HI on large and small scales in starburst radio galaxies

The study of the optical continuum of radio galaxies shows that about 30% have a young stellar population component. Among them are the most far-IR bright radio galaxies. A further indication of the relatively gas rich environment of these galaxies (possibly related to the recent merger from which they originate) is the high fraction being detected in H I. Here we present recent results obtained from the study of neutral hydrogen (detected either in emission or absorption) in a group of starburst radio galaxies. In some objects, large-scale (tens of kpc) structures involving H I masses exceeding 10 M⊙ are observed. In these cases, the H I can be used to study the origin and evolution of these systems and the timescales involved. In this respect, the parameters obtained from the study of the stellar populations and from the H I can be complementary. In other objects, very broad ( > ∼ 1000 km/s), mostly blueshifted H I is detected in absorption. This result shows that, despite the extremely energetic phenomena occurring near an AGN including the powerful radio jet some of the outflowing gas remains, or becomes again, neutral. This can give new and important insights in the physical conditions of the gaseous medium around an AGN. The possible origin of the extreme kinematics is discussed. 1. Preamble: Starburst radio galaxies What we will call starburst radio galaxies in this brief review are radio sources hosted by galaxies that spectroscopically show a young stellar population component in addition to the old component typical of elliptical galaxies. These