arXiv subjects

R. Maiolino

Publications and source records attributed to R. Maiolino.

At least 379 records · Page 21Linked to original sources

arXiv2000

The hard X-ray emission of luminous infrared galaxies

We present a study of the hard X-ray properties of a sample that includes all the Luminous Infrared Galaxies (LIGs, L_IR > 10^11 L_sun) observed in the 2-10 keV energy band (new and archival data). We find that a significant fraction of the sources optically classified as AGNs do not show any indication of nuclear activity in the X rays, thus suggesting heavy absorption along our line of sight. The absence of strong emission in the 20-200 keV band in a subsample of LIGs observed with BeppoSAX suggests that in many cases these sources are completely Compton thick (N_H > 10^25 cm^(-2)). From a comparison between the infrared and the X-ray emission we deduce that the mid-IR emission is absorbed by a lower column density than the X-ray emission or, alternatively, that the dust-to-gas ratio is lower than Galactic. We describe a simple model that reproduces the IR-X correlation by means of mixed AGN and starburst contributions and we compare the predictions of this model with the observational data at X-ray and optical wavelengths. Finally, we discuss the biases that affect the currently available samples of LIGs and briefly analyze a small unbiased sample, finding that at least 50% of the sources host a (weak) AGN.

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arXiv2000

Optical, near-infrared and hard X-ray observations of SAXJ1353.9+1820: a red quasar

We present the results of a follow-up ASCA observation and multicolour optical and near-infrared photometry carried out at the 3.5-m Italian National Telescope Galileo of SAXJ1353.9+1820. This object, serendipitously discovered by BeppoSAX in the 5-10 keV band, has been spectroscopically identified as a red quasar at z=0.217. The combined X-ray and optical-infrared data reveal the presence of a moderately luminous X-ray source (~ 10^{44} erg s^{-1}) obscured by a column density of the order of 10^{22} cm^{-2} in a otherwise optically passive early-type galaxy. The implications for the nature of red quasars and their possible contribution to the hard X-ray background are briefly outlined.

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arXiv2000

The Elusive Active Nucleus of NGC 4945

We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. The HST data are complemented with ground based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is detected in Pa alpha, while H_2 traces the walls of a super bubble opened by supernova-driven winds. The conically shaped cavity is particularly prominent in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images are heavily affected by dust extinction and the nucleus of the galaxy is located in a highly reddened region with an elongated, disk-like morphology. No manifestation of the active nucleus is found, neither a strong point source nor dilution in CO stellar features, which are expected tracers of AGN activity. Even if no AGN traces are detected in the near-IR, with the currently available data it is still not possible to establish whether the bolometric luminosity of the object is powered by the AGN or by the starburst: we demonstrate that the two scenarios constitute equally viable alternatives. However, the absence of any signature other than in the hard X-rays implies that, in both scenarios, the AGN is non-standard: if it dominates, it must be obscured in all directions, conversely, if the starburst dominates, the AGN must lack UV photons with respect to X-rays. An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations.

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arXiv2000

The hard X-ray properties of the Seyfert nucleus in NGC 1365

We present BeppoSAX observations of the Seyfert 1.8 galaxy NGC1365 in the 0.1--100 keV range. The source was 6 times brighter than during an ASCA observation 3 years earlier. The 4--10 keV flux is highly variable during the BeppoSAX observation, while the soft (0.1-4 keV) emission is constant within the errors. Both a cold and a warm reflector and a cold absorber are required to explain the observed spectrum. The comparison between ASCA and BeppoSAX spectra strongly suggests that the circumnuclear material has a more complex structure than a simple homogeneous torus, with quite different absorbing gas columns along different lines of sight. A broad iron K_alpha line is also present in the spectrum, with the peak energy significantly redshifted. This can be explained by means of a relativistic disk line model. Alternatively, a warm absorption Fe line system with N_H ~ 10^23 cm^(-2) could account for the observed line profile.

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arXiv2000

Where are the type 2 AGNs?

Most of the energy of the X-ray background is contributed by sources with a hard X-ray spectrum, harder than the spectrum observed in nearby type 1 AGNs. The hard background contributors are conventionally identified with type 2 AGNs, obscured by circumnuclear matter both in the optical and in the soft X rays. However, preliminary results of identification programs seem to find systematically few type 2 AGNs in samples selected with obscuration insensitive criteria. We discuss this issue, and assess the various scenarios which are being proposed to solve the discrepancy.

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arXiv2000

The variability of the Seyfert galaxy NGC 2992: the case for a revived AGN

We report the transition to an active state of the nucleus in the Seyfert 1.9 galaxy NGC 2992, discovered by means of new hard X-ray data. While the 2-10 keV flux declined by a factor of ~ 20 from 1978 to 1994, two recent BeppoSAX observations in 1997 and in 1998 caught the nuclear emission raising back to the same level of activity observed in 1978. In both BeppoSAX observations the X-ray spectrum of the source is well represented by a power law with spectral index Gamma ~ 1.7, absorbed by a column density of N_H ~ 10^{22} cm^{-2} and characterized by a prominent iron Kαline. While in the second BeppoSAX data set the line properties appear to be consistent with those expected from accretion disc models, in the first BeppoSAX data set the iron feature is rather peculiar. The broadening is not significant and the line energy is E_{Kα}=6.62 +/- 0.07 keV, indicating emission from highly ionized iron. The line has too high equivalent width (~ 700 eV) to be produced by a hot scattering medium. By comparing these data with data previously in the literature, we interpret the spectral and flux changes in terms of different phases of rebuilding an accretion disc. The timescale for the disc rebuilding is estimated to range between 1 and 5 years. The X-ray data are complemented with optical and near-infrared followup spectra taken 1.5 months after the discovery of the X-ray burst. The spectra are characterized by prominent broad emission lines. There is also evidence for hot dust emission in the H and K bands that, however, is probably still in the process of increasing.

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arXiv1999

Discovery of a nuclear gas bar feeding the active nucleus in Circinus

We report the discovery of gas inflow motions towards the active nucleus of the Circinus galaxy caused by the non-axisymmetric potential of a nuclear gas bar. Evidence for dust associated with the bar comes from the HST/NICMOS H-K color map, whereas the streaming motions along the gas bar are seen in the velocity field of the H2 S(1)(1-0) emission line. The gas bar is about 100 pc long with a visual extinction in excess of 10 mag. Indication for the gaseous nature of this bar comes from the lack of a stellar counterpart even in the K band where the extinction is greatly reduced. We also use the NICMOS emission line images (Pa-alpha, [SiVI], and [FeII]) to study the innermost region of the ionization cones and the nuclear star forming activity. We discuss the possible relationship of these components with the gaseous bar.

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arXiv1999

BeppoSAX unveils the nuclear component in NGC 6240

The direct nuclear X-ray emission of the ultraluminous IRAS galaxy NGC 6240 has been observed for the first time by BeppoSAX. It is seen through an absorber with N_H=2*10^{24} cm^{-2}. The 2-10 keV X-ray luminosity >10^{44} erg s^{-1} definitely proves that a powerful AGN is hosted in this galaxy, despite the LINER optical appearance. Assuming the typical L_X/L_bol ratio for quasars, our result implies that it is the AGN and not the starburst that dominates the energy output of NGC 6240.

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arXiv1999

Starbursts in active galaxy nuclei: observational constraints from IR stellar absorption lines

High quality infrared spectra of active galaxies including the stellar absorption features of Si at 1.59 um, CO(6,3) at 1.62 um, and CO(2,0) at 2.29 um are used to measure the stellar mass to light ratio at 1.65 um (M_H/L) and investigate the occurrence circum--nuclear starbursts. We find that old and powerful starbursts are relatively common in obscured AGNs (5 objects out of 13) while absent in genuine Seyfert 1's (0 objects out of 8). The data are also used to derive the non-stellar continuum which is very red and compatible with emission from warm (~1000 K) dust even in bare Sy1's, thus indicating that the near IR nuclear continuum is reprocessed radiation. Hot dust (~800 K) emission is also detected in a few obscured AGNs, including the Seyfert 2 prototype NGC1068. The observed non-stellar flux is too high to be accounted for by scattered light and therefore indicates that the material obscuring the AGN must have a quite small (<~1 pc) projected size.

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arXiv1999

Infrared 3D Observations of Nearby Active Galaxies

We present multi-wavelength imaging observations of three nearby and famous active galaxies obtained with NICMOS, ISOCAM and the MPE near-IR integral field spectrometer. The data reveal a variety of features and properties that are missed in optical studies and in traditional IR monodimensional spectroscopy.

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arXiv1999

Soft X-ray spectroscopy of Compton-thick Seyfert 2 galaxies with BeppoSAX

We present a X-ray spectroscopic study of the bright Compton-thick Seyfert 2s NGC1068 and the Circinus Galaxy, performed with BeppoSAX. Matt et al. (1997 and 1998) interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC1068 spectrum is rich in emission lines, mainly due to K-alpha transitions of He-like elements from oxygen to iron, plus a K-alpha fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, of the warm scatterer less than a few 10^21 atoms/cm/cm. In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with ionization parameter U<5 and density about a few times 10^22 atoms/cm/cm. In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC1068 an optically thin plasma emission with kT~500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed.

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arXiv1999

The distribution of absorbing column densities among Seyfert 2 galaxies

We use hard X-ray data for an "optimal" sample of Seyfert 2 galaxies to derive the distribution of the gaseous absorbing column densities among obscured active nuclei in the local Universe. Of all Seyfert 2 galaxies in the sample, 75% are heavily obscured (N_H > 10^23 cm^-2) and about half are Compton thick (N_H > 10^24 cm^-2). Intermediate type 1.8-1.9 Seyferts are characterized by an average N_H much lower than "strict" Seyfert 2s. No correlation is found between N_H and the intrinsic luminosity of the nuclear source. This N_H distribution has important consequences for the synthesis of the cosmic X-ray background. Also, the large fraction of Compton thick objects implies that most of the obscuring gas is located within a radius of a few 10 pc from the nucleus.

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arXiv1999

BeppoSAX observations of Mkn 3: Piercing through the torus of a Seyfert 2 galaxy

A new BeppoSAX broad-band (0.6-150 keV) spectrum of the Seyfert 2 galaxy Mkn 3 is presented. The spectrum provides a direct measurement of a large, neutral column of gas with Nh~10**24 cm**-2 in the source direction. The source, as bright as 3C 273 above 10 keV, has a steep (Gamma~1.8) spectrum without any evidence of a high-energy cutoff up to at least 150 keV. At lower energies, the data are best modeled with the addition of an unabsorbed reflection component. Combining these data with previous Ginga and ASCA observations, the Fe K$_α$ and reflection continuum indicate that the reprocessed emission is responding slower than the intrinsic continuum variations suggesting a size of the reprocessor >= 2 pc. Identifying such a reprocessor with a (close to edge-on) obscuring torus, the overall result fits well into unified models since, presumably, one can interpret the strong absorption as due to transmission through the rim of the torus and the unabsorbed (directly viewed) reflection component as due to reprocessing from the torus inner surface.

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arXiv1999

BeppoSAX uncovers the hidden Seyfert 1 nucleus in the Seyfert 2 galaxy NGC 2110

The Seyfert 2 galaxy NGC 2110 has been observed with BeppoSAX between 0.5 and 150 keV. The high energy instrument onboard, PDS, has succeeded in measuring for the first time the spectrum of this source in the 13-150 keV range. The PDS spectrum, having a photon index Gamma~1.86 is fully compatible with that expected from a Seyfert 1 nucleus. In the framework of unified models, the harder (Gamma~1.67) 2-10 keV spectrum is well explained assuming the presence of a complex partial + total absorber (Nh~30x10^22 cm^-2 x25% + Nh~4x10^22 cm^-2 x100%). The high column density of this complex absorber is consistent both with the FeK_alpha line strength and with the detection of an absorption edge at E~7.1 keV in the power-law spectrum.

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arXiv1998

One more surprise from the Circinus Galaxy: BeppoSAX discovery of a transmission component in hard X-rays

The Circinus Galaxy has been observed, for the first time above 10 keV, by BeppoSAX. An excess emission above the extrapolation of the best fit 0.1--10 keV spectrum is apparent in the PDS data. The most likely explanation is that we are observing the nucleus through a column density of about 4x10**24 cm-2, i.e. large enough to completely block the radiation below 10 keV but small enough to permit partial transmission above this energy.

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arXiv1998

The effect of bars on the obscuration of active nuclei

We present evidence for a strong correlation between the gaseous absorbing column density towards type 2 Seyfert nuclei and the presence of a stellar bar in their host galaxies. Strongly barred Seyfert 2 galaxies have an average Nh that is two orders of magnitude higher than non-barred Sy2s. More than 80% of Compton thick (Nh>10^24 cm^-2) Seyfert 2s are barred. This result indicates that stellar bars are effective in driving gas in the vicinity of active nuclei.

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arXiv1998

A 3 dimensional diagnostic diagram for Seyfert 2s: probing X-ray absorption and Compton thickness

We present and discuss a "3-dimensional" diagnostic diagram for Seyfert2 galaxies obtained by means of X-ray and [OIII] data on a large sample of objects (reported in the Appendix). The diagram shows the Kalpha iron line equivalent width as a function of both the column density derived from the photoelectric cutoff and the 2-10 keV flux normalized to the [OIII] optical line flux (the latter corrected for extinction and assumed to be a true indicator of the source intrinsic luminosity). We find that the hard X-ray properties of type 2 objects depend on a single parameter, the absorbing column density along the line of sight,in accordance with the unified model. The diagram can be used to identify Compton thick sources and to isolate and study peculiar objects. From this analysis we have obtained a column density distribution of Seyfert 2 galaxies which is thought to be a good approximation of the real distribution. A large population of heavily absorbed objects is discovered, including many Compton thick candidates. Our results indicate that the mean Log Nh/cm^(-2)in type 2 Seyferts is 23.5 and that as much as 23-30% of sources have Nh > 10^24 cm^(-2).

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arXiv1998

BeppoSAX Observations of Bright Seyfert 2 Galaxies: Measuring the Intrinsic Continuum Emission

We report broad band (0.1-200 keV) X-ray observations, made by BeppoSAX, of a sample of bright Seyfert 2 galaxies: NGC7172, NGC2110, NGC4507, Mkn 3 and NGC7674. These spectra provide a better understanding of the effects of X-ray reprocessing by cold material in the source and allow to put tighter constraints on the various spectral parameters involved. In particular, the data are used to determine, with less ambiguities than in the past, the shape of the intrinsic continuum emission by means of the high energy data. Within the small sample both Compton thin and Compton thick sources are found according to the expectations of the unified theory.

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