Alessandro Capetti

The Chandra view of the 3C/FR I sample of low luminosity radio-galaxies

We present results from Chandra observations of the 3C/FR I sample of low luminosity radio-galaxies. We detected a power-law nuclear component in 12 objects out of the 18 with available data. In 4 galaxies we detected nuclear X-ray absorption at a level of N H ∼ (0.2-6) x 10 22 cm -2 . X-ray absorbed sources are associated with the presence of highly inclined dusty disks (or dust filaments projected onto the nuclei) seen in the HST images. This suggests the existence of a flattened X-ray absorber, but of much lower optical depth than in classical obscuring tori. We thus have an unobstructed view toward most FR I nuclei, while absorption plays only a marginal role in the remaining objects. Three pieces of evidence support a jet origin for the X-ray cores: i) the presence of strong correlations between the nuclear luminosities in the radio, optical, and X-ray bands, extending over 4 orders of magnitude and having a much smaller dispersion (∼0.3 dex) when compared to similar trends found for other classes of AGNs, all of which points to a common origin for the emission in the three bands; ii) the close similarity of the broad-band spectral indices with the sub-class of BL Lac objects sharing the same range of extended radio-luminosity, in accord with the FR I/BL Lacs unified model; iii) the presence of a common luminosity evolution of spectral indices in both FR I and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their origin, strengthens the interpretation of low efficiency accretion in low luminosity radio-galaxies.

Streaming Motions toward the Supermassive Black Hole in NGC 1097

We have used GMOS-IFU and high-resolution HST-ACS observations to map, in unprecedented detail, the gas velocity field and structure within the 0.7 kpc circumnuclear ring of the SBb LINER/Seyfert 1 galaxy NGC 1097. We find clear evidence of radial streaming motions associated with spiral structures leading to the unresolved (<3.5 pc) nucleus, which we interpret as part of the fueling chain by which gas is transported to the nuclear starburst and supermassive black hole.

Unveiling the Active Nucleus of Centaurus A

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near–IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by AV ∼ 14, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII]λ1.64� m line and presents a morphology similar to that observed in Paα with a [FeII]/Paα ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Paα observations in a 50 ′′ ×50 ′′ circumnuclear region suggest enhanced star formation (∼ 0.3M⊙ yr −1 ) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ∼ 4 ′′ and suggests a black–hole mass of ∼ 10 9 M⊙. A linear blue structure aligned with the radio/X–ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.

The accretion mechanism in low-power radio galaxies.

We study a sample of 44 low-luminosity radio-loud AGN, which represent a range of nuclear radio-power spanning 5 orders of magnitude, to unveil the accretion mechanism in these galaxies. We estimate the accretion rate of gas associated with their hot coronae by analyzing archival Chandra data, to derive the deprojected density and temperature profiles in a spherical approximation. Measuring the jet power from the nuclear radio-luminosity, we find that the accretion power correlates linearly with the jet power, with an efficiency of conversion from rest mass into jet power of ∼ 0.012. These results strengthen and extend the validity of the results obtained by Allen and collaborators for 9 radio galaxies, indicating that hot gas accretion is the dominant process in FR I radio galaxies across their full range of radio-luminosity. We find that the different levels of nuclear activity are driven by global differences in the structure of the galactic hot coronae. A linear relation links the jet power with the host X-ray surface brightness. This implies that a substantial change in the jet power must be accompanied by a global change in its ISM properties, driven for example by a major merger. This correlation provides a simple widely applicable method to estimate the jet-power of a given object by observing the intensity of its host X-ray emission. To maintain the mass flow in the jet, the fraction of gas that crosses the Bondi radius reaching the accretion disk must be & 0.002. This implies that the radiative efficiency of the disk must be η . 0.005, an indication that accretion in these objects occurs not only at a lower rate, but also at lower efficiency than in standard accretion disks.We study a sample of 44 low-luminosity radio-loud AGN, which represent a range of nuclear radio-power spanning 5 orders of magnitude, to unveil the accretion mechanism in these galaxies. We estimate the accretion rate of gas associated with their hot coronae by analyzing archival Chandra data, to derive the deprojected density and temperature profiles in a spherical approximation. Measuring the jet power from the nuclear radio-luminosity, we find that the accretion power correlates linearly with the jet power, with an efficiency of conversion from rest mass into jet power of ∼0.012. These results strengthen and extend the validity of the results obtained by Allen and collaborators for 9 radio galaxies, indicating that hot gas accretion is the dominant process in FR I radio galaxies across their full range of radio-luminosity. We find that the different levels of nuclear activity are driven by global differences in the structure of the galactic hot coronae. A linear relation links the jet power with the host X-ray surface brightness. This implies that a substantial change in the jet power must be accompanied by a global change in its ISM properties, driven for example by a major merger. This correlation provides a simple widely applicable method to estimate the jet-power of a given object by observing the intensity of its host X-ray emission. To maintain the mass flow in the jet, the fraction of gas that crosses the Bondi radius reaching the accretion disk must be 0.002. This implies that the radiative efficiency of the disk must be η 0.005, an indication that accretion in these objects occurs not only at a lower rate, but also at lower efficiency than in standard accretion disks.

The BL Lac heart of Centaurus A

Emission from the nucleus of the closest radio galaxy, Centaurus A, is observed from the radio to the gamma ray band. We build, for the first time, its overall Spectral Energy Distribution (SED) that appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ~0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac and provide strong support in favour of the unification scheme for low luminosity radio-loud AGNs. Modeling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-pc scale jets.

An optical spectroscopic survey of the 3CR sample of radio galaxies with z < 0.3. I. Presentation of the data ⋆

We present a homogeneous and 92% complete dataset of optical nuclear spectra for the 113 3CR radio sources with redshifts <0.3, obtained with the Telescopio Nazionale Galileo. For these sources we could obtain uniform and uninterrupted coverage of the key spectroscopic optical diagnostics. The observed sample, including powerful classical FR II radio-galaxies and FR I, together spanning four orders of magnitude in radio-luminosity, provides a broad representation of the spectroscopic properties of radio galaxies. In this first paper we present an atlas of the spectra obtained, provide measurements of the diagnostic emission line ratios, and identify active nuclei with broad line emission. These data will be used in follow-up papers to address the connection between the optical spectral characteristics and the multiwavelength properties of the sample.We present a homogeneous and 92% complete dataset of optical nuclear spectra for the 113 3CR radio sources with redshifts < 0.3, obtained with the Telescopio Nazionale Galileo. For these sources we could obtain uniform and uninterrupted coverage of the key spectroscopic optical diagnostics. The observed sample, including powerful classical FR II radio-galaxies and FR I, together spanning four orders of magnitude in radio-luminosity, provides a broad representation of the spectroscopic properties of radio galaxies. In this first paper we present an atlas of the spectra obtained, provide measurements of the diagnostic emission line ratios, and identify active nuclei with broad line emission. These data will be used in follow-up papers to address the connection between the optical spectral characteristics and the multiwavelength properties of the sample.

Understanding the nature of FR II optical nuclei: a new diagnostic plane for radio galaxies ⋆

We extend our study of the nuclei of 3CR FR II radio galaxies through HST optical images up to z= 0:3. In the majority of them an unresolved nucleus (central compact core, CCC) is found. We analyze their position in the plane formed by the radio and optical nuclear luminosities in relation to their optical spectral properties. The broad-lined objects (BLO) have the brightest nuclei: they are present only at optical luminositiesL > 410 42 erg s 1 which we suggest might represent a threshold in the radiative eciency combined with a small range of black hole masses. About 40% of the high and low excitation galaxies (HEG and LEG) show CCC which resemble those previously detected in FR I galaxies, in apparent contrast to the unification model. The equivalent width of the (OIII) emission line (with respect to the nuclear luminosity) reveals the nature of these nuclei, indicating that the nuclei of HEG are obscured to our line of sight and only scattered radiation is observed. This implies that the population of FR II is composed of objects with dierent nuclear properties, and only a fraction of them can be unified with quasars.

The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?*

We study a complete, distance-limited sample of 25 LINERs, 21 of which have been imaged with the Hubble Space Telescope. In nine objects we detect an unresolved nucleus. To study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local active galactic nuclei (AGNs), namely, Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our results show that the LINER population is not homogeneous, as there are two subclasses: (1) the first class is similar to the LLRG class, as it extends the population of radio-loud nuclei to lower luminosities; (2) the second is similar to Seyfert galaxies and extends the properties of radio-quiet nuclei toward the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass versus nuclear radio loudness: all radio-loud LINERs have MBH 108 M☉, while Seyfert galaxies and radio-quiet LINERs have MBH 108 M☉. The different nature of the various classes of local AGNs are best understood when the fraction of the Eddington luminosity they irradiate, Lo/LEdd, is plotted against the nuclear radio-loudness parameter: Seyfert galaxies are associated with relatively high radiative efficiencies Lo/LEdd 10-4 (and high accretion rates onto low-mass black holes); LLRGs are associated with low radiative efficiencies (and low accretion rates onto high-mass black holes); all LINERs have low radiative efficiency (and accretion rates) and can be radio-loud or radio-quiet, depending on their black hole mass.

Is There Really a Black Hole at the Center of NGC 4041? Constraints from Gas Kinematics

We present Space Telescope Imaging Spectrograph spectra of the Sbc spiral galaxy NGC 4041, which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r 04 40 pc), high surface brightness, rotating nuclear disk cospatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak-to-peak amplitude of ~40 km s-1 and is systematically blueshifted by ~10-20 km s-1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1) × 107 M☉ is needed to reproduce the observed rotation curve. However, the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning, we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ~2 or the inclination is allowed to vary. This model results in a 3 σ upper limit of 6 × 106 M☉ on the mass of any nuclear black hole (BH). Overall, our analysis only allows us to set an upper limit of 2 × 107 M☉ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of -17.7 and with a central stellar velocity dispersion of 95 km s-1, then these results are not inconsistent with both the MBH-Lsph and the MBH-σ* correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce; therefore, the present result adds an important new data point to our understanding of BH demography.

The supermassive black hole in Centaurus A: a benchmark for gas kinematical measurements

We present new HST Space Telescope Imaging Spectrograph observations of the nearby radio galaxy NGC 5128 (Centaurus A). The bright emission line with longest wavelength accessible from HST, [S III]λ9533 A, was used to study the kinematics of the ionized gas in the nuclear region with a 0.