Star Formation in Active Galaxies: A Spectroscopic Perspective

Abstract

I review the relationship between star formation and black hole building, based on spectroscopic observations of the stellar population in active galactic nuclei (AGNs) and their host galaxies. My emphasis is on large, well-defined local samples of AGNs, whose optical continua are dominated by starlight. I summarize the spectroscopic tools used to characterize their stellar content. Surveys of the nuclei of the nearest galaxies show that the stellar population in most low-power AGNs is predominantly old. In contrast, young ($< 1$ Gyr) stars are detected in about half of the powerful type 2 Seyfert nuclei. I summarize Sloan Digital Sky Survey spectroscopy of the host galaxies of 22,000 AGNs. The AGN phenomenon is commonplace only in galaxies with high mass, high velocity dispersion, and high stellar surface mass density. Most normal galaxies with these properties have old stellar populations. However, the hosts of powerful AGNs (Seyfert 2s) have young stellar populations. The fraction of AGN hosts that have recently undergone a major starburst also increases with AGN luminosity. A powerful AGN presumably requires a massive black hole (host with a substantial bulge) plus an abundant fuel supply (a star-forming ISM). This combination is rare today, but would have been far more common at early epochs.