Testing the FR I/BL Lac unifying model with HST observations

Hubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predictions. As a pilot project, we selected five radio galaxies which show extended nuclear discs in the HST images. The relative orientation of the projected radio-jets and of the extended nuclear discs indicates that they are not perpendicular, as the simplest geometrical model would suggest, but that they form an angle of ~ 20 - 40 degrees with the symmetry axis of the disc: a significant change of orientation occurs between the innermost AGN structure and the kpc-scale. Nevertheless, the discs appear to be useful indicators of the radio sources orientation since the angles formed by the disc axis and the jet with the line of sight differ by only ~ 10 - 20 degrees. At the center of each disc an unresolved nuclear source is present. We compared its luminosity with the optical core luminosity of BL Lacs selected for having similar host galaxy magnitude and extended radio luminosity. The BL Lac cores are between 2 E2 and 3 E5 times brighter than the corresponding radio galaxies ones. The FR I/BL Lac core luminosity ratio shows a suggestive correlation with the orientation of the radio galaxies with respect to the line of sight. The behavior of this ratio is quantitatively consistent with a scenario in which the emission in the FR I and BL Lac is dominated by the beamed radiation from a relativistic jet with Doppler factor ~ 5 - 10, thus supporting the basic features of the proposed unification schemes.Comment: 11 pages, 10 figures, submitted to MNRAS, revised versio

Recent star formation in nearby 3CR radio-galaxies from UV HST observations

We analyzed HST images of 31 nearby (z <~ 0.1) 3CR radio-galaxies. We compared their UV and optical images to detect evidence of recent star formation. Six objects were excluded because they are highly nucleated or had very low UV count rates. After subtracting the emission from their nuclei and/or jets, 12 of the remaining 25 objects, presenting an UV/optical colors NUV - r < 5.4, are potential star-forming candidates. Considering the contamination from other AGN-related processes (UV emission lines, nebular continuum, and scattered nuclear light), there are 6 remaining star-forming "blue" galaxies. We then divide the radio galaxies, on the basis of the radio morphology, radio power, and diagnostic optical line ratios, into low and high excitation galaxies, LEG and HEG. While there is no correlation between the FR type (or radio power) and color, the FR type is clearly related to the spectroscopic type. In fact, all HEG (with one possible exception) show morphological evidence of recent star formation in UV compact knots, extended over 5-20 kpc. Conversely, there is only 1 "blue" LEG out of 19, including in this class also FR I galaxies. The picture that emerges, considering color, UV, optical, and dust morphology, is that only in HEG recent star formation is associated with these relatively powerful AGN, which are most likely triggered by a recent, major, wet merger. Conversely, in LEG galaxies the fraction of actively star-forming objects is not enhanced with respect to quiescent galaxies. The AGN activity in these sources can be probably self-sustained by their hot interstellar medium.Comment: Accepted for publication in A&

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.Comment: 5 pages, 2 figures, accepted for publication in MNRAS, pink page

Origin of X-shaped radio-sources: further insights from the properties of their host galaxies

We analyze the properties of a sample of X-shaped radio-sources (XRSs). These objects show, in addition to the main lobes, a pair of wings producing their peculiar radio morphology. We obtain our sample by selecting from the initial list of Cheung (2007, AJ, 133, 2097) the 53 galaxies with the better defined wings and with available SDSS images. We identified the host galaxies and measured their optical position angle, obtaining a positive result in 22 cases. The orientation of the secondary radio structures shows a strong connection with the optical axis, with all (but one) wing forming a angle larger than 40 degrees with the host major axis. The probability that this is compatible with a uniform distribution is P = 0.9 10E-4. Spectra are available from the SDSS for 28 XRSs. We modeled them to extract information on their emission lines and stellar population properties. The sample is formed by approximately the same number of high and low excitation galaxies (HEG and LEG); this classification is essential for a proper comparison with non-winged radio-galaxies. XRSs follow the same relations between radio and line luminosity defined by radio-galaxies in the 3C sample. While in HEGs a young stellar population is often present, this is not detected in the 13 LEGs, again in agreement with the properties of non XRSs. The lack of young stars in LEGs support the idea that they did not experiences a recent gas rich merger. The connection between the optical axis and the wings orientation, as well as the stellar population and emission lines properties, provide further support for an hydro-dynamic origin of the radio-wings (for example associated with the expansion of the radio cocoon in an asymmetric external medium) rather than with a change of orientation of the jet axis.Comment: Accepted for publication in A&

Revisiting the census of low-luminosity AGN

The aim of this paper is to revisit critically the current census of AGN as derived from optical spectroscopy. We considered the spectra of nearby (z<0.1) galaxies from the Sloan Digital Sky Survey (SDSS). The equivalent width (EW) distribution of the [O III]5007 emission line is strongly clustered around ~0.6 A, extending the validity of the results we obtained for red giant ellipticals. The close connection between emission lines and stellar continuum points to stellar processes as the most likely source of the bulk of the ionizing photons in these galaxies although their emission line ratios are similar to those of active nuclei. Genuine AGN might be sought mainly among the minority (~5-10%) of outliers, i.e., galaxies with EW>~2 A. The galaxies located in the AGN region of the spectroscopic diagnostic diagrams outnumber outliers by a factor 5-10 which casts doubts on the accuracy of the current identification of active galaxies, particularly those of LINERs of low line luminosity, <~ 10^39-10^40 erg/s. This conclusion can be tested by using spectra that covers smaller physical regions such as those that are already available in the literature of the ~500 nearest bright galaxies, with a stellar continuum reduced by a factor of 20-100 with respect to SDSS galaxies. If the emission lines were mainly of AGN origin, their contrast against the continuum should be enhanced. Instead, their EW distribution is similar to that of the SDSS sample, with just an increase of the outlier fraction. We conclude that the number low-luminosity AGN is currently largely overestimated with a sample purity as low as ~10%. As a consequence the properties of low-luminosity AGN should be fundamentally revised.Comment: Accepted for publication in A&

High-resolution VLA observations of FR0 radio galaxies: properties and nature of compact radio sources

We present the results of Karl G. Jansky Very Large Array (VLA) observations to study the properties of FR0 radio galaxies, the compact radio sources associated with early-type galaxies which represent the bulk of the local radio-loud AGN population. We obtained A-array observations at 1.5, 4.5, and 7.5 GHz for 18 FR0s from the FR0CAT sample: these are sources at $z<0.05$, unresolved in the FIRST images and spectroscopically classified as low excitation galaxies (LEG). Although we reach an angular resolution of $\sim$0.3 arcsec, the majority of the 18 FR0s is still unresolved. Only four objects show extended emission. Six have steep radio spectra, 11 are flat cores, while one shows an inverted spectrum. We find that 1) the ratio between core and total emission in FR0s is $\sim$30 times higher than in FRI and 2) FR0s share the same properties with FRIs from the nuclear and host point of view. FR0s differ from FRIs only for the paucity of extended radio emission. Different scenarios were investigated: 1) the possibility that all FR0s are young sources eventually evolving into extended sources is ruled out by the distribution of radio sizes; 2) similarly, a time-dependent scenario, where a variation of accretion or jet launching prevents the formation of large-scales radio structures, appears to be rather implausible due to the large abundance of sub-kpc objects 3) a scenario in which FR0s are produced by mildly relativistic jets is consistent with the data but requires observations of a larger sample to be properly tested.Comment: accepted for publication on MNRAS (12 pages, 8 figures

Discovery of a FR0 radio galaxy emitting at γ\gamma-ray energies

We present supporting evidence for the first association of a Fermi source, 3FGLJ1330.0-3818, with the FR0 radio galaxy Tol1326-379. FR0s represent the majority of the local radio loud AGN population but their nature is still unclear. They share the same properties of FRIs from the point of view of the nuclear and host properties, but they show a large deficit of extended radio emission. Here we show that FR0s can emit photons at very high energies. Tol1326-379 has a GeV luminosity of $L_{>1~{\rm GeV}} \sim 2\times10^{42}$ erg s$^{-1}$, typical of FRIs, but with a steeper $\gamma$-ray spectrum ($\Gamma=2.78\pm 0.14$). This could be related to the intrinsic jet properties but also to a different viewing angle.Comment: 7 pages, 5 figures , accepted for publication on MNRA