GMRT observations of extragalactic radio sources with steeply inverted spectra

Mhaskey, Mukul; Gopal-Krishna, . .; Dabhade, Pratik; Paul, Surajit; Salunkhe, Sameer; Sirothia, S. K.

doi:10.1093/mnras/stz335

Cited by 14 publications

(19 citation statements)

References 51 publications

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“…Also, the dense environment indicated by the high optical depth would be consistent with the possibility of FFA mechanism being responsible for the sharp spectral turnover witnessed for the source J1209−2032, as inferred for GPS and CSS sources (Pihlström et al 2003;Bicknell et al 2018). A discussion on the FFA process operating in such extremely inverted sources is provided in Mhaskey et al (2019b).…”

Section: Discussionsupporting

confidence: 63%

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uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

Mhaskey

Paul

Gupta

et al. 2020

A&A

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We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit of α = +2.5 (S(ν) ∝ να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z = 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020 cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z = 2.171), no HI absorption was detected in our observations.

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Section: Discussionsupporting

confidence: 63%

“…For the source J1549+5038 Pasetto et al (2016 inferred a large (restframe) rotation measure (RM = 1400 ± 500 rad m −2 ). The available radio continuum data for J1549+5038 confirms a strong flux variability at centimetre wavelengths, as cited in Mhaskey et al (2019b).…”

Section: J1209−2032 and J1549+5038supporting

confidence: 63%

uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

Mhaskey

Paul

Gupta

et al. 2020

A&A

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“…We find a spectral index difference of ∆α = α 2 − α 1 = 0.45 ± 0.1, which is less than the theoretical limit imposed by pure synchrotron self-absorption (Mhaskey et al 2019). This indicates that other absorption processes may also influence the flat spectrum radio SED, such as free-free absorption (Kameno et al 2005;Tingay & de Kool 2003).…”

Section: Flat Spectrum Sourcescontrasting

confidence: 57%

The VLA-COSMOS 3 GHz Large Project: Average radio spectral energy distribution of active galactic nuclei

Tisanić

Smolčić

Imbrišak

et al. 2020

A&A

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Context. As the Square Kilometer Array (SKA) is expected to be operational in the next decade, investigations of the radio sky in the range of 100 MHz–10 GHz have become important for simulating SKA observations. In determining physical properties of galaxies from radio data, the radio spectral energy distribution (SED) is often assumed to be described by a simple power law, usually with a spectral index of 0.7 for all sources. Even though radio SEDs have been shown to exhibit deviations from this assumption, both in differing spectral indices and complex spectral shapes, it is often presumed that their individual differences can be canceled out in large samples. Aims. Since the average spectral index around 1 GHz (observed-frame) is important for determining physical properties of large samples of galaxies, we aim to test whether individual differences in the spectra of radio-identified active galactic nuclei align with the simple assumption of α = 0.7 and test the evolution of the parameters of the synchrotron aging model with redshift and radio luminosity. Methods. We use a sample of 744 radio-excess active galactic nuclei (RxAGN), defined as those that exhibit more than a 3σ radio luminosity excess with respect to the value expected only from the contribution from star formation, out to z ∼ 4. We constructed their average radio SED by combining Very Large Array (VLA) observations of the COSMOS field at 1.4 GHz and 3 GHz with Giant Meterwave Radio Telescope (GMRT) observations at 325 MHz and 610 MHz. To account for nondetections in the GMRT maps, we employed the survival analysis technique. We binned the RxAGN sample into luminosity- and redshift-complete subsamples. In each bin, we constrained the shape of the average radio SED by fitting a broken power-law model. Results. We find that the RxAGN sample can be described by a spectral index of α1 = 0.28 ± 0.03 below the break frequency νb = (4.1 ± 0.2) GHz and α2 = 1.16 ± 0.04 above it, while a simple power-law model, capturing fewer spectral features, yields a single spectral index of 0.64 ± 0.07. By binning in 1.4 GHz of radio luminosity and redshift, we find that the power-law spectral index is positively correlated with redshift and that the broken power-law spectral index above 4 GHz is positively correlated with both the redshift and source size. By selecting sources with sizes less than 1 kpc, we find a subsample of flat-spectrum sources, which can be described by a spectral index of α = 0.41 ± 0.07 and a broken power-law spectral index of α1 = 0.1 ± 0.1 (α2 = 0.55 ± 0.09) below (above) a break frequency of νb = (2.7 ± 0.5) GHz. Conclusions. We have constrained the radio SED for a sample of RxAGN in the COSMOS field using available VLA and GMRT data, corresponding to the rest-frame frequency range from ∼0.3 GHz to ∼10 GHz. We describe our derived average radio SED of RxAGN using power-law and broken power-law models, yielding a radio SED that steepens above ∼4 GHz.

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“…Our sources have optically thick spectral index constraints that are consistent with SSA, though improved spectral coverage at low frequencies (<1 GHz) will ultimately be needed (Callingham et al 2015;Collier et al 2018;Keim et al 2019;Mhaskey et al 2019aMhaskey et al , 2019b. The identification of FFA associated with any of the newly radio-loud AGNs in our sample would be of particular interest in the context of galaxy evolution since simulations have shown that it may arise from jet−ISM interactions (Bicknell et al 1997(Bicknell et al , 2018.…”

Section: Radio Color-color Diagrammentioning

confidence: 66%

“…In a future study, we will incorporate new data from forthcoming VLA and Giant Metre-wave Radio Telescope (GMRT) observations spanning a broader frequency range (the full contiguous frequency range of the VLA from 1 to 50 GHz and deep measurements below 1 GHz using the VLA and GMRT). This will enable a more rigorous radio SED modeling analysis that will allow us to test whether SSA or FFA is responsible for the observed spectral curvature (Mhaskey et al 2019a(Mhaskey et al , 2019b.…”

Section: Radio Sed Modelingmentioning

confidence: 99%

Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal Timescales Revealed by VLASS and FIRST

Nyland

Dong

Patil

et al. 2020

ApJ

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We have performed a search over 3440 deg 2 of Epoch 1 (2017-2019) of the Very Large Array Sky Survey to identify unobscured quasars in the optical (0.2<z<3.2) and obscured active galactic nuclei (AGNs) in the infrared that have brightened dramatically in the radio over the past one to two decades. These sources would have been previously classified as "radio-quiet" quasars based on upper limits from the Faint Images of the Radio Sky at Twenty cm survey (1993-2011), but they are now consistent with "radio-loud" quasars ( -= -L 10 erg s 3 GHz 40 42 1). A quasi-simultaneous, multiband (∼1-18 GHz) follow-up study of 14 sources with the VLA has revealed compact sources (<0 1 or <1 kpc) with peaked radio spectral shapes. The highamplitude variability over decadal timescales at 1.5 GHz (100% to >2500%) but roughly steady fluxes over a few months at 3 GHz are inconsistent with extrinsic variability due to propagation effects, thus favoring an intrinsic origin. We conclude that our sources are powerful quasars hosting compact/young jets. This challenges the generally accepted idea that "radio-loudness" is a property of the quasar/AGN population that remains fixed on human timescales. Our study suggests that frequent episodes of short-lived AGN jets that do not necessarily grow to large scales may be common at high redshift. We speculate that intermittent but powerful jets on subgalactic scales could interact with the interstellar medium, possibly driving feedback capable of influencing galaxy evolution.

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GMRT observations of extragalactic radio sources with steeply inverted spectra

Cited by 14 publications

References 51 publications

uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

The VLA-COSMOS 3 GHz Large Project: Average radio spectral energy distribution of active galactic nuclei

Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal Timescales Revealed by VLASS and FIRST

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