Abstract
Remnant radio galaxies (RRGs), characterized by the cessation of AGN activity, represent a short-lived last phase of radio galaxy’s life-cycle. Hitherto, searches for RRGs, mainly based on the morphological criteria, have identified large angular size sources resulting into a bias towards the remnants of powerful FR-II radio galaxies. In this study we make the first attempt to perform a systematic search for RRGs of small angular sizes (<30″) in the XMM–LSS field. By using spectral curvature criterion we discover 48 remnant candidates exhibiting strong spectral curvature i.e., α150MHz325MHz−α325MHz1.4GHz ≥ 0.5. Spectral characteristics at higher frequency regime (>1.4 GHz) indicate that some of our remnant candidates can depict recurrent AGN activity with an active core. We place an upper limit on the remnant fraction (frem) to be 3.9%, which increases to 5.4% if flux cutoff limit of S150MHz ≥ 10 mJy is considered. Our study unveils, hitherto unexplored, a new population of small-size (<200 kpc) remnant candidates that are often found to reside in less dense environments and at higher redshifts (z) > 1.0. We speculate that a relatively shorter active phase and/or low jet power can be plausible reasons for the small size of remnant candidates.
Highlights
Radio galaxies, a subclass of Active Galactic Nuclei (AGN), emit copiously at radio wavelengths and exhibit well defined radio structures—a radio core, highly collimated outflowing bipolar jets eventually terminating into radio lobes
To identify radio sources showing curved radio spectrum we examine the difference between low-frequency and high-frequency spectral indices i.e., ∆α = αlow − αhigh
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Summary
A subclass of Active Galactic Nuclei (AGN), emit copiously at radio wavelengths and exhibit well defined radio structures—a radio core, highly collimated outflowing bipolar jets eventually terminating into radio lobes. According to the evolutionary models infancy phase of a radio galaxy can be depicted by compact sources with Linear-Angular-Size (LAS) less than a kpc Based on their spectral characteristics these sources are known as High-Frequency-Peakers (HFP). The cutoff limit placed on the angular size introduces a bias towards large and powerful radio galaxies In their sample of RRGs with LAS ≥ 6000 [8] found that all RRGs show double-lobe morphology with high radio luminosity (L150 MHz > 1025 W Hz−1 ) suggesting their progenitors to be powerful FR-II radio galaxies. To identify RRG candidates and their host galaxies we use deep multi-frequency radio and optical data available in the XMM−LSS field. In following subsections we provide brief details of the surveys that are used in our study
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