arXiv subjects

Barbara Balmaverde

Publications and source records attributed to Barbara Balmaverde.

24 records · Page 2Linked to original sources

arXiv2015

The naked nuclei of LINERs

We analyze HST spectra and Chandra observations of a sample of 21 LINERs, at least 18 of which genuine AGN. We find a correlation between the X-rays and emission lines luminosities, extending over three orders of magnitude and with a dispersion of 0.36 dex; no differences emerge between LINERs with and without broad lines, or between radio-loud and radio-quiet sources. The presence of such a strong correlation is remarkable considering that for half of the sample the X-ray luminosity can not be corrected for local absorption. This connection is readily understood since the X-ray light is associated with the same source producing the ionizing photons at the origin of the line emission. This implies that we have a direct view of the LINERs nuclei in the X-rays: the circumnuclear, high column density structure (the torus) is absent in these sources. Such a conclusion is also supported by mid-infrared data. We suggest that this is due to the general paucity of gas and dust in their nuclear regions that causes also their low rate of accretion and low bolometric luminosity.

astro-ph.GA↗

arXiv2008

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 < 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.

astro-ph↗

arXiv2007

The host galaxy/AGN connection. Brightness profiles of early-type galaxies hosting Seyfert nuclei

[ABRIDGED] We recently presented evidence of a connection between the brightness profiles of nearby early-type galaxies and the properties of the AGN they host. The radio loudness of the AGN appears to be univocally related to the host's brightness profile: radio-loud nuclei are only hosted by ``core'' galaxies while radio-quiet AGN are only found in ``power-law'' galaxies. We extend our analysis here to a sample of 42 nearby (V < 7000 km/s) Seyfert galaxies hosted by early-type galaxies. We used the available HST images to study their brightness profiles. Having excluded complex and highly nucleated galaxies, in the remaining 16 objects the brightness profiles can be successfully modeled with a Nuker law with a steep nuclear cusp characteristic of ``power-law'' galaxies (with logarithmic slope 0.51 - 1.07). This result is what is expected for these radio-quiet AGN based on our previous findings, thus extending the validity of the connection between brightness profile and radio loudness to AGN of a far higher luminosity. We explored the robustness of this result against a different choice of the analytic form for the brightness profiles, using a Sersic law. In no object could we find evidence of a central light deficit with respect to a pure Sersic model, the defining feature of ``core'' galaxies in this modeling framework. We conclude that, regardless of the modeling strategy, the dichotomy of AGN radio loudness can be univocally related to the host's brightness profile. Our general results can be re-phrased as ``radio-loud nuclei are hosted by core galaxies, while radio-quiet AGN are found in non-core galaxies''.

astro-ph↗

arXiv2006

The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?

[ABRIDGED] This is the third in a series of three papers exploring the connection between the multiwavelength properties of AGN in nearby early-type galaxies and the characteristics of their hosts. We selected 116 AGN candidates requiring a radio flux of 1 mJy. We classified the objects with HST images into ``core'' and ``power-law'' galaxies, on the basis of the nuclear slope of their brightness profiles. We used HST and Chandra data to isolate their nuclear emission to study the multiwavelength behaviour of their nuclei. The properties of the nuclei hosted by the 29 core galaxies were presented in Paper II. Core galaxies invariably host a radio-loud nucleus, with a median radio-loudness of Log R = 3.6 and an X-ray based radio loudness parameter of Log R,X = -1.3. Here we discuss the properties of the nuclei of the 22 ``power-law'' galaxies. They show a substantial excess of optical and X-ray emission with respect to core galaxies at the same level of radio luminosity. Conversely, their radio-loudness parameters, Log R ~ 1.6 and Log R,X ~ -3.3, are similar to those measured in Seyfert galaxies. Thus the radio-loudness of AGN hosted by early-type galaxies appears to be univocally related to the host's brightness profile: radio-loud AGN are only hosted by core galaxies, while radio-quiet AGN are found only in power-law galaxies. The brightness profile is determined by the galaxy's evolution, through its merger history; our results suggest that the same process sets the AGN flavour.

astro-ph↗

arXiv2005

The host galaxy/AGN connection in nearby early-type galaxies. Sample selection and hosts brightness profiles

This is the first of a series of three papers exploring the connection between the multiwavelength properties of AGNs in nearby early-type galaxies and the characteristics of their hosts. We selected two samples, both with high resolution 5 GHz VLA observations available and providing measurements down to 1 mJy level, reaching radio-luminosities as low as 10^19 W/Hz. We focus on the 116 radio-detected galaxies as to boost the fraction of AGN with respect to a purely optically selected sample. Here we present the analysis of the optical brightness profiles based on archival HST images, available for 65 objects. We separate early-type galaxies on the basis of the slope of their nuclear brightness profiles, into core and power-law galaxies following the Nuker's scheme, rather than on the traditional morphological classification (i.e. into E and S0 galaxies). Our sample of AGN candidates is indistinguishable, when their brightness profiles are concerned, from galaxies of similar optical luminosity but hosting weaker (or no) radio-sources. We confirm previous findings that relatively bright radio-sources (L_r > 10^(21.5) W/Hz) are uniquely associated to core galaxies. However, below this threshold in radio-luminosity core and power-law galaxies coexist and they do not show any apparent difference in their radio-properties. Not surprisingly, since our sample is deliberately biased to favour the inclusion of active galaxies, we found a higher fraction of optically nucleated galaxies. Addressing the multiwavelength properties of these nuclei will be the aim of the two forthcoming papers.

astro-ph↗

arXiv2005

The infrared-dominated jet of 3C401

We present a Hubble Space Telescope image of the FRII radio galaxy 3C 401, obtained at 1.6 microns with the NICMOS camera in which we identify the infrared counterpart of the brightest region of the radio jet. The jet has a complex radio structure and brightens where bending occurs, most likely as a result of relativistic beaming. We analyze archival data in the radio, optical and X-ray bands and we derive its spectral energy distribution. Differently from all of the previously known optical extragalactic jets, the jet in 3C401 is not detected in the X-rays even in a long 48ksec X-ray Chandra exposure and the infrared emission dominates the overall SED. We propose that the dominant radiation mechanism of this jet is synchrotron. The low X-ray emission is then caused by two different effects: i) the lack of any strong external photon field and ii) the shape of the electron distribution. This affects the location of the synchrotron peak in the SED, resulting in a sharp cut-off at energies lower than the X-rays. Thus 3C401 shows a new type of jet which has intermediate spectral properties between those of FRI, which are dominated by synchrotron emission up to X-ray energies, and FRII/QSO, which show a strong high-energy emission due to inverse-Compton scattering of external photons. This might be a clue for the presence of a continuous ``sequence'' in the properties of large scale jets, in analogy with the ``blazar sequence'' already proposed for sub-pc scale jets.

astro-ph↗