaney et al. 2011; Ranalli et al. 2003). The luminosity of star forming submillimeter galaxies (SMGs) and AGN SMGs, base on their X-ray spectrum fitting, are represented by filled and open purple circles receptively (Laird et al. 2010).

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small value of dispersion would may be due to statistical uncertainty rather than physical reasons (Lutz et al. 2004). Based on the intrinsic AGN/quasar IR SED from Netzer et al. (2007), Mullaney et al. (2011) related the 12.3µm luminosity to the total infrared luminosity (LIR) (their equation 5). By combining the Gandhi et al. (2009) and Mullaney et al. (2011) relations, we derived the unbiased LX vs. LIR relation for AGNs, free of any contamination from the host galaxy. This “pure” AGN LX vs. LIR relation is represented by the dotted line in Figure 8. Such relation has been confirmed by swift-BAT X-ray selected AGN population (Tueller et al. 2010), together with IRAS infrared measurements (BAT/IRAS AGN). The energy band of 14-195keV in swift-BAT observation could select the galaxies with independent of obscuration. Indeed, their 60µm luminosity is more likely to be powered by the AGN rather than star formation activity from the host galaxy because of linearly increasing correlation between LIR and LX (Mullaney et al. 2012). For our COSMOS sample, the LX vs. LIR distribution of 70µm/X-ray galaxies deviates from AGN relation and star forming relation by 0.5− 1 dex and 1 − 2 dex, respectively. From the infrared and X-ray continuum perspective, AGNs approximately dominate the entire system.

Submillimeter galaxies (SMGs) are a population of objects selected according to their detection in submillimeter wavelengths. This population is dominated by strongly star-forming galaxies at high redshift, with this star formation producing a large amount of cold dust (Chapman et al. 2005). The concomitance in redshifts and infrared luminosities with ULIRGs indicates that SMGs may be an early stage of evolution of the merger scenario (Greve et al. 2005; Biggs & Ivison 2008; Engel et al. 2010). Ultra-deep X-ray observations for those distant star-forming galaxies indicate that 20-30% of SMGs host an AGN (Alexander et al. 2005). Laird et al. (2010) have studied the X-ray spectral properties of SMGs to classify them as AGN or starburst. These objects are represented in Figure 8 with open and closed purple circles, respectively. Our 70µm/X-ray galaxies appear to share

the same LX vs. LIR relation as the AGN-confirmed SMGs.

In summary, all indicators in the LX vs. LIR distribution of our 70µm/X-ray galaxies cover toward a higher AGN activity relative to star formation.