Hardness ratio of 70µm/X-ray galaxies

To explain the diversity of AGNs (e.g., narrow-lines vs. broad-lines), a unified model, based on the variation of obscuration due to the orientation of the dust torus surrounding the SMBH accretion disk, has been proposed. In such model, the presence of an edge-on dust torus not only blocks the broad-line emissions, but also increases the absorption in soft X-ray because of the higher hydrogen column density (Antonucci 1993).

In addition, for on-going galaxy mergers, the star formation trigger the additional amount of neutral hydrogen formation, absorbing soft X-ray (e.g., Esquej et al. 2012).

Indeed, using the Extended Chandra Deep Field-South (ECDF-S) dataset, Treister et al.

(2009) observed large obscuration of the soft X-ray emission in star-forming galaxies hosting an AGN at high redshifts, revealing a large amount of neutral hydrogen on the line-of-sight.

Our aim is investigating if there is a direct connection between the obscuration properties of the AGN and those of the host galaxy. As absorption affects the soft X-ray emissions more than the hard X-ray emissions, the HR is a good tracer of obscuration, with higher value of HR corresponding to larger absorption. To test whether our two samples, X-ray selected galaxies and 70µm/X-ray galaxies, are consistent with being drawn from the same sample (i.e. our null hypothesis), we apply Kolmogorov-Smirnov (K-S) test (Numerical Recipes, Press et al. 1992).

As an initial test, we examined the HR of XMM and Chandra X-ray selected sources, to test the robustness of our conversion between XMM HR and Chandra HR (conversion factor for Chandra HRs, see section 3.4). The results of our K-S test for these populations are shown in Figure 10, with the HR cumulative probability distribution of XMM selected sample in black solid line and Chandra selected sample in red dotted line, and the K-S parameters are summarized in Table 3. The probability is only 3% that XMM sample and Chandra sample are drawn from same parent samples, which is not significant enough to

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reject the null hypothesis (our threshold to reject the null hypothesis is 99%). Although two X-ray facilities have different sky coverage and depth, XMM HR distribution is roughly identical to the Chandra HR distribution.

Then, we apply the K-S test between the 70µm/X-ray and XMM samples. This time, the null hypothesis is rejected at a significance of 99% confidence level, implying that 70µm/X-ray galaxies and XMM galaxies are drawn from different populations. The K-S test between 70µm/X-ray and Chandra selected samples also rejects the hypothesis. The probability of drawing from same population for XMM & 70µm/X-ray is ∼ 0.3%, for Chandra & 70µm/X-ray is 0.2%. Figure 10 displays the cumulative probability distribution of HR from XMM (black solid line), Chandra (red solid line), and 70µm/X-ray (green solid line) selected samples. The K-S parameters resulting from the three tests are summarized in Table 3.

In fact, the probability distribution fraction below HR ≤ 0.3 of XMM, Chandra, and 70µm/X-ray galaxies is similar, indicates the unobscured AGN fraction is uniform. The obvious diversity appears above the HR > -0.3, in terms of the probability distribution of XMM and Chandra, there is 80% AGNs with HR ≤ 0, however, there is only 60% AGNs with HR≤ 0 from 70micron/X-ray selection. According to column density derived from HR, the mean value of NH for overall 70µm/X-ray galaxies corresponds to 10²² cm⁻² which is consistent with mildly absorbed AGN population definition. But there are 21 70µm/X-ray galaxies lacking in soft X-ray among our sample, which could be candidates of Compton thick AGN we did not count in K-S tests. In order to derive the column density for lacking soft X-ray detection 70µm/X-ray galaxies, we assumed their soft X-ray count rate from flux limit, and then calculated the hardness ratio as a lower limit. Applying the same method (section x.x), we obtain the mean value of column density is at least ∼ 10²³ cm⁻² for 21 out of 70µm/X-ray galaxies. From lacking soft X-ray detection sample, there is one object

(X-ray ID = 5042, 70µm ID = 1539) could be a Compton thick AGN (NH ∼ 1.45 x 10²⁴ cm⁻²), its photometric redshift corresponds to 2.36 with total infrared luminosity exceed 10¹³ L⊙, belonging to hyper ULIRG. Even we neglected the lacking soft X-ray detection 70µm/X-ray galaxies, the K-S tests for well-defined HR 70µm/X-ray galaxies is significant enough to show there is additional neutral hydrogen exists with 70µm/X-ray selection.

The tests indicate that 70µm/X-ray galaxies include more obscured AGNs than purely X-ray selected samples. This implies that, in the case of AGNs hosted by dust-enshrouded galaxies, the presence of dust would not only associated to AGN unified model scheme but also additionally physical process. Furthermore, the NH of dust-enshrouded galaxies from 70µm/X-ray selection is not heavily obscured like Compton thick AGNs, predicted from X-ray background synthesis. We speculate that the excess of obscuration is not only attributed to absorption on the line-of-sight by continuum or clumpy dust torus around the SMBH, but also is associated with an additional more diffuse dust component generated by strong star formation in the host galaxy. The starburst can be subsequence of an event involving the whole system (e.g. merger) or generated by the circumnuclear region (e.g.

bar). Because of tight symbiosis between AGN and host galaxy, hence we can not neglect the influence of the host galaxy on AGN properties. Only X-ray observations with very high spatial resolution will enable us to identify the origin of the extra amount of neutral hydrogen.

5. DISSUSSION

5.1. 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.

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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

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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).

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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.

5.2. AGN obscuration from starburst?

According to the major merger scenario of galaxy evolution, developed to interpret a potential ULIRG-QSO connection, galaxies undergo an obscuration phase after the merger occurred due to an enhancement of star formation and dust production. This obscured phase is thought to be ended by a consecutive AGN phase, during which feedback from the central SMBH repels the dust (Hopkins et al. 2008). Giving an illustration, the NGC 6240, low-ionization nuclear emission-line regions (LINER), is a well-studied ULIRG, with high infrared luminosity indicating a starburst activity that generates a large amount of dust and molecular gas (Iono et al. 2007). A detailed X-ray spectrum from BeppoSAX of NGC 6240 revealed a very strong absorption from the neutral gas column density NH ∼ 2 x 10²⁴ cm⁻² (Vignati et al. 1999). Thanks to the higher spatial resolution, Chandra observations of NGC 6240 identified the presence of a double AGNs system, hidden in the core of the galaxy (Komossa et al. 2003). These observations are consistent with the idea that galaxy was formed consequentially of a merger .

To efficiently investigate the evolutionary stages of the AGN concomitance with star formation, our approach is to study the properties galaxies detected in both infrared and X-ray. Rafferty et al. (2011) had a similar approach to ours, looking for the X-ray counterpart of 70µm-selected galaxies in the ECDF-S, CDF-S, and EGS fields. Performing both a X-ray spectrum and a X-ray band ratio analysis, assuming a power-low spectrum derives different column densities, they found the column density derived from X-ray band ratio that are systematically low. From the X-ray spectrum confirmed sample (Tozzi et al.

2006), they did not identify any excess of neutral hydrogen in 70µm-selected X-ray sources

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compared to other X-ray selected AGNs (Figure 4 in Rafferty et al. (2011) paper). They concluded that the absence of obscured AGN among the star-forming galaxies contradicts to the current AGN and galaxy co-evolution model (Hopkins et al. 2008). Actually, the main reason why Rafferty et al. (2011) did not detect any additional obscured AGN from star-forming galaxies is probably the statistical insufficiency of their sample. Although they have 158 70µm/X-ray galaxies with 108 were identified as AGN, for the purpose of accuracy, they only had accessed to 17 of 70µm/X-ray galaxies, with hydrogen column density NH derived by X-ray spectral analysis, comparing with whole AGN population (Tozzi et al. 2006).

In addition, Rafferty et al. (2011) have used HRs to measure the column density for correcting absorbed X-ray luminosity, most of 70µm/X-ray galaxies with NH ∼ 10²⁰ - 10²³ that is consistent with our mean value of NH ∼ 10²². Both Rafferty et al. (2011) and our works have identified one possible Compton-thick AGN comparable to the X-ray properties of ULIRGs in local universe (e.g. Mrk231, NGC6240). We speculated it is due to the selection effect, Compton-thick AGN with large amount of neutral hydrogen that even the hard X-ray photon cannot penetrate. Either checking the Fe Kα line or observing higher energy band in X-ray could seek out more Compton-thick AGNs, however, this issue is beyond our scope of this paper.

Our large sample (142 70µm/X-ray galaxies) enables us a proper statistical study. We therefore took a different approach by applying a K-S test on the distribution of hardness ratio to investigate if 70µm/X-ray galaxies and X-ray selected AGNs are drawn from the same sample (see section 4.3). Trichas et al. (2009) attempted to use the X-ray sources with 70µm counterparts counterparts in the redshift range 0.5 < z < 1.3 from the Spitzer Wide Area Infrared Extragalactic (SWIRE) survey, but they have 3% sufficient probability to conclude that X-ray sources with 70µm detection were drawn from the global X-ray

population. In contrast, we used the opposite selection method - 70µm sources with X-ray counterparts since the redshift to 3 in the COSMOS field. Owing to a factor of 2 deeper 70µm observations in the COSMOS field, we could extract a larger sample includes more star forming galaxies with 10¹⁰L_⊙ ≤ L^IR < 10¹¹L_⊙. Our K-S test shows in less than 0.2%

of probability for 70µm galaxies with X-ray detection to be drawn from the global X-ray population. The mean value (1-sigma) of hardness ratio for XMM selected sample, Chandra selected sample, and 70µm/X-ray sample is -0.26 (∓ 0.28), -0.28(∓0.29), and -0.16(∓0.39), respectively. Such result is in agreement with the merger galaxy formation scenario: the neutral hydrogen obscuration in infrared-luminous AGN not only comes from the dust torus component around the AGN, but also from additionally physical process such as extreme star formation region in the host galaxy.

6. CONCLUSION

We have investigated the properties of 142 galaxies both detected in X-ray and 70µm in the COSMOS field. X-ray data are obtained from both XMM and Chandra point source catalogs, and 70µm photometry is drawn from Spitzer-MIPS 70µm point source catalog.

We classified our sample into three distinct subsamples according to their respective total infrared luminosity (LIR): star-forming galaxies (LIR < 10¹¹L_⊙), luminous infrared galaxies (LIRGs, 10¹¹L_⊙ ≤ L^IR< 10¹²L_⊙), and ultra-luminous infrared galaxies (ULIRGs, LIR≥ 10¹²L_⊙), with median redshifts of z∼ 0.168, 0.518 and 1.268, respectively. The major conclusions for this study are as follows:

1. We applied two methods to determine which mechanism dominates the SED, star formation or AGN:

i) Using Spitzer-IRAC colors, we have shown that the majority of our sample is dominated by AGN. Although the higher AGN fraction accompanies with higher

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total infrared luminosity (LIR), the presence of PAH emission lines (e.g. 3.3µm, 6.2µm, 7.7µm, 8.6µm, and 11.2µm) related to star formation could disturb AGN identification.

ii) Using the relation between intrinsic rest-frame hard X-ray (2-10keV) and total infrared luminosity LIR (8-1000µm), we found that our samples are dominated by AGN, with property comparable to AGN detected SMGs.

2. We provided evidences for additional X-ray obscuration in X-ray detected 70µm galaxies, in agreement with current AGN/starburst co-evolution model (Hopkins et al. 2008). A K-S test on the modified HR between 70µm/X-ray galaxies and whole X-ray samples rejected the assumption of them to be originated from the same population within 99% confidence level, supporting the idea that the excess of X-ray absorption does not come from the AGN dusty torus obscuration, but probably from additional diffuse obscuration generated by the star formation in the galaxy.

3. We estimated the dust temperature of our 70µm galaxy samples by fitting their far-infrared photometries with fixed emissivity models. Despite the presence of a warm dust component, the cold dust shows a similar temperature in host galaxy with and without AGN, indicating that the longer wavelengths are still dominated by star formation. This evidence conflicts with a scenario where radiative feedback from the AGN truncates the star formation in the host galaxy.

Acknowledgements

I would like to express my deepest appreciation to all people who help me the possibility to finish this master thesis. I would like to dedicate this thesis to my parents, Grace Chang and Ching-Yuan Lin, for their unconditional supports and allowing me to be

as ambitious as I wanted. Thanks to my dog - Pocky, he brings a lot of fun for me through my research process. A great gratitude I give to my supervisors, Yasuhiro Hashimoto and S´ebastien Foucaud, who provide plentiful advices and help me to coordinate my project especially in writing this report. A special thanks to Lin-Wen Chen, with his solid training for astronomical courses that inspired me to put more efforts on my master project.

We would like to acknowledge the wonderful work made by Jeyhan S. Kartaltepe on the Spitzer-COSMOS 70µm point source catalog on which our analyses are based. We would like to thank the following people for very useful discussions, suggestions, and comments:

Jeyhan S. Kartaltepe, Wei-Hao Wang, Albert Kong, Matthew A. Malkan, Nick Scoville, and David B. Sanders.

The work presented here is supported by the National Science Council of Taiwan under the grants NSCM-003-002-MY2, NSC99-2119-M-003-005, and NSC 99-2112-M-003-001-MY2. This research used extensively the COSMOS survey archive data of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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