Giant Metrewave Radio Telescope detections of two high-opacity HI 21cm absorbers at $z \approx 1.2$
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We report the discovery of two remarkable high-opacity HI 21cm absorbers against low-luminosity active galactic nuclei (AGNs), at $z = 1.2166$ towards J0229+0044 and at $z=1.1630$ towards J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for HI 21cm absorption against radio sources in the DEEP2 survey fields, covering $z \approx 0.73-1.53$, and including sources without known redshifts. The velocity-integrated HI 21cm optical depths are $(74.2 \pm 7.8)$ km s$^{-1}$ (J0229+0044) and $(78.41 \pm 0.81)$ km s$^{-1}$ (J0229+0053), higher than that of any known redshifted HI 21cm absorber at $z > 0.12$, and implying high H{\sc i} column densities, $> 10^{22}$ cm$^{-2}$. The emission redshift of J0229+0044 is consistent with the HI 21cm absorption redshift, while the strength and velocity spread of the absorption against J0229+0053 suggest that it too arises from gas in the AGN environment: both absorbers are thus likely to be "associated" systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215 Angstrom ultraviolet luminosities ($\lesssim 10^{26.1}$ W Hz$^{-1}$ and $\lesssim 10^{21.7}$ W Hz$^{-1}$, respectively), both significantly lower than the typical luminosities of AGNs against which HI 21cm searches have hitherto been carried out at $z \gtrsim 1$. The paucity of HI 21cm absorbers at $z \gtrsim 1$ may be due to a luminosity bias in high-$z$ AGN samples that have been searched for HI 21cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.Keywords:
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Observations of the Spitzer extragalactic First Look Survey field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Seven individual pointings were observed, covering an area of 4 square degrees with a resolution of 5.8'' x 4.7'', PA 60 deg. The r.m.s. noise at the centre of the pointings is between 27 and 30 microJy before correction for the GMRT primary beam. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic, along with a catalogue of 3944 sources detected above 5 sigma, are presented. The survey complements existing radio and infrared data available for this region.
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Abstract Arp 187 is one of the fading active galactic nuclei (AGNs), whose AGN activity is currently decreasing in luminosity. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the long-term luminosity change over 10 4 yr. The Very Large Array 5 GHz and 8 GHz images and the ALMA 133 GHz images reveal bimodal jet lobes with ∼5 kpc size and the absence of the central radio core. The 6dF optical spectrum shows that Arp 187 hosts a narrow line region with the estimated size of ∼1 kpc, and the line strengths give the AGN luminosity of L bol = 1.5 × 10 46 erg s −1 . On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of L bol < 2.2 × 10 43 erg s −1 . The absence of the radio core gives the more strict upper bound of the current AGN luminosity of L bol < 8.0 × 10 40 erg s −1 , suggesting that the central engine is already quenched. These multiwavelength signatures indicate that Arp 187 hosts a “dying” AGN: the central engine is already dead, but the large-scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of ∼10 3–5 fainter within ∼10 4 yr, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within ∼500 yr.
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We present results from a study of X-shaped sources based on observations using the Giant Metrewave Radio Telescope (GMRT). These observations were motivated by our low frequency study of 3C 223.1 (Lal & Rao 2005), an X-shaped radio source, which showed that the wings (or low-surface-brightness jets) have flatter spectral indices than the active lobes (or high-surface-brightness jets), a result not easily explained by most models. We have now obtained GMRT data at 240 and 610 MHz for almost all the known X-shaped radio sources and have studied the distribution of the spectral index across the sources. While the radio morphologies of all the sources at 240 and 610 MHz show the characteristic X-shape, the spectral characteristics of the X-shaped radio sources, seem to fall into three categories, namely, sources in which (A) the wings have flatter spectral indices than the active lobes, (B) the wings and the active lobes have comparable spectral indices, and (C) the wings have steeper spectral indices than the active lobes. We discuss the implications of the new observational results on the various formation models that have been proposed for X-shaped sources.
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AbstractA Giant Metrewave Radio Telescope (GMRT) is being set up by TIFR as a National Facility at Khodad, near Narayangaon, 80 km north of Pune and 200 km east of Bombay. The academic headquarters of the GMRT is to be located in the University of Poona Campus.When completed in 1992, GMRT will become the most powerful facility in the world for astronomical research in the metre and decimetre wave-bands (~30 to 1420 MHz). It is being designed to investigate a wide variety of celestial objects, reaching from our solar system to the very edge of the observable Universe. An important scientific goal will be to verify the prediction of the Big Bang Model for the origin of the Universe, according to which galaxies must have condensed out of massive clouds of neutral hydrogen, which can be observed only at metre wavelengths, through their highly redshifted λ 21-cm line radiation.
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Abstract The Giant Metrewave Radio Telescope (GMRT) is being set up near Pune in India consisting of 30 numbers of 45-m diameter parabolic dishes. Twelve of these are being placed in a central array, about 1 × 1 km 2 in size, and the remaining eighteen along three 14-km long arms forming a Y-shaped array. GMRT will operate at six frequency bands near 38, 153, 233, 327, 611 and 1420 MHz. It is expected to be completed progressively by 1993, and will be the world’s largest radio telescope operating at metre and decimetre wavelengths.
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In this paper we describe a method for measuring the effective receiver temperatureTerc and its variation for the entire receiver chain of a radio telescope, and use it to make a radio-continuum map of the sky at 240 MHz using the Giant Metrewave Radio Telescope (GMRT). We also show that in the case of GMRT, Terc varies mainly with elevation and ambient temperature. The calibration techniques evolved here are applicable to similar interferometers with a large number of antennas, several frequency bands and a number of receiver systems at room temperature (where conventional methods are time-consuming). This method ideally requires just one complete day of observations in a frequency band.
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Abstract We report the discovery of two remarkable high-opacity H i 21 cm absorbers against low-luminosity active galactic nuclei (AGNs), at z = 1.2166 toward J0229+0044 and at z = 1.1630 toward J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for H i 21 cm absorption against radio sources in the DEEP2 survey fields, covering z ≈ 0.73–1.53, and including sources without known redshifts. The velocity-integrated H i 21 cm optical depths are (74.2 ± 7.8) km s −1 (J0229+0044) and (78.41 ± 0.81) km s −1 (J0229+0053), higher than that of any known redshifted H i 21 cm absorber at z > 0.12, and implying high H i column densities, >10 22 cm −2 . The emission redshift of J0229+0044 is consistent with the H i 21 cm absorption redshift, while the strength and velocity spread of the absorption against J0229+0053 suggest that it too arises from gas in the AGN environment: both absorbers are thus likely to be “associated” systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215 Å ultraviolet luminosities (≲10 26.1 W Hz −1 and ≲10 21.7 W Hz −1 , respectively), both significantly lower than the typical luminosities of AGNs against which H i 21 cm searches have hitherto been carried out at z ≳ 1. The paucity of H i 21 cm absorbers at z ≳ 1 may be due to a luminosity bias in high- z AGN samples that have been searched for H i 21 cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.
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