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Register a new user325-MHz observations of the ELAIS-N1 field using the Giant Metrewave Radio Telescope
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S. K. Sirothia
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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 $approx40 mu$Jy beam$^{-1}$, which is the lowest that has been achieved at this frequency. We detect 1286 sources with a total flux density above $approx270 mu$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.
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Context: Observations at the radio continuum band below the gigahertz band are key when the nature and properties of nonthermal sources are investigated because their radio radiation is strongest at these frequencies. The low radio frequency range is therefore the best to spot possible counterparts to very high-energy (VHE) sources: relativistic particles of the same population are likely to be involved in radio and high-energy radiation processes. Some of these counterparts to VHE sources can be stellar sources. Aims: The Cygnus region in the northern sky is one of the richest in this type of sources that are potential counterparts to VHE sources. We surveyed the central ~15sq deg of the Cygnus constellation at the 325 and 610~MHz bands with angular resolutions and sensitivities of 10 and 6, and 0.5 and 0.2 mJy/beam, respectively. Methods: The data were collected during 172 hours in 2013 - 2017, using the Giant Metrewave Radio Telescope (GMRT) with 32MHz bandwidth, and were calibrated using the SPAM routines. The source extraction was carried out with the PyBDSF tool, followed by verification through visual inspection of every putative catalog candidate source in order to determine its reliability. Results: In this first paper we present the catalog of sources, consisting of 1048 sources at 325 MHz and 2796 sources at 610MHz. By cross-matching the sources from both frequencies with the objects of the SIMBAD database, we found possible counterparts for 143 of them. Most of the sources from the 325-MHz catalog (993) were detected at the 610MHz band, and their spectral index alpha was computed adopting S(nu) ~ nu^alpha. The maximum of the spectral index distribution is at alpha=-1, which is characteristic of nonthermal emitters and might indicate an extragalactic population.
In this first paper of the series, we present initial results of newly upgraded Giant Meterwave Radio Telescope (uGMRT) observation of European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz with 32 MHz bandwidth. Precise measurement of fluctuations in Galactic and extragalactic foreground emission as a function of frequency as well as angular scale is necessary for detecting redshifted 21-cm signal of neutral hydrogen from Cosmic Dawn, Epoch of Reionization (EoR) and post-reionization epoch. Here, for the first time we have statistically quantified the Galactic and extragalactic foreground sources in the ELAIS-N1 field in the form of angular power spectrum using the newly developed Tapered Gridded Estimator (TGE). We have calibrated the data with and without direction-dependent calibration techniques. We have demonstrated the effectiveness of TGE against the direction dependent effects by using higher tapering of field of view (FoV). We have found that diffuse Galactic synchrotron emission (DGSE) dominates the sky, after point source subtraction, across the angular multipole range $ 1115 leqslant mathcal{ell} leqslant 5083 $ and $ 1565 leqslant mathcal{ell} leqslant 4754 $ for direction-dependent and -independent calibrated visibilities respectively. The statistical fluctuations in DGSE has been quantified as a power law of the form $mathcal{C}_{mathcal{ell}}= A mathcal{ell}^{-beta} $. The best fitted values of (A, $beta$) are ($ 62 pm 6$ $mK^{2}$, $2.55 pm 0.3 $) and ($ 48 pm 4$ $mK^{2}$, $2.28 pm 0.4 $ ) for the two different calibration approaches. For both the cases, the power law index is consistent with the previous measurements of DGSE in other parts of sky.
With 30 antennas and a maximum baseline length of 25 km, the Giant Metrewave Radio Telescope (GMRT) is the premier low-frequency radio interferometer today. We have carried out a study of possible expansions of the GMRT, via adding new antennas and installing focal plane arrays (FPAs), to improve its point-source sensitivity, surface brightness sensitivity, angular resolution, field of view, and U-V coverage. We have carried out array configuration studies, aimed at minimizing the number of new GMRT antennas required to obtain a well-behaved synthesized beam over a wide range of angular resolutions for full-synthesis observations. This was done via two approaches, tomographic projection and random sampling, to identify the optimal locations for the new antennas. We report results for the optimal locations of the antennas of an expanded array (the EGMRT), consisting of the existing 30 GMRT antennas, 30 new antennas at short distances, $leq 2.5$ km from the array centre, and 26 new antennas at long distances, $approx 5-25$ km from the array centre. The collecting area and the field of view of the proposed EGMRT array would be larger by factors of, respectively, $approx 3$ and $approx 30$, than those of the GMRT. Indeed, the EGMRT continuum sensitivity and survey speed with 550-850 MHz FPAs installed on the 45 antennas within a distance of $approx 2.5$ km of the array centre would be far better than those of any existing interferometer, and comparable to the sensitivity and survey speed of Phase-1 of the Square Kilometre Array.
We present results from a study of seven large known head-tail radio galaxies based on observations using the Giant Metrewave Radio Telescope at 240 and 610 MHz. These observations are used to study the radio morphologies and distribution of the spectral indices across the sources. The overall morphology of the radio tails of these sources is suggestive of random motions of the optical host around the cluster potential. The presence of the multiple bends an d wiggles in several head-tail sources is possibly due to the precessing radio jets. We find steepening of the spectral index along the radio tails. The prevailing equipartition magnetic field also decreases a long the radio tails of these sources. These steepening trends are attributed to the synchrotron aging of plasma toward the ends of the tails. The dynamical ages of these sample sources have been estimated to be ~100 Myr, which is a factor of six more than the age estimates from the radiative losses due to synchrotron cooling.
We present deep polarimetric observations at 1420 MHz of the European Large Area ISO Survey North 1 region (ELAIS N1) as part of the Dominion Radio Astrophysical Observatory Planck Deep Fields project. By combining closely spaced aperture synthesis fields, we image a region of 7.43 square degrees to a maximum sensitivity in Stokes Q and U of 78 microJy/beam, and detect 786 compact sources in Stokes I. Of these, 83 exhibit polarized emission. We find that the differential source counts (log N - log p) for polarized sources are nearly constant down to p > 500 microJy, and that these faint polarized radio sources are more highly polarized than the strong source population. The median fractional polarization is (4.8 +/- 0.7)% for polarized sources with Stokes I flux density between 1 and 30 mJy; approximately three times larger than sources with I > 100 mJy. The majority of the polarized sources have been identified with galaxies in the Spitzer Wide Area Infrared Extragalactic Survey (SWIRE) image of ELAIS N1. Most of the galaxies occupy regions in the IRAC 5.8/3.6 micron vs. 8.0/4.5 micron color-color diagram associated with dusty AGNs, or with ellipticals with an aging stellar population. A few host galaxies have colors that suggests significant PAH emission in the near-infrared. A small fraction, 12%, of the polarized sources are not detected in the SWIRE data. None of the polarized sources in our sample appears to be associated with an actively star-forming galaxy.
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