Changes: The Past, Present, and Future of the Nearby Dwarf Galaxy NGC 55^∗ †

Abstract

Spectra that cover wavelengths from 0.6 to 1.1µm are used to examine the behavior of emission and absorption features in a contiguous 22 × 300 arcsec region centered on the nearby dwarf galaxy NGC 55. This area includes the two largest star-forming complexes in the galaxy, as well as other starforming structures in the little-explored north west part of the disk. Based on the relative strengths of various emission features measured over spatial scales of many tens of parsecs, it is concluded that the ionization states and sulphur abundances in most of the star-forming regions near the center of NGC 55 are similar. However, a large star-forming region is identified in the north west part of the disk at a projected distance of ∼ 1 kpc from the galaxy center that has distinct ionization properties. In addition to tracing areas of present-day star formation, the spectra are also used to identify fossil star-forming regions by mapping the depth of the near-infrared Ca triplet. One such area is identified near the intersection of the major and minor axes. There is a corresponding concentration of bright red stars in archival [3.6] and [4.5] images that are part of a mass concentration that is structurally distinct from the surrounding disk. It is suggested that the area near the intersection of the major and minor axes in NGC 55 is a proto-nucleus. The spectra of bright unresolved sources that are blended stellar asterisms, compact HII regions, and star clusters are also discussed. The spectra of some of the HII regions contain Ca triplet absorption lines, signalling a concentration of stars in the resolution element that span many Myr. Six of the unresolved sources have spectroscopic characteristics that are indicative of C stars embedded in intermediate age clusters, and these are likely compact star clusters that are analogous to those in the Large Magellanic Cloud. The peculiar properties of NGC 55 have been well documented in the literature, and it is argued that these may indicate that NGC 55 is transforming into a dwarf lenticular galaxy.

Intermediate-mass Early-type Disk Galaxies in the Virgo Cluster. II. Near-Infrared Spectra and Evidence for Differences in Evolution* † ‡ † ‡

“…The radial metallicity profile of NGC 5102 suggests that the current morphology might be the result of the buckling of the bar in an LMC-like galaxy (Davidge 2015a). NGC 55 is a nearby irregular galaxy in a low density environment that may be transitioning into a dwarf lenticular morphology (Davidge 2018b;2019). If moderate mass early-type disk galaxies in the Virgo cluster and the field have been subject to similar evolutionary processes then the former might show evidence of (1) a young stellar component, and (2) prolonged evolution in an environment that has a lower density than the present-day Virgo cluster.…”