Tailed Radio Galaxies from the TIFR GMRT Sky Survey

Abstract: We present a list of candidate tailed radio galaxies using the TIFR GMRT Sky Survey Alternative Data Release 1 (TGSS ADR1) at 150 MHz. We have visually examined 5336 image fields and found 264 candidates for the tailed galaxy. Tailed radio galaxies are classified as Wide Angle Tailed (WAT) galaxies and Narrow-Angle Tailed (NAT) galaxies, based on the angle between the two jets of the galaxy. We found a sample of tailed radio galaxies which includes 203 WAT' and 61NAT' type sources. These newly identified tailed sources are significant additions to the list of known tailed radio galaxies. The source morphology and luminosity features of the different candidate galaxies and their optical identifications are presented in the paper. Other radio properties and general features of the sources are also discussed.

• The paper systematically identifies 264 tailed radio galaxies—203 WATs and 61 NATs—using explicit, morphology-driven criteria from TGSS ADR1.
• It employs multi-wavelength cross-associations and spectral index measurements to robustly classify and confirm host galaxies.
• Results indicate distinct cluster-centric distributions and AGN-ICM interactions, establishing tailed sources as effective signposts for galaxy clusters.

An Expert Overview of "Tailed Radio Galaxies from the TIFR GMRT Sky Survey" (2110.05484)

Introduction and Context

This work provides a systematic identification and analysis of tailed radio galaxies using the 150 MHz TIFR GMRT Sky Survey Alternative Data Release 1 (TGSS ADR1). Tailed radio galaxies are a prominent class of AGN where dual bent jets ("tails") originate from the core, subdivided morphologically into wide-angle-tailed (WAT; $90^\circ < \theta \leq 180^\circ$) and narrow-angle-tailed (NAT; $\theta \leq 90^\circ$) galaxies, with the classification fundamentally based on the opening angle between bent jets. Such sources probe cluster dynamics, the intracluster medium (ICM), and galaxy evolution processes.

The paper identifies 264 candidate tailed radio galaxies via manual examination of 5336 TGSS fields, reporting 203 WATs and 61 NATs. This is the largest, uniform low-frequency sample of its kind to date. The work includes an extensive morphological, photometric, and spectroscopic analysis, emphasizing comparative features and environmental associations.

Methodology

The candidate selection is based on explicit, morphologically-driven criteria: radio sources with two-jet structures exhibiting significant, symmetric bending are catalogued as tailed candidates. Confirmation involves:

• Flat spectral index in the compact "head" region to ascertain core location.
• Presence of "warmspots" near the core exceeding the tail flux, demarcating the jet-ICM interface.
• Measurement of the opening angle, with WATs and NATs distinguished via a quantitative threshold.
• Angular extension above the TGSS synthesized beam.

Optical and infrared counterpart identification utilizes likelihood ratio cross-association with Pan-STARRS1 and WISE, consolidating deep multiwavelength data for robust host galaxy matching. Redshift information is compiled from SDSS, 2MASS, and the literature, with positional accuracy and false-match probabilities rigorously controlled.

Main Results

Numerical Highlights:

• Number of New Identifications: 264 (203 WAT, 61 NAT), with optical/IR counterparts for 261 and redshift data for 112 sources.
• Flux and Spectral Properties: Integrated 150 MHz flux densities range 0.1–20.1 Jy; most sources have steep spectra ($\alpha_{150}^{1400} > 0.5$); spectral indices peak near 0.75 for both classes, encompassing core-jet and lobe-dominated systems.
• Redshift and Luminosity: Redshifts span $0.01–0.68$; radio luminosities at 150 MHz range up to $1.9 \times 10^{27}$ W Hz$^{-1}$ for WATs, consistent with transition between FR I and FR II classes.

Significant Claims:

• Only about half the sources are associated with known clusters; a substantial fraction lack known overdensities, suggesting potential for tracking poorly studied galaxy concentrations.
• The cluster association analysis shows WATs preferentially located near cluster centers (<250 kpc), while NATs are more peripherally distributed, in agreement with ram pressure and ICM-dynamics models.
• No evidence is found for systematic distinctions between the radio luminosity distributions of WATs and NATs, suggesting comparable nuclear engine conditions.

Implications

The enlarged catalog of bent radio galaxies at low frequencies yields direct implications for studies of cluster finding, environment-driven AGN feedback, and radio-mode AGN physics:

• Cluster Tracing and Large-Scale Structure: The numerous tailed galaxies unassociated with known clusters suggest these radio morphologies are highly efficient signposts for cluster or group-scale overdensities, particularly valuable for high-redshift cluster searches.
• AGN/ICM Interactions: The observed distribution of bending angles and cluster-centric radii aligns with hydrodynamical models where ram pressure (NATs) and cluster weather/ICM wind/electromagnetic forces (WATs) shape jet morphology.
• Spectral and Population Studies: Uniform low-frequency data facilitate unbiased statistical studies, useful for constraining the energetics and lifetimes of jets, spectral aging models, and ICM density/magnetic field reconstructions.

The work affirms that tailed morphology is present across a wide range of cluster masses (down to a few $10^{14} M_\odot$), consistent with simulations predicting suppression of tailed sources in lower-mass clusters, but not an exclusive high-mass phenomenon.

Theoretical and Practical Perspectives

This catalog, coupled with the high sensitivity and resolution of TGSS ADR1, offers a robust framework for future population studies, simulation calibrations (jet/ICM models), and automated detection pipelines combining morphology, spectral, and multi-wavelength association. The findings refine constraints on both the environmental (e.g., ICM wind, cluster weather) and nuclear (jet power, intermittency) drivers of radio galaxy morphology. The increased sample size further enhances the statistical basis for probing the evolution of radio AGN and the connection between radio-mode activity and the cosmic web.

Future Directions

Key avenues include:

• Deep Multi-Frequency Follow-Up: To map total spectral curvature, spectral aging, and probe possible episodic AGN activity.
• Cluster and Group Identification: Cross-correlation with deep X-ray/SZ/IR datasets to unveil the cluster environments for the numerous sources lacking known associations.
• Automated Source Identification: Application and benchmarking of advanced pattern recognition (e.g., deep learning methodologies) for morphologically complex sources informed by this expanded manual selection.
• Statistical Modeling: Incorporating these data into cosmological simulations of AGN feedback and cluster weather to refine models of radio-loud AGN population demographics.

Conclusion

This paper establishes a comprehensive catalog of 264 tailed radio galaxies using the TGSS ADR1, effectively doubling the population available for low-frequency morphological AGN studies. The uniformity and depth enable exploration of the impact of the ICM and environment on radio galaxy evolution, and offer new insight into the statistical properties of bent-tail AGN as cosmic tracers. The results underscore the significance of low-frequency radio surveys for identifying AGN-ICM interplay and mapping large-scale structure. The presented sample is poised to inform and stimulate multi-wavelength and simulation-driven investigations of galaxy clusters and radio AGN evolution.