CosmoDRAGoN III: Shaping the Afterlife -- How Progenitors and Environments Sculpt Radio Galaxy Remnants

TL;DR

This study uses 3D hydrodynamic simulations to explore how progenitor properties and environments influence the morphology and spectral evolution of radio galaxy remnants, highlighting observational biases and detection prospects.

Contribution

It provides detailed simulation-based insights into the morphological diversity and spectral evolution of radio galaxy remnants across different environments.

Findings

Remnant morphology varies from amorphous to double-lobed.

Surface brightness depends strongly on environment, affecting detectability.

Spectral indices follow a consistent evolution sequence with curvature and ultra-steep phases.

Abstract

Identifying remnant radio-loud active galactic nuclei (AGNs) is challenging due to their diverse morphological and spectral characteristics. Using three-dimensional hydrodynamic simulations of 15 radio galaxies, we investigate how the spectral evolution of remnants depends on progenitor power, active lifetime, environment, and underlying dynamics. The simulations span low-density group and high-density cluster environments re-gridded from smooth-particle-hydrodynamic cosmological simulations. The resulting remnants exhibit a wide range of morphologies, from amorphous structures to double-lobed forms. We find that jet power correlates with the spectral slope. As the remnant lobes evolve, we find surface brightness depends strongly on environment: group remnants are systematically dimmer and more amorphous than cluster remnants, highlighting a potential observational bias against these low-surface-brightness sources. In our models, we estimate that the peak surface brightness of a low-redshift, 50 Myr-old remnant from a low-power progenitor in a 10^{13} M_sun group environment should be routinely detectable at the 3{\sigma} level with LOFAR, although 20-30% of the emission would remain undetectable within a reasonable integration time. We find young remnants exhibit low-frequency (150-1400 MHz) spectral indices that overlap with active sources, and follow a consistent and established spectral-evolution sequence: significant curvature ({\alpha}_{1400}^{6000} - {\alpha}_{150}^{1400} > 0.5) develops before an ultra-steep low-frequency index ({\alpha}_{150}^{1400} > 1.2). The results presented in this work are intended as a reference point for current and upcoming low-frequency studies of radio remnants.