Radio spectral properties and aging of two tailed radio galaxies in a galaxy group at z=0.35

Vollständiger Abstract

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We present a study of two tailed radio galaxies located in the core of a massive, dynamically young galaxy group, previously identified as an early group-group merger. Using broadband VLA 3 GHz and 1.4 GHz, MeerKAT 1.35 GHz, and GMRT 610 MHz and 325 MHz observations, we investigate the radio spectral properties as well as the spectral and dynamical ages of both sources. The radio morphologies show clear evidence of an interaction with the intragroup medium (IGM). One galaxy is a wide-angle tail (WAT) source with well-collimated, megaparsec-scale bent jets terminating in bright lobes, while the other is most likely a head–tail (HT) galaxy characterized by a compact core and a mostly straight tail that bends slightly near its end. Both galaxies exhibit high radio luminosities across all five frequencies, and we find spectral indices of α=0.8± 0.1 for the WAT and α=0.6± 0.2 for the HT galaxy. Spatially resolved spectral index maps and spectral analysis of individual galaxy components reveal steepening of the radio spectrum with increasing distance from the core in both galaxies, with localized spectral flattening in the WAT lobes and hotspots along the northern jet, and indications of such flattening in the middle of the HT tail. Spectral ages derived using the Jaffe-Perola model are 33.80 -7.23 Myr for the WAT and 20.86 -17.17 Myr for the HT, which are significantly lower than the corresponding dynamical age estimates of 420 to 700 Myr (for WAT) and 140 Myr (and possibly up to 280 Myr, for HT), yielding dynamical-to-spectral age ratios of sim12-20 and sim7 (and possibly up to sim14), respectively. The discrepancy may be reduced in future studies through the use of more complex dynamical aging models that incorporate interactions between the radio lobes and the IGM, which require deeper X-ray observations of the group. Spectral age estimates may also be affected by the mixing of electron populations, and they could be better constrained with future deep, high-resolution broadband radio observations of this system at both megahertz and frequencies above 3 GHz. The combination of extended radio structures, spectral signatures of radiative aging with localized reacceleration, and activity timescales of tens to hundreds of megayears indicates that both galaxies are actively interacting with, and likely mechanically depositing energy into their environment.