Molecular hydrogen and [Fe II] in Active Galactic Nuclei

Abstract

Abstract. Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H 2 and [Fe ] lines in a sample of mostly Seyfert 1 galaxies. The spectral coverage allows simultaneous observation of the JHK bands, thus eliminating the aperture and seeing effects that have usually plagued previous works. The H 2 lines are unresolved in all objects in which they were detected while the [Fe ] lines have widths implying gas velocities of up to 650 km s −1 . This suggests that, very likely, the H 2 and [Fe ] emission does not originate from the same parcel of gas. Molecular H 2 lines were detected in 90% of the sample, including PG objects, indicating detectable amounts of molecular material even in objects with low levels of circumnuclear starburst activity. Analysis of the observations favors thermal excitation mechanisms for the H 2 lines. Indeed, in NGC 3227, Mrk 766, NGC 4051 and NGC 4151, the molecular emission is found to be purely thermal but with heating processes that vary between the objects. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects for which data were available. [Fe ] lines are detected in all of the sample AGN. The [Fe ] 1.254 µm/Paβ ratio is compatible with excitation of the [Fe ] lines by the active nucleus in most Seyfert 1 galaxies, but in Mrk 766 the ratio implies a stellar origin. A correlation between H 2 /Brγ and [Fe ]/Paβ is found for our sample objects supplemented by data from the literature. The correlation of these line ratios is a useful diagnostic tool in the NIR to separate emitting line objects by their level of nuclear activity. X-ray excitation models are able to explain the observed H 2 and part of the [Fe ] emission but fail to explain the observations in Seyfert 2 galaxies. Most likely, a combination of X-ray heating, shocks driven by the radio jet and circumnuclear star formation contributes, in different proportions, to the H 2 and [Fe ] lines observed. In most of our sample objects, the [Fe ] 1.257 µm/1.644 µm ratio is found to be 30% lower than the intrinsic value based on current atomic data. This implies either that the extinction towards the [Fe ]-emitting clouds is very similar in most objects or there are possible inaccuracies in the A-values in the Fe  transitions.

VLT-SINFONI integral field spectroscopy of low-zluminous and ultraluminous infrared galaxies

“…While low H 2 1−0S(1)/Brγ values appear to be characteristic of starbursts (H 2 1−0S(1)/Brγ < ∼ 0.6), classical Seyfert 1 and 2 galaxies also display a range of values (Rodríguez-Ardila et al 2004, 2005Riffel et al 2010) that are compatible with those measured in our sample. The [FeII]/Brγ ratio also shows values compatible with those observed in starbursts, although higher values would have been expected for Seyfert galaxies, such as NGC 5135 and NGC 7130 (Rodríguez-Ardila et al 2004;Riffel et al 2010;Valencia-S et al 2012). These galaxies show the highest values of the [FeII]/Brγ ratio of the whole sample, close to ∼2.0, and are similar to values reported for other Seyfert 2 galaxies (Blietz et al 1994).…”

Understanding the two-dimensional ionization structure in luminous infrared galaxies

“…Larkin and coworkers reported a strong linear relation in the (log−log) [FeII]1.26 µm/Paβ−H 2 2.12 µm/Brγ plane, with starforming galaxies showing the lowest ratios, Seyferts with intermediate ratios, and LINERs with the highest ratios. Later studies of Seyfert 1 and Seyfert 2 galaxies (Rodríguez-Ardila et al 2004Riffel et al 2006) confirmed the trend first reported by Larkin and coworkers. A more recent analysis on a larger compilation of Seyfert 1 and 2 galaxies, including star-forming galaxies and a few LINERs (Riffel et al 2013a), have shown that activity in galaxies seems to follow a unique linear relation log ([FeII]1.26 µm/Paβ) = 0.749 × log(H 2 2.12 µm/Brγ) − 0.207, with star-forming galaxies showing the lowest ratios and LINERs apparently the largest.…”

Nearby AGN and their hosts in the near infrared

“…Reunanen et al (2002) present NIR-spectroscopy of two Seyfert 1, three Seyfert 2 and one Seyfert 1.5 and show that in Seyfert 2 galaxies Fe II is generally stronger than Brγ or H 2 lines, while apparently in Seyfert 1 galaxies Brγ is stronger. Rodríguez-Ardila et al (2004) also studied H 2 and Fe II line emission in a sample of 22 (mostly) Seyfert galaxies and concluded that these lines originate in different regions due to systematically different linewidths. They detected molecular hydrogen in 90% of their sources, with the majority being excited by thermal excitation processes.…”