Co-authors' Institutions: IF/UFRGS; LNA/MCT University of Oxford

Abstract:

It is widely known that circumnuclear star formation is commonly detected in active galactic nuclei (AGN). In fact, in the last years, increasing observational evidence has confirmed that nuclear/circumnuclear starbursts coexists in objects harboring an AGN, giving support to the so called AGN-SB connection. Additional support to this hypothesis comes from recent investigations of nearby AGN, where it was found that a significant fraction of their emitted energy might be provided by starburst activity. In the other extreme, it has been found that some SB galaxies display no signs of nuclear activity in the optical domain but reveals compact AGN cores at other wavelengths, either in X-rays or in the near or mid-infrared, where the effects of extinction is low or negligible. Stellar absorption features in the infrared are widely believed to provide a way for recognizing red supergiants, they arise as prime indicators for tracing starburst in galaxies. In this line of thought, the evolutionary population synthesis calculation recently presented by Maraston, 2005 (MNRAS, 362,799) foresees the presence of molecular features like CH, CN and C_2. They arise in young/intermediate stellar populations and their spectral signatures are particularly enhanced in the NIR. These molecules are produced using the residual carbon available, after binding all the oxygen into CO molecules. Of particular importance are the CN bands, which arise according to the models, from stars with ages in the range between 0.3 and 2 Gyrs and are attributed to stars in the thermally pulsing asymptotic giant branch (TP-AGB) phase. Here we present the first detection of the near-infrared CN absorption band in the nuclear spectra of an active galactic nuclei (AGN). This feature is a starburst activity tracer. We measure the equivalent width (W) of the 1.1 microns CN band. The values of the W of these band are between 0.019 mag and 0.085 mag. The W of the CN line is weakly correlated with that of the bluer line of the 2.3 microns CO band. The presence of the 1.1 microns CN band in the spectra of the sources is taken as an unambiguous evidence of the presence of young/intermediate stellar populations close to the central source of the AGN.