Unveiling obscured accretion in the Chandra Deep Field South
F. Fiore, A. Grazian, P. Santini, S. Puccetti, M. Brusa, C. Feruglio, A. Fontana, E. Giallongo, A. Comastri, C. Gruppioni, F. Pozzi, G. Zamorani, C. Vignali
备注：
Revised version, to be published by The Astrophysical Journal
A large population of heavily obscured, Compton Thick AGNs is predicted by models of galaxy formation, models of Cosmic X-ray Background and by the ``relic'' super-massive black-hole mass function measured from local bulges. However, so far only a handful of Compton thick AGNs have been possibly detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can be recovered thanks to the reprocessing of the AGN UV emission in the infrared by selecting sources with AGN luminosity's in the mid-infrared and faint near-infrared and optical emission. To this purpose, we make use of deep HST, VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the number of Compton thick AGN in this field. We show that sources with high 24$μ$m to optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80$\pm15%$) are indeed likely to be highly obscured, Compton thick AGNs. The number of infrared selected, Compton thick AGNs with 5.8$μ$m luminosity higher than $10^{44.2}$ erg s$^{-1}$ turns out to be similar to that of X-ray selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity higher than $10^{43}$ erg s$^{-1}$ in the redshift bin 1.2-2.6. This ``factor of 2'' source population is exactly what it is needed to solve the discrepancies between model predictions and X-ray AGN selection.
We present the latest determination of the X-ray (2-10 keV) AGN luminosity function accounting for the selection effect due to X-ray absorption. The main results are: 1) the inclusion of obscured AGN confirms the AGN differential luminosity evolution, but makes it less extreme than what is found selecting unobscured AGN in soft X-rays, and more similar to a pure luminosity evolution; 2) significant correlations are found between the fraction of obscured sources, the luminosity and the redshift, this fraction increasing toward both low AGN luminosities and high redshifts. We discuss our finding…
We present results from an XMM-Newton observation of the nearby Seyfert 2 galaxy NGC5643. The nucleus exhibits a very flat X-ray continuum above 2 keV, together with a prominent K-alpha fluorescent iron line. This indicates heavy obscuration. We measure an absorbing column density N_H in the range 6-10 x 10^{23} atoms/cm/cm, either directly covering the nuclear emission, or covering its Compton-reflection. In the latter case, we might be observing a rather unusual geometry for the absorber, whereby reflection from the inner far side of a torus is in turn obscured by its near side outer atmosph…
We investigate the optical colours of X-ray sources from the Extended Chandra Deep Field South (ECDFS) using photometry from the COMBO-17 survey, aiming to explore AGN - galaxy feedback models. The X-ray sources populate both the ``blue'' and the ``red sequence'' on the colour-magnitude diagram. However, sources in the ``red sequence'' appear systematically more obscured. HST imaging from the GEMS survey demonstrates that the nucleus does not affect significantly the observed colours, and therefore red sources are early-type systems. In the context of AGN feedback models, this means that there…
Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available …
We define a sample of 27 radio-excess AGN in the Chandra Deep Field North by selecting galaxies that do not obey the radio/infrared correlation for radio-quiet AGN and star-forming galaxies. Approximately 60% of these radio-excess AGN are X-ray undetected in the 2 Ms Chandra catalog, even at exposures of > 1 Ms; 25% lack even 2-sigma X-ray detections. The absorbing columns to the faint X-ray-detected objects are 10^22 cm^-2 < N_H < 10^24 cm^-2, i.e., they are obscured but unlikely to be Compton thick. Using a local sample of radio-selected AGN, we show that a low ratio of X-ray to radio emissi…
Context. The detection, thanks to the INTEGRAL satellite, of a number of new hard X-ray sources calls for the identification of their nature by means of observations at longer wavelengths. Aims. In our continuing campaign to identify longer-wavelength counterparts of these newly-discovered hard X-ray sources detected by INTEGRAL, we observed the putative optical counterparts of seven southern sources at the South African Astronomical Observatory and at the European Southern Observatory. Methods. Optical spectroscopy was performed on all candidates of these X-ray sources. For two of these objec…
In this letter we present clear evidence that Mrk 573 is an obscured Narrow-Line Seyfert 1 (NLSy1) and not an archetypal Seyfert 2, as it has been classified until now. Only three galaxies have been proposed as members of this class, prior to this work. Here we report near-infrared spectroscopic data taken with LIRIS on the 4.2 m William Herschel Telescope (WHT). Our high quality near-infrared nuclear spectrum in the 0.88-1.35 micron wavelength range shows the permitted O I 1.128,1.317 micron narrow lines, the Fe II 9200-Ålines and the Fe II 1-micron lines, together with a relatively broad com…
We investigate the nature of a sample of 92 Spitzer/MIPS 24 micron selected galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6-8 micron bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGN not detected even in deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z>1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical to IR spectral energy distributions (SEDs) of the X-ray detected galaxies are almost equally …
The infrared is a key wavelength regime for probing the dusty, obscured nuclear regions of active galaxies. We present results from an infrared study of 87 nearby Seyfert galaxies using the Spitzer Space Telescope and ground-based telescopes. Combining detailed modelling of the 3 - 100 micron spectral energy distributions with mid-IR spectral diagnostics and near-infrared observations, we find broad support for the unified model of AGNs. The IR emission of Seyfert 1s and 2s is consistent with their having the same type of central engine viewed at a different orientation. The nature of the puta…
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the AGN host galaxies have on average redder rest-frame optical colors than non-active galaxies, and that they live mostly in the "green valley". The dependence of the fraction of AGN t…
We present Chandra and Spitzer data of the 186, extragalactic, hard 2-10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%) there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, according to strictly X-ray criteria, i.e. defined as having large hydrogen column densities (N_H>10^22 cgs) and luminosities (Lx>10^44 cgs). The surface density of X-ray obscured QSOs is ~210 sq. deg. We find 18 candidate Compton thick N_H>10^24 cgs sources, of which three have Q…
We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infrared-selected sources to the unresolved, hard (6-8 keV) cosmic X-ray background (CXB). We use the publicly available, 24 micron Spitzer Space Telescope MIPS catalogs from the Great Observatories Origins Deep Survey (GOODS) - North and South fields, which are centered on the 2 Ms Chandra Deep Field-North and the 1 Ms Chandra Deep Field-South, to identify bright (S_24 > 80 microJy) mid-infrared sources that may be powered by heavily obscured AGNs. We measure a significant stacked X-ray signal in all of…
The deep X-ray, optical, and far-infrared fields that constitute GOODS are sensitive to obscured AGN (N_H>10^{22} cm^{-2}) at the quasar epoch (z~2-3), as well as to unobscured AGN as distant as z~7. Luminous X-ray emission is a sign of accretion onto a supermassive black hole and thus reveals all but the most heavily obscured AGN. We combine X-ray luminosity functions with appropriate spectral energy distributions for AGN to model the X-ray, optical and far-infrared flux distributions of the X-ray sources in the GOODS fields. A simple model based on the unified paradigm for AGN, with ~3 times…
Measuring the population of obscured quasars is one of the key issues to understand the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this letter we study the population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV band selected samples also filter out most of the X-ray faint sources with too few counts, for which the measurements of N_H and L_X have very la…
In this work we use the Hubble Deep Field South (HDF-S) version 2 images to assess the galaxy population out to z ~ 2. We have used two methods of templates fitting of the spectral energy distributions to obtain photometric redshifts and classify the objects. The Bayesian photometric redshifts gave better results when compared with 54 spectroscopic redshifts available in the literature. Analysis of the rest-frame colour distribution shows a bimodality out to z ~ 1.4. We separated our sample in a blue and a red population at B-V = 0.29. At low redshifts (0.2<z<0.6), ~ 60% of the galaxies have B…
Extremely Red Objects (EROs, R-K>5) constitute a heterogenous class of extragalactic sources including high redshift elliptical galaxies, dusty star-forming systems and heavily obscured AGNs. Hard X-ray observations provide an unique and powerful tool to uncover obscured nuclear or star-forming activity. We present the results of XMM-Newton observations of the largest sample of EROs available to date (about 450 objects over a contiguous area of 700 arcmin^2). Five of the 46 hard X-ray selected sources brighter than 3x10^{-15} cgs in the 2-10 keV band, are associated with EROs. All of the X-ray…
The Chandra X-ray Observatory is finding a large number of cavities in the X-ray emitting intra-cluster medium which often coincide with the lobes of the central radio galaxy. We propose high-resolution Sunyaev-Zel'dovich (SZ) observations in order to infer the still unknown dynamically dominating component of the radio plasma bubbles. This work calculates the thermal and relativistic SZ emission of different compositions of these plasma bubbles while simultaneously allowing for the cluster's kinetic SZ effect. As examples, we present simulations of an Atacama Large Millimeter Array (ALMA) obs…
The X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003, January 29. We carried out optical and near-infrared (NIR) observations at the European Southern Observatory (ESO La Silla) in the course of a Target of Opportunity (ToO) programme. We discovered the optical counterpart and confirmed an already proposed NIR candidate. NIR spectroscopy revealed a large amount of emission lines, including forbidden iron lines and P-Cygni profiles. The spectral energy distribution of the source points towards a high luminosity and a high temperature, with an absorption greater t…
(Abridged) This work explores the existence of high redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations. To this end, we use the multi-wavelength imaging data available for the GOODS-South field, and select a flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our study to the galaxies undetected by the optical HST+ACS imaging and close to the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20 galaxies on which we performed a detailed photometric analys…
We investigate the use of optical variability to identify and study Active Galactic Nuclei (AGN) in the GOODS-South field. A sample of 22 mid-infrared power law sources and 102 X-ray sources with optical counterparts in the HST ACS images were selected. Each object is classified with a variability significance value related to the standard deviation of its magnitude in five epochs separated by 45-day intervals. The variability significance is compared to the optical, mid-IR, and X-ray properties of the sources. We find that 26% of all AGN candidates (either X-ray- or mid-IR-selected) are optic…
We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field South (CDFS). 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R-K colours are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external iden…
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 um) ISO image presented by Mirabel et al (1998). We obtained high signal-to-noise 2.03-2.45 um spectra of the nucleus and the obscured star cluster at R = 1900. The cl…
We present a detailed and self-consistent modeling of the cosmic X-ray background (XRB) based on the most up-to-date X-ray luminosity functions (XLF) and evolution of Active Galactic Nuclei (AGN). The large body of observational results collected by soft (0.5-2 keV) and hard (2-10 keV) X-ray surveys are used to constrain at best the properties of the Compton-thin AGN population and its contribution to the XRB emission. The number ratio R between moderately obscured (Compton-thin) AGN and unobscured AGN is fixed by the comparison between the soft and hard XLFs, which suggests that R decreases f…
To better understand the early stages of massive star cluster evolution we have obtained J,H,K', and N (10.8 micron) images of the nuclear region of the starburst galaxy He 2-10. The N-band images were obtained with the Gemini North telescope and reveal four of the five enshrouded clusters, or "ultradense HII regions" (UDHIIs), recently discovered in radio maps. None of these sources appears in either the optical HST images or the near-infrared images. They comprise about 60% of the total N-band flux from He 2-10 and, we suspect, a similar fraction of the total far infrared flux measured by IR…
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 micron) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 - 2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1…
We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk~273 Using the ACIS-S3 instrument on board Chandra. The high resolution X-ray image, for the first time, reveals a compact hard X-ray nucleus in Mrk~273. Its X-ray energy distribution is well described by a heavily obscured power-law spectrum plus a narrow $\Feka$ emission line at 6.4 keV. The neutral hydrogen column density is about $4\times10^{23}\cm^{-2}$, implying an absorption -corrected X-ray luminosity (0.1--10 keV) for the nucleus of $\Lx\approx 6.5\times 10^{43} \ergs$. There are also bright soft X-r…
VLT J- to M\p-band adaptive optics observations of the Circinus Galaxy on parsec scales resolve a central bright Ks-band source with a FWHM size of 1.9 $\pm$ 0.6 pc. This source is only visible at wavelengths longward of 1.6 $μ$m and coincides in position with the peak of the [Si VII]~2.48 $μ$m coronal line emission. With respect to the peak of the central optical emission, the source is shifted by $\sim$ 0.15\arcsec (2.8 pc) to the south-east. Indeed, it defines the vertex of a fairly collimated beam which extends for $\sim$ 10 pc, and which is seen in both continuum light shortward of 1.6 $μ…
We combine bright XMM data with the Chandra Deep Field South observations in order to explore the behavior of the intrinsic AGN absorption, as a function of redshift and luminosity.Our sample consists of 359 sources selected in the hard 2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1 cm^-2 with a high rate of spectroscopic or photometric redshift completeness (100 and 85 per cent respectively for the Chandra and XMM data. We derive the column density values using X-ray spectral fits. We find that the fraction of obscured AGN falls with increasing luminosity in a…
Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is poss…
We explore the nature of the radio emission of X-ray selected AGN by combining deep radio (1.4GHz; 60micro-Jy) and X-ray data with multiwavelength (optical, mid-infrared) observations in the Extended Chandra Deep Field South (E-CDFS). The fraction of radio detected X-ray sources increases from 9% in the E-CDFS to 14% in the central region of this field, which has deeper X-ray coverage from the 1Ms CDFS. We find evidence that the radio emission of up to 60% of the hard X-ray/radio matched AGN is likely associated with star-formation in the host galaxy. Firstly, the mid-IR (24micron) properties …
ABRIDGED: We use deep optical, IR and radio observations to explore the symbiosis between the nuclear activity and galaxy evolution in the southern compact radio source PKS1549-79 (z=0.1523). The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. This is further supported by the detection of a young stellar population which was formed 50-250Myr ago and makes up 1-30% of the total stellar mass. Despite the core-jet structure of the radio source we detect HI absorption associa…
We present a pilot study of the stellar populations of 8 AGN hosts at z~1 and compare to (1) lower redshift samples and (2) a sample of nonactive galaxies of similar redshift. We utilize K' images in the GOODS South field obtained with the laser guide star adaptive optics (LGSAO) system at Keck Observatory. We combine this K' data with B, V, i, and z imaging from the ACS on HST to give multi-color photometry at a matched spatial resolution better than 100 mas in all bands. The hosts harbor AGN as inferred from their high X-ray luminosities (L_X > 10^42 ergs/s) or mid-IR colors. We find a corre…
We have combined multi-wavelength observations of a selected sample of starforming galaxies with galaxy evolution models in order to compare the results obtained for different SFR tracers and to study the effect that the evolution of the starforming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field South (CDFS) and chose the objects that also have UV and IR data from GALEX and GOODS-Spitzer respectiv…
We present results from an HST study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into seperate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K-band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a …
We report the cumulative results of five surveys for water maser emission at 1.35 cm wavelength in 131 active galactic nuclei (AGNs) and star-forming galaxies, conducted at the Parkes Observatory between 1993 and 1998. We detected one new maser, in the edge-on galaxy IRASF01063-8034, which exhibits a single, about 0.1 Jy spectral feature at 4282+/-6 km/s (heliocentric) with an unusually large 54+/-16 km/s half-power full width. The centroid velocity of the emission increased to 4319.6+/-0.6 km/s (38+/-2 km/s width) over the 13 days between discovery and confirmation of the detection. A similar…
The X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003, January 29. The high energy spectrum exhibits such a high column density that the source is undetectable in X-rays below 2 keV. On 2003, February, 23--25 we triggered a Target of Opportunity (ToO) program using the EMMI and SOFI instruments on the New Technology Telescope of the European Southern Observatory (La Silla) to get optical and near-infrared (NIR) observations. We discovered the optical counterpart, and confirmed the already proposed candidate in the NIR. We report here photometric measurements in t…
The Extended Chandra Deep Field-South (ECDFS) survey consists of 4 Chandra ACIS-I pointings and covers $\approx$ 1100 square arcminutes ($\approx$ 0.3 deg$^2$) centered on the original CDF-S field to a depth of approximately 228 ks. This is the largest Chandra survey ever conducted at such depth, and only one XMM-Newton survey reaches a lower flux limit in the hard 2.0--8.0 keV band. We detect 651 unique sources -- 587 using a conservative source detection threshold and 64 using a lower source detection threshold. These are presented as two separate catalogs. Of the 651 total sources, 561 are …
We present interferometric observations of three ultraluminous infrared galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs were derived and compared with those of lower luminosity galaxies know…
Deep X-ray surveys are providing crucial information on the evolution of AGN and galaxies. We review some of the latest results based on the X-ray spectral analysis of the sources detected in the Chandra Deep Field South, namely: i) constraints on obscured accretion; ii) constraints on the missing fraction of the X-ray background; iii) the redshift distribution of Compton-thick sources and TypeII QSO; iv) the detection of star formation activity in high-z galaxies through stacking techniques; v) the detection of large scale structure in the AGN distribution and its effect on nuclear activity. …
