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
