A high space density of L* Active Galactic Nuclei at z~4 in the COSMOS field

Identifying the source population of ionizing radiation, responsible for the reionization of the universe, is currently a hotly debated subject with conflicting results. Studies of faint, high-redshift star-forming galaxies, in most cases, fail to detect enough escaping ionizing radiation to sustain the process. Recently, the capacity of bright quasi-stellar objects to ionize their surrounding medium has been confirmed also for faint active galactic nuclei (AGNs), which were found to display an escaping fraction of ~74% at z~4. Such levels of escaping radiation could sustain the required UV background, given the number density of faint AGNs is adequate. Thus, it is mandatory to accurately measure the luminosity function of faint AGNs (L~L*) in the same redshift range. For this reason we have conducted a spectroscopic survey, using the wide field spectrograph IMACS at the 6.5m Baade Telescope, to determine the nature of our sample of faint AGN candidates in the COSMOS field. This sample was assembled using photometric redshifts, color, and X-ray information. We ended up with 16 spectroscopically confirmed AGNs at 3.6<z<4.2 down to a magnitude of i$_{AB}$=23.0 for an area of 1.73 deg$^{2}$. This leads to an AGN space density of ~1.6$\times10^{-6} Mpc^{-3}$ (corrected) at z~4 for an absolute magnitude of M$_{1450}$=-23.5. This is higher than previous measurements and seems to indicate that AGNs could make a substantial contribution to the ionizing background at z~4. Assuming that AGN physical parameters remain unchanged at higher redshifts and fainter luminosities, these sources could be regarded as the main drivers of cosmic reionization.Comment: 10 pages, 3 figures, accepted for publication by Ap

A population of intermediate-mass black holes in dwarf starburst galaxies up to redshift=1.5

We study a sample of $\sim$50,000 dwarf starburst and late-type galaxies drawn from the COSMOS survey with the aim of investigating the presence of nuclear accreting black holes (BHs) as those seed BHs from which supermassive BHs could grow in the early Universe. We divide the sample into five complete redshift bins up to $z=1.5$ and perform an X-ray stacking analysis using the \textit{Chandra} COSMOS-Legacy survey data. After removing the contribution from X-ray binaries and hot gas to the stacked X-ray emission, we still find an X-ray excess in the five redshift bins that can be explained by nuclear accreting BHs. This X-ray excess is more significant for $z<0.5$. At higher redshifts, these active galactic nuclei could suffer mild obscuration, as indicated by the analysis of their hardness ratios. The average nuclear X-ray luminosities in the soft band are in the range 10$^{39}-10^{40}$ erg s$^{-1}$. Assuming that the sources accrete at $\geq$ 1\% the Eddington rate, their BH masses would be $\leq$ 10$^{5}$ M$_{\odot}$, thus in the intermediate-mass BH regime, but their mass would be smaller than the one predicted by the BH-stellar mass relation. If instead the sources follow the correlation between BH mass and stellar mass, they would have sub-Eddington accreting rates of $\sim$ 10$^{-3}$ and BH masses 1-9 $\times$ 10$^{5}$ M$_{\odot}$. We thus conclude that a population of intermediate-mass BHs exists in dwarf starburst galaxies, at least up to $z$=1.5, though their detection beyond the local Universe is challenging due to their low luminosity and mild obscuration unless deep surveys are employed.Comment: 10 pages, 7 figures, ApJ in pres

The high-redshift Universe with the International X-ray Observatory

We discuss some of the main open issues related to the light-up and evolution of the first accreting sources powering high redshift luminous quasars. We discuss the perspectives of future deep X-ray surveys with the International X-ray Observatory and possible synergies with the Wide Field X-ray Telescope.Comment: 6 pages, 6 figures. Proceedings of "The Wide Field X-ray Telescope Workshop", held in Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societ\`a Astronomica Italiana 2010 (arXiv:1010.5889

Intermediate-mass black holes in dwarf galaxies out to redshift ∼\sim 2.4 in the Chandra COSMOS Legacy Survey

We present a sample of 40 AGN in dwarf galaxies at redshifts $z \lesssim$ 2.4. The galaxies are drawn from the \textit{Chandra} COSMOS-Legacy survey as having stellar masses $10^{7}\leq M_{*}\leq3 \times 10^{9}$ M$_{\odot}$. Most of the dwarf galaxies are star-forming. After removing the contribution from star formation to the X-ray emission, the AGN luminosities of the 40 dwarf galaxies are in the range $L_\mathrm{0.5-10 keV} \sim10^{39} - 10^{44}$ erg s$^{-1}$. With 12 sources at $z > 0.5$, our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid\_1192, at $z = 2.39$ and with $L_\mathrm{0.5-10 keV} \sim 10^{44}$ erg s$^{-1}$. One of the dwarf galaxies has $M_\mathrm{*} = 6.6 \times 10^{7}$ M$_{\odot}$ and is the least massive galaxy found so far to host an AGN. All the AGN are of type 2 and consistent with hosting intermediate-mass black holes (BHs) with masses $\sim 10^{4} - 10^{5}$ M$_{\odot}$ and typical Eddington ratios $> 1\%$. We also study the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to $z$ = 0.7. We find that the AGN fraction for $10^{9}< M_{*}\leq3 \times 10^{9}$ M$_{\odot}$ and $L_\mathrm{X} \sim 10^{41}-10^{42}$ erg s$^{-1}$ is $\sim$0.4\% for $z \leq$ 0.3 and that it decreases with X-ray luminosity and decreasing stellar mass. Unlike massive galaxies, the AGN fraction seems to decrease with redshift, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. Mindful of potential caveats, the results seem to favor a direct collapse formation mechanism for the seed BHs in the early Universe.Comment: 16 pages, 10 figures, accepted for publication in MNRA

Hidden AGNs in Early-Type Galaxies

We present a stacking analysis of the complete sample of Early Type Galaxies (ETGs) in the \textit{Chandra} COSMOS (C-COSMOS) survey, to explore the nature of the X-ray luminosity in the redshift and stellar luminosity ranges $0<z<1.5$ and {10}^{9}. Using established scaling relations, we subtract the contribution of X-ray binary populations, to estimate the combined emission of hot ISM and AGN. To discriminate between the relative importance of these two components, we (1) compare our results with the relation observed in the local universe $L_{X,gas}\propto L_K^{4.5}$ for hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of each stacked bin. We find two regimes where the non-stellar X-ray emission is hard, consisten t with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z ($\sim 1.2$) ETGs with average high X-ray luminosity (L_{X-LMXB}\gtrsim 6\times{10}^{42}\mbox{ erg}/\mbox{s}). These luminosities are consistent with the presence ofhighly absorbed "hidden" AGNs in these ETGs, which are not visible in their optical-IR spectra and spectral energy distributions. Second, confirming the early indication from our C-COSMOS study of X-ray detected ETGs, we find significantly enhanced X-ray luminoaity in lower stellar mass ETGs (L_K\lesssim{10}^{11}L_{\astrosun}), relative to the local $L_{X,gas}\propto L_K^{4.5}$ relation. The stacked spectra of these ETGs also suggest X-ray emission harder than expected from gaseous hot halos. This emission is consistent with inefficient accretion ${10}^{-5}-{10}^{-4}\dot{M}_{Edd}$ onto M_{BH}\sim {10}^{6}-{10}^{8}\,M_{\astrosun}.Comment: 22 pages, 7 figures, 2 tables. Accepted for publications on Ap

Early-type galaxies in the Chandra COSMOS Survey

We study a sample of 69 X-ray detected Early Type Galaxies (ETGs), selected from the Chandra COSMOS survey, to explore the relation between the X-ray luminosity of hot gaseous halos (L_X, gas) and the integrated stellar luminosity (L_K) of the galaxies, in a range of redshift extending out to z=1.5. In the local universe a tight steep relationship has been stablished between these two quantities (L_X,gas~ L_K^4.5) suggesting the presence of largely virialized halos in X-ray luminous systems. We use well established relations from the study of local universe ETGs, together with the expected evolution of the X-ray emission, to subtract the contribution of low mass X-ray binary populations (LMXBs) from the X-ray luminosity of our sample. Our selection minimizes the presence of active galactic nuclei (AGN), yielding a sample representative of normal passive COSMOS ETGs; therefore the resulting luminosity should be representative of gaseous halos, although we cannot exclude other sources such as obscured AGN, or enhanced X-ray emission connected with embedded star formation in the higher z galaxies. We find that most of the galaxies with estimated L_X<10^42 erg/s and z<0.55 follow the L_X,gas- L_K relation of local universe ETGs. For these galaxies, the gravitational mass can be estimated with a certain degree of confidence from the local virial relation. However, the more luminous (10^42<L_X<10^43.5 erg/s) and distant galaxies present significantly larger scatter; these galaxies also tend to have younger stellar ages. The divergence from the local L_X,gas - L_K relation in these galaxies implies significantly enhanced X-ray emission, up to a factor of 100 larger than predicted from the local relation. We discuss the implications of this result for the presence of hidden AGN, and the evolution of hot halos, in the presence of nuclear and star formation feedback.Comment: 29 pages, 10 figures, accepted for publication on ApJ on May 27 201

The Xmm-Newton Spectrum of a Candidate Recoiling Supermassive Black Hole: An Elusive Inverted P-Cygni Profile

We present a detailed spectral analysis of new XMM-Newton data of the source CXOC J100043.1+020637, also known as CID-42, detected in the COSMOS survey at z = 0.359. Previous works suggested that CID-42 is a candidate recoiling supermassive black hole (SMBH) showing also an inverted P-Cygni profile in the X-ray spectra at ~6 keV (rest) with an iron emission line plus a redshifted absorption line (detected at 3σ in previous XMM-Newton and Chandra observations). Detailed analysis of the absorption line suggested the presence of ionized material flowing into the black hole at high velocity. In the new long XMM-Newton observation, while the overall spectral shape remains constant, the continuum 2-10 keV flux decrease of ~20% with respect to previous observation and the absorption line is undetected. The upper limit on the intensity of the absorption line is EW \u3c 162 eV. Extensive Monte Carlo simulations show that the nondetection of the line is solely due to variation in the properties of the inflowing material, in agreement with the transient nature of these features, and that the intensity of the line is lower than the previously measured with a probability of 98.8%. In the scenario of CID-42 as a recoiling SMBH, the absorption line can be interpreted as being due to an inflow of gas with variable density that is located in the proximity of the SMBH and recoiling with it. New monitoring observations will be requested to further characterize this line