The Disk and Extraplanar Regions of NGC 55

Abstract

The stellar content of the nearby SB(s)m galaxy NGC 55 is investigated using images covering the visible and near-infrared wavelength regions. A well-defined plume, which stellar evolution models suggest contains stars with masses near the RSG - AGB transition, is detected in CMDs of the disk, and it is concluded that star formation in the thin disk of NGC 55 has ocurred at a significant rate for at least the past 0.1 - 0.2 Gyr. The disk also contains a large population of old (log(t_yr) ~ 10) stars, and it is argued that a stable disk has been in place in NGC 55 for a significant fraction of the age of the Universe. At projected distances in excess of 2 kpc off of the disk plane the brightest AGB stars have ages 10(+3)(-2) Gyr. Thus, despite indications that dust and gas are present in the envelope surrounding the NGC 55 disk, the AGB content suggests that recently formed stars do not occur in large numbers in the extraplanar region. The mean metallicity of extraplanar RGB stars is in excellent agreement with that measured in the extraplanar HII regions EHR 1 and 2, suggesting that the age-metallicity relation in this part of NGC 55 has been flat for at least a few Gyr. Finally, the RGB-tip occurs near i' = 23.1 in the extraplanar region, and a distance modulus of 26.5 is computed from this feature.

27 pages of text, 20 postscript figures. Accepted for publication in the ApJ

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• The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
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• The Disk and Extraplanar Environment of NGC 247
• Structure and Population of the NGC55 Stellar Halo from a Subaru/Suprime-Cam Survey
• Gas and dark matter in the Sculptor group: NGC 55
• The Disk and Extraplanar Regions of NGC 2403
• Physical properties of the first spectroscopically confirmed red supergiant stars in the Sculptor Group galaxy NGC 55
• The X-ray properties of the dwarf Magellanic-type galaxy NGC 55
• Red Supergiants in the Disk of M81: Tracing the Spatial Distribution of Star Formation 25 Million Years in the Past
• Seeing Red in NGC 1978, NGC 55, and NGC 3109
• Changes: The Past, Present, and Future of the Nearby Dwarf Galaxy NGC 55