“O ELO PERDIDO NA EVOLUÇÃO DE RÁDIO-GALÁXIAS: INVESTIGANDO EMISSÕES ESTENDIDAS EM BAIXA FREQUÊNCIA”

Abstract

Active galactic nuclei (AGNs) with jets are morphologically classified according to their evolutionary stage, with Gigahertz Peaked-Spectrum (GPS) and Compact SteepSpectrum (CSS) sources traditionally understood as representing the young and confined phase of jets. However, the paradigm of a linear evolution has been challenged by an abundance of compact sources and the discovery of objects featuring a young nucleus embedded within a structure related to extended and aged emission, indicating multiple episodes of activity (“restarted” jets). This work aims to re-evaluate a sample of 84 sources previously classified as GPS/CSS by leveraging the unparalleled sensitivity of the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz to detect extended, low-surface-brightness emission undetectable in surveys above 1 GHz. We combined multi-frequency data (from 144 MHz to 4.85 GHz) to perform a comparative morphological analysis and construct Spectral Energy Distributions (SEDs). Our results show that 9 sources (17.3%) display clear morphological evidence of extended emission, being classified as restarted. Another 9 sources (17.3%) are classified as restarted jet candidates, and 28 (53.8%) remain compact. Spectral analysis reveals a diversity of classes (GPS, CSS, HFP, and spectra without peaks or flattened at low frequencies), with a tendency for restarted sources to have steeper (CSS-like) spectra. We conclude that a significant fraction of compact sources are, in fact, restarted jets, suggesting that jet activity is a cyclic and intermittent process. This result has profound implications for evolutionary models of radio galaxies and for the understanding of feedback exerted by multiple episodes of activity