103 research outputs found
Low radio frequency signatures of ram pressure stripping in Virgo spiral NGC 4254
We report the detection of extended low radio frequency continuum emission
beyond the optical disk of the spiral galaxy NGC 4254 using the Giant Metrewave
Radio Telescope. NGC 4254, which has an almost face-on orientation, is located
in the outskirts of the Virgo cluster. Since such extended emission is uncommon
in low inclination galaxies, we believe it is a signature of magnetised plasma
pushed out of the disk by ram pressure of the intracluster medium as NGC 4254
falls into the Virgo cluster. The detailed spectral index distribution across
NGC 4254 shows that the steepest spectrum alpha < -1 (S propto nu^{alpha})
arises in the gas beyond the optical disk. This lends support to the ram
pressure scenario by indicating that the extended emission is not from the disk
gas but from matter which has been stripped by ram pressure. The steeper
spectrum of the extended emission is reminiscent of haloes in edge-on galaxies.
The sharp fall in intensity and enhanced polarization in the south of the
galaxy, in addition to enhanced star formation reported by others provide
evidence towards the efficacy of ram pressure on this galaxy. HI 21cm
observations show that the gas in the north lags in rotation and hence is
likely the atomic gas which is carried along with the wind. NGC 4254 is a
particularly strong radio emitter with a power of 7x10^{22} Watts/Hz at 240
MHz. We find that the integrated spectrum of the galaxy flattens at lower
frequencies and is well explained by an injection spectrum with
alpha_0=-0.45+-0.12. We end by comparing published simulation results with our
data and conclude that ram pressure stripping is likely to be a significant
contributor to evolution of galaxies residing in X-ray poor groups and cluster
outskirts.Comment: 10 pages, 7 figures (figures 1,2 and 4 are in jpg format) and 2
tables; Accepted for publication in MNRA
Hint of 150 MHz radio emission from the Neptune-mass extrasolar transiting planet HAT-P-11b
Since the radio-frequency emission from planets is expected to be strongly
influenced by their interaction with the magnetic field and corona of the host
star, the physics of this process can be effectively constrained by making
sensitive measurements of the planetary radio emission. Up to now, however,
numerous searches for radio emission from extrasolar planets at radio
wavelengths have only yielded negative results. Here we report deep radio
observations of the nearby Neptune-mass extrasolar transiting planet HAT-P-11b
at 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). On July 16, 2009,
we detected a 3-sigma emission whose light curve is consistent with an eclipse
when the planet passed behind the star. This emission is at a position 14
arcsec from the transiting exoplanet's coordinates; thus, with a synthetized
beam of FWHM~16 arcsec, the position uncertainty of this weak radio signal
encompasses the location of HAT-P-11. We estimate a 5% false positive
probability that the observed radio light curve mimics the planet's eclipse
light curve. If the faint signature is indeed a radio eclipse event associated
with the planet, then its flux would be 3.87 mJy +/- 1.29 mJy at 150 MHz.
However, our equally sensitive repeat observations of the system on November
17, 2010 did not detect a significant signal in the radio light curve near the
same position. This lack of confirmation leaves us with the possibility of
either a variable planetary emission, or a chance occurrence of a false
positive signal in our first observation. Deeper observations are required to
confirm this hint of 150 MHz radio emission from HAT-P-11b.Comment: Accepted for publication in Astronomy & Astrophysic
J021659-044920: a relic giant radio galaxy at z ~ 1.3
We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at
redshift that exhibits large-scale extended, nearly co-spatial,
radio and X-ray emission from radio lobes, but no detection of Active Galactic
Nuclei core, jets and hotspots. The total angular extent of the GRG at the
observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is
found to be 2.4 arcmin, that corresponds to a total projected linear
size of 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz
shows high spectral curvature ( 1.19) with sharp steepening above
0.325 GHz, consistent with relic radio emission that is 8
10 yr old. The radio spectral index map between observed frame 0.325 and
1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from
outer-end to inner-end, indicating backflow of plasma in the lobes. The
extended X-ray emission characterized by an absorbed power-law with photon
index 1.86 favours inverse-Compton scattering of the Cosmic Microwave
Background (ICCMB) photons as the plausible origin. Using both X-ray and radio
fluxes under the assumption of ICCMB we estimate the magnetic field in the
lobes to be 3.3 G. The magnetic field estimate based on energy
equipartition is 3.5 G. Our work presents a case study of a rare
example of a GRG caught in dying phase in the distant Universe.Comment: 10 pages, 5 figures, 3 tables. Published in MNRAS. Corrected typos
and added a referenc
A search for radio emission from exoplanets around evolved stars
The majority of searches for radio emission from exoplanets have to date
focused on short period planets, i.e., the so-called hot Jupiter type planets.
However, these planets are likely to be tidally locked to their host stars and
may not generate sufficiently strong magnetic fields to emit electron cyclotron
maser emission at the low frequencies used in observations (typically >150
MHz). In comparison, the large mass-loss rates of evolved stars could enable
exoplanets at larger orbital distances to emit detectable radio emission. Here,
we first show that the large ionized mass-loss rates of certain evolved stars
relative to the solar value could make them detectable with the Low Frequency
Array (LOFAR) at 150 MHz ( = 2 m), provided they have surface magnetic
field strengths >50 G. We then report radio observations of three long period
(>1 au) planets that orbit the evolved stars Gem, Dra, and
UMi using LOFAR at 150 MHz. We do not detect radio emission from any
system but place tight 3 upper limits of 0.98, 0.87, and 0.57 mJy on
the flux density at 150 MHz for Gem, Dra, and UMi,
respectively. Despite our non-detections these stringent upper limits highlight
the potential of LOFAR as a tool to search for exoplanetary radio emission at
meter wavelengths.Comment: 9 pages, 3 figure
On the nature of infrared-faint radio sources in the SXDF and VLA-VVDS fields
Infrared-Faint Radio Sources (IFRSs) are an unusual class of objects that are
relatively bright at radio wavelengths but have faint or undetected infrared
counterparts even in deep surveys. We identify and investigate the nature of
IFRSs using deep radio (S 100 Jy beam at
5), optical (m 26 - 27.7 at 5), and near-IR
(S 1.3 - 2.0 Jy beam at 5) data
available in two deep fields namely the Subaru X-ray Deep Field (SXDF) and the
Very Large Array - VIMOS VLT Deep Survey (VLA-VVDS) field. In 1.8 deg of
the two fields we identify a total of nine confirmed and ten candidate IFRSs.
We find that our IFRSs are high-redshift radio-loud AGN, with 12/19 sources
having redshift estimates in the range of 1.7 - 4.3, while a limit
of 2.0 is placed for the remaining seven sources. Notably, our study
finds, for the first time, IFRSs with measured redshift 3.0, and also, the
redshift estimates for IFRSs in the faintest 3.6 m flux regime i.e.,
S 1.3 Jy. Radio observations show that our IFRSs
exhibit both compact unresolved as well as extended double-lobe morphologies,
and have predominantly steep radio spectra between 1.4 GHz and 325 MHz. The
non-detection of all but one IFRSs in the X-ray band and the optical-to-MIR
colour (m - m) suggest that a significant fraction
of IFRSs are likely to be hosted in dusty obscured galaxies.Comment: 20 pages, 8 figures, 4 tables, accepted for publication in MNRA
Deep GMRT 150 MHz observations of the DEEP2 fields: Searching for High Red-shift Radio Galaxies Revisited
High red-shift radio galaxies are best searched at low radio frequencies, due
to its steep radio spectra. Here we present preliminary results from our
programme to search for high red-shift radio galaxies to ~ 10 to 100 times
fainter than the known population till date. We have extracted ultra-steep
spectrum (USS) samples from deep 150 MHz Giant Meter-wave Radio Telescope
(GMRT) observations from one of the three well-studied DEEP2 fields to this
affect. From correlating these radio sources w.r.t to the high-frequency
catalogues such as VLA, FIRST, and NVSS at 1.4 GHz, we find ~ 100 steep
spectrum (spectral index, 1) radio sources, which are good
candidates for high red-shift radio galaxies.Comment: 3 pages, 1 figures, Revised version under review in the Journal of
Astrophysics and Astronom
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