A Malin 1 'cousin' with counter-rotation: internal dynamics and stellar content of the giant low surface brightness galaxy UGC 1922

TL;DR

This study provides a detailed analysis of the giant low surface brightness galaxy UGC 1922, revealing a counter-rotating core and suggesting its formation through galaxy merger simulations, enhancing understanding of such rare systems.

Contribution

It offers the first detailed observational and simulation-based insight into UGC 1922's internal dynamics and formation history, highlighting a counter-rotating core and merger origin.

Findings

Presence of a kinematically decoupled, counter-rotating core.

Radial metallicity gradient consistent with moderate-size LSB galaxies.

A large, old, metal-rich stellar population resembling a giant elliptical.

Abstract

The formation scenario for giant low surface brightness (gLSB) galaxies with discs as large as 100 kpc still remains unclear. These stellar systems are rare and very hard to observe, therefore a detailed insight on every additional object helps to understand their nature. Here we present a detailed observational study of the gLSB UGC 1922 performed using deep optical imaging and spectroscopic observations combined with archival ultraviolet data. We derived spatially resolved properties of stellar and ionized gas kinematics and characteristics of stellar populations and interstellar medium. We reveal the presence of a kinematically decoupled central component, which counter rotates with respect to the main disc of UGC 1922. The radial metallicity gradient of the ionised gas is in agreement with that found for moderate-size LSB galaxies. At the same time, a slowly rotating and dynamically hot central region of the galaxy hosts a large number of old metal-rich stars, which creates an appearance of a giant elliptical galaxy, that grew an enormous star forming disc. We reproduce most of the observed features of UGC 1922 in N-body/hydrodynamical simulations of an in-plane merger of giant Sa and Sd galaxies. We also discuss alternative formation scenarios of this unusual system.