The X-ray emission of z>2.5 active galactic nuclei can be obscured by their host galaxies

We present a multi-wavelength study of seven AGN at spectroscopic redshift >2.5 in the 7 Ms Chandra Deep Field South, selected to have good FIR/sub-mm detections. Our aim is to investigate the possibility that the obscuration observed in the X-rays can be produced by the interstellar medium (ISM) of the host galaxy. Based on the 7 Ms Chandra spectra, we measured obscuring column densities N$_{H, X}$ in excess of 7x10$^{22}$ cm$^{-2}$ and intrinsic X-ray luminosities L$_{X}$>10$^{44}$ erg s$^{-1}$ for our targets, as well as equivalent widths for the Fe K$\alpha$ emission line EW>0.5-1 keV. We built the UV-to-FIR spectral energy distributions by using broad-band photometry from CANDELS and Herschel catalogs. By means of an SED decomposition technique, we derived stellar masses (M$_{*}$~10$^{11}$ Msun), IR luminosities (L$_{IR}$>10$^{12}$ Lsun), star formation rates (SFR~190-1680 Msun yr$^{-1}$) and AGN bolometric luminosities (L$_{bol}$~10$^{46}$ erg s$^{-1}$) for our sample. We used an empirically-calibrated relation between gas masses and FIR/sub-mm luminosities and derived M$_{gas}$~0.8-5.4x10$^{10}$ Msun. High-resolution (0.3-0.7'') ALMA data (when available, CANDELS data otherwise) were used to estimate the galaxy size and hence the volume enclosing most of the ISM under simple geometrical assumptions. These measurements were then combined to derive the column density associated with the ISM of the host, on the order of N$_{H, ISM}$~10$^{23-24}$ cm$^{-2}$. The comparison between the ISM column densities and those measured from the X-ray spectral analysis shows that they are similar. This suggests that, at least at high redshift, significant absorption on kpc scales by the dense ISM in the host likely adds to or substitutes that produced by circumnuclear gas on pc scales (i.e., the torus of unified models). The lack of unobscured AGN among our ISM-rich targets supports this scenario.Comment: 15 pages, 3 figures. Accepted for publication in A&

Spectroscopy of an extreme [OIII] emitting active galactic nucleus at z = 3.212: implications for the reionisation era

Reionization-era galaxies often display intense nebular emission lines, both in rest-frame optical ([O III] + H β) and ultraviolet (UV; C III], C IV). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGNs) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue (β −1) than typical star-forming systems in the reionization era. To investigate the properties of AGNs in the reionization era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift (⁠z ∼ 2−3). Our goals are to determine whether AGNs with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper, we report the detection of an X-ray luminous AGN at z = 3.21 (UDS-24561) with extreme [O III] + H β line emission (equivalent width = 1300 Å) and a blue UV continuum slope (β = −2.34). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionization models and rest-frame optical lines with both a narrow component [full width at half-maximum (FWHM) =154kms−1] and extended broad wings (FWHM =977kms−1⁠), consistent with outflowing gas. We describe how such objects can be identified in future James Webb Space Telescope emission line surveys in the reionization era, thereby providing a valuable census of AGN activity at z > 6 and understanding their contribution to cosmic reionization

BEAGLE-AGN I: Simultaneous constraints on the properties of gas in star-forming and AGN narrow-line regions in galaxies

We present the addition of nebular emission from the narrow-line regions (NLR) surrounding active galactic nuclei (AGN) to BEAGLE (BayEsian Analysis of GaLaxy sEds). Using a set of idealised spectra, we fit to a set of observables (emission-line ratios and fluxes) and test the retrieval of different physical parameters. We find that fitting to standard diagnostic-line ratios from Baldwin et al. (1981) plus [O II]3726,3729/[O III]5007, Hbeta/ Halpha, [O I]6300/[O II]3726,3729 and Halpha flux, degeneracies remain between dust-to-metal mass ratio and ionisation parameter in the NLR gas, and between slope of the ionizing radiation (characterising the emission from the accretion disc around the central black hole) and total accretion-disc luminosity. Since these degeneracies bias the retrieval of other parameters even at maximal signal-to-noise ratio (S/N), without additional observables, we suggest fixing the slope of the ionizing radiation and dust-to-metal mass ratios in both NLR and HII regions. We explore the S/N in Hbeta required for un-biased estimates of physical parameters, finding that S/N(Hbeta)~10 is sufficient to identify a NLR contribution, but that higher S/N is required for un-biased parameter retrieval (~20 for NLR-dominated systems, ~sim30 for objects with approximately-equal Hbeta contributions from NLR and HII regions). We also compare the predictions of our models for different line ratios to previously-published models and data. By adding [He II]4686-line measurements to a set of published line fluxes for a sample of 463 AGN NLR, we show that our models with $-4<$ionisation parameter in the NLR gas$<-1.5$ can account for the full range of observed AGN properties in the local Universe.Comment: 22 pages, 18 figures. Submitted to MNRA

ALMA reveals a warm and compact starburst around a heavily obscured supermassive black hole at z=4.75

We report ALMA Cycle 0 observations at 1.3mm of LESS J033229.4-275619 (XID403), an Ultraluminous Infrared Galaxy at $z=4.75$ in the Chandra Deep Field South hosting a Compton-thick QSO. The source is not resolved in our data at a resolution of $\sim$0.75 arcsec, placing an upper-limit of 2.5 kpc to the half-light radius of the continuum emission from heated-dust. After deconvolving for the beam size, however, we found a $\sim3\sigma$ indication of an intrinsic source size of $0.27\pm0.08$ arcsec (Gaussian FWHM), which would correspond to $r_{half}\sim0.9\pm0.3$ kpc. We build the far-IR SED of XID403 by combining datapoints from both ALMA and Herschel and fit it with a modified blackbody spectrum. For the first time, we measure the dust temperature $T_d=58.5\pm5.3$ K in this system, which is comparable to what has been observed in other high-z submillimeter galaxies. The measured star formation rate is SFR=$1020\pm150$ $M_{\odot}$ yr$^{-1}$, in agreement with previous estimates at lower S/N. Based on the measured SFR and source size, we constrain the SFR surface density to be $\Sigma_{SFR}>26\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ ($\sim200\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ for $r_{half}\sim0.9$ kpc). The compactness of this starburst is comparable to what has been observed in other local and high-z starburst galaxies. If the gas mass measured from previous [CII] and CO(2-1) observations at low resolution is confined within the same dust region, assuming $r_{half}\sim0.9\pm0.3$ kpc, this would produce a column density of $N_H\sim0.3-1.1\times10^{24}$cm$^{-2}$ towards the central SMBH, similar to the column density of $\approx1.4\times10^{24}$cm$^{-2}$ measured from the X-rays. Then, in principle, if both gas and dust were confined on sub-kpc scales, this would be sufficient to produce the observed X-ray column density without any need of a pc-scale absorber [abridged].Comment: 11 pages, 5 figures. Accepted for publication in A&

Simulating the infrared sky with a S PRITZ

Aims. Current hydrodynamical and semi-empirical simulations of galaxy formation and evolution have difficulties in reproducing the number densities of infrared-detected galaxies. Therefore, a phenomenological simulation tool that is new and versatile is necessary to reproduce current and predict future observations at infrared (IR) wavelengths. Methods. In this work we generate simulated catalogues starting from the Herschel IR luminosity functions of different galaxy populations to consider different populations of galaxies and active galactic nuclei (AGN) in a consistent way. We associated a spectral energy distribution and physical properties, such as stellar mass, star formation rate, and AGN contribution, with each simulated galaxy using a broad set of empirical relations. We compared the resulting simulated galaxies, extracted up to z = 10, with a broad set of observational relations. Results. Spectro-Photometric Realisations of IR-Selected Targets at all-z (SPRITZ) simulations allow us to obtain, in a fully consistent way, simulated observations for a broad set of current and future facilities with photometric capabilities as well as low-resolution IR spectroscopy, such as the James Webb Space Telescope (JWST) or the Origin Space Telescope (OST). The derived simulated catalogue contains galaxies and AGN that by construction reproduce the observed IR galaxy number density, but this catalogue also agrees with the observed number counts from UV to far-IR wavelengths, the observed stellar mass function, the star formation rate versus stellar mass plane, and the luminosity function from the radio to X-ray wavelengths. The proposed simulation is therefore ideal to make predictions for current and future facilities, in particular, but not limited to, those operating at IR wavelengths

Simulating the infrared sky with a Spritz

Current hydrodynamical and semi-empirical simulations of galaxy formation and evolution have difficulties in reproducing the number densities of IR-detected galaxies. Therefore, a versatile, phenomenological new simulation tool is necessary to reproduce current and predict future observations at IR wavelengths. In this work we generate simulated catalogues starting from the Herschel infrared luminosity functions of different galaxy populations, in order to consider in a consistent way different populations of galaxies and active galactic nuclei. We associated a spectral energy distribution and physical properties, such as stellar mass, star-formation-rate and AGN contribution, to each simulated galaxy using a broad set of empirical relations. We compare the resulting simulated galaxies, extracted up to z$=$10, with a broad set of observational relations. The Spectro-Photometric Realisations of Infrared-selected Targets at all-z (SPRITZ) simulation will allow us to obtain in a fully consistent way simulated observations for a broad set of current and future facilities with photometric capabilities as well as low-resolution IR spectroscopy, like the James Webb Space Telescope (JWST) or the Origin Space Telescope (OST). The derived simulated catalogue contains galaxies and active galactic nuclei that by construction reproduce the observed IR galaxy number density, but it is also in agreement with the observed number counts from UV to far-IR wavelengths, the observed stellar mass function, the star-formation-rate vs. stellar mass plane and the luminosity function from the radio to the X-ray. The proposed simulation is therefore ideal to make predictions for current and future facilities, in particular, but not limited to, those operating at IR wavelengths. The SPRITZ simulation will be publicly available.Comment: Accepted for publication in A&A, 32 pages, 30 figure

The AGN content in luminous IR galaxies at z\sim2 from a global SED analysis including Herschel data

We use Herschel-PACS far-infrared data, combined with previous multi-band information and mid-IR spectra, to properly account for the presence of an active nucleus and constrain its energetic contribution in luminous infrared (IR) sources at z\sim2. The sample is composed of 24 sources in the GOODS-South field, with typical IR luminosity of 10^{12} Lo. Data from the 4 Ms Chandra X-ray imaging in this field are also used to identify and characterize AGN emission. We reproduce the observed spectral energy distribution (SED), decomposed into a host-galaxy and an AGN component. A smooth-torus model for circum-nuclear dust is used to account for the direct and re-processed contribution from the AGN. We confirm that galaxies with typical L_{8-1000um}\sim10^{12}Lo at z\sim2 are powered predominantly by star-formation. An AGN component is present in nine objects (\sim35% of the sample) at the 3sigma confidence level, but its contribution to the 8-1000 um emission accounts for only \sim5% of the energy budget. The AGN contribution rises to \sim23% over the 5-30 um range (in agreement with Spitzer IRS results) and to \sim60% over the narrow 2-6 um range. The presence of an AGN is confirmed by X-ray data for 3 (out of nine) sources, with X-ray spectral analysis indicating the presence of significant absorption, i.e. NH\sim10^{23}-10^{24} cm^{-2}. An additional source shows indications of obscured AGN emission from X-ray data. The comparison between the mid-IR--derived X-ray luminosities and those obtained from X-ray data suggests that obscuration is likely present also in the remaining six sources that harbour an AGN according to the SED-fitting analysis.Comment: 12 pages, including 5 figures. Accepted for publication in MNRA

AKARI-NEP: effects of AGN presence on SFR estimates of galaxies

How does the presence of an AGN influence the total SFR estimates of galaxies and change their distribution with respect to the Galaxy Main Sequence? To contribute to solving this question, we study a sample of 1133 sources detected in the North Ecliptic Pole field (NEP) by AKARI and Herschel. We create a multi-wavelength dataset for these galaxies and we fit their multi-wavelength Spectral Energy Distribution (SED) using the whole spectral regime (from 0.1 to 500 µm). We perform the fit using three procedures: LePhare and two optimised codes for identifying AGN tracers from the SED analysis. In this work we present an overview of the comparison between the estimates of the Infrared bolometric luminosities (between 8 and 1000 µm) and the AGN fractions obtained exploiting these different procedures. In particular, by estimating the AGN contribution in four different wavelength ranges (5-40 µm, 10-20 µm, 20-40 µm and 8-1000 µm) we show how the presence of an AGN affects the PAH emission by suppressing the ratio L8 µm/ L4.5 µm as a function of the considered wavelength range

The lesser role of starbursts for star formation at z=2

Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5<z<2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than 4 times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star forming galaxies and account for only 10% of the cosmic SFR density at z~2. Only when limited to SFR>1000M(sun)/yr, off-sequence sources significantly contribute to the SFR density (46+/-20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies.Comment: Accepted for publication in ApJ Letter