Correction of X-ray properties

With the help of combining both XMM and Chandra X-ray catalogs, we extend our total sample of 70µm/X-ray galaxies to 142 sources, of which 58 are listed in both X-ray catalogs. The count rates for these sources have been converted into 0.5− 2keV (soft band) and 2− 10keV (hard band) flux for both XMM and Chandra observations by assuming different power-law indices. Indeed, XMM and Chandra used 2-8 keV and 2-7 keV energy band for collecting hard X-ray counts. In order to estimate the hard band flux (e.g.

2-10 keV) from count rates, XMM and Chandra observations assumed Γ ∼ 1.7 and 1.4, respectively (Cappelluti et al. 2009; Elvis et al. 2009). For the 58 sources with overlapping detections between the two catalogs, we confirmed both hard and soft band flux from XMM and Chandra are consistent. The correlation in soft band follows the 1:0.9 relation with 1 σ dispersion of 0.063 in log scale, the correlation in hard band follows the 1:1.1 relation with 1 σ dispersion of 0.034 in log scale. Figure 2 show the correlation of hard X-ray luminosity between XMM and Chandra observations.

Given the large redshift range covered by our sample (see Figure 1), it is necessary to apply k-corrections to compute the X-ray rest-frame luminosities of our 70µm/X-ray galaxy sample. We assumed that the spectrum of our galaxies in X-ray follows a simple power law with photon index Γ = 1.7, and derived the ratio between rest-frame and observe-frame flux integration, to compute the rest-frame flux. Although the different Γ assumption would affect k-correction factor, the luminosity of hard X-ray does not have significant change, we still applied Γ = 1.7 for 70µm/X-ray galaxy sample whether they extracted from Chandra catalog or not.

One of the main goals of our study is to explore the relationship between the galaxy host extinction and the central region obscuration. However, the neutral hydrogen column density identified in X-ray is integrated on the line-of-sight, therefore estimations of the

Fig. 2.— Hard X-ray luminosity from the Chandra catalog (Elvis et al. 2009) versus hard X-ray luminosity from the XMM catalog (Brusa et al. 2010) for our 70µm/X-ray galaxies.

The correlation factor follows a well-fit 1:1.1 relation with 1 σ dispersion of 0.034 in log scale.

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intrinsic column density from X-ray spectrum fitting become model-dependent (Akylas et al. 2006). In order to simplify our analysis, we decided to use the hardness ratio (hereafter HR) as a good proxy of central region obscuration, define as:

HR = H− S

H + S (3)

where H is X-ray hard band counts and S is X-ray soft band counts.

Our HR values are extracted from Brusa et al. (2010) for XMM observations and Elvis et al. (2009) for Chandra observations. To calibrate our empirical conversion factor, we take advantage of our sources detected in both observatories. In Figure 3, we compared the HR from XMM with the HR from Chandra, for the overlapping detected sources. We have employed a linear fitting for 38 of 70µm/X-ray galaxies, which have both HR from XMM and Chandra. The result followed a 1.03:1 correlation with 1 σ dispersion∼ 0.06 (χ²∼ 0.4).

The spectral variability could induce the inconsistency in different observations, Mateos et al. (2007) have summarized the ∆ Γ∼ 0.2 changes in observed X-ray color of ∆ HR ∼ 0.1.

In fact, our typical HR dispersion is∼ 0.05 - 0.1, it cannot rule out the influence of spectral variability among our samples. Flux variable AGNs are another uncertainty, HR suffers the flux variability that will hard to derived the value of absorption accurately. According to former paragraph, the hard and soft band luminosity from XMM and Chandra are almost keeping the consistency that we could conclude there is less contribution in our samples from flux variation. In order to unify HRs from different observations, all of our HR values are in XMM band system.

There is 96/142 (68%) of the 70µm/X-ray sources are detected in both soft and hard X-ray band that provided the well-defined HR values. For the rest 46/142(32%) objects, 24 are only soft band detection, 21 are only hard band detection, and 1 is full band detection (neither detected in soft band nor hard band). For the purpose of presenting intrinsic hard X-ray luminosity, correcting the absorption by neutral hydrogen is obligatory. We

Fig. 3.— Hardness ratio extracted from the XMM catalog (Brusa et al. 2010) vs. value from the Chandra catalog (Elvis et al. 2009) for our 38 sources detected with both observatories.

The different hard X-ray energy bands between the two observations, influence the hardness ratio measurements. We fit a simple line to estimate a conversion factor between the two different measurements. The fitted conversion factor is of 1.03 with small dispersion 1 σ ∼ 0.06 (χ² ∼ 0.4).

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Fig. 4.— The hardness ratio (HR) versus redshift of 70µm/X-ray galaxies. Error bar indi-cates 1 σ dispersion from either XMM or Chandra catalogs. The solid lines from bottom to top present different amount of NH = 1 x 10²¹, 3 x 10²¹, 1 x 10²², 3 x 10²², 1 x 10²³, 3 x 10²³, and 1 x 10²⁴ cm⁻², respectively. Because of negative k-correction effect in X-ray wavelength, higher energy photon can penetrate a large amount of neural hydrogen indicates the HRs decrease with increased redshift. The HR value below the -0.5 indicates there is no obscuration in X-ray emission.

attempted to use HRs as an indicator for column density of NH along light-of-sight that could provide the clue to derive unabsorbed hard X-ray luminosity. First, in order to prevent degeneracy among photon index and column density, we fixed the intrinsic photon index of power law to 1.7 as general broad-line AGN. To simulate the observed HRs, we used Portable, Interactive, Multi-Mission Simulator (PIMMS)⁴ from HEASARC and set a fake source with varied amount of NH, covering a redshift interval 0 < z < 3. Then comparing the expected HRs to those well-defined ones (96/142; ∼ 68%), we could obtain the appropriate quantity of column density. Figure 4 displays the value of HR versus the redshfit, the solid lines from bottom to top present different amount of NH = 1 x 10²¹, 3 x 10²¹, 1 x 10²², 3 x 10²², 1 x 10²³, 3 x 10²³, and 1 x 10²⁴ cm⁻², respectively. The mean value of HR for 70µm/X-ray galaxies in COSMOS field is -0.16 with corresponding NH = 2.7 x 10²² cm⁻². The histogram of NH for 70µm/X-ray galaxies is shown in Figure 5.

Second, we inputted the best-fit column density with known redshift and the assumption of photon index Γ ∼ 0.7, we could measure the correction factor between absorbed and unabsorbed hard X-ray luminosity for well-defined HR 70µm/X-ray galaxy.

Figure 6 shows the absorption correction factor versus redshift, the mean value of Frest/Fobs

is about 1.05. In fact, the intrinsic rest frame 2-10keV X-ray luminosity is strongly dependent upon the precise quantity of absorption and initial photon index (Γ) assumption (Comastri 2004).

To ensure the intrinsic 2-10keV luminosity robustness, we only estimated the absorption correction for those samples that have both soft and hard band X-ray detection. Similarly, in the latter statistical analyze, we have excluded any sample with no counts in hard band or soft band (i.e. HR = -1 or 1). However, if we set a flux limit as lower limit, lacking soft band counts samples (i.e. HR = 1) could imply the presence of Compton thick

4http://heasarc.gsfc.nasa.gov/Tools/w3pimms.html

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Fig. 5.— The histogram of NH for 70µm/X-ray galaxies. The column density of neutral hydrogen is derived from well-defined HRs by assuming photon index (Γ) = 1.7. The mean value of HR for 70µm/X-ray galaxies in COSMOS field is 2.7 x 10²² cm⁻². The samples with NH< 1.0 x 10²¹cm⁻²are not shown in Figure 4 because they are derived from extrapolation.

Fig. 6.— The absorption correction factor (Frest(2-10keV)/Fobs(2-10keV)) versus redshift.

Error bar comes from 1 σ dispersion of HR. The solid lines from bottom to top present different amount of NH = 1 x 10²¹, 3 x 10²¹, 1 x 10²², 3 x 10²², 1 x 10²³, 3 x 10²³, and 1 x 10²⁴ cm⁻², respectively. Absorption correction factors with < 1 are set to 1.

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AGNs (1 x 10²⁴> NH), a notable population in X-ray background synthesis model for the interpretation of intensity peak about 30 keV (Comastri 2004). We will discuss the samples with lacking soft X-ray emission in section 4.3. Previous studies have shown that the star-forming galaxies and infrared excess galaxies are likely to host Compton-thick AGN (Daddi et al. 2007; Fiore et al. 2008). But, in this paper, our aim is to study the mutual influence between AGN and host galaxies with 70µm and X-ray selected technique rather than finding the heavily obscured AGNs.