The Extraordinary Infrared Spectrum of NGC 1222 (Mkn 603)

TL;DR

This study analyzes the infrared spectrum of NGC 1222, revealing it as the first solar-metallicity starburst galaxy dominated by high-excitation lines, indicating the presence of very high-mass stars up to 100 Msun.

Contribution

It provides the first detailed infrared spectral analysis of a solar-metallicity starburst galaxy dominated by high-excitation lines, suggesting the presence of stars larger than 35 Msun.

Findings

NGC 1222's infrared emission is dominated by the [NeIII] line.

The upper mass cutoff in NGC 1222 is estimated at 40-100 Msun.

This galaxy is the first known starburst with solar metallicity showing high-excitation line dominance.

Abstract

The infrared spectra of starburst galaxies are dominated by the low-excitation lines of [NeII] and [SIII], and the stellar populations deduced from these spectra appear to lack stars larger than about 35 Msun. The only exceptions to this result until now were low metallicity dwarf galaxies. We report our analysis of the mid-infrared spectra obtained with IRS on Spitzer of the starburst galaxy NGC 1222 (Mkn 603). NGC 1222 is a large spheroidal galaxy with a starburst nucleus that is a compact radio and infrared source, and its infrared emission is dominated by the [NeIII] line. This is the first starburst of solar or near-solar metallicity, known to us, which is dominated by the high-excitation lines and which is a likely host of high mass stars. We model the emission with several different assumptions as to the spatial distibution of the high- and low-excitation lines and find that the upper mass cutoff in this galaxy is 40-100 Msun.