Seeing Red in NGC 1978, NGC 55, and NGC 3109

TL;DR

This study analyzes the integrated red spectra of star clusters and dwarf galaxies NGC 1978, NGC 55, and NGC 3109, revealing stellar population characteristics and effects of stochastic variations on spectral features.

Contribution

It provides new insights into the stellar populations and spectral signatures of these objects, including effects of stochasticity and comparisons with models and star formation histories.

Findings

CN band near 0.79um indicates presence of C stars.

Ca triplet lines are stronger than expected, suggesting extra mixing.

Red light in NGC 55 dominated by 1-2 Gyr old stars.

Abstract

Spectra of the intermediate age star cluster NGC 1978 and the dwarf irregular galaxies NGC 55 and NGC 3109 that span the 0.7 to 1.1um wavelength interval are discussed. The NGC 1978 spectra are used to examine stochastic effects on the integrated red light from an intermediate age cluster. The removal of either the brightest M giant or the brightest C star from the co-added NGC 1978 spectrum has minor effects on the equivalent withs of the Ca triplet. The most robust signature of C stars in the integrated cluster spectrum at these wavelengths is the CN band head near 0.79um. The equivalent widths of Ca triplet lines in the NGC 1978 spectrum and in the spectra of individual cluster stars are larger than expected for a scaled-solar abundance system, and it is suggested that these stars have been subject to extra mixing processes. Ca lines weaken with increasing distance from the disk plane in the NGC 55 spectra. Comparisons with models suggest that the red light from NGC 55 is dominated by stars with ages 1 - 2 Gyr, in agreement with star-forming histories (SFHs) obtained from the analysis of CMDs. The NGC 3109 observations sample three different parts of that galaxy but have a low signal-to-noise ratio. Comparisons with models suggest that the light from the NGC 3109 disk at red wavelengths is dominated by RSGs with ages of at most a few tens of Myr, in qualitative agreement with SFHs that are based on photometric measurements.