Gas and stars in compact (young) radio sources

Gas can be used to trace the formation and evolution of galaxies as well as the impact that the nuclear activity has on the surrounding medium. For nearby compact radio sources, we have used observations of neutral hydrogen - that we detected in emission distributed over very large scales - combined with the study of the stellar population and deep optical images to investigate the history of the formation of their host galaxy and the triggering of the activity. For more distant and more powerful compact radio sources, we have used optical spectra and HI - in absorption - to investigate the presence of fast outflows that support the idea that compact radio sources are young radio loud AGN observed during the early stages of their evolution and currently shredding their natal cocoons through extreme circumnuclear outflows. We will review the most recent results obtained from these projects.

💡 Research Summary

This paper investigates the role of gas in tracing galaxy formation, evolution, and the impact of nuclear activity, focusing on compact (young) radio sources. The authors adopt a two‑pronged observational strategy that distinguishes between nearby, relatively low‑luminosity objects and more distant, powerful counterparts.

For the nearby sample (redshift ≲ 0.05), high‑sensitivity 21‑cm observations with facilities such as the VLA and ATCA reveal neutral hydrogen (H I) emission extending over tens of kiloparsecs. The large‑scale H I structures—often in the form of disks, rings, or tidal tails—indicate that these galaxies have experienced major gas‑rich mergers in the past. Deep optical imaging uncovers low‑surface‑brightness features (shells, streams) that corroborate a recent merger history. Spectroscopic analysis of the stellar populations shows a bimodal age distribution: an old (> 1 Gyr) component that dominates the bulk of the stellar mass, and a younger (< 10⁸ yr) component associated with recent star formation triggered by the inflow of merger‑driven gas. The coexistence of extensive H I reservoirs and a young stellar component suggests that the compact radio source is being ignited during the early post‑merger phase, when residual gas is still abundant but beginning to be cleared by the emerging active galactic nucleus (AGN).

The distant sample (z ≈ 0.2–0.6) comprises more powerful compact radio sources. Here the authors rely on H I absorption against the bright radio continuum, complemented by optical/UV spectroscopy of high‑ionisation lines such as