135 research outputs found
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 ,
unresolved in the FIRST images and spectroscopically classified as low
excitation galaxies (LEG). Although we reach an angular resolution of 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 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 -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 erg
s, typical of FRIs, but with a steeper -ray spectrum
(). 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
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