A correlation between the stellar and [Fe II] velocity dispersions in active galaxies

Abstract

We use near-infrared (near-IR) spectroscopic data from the inner few hundred parsecs of a sample of 47 active galaxies to investigate possible correlations between the stellar velocity dispersion (σ ), obtained from the fit of the K-band CO stellar absorption bands, and the gas velocity dispersion (σ), obtained from the fit of the emission-line profiles of [S III] λ0.953 μm, [Fe II] λ1.257 μm, [Fe II] λ1.644 μm and H2 λ2.122 μm. While no correlations with σ were found for H2 and [S III], a good correlation was found for the two [Fe II] emission lines, expressed by the linear fit σ = 95.4 ± 16.1 + (0.25 ± 0.08) × σ[Fe II]. Excluding barred objects from the sample, a better correlation is found between σ and σ[Fe II], with a correlation coefficient of R = 0.80 and fitted by the following relation: σ = 57.9 ± 23.5 + (0.42 ± 0.10) × σ[Fe II]. This correlation can be used to estimate σ in cases where it cannot be directly measured and the [Fe II] emission lines are present in the spectra, allowing us to obtain the mass of the supermassive black hole (SMBH) from the M•–σ relation. The scatter from a one-to-one relationship between σ and its value derived from σ[Fe II] using the equation above for our sample is 0.07 dex, which is smaller than that obtained in previous studies which use σ[O III] in the optical as a proxy for σ . The use of σ[Fe II] in the near-IR instead of σ[O III] in the optical is a valuable option for cases in which optical spectra are not available or are obscured, as in the case of many active galactic nuclei. The comparison between the SMBH masses obtained using the M•–σ relation in which σ was directly measured with those derived from σ[Fe II] reveals only a small average difference of ΔlogM• = 0.02 with a scatter of 0.32 dex for the complete sample and ΔlogM• = 0.00 with a scatter of 0.28 dex for a subsample excluding barred galaxies.