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 O’Dea (PASP 110:493–532, https://doi.org/10.1086/316162, 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. Multi-frequency 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. Also, 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.

https://link.springer.com/content/pdf/10.1007/s00159-021-00131-w.pdf

Compact steep-spectrum and peaked-spectrum radio sources

A long-period radio transient active for three decades

Source classifications, stellar masses and star formation rates are presented for 80,000 radio sources from the first data release of the Low Frequency Array Two-metre Sky Survey (LoTSS) Deep Fields, which represents the widest deep radio survey ever undertaken. Using deep multi-wavelength data spanning from the ultraviolet to the far-infrared, spectral energy distribution (SED) fitting is carried out for all of the LoTSS-Deep host galaxies using four different SED codes, two of which include modelling of the contributions from an active galactic nucleus (AGN). Comparing the results of the four codes, galaxies that host a radiative AGN are identified, and an optimised consensus estimate of the stellar mass and star-formation rate for each galaxy is derived. Those galaxies with an excess of radio emission over that expected from star formation are then identified, and the LoTSS-Deep sources are divided into four classes: star-forming galaxies, radio-quiet AGN, and radio-loud high-excitation and low-excitation AGN. Ninety-five per cent of the sources can be reliably classified, of which more than two-thirds are star-forming galaxies, ranging from normal galaxies in the nearby Universe to highly-starbursting systems at z>4. Star-forming galaxies become the dominant population below 150-MHz flux densities of about 1 mJy, accounting for 90 per cent of sources at a 150-MHz flux density of 100 microJy. Radio-quiet AGN comprise around 10 per cent of the overall population. Results are compared against the predictions of the SKADS and T-RECS radio sky simulations, and improvements to the simulations are suggested. 

/pdf/the-lofar-two-metre-sky-survey-deep-fields-data-release-1-v-2vu9z8kv.pdf

The LOFAR Two-metre Sky Survey: Deep Fields data release 1. V. Survey description, source classifications, and host galaxy properties

We present a 324.5MHz image of the NOAO Bootes field that was made using Very Large Array (VLA) P-band observations. The image has a resolution of 5.6x5.1arcsec, a radius of $2.05^\circ$ and a central noise of ~0.2mJy\beam. Both the resolution and noise of the image are an order of magnitude better than what was previously available at this frequency and will serve as a valuable addition to the already extensive multiwavelength data that are available for this field. The final source catalogue contains 1370 sources and has a median 325 to 1400MHz spectral index of -0.72. Using a radio colour-colour diagram of the unresolved sources in our catalogue, we identify 33 megahertz peaked-spectrum (MPS) sources. Based on the turnover frequency linear size relation for the gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources, we expect that the MPS sources that are compact on scales of tens of milliarcseconds should be young radio loud active galactic nuclei at high (z>2) redshifts. Of the 33 MPS sources, we were able to determine redshifts for 24, with an average redshift of 1.3. Given that five of the sources are at z>2, that the four faint sources for which we could not find redshifts are likely at even higher redshifts and that we could only select sources that are compact on a scale of ~5arcsec, there is encouraging evidence that the MPS method can be used to search for high-redshift sources.

Megahertz peaked-spectrum sources in the Bo\"otes field I - a route towards finding high-redshift AGN?

Abstract We describe a new low-frequency wideband radio survey of the southern sky. Observations covering 72–231 MHz and Declinations south of 
$+30^\circ$
 have been performed with the Murchison Widefield Array “extended” Phase II configuration over 2018–2020 and will be processed to form data products including continuum and polarisation images and mosaics, multi-frequency catalogues, transient search data, and ionospheric measurements. From a pilot field described in this work, we publish an initial data release covering 1,447 
$\mathrm{deg}^2$
 over 
$4\,\mathrm{h}\leq \mathrm{RA}\leq 13\,\mathrm{h}$
 , 
$-32.7^\circ \leq \mathrm{Dec} \leq -20.7^\circ$
 . We process twenty frequency bands sampling 72–231 MHz, with a resolution of 2′–45′′, and produce a wideband source-finding image across 170–231 MHz with a root mean square noise of 
$1.27\pm0.15\,\mathrm{mJy\,beam}^{-1}$
 . Source-finding yields 78,967 components, of which 71,320 are fitted spectrally. The catalogue has a completeness of 98% at 
${{\sim}}50\,\mathrm{mJy}$
 , and a reliability of 98.2% at 
$5\sigma$
 rising to 99.7% at 
$7\sigma$
 . A catalogue is available from Vizier; images are made available via the PASA datastore, AAO Data Central, and SkyView. This is the first in a series of data releases from the GLEAM-X survey.

/pdf/galactic-and-extragalactic-all-sky-murchison-widefield-array-11erb6j1.pdf

GaLactic and Extragalactic All-sky Murchison Widefield Array survey eXtended (GLEAM-X) I: Survey description and initial data release

Abstract Spectral variability offers a new technique to identify small scale structures from scintillation, as well as determining the absorption mechanism for peaked-spectrum (PS) radio sources. In this paper, we present very long baseline interferometry (VLBI) imaging using the long baseline array (LBA) of two PS sources, MRC 0225–065 and PMN J0322–4820, identified as spectrally variable from observations with the Murchison Widefield Array (MWA). We compare expected milliarcsecond structures based on the detected spectral variability with direct LBA imaging. We find MRC 0225–065 is resolved into three components, a bright core and two fainter lobes, roughly 430 pc projected separation. A comprehensive analysis of the magnetic field, host galaxy properties, and spectral analysis implies that MRC 0225–065 is a young radio source with recent jet activity over the last 
$10^2$
 – 
$10^3$
 yr. We find PMN J0322–4820 is unresolved on milliarcsecond scales. We conclude PMN J0322–4820 is a blazar with flaring activity detected in 2014 with the MWA. We use spectral variability to predict morphology and find these predictions consistent with the structures revealed by our LBA images.

Milliarcsecond structures of variable-peaked spectrum sources

Spectral variability of radio sources encodes information about the conditions of intervening media, source structure, and emission processes. With new low-frequency radio interferometers observing over wide fractional bandwidths, studies of spectral variability for a large population of extragalactic radio sources are now possible. Using two epochs of observations from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey that were taken one year apart, we search for spectral variability across 100--230 MHz for 21,558 sources. We present methodologies for detecting variability in the spectrum between epochs and for classifying the type of variability: either as a change in spectral shape or as a uniform change in flux density across the bandwidth. We identify 323 sources with significant spectral variability over a year-long timescale. Of the 323 variable sources, we classify 51 of these as showing a significant change in spectral shape. Variability is more prevalent in peaked-spectrum sources, analogous to gigahertz-peaked spectrum and compact steep-spectrum sources, compared to typical radio galaxies. We discuss the viability of several potential explanations of the observed spectral variability, such as interstellar scintillation and jet evolution. Our results suggest that the radio sky in the megahertz regime is more dynamic than previously suggested.

/pdf/spectral-variability-of-radio-sources-at-low-frequencies-39t5c3ualp.pdf

Spectral variability of radio sources at low frequencies

Characterising spectral variability of radio sources is a technique that offers the ability to determine the astrophysics of the intervening media, source structure, emission and absorption processes. We present broadband (0.072–10 GHz) spectral variability of 15 peaked-spectrum (PS) sources with the Australia Telescope Compact Array (ATCA) and the Murchison Widefield Array (MWA). These 15 PS sources were observed quasi-contemporaneously with ATCA and the MWA four to six times during 2020 with approximately a monthly cadence. Variability was not detected at 1–10 GHz frequencies but 13 of the 15 targets show significant variability with the MWA at megahertz frequencies. We conclude the majority of variability seen at megahertz frequencies is due to refractive interstellar scintillation of a compact component∼ 25 mas across. We also identify four PS sources that show a change in their spectral shape at megahertz frequencies. Three of these sources are consistent with a variable optical depth from an inhomogeneous free-free absorbing cloud around the source. One PS source with a variable spectral shape at megahertz frequencies is consistent with an ejection travelling along the jet. We present spectral variability as a method for determining the physical origins of observed variability and for providing further evidence to support absorption models for PS sources where spectral modelling alone is insufficient.

/pdf/wide-band-spectral-variability-of-peaked-spectrum-sources-2z7ub18r.pdf

Wide-Band Spectral Variability of Peaked Spectrum Sources

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GaLactic and Extragalactic All-sky Murchison Widefield Array survey eXtended (GLEAM-X) I: Survey description and initial data release

Milliarcsecond structures of variable-peaked spectrum sources

Spectral variability of radio sources at low frequencies

Wide-Band Spectral Variability of Peaked Spectrum Sources