Starbursts around AGN - What do we know about them?


Paper:	Starbursts around AGN - What do we know about them?
Volume:	290, Active Galactic Nuclei: from Central Engine to Host Galaxy
Page:	461
Authors:	Cid Fernandes, R.; González Delgado, R. Maria; Schmitt, H.; Strochi-Bergmann, T.; Pires Martins, L.; Heckman, T.; Leitherer, C.
Abstract:	We present a comparative study of 15 composite starburst + Seyfert 2 galaxies and 20 ``pure''/``boring'' Seyfert 2's (those where star-formation has not been detected). Composites are defined as nuclei which show the direct (but difficult-to-detect) spectroscopic signatures of the hot massive stars that power circum-nuclear starbursts: UV stellar wind lines, WR features and/or high order Balmer absorption lines. We examine a variety of more-easily-measured quantities for this sample, such as the equivalent widths of strong absorption features, continuum colors, emission-line equivalent widths, emission line ratios and profiles, far-IR luminosities, and near-UV surface brightness. Our goals are to verify whether these properties of the composite nuclei are consistent with the expected impact of a starburst, and to investigate alternative less-demanding methods to infer the presence of starbursts in Seyfert 2 nuclei, applicable to larger samples. We find that starbursts do indeed leave clear and easily quantifiable imprints on the near-UV to optical continuum and emission line properties of Seyfert 2's. Composite systems can be recognized by: (1) a strong ``Featureless Continuum'', which dilutes the Ca K line from old bulge stars to W_K < 10 Å; (2) emission lines whose equivalent widths are intermediate between Starburst galaxies and ``pure'' Seyfert 2's; (3) relatively low excitation line-ratios, which indicate that part of the gas ionization in these Seyfert 2's (typically ∼50% of Hβ) is due to photoionization by OB stars; (4) large far IR luminosities (> 10¹⁰ L_odot); (5) High near-UV surface brightness (∼10³ L_odot pc^-2). These characteristics are all consistent with the expected impact of circum-nuclear starbursts on the observed properties of Seyfert 2's. Furtheremore, they offer cheap and easy-to-aply alternative empirical diagnostics of the presence of circum-nuclear starbursts. We also find strong evidence for an evolutionary scenario, in which luminous composite systems evolve towards weak and ``pure'' Seyfert 2's. For various and technical and (mostly) historical reasons, other classes of AGN have not been carefully scrutinized for the presence of circum-nuclear starbursts, hindering a more panoramic understanding of the starburst-AGN connection. Do LINERs, transition objects and type 1 Seyferts also have circum-nuclear starbursts? We present some ``hot-off-the-dome'' observational results which answer this question. We conclude that, as a whole, the once herectic subject of the starburst-AGN connection has finally come off age. Slowly but syrely, these new evidence is helping us figure the roles of starbursts and black-holes in AGN.