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A. Capetti

Publications and source records attributed to A. Capetti.

89 records · Page 5Linked to original sources
arXiv2002

On the origin of X-shaped radio-sources: new insights from the properties of their host galaxies

A significant fraction of extended radio sources presents a peculiar X-shaped radio morphology: in addition to the classical double lobed structure, radio emission is also observed along a second axis of symmetry in the form of diffuse wings or tails. We re-examine the origin of these extensions relating the radio morphology to the properties of their host galaxies. The orientation of the wings shows a striking connection with the structure of the host galaxy as they are preferentially aligned with its minor axis. Furthermore, wings are only observed in galaxies of high projected ellipticity. Hydrodynamical simulations of the radio-source evolution show that X-shaped radio-sources naturally form in this geometrical situation: as a jet propagates in a non-spherical gas distribution, the cocoon surrounding the radio-jets expands laterally at a high rate producing wings of radio emission, in a way that is reminiscent of the twin-exhaust model for radio-sources.

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arXiv2001

The HST survey of the B2 sample of radio-galaxies: optical nuclei and the FRI/BL Lac unified scheme

We examine the optical properties of the nuclei of low luminosity radio-galaxies using snapshot HST images of the B2 sample. In agreement with the results obtained from the analysis of the brighter 3C/FRI sample, we find a correlation between fluxes (and luminosities) of the optical and radio cores. This provides further support for the interpretation that the optical nuclear emission in FRI is dominated by synchrotron emission and that accretion in these sources takes place in a low efficiency radiative regime. In the framework of the FRI/BL Lacs unified scheme, we find that the luminosity difference between FRI and BL Lac nuclei can be reproduced with a common beaming factor in both the radio and the optical band, independent of the extended radio luminosity, thus supporting such a scenario. The corresponding bulk Lorentz factor is significantly smaller than is expected from observational and theoretical considerations in BL Lacs: this can be interpreted as due to a velocity structure in the jet, with a fast spine surrounded by a slower layer.

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arXiv2000

The black hole mass vs bulge mass relationship in spiral galaxies

We describe an on-going HST program aimed at determining the relationship between the nuclear black hole mass and bulge mass in spiral galaxies. We have selected a volume limited sample of 54 nearby spiral galaxies for which we already have ground based emission line rotation curves, CCD surface photometry and radio maps. We are now obtaining HST/STIS longslit observations of each of the galaxies in the sample in order to determine the nuclear Halpha rotation curve at high (~0.1") spatial resolution. We will use these data to measure the unresolved dark mass concentration at the nucleus of each object. Here we show the first results from observations of objects in the sample.

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arXiv2000

The HST snapshot survey of the B2 sample of low luminosity radio-galaxies: a picture gallery

A Hubble Space Telescope snapshot survey of the B2 sample of low luminosity radio galaxies has, at present, produced V and I images of 41 objects. Together with 16 images of B2 sources taken from the HST archive, there are now high resolution optical data for 57 % of the sample. All host galaxies are luminous ellipticals, except one which is a spiral galaxy, while another one turns out to be a misidentification. We present an album of the images of the B2 radio galaxies observed so far, and give a brief description of the optical morphology of the galaxies. Dust features (in the form of disks, lanes or irregular patches) are seen in most of the galaxies of the sample, 58 %. Compact optical cores are also very common (18/57). A preliminary analysis has revealed the presence of an optical jet in three objects, indicating they can be detected in a sizeable percentage in these low luminosity radio sources. Brightness profiles of dust-free galaxies are well represented by a Nuker law and all shows the existence of a resolved shallow cusp.

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arXiv2000

Spectral energy distributions of FRI nuclei and the FRI/BL Lac unifying model

We consider archival ROSAT and HST observations of five FRI radio galaxies and isolate their nuclear emission from that of the host galaxy. This enable us to determine the Spectral Energy Distributions (SED) of their nuclei spanning from the radio to the X-ray band. They cannot be described as single power-laws but require the presence of an emission peak located between the IR and soft X-ray band. We found consistency between the SED peak position and the values of the broad band spectral indices of radio galaxies with those of BL Lac, once the effects of beaming are properly taken into account. FRI SED are thus qualitatively similar to those of BL Lacs supporting the identification of FRI sources as their mis-oriented counterparts. No dependence of the shape of the SED on the FR~I orientation is found.

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arXiv2000

HST infrared imaging polarimetry of Centaurus A: implications for the unified scheme and the existence of a mis-directed BL Lac nucleus

We report results from HST/NICMOS 2 micron imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region which extends over ~ 70 degrees and thus it does not originate from a highly collimated beam, but is associated with more omni-directional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1 %) nuclear source whose polarization angle PA = 148.2 degrees is perpendicular to the jet axis. We set an upper limit of 0.04'' (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~ 1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low luminosity radio-galaxies observed with HST. We discuss these results in the framework of the FRI / BL Lac unifying model.

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arXiv2000

The morphology of the emission line region of Compact Steep Spectrum radio sources

We present the results of HST narrow band imaging of eleven Compact Steep Spectrum (CSS) radio sources. Five of them (3C 48, 3C 147, 3C303.1, 3C 277.1 and 4C 12.50) were part of a dedicated ``pointed'' program of deep line imaging ([OIII]). For six additional sources (3C 49, 3C 93.1, 3C 138, 3C 268.3, 3C305.1 and 3C343.1) ``snapshot'' images ([OIII] or [OII]) were taken from the HST archive. In all but one of the targets (3C 49) line emission has been detected and only in one case (3C 138) is unresolved. In four out five of the sources with deep observations, the line emission extends well beyond the size of the radio source but along the radio axis. Structures of similar surface brightness would have not been seen in the snapshot images. These emission line structures extend to scales of 10 to 30 kpc and cover a projected angle, when seen from the nucleus, of 30 to 110 degrees, indicating that the nuclear illumination is anisotropic. Photon counting arguments also support this interpretation. In six objects the radio emission extends over more than 1 arcsec. In these cases the line emission has an elongated structure, linking the nucleus to the radio-lobes, possibly tracing the path of the invisible radio jets. Nevertheless the emission line morphologies do not show the bow shocks at the extremities of the radio lobes expected if they are sources whose expansion is frustrated by a dense external medium. Our data favour the alternative model in which CSSs are the young phase of the large size radio sources. When ``pointed'' pure continuum images are available, there appears to be no alignment between radio and continuum emission which contradicts previous suggestions based on broad-band HST imaging.

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arXiv2000

Understanding NLR in Seyfert Galaxies: numerical simulation of jet-cloud interaction

Recent HST observations suggest that the NLR in Seyfert Galaxies can be the result of interaction between jet and external inhomogeneous medium; following this suggestion we perform numerical simulations considering the impact of a radiative jet on a dense cloud. We approach the problem adopting a hydrodynamical code, that consents us to study in detail the jet hydrodynamics, while we choose a more simplified treatment of radiative processes, in order to give a qualitatively good interpretation of the emission processes. Our three main purposes are: i) to reproduce in our simulations the physical conditions observed in the NLR of Seyfert Galaxies, ii) to obtain physical constraints of the jet parameters and iii) to study the jet capability to photoionize the surrounding medium.

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arXiv1999

HST FOC spectroscopy of the NLR of NGC 4151. I. Gas kinematics

We present the results from a detailed kinematic analysis of both ground-based, and Hubble Space Telescope/Faint Object Camera long-slit spectroscopy at sub-arcsec spatial resolution, of the narrow-line region of NGC 4151. In agreement with previous work, the extended emission gas (R > 4") is found to be in normal rotation in the galactic plane, a behaviour that we were able to trace even across the nuclear region, where the gas is strongly disturbed by the interaction with the radio jet, and connects smoothly with the large scale rotation defined by the neutral gas emission. The HST data, at 0.029" spatial resolution, allow us for the first time to truly isolate the kinematic behaviour of the individual clouds in the inner narrow-line region. We find that, underlying the perturbations introduced by the radio ejecta, the general velocity field can still be well represented by planar rotation down to a radius of ~ 0.5" (30 pc), distance at which the rotation curve has its turnover. The most striking result that emerges from our analysis is that the galaxy potential derived fitting the rotation curve changes from a "dark halo" at the ENLR distances to dominated by the central mass concentration in the NLR, with an almost Keplerian fall-off in the 1"< R < 4" interval. The observed velocity of the gas at 0.5" implies a mass of M ~ 10E9 M(sol) within the inner 60 pc. The presence of a turnover in the rotation curve indicates that this central mass concentration is extended. The first measured velocity point (outside the region saturated by the nucleus) would imply an enclosed mass of ~ 5E7 M(sol) within R ~ 0.15" (10 pc) which represents an upper limit to any nuclear point mass.

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arXiv1998

Radiative ages in a representative sample of low luminosity radio galaxies

Two frequency observations, mainly at 1.4 and 5 GHz from the VLA, have been used to study spectral variations along the lobes of some nearby low luminosity radio galaxies that constitute a representative sample selected from the B2 catalogue. The variations of the spectral index have been interpreted as being due to synchrotron and inverse Compton losses and characteristic spectral ages are deduced for the relativistic electrons. The radiative ages are in the range of several 10^7 years. These ages correlate well with the source sizes. They also appear to be consistent with dynamical ages determined from ram-pressure arguments, if we make reasonable assumptions about the ambient gas density and allow for very moderate deviations from the equipartition conditions. There appears to be a significant difference between the radiative ages of sources in our sample and those of more powerful 3CR radio sources. We briefly discuss the possibility of re-acceleration processes and indicate some objects where these may occur.

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arXiv1998

The origin of the Narrow Line Region of Mrk 3: an overpressured jet cocoon

We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the radio-jet the velocity field is highly perturbed and shows two velocity systems separated by as much as 1700 km/s. We interpret this to be the consequence of the rapid expansion of a cocoon of hot gas, shocked and heated by the radio-emitting outflow, which compresses and accelerates the ambient gas. The NLR itself is essentially a cylindrical shell expanding supersonically. From the size and velocity of the expanding region, we derive an upper limit to the radio-source age, ~< 1.5 E5 years, and a lower limit for the jet power, ~> 2 E42 erg/s required to inflate the cocoon and estimate that the jet minimum advance speed is 3 E-3 pc per year. The total kinetic energy of the high velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy carried by the jet over its lifetime and this quantitatively supports the idea that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is representative of the Seyfert population then these sources must also be short lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2 and 3 orders of magnitude smaller than that derived for radio-loud AGNs with similar emission-line luminosity. On the other hand, the fraction of jet power dissipated in radio-emission is similar. We speculate that the main distinction between radio-quiet and radio-loud AGN is ascribed to a difference in jet power rather than to a different efficiency in synchrotron emission production.

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arXiv1998

Jet driven motions in the Narrow Line region of NGC1068

We have obtained HST FOC f/48 long-slit spectroscopy of the inner 4" of the Narrow Line Region of NGC 1068 between 3500-5400A with a spectral resolution of 1.78A/pixel. At a spatial scale of 0.0287" per pixel these data provide an order of magnitude improvement in resolution over previous ground based spectra and allow us to trace the interaction between the radio jet and the gas in the NLR. Our results show that, within +/-0.5" of the radio-jet the emission lines are kinematically disturbed and split into two components whose velocity separation is 1500 km/sec. The filaments associated with the radio lobe also show a redshifted kinematic disturbance of the order of 300 km/sec which probably is a consequence of the expansion of the radio plasma. Furthermore, the material enveloping the radio-jet is in a much higher ionization state than that of the surrounding NLR gas. The highest excitation is coincident with the jet axis where emission in the coronal line of [FeVII] 3769A is detected and the HeII 4686A is strong but where [OII] 3727A is depressed. This large localized increase in ionization on the jet axis is accompanied by the presence of an excess continuum. Because the electron density is substantially larger in the jet compared to the surrounding NLR, these results can only be explained if there is a more intense ionizing continuum associated with the jet. This can be accomplished in a variety of ways which include an intrinsically anisotropic nuclear radiation field, a reduced gas covering factor or the presence of a local ionization source. The morphology, kinematics and, possibly, the ionization structure of the NLR in the vicinity of the jet of NGC 1068 are a direct consequence of the interaction with the radio outflow.

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arXiv1997

The Narrow Line Region of NGC 4151: a Turbulent Cauldron

We present the first results of the Hubble Space Telescope/Faint Object Camera long-slit spectroscopy of the inner 8" of the Narrow Line Region of NGC 4151 at a spatial resolution of 0".029. The emission gas is characterized by an underlying general orderly behaviour, consistent with galactic rotation, over which are superposed kinematically distinct and strongly localized emission structures. High velocity components shifted up to ~ 1500 km/s from the systemic velocity are seen, associated with individual clouds located preferentially along the edges of the radio knots. Off-nuclear blue continuum emission is also observed, associated with the brightest emission line clouds. Emission line ratios like [NeIII]3869/[OII]3727, and [OII]3727/Hbeta are observed to vary substantially between individual clouds. We advance the general picture that, as in other Seyfert galaxies observed with HST (e.g., NGC 1068, Mrk 573), the interaction of the radio jet with the ambient gas strongly influences both the morphology and the physical conditions of the NLR.

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arXiv1997

The supermassive black hole of M87 and the kinematics of its associated gaseous disk

We have obtained long-slit observations of the circumnuclear region of M87 at three different locations, with a spatial sampling of 0.028" using the Faint Object Camera f/48 spectrograph on board HST. These data allow us to determine the rotation curve of the inner ~1" of the ionized gas disk in [OII]3727 to a distance as close as 0.07" (~5pc) to the dynamic center, thereby significantly improving on both the spatial resolution and coverage of previous FOS observations. We have modeled the kinematics of the gas under the assumption of the existence of both a central black hole and an extended central mass distribution, taking into account the effects of the instrumental PSF, the intrinsic luminosity distribution of the line, and the finite size of the slit. We find that the central mass must be concentrated within a sphere whose maximum radius is 0.05" (~3.5pc) and show that both the observed rotation curve and line profiles are consistent with a thin--disk in keplerian motion. We conclude that the most likely explanation for the observed motions is the presence of a supermassive black hole and derive a value of M_{BH} = (3.2+/-0.9) 10^9 M_{sun} for its mass.

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arXiv1997

Is there really a super-massive black hole in M87?

We present the first HST long-slit spectrum of a gaseous disk around a candidate super-massive black-hole. The results of this study on the kinematics of the gaseous disk in M87 are a considerable improvement in both spatial resolution and accuracy over previous observations and requires a projected mass of M_{BH}(sin i)^2 = (2.0+/- 0.5) 10^9 M_{sun} (M_{BH}=3.2 10^9 M_{sun} for a disk inclination i=52deg) concentrated within a sphere whose radius is less than 0.05" (3.5 pc) to explain the observed rotation curve. The kinematics of the ionized gas is well described by a thin disk in keplerian motion. A lower limit to the mass-to-light ratio of this region is M/L_{V}~110, significantly strengthening the claim that this mass is due to the presence of a central black-hole in M87.

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