Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

TL;DR

This study detects and analyzes warm molecular hydrogen emission in ten regions of the Large Magellanic Cloud, revealing its excitation conditions, correlation with infrared emission, and implications for photodissociation region heating.

Contribution

First detection of H$_2$ emission in multiple LMC regions using Spitzer IRS, with analysis of excitation conditions and correlation with dust and infrared properties.

Findings

Warm H$_2$ temperatures range 86-137 K.

H$_2$ surface brightness correlates with PAH and infrared emission.

H$_2$ excitation driven by photoelectric heating in PDRs.

Abstract

We present the detection and analysis of molecular hydrogen emission toward ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of twelve regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0--0 transitions of H$_2$ at 28.2 and 17.1${\,\rm \mu m}$ are detected in the IRS spectra for ten regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3$\sigma$ detection threshold is achieved for lines at 12.2 and 8.6${\,\rm \mu m}$. The excitation diagrams of the detected H$_2$ transitions are used to determine the warm H$_2$ gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range $86-137\,{\rm K}$. The bulk of the excited H$_2$ gas is found at these temperatures and contributes $\sim$5-17% to the total gas mass. We find a tight correlation of the H$_2$ surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photo-electric heating in photodissociation regions. We find the excitation of H$_2$ by this process is equally efficient in both atomic and molecular dominated regions. We also present the correlation of the warm H$_2$ physical conditions with dust properties. The warm H$_2$ mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H$_2$ excitation in photodissociation regions.