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Astron. Astrophys. 350, 9-16 (1999)

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Starbursts in active galaxy nuclei: observational constraints from IR stellar absorption lines *

E. Oliva 1, L. Origlia 2, R. Maiolino 1 and A.F.M. Moorwood 3

1 Osservatorio di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
2 Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy
3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85478 Garching, Germany

Received 14 April 1999 / Accepted 19 July 1999


High quality infrared spectra of active galaxies including the stellar absorption features of Si at 1.59 µm, CO(6,3) at 1.62 µm, and CO(2,0) at 2.29 µm are used to measure the stellar mass to light ratio at 1.65 µm ([FORMULA]) and investigate the occurrence of circum-nuclear starbursts. We find that old and powerful starbursts are relatively common in obscured AGNs (5 objects out of 13) while absent in genuine Seyfert 1's (0 objects out of 8).

The data are also used to derive the non-stellar continuum which is very red and compatible with emission from warm ([FORMULA]1000 K) dust even in bare Sy1's, thus indicating that the near IR nuclear continuum is reprocessed radiation. Hot dust ([FORMULA]800 K) emission is also detected in a few obscured AGNs, including the Seyfert 2 prototype NGC1068. The observed non-stellar flux is too high to be accounted for by scattered light and therefore indicates that the material obscuring the AGN must have a quite small ([FORMULA]1 pc) projected size.

Key words: galaxies: stellar content – galaxies: starburst – galaxies: Seyfert – galaxies: active

* Based on observations collected at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: E. Oliva

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© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999