The stellar content of low redshift radio galaxies from near-infrared spectroscopy

Context. We present medium spectral resolution near-infrared (NIR) HK-band spectra for eight low redshift (z 0.06) radio galaxies to study the NIR stellar properties of their host galaxies. The sample was selected from a radio galaxy sample imaged previously in the B- and R-band. They were found to be bluer than inactive elliptical galaxies, possibly indicating a recent star formation episode. As a homogeneous comparison sample, we used nine inactive elliptical galaxies that were observed with the same telescope and detector with similar resolution and wavelength range. Aims. The aim of the study is, by using the advantage of NIR absorption features, to compare the NIR spectral properties of radio galaxies to those of inactive early-type galaxies and, furthermore, produce the first NIR HK-band spectra for low redshift radio galaxies. Methods. For the radio galaxy and inactive elliptical samples, spectral indices of several diagnostic absorption features, namely SiI(1.589 mu m), CO(1.619 mu m) in the H-band and NaI(2.207 mu m), CaI(2.263 mu m), CO(> 2.29 mu m) in the K-band, were measured. The strength of absorption lines depends on the luminosity and/or temperature of stars and, therefore, spectral indices can be used to trace the stellar population of galaxies. To characterize the age of the populations, the measured EWs of the absorption features were fitted with the corresponding theoretical evolutionary curves of the EWs calculated by the stellar synthesis model. Results. On average, EW(CO 2.29) of radio galaxies is somewhat greater than that of inactive ellipticals. Most likely, EW(CO 2.29) is not significantly affected by dilution, thus indicating that elliptical galaxies containing AGN are in a different stage in their evolution than inactive ellipticals. This is also supported by comparing other NIR absorption line features, such as CaI and NaI, with each other. Based on the theoretical evolutionary curves of EWs, absorption features are consistent with the intermediate age stellar population, suggesting that host galaxies contain both old and intermediate age components. An intermediate age population is also consistent with previous optical spectroscopy studies, which have shown evidence of an intermediate age (similar to 2 Gyr) stellar population in radio galaxies, and also in some of the early-type galaxies. Conclusions. An intermediate stellar population component indicates that radio galaxies have experienced a star formation epoch relatively recently. The existence of an intermediate stellar population is a link between the star formation episode, possibly induced by an interaction or merging event, and triggering of nuclear activity.