Powerful Activity in Galaxies - the Wonder Years (aka Generation Alpha)
Powerful extragalactic radio sources hosted in galaxies come in all shapes, sizes, powers, and presumably ages. What might a young or nascent powerful radio source look like? The class of compact powerful extragalactic radio sources are defined to have linear sizes < 20 kpc and be sub-galactic in scale. The smallest of these have uniquely peaked radio spectra. These sources might be very young radio galaxies which will evolve into the traditional large radio galaxies with jets and lobes reaching to 100s of kpc to Mpc beyond the host galaxy. Or, they might simply be compact because they are confined (and enhanced in radio power) by interaction with particularly dense gas in their environments which make it difficult for the jets to escape. Alternately, the compact sources might be transient or intermittent sources, which turn on and off with a quicker cadence. Then again, different of these hypotheses may apply to individual objects or at different stages in a given object's evolution. The relative number in each population, and the accompany lifetimes, have significant implications for understanding the life cycles of activity in galaxies. Proper motion studies over long time baselines have helped determine hotspot speeds for over three dozen sources and establish that some objects are indeed young. Multi-frequency observations have demonstrated that many compact sources are indeed embedded in a dense interstellar medium and vigorously interacting with it. The detection of emission line gas aligned with the radio source, and blue-shifted HI absorption and [OIII] emission lines indicates that AGN feedback is present in these objects and is likely critical to the evolution of activity and the host galaxy supporting the AGN feedback model.