891 research outputs found
Multi-band constraints on the nature of emission line galaxies
Our aim is to explore the nature of emission line galaxies by combining
high-resolution observations obtained in different bands to understand which
objects are powered by an Active Galactic Nucleus(AGN). From the spectroscopic
Palomar survey of nearby bright galaxies, we selected a sample of 18 objects
observed with HST, Chandra, and VLA.
No connection is found between X-ray and emission line luminosities from
ground-based data, unlike what is found for brighter AGN. Conversely, a strong
correlation emerges when using the HST spectroscopic data, which are extracted
on a much smaller aperture. This suggests that the HST data better isolate the
AGN component when one is present, while ground-based line measurements are
affected by diffuse emission from the host galaxies.
The sample separates into two populations. The 11 objects belonging to the
first class have an equivalent width of the [OIII] emission line measured from
HST data EW([OIII])>~2 A and are associated with an X-ray nuclear source; in
the second group we find seven galaxies with EW([OIII])<~1 A that generally do
not show any emission related to an active nucleus (emission lines, X-ray, or
radio sources). This latter group includes about half of the Low Ionization
Nuclear Emission-line region (LINERs) or transition galaxies of the sample, all
of which are objects of low [OIII] line luminosity (<~1E38 erg s-1) and low
equivalent width (<~1 A) in ground-based observations. These results strengthen
the suggestion that the EW([OIII]) value is a robust predictor of the nature of
an emission line galaxy.Comment: 6 pages. Accepted for publication in A&
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&
On the nature of optical nuclei in FR I radio-galaxies from ACS/HST imaging polarimetry
We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of
the 9 closest radio-galaxies in the 3C catalogue with an FR I morphology. The
nuclear sources seen in direct HST images in these galaxies are found to be
highly polarized with levels in the range ~2-11 % with a median value of 7 %.
We discuss the different mechanisms that produce polarized emission and
conclude that the only viable interpretation is a synchrotron origin for the
optical nuclei. This idea is strengthened by the analogy with the polarization
properties of BL Lac objects, providing also further support to the FRI/BL Lac
unified model. This confirms previous suggestions that the dominant emission
mechanism in low luminosity radio-loud AGN is related to non-thermal radiation
produced by the base of their jets. In addition to the nuclear polarization
(and to the large scale optical jets), polarization is detected co-spatially
with the dusty circumnuclear disks, likely due to dichroic transmission; the
polarization vectors are tangential to the disks as expected when the magnetic
field responsible for the grains alignment is stretched by differential
rotation. We explored the possibility to detect the polarimetric signature of a
misaligned radiation beam in FR I, expected in our sources in the frame of the
FR I/ BL Lac unification. We did not find this effect in any of the galaxies,
but our the results are not conclusive on whether a misaligned beam is indeed
present in FR I.Comment: 16 page, 10 figures, accepted for publication in A&
The Chandra view of the 3C/FRI sample of low luminosity radio-galaxies
We present results from Chandra observations of the 3C/FRI 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 about N_H= (0.2-6)e22 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 un-obstructed view toward
most FR~I nuclei while absorption plays only a marginal role in the remaining
objects. Three pieces of evidence support an interpretation for 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 with a much smaller dispersion (about 0.3 dex) when compared to
similar trends found for other classes of AGNs, pointing 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 FRI/BL Lacs unified model; iii)
the presence of a common luminosity evolution of spectral indices in both FRI
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.Comment: Accepted 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&
FR0CAT: a FIRST catalog of FR0 radio galaxies
With the aim of exploring the properties of the class of FR0 radio galaxies,
we selected a sample of 108 compact radio sources, called FR0CAT, by combining
observations from the NVSS, FIRST, and SDSS surveys. The catalog includes
sources with z, with a radio size 5 kpc, and with an
optical spectrum characteristic of low-excitation galaxies. Their 1.4-GHz radio
luminosities range erg/s. The
FR0CAT hosts are mostly (86%) luminous () red
early-type galaxies with black hole masses : similar to the hosts of FRI radio galaxies, but they are on
average a factor 1.6 less massive. The number density of FR0CAT sources
is 5 times higher than that of FRIs, and thus they represent the dominant
population of radio sources in the local Universe. Different scenarios are
considered to account for the smaller sizes and larger abundance of FR0s with
respect to FRIs. An age-size scenario that considers FR0s as young radio
galaxies that will all eventually evolve into extended radio sources cannot be
reconciled with the large space density of FR0s. However, the radio activity
recurrence, with the duration of the active phase covering a wide range of
values and with short active periods strongly favored with respect to longer
ones, might account for their large density number. Alternatively, the jet
properties of FR0s might be intrinsically different from those of the FRIs, the
former class having lower bulk Lorentz factors, possibly due to lower black
hole spins. Our study indicates that FR0s and FRI/IIs can be interpreted as two
extremes of a continuous population of radio sources that is characterized by a
broad distribution of sizes and luminosities of their extended radio emission,
but shares a single class of host galaxies.Comment: 11 pages, 6 figures, accepted for publication on A&
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