The warm molecular gas and dust of Seyfert galaxies: two different phases of accretion?

TL;DR

This study images warm molecular gas in Seyfert galaxy centers, revealing two distinct accretion phases: colder dust filaments and warmer inner gas, with X-ray heating as the primary excitation mechanism.

Contribution

It provides high-resolution imaging of warm molecular gas, demonstrating the existence of two different accretion phases and their distinct morphologies in Seyfert galaxies.

Findings

Warm H$_2$ gas is concentrated within 100 pc of the nucleus.

Dust filaments and warm gas do not always spatially correlate.

X-ray heating is the dominant excitation mechanism for the H$_2$ gas.

Abstract

The distribution of warm molecular gas (1000--3000 K), traced by the near-IR H$_2$ 2.12 $\mu$m line, has been imaged with a resolution $<0.5$ arcsec in the central 1 kpc of seven nearby Seyfert galaxies. We find that this gas is highly concentrated towards the central 100 pc and that its morphology is often symmetrical. Lanes of warm H$_2$ gas are observed only in three cases (NGC\,1068, NGC\,1386 and Circinus) for which the morphology is much wider and extended than the dust filaments. We conclude that there is no one-to-one correlation between dust and warm gas. This indicates that, if the dust filaments and lanes of warm gas are radial streaming motions of fuelling material, they must represent \textit{two different phases of accretion}: the dust filaments represent a colder phase than the gas close to the nucleus (within $\sim$100 pc). We predict that the morphology of the nuclear dust at these scales should resemble that of the cold molecular gas (e.g. CO at 10--40 K), as we show for CenA and NGC\,1566 by Atacama Large Millimeter/submillimeter Array (ALMA) observations, whereas the inner H$_2$ gas traces a much warmer phase of material identified with warmer (40-500 K) molecular gas such as CO(6-5) or HCN (as shown by ALMA for NGC\,1068 and NGC\,1097). We also find that X-ray heating is the most likely dominant excitation mechanism of the H$_{2}$ gas for most sources.