Warm molecular gas temperature distribution in six local infrared bright Seyfert galaxies

TL;DR

This study analyzes the temperature distribution of warm molecular gas in six local infrared bright Seyfert galaxies using combined CO and H2 spectral data, revealing different excitation mechanisms and effects of galaxy interactions.

Contribution

It introduces a unified model with a broken power-law temperature distribution that fits both CO and H2 SLEDs in Seyfert galaxies, highlighting the roles of shocks and PDRs.

Findings

Shallower temperature distributions in mergers suggest higher shock contributions.

A broken power-law model effectively reproduces CO and H2 SLEDs.

Active galactic nuclei may influence molecular gas temperatures.

Abstract

We simultaneously analyze the spectral line energy distributions (SLEDs) of CO and H2 of six local luminous infrared (IR) Seyfert galaxies. For the CO SLEDs, we used new Herschel/SPIRE FTS data (from J=4-3 to J=13-12) and ground-based observations for the lower-J CO transitions. The H2 SLEDs were constructed using archival mid-IR Spitzer/IRS and near-IR VLT/SINFONI data for the rotational and ro-vibrational H2 transitions, respectively. In total, the SLEDs contain 26 transitions with upper level energies between 5 and 15000 K. A single, constant density, model (n$_{H_2}$ ~ 10$^{4.5-6}$ cm$^{-3}$) with a broken power-law temperature distribution reproduces well both the CO and H2 SLEDs. The power-law indices are $\beta_1$ ~ 1-3 for warm molecular gas (20 K < T < 100 K) and $\beta_2$ ~ 4-5 for hot molecular gas (T > 100 K). We show that the steeper temperature distribution (higher $\beta$) for hot molecular gas can be explained by shocks and photodissociation region (PDR) models, however, the exact $\beta$ values are not reproduced by PDR or shock models alone and a combination of both is needed. We find that the three major mergers among our targets have shallower temperature distributions for warm molecular gas than the other three spiral galaxies. This can be explained by a higher relative contribution of shock excitation, with respect to PDR excitation, for the warm molecular gas in these mergers. For only one of the mergers, IRASF 05189-2524, the shallower H2 temperature distribution differs from that of the spiral galaxies. The presence of a bright active galactic nucleus in this source might explain the warmer molecular gas observed.