63 research outputs found
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&
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
X-ray study of a sample of FR0 radio galaxies: unveiling the nature of the central engine
FR0s are compact radio sources that represent the bulk of the Radio-Loud (RL)
AGN population, but they are still poorly understood. Pilot studies on these
sources have been already performed at radio and optical wavelengths: here we
present the first X-ray study of a sample of 19 FR0 radio galaxies selected
from the SDSS/NVSS/FIRST sample of Best & Heckman (2012), with redshift
0.15, radio size 10 kpc and optically classified as low-excitation
galaxies (LEG). The X-ray spectra are modeled with a power-law component
absorbed by Galactic column density with, in some cases, a contribution from
thermal extended gas. The X-ray photons are likely produced by the jet as
attested by the observed correlation between X-ray (2-10 keV) and radio (5 GHz)
luminosities, similar to FRIs. The estimated Eddington-scaled luminosities
indicate a low accretion rate. Overall, we find that the X-ray properties of
FR0s are indistinguishable from those of FRIs, thus adding another similarity
between AGN associated with compact and extended radio sources. A comparison
between FR0s and low luminosity BL Lacs, rules out important beaming effects in
the X-ray emission of the compact radio galaxies. FR0s have different X-ray
properties with respect to young radio sources (e.g. GPS/CSS sources),
generally characterized by higher X-ray luminosities and more complex spectra.
In conclusion, the paucity of extended radio emission in FR0s is probably
related to the intrinsic properties of their jets that prevent the formation of
extended structures, and/or to intermittent activity of their engines.Comment: Accepted for publication in MNRAS (18 pages, 4 figures
The large-scale environment of FR 0 radio galaxies
We explore the properties of the large-scale environment of FR0 radio
galaxies belonging to the FR0CAT sample which includes 104 compact radio
sources associated with nearby (z<0.05) early-type galaxies. By using various
estimators we find that FR0s live in regions of higher than the average
galaxies density and a factor two lower density, on average, with respect to
FRI radio galaxies. This latter difference is driven by the large fraction
(63%) of FR0s located in groups formed by less than 15 galaxies, an environment
which FRIs rarely (17%) inhabit. Beside the lack of substantial extended radio
emission defining the FR0s class, this is the first significant difference
between the properties of these two populations of low power radio galaxies. We
interpret the differences in environment between FR0s and FRIs as the due to an
evolutionary link between local galaxies density, BH spin, jet power, and
extended radio emission.Comment: 11 pages, 6 figures, 3 tables (Pre-proofs version - Published in A&A
HST emission-line images of nearby 3CR radio galaxies: two photoionization, accretion and feedback modes
We present HST/ACS narrow-band images of a low-z sample of 19 3C radio
galaxies to study the H and [OIII] emissions from the narrow-line
region (NLR). Based on nuclear emission line ratios, we divide the sample into
High and Low Excitation Galaxies (HEGs and LEGs). We observe different line
morphologies, extended line emission on kpc scale, large [OIII]/H
scatter across the galaxies, and a radio-line alignment. In general, HEGs show
more prominent emission line properties than LEGs: larger, more disturbed, more
luminous, and more massive regions of ionized gas with slightly larger covering
factors. We find evidence of correlations between line luminosities and (radio
and X-ray) nuclear luminosities. All these results point to a main common
origin, the active nucleus, which ionize the surrounding gas. However, the
contribution of additional photoionization mechanism (jet shocks and star
formation) are needed to account for the different line properties of the two
classes. A relationship between the accretion, photoionization and feedback
modes emerges from this study. For LEGs (hot-gas accretors), the synchrotron
emission from the jet represents the main source of ionizing photons. The lack
of cold gas and star formation in their hosts accounts for the moderate
ionized-gas masses and sizes. For HEGs (cold-gas accretors), an ionizing
continuum from a standard disk and shocks from the powerful jets are the main
sources of photoionization, with the contribution from star formation. These
components, combined with the large reservoir of cold/dust gas brought from a
recent merger, account for the properties of their extended emission-line
regions.Comment: accepted for publication on ApJ (22 pages, 12 figures
Molecular gas and nuclear activity in early-type galaxies: any link with radio-loudness?
Aims. We want to study the amount of molecular gas in a sample of nearby
early-type galaxies (ETGs) which host low-luminosity Active Galactic Nuclei
(AGN). We look for possible differences between the radio-loud (RL) and
radio-quiet (RQ) AGN. Methods. We observed the CO(1-0) and CO(2-1) spectral
lines with the IRAM 30m and NRO 45m telescopes for eight galaxies. They belong
to a large sample of 37 local ETGs which host both RQ and RL AGN. We gather
data from the literature for the entire sample. Results. We report the new
detection of CO(1-0) emission in four galaxies (UGC0968, UGC5617, UGC6946, and
UGC8355) and CO(2-1) emission in two of them (UGC0968 and UGC5617). The
CO(2-1)/CO(1-0) ratio in these sources is . Considering both the
new observations and the literature, the detection rate of CO in our sample is
55 9%, with no statistically significant difference between the hosts of
RL and RQ AGNs. For all the detected galaxies we converted the CO luminosities
into the molecular masses, , that range from 10 to 10
M, without any statistically significant differences between RL and
RQ galaxies. This suggests that the amount of molecular gas does not likely set
the radio-loudness of the AGN. Furthermore, despite the low statistical
significance, the presence of a weak trend between the H mass with
various tracers of nuclear activity (mainly [O III] emission line nuclear
power) cannot be excluded.Comment: Accepted for publication on A&A, 9 pages, 5 figure
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