New Radio Data on Sources of the Big Trio Program for Searching for Distant Radio Galaxies

TL;DR

This work revisits the Big Trio program’s steep-spectrum radio sources to search for distant HzRGs, integrating new radio, optical, and infrared data to refine morphologies, environments, and redshifts for 113 SS sources observed with the 6 m BTA telescope. By combining multi-epoch flux densities across a broad frequency range and applying robust variability metrics, the study differentiates instrumental effects from intrinsic variability and identifies sources with spectral peaks, young ages, or restarted activity. The improved low-frequency data (e.g., GLEAM) reveal a lower fraction of steep-spectrum sources than earlier assessments and highlight spectral evolution in many objects, while morphological analysis shows a substantial fraction associated with dense environments and episodic activity. These results refine the census of HzRG candidates, emphasize environmental influences on radio morphology, and illustrate how combining historical and contemporary surveys enhances interpretation of radio source evolution over decades. The findings have implications for understanding the formation of massive galaxies and cluster environments at high redshift.

Abstract

Radio sources with steep and ultra-steep spectra, identified through a series of surveys of the Cold experiment using the RATAN-600 radio telescope, formed the basis of the Big Trio program aimed at discovering distant radio galaxies. With the advent of new radio, optical, and infrared sky surveys, it became possible to carry out additional studies of the 113 radio sources observed within this program using the 6-meter BTA telescope. Morphological features indicative of the evolutionary state of radio sources revealed that 9 objects exhibit signs of restarted activity, 12 can be classified as young, and 2 as dying. Based on their WAT/NAT and WRS morphologies, 24 sources are likely located within galaxy groups or clusters, or show evidence of jet reorientation. Additionally, four objects in the sample represent pairs of radio sources whose host galaxies are separated by only a few tens of kiloparsecs. Continuum spectra were constructed, and variability indices were calculated. A high proportion of sources with a variability index ($V>3$) is attributed to significant differences in angular resolution between the TXS and VCSS surveys, as well as to underestimated flux densities for certain double-lobed sources. A comparison of spectral indices derived from older and newer data indicates a decrease in the number of steep-spectrum sources in the sample. This reduction is most likely due to improved characterization of the low-frequency portion of the continuum spectra following the incorporation of GLEAM data. However, for some radio sources, the observed spectral changes may reflect internal evolution, that observed in a shift of the spectral peak or a roll-off in the radio spectrum towards lower frequencies, as evidenced by flux density measurements from various catalogues covering a period of up to forty years.

Figures (6)

• Figure 1: Examples of radio sources: a) X-shaped radio source RC J0213+0516. Near its host galaxy ($z_{sp}$=0.935) there is a quasar ($z_{sp}$=0.934), associated with a weak radio source. The distance between the galaxy and the quasar is $7.9^{\prime\prime}$ or 61 kpc; b) RC J0519+0510 is a DDRG-type radio source, which can also be classified as a rare morphological type of HSR sources.
• Figure 2: Histograms of variability index distributions: (a) $V_{340}$ (grey) and $V_{1400}$ (red); (b) $V_{3940}$ (grey) and $V_{4850}$ (red).
• Figure 3: Distributions of variability indices: (a) $V_{340}$ (gray) for sources with nearby companions, and $V_{340}^{c}$ (red), where the contribution of neighbors is accounted for; (b) $V_{340}$ (gray) and $V_{340}^{cc}$ (red) for double sources. The index $V_{340}^{cc}$ incorporates both the underestimated flux $S_{340}$ of double radio sources and the contribution of neighbors. Gaussian fits to the distributions are shown as dashed lines in black and red, respectively.
• Figure 4: Radio spectra of sources exhibiting a spectral peak: (a) RC J0133+0459, (b) RC J0907+0439, (c) RC J1100+0444. The spectra were constructed using archival data from 1996BSAO...42....5B and newly obtained measurements from the GLEAM, VCSS, RACS, and VLASS surveys (marked in red).
• Figure 5: Comparison of spectral indices $\alpha_{365}^{3940}$ calculated with linear approximation of "old" data, $\alpha_{340}^{3000}$ -- of "new" data, and $\alpha_{74}^{231}$ -- of "glm" data for radio sources of the Big Trio program. Distributions and their approximation by Gaussians for spectral indices: (a) $\alpha_{365}^{3940}$ (grey) and $\alpha_{340}^{3000}$ (red line); (b) distributions of spectral indices $\alpha_{365}^{3940}$ (grey) and $\alpha_{76}^{227}$ (red line).