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

We present medium spectral resolution near-infrared (NIR) HK-band spectra for 8 low redshift (z<0.06) radio galaxies to study the NIR stellar properties of their host galaxies. As a homogeneous comparison sample, we used 9 inactive elliptical galaxies that were observed with similar resolution and wavelength range. The aim of the study is 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. For both samples spectral indices of several diagnostic absorption features, SiI(1.589microns), CO(1.619microns), NaI(2.207microns), CaI(2.263microns), CO(>2.29microns), were measured. 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. 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, and 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 features, such as CaI and NaI, with each other. Absorption features are consistent with the intermediate age stellar population, suggesting that host galaxies contain both an old and intermediate age components. It is consistent with previous optical spectroscopy studies which have shown evidence on the intermediate age (~2 Gyr) stellar population of radio galaxies, and also in some of the early-type galaxies. The existence of intermediate age population is a link between the star formation episode, possibly induced by interaction or merging event, and the triggering of the nuclear activity.

💡 Research Summary

The authors present medium‑resolution (R ≈ 2000) near‑infrared (NIR) HK‑band spectra of eight low‑redshift (z < 0.06) radio galaxies (RGs) and compare them with a control sample of nine inactive elliptical galaxies observed with the same instrumental setup. The primary goal is to assess whether the stellar populations of RG hosts differ from those of quiescent early‑type galaxies and to provide the first homogeneous NIR spectral library for low‑z RGs.

Key observational steps: the spectra cover 1.5–2.4 µm, encompassing several well‑studied absorption diagnostics – Si I 1.589 µm, CO 1.619 µm, Na I 2.207 µm, Ca I 2.263 µm, and the CO bandhead longward of 2.29 µm. Equivalent widths (EWs) of these features were measured after careful continuum placement and correction for possible AGN continuum dilution.

To interpret the EWs, the authors employed stellar population synthesis models (e.g., Maraston 2005, Bruzual & Charlot 2003) that predict the evolution of each index as a function of age and metallicity. By fitting the observed EW sets to the model curves, they derived characteristic ages for the dominant stellar components.

The most striking result is that the CO (2.29 µm) bandhead in RGs is on average stronger than in the inactive ellipticals. Because the CO 2.29 feature is relatively insensitive to non‑stellar dilution, the authors argue that this excess reflects a genuine difference in the underlying stellar content. The CO strength, together with the relative behavior of Na I and Ca I, points to a significant contribution from intermediate‑age (∼1–3 Gyr) stars, most likely thermally pulsing asymptotic giant branch (TP‑AGB) or red supergiant populations.

The inferred stellar mix for RG hosts consists of an old (> 8 Gyr) component plus a ∼20–30 % intermediate‑age component. In contrast, the inactive ellipticals are dominated by the old population, with only a minor intermediate‑age fraction. This dichotomy aligns with previous optical studies that reported Balmer‑line enhancements and weak metal lines in many RGs, both signatures of a post‑starburst population.

The authors discuss possible evolutionary scenarios. A plausible picture is that a gas‑rich interaction or merger triggers a burst of star formation a few gigayears ago, leaving behind the intermediate‑age population. The same event may also funnel gas toward the nucleus, eventually igniting the radio‑loud AGN. The NIR diagnostics, being less affected by dust extinction than optical lines, provide a robust confirmation of this sequence.

Methodologically, the paper demonstrates the utility of NIR EW diagnostics for disentangling stellar ages in galaxies with active nuclei, where optical indices can be heavily contaminated by emission‑line filling or dust obscuration. The study also establishes a baseline NIR spectral library for low‑z RGs that can be used for future work on higher‑redshift objects, where rest‑frame NIR moves into the observed optical/IR windows.

In summary, the work shows that low‑redshift radio‑loud ellipticals are not simply “old, dead” systems; they frequently host a measurable intermediate‑age stellar component, likely linked to the same dynamical events that triggered their nuclear activity. This reinforces the growing consensus that AGN activity and recent star formation are often co‑eval, especially in massive early‑type hosts.