THE FIRST DETECTION OF THE NEAR INFRARED CN BAND IN ACTIVE GALACTIC NUCLEI Rogério Riffel IF/UFRGS Miriani G. Pastoriza IF/UFRGS Alberto Rodríguez Ardila LNA Claudia Maraston University of Oxford
ABSTRACT We present the first detection of the near infrared CN absorption band in the nuclear spectra of active galactic nuclei (AGN). This feature is a recent star formation tracer, being particularly strong in carbon stars. The equivalent width of the CN line correlates with that of the CO at 2.3 m, as expected in stellar populations with ages between ~0.2 and ~2 Gyrs. The presence of the 1.1 m 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. Near infrared bands can be powerful age indicators for star formation connected to AGN, the understanding of which is crucial in the context of galaxy formation and AGN feedback.
INTRODUCTION It is widely known that circumnuclear star formation is commonly detected in 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. 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 of Maraston (2005) 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. Of particular importance are the CN bands, which arise according to the models, from stars with ages in the range between 0.2 and 2 Gyr and are attributed to stars in the thermally pulsing asymptotic giant branch (TP AGB) phase. Here we present the first detection of the nearinfrared CN absorption band in the nuclear spectra of AGNs.
THE DATA The Galaxies chosen for this work are the ones presented by Riffel et al. (2006); The spectra were obtained at the NASA Infrared Telescope Facility, using the 3 m infrared optimized telescope; The SpeX spectrograph was used in the short cross dispersed mode; A 0.8 x 15 slit was employed giving a spectral resolution of 360 Km/s;
Zoom of the 1.1 µm CN and 2.2 µm CO bands.
THE STRENGTHS OF THE CN BAND We have modeled the emission lines that affect the CN band; Pa width and flux was constrained to those of Pa, using the calculations of Hummer & Storey (1987). For the He I emission line we have adjusted a Gaussian to the emission line profile; The model was subtracted Model for the emission lines around the CN band and the band free from emission lines. from the spectra.
OBSERVATIONS VS MODELS CN equivalent widths vs CO equivalent widths. Observations vs models. The observed values are indeed consistent with the models prediction; The strengths of two bands are correlated; The CO band is also present in the TP AGB spectra, explaining the correlation between the W of CN and CO; The galaxies can be separated into two main groups according to their chemical composition.
Final Remarks We present the first detection of the 1.1 m CN band in the inner 300 pc of an AGN; The presence of such band can be taken as an unambiguous evidence of stellar populations with ages in the range of ~0.2 to ~2 Gyr; We have modeled the emission line spectrum in order to quantify properly the strengths of the NIR CN absorption band; We find a nice consistency of the observed and predicted values for ages around 1 Gyr;