Both observational and simulation evidences over the past decades strongly suggest the existence of supermassive black hole (SMBH) within the spherical bulge of galaxies. The tight connection between the SMBH mass and the bulge properties, such as bulge luminosity or stellar velocity dispersion, indicates a scenario of co-evolution for central black hole and its host galaxy (Kormendy & Richstone 1995; Magorrian et al. 1998). Galaxy merger is a plausible interpretation to explain the concomitant growth of black hole and host galaxy under the ΛCDM model (Treister et al. 2010). Such interpretation suggests the mergers distort galaxy morphology, reducing angular momentum that induces gas accumulation around circumnuclear region easily (Barnes & Hernquist 1991). During the starburst epoch, a large amount of dust and molecular gas around central region (Chung et al. 2011;
Magdis et al. 2011) can be interpreted as remnants of supernova explosions and stellar wind from evolved stars. For instance, the local infrared luminous galaxy Arp 220 has been confirmed higher supernova rate from radio observation that supports star formation drives its enormous infrared luminosity (Lonsdale et al. 2006a). Meanwhile, interstellar medium (ISM) is also funneled onto the SMBH in the galactic nucleus and consequently triggers a phase of active galactic nuclei (AGNs), with its powerful electromagnetic radiation spanning from radio to gamma-ray. In the later galaxy evolution stage, the merger scenario suggests that the radiation-pressure or kinematic-wind feedback from AGN may quench the star formation, disrupting the remnant gas and dust. At the same time, central AGN turns into an optically unobscured quasar, a class of luminous AGN. Final outcome in galaxy evolution is a red elliptical galaxy (Hopkins et al. 2008, and references therein).
Sanders et al. (1988a) first proposed a connection between the Ultra Luminous Infrared Galaxies (ULIRGs; LIR ≥ 1012L⊙) and quasars as two successive snapshots of merger event between two gas-rich spiral galaxies. Indeed, from IRAS 1 Jy ULIRGs studies, the
merger-ULIRG connection has been revealed by a large fraction of interacting galaxies at z < 0.1 (Borne et al. 2000; Cui et al. 2001; Veilleux et al. 2002). Besides, the merger fraction has been shown to increase as infrared luminosity raises, merging galaxy being prevalent population amongst ULIRGs to redshift∼ 1 (Shi et al. 2009). However at higher redshift ∼ 2, the merging galaxy fraction drops slightly and non-interacting disk galaxy fraction increases (Kartaltepe et al. 2012).
On the other hand, the merger-AGN connection is more difficult to observe. Although, in the local universe, optical luminous quasars are considered to be related to post-starburst merger stage (Canalizo & Stockton 2001; Bennert et al. 2008), at 1.5 < z < 2.5, AGNs are more likely to occur inside of normal disk or bulge galaxy, instead of interacting systems, suggesting that secular processes (such as bars or nucleus rings) may trigger nuclear activity instead of major-mergers (Schawinski et al. 2011; Kocevski et al. 2012). The connection between ULIRGs and AGNs has been investigated for the past few years. The AGN fraction seems to rise with the infrared luminosity increased (Genzel & Cesarsky 2000; Kartaltepe et al. 2010). The AGN activities couple with their infrared spectral energy distribution (SED) profile, the higher mid-infrared color ratio (i.e. F25/F60≥ 0.2), which classified as warm ULIRGs, is believed to dominate by AGN. Otherwise, lower mid-infrared color ratio (i.e. F25/F60≤ 0.2) is classified as cold ULIRGs, dominating by star formation (Sanders et al. 1988b; Veilleux et al. 2009).
In order to determine the exact role of the AGN in the overall galaxy evolution, it is essential to separate the AGN contribution from the normal star formation activity. To decouple two components, three distinct methods can be used:
(i) Specific emission lines ratio - Fine structure radiation in optical and infrared wavelength are reliable tracers of excitation state of the ISM in galaxies. A classification based on two independent line ratios can help to segregate AGN and star-forming galaxies. For
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instance, investigating [OIII]5007˚A/Hβ versus [NII]6584˚A/Hα (Baldwin et al. 1981, BPT) diagram will lead to distinguish the photoionization, derived from normal HII regions, from power-law continuum generated by AGNs. Unfortunately, galaxies undergoing merger event suffer from dust obscuration in optical wavelength that limits the efficiency of such diagnostics, but not likely misclassify the AGN identification (Veilleux 2002; LaMassa et al.
2012).
By the advent of the infrared spectrometer on the Infrared Space Observatory (ISO) and the Spitzer telescope, additional emission lines were identified for AGN diagnostics in the mid-infrared that are less sensitive to extinction. For example, the line ratio [O IV]25.9µm/[Nell]12.8µm or [NeV]14.3µm/[Nell]12.8µm is a reliable indicator to separate AGNs from star-forming galaxies (Genzel et al. 1998; Armus 2006). The different transition lines of 6.2µm 7.7µm and 11.3 µm Polycyclic Aromatic Hydrocarbon (PAH) molecules are other reliable star formation tracers, with weaker PAH emission corresponds to higher AGN contribution.(Voit 1992; Veilleux et al. 2009);
(ii) Continuum slope - In contrast with cold dust components in far-infrared are powered by star formation, hot dust components around 10µm are considered to be associated with AGN influence. As the influence of the AGN becomes significant in the galaxy, the infrared spectral indexes start to be dominated by a power-law continuum∼ 10µm (Alonso-Herrero et al. 2006; Donley et al. 2007). Therefore, the mid-infrared to far-infrared ratio is a simple measure to quantify the relative contributions of AGNs and star-forming galaxies (Veilleux et al. 2009);
(iii) Spectral energy distribution fitting - With the development of deep multi-wavelength photometric surveys, panchromatic Spectral Energy Distribution (SED) studies are becoming increasingly popular. Theoretical or empirical templates are commonly used to fit the SED and determine the photometric redshifts of galaxies. Multi-component fitting, including the contribution from stars, dust, and AGNs is now possible thanks to very broad
photometric coverage. In the case of heavily obscured AGNs, although star formation of host galaxy dominates the flux at most wavelengths, a bump in mid-infrared produced by the circumnuclear dust radiation, can be an indicator of an obscured AGN. Therefore, identifying hot dust component from the galaxy SED is a reliable way to determine the AGN contribution to total infrared luminosity (Mullaney et al. 2011; Pozzi et al. 2012).
Although the different methods may not agree completely on defining a pure sample (i.e. pure-AGN or pure starburst galaxy), they all provide evidences that, for the major part of the samples, AGN and star formation occur concomitantly. Except pursuing the intrinsic AGN contribution fraction on the total infrared luminosity, what is essential here is to understand the mutual influence on the properties of AGNs and hosts star-forming galaxies. We tried to investigate whether the dust from star formation in host galaxy could obscure central AGN and the presence of AGN radiation could change the far-infrared SED.
The SEDs of Luminous Infrared Galaxies (LIRGs; 1011L⊙ ≤ LIR < 1012L⊙) and ULIRGs are peaked in the range of 40-200µm (Sanders & Mirabel 1996). In fact, the 70µm Spitzer band is ideal to unambiguously trace star-forming galaxies as it is little affected by PAH emissions, silicate absorption, and stellar flux. Central AGNs are usually identified by their strong X-ray continuum emission. Indeed, high energy X-ray photons emitted by hot corona of accretion disk (e.g., Haardt & Maraschi 1993) around the central black-hole are usually little absorbed by dust and gas from host galaxies, compared to lower energy UV-photons. One noticeable exception are compton thick objects (NH ≥ 1024 cm−2) for which even hard X-ray photons are absorbed. Compton thick AGNs are not uncommon existence among ULIRG population. In local universe, Lonsdale et al. (2006b) have concluded that the presence of Fe Kα lines and flat spectrum in ULIRGs indicate the Compton-thick AGN harbors inside the central region (e.g. Mrk 273 in Balestra et al. (2005) paper). In higher redshift, although different Compton-thick candidate selections (e.g.
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24µm excess, mid-infrared spectral slope, and radio excess) are not complete coincidence (Bauer et al. 2010), the Compton-thick fraction is significantly increased in early universe (Brightman & Ueda 2012).
Several studies have investigated the nature of the 70µm galaxy population. Patel et al. (2011) lead a spectroscopic follow-up in optical wavelength of 70µm galaxies selected from the Spitzer Wide-area Infrared Extragalactic Legacy Survey. Their results suggest that the most of the IR photons are powered by star formation, while contribution from AGN dusty torus emission are negligible for the non-QSO-dominated samples. From the SED study of 61 out of 70µm selected galaxies from the 0.5 deg2 wide Extended Groth Strip (EGS) field, Symeonidis et al. (2010) concluded that, even in the presence of powerful hard X-ray emission originated from AGNs, dust emission components are required to explain the observed strong far-infrared luminosity. To reveal a potential starburst-AGN connection, Trichas et al. (2009) used 28 X-ray sources with 70µm detection galaxies and applied a statistical K-S test (Kolmogorov-Smirnov test) to assess divergency in hardness ratio between X-ray detected 70µm galaxies and the whole X-ray population in the redshift interval 0.5 < z <1.5. However, the hardness ratio result leads to a less significant probability of the K-S test, in contrast to current co-evolution of AGN and host galaxy that the central region is obscured by dust and gas due to circumnuclear starburst (Hopkins et al. 2008).
In this paper, we aim to dissect the role of AGN in the infrared (ultra)luminous phase of galaxy evolution. We extracted the X-ray detected sub-sample from unconfused 70µm catalog. Our main 70µm galaxy catalog was published by Kartaltepe et al. (2010), including total infrared luminosity, multi-wavelength photometry and redshift. We describe the data used for this work in section 2. As we are interested in physical property of AGN and far-infrared selected host galaxy, we cross-match X-ray and 70µm datasets. In section 3, we
depict our matching method as well as the methods used to estimate and calibrate different physical parameter measurements. We present our results in section 4, and then conduct a detailed discussions in section 5. Throughout this paper, we adopt a fiducial cosmological model with the following parameters: H0 = 70 km s−1 Mpc−1, and ΩM = 0.3, ΩΛ = 0.7.
Unless otherwise stated, all magnitudes in this paper are in the Vega system.