# Changes: The Past, Present, and Future of the Nearby Dwarf Galaxy NGC 55

## TL;DR

This study uses spectral analysis to explore the emission, absorption features, and star formation history of NGC 55, revealing distinct regions, stellar populations, and potential galaxy evolution processes.

## Contribution

It provides new insights into the spatial variation of ionization, star formation, and stellar populations in NGC 55 through detailed spectral observations.

## Key findings

- Identification of a large star-forming region with unique ionization properties.
- Detection of fossil star-forming regions and a potential proto-nucleus.
- Evidence of intermediate-age stellar populations and galaxy transformation signs.

## Abstract

Spectra that cover wavelengths from 0.6 to 1.1um are used to examine the behavior of emission and absorption features in a contiguous 22 x 300 arcsec region centered on the nearby dwarf galaxy NGC 55. 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. 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. Fossil star-forming regions are also identified using the depth of the near-infrared Ca triplet. One such area is identified near the intersection of the major and minor axes, and it is suggested that this area 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. 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.