arXiv subjects

Roberto Maiolino

Publications and source records attributed to Roberto Maiolino.

At least 397 records · Page 22Linked to original sources

arXiv2010

Confirmation of X-Ray Absorption by WHIM in the Sculptor Wall

In a previous paper we reported a 3-sigma detection of an absorption line from the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight-line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted OVII WHIM line at a significance level of 3.4-sigma, a substantial improvement over the 1.7-sigma level reported previously when using only the Cycle-8 data. The significance increases to 4.0-sigma when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a 90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upper limit on the Doppler-b parameter of 460 km/s. Assuming the absorber is uniformly distributed throughout the ~ 15 Mpc portion of the blazar's sight-line that intercepts the Sculptor Wall, that the OVII column density is ~ 2 10^16 cm^-2 (corresponding to b > 150 km/s where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~ 30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of OVII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.

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arXiv2010

The Building Up of the Black Hole Mass - Stellar Mass Relation

We derive the growth of SMBHs relative to the stellar content of their host galaxy predicted under the assumption of BH accretion triggered by galaxy encounters occurring during their merging histories. We show that, within this framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole mass and the galactic stellar mass (normalized to the local value) depends on both BH mass and redshift. While the average value and the spread of Gamma(z) increase with z, such an effect is larger for massive BHs, reaching values Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with recent observations of high-redshift QSOs; this is due to the effectiveness of interactions in triggering BH accretion in high-density environments at high redshifts. To test such a model against observations, we worked out specific predictions for sub-samples of the simulated galaxies corresponding to the different observational samples for which measurements of Gamma have been obtained. We found that for Broad Line AGNs at 1 4.

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arXiv2009

Spitzer Infrared Low-Resolution Spectroscopic Study of Buried AGNs in a Complete Sample of Nearby Ultraluminous Infrared Galaxies

We present the results of Spitzer IRS low-resolution infrared 5-35 micron spectroscopy of 17 nearby ULIRGs at z < 0.2, optically classified as non-Seyferts. The presence of optically elusive, but intrinsically luminous, buried AGNs is investigated, based on the strengths of polycyclic aromatic hydrocarbon emission and silicate dust absorption features detected in the spectra. The signatures of luminous buried AGNs, whose intrinsic luminosities range up to ~10^12 Lsun, are found in eight sources. We combine these results with those of our previous research to investigate the energy function of buried AGNs in a complete sample of optically non-Seyfert ULIRGs in the local universe at z < 0.3 (85 sources). We confirm a trend that we previously discovered: that buried AGNs are more common in galaxies with higher infrared luminosities. Because optical Seyferts also show a similar trend, we argue more generally that the energetic importance of AGNs is intrinsically higher in more luminous galaxies, suggesting that the AGN-starburst connections are luminosity-dependent. This may be related to the stronger AGN feedback scenario in currently more massive galaxy systems, as a possible origin of the galaxy downsizing phenomenon.

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arXiv2009

Chemical Evolution of High-Redshift Radio Galaxies

We present new deep optical spectra of 9 high-z radio galaxies (HzRGs) at z > 2.7 obtained with FORS2 on VLT. These rest-frame ultraviolet spectra are used to infer the metallicity of the narrow-line regions (NLRs) in order to investigate the chemical evolution of galaxies in high-z universe. We focus mainly on the CIV/HeII and CIII]/CIV flux ratios that are sensitive to gas metallicity and ionization parameter. Although the NV emission has been widely used to infer the gas metallicity, it is often too weak to be measured accurately for NLRs. By combining our new spectra with data from the literature, we examine the possible redshift evolution of the NLR metallicity for 57 HzRGs at 1 < z < 4. Based on the comparison between the observed emission-line flux ratios and the results of our photoionization model calculations, we find no significant metallicity evolution in NLRs of HzRGs, up to z ~ 4. Our results imply that massive galaxies had almost completed their chemical evolution at much higher redshift (z > 5). Finally, although we detect strong NV emission lines in 5 HzRGs at z > 2.7, we point out that high NV/HeII ratios are not indicative of high metallicities but correspond to high ionization parameters of gas clouds in NLRs.

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arXiv2008

The Hard X-ray Spectrum as a Probe for Black-Hole Growth in Radio-Quiet Active Galactic Nuclei

We study the hard-X-ray spectral properties of ten highly luminous radio-quiet (RQ) active galactic nuclei (AGNs) at z=1.3-3.2, including new XMM-Newton observations of four of these sources. We find a significant correlation between the normalized accretion rate (L/L_Edd) and the hard-X-ray photon index (Gamma) for 35 moderate-high luminosity RQ AGNs including our ten highly luminous sources. Within the limits of our sample, we show that a measurement of Gamma and L_X can provide an estimate of L/L_Edd and black-hole (BH) mass (M_BH) with a mean uncertainty of a factor of <~3 on the predicted values of these properties. This may provide a useful probe for tracing the history of BH growth in the Universe, utilizing samples of X-ray-selected AGNs for which L/L_Edd and M_BH have not yet been determined systematically. It may prove to be a useful way to probe BH growth in distant Compton-thin type 2 AGNs. We also find that the optical-X-ray spectral slope (a_ox) depends primarily on optical-UV luminosity rather than on L/L_Edd in a sample of RQ AGNs spanning five orders of magnitude in luminosity and over two orders of magnitude in L/L_Edd. We detect a significant Compton-reflection continuum in two of our highly luminous sources, and in the stacked X-ray spectrum of seven other sources with similar luminosities, we obtain a mean relative Compton reflection of R=0.9^{+0.6}_{-0.5} and an upper limit on the rest-frame equivalent width of a neutral Fe Ka line of 105 eV. We do not detect a significant steepening of the X-ray power-law spectrum below rest-frame 2 keV in any of our highly luminous sources, suggesting that a soft-excess feature, commonly observed in local AGNs, either does not depend strongly on L/L_Edd, or is not accessible at high redshifts using current X-ray detectors. [Abridged]

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arXiv2008

A photometric survey for Lyalpha-HeII dual emitters: Searching for Population III stars in high-redshift galaxies

We present a new photometric search for high-z galaxies hosting Population III (PopIII) stars based on deep intermediate-band imaging observations obtained in the Subaru Deep Field (SDF), by using Suprime-Cam on the Subaru Telescope. By combining our new data with the existing broad-band and narrow-band data, we searched for galaxies which emit strongly both in Ly_alpha and in HeII 1640 (``dual emitters'') that are promising candidates for PopIII-hosting galaxies, at 3.93 2 Msun/yr was found by our photometric search in 4.03 x 10^5 Mpc^3 in the SDF. This result disfavors low feedback models for PopIII star clusters, and implies an upper-limit of the PopIII SFR density of SFRD_PopIII < 5 x 10^-6 Msun/yr/Mpc^3. This new selection method to search for PopIII-hosting galaxies should be useful in future narrow-band surveys to achieve the first observational detection of PopIII-hosting galaxies at high redshifts.

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arXiv2008

A deep X-ray observation of M82 with XMM-Newton

We report on the analysis of a deep (100 ks) observation of the starburst galaxy M82 with the EPIC and RGS instruments on board the X-ray telescope XMM-Newton. The broad-band (0.5-10 keV) emission is due to at least three spectral components: i) continuum emission from point sources; ii) thermal plasma emission from hot gas; iii) charge exchange emission from neutral metals (Mg and Si). The plasma emission has a double-peaked differential emission measure, with the peaks at ~0.5 keV and ~7 keV. Spatially resolved spectroscopy has shown that the chemical absolute abundances are not uniformly distributed in the outflow, but are larger in the outskirts and smaller close to the galaxy centre. The abundance ratios also show spatial variations. The X-ray derived Oxygen abundance is lower than that measured in the atmospheres of red supergiant stars, leading to the hypothesis that a significant fraction of Oxygen ions have already cooled off and no longer emit at energies > ~0.5 keV.

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arXiv2008

The effect of radiation pressure on virial black hole mass estimates and the case of Narrow Line Seyfert 1 galaxies

We consider the effect of radiation pressure from ionizing photons on black hole (BH) mass estimates based on the application of the virial theorem to broad emission lines in AGN spectra. BH masses based only on the virial product V^2R and neglecting the effect of radiation pressure can be severely underestimated especially in objects close to the Eddington limit. We provide an empirical calibration of the correction for radiation pressure and we show that it is consistent with a simple physical model in which BLR clouds are optically thick to ionizing radiation and have average column densities of NH~10^23 cm^-2. This value is remarkably similar to what is required in standard BLR photoionization models to explain observed spectra. With the inclusion of radiation pressure the discrepancy between virial BH masses based on single epoch spectra and on reverberation mapping data drops from 0.4 to 0.2 dex rms. The use of single epoch observations as surrogates of reverberation mapping campaigns can thus provide more accurate BH masses than previously thought. Finally, we show that Narrow Line Seyfert 1 (NLS1) galaxies have apparently low BH masses because they are radiating close to their Eddington limit. After the radiation pressure correction, NLS1 galaxies have BH masses similar to other broad line AGNs and follow the same MBH-sigma/L relations as other active and normal galaxies. Radiation forces arising from ionizing photon momentum deposition constitute an important physical effect which must be taken into account when computing virial BH masses.

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arXiv2007

High-redshift Ly alpha emitters with a large equivalent width: Properties of i-dropout galaxies with an NB921-band depression in the Subaru Deep Field

We report new follow-up spectroscopy of i-dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed i-dropout selection method is expected to select galaxies with large equivalent width Ly alpha emission over a wide redshift range, 6.0<z<6.5. Two of four observed targets show a strong emission line with a clear asymmetric profile, identified as Ly alpha emitters at z=6.11 and 6.00. Their rest-frame equivalent widths are 153A and 114A, which are lower limits on the intrinsic equivalent widths. Through our spectroscopic observations (including previous ones) of NB921-depressed i-dropout galaxies, we identified 5 galaxies in total with a rest-frame equivalent width larger than 100A at 6.0<z<6.5 out of 8 photometric candidates, which suggests that the NB921-depressed i-dropout selection method is possibly an efficient way to search for Ly alpha emitters with a large Ly alpha equivalent width, in a wider redshift range than usual narrow-band excess techniques. By combining these findings with our previous observational results, we infer that the fraction of broad-band selected galaxies having a rest-frame equivalent width larger than 100A is significantly higher at z~6 (the cosmic age of ~1 Gyr) than that at z~3 (~2 Gyr), being consistent with the idea that the typical stellar population of galaxies is significantly younger at z~6 than that at z~3. The NB921-depressed i-dropout galaxies may be interesting candidates for hosts of massive, zero-metallicity Population III stars.

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arXiv2007

A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies - A Constraint on Geometry between Energy Sources and Dust -

We present the results of Spitzer IRS low-resolution infrared 5-35 micron spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. We focus on the search for the signatures of buried active galactic nuclei (AGNs) in the complete sample of ULIRGs classified optically as non-Seyferts (LINERs or HII-regions). In addition to polycyclic aromatic hydrocarbon (PAH) emission features at 6.2 micron, 7.7 micron, and 11.3 micron, the conventional tool of starburst-AGN separation, we use the optical depths of the 9.7 micron and 18 micron silicate dust absorption features to infer the geometry of energy sources and dust at the nuclei of these ULIRGs, namely, whether the energy sources are spatially well mixed with dust(a normal starburst) or are more centrally concentrated than the dust (a buried AGN). Infrared spectra of at least 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs are naturally explained by emission consisting of (1) energetically insignificant, modestly obscured (Av < 20-30 mag) PAH-emitting normal starbursts, and (2) energetically dominant, highly dust-obscured, centrally concentrated energy sources with no PAH emission. We interpret the latter component as a buried AGN. The fraction of ULIRGs showing some buried AGN signatures is higher in LINER ULIRGs than in HII-region ULIRGs. Most of the luminous buried AGN candidates are found in ULIRGs with cool far-infrared colors. Where the absorption-corrected intrinsic AGN luminosities are derivable with little uncertainty, they are found to be of the order of 10^12Lsun, accounting for the bulk of the ULIRGs' luminosities. The 5-35 micron spectroscopic starburst/AGN classifications are generally consistent with our previous classifications based on 3-4 micron spectra for the same sample.

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arXiv2006

Metallicity Evolution of Active Galactic Nuclei

In this contribution we report our recent investigation of the gas metallicity in active galactic nuclei and its dependence on luminosity and redshift. We compile large spectroscopic datasets of broad-line and narrow-line AGNs, and compare them with the results of our photoionization models. Through the analysis of both the broad and the narrow emission-line regions, we find that: (1) for a given luminosity, there is no redshift dependence of the gas metallicity; (2) for a given redshift, there is a significant correlation between gas metallicity and luminosity; (3) the luminosity-metallicity relation does no show any evolution in the redshift range 2 < z < 4.

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arXiv2006

Gas metallicity diagnostics in star-forming galaxies

Generally the gas metallicity in distant galaxies can only be inferred by using a few prominent emission lines. Various theoretical models have been used to predict the relationship between emission line fluxes and metallicity, suggesting that some line ratios can be used as diagnostics of the gas metallicity in galaxies. However, accurate empirical calibrations of these emission line flux ratios from real galaxy spectra spanning a wide metallicity range are still lacking. In this paper we provide such empirical calibrations by using the combination of two sets of spectroscopic data: one consisting of low-metallicity galaxies with a measurement of [OIII]4363, and the other one consisting of galaxies in the SDSS DR4 whose metallicity has been determined from various strong emission lines in their spectra. This combined data set constitutes the largest sample of galaxies with information on the gas metallicity available so far and spanning the widest metallicity range. Our empirical diagrams show that the line ratio [OIII]/[OII] is a useful tool to break the degeneracy in the R_23 parameter when no information on the [NII] line is available. The line ratio [NeIII]/[OII] also results to be a useful metallicity indicator for high-z galaxies. Finally, we compare these empirical relations with photoionization models. We find that the empirical R_23-metallicity sequence is strongly discrepant with respect to the trend expected by models with constant ionization parameter, which is interpreted as a consequence of a strong metallicity dependence of the average ionization parameter in galaxies. This result should warn about the use of theoretical models with constant ionization parameter to infer metallicities from observed line ratios. (abbreviated)

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arXiv2006

New insights on chemical evolution of galaxies: XMM-Newton observations of M82

We report on the ongoing data analysis of a very deep (~100 ks) XMM-Newton observation of the starburst galaxy M82. We show some details of data analysis and a few results from spatially-resolved spectroscopy with the EPIC cameras. Since M82 is a bright object with a complex spectrum, the data reduction of such a deep observation has posed many challenges both about the involved astrophysical processes and the available data analysis techniques. Vertical (with respect to the galaxy plane) abundance gradients are discovered. The hints for an under-abundance of Oxygen stemming from our previous study are confirmed. The hot X-ray emitting gas is shown to have a multi-temperature distribution.

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arXiv2006

The Hard X-ray Spectral Slope as an Accretion-Rate Indicator in Radio-Quiet Active Galactic Nuclei

We present new XMM-Newton observations of two luminous and high accretion-rate radio-quiet active galactic nuclei (AGNs) at z~2. Together with archival X-ray and rest-frame optical spectra of three sources with similar properties as well as 25 moderate-luminosity radio-quiet AGNs at z<0.5, we investigate, for the first time, the dependence of the hard (>~2 keV) X-ray power-law photon index on the broad H_beta emission-line width and on the accretion rate across ~3 orders of magnitude in AGN luminosity. Provided the accretion rates of the five luminous sources can be estimated by extrapolating the well-known broad-line region size-luminosity relation to high luminosities, we find that the photon indices of these sources, while consistent with those expected from their accretion rates, are significantly higher than expected from the widths of their H_beta lines. We argue that, within the limits of our sample, the hard-X-ray photon index depends primarily on the accretion rate.

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arXiv2005

Gaseous abundances in M82

We present the preliminary analysis of a deep (100ks) XMM-Newton observation of M82. The spatial distribution of the abundances of chemical elements (Fe, O, Ne, Mg, Si, S) is investigated through narrow-band imaging analisys and spatially-resolved spectroscopy. We find that the abundances of alpha-elements follow a bipolar distribution, these elements being more abundant in the gaseous outflow than in the galaxy centre. This behaviour is found to be more marked for lighter elements (O, Ne) than for heavier elements.

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arXiv2005

The Evolution of the Broad-Line Region among SDSS Quasars

Based on 5344 quasar spectra taken from the SDSS Data Release 2, the dependences of various emission-line flux ratios on redshift and quasar luminosity are investigated in the ranges 2.0 < z < 4.5 and -24.5 > M_B > -29.5$. We show that the emission lines in the composite spectra are fitted better with power-law profiles than with double Gaussian or modified Lorentzian profiles, and in particular we show that the power-law profiles are more appropriate to measure broad emission-line fluxes than other methods. The composite spectra show that there are statistically significant correlations between quasar luminosity and various emission-line flux ratios, such as NV/CIV and NV/HeII, while there are only marginal correlations between quasar redshift and emission-line flux ratios. We obtain detailed photoionization models to interpret the observed line ratios. The correlation of line ratios with luminosity is interpreted in terms of higher gas metallicity in more luminous quasars. For a given quasar luminosity, there is no metallicity evolution for the redshift range 2.0 < z < 4.5. The typical metallicity of BLR gas clouds is estimated to be Z ~ 5 Z_sun, although the inferred metallicity depends on the assumed BLR cloud properties, such as their density distribution function and their radial distribution. The absence of a metallicity evolution up to z ~ 4.5 implies that the active star-formation epoch of quasar host galaxies occurred at z > 7.

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arXiv2005

Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei

We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrow-line regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0), or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range 1.2 < z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at z > 4.

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arXiv2005

An Observational Pursuit for Population III Stars in a Ly_alpha Emitter at z=6.33 through HeII Emission

We present a very deep near-infrared spectroscopic observation of a strong Ly_alpha emitter at z=6.33, SDF J132440.6+273607, which we used to search for HeII 1640. This emission line is expected if the target hosts a significant number of population III stars. Even after 42 ksec of integration with the Subaru/OHS spectrograph, no emission-line features are detected in the JH band, which confirms that SDF J132440.6+273607 is neither an active galactic nucleus nor a low-$z$ emission-line object. We obtained a 2sigma upper-limit of 9.06e-18 ergs/s/cm^2 on the HeII 1640 emission line flux, which corresponds to a luminosity of 4.11e42 ergs/s. This upper-limit on the HeII 1640 luminosity implies that the upper limit on population III star-formation rate is in the range 4.9--41.2 M_sun/yr if population III stars suffer no mass loss, and in the range 1.8--13.2 M_sun/yr if strong mass loss is present. The non-detection of HeII in SDF J132440.6+273607 at z=6.33 may thus disfavor weak feedback models for population III stars.

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