3,994 research outputs found
PKS1932-46: a radio source in an interacting group?
We present the results of a multiwavelength study of the z=0.23 radio source
PKS1932-46. VIMOS IFU spectroscopy is used to study the morphology, kinematics
and ionisation state of the EELR surrounding this source, and also a companion
galaxy at a similar redshift. Near- and far-IR imaging observations obtained
using the NTT and SPITZER are used to analyse the underlying galaxy
morphologies and the nature of the AGN. The host galaxy is identified as an ~M*
elliptical. Combining Spitzer mid-IR with X-ray, optical and near-IR imaging
observations of this source, we conclude that its AGN is underluminous for a
radio source of this type, despite its status as a BLRG. However, given its
relatively large [OIII] luminosity it is likely that the AGN was substantially
more luminous in the recent past (<10^4 years ago). The EELR is remarkably
extensive and complex, reminiscent of the systems observed around sources at
higher redshifts/radio powers, and the gas is predominantly ionised by a
mixture of AGN photoionisation and emission from young stars. We confirm the
presence of a series of star-forming knots extending N-S from the host galaxy,
with more prodigious star formation occuring in the merging companion galaxy to
the northeast, which has sufficient luminosity at mid- to far-IR wavelengths to
be classified as a LIRG. The most plausible explanation of our observations is
that PKS1932-46 is a member of an interacting galaxy group, and that the
impressive EELR is populated by star-forming, tidal debris. We suggest that the
AGN itself may currently be fuelled by material associated either with the
current interaction, or with a previous merger event. Surprisingly, it is the
companion object, rather than the radio source host galaxy, which is undergoing
the bulk of the star formation activity within the group.Comment: 15 pages, 14 figures (compressed for astro-ph, 1 colour). Accepted
for publication in MNRAS. Abstract abridge
The impact of the early stages of radio source evolution on the ISM of the host galaxies
The study of both neutral and ionized gas in young radio sources is providing
key information on the effect the radio plasma has on the ISM of these objects.
We present results obtained for the compact radio sources PKS1549-79, 4C12.50
and PKS1814-63 and for the intermediate-size radio galaxy 3C459. At least in
the first two, low ionisation optical emission lines and HI absorption appear
to be associated with the extended, but relatively quiescent, dusty cocoon
surrounding the nucleus. The [OIII] lines are, on the other hand, mostly
associated with the region of interaction between the radio plasma and the ISM,
indicating a fast outflow from the canter. A case of fast outflow (up to ~1000
km/s) is also observed in HI in the radio source 4C12.50. As the radio source
evolves, any obscuring material along the radio axis is swept aside until,
eventually, cavities (of the same kind as observed e.g. in Cygnus A) are
hollowed out on either side of the nucleus. We may witness this phase in the
evolution of a radio source in the radio galaxy 3C459.Comment: 8 pages (5 figures of which one in gif format). Will appear in
Publications of the Astronomical Society of Australia, as part of the
proceedings of the 3rd GPS/CSS workshop, ed. T. Tzioumis, W. de Vries, I.
Snellen, A. Koekemoe
Starburst radio galaxies: general properties, evolutionary histories and triggering
In this paper we discuss the results of a programme of spectral synthesis
modelling of a sample of starburst radio galaxies in the context of scenarios
for the triggering of the activity and the evolution of the host galaxies. The
starburst radio galaxies -- comprising ~15 - 25% of all powerful extragalactic
radio sources -- frequently show disturbed morphologies at optical wavelengths,
and unusual radio structures, although their stellar masses are typical of
radio galaxies as a class. In terms of the characteristic ages of their young
stellar populations (YSP), the objects can be divided into two groups: those
with YSP ages t_ysp < 0.1 Gyr, in which the radio source has been triggered
quasi-simultaneously with the main starburst episode, and those with older YSP
in which the radio source has been triggered or re-triggered a significant
period after the starburst episode. Combining the information on the YSP with
that on the optical morphologies of the host galaxies, we deduce that the
majority of the starburst radio galaxies have been triggered in galaxy mergers
in which at least one of the galaxies is gas rich. However, the triggering (or
re-triggering) of the radio jets can occur immediately before, around, or a
significant period after the final coalescence of the merging nuclei,
reflecting the complex gas infall histories of the merger events. Overall, our
results provide further evidence that powerful radio jet activity can be
triggered via a variety of mechanisms, including different evolutionary stages
of major galaxy mergers; clearly radio-loud AGN activity is not solely
associated with a particular stage of a unique type of gas accretion event.Comment: 16 pages, 3 Figures, accepted for publication in MNRA
The Declined Activity in the Nucleus of NGC 1316
NGC 1316 (Fornax A) is a radio galaxy with prototypical double lobes, where
the magnetic field intensity is accurately measured via the inverse-Compton
technique. The radio-emitting electrons in the lobes are inferred to have a
synchrotron life time of 0.1 Gyr. Considering the lobe energetics, we estimate
the past nuclear X-ray luminosity of NGC 1316 to be at least 4 times 10^{34} W
(4 times 10^{41} erg s^{-1}). Thus, the nucleus was rather active at least 0.1
Gyr ago. In contrast, we confirmed with ASCA and ROSAT that the nucleus of NGC
1316 is very faint in X-rays at present, with the 2--10 keV luminosity of any
AGN-like hard component being < 2 times 10^{33} W (2 times 10^{40} erg s^{-1})
even assuming a nuclear obscuration up to 10^{28} m^{-2} (10^{24} cm^{-2}).
This is at least an order of magnitude lower than the estimated past activity,
indicating that the nucleus is presently very inactive. From these two results,
we conclude that the nucleus of NGC 1316 has become dormant during the last 0.1
Gyr. This suggests the possible abundance of ``dormant'' quasars in nearby
galaxies.Comment: 7 pages, 2 figures, to be published in the Astrophysical Journal
Letter
A Very Radio-Loud Narrow-Line Seyfert 1: PKS 2004-447
We have discovered a very radio-loud Narrow-Line Seyfert 1 candidate: PKS
2004-447. This Seyfert is consistent with the formal definition for NLS1s,
although it does not have quite the same spectral features as some typical
members of this subclass. Only ROSAT survey data is available at X-ray
wavelengths, so it has not been possible to compare this source with other
NLS1s at these wavelengths. A full comparison of this source with other members
of the subclass will improve our physical understanding of NLS1s. In addition,
using standard calculations, we estimate the central black hole to have a mass
of . This does not agree with predictions in the
literature, that radio-loud AGN host very massive black holes.Comment: 15 pages, 4 figures, accepted for publication in ApJ, minor typos
change
PKS2250-41: a case study for triggering
We present the results of a multiwavelength study of the z = 0.31 radio
source PKS2250-41. Integral field unit and long-slit spectroscopy obtained
using VIMOS and FORS1 on the VLT, and archival HST optical imaging observations
are used to study the morphology, kinematics and ionisation state of the
extended emission line region (EELR) surrounding this source, and also a
companion galaxy at a similar redshift. Near-infrared imaging observations
obtained using the NTT are used to analyse the underlying galaxy morphologies.
The EELR displays a complex variety of different gas kinematics and ionization
states, consistent with a mixture of radio source shocks and AGN
photoionization. The radio galaxy is likely to lie within a group environment,
and is plausibly undergoing interactions with one or more other objects. The
disk-like galaxy to the northeast of the radio source lies at a similar
redshift to the radio galaxy itself, and has its major axis position angle
aligned with the filamentary continuum and line emission extending outwards
from the radio galaxy. This filamentary structure is most plausibly interpreted
as a tidal structure associated with an interaction involving the radio source
host galaxy and the aligned companion galaxy to the north-east; this encounter
may have potentially triggered the current epoch of radio source activity.
Overall, PKS2250-41 displays some of the best evidence that radio source
activity can be triggered in this manner. [abridged]Comment: 16 pages, 13 figures (some colour). Accepted for publication in
MNRAS. Abstract abridge
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift
Remnant radio galaxies represent the final dying phase of radio galaxy
evolution, in which the jets are no longer active. Due to their rarity in flux
limited samples and the difficulty of identification, this dying phase remains
poorly understood and the luminosity evolution largely unconstrained. Here we
present the discovery and detailed analysis of a large (700 kpc) remnant radio
galaxy with a low surface brightness that has been identified in LOFAR images
at 150 MHz. By combining LOFAR data with new follow-up Westerbork observations
and archival data at higher frequencies, we investigated the source morphology
and spectral properties from 116 to 4850 MHz. By modelling the radio spectrum
we probed characteristic timescales of the radio activity. The source has a
relatively smooth, diffuse, amorphous appearance together with a very weak
central compact core which is associated with the host galaxy located at
z=0.051. From our ageing and morphological analysis it is clear that the
nuclear engine is currently switched off or, at most, active at a very low
power state. The host galaxy is currently interacting with another galaxy
located at a projected separation of 15 kpc and a radial velocity offset of 300
km/s. This interaction may have played a role in the triggering and/or shut
down of the radio jets. The spectral shape of this remnant radio galaxy differs
from the majority of the previously identified remnant sources, which show
steep or curved spectra at low to intermediate frequencies. In light of this
finding and in preparation for new-generation deep low-frequency surveys, we
discuss the selection criteria to be used to select representative samples of
these sources.Comment: 10 pages, 6 figures, 5 tables, A&A accepte
- …