Search for gas accretion imprints in voids: II. The galaxy Ark 18 as a result of a dwarf-dwarf merger

TL;DR

This study investigates Ark 18, a low-surface brightness galaxy in the Eridanus void, revealing it as a product of a dwarf-dwarf merger and analyzing its complex structure, kinematics, and star formation history.

Contribution

It provides the first detailed analysis of Ark 18, demonstrating its formation through a dwarf-dwarf merger and characterizing its gas dynamics and stellar populations.

Findings

Ark 18 is dark matter dominated and gas-rich.

The galaxy shows a velocity field with two rotating components.

The merger occurred over 300 million years ago.

Abstract

The low-mass low-surface brightness (LSB) disc galaxy Arakelian 18 (Ark 18) resides in the Eridanus void and because of its isolation represents an ideal case to study the formation and evolution mechanisms of such a galaxy type. Its complex structure consists of an extended blue LSB disc and a bright central elliptically-shaped part hosting a massive off-centered star-forming clump. We present the in-depth study of Ark 18 based on observations with the SCORPIO-2 long-slit spectrograph and a scanning Fabry-Perot interferometer at the Russian 6-m telescope complemented by archival multi-wavelength images and SDSS spectra. Ark 18 appears to be a dark matter dominated gas-rich galaxy without a radial metallicity gradient. The observed velocity field of the ionised gas is well described by two circularly rotating components moderately inclined with respect to each other and a possible warp in the outer disc. We estimated the age of young stellar population in the galaxy centre to be ~140 Myr, while the brightest star-forming clump appears to be much younger. We conclude that the LSB disc is likely the result of a dwarf-dwarf merger with a stellar mass ratio of the components at least ~5:1 that occurred earlier than 300 Myr ago. The brightest star forming clump was likely formed later by accretion of a gas cloud.