77 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
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The <i>AKARI</i> deep fields: early results from multi-wavelength follow-up campaigns
We present early results from our multi-wavelength follow-up campaigns of the AKARI Deep Fields at the North and South Ecliptic Poles. We summarize our campaigns in this poster paper, and present three early outcomes. (a) Our AAOmega optical spectroscopy of the Deep Field South at the AAT has observed over 550 different targets, and our preliminary local luminosity function at 90 ?m from the first four hours of data is in good agreement with the predictions from Serjeant & Harrison (2005). (b) Our GMRT 610 MHz imaging in the Deep Field North has reached ?30 ?Jy RMS, making this among the deepest images at this frequency. Our 610 MHz source counts at >200 ?Jy are the deepest ever derived at this frequency. (c) Comparing our GMRT data with our 1.4 GHz WSRT data, we have found two examples of radio-loud AGN that may have more than one epoch of activity
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
Extragalactic radio sources with sharply inverted spectrum at metre wavelengths
We present the first results of a systematic search for the rare
extragalactic radio sources showing an inverted (integrated) spectrum, with
spectral index , a previously unexplored spectral domain. The
search is expected to yield strong candidates for , for which
the standard synchrotron self-absorption (characterized by a single power-law
energy distribution of relativistic electron population) would not be a
plausible explanation, even in an ideal case of a perfectly homogeneous source
of incoherent synchrotron radiation. Such sharply inverted spectra, if found,
would require alternative explanations, e.g., free-free absorption, or
non-standard energy distribution of relativistic electrons which differs from a
power-law (e.g., Maxwellian).
The search was carried out by comparing two sensitive low-frequency radio
surveys made with sub-arcminute resolution, namely, the WISH survey at 352 MHz
and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains
7056 WISH sources classified as `single' and brighter than 100 mJy at 352 MHz.
We focus here on the seven of these sources for which we find .
Two of these are undetected at 150 MHz and are particularly good candidates for
. Five of the seven sources exhibit a
`Gigahertz-Peaked-Spectrum' (GPS).Comment: 8 pages, 2 figures, accepted for publication in MNRA
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
325-MHz observations of the ELAIS-N1 field using the Giant Metrewave Radio Telescope
We present observations of the European Large-Area {\it ISO} Survey-North 1
(ELAIS-N1) at 325 MHz using the Giant Metrewave Radio Telescope (GMRT), with
the ultimate objective of identifying active galactic nuclei and starburst
galaxies and examining their evolution with cosmic epoch. After combining the
data from two different days we have achieved a median rms noise of Jy beam, which is the lowest that has been achieved at this
frequency. We detect 1286 sources with a total flux density above Jy. In this paper, we use our deep radio image to examine the spectral
indices of these sources by comparing our flux density estimates with those of
Garn et al. at 610 MHz with the GMRT, and surveys with the Very Large Array at
1400 MHz. We attempt to identify very steep spectrum sources which are likely
to be either relic sources or high-redshift objects as well as inverted-spectra
objects which could be Giga-Hertz Peaked Spectrum objects. We present the
source counts, and report the possibility of a flattening in the normalized
differential counts at low flux densities which has so far been reported at
higher radio frequencies.Comment: The paper contains 15 figures and 5 tables. Accepted for publication
in MNRA
Low-frequency radio emission in the massive galaxy cluster MACS J0717.5 + 3745
International audienceTo investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5 + 3745, located at z = 0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5 + 3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process. Methods: Continuum observations were carried out using GMRT at 0.235 and 0.61 GHz on MACS J0717.5 + 3745 and archival data from the VLA (0.074 and 1.42 GHz) and WSRT (0.325 GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used. Results: A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index α0.2350.61˜-1.17±0.37. We have detected a giant radio halo within the cluster system with a linear size of 1.58 Mpc and a "Chair-shaped" filament structure between the merging subclusters of linear size 853 kpc at 0.235 GHz. This is the most powerful halo ever observed with P1.4 = 9.88 × 1025 WHz-1 and an equipartition magnetic field estimate of ~6.49 μG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events
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