arXiv subjects

R. Maiolino

Publications and source records attributed to R. Maiolino.

At least 361 records · Page 20Linked to original sources

arXiv2002

The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.

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arXiv2001

The TNG Near Infrared Camera Spectrometer

NICS (acronym for Near Infrared Camera Spectrometer) is the near-infrared cooled camera-spectrometer that has been developed by the Arcetri Infrared Group at the Arcetri Astrophysical Observatory, in collaboration with the CAISMI-CNR for the TNG (the Italian National Telescope Galileo at La Palma, Canary Islands, Spain). As NICS is in its scientific commissioning phase, we report its observing capabilities in the near-infrared bands at the TNG, along with the measured performance and the limiting magnitudes. We also describe some technical details of the project, such as cryogenics, mechanics, and the system which executes data acquisition and control, along with the related software.

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arXiv2001

Optical identification of sources from the HELLAS2XMM survey

We present preliminary results on the optical identifications of sources from the High Energy Large Area Survey performed with XMM-Newton (HELLAS2XMM). This survey covers about 3 square degrees of sky down to a 2-10 keV flux of 7 x 10^-15 erg cm-2 s-1 (Baldi & Molendi these proceedings, Baldi et al. 2001). The survey good sensitivity over a large area allows us to confirm and extend previous Chandra results about a population of X-ray luminous but optically dull galaxies emerging at 2-10 keV fluxes of about 10-14 erg cm-2 s-1. Although the statistics are still rather poor, it appears that at these X-ray fluxes the fraction of these galaxies is similar or even higher than that of narrow line AGN.

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arXiv2001

X-ray bright optically quiet galaxies: the case of P3

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at X-ray fluxes of the order of 10-14 erg cm-2 s-1. The nature of these objects is still unknown. We present the results of multiwavelength observations of what can be considered the best studied example: the Chandra source CXOUJ031238.9-765134 optically identified by Fiore et al. (2000) with an apparently normal galaxy at z=0.159 and called ``Fiore P3''.

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arXiv2001

The Ly-edge paradox and the need for obscured QSOs

Based on the most recent QSO ultraviolet spectra, the covering factor of the clouds of the Broad Line Region (BLR) is about 30%, or larger. This value would imply that in at least 30% of the QSOs our line of sight crosses one, or more, BLR clouds and, in the latter case, the UV spectrum should show a sharp Ly-edge in absorption. This Ly-edge in absorption is never observed. This paradox is solved if, as suggested by various authors, the BLR is flattened and the dusty gas in the outer parts, on the same plane, prevents the observation along the lines of sight passing through the BLR clouds. The objects observed edge-on (with respect to the flattened BLR) would be classified as obscured QSOs or, within the framework of the unified model, type 2 QSOs. The covering factor of the BLR constrains the fraction of obscured QSOs to be QSO2/QSO1 > 0.5. This lower limit is already high with respect to the number of candidate type 2 QSOs claimed so far. We discuss this constraint in relation to recent AGN surveys.

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arXiv2001

Discovery of strong CIV absorption in the highest redshift quasar

We report the near-IR detection of a prominent CIV absorption in the rest-frame UV spectrum of the most distant known QSO, SDSS J104433.04-012502.2, at z=5.80. This QSO was recently observed with XMM-Newton and it was found to be notably X-ray weak. The equivalent width of the CIV absorption feature (~10 A) strongly supports the idea that the X-ray faintness of this QSO is due to heavy absorption by gas with a column density N_H > 10^{24} cm^-2. The shape of the CIV feature suggests that this is a Broad Absorption Line QSO. Although absorbed by a huge column of gas, the observed continuum in the 0.9-2.4um range (~1300-3500 A rest frame) exactly matches the template of unabsorbed QSOs without invoking any reddening (E(B-V)<0.08 mag), indicating that dust in the absorbing gas is either absent or composed of large grains.

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arXiv2001

NICS-TNG low-resolution 0.85-2.45um spectra of L-Dwarfs: a near-infrared spectral classification scheme for faint dwarfs

We present complete near-infrared (0.85-2.45 um), low-resolution (~100) spectra of a sample of 26 disk L-dwarfs with reliable optical spectral type classification. The observations have been obtained with NICS at the TNG using a prism-based optical element (the Amici device) that provides a complete spectrum of the source on the detector. Our observations show that low-resolution near-infrared spectroscopy can be used to determine the spectral classification of L-dwarfs in a fast but accurate way. We present a library of spectra that can be used as templates for spectral classification of faint dwarfs. We also discuss a set of near-infrared spectral indices well correlated with the optical spectral types that can be used to accurately classify L-dwarfs earlier than L6.

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arXiv2001

N-band Imaging of Seyfert Nuclei and the MIR-X-ray correlation

We present new mid-infrared (N-band) images of a sample of eight nearby Seyfert galaxies. In all of our targets, we detect a central unresolved source, which in some cases has been identified for the first time. In particular, we have detected the mid-infrared emission from the active nucleus of NGC 4945, which previously remained undetected at any wavelength but hard X-rays. We also detect circumnuclear extended emission in the Circinus galaxy along its major axis, and find marginal evidence for extended circumnuclear emission in NGC 3281. The high spatial resolution (1.7") of our data allows us to separate the flux of the nuclear point sources from the extended circumnuclear starburst (if present). We complement our sample with literature data for a number of non-active starburst galaxies, and relate the nuclear N-band flux to published hard (2-10 kev) X-ray fluxes. We find tight and well-separated correlations between nuclear N-band flux and X-ray flux for both Seyfert and starburst nuclei which span over 3 orders of magnitude in luminosity. We demonstrate that these correlations can be used as a powerful classification tool for galactic nuclei. For example, we find strong evidence against NGC 1808 currently harbouring an active Seyfert nucleus based on its position in the mid-infrared-X-ray diagram. On the other hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.

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arXiv2001

Properties of the circumnuclear medium in Active Galactic Nuclei

We propose a classification scheme for the circumnuclear medium of AGNs based on their location in the A_V - N_H plane (related to dust absorption and gas absorption, respectively). We present evidence that the A_V/N_H ratio is often lower than the Galactic value. This is shown by the broad line ratios of a sample of intermediate Seyferts, and by the optical/X-ray ratio of line selected (as opposed to color selected) AGNs. We point out the implications for the physical properties of the absorber. A complete classification would require a third axis, related to the density of the absorber and/or its distance from the AGN.

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arXiv2001

A new population of soft X-ray weak quasars

Quasars selected in optical surveys by means of their blue or UV excess are known to be strong emitters in the X-rays, except for Broad Absorption Line (BAL) objects. In this paper we study the X-ray emission of quasars selected through their emission line spectrum rather than their blue excess. X-ray data are obtained by cross-correlating two optical samples (the Hamburg Survey Catalogue and the Palomar Transit Catalogue) with the WGA catalogue of ROSAT observations. We find that a significant fraction of objects are strongly underluminous in the X-rays. We discuss the physical nature of these sources and we propose an interpretation for their optical and X-ray properties based on a dust-poor and gas-rich absorber. Our results suggest the existence of a population of AGNs having a type 1 optical spectrum and a type 2 X-ray emission

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arXiv2001

NICS-TNG infrared spectroscopy of NGC1068: the first extragalactic measurement of [PII] and a new tool to constrain the origin of [FeII] line emission in galaxies

We report 0.9-1.4 micron spectroscopic observations of NGC1068 collected during the commissioning phase of the near infrared camera spectrometer (NICS) of the Telescopio Nazionale Galileo (TNG). These yielded the first extragalactic measurement of [PII] (1.188 micron) line emission. In the central 0.75"x2" the [FeII]/[PII] line-intensity ratio is close to unity, similar to that measured in the Orion bar and a factor of about 20 smaller than in supernova remnants. This indicates that most of iron is locked into grains and, therefore, argues against shock excitation being the primary origin of [FeII] line emission in the central regions of NGC1068. We propose the [FeII]/[PII] ratio as a simple and effective tool to study and perhaps resolve the long debated questions related to the origin of [FeII] line emission and, more generally, to constrain the role of shock excitation in active galaxies.

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arXiv2000

An X-ray and Near-IR spectroscopic analysis of the ULIRG IRAS 05189-2524

We present new, quasi-simultaneous hard X-ray and near-IR spectra of the ultraluminous infrared galaxy IRAS 05189-2524, and discuss them together with archival and literature data. The 1.9 Seyfert nucleus is Compton-thin. The near-IR broad lines are seen in transmission, similarly to the X rays, and the medium along the line of sight has an Av/Nh ratio definitely lower than Galactic. The increase in obscuration observed at the latter epoch has (Delta Av)/(Delta Nh) also less than Galactic, supporting a correlation between the properties of the obscuring matter and its proximity to the center. The measured Av is compatible with the broad component of Halpha being seen in transmission, as opposed to scattering, so that most of the observed polarization must be due to dichroism. The bolometric luminosity of the AGN, deduced from the X-ray and line luminosities, falls definitely short of accounting for the IR luminosity of the galaxy, consistent with its coolish infrared color.

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arXiv2000

Dust in active nuclei. II. Powder or gravel?

In a companion paper, Maiolino et al. (2000) presented various observational evidences for "anomalous" dust properties in the circumnuclear region of AGNs and, in particular, the reduced E(B-V)/N_H and Av/N_H ratios, the absence of the silicate absorption feature in mid-IR spectra of Sy2s and the absence of the carbon dip in UV spectra of reddened Sy1s. In this paper we discuss various explanations for these facts. The observational constraints favor a scenario where coagulation, catalyzed by the high densities in the circumnuclear region, yields to the formation of large grains. The resulting extinction curve is featureless, flatter than Galactic and the E(B-V)/N_H and Av/N_H ratios are significantly reduced. These results should warn about an unappropriate use of the standard Galactic extinction curve and Av/N_H ratio when dealing with the extreme gas conditions typical of the circumnuclear clouds of AGNs. We also investigated alternative scenarios for the observed anomalous properties of dust in AGNs. Some of these scenarios might explain some of the observed properties for a few objects, but they generally fail to account for all of the observational constraints obtained for the large sample of AGNs studied in these works.

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arXiv2000

Dust in active nuclei. I. Evidence for "anomalous" properties

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the E(B-V)/N_H ratio is nearly always lower than Galactic by a factor ranging from ~3 up to ~100. Other observational results indicate that the Av/N_H ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7um (silicates) and at 2175A (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the E(B-V)/N_H and Av/N_H ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

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arXiv2000

Obscured Active Galactic Nuclei

The properties of the absorption in type 2, narrow line AGNs are reviewed by focusing on the X-ray indicators. I discuss the properties of the cold absorbing medium (the putative torus) and of the reprocessed components, as well as their implications for the unified model. The relation between optical classification and X-ray absorption is examined. The case of "fossil" AGNs, whose type 2 classification is not due to absorption effects, is also discussed. Although this review is mainly focused on nearby Seyfert 2 galaxies, I also shortly discuss the effects of absorption at higher luminosities and higher redshift and the implications for the X-ray background.

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arXiv2000

The BeppoSAX HELLAS survey: on the nature of faint hard X-ray selected sources

The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80 square degrees of sky down to a flux of F(5-10keV)~5E-14 cgs. Optical spectroscopic identification of about half of the sources in the sample (62) shows that many (~50%) are highly obscured AGN, in line with the predictions of AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et al. 1995). The X-ray data, complemented by optical, near-IR and radio follow-up, indicate that the majority of these AGN are ``intermediate'' objects, i.e. type 1.8-1.9 AGN,`red' quasars, and even a few broad line, blue continuum quasars, obscured in X-rays by columns of the order of logNH=22.5-23.5 cm-2, but showing a wide dispersion in optical extinction. The optical and near-IR photometry of the obscured objects are dominated by galaxy starlight, indicating that a sizeable fraction of the accretion power in the Universe may actually have been missed in optical color surveys. This also implies that multicolor photometry techniques may be efficiently used to assess the redshift of the hard X-ray selected sources.

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arXiv2000

Sub-mm and X-ray background: two unrelated phenomena?

Obscured AGNs are thought to contribute a large fraction of the hard X-ray background (2-10 keV), and have also been proposed as the powerhouse of a fraction of the SCUBA sources which make most of the background at 850um, thus providing a link between the two spectral windows. We have tackled this issue by comparing data at 2-10 keV and at 850um for a sample of 34 sources at fluxes (or limiting fluxes) which resolve most of the background in the two bands. We present here new SCUBA observations, and new correlations between separate data sets retrieved from the literature. Similar correlations presented by others are added for completeness. None of the 11 hard X-ray (2-10 keV) sources has a counterpart at 850um, with the exception of a Chandra source in the SSA13 field, which is a candidate type 2, heavily absorbed QSO at high redshift. The ratios F(850um)/F(5keV) (mostly upper limits) of the X-ray sources are significantly lower than the value observed for the cosmic background. In particular, we obtain that 2-10 keV sources brighter than 10^-15 erg s^-1 cm^-2, which make at least 75% of the background in this band, contribute for less than 7% to the submillimeter background. Out of the 24 SCUBA sources, 23 are undetected by Chandra. The ratios F(850um)/F(5keV) (mostly lower limits) of these SCUBA sources indicate that most of them must be powered either by starburst activity, or by an AGN which is obscured by a column Nh > 10^25 cm^-2, with a reflection efficiency in the hard X rays significantly lower than 1% in most cases. However, AGNs of this type could not contribute significantly to the 2-10 keV background.

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arXiv2000

Optically dim counterparts of hard X-ray selected AGNs

We present near-IR photometry and imaging observations of a small sample of sources identified in the BeppoSAX 5-10 keV survey (HELLAS) which resolves ~ 20-30% of the X-ray background at these energies. The near-IR data are combined with optical spectra and photometry. Only 40% of the sources in our sample have the blue, power law continuum typical of color-selected QSOs. The remaining 60% are dominated by a galactic component which, on the basis of the continuum colors and shape, have ages ranging from 10^9 to 10^10 years. The images show that the blue QSOs are pointlike at our angular resolution, while all the other sources are extended, consistent with their spectral appearance and low redshift. Since down to R=20 only about two thirds of the HELLAS sources have a counterpart, the preliminary HELLAS census comprises in roughly equal parts: i) blue QSOs (mostly at high redshifts); ii) optically dim, galaxy-dominated active nuclei (mostly at modest redshifts); and iii) empty fields (possibly highly absorbed QSOs at high redshifts).

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