28 research outputs found
Discovery of X-shaped Morphology of the Giant Radio Galaxy GRG 0503-286
The high surface-brightness sensitivity of the GLEAM survey image of the
giant radio galaxy GRG 0503-28 at 70-230 MHz has revealed an
inversion-symmetric bending of its two lobes, while maintaining between their
bent portions a ~200 kpc wide strip-like radio emission gap. This lends the
source the appearance of a mega-sized X-shaped radio galaxy. Identifying the
emission gap with the presence of a gaseous layer, probably a WHIM-filled sheet
in the cosmic web, we suggest that the layer is the most likely cause of the
inversion-symmetric bending of the two radio lobes. Multiple observational
manifestations of such gaseous layers are noted. The two lobes of this GRG,
known to extend very asymmetrically from the host galaxy, are remarkably
symmetric about the emission gap, confirming a curious trend noted earlier for
double radio sources of normal dimensions. The anomalous radio spectral
gradient reported for the northern lobe of this GRG is not substantiated.Comment: Accepted for publication in A&A Letters | 5 pages and 3 figures |
Comments are welcome | Dedicated to Prof Govind Swarup, FR
Decoding the giant extragalactic radio sources
Giant radio sources (GRSs) defined to be > 0.7 Mpc are the largest single
objects in the Universe and can be associated with both galaxies (GRGs) and
quasars (GRQs). They are important for understanding the evolution of radio
galaxies and quasars whose sizes range from pc to Mpc scales and are also
valuable probes of their environment. These radio-loud active galactic nuclei
(RLAGN) interact with the interstellar medium of the host galaxy on small
scales and the large-scale intracluster or intergalactic medium for the GRSs.
With several new and sensitive surveys over the last few years, the number of
known GRSs has increased many fold which has led a resurgence of interest in
the field. This review article summarises our current understanding of these
sources based on nearly five decades of research, and discusses the importance
of the Square Kilometer Array (SKA) in addressing some of the outstanding
questions.Comment: Accepted for publication in the Journal of Astrophysics and Astronomy
for Special Issue on Indian Participation in the SKA. Guest Editors: Abhirup
Datta, Nirupam Roy, Preeti Kharb and Tirthankar Roy Choudhury). 31 pages, 8
figures, and 4 tables. Comments are welcom
GMRT observations of extragalactic radio sources with steeply inverted spectra
We report quasi-simultaneous GMRT observations of seven extragalactic radio
sources at 150, 325, 610 and 1400 MHz, in an attempt to accurately define their
radio continuum spectra, particularly at frequencies below the observed
spectral turnover. We had previously identified these sources as candidates for
a sharply inverted integrated radio spectrum whose slope is close to, or even
exceeds = +2.5, the theoretical limit due to synchrotron
self-absorption (SSA) in a source of incoherent synchrotron radiation arising
from relativistic particles with the canonical (i.e., power-law) energy
distribution. We find that four out of the seven candidates have an inverted
radio spectrum with a slope close to or exceeding +2.0, while the critical
spectral slope is exceeded in at least one case. These sources,
together with another one or two reported in very recent literature, may well
be the archetypes of an extremely rare class, from the standpoint of violation
of the SSA limit in compact extragalactic radio sources. However, the
alternative possibility that free-free absorption is responsible for their
ultra-sharp spectral turnover cannot yet be discounted.Comment: Accepted for publication in MNRAS. Comments are welcom
GMRT observations of a first sample of Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) candidates in the Northern sky
We present an extension of our search for Extremely Inverted Spectrum
Extragalactic Radio Sources (EISERS) to the northern celestial hemisphere. With
an inverted radio spectrum of slope > +2.5, these rare sources would
either require a non-standard particle acceleration mechanism (in the framework
of synchrotron self-absorption hypothesis), or a severe free-free absorption
which attenuates practically all of their synchrotron radiation at metre
wavelengths. By applying a sequence of selection filters, a list of 15 EISERS
candidates is extracted out by comparing two large-sky radio surveys, WENSS
(325 MHz) and TGSS-ADR1 (150 MHz), which overlap across 1.03 steradian of
the sky. Here we report quasi-simultaneous GMRT observations of these 15 EISERS
candidates at 150 MHz and 325 MHz, in an attempt to accurately define their
spectra below the turnover frequency. Out of the 15 candidates observed, two
are confirmed as EISERS, since the slope of the inverted spectrum between these
two frequencies is found to be significantly larger than the critical value
= +2.5: the theoretical limit for the standard case of synchrotron
self-absorption (SSA). For another 3 sources, the spectral slope is close to,
or just above the critical value . Nine of the sources have GPS type
radio spectra. The parsec-scale radio structural information available for the
sample is also summarised.Comment: Accepted for publication in MNRAS. Comments are welcom
SAGAN -- III: New insights into giant radio quasars
Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs),
propelling megaparsec-scale jets. In order to understand GRQs and their
properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset
of small (size <700 kpc) radio quasars (SRQs) from the literature. In this
process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z <
1.72, which we include in the GRQ catalogue. Using the above samples, we have
carried out a systematic comparative study of GRQs and SRQs, using optical and
radio data. Our results show that the GRQs and SRQs statistically have similar
spectral index and black hole mass distributions. However, SRQs have higher
radio core power, core dominance factor, total radio power, jet kinetic power
and Eddington ratio compared to GRQs. On the other hand, when compared to giant
radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio.
The high core dominance factor of SRQs is an indicator of them lying closer to
the line of sight than GRQs. We also find a correlation of the accretion disc
luminosity with the radio core and jet power of GRQs, which provides evidence
for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of
GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies
(SRGs) and SRQs, which indicate that size is not strongly dependent on the
accretion state. Using all of these, we provide a basic model for the growth of
SRQs to GRQs.Comment: Accepted for publication in Astronomy & Astrophysics. 9 figures, and
2 tables. This is the third paper in the series. Comments are welcome. "SAGAN
Project website this http https://sites.google.com/site/anantasakyatta/sagan
Search and analysis of giant radio galaxies with associated nuclei (SAGAN) -- I : New sample and multi-wavelength studies
We present the first results of a project called SAGAN, which is dedicated
solely to the studies of relatively rare megaparsec-scale radio galaxies in the
Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs
primarily from the NVSS with sizes ranging from ~0.71 Mpc to 2.82 Mpc in the
redshift range of ~0.03 - 0.95, of which 23 are hosted by quasars (giant radio
quasars, GRQs). As part of the project SAGAN, we have created a database of all
known GRGs, the GRG catalogue, from the literature (including our new sample);
it includes 820 sources. For the first time, we present the multi-wavelength
properties of the largest sample of GRGs. Our results establish that the
distributions of the radio spectral index and the black hole mass of GRGs do
not differ from the corresponding distributions of normal-sized radio galaxies
(RGs). However, GRGs have a lower Eddington ratio (ER) than RGs. Using the
mid-infrared data, we classified GRGs in terms of their accretion mode: either
a high-power radiatively efficient high-excitation state, or a radiatively
inefficient low-excitation state. We find that GRGs in high-excitation state
statistically have larger sizes, stronger radio power, jet kinetic power, and
higher ER than those in low-excitation state. Our analysis reveals a strong
correlation between the ER and the scaled jet kinetic power, which suggests a
disc-jet coupling. Our environmental study reveals that ~10% of all GRGs may
reside at the centres of galaxy clusters, in a denser galactic environment,
while the majority appears to reside in a sparse environment. We find that the
probability of BCG as a GRG is quite low. We present new results for GRGs that
range from black hole mass to large-scale environment properties. We discuss
their formation and growth scenarios, highlighting the key physical factors
that cause them to reach their gigantic size. Abridged.Comment: Accepted for publication in Astronomy & Astrophysics. 14 figures, 7
tables and 7 montages. Comments are welcome. "SAGAN Project website
https://sites.google.com/site/anantasakyatta/sagan
`Zwicky's Nonet': a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets
We report the results of our radio, optical and infra-red studies of a
peculiar radio source 4C~35.06, an extended radio-loud AGN at the center of
galaxy cluster Abell 407 (). The central region of this cluster hosts
a remarkably tight ensemble of nine galaxies, the spectra of which resemble
those of passive red ellipticals, embedded within a diffuse stellar halo of
1~arcmin size. This system (named the `Zwicky's Nonet') provides unique
and compelling evidence for a multiple-nucleus cD galaxy precursor.
Multifrequency radio observations of 4C~35.06 with the Giant Meterwave Radio
Telescope (GMRT) at 610, 235 and 150 MHz reveal a system of 400~kpc scale
helically twisted and kinked radio jets and outer diffuse lobes. The outer
extremities of jets contain extremely steep spectrum (spectral index -1.7 to
-2.5) relic/fossil radio plasma with a spectral age of a few yr. Such ultra-steep spectrum relic radio lobes without definitive
hot-spots are rare, and they provide an opportunity to understand the
life-cycle of relativistic jets and physics of black hole mergers in dense
environments. We interpret our observations of this radio source in the context
of the growth of its central black hole, triggering of its AGN activity and jet
precession, all possibly caused by galaxy mergers in this dense galactic
system. A slow conical precession of the jet axis due to gravitational
perturbation between interacting black holes is invoked to explain the unusual
jet morphology.Comment: Published in MNRAS | No. of pages 12, 10 figures and 4 tables.
Comments are welcom