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.

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.

Full text

1. INTRODUCTION

Studies of nearby galaxies lay the foundation for understanding the evolution of more distant systems. Given the wide diversity of galaxy types throughout the Universe, it is then important to examine the evolution of as many nearby galaxies as possible. At a distance of $\sim 2.3$ Mpc (Kudritzki et al. 2016 and references therein), NGC 55 is one of the nearest galaxies that is not a member of the Local Group. However, many of its properties remain uncertain, in large part because it is viewed with an almost edge-on orientation.

While NGC 55 has been classified as a Magellanic Irregular (de Vaucouleurs et al. 1991), the distribution of light at red and near-infrared wavelengths has a lenticular shape, and so differs from that of the edge-on Magellanic galaxy NGC 4236, which has a similar luminosity and distance (Davidge 2018b). The disk of NGC 55 is lop-sided, with what appears to be a tidal arm extending in the general direction of its nearest large companion – NGC 300. Isolated HI clouds are found along the disk plane at projected distances of $\sim 30$ kpc from the main body of the galaxy (Westmeier et al. 2013). The HI disk of NGC 55 also shows signs of being disturbed, hinting at a chaotic past (Westmeier et al. 2013).

Despite having features that might be associated with tidal interactions, NGC 55 is not in a dense environment. NGC 55 and NGC 300 are the dominant members of a sub-system that is on the near side of the so-called Sculptor Group. The dwarf galaxies ESO294-010 and ESO410–005 are other possible members of this sub-group, which has an estimated crossing time that exceeds the Hubble time (Karanchentsev et al. 2003). Other members of the so-called Sculptor Group are at larger distances from NGC 55 than might be inferred simply from their projected location on the sky.

Source confusion that is exacerbated by the orientation of NGC 55 on the sky makes it challenging to resolve individual stars except in the outer areas of the stellar disk and in the extraplanar regions. Still, NGC 55 was one of the first galaxies outside of the Local Group for which the detection of individual stars was claimed (Graham 1982). NGC 55 was also an early target of narrow-band photometric surveys to identify C stars (Pritchet et al. 1987). The C star frequency deduced from those data is significantly higher than in the LMC and SMC (Battinelli & Demers 2005), suggesting levels of star-forming activity in NGC 55 during intermediate epochs that exceed those in the LMC and SMC. Davidge (2018a) examined the spectroscopic properties of the northern half of the NGC 55 minor axis, and found that the Ca triplet lines near the disk plane have depths that are consistent with [Fe/H] $=-0.35$ and a luminosity-weighted age of 1 – 2 Gyr. A large C star frequency might then be expected.

Magrini, Goncalves, & Vajgel (2017) measure oxygen abundances in HII regions along the NGC 55 disk plane that are intermediate between those of the LMC and SMC (e.g. Carlos Reyes et al. 2015). The analysis of the spectra of bright supergiants in NGC 55 reveals LMC-like abundances (Castro et al. 2012; Kudritzki et al. 2016; Patrick et al. 2017). While there appears not to be a radial abundance gradient in the LMC (e.g. Toribio San Cipriano et al. 2017; Pagel et al. 1978), there is conflicting evidence about radial abundance trends in NGC 55. Kudritzki et al. (2016) find evidence for a metallicity gradient in the interstellar medium (ISM), while Magrini et al. (2017) find a flat radial abundance profile. Kudritzki et al. (2016) conclude that NGC 55 is accreting large amounts of gas. If this is the case then the chemical content of HII regions may not mirror that of older stars in the same part of the galaxy.

While HII regions are found throughout NGC 55, much of the present-day star-forming activity is concentrated in two star-forming complexes in the central kpc of the galaxy that have 30 Dor-like luminosities (Ferguson et al. 1996). Otte & Dettmar (1999) label these as H2 and H4 in their Figure 2. Recent star formation in these – and other – complexes has undoubtedly helped to shape the present-day spatial distribution of the ISM in NGC 55, including the excitation of a diffuse interstellar component that is found over much of the galaxy (e.g. Graham & Lawrie 1982; Hoopes et al. 1996; Ferguson et al. 1996; Otte & Dettmar 1999).

A substantial extraplanar ISM is present in NGC 55, and the properties of this gas provides additional insights into the evolution of the galaxy. The HI distribution of NGC 55 is not symmetric about the major axis (Westmeier et al. 2013), and the presence of bubbles, shells, and chimney structures in the ISM indicate that material is being heated and expelled from the disk. HII regions are found well off of the disk plane (Tullmann et al. 2003; Kudritzki et al. 2016), and Tullmann et al. (2003) measure metallicities that are lower than in the disk. Such low metallicities indicate that mechanisms other than the ejection of gas from the disk contribute to the extraplanar gas and dust. A population of HI clouds have been found around NGC 55 (Westmeier et al. 2013), and these might serve as a reservoir for fueling extraplanar star formation.

Davidge (2005) used deep GMOS images to probe the stellar content off of the disk plane, and found that the extraplanar environment is dominated by old stars. While finding old stars in this part of NGC 55 is not unexpected, the color of the extraplanar RGB is consistent with [M/H] $\sim-1.2$ to $-1.3$. This is similar to the metallicity found in extraplanar HII regions, hinting at a flat age-metallicty relation in this part of the galaxy. Tikhonov et al. (2005) estimate a similar metallicity from deep HST images of the same area.

It is apparent that NGC 55 has characteristics that may not be typical of Magellanic Irregular galaxies. In the present paper we discuss a grid of long-slit spectra of NGC 55 recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South (GS). A contiguous $0.4\times 5$ arcmin area was observed, centered near the intersection of the major and minor axes. The target area covers regions to the south east of the minor axis that have been the subject of many previous studies (see above), as well as the less-studied area to the north west of the minor axis.

The primary goal is to examine the luminosity-weighted properties of the central regions of the galaxy, which is an environment that is expected to be rich in information about its past evolution. Integral field observations of this nature are a powerful means of investigating stellar content and galactic structure in complicated environments (e.g. Roth et al. 2018). The scientific benefits of integral field observations has motivated the development of instruments such as MUSE (e.g. Bacon et al. 2010), as well as software packages such as TYPHOON (Sturch & Madore 2012).

Spectra of compact sources with a FWHM that is comparable to that of the seeing disk, and so are unresolved, are also examined. With an image scale of 1 arcsec $\sim 10$ parsecs, then the majority of these sources are almost certainly blended asterisms, star clusters, or compact HII regions. The GMOS spectra are supplemented with archival GALEX (Martin et al. 2005), 2MASS (Skrutskie et al. 2006), and SPITZER (Werner et al. 2004) images. The GALEX images trace star formation over the past few tenths of a Gyr, while the 2MASS and SPITZER images trace stars that formed over a much longer time span and are the dominant contributor to overall stellar mass.

Details of the observations and the steps used to remove instrumental and atmospheric signatures are reviewed in Section 2. The GALEX, 2MASS, and Spitzer observations are discussed in Section 3, while the projected distribution of emission and absorption lines in the area surveyed with GMOS, including the spectroscopic properties of the largest star-forming complexes, are the subject of Section 4. The nature of unresolved sources are examined in Section 5. The paper closes in Section 6 with a discussion and summary of the results.

2. OBSERVATIONS & REDUCTIONS

2.1. Description of the Observations