The first detection of near-infrared CN bands in active galactic nuclei: signature of star formation

We present the first detection of the near-infrared CN absorption band in the nuclear spectra of active galactic nuclei (AGNs). 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 μm as expected in stellar populations (SPs) with ages between ~0.2 and ~2 Gyr. The presence of the 1.1 μm CN band in the spectra of the sources is taken as unambiguous evidence of the presence of young/intermediate SPs close to the central source of the AGNs. Near-infrared bands can be powerful age indicators for star formation connected to AGNs, the understanding of which is crucial in the context of galaxy formation and AGN feedback.