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Register a new userMultiwavelength characterization of faint Ultra Steep Spectrum radio sources : A search for high-redshift radio galaxies
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Ultra Steep Spectrum (USS) radio sources are one of the efficient tracers of powerful High-z Radio Galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts. Using our 325 MHz GMRT observations (5-sigma ~ 800 microJy) and 1.4 GHz VLA observations (5-sigma ~ 80 - 100 microJy) available in two subfields (viz., VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature. The optical, IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Redshift estimates are available for 86/116 (~ 74%) USS sources in the VLA-VVDS field and for 39/44 (~ 87%) USS sources in the SXDF fields with median values (z_median) ~ 1.18 and ~ 1.57, which are higher than that for non-USS radio sources (z_median non-USS ~ 0.99 and ~ 0.96), in the two subfields, respectively. The flux ratio of radio to mid-IR (S_1.4 GHz/S_3.6 micron) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ~ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (~ 26% in the VLA-VVDS and ~ 13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and thus, can be considered as potential HzRG candidates.
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We report on a time-domain search for pulsars in 44 steep spectrum radio sources originally identified from recent imaging surveys. The time-domain search was conducted at 327 MHz using the Ooty radio telescope, and utilized a semi-coherent dedispersion scheme retaining the sensitivity even for sub-millisecond periods up to reasonably high dispersion measures. No new pulsars were found. We discuss the nature of these steep spectrum sources and argue that majority of the sources in our sample should either be pulsars or a new category of Galactic sources. Several possibilities that could hinder detection of these sources as pulsars, including anomalously high scattering or alignment of the rotation and magnetic axes, are discussed in detail, and we suggest unconventional search methods to further probe these possibilities.
We present optical spectroscopy of 62 objects selected from several samples of ultra steep spectrum (USS) radio sources. 46 of these are from our primary catalog, consisting of 669 sources with radio spectral indices alpha < -1.30 (S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on the basis of their faint optical and near-IR identifications. Most are identified as narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19. Ten objects are at z>3, nearly doubling the number of such sources known to date. Four of the USS radio sources are identified with quasars, of which at least three have very red spectral energy distributions. The source TN J0936-2242 is identified with an extremely red object (ERO, R-K>5); both it and a close companion are at z=1.479. The spectrum of the ERO closely resembles that of previously discovered radio galaxies at z~1.5. Five sources show continuum emission, but fail to show any clear emission or absorption features, despite integrations of ~1 h with the Keck telescope. We suggest that these objects could be (i) radio galaxies with faint emission lines in the ``redshift desert at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, which are dominated by a stellar continuum observed with insufficient S/N, or (iii) pulsars. Three radio sources identified with faint objects in the K-band images remain undetected in 50-90 min spectroscopic integrations with the Keck telescope, and are possible z>7 candidates.
Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are compact, powerful radio sources. The multi-frequency observational properties and current theories are reviewed with emphasis on developments since the earlier review of ODea (1998). There are three main hypotheses for the nature of PS and CSS sources. (1) The PS sources might be very young radio galaxies which will evolve into CSS sources on their way to becoming large radio galaxies. (2) The PS and CSS sources might be compact because they are confined (and enhanced in radio power) by interaction with dense gas in their environments. (3) Alternately, the PS sources might be transient or intermittent sources. Each of these hypotheses may apply to individual objects. The relative number in each population will have significant implications for the radio galaxy paradigm. Proper motion studies over long time baselines have helped determine hotspot speeds for over three dozen sources and establish that these are young objects. Multifrequency polarization observations have demonstrated that many CSS/PS sources are embedded in a dense interstellar medium and vigorously interacting with it. The detection of emission line gas aligned with the radio source, and blue-shifted HI absorption and [OIII] emission lines indicates that AGN feedback is present in these objects -- possibly driven by the radio source. CSS/PS sources with evidence of episodic AGN over a large range of time-scales have been discussed. The review closes with a discussion of open questions and prospects for the future.
Compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources represent a large fraction of the extragalactic objects in flux density-limited samples. They are compact, powerful radio sources whose synchrotron peak frequency ranges between a few hundred MHz to several GHz. CSS and GPS radio sources are currently interpreted as objects in which the radio emission is in an early evolutionary stage. In this contribution I review the radio properties and the physical characteristics of this class of radio sources, and the interplay between their radio emission and the ambient medium of the host galaxy.
Ultra-steep spectrum (USS) radio sources are good tracers of powerful radio galaxies at $z > 2$. Identification of even a single bright radio galaxy at $z > 6$ can be used to detect redshifted 21cm absorption due to neutral hydrogen in the intervening IGM. Here we describe a new sample of high-redshift radio galaxy (HzRG) candidates constructed from the TGSS ADR1 survey at 150 MHz. We employ USS selection ($alpha le -1.3$) in $sim10000$ square degrees, in combination with strict size selection and non-detections in all-sky optical and infrared surveys. We apply flux density cuts that probe a unique parameter space in flux density ($50 < S_{textrm{150}} < 200$ mJy) to build a sample of 32 HzRG candidates. Follow-up Karl G. Jansky Very Large Array (VLA) observations at 1.4 GHz with an average beam size of $1.3$ arcseconds ($$) revealed $sim 48%$ of sources to have a single radio component. P-band (370 MHz) imaging of 17 of these sources revealed a flattening radio SED for ten sources at low frequencies, which is expected from compact HzRGs. Two of our sources lie in fields where deeper multi-wavelength photometry and ancillary radio data are available and for one of these we find a best-fit photo-z of $4.8 pm 2.0$. The other source has $z_{textrm{phot}}=1.4 pm 0.1$ and a small angular size ($3.7$), which could be associated with an obscured star forming galaxy or with a dead elliptical. One USS radio source not part of the HzRG sample but observed with the VLA nonetheless is revealed to be a candidate giant radio galaxy with a host galaxy photo-z of $1.8pm0.5$, indicating a size of 875 kpc.
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