X-ray - Infrared relation of AGNs and search for highly obscured accretion in the AKARI NEP Field

TL;DR

This study investigates the relationship between X-ray and infrared emissions of AGNs using AKARI and Chandra data, aiming to identify highly obscured and Compton-thick AGNs, and discusses future prospects with Spectrum-RG.

Contribution

It utilizes AKARI's unique mid-infrared coverage combined with Chandra X-ray data to detect and analyze heavily obscured AGNs, including Compton-thick sources.

Findings

Identification of heavily obscured AGNs through X-ray and IR comparison

Detection of AGN candidates at z~1-2 using AKARI's IR bands

Discussion of future observations with Spectrum-RG

Abstract

The infrared Astronomical Satellite AKARI conducted deep ($\sim$ 0.4 deg$^2$) and wide ($\sim 5.4$ deg$^2$) surveys around the North Ecliptic Pole (NEP) with its InfraRed Camera (IRC) with nine filters continuously covering the 2-25 $\mu$m range. These photometric bands include three filters that fill the `Spitzer gap' between the wavelength coverages of IRAC and MIPS. This unique feature has enabled us to make sensitive mid-infrared detection of AGN candidates at z$\sim$ 1-2, based on the Spectral Energy Distribution (SED) fitting including hot dust emission in the AGN torus. This enables us to compare X-rays and the AGN torus component of the infrared emission to help us identify highly absorbed AGNs, including Compton-thick ones. We report our results of the Chandra observation of the AKARI NEP Deep Field and discuss the prospects for upcoming Spectrum-RG (eROSITA+ART-XC) on the AKARI Wide field.

Full text

††volume: 341††journal: PanModel2018 : Challenges in Panchromatic Galaxy Modelling with Next Generation Facilities††editors: M. Boquien, E. Lusso, C. Gruppioni, and P. Tissera eds.

1 Introduction

There are three major bumps in the electromagnetic continuum in a typical AGN: 1) optically-thick thermal emission from the accretion disk in the ultraviolet, 2) the primary X-ray continuum by the inverse-Compton scattering of the disk ultraviolet photons by hot coronae, and 3) the infrared emission from hot dust in the circum-nuclear torus. Additionally, reprocessed primary X-ray continuum by reflections at surrounding material produces a characteristic “Comption hump” in hard X-rays. Observational determination of how these three components are inter-related is a key to panchromatic modelings of the AGN component in galaxies.

In multiwavelength extragalactic surveys, X-rays are considered the most efficient marker of the AGN activity. However it is subject to absorption by intervening gas by photo-electric absorption, which occurs mainly at photon energies $E<10$ keV. If the line of sight column density becomes N_{\rm H}\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{>}}}10^{24}{\rm cm^{-2}}, the effects Compton scattering further attenuates X-ray emission even at E\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{>}}}10 keV.

While the infrared emission from hot dust heated by AGNs is less subject to absorption by intervening material, it has to be separated from stars and warm dust from star-formation regions by an SED decomposition to measure the IR luminosity of the AGN component alone.

The continuous 9-band 2-25 $\mu$m photometric data available from AKARI NEP Survey are ideal for such SED decomposition of galaxies. The availability of three photometric bands in the 11-20 $\mu$m range, which is the wavelength gap between IRAC and MIPS instruments of Spitzer, makes the AKARI IRC dataset a very strong tool for separating AGN hot dust emission from stellar+star-formation components at 1\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{<}}}z\leq 2 (the “Cosmic Noon”).

In this proceedings article, we summarize the results and prospects of our X-ray followup studies of the AKARI NEP field with emphasis on X-ray data and the connection between X-rays and infrared. These include our Chandra followup studies of the AKARI NEP Deep Field and prospects of upcoming Spectrum-RG (eROSITA+ART-XC) observations.

2 The AKARI NEP Survey

AKARI was a Japanese IR astronomical satellite ([matsuhara05, Matsuhara et al. 2005]; see also Goto et al., this symposium), which was operated from 2006 to 2011. One of the unique features of the observatory was the availability of continuous wavelength coverage over 2–25 $\mu$m in 9 filters with its IR Camera (IRC), including three filters in the 11–19 $\mu$m range. These filters fill the 9–20 $\mu$m gap between the Spitzer IRAC and MIPS instruments and allows an efficient selection of AGN at 1\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{<}}}z\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{<}}}2 in the MIR. This redshift range (the so-called Cosmic Noon) is particularly important to study since that is where both the X-ray detected number and luminosity densities peak.

Figure 1 demonstrates the advantage of the continuous wavelength coverage of AKARI 9-band photometry (including 10–19 $\mu$m) over the Spizer IRAC+MIPS photomtetric bands in detecting the AGN torus component. (See caption.)

As a legacy program of AKARI, a major portion of all pointed observations during its liquid helium phase was devoted to surveys around North Ecliptic Pole (NEP), namely the AKARI NEP Deep ($\sim 0.4$ deg2; hereafter ANEPD) and Wide ($\sim 5.4$ deg2; ANEPW) field surveys with IRC. At ANEPD, the AKARI data have 5$\sigma$ sensitivity limits of, e.g., $\sim$9 and $87$ $\mu$Jy at 3.2 and 15 $\mu$m respectively ([murata13, Murata et al. 2013]), while the limiting fluxes are approximately twice as large in NEPW ([Kim et al.(2012), Kim et al. 2012]).

3 Chandra Observations of the AKARI NEP Deep Field

We were granted a 250 ks dense tiling of a 3 $\times$ 4 Chandra ACIS-I mosaic survey in the ANEPD field ([Krumpe et al.(2015), Krumpe et al. 2015]; hereafter K15). Including additional $\sim 50$ ks of data from archive, we cover the field with exposure times ranging from $\sim$ 40 – 80 ks. The source catalog and the first analysis of 457 X-ray sources is presented in K15.

In K15, (see Fig. 2 left), we compare the observed X-ray (2-7 keV) luminosity $L_{\rm OBS,2-7keV}$, i.e. luminosity calculated without K-correction or absorption correction) and the AGN component of the IR luminosity $L_{\rm IR,AGN}$ obtained from SED fitting using an updated version of the [Hanami et al.(2012), Hanami et al. (2012)]’s procedure. By this comparison, we selected 28 strong Compton-thick AGN (CTAGN) candidates, i.e., AGNs with a line of sight column density of $N_{\rm H}>10^{24}{\rm[cm^{-2}]}$.

The CTAGNs are characterized by a large equivalent width of the Fe K$\alpha$ line (EW\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower 4.0pt\hbox{\sim}}\hss}\hbox{>}}}1 keV). Using the CSTACK utility111http://lambic.astrosen.unam.mx/cstack, we have made a rest-frame stacking of the X-ray spectra of the 28 strong CT AGN candidates (Fig. 2 right), obtaining an equivalent width of the Fe K$\alpha$ of $EW=1.0\pm 0.6$ keV. This suggests that a majority of these candidates are CTAGNs with possible contamination from non-CT AGNs.

4 Prospects for eROSITA/ART-XC observations

The spacecraft Spectrum-RG, which is planned to be launched in the near future (Merloni, this symposium), carries two instruments, eROSITA (sensitive in 0.2-10 keV) and ART-XC (5-30 keV). The plan of the mission is to perform 8 all-sky surveys over four years and the every great circle scan will go through the Ecliptic pole regions. Spectrum-RG will provide with us the best-matched dataset for our studies of highly obscured AGNs in the $\sim 5.4$ deg2 ANEPW field, which is centered at the exact NEP. The combination of the ANEPW survey with the eROSITA dataset, we expect to probe a high luminosity - highly absorbed AGN population.

Using the current determination of the X-ray luminosity function of AGNs at $0.4<z<2$ ([Miyaji et al.(2015), Miyaji et al. 2015]), $N_{\rm H}$ function ([ueda14, Ueda et al. 2014]), and the expected flux limit of eROSITA in the 2-10 keV band ([merloni12, Merloni et al. 2012]), we expect to identify $\sim 270$ CTAGN/semi-CTAGN candidates using the method described above. The ART-XC instrument is sensitive in 5-30 keV, we expect to improve the distinction between non-CT AGNs and CT-AGNs by adding the ART-XC dataset.

5 Concluding Remarks

We are in the process of improving the investigation. The improvements include:

• •

Spectroscopic followup of ANEPD X-ray sources, IR-selected AGNs and and CTAGN candidates, using large telescopes including Gran Telescopio Canarias, Keck, and Large Binocular Telescope.

• •

We will use updated models for the AGN torus. These include the Clumpy Torus model ([nenkova15, Nenkova et al. 2015]) on the infrared side. An X-ray torus clumpy model that has the consistent geometrical configurations/parameter definitions as the [nenkova15, Nenkova et al. (2015)] model is being developed by a group at Kyoto University (Tanimoto et al.,Ogawa et al. this symposium). We will use these models for a consistent IR-X-ray modeling in selecting CT-AGNs.

TM is supported by UNAM-DGAPA PAPIIT (IN104216, IN111319) and CONACyT (252531). He thanks the conference organizers/NAOJ for a travel support.