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

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.

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Cited by in corpus (6)

• An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems
• Luminous AGB variables in the dwarf Irregular Galaxy, NGC 3109
• The Stellar Contents of Intermediate Mass Disk Galaxies in the Virgo Cluster. I. GMOS Spectra
• Search for a metallicity spread in the multiple population Large Magellanic Cloud cluster NGC 1978
• A machine-learning photometric classifier for massive stars in nearby galaxies II. The catalog
• Changes: The Past, Present, and Future of the Nearby Dwarf Galaxy NGC 55