Satellite accretion in action: a tidally disrupting dwarf spheroidal around the nearby spiral galaxy NGC 253

TL;DR

This paper reports the discovery of a tidally disrupting dwarf spheroidal galaxy around NGC 253, marking the first such event beyond the Local Group, and demonstrates the effectiveness of small telescopes in such discoveries.

Contribution

First identification of a tidally disrupting dwarf spheroidal galaxy beyond the Local Group using small telescopes and follow-up observations, providing insights into galaxy accretion processes.

Findings

Discovered NGC 253-dw2, a disrupting dwarf spheroidal galaxy.

Determined its distance, shape, and luminosity.

Provided evidence for ongoing galaxy accretion and disruption.

Abstract

We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing tidal disruption around a nearby spiral galaxy, NGC 253 in the Sculptor group: the first such event identified beyond the Local Group. The dwarf was found using small-aperture amateur telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve its brightest stars. Using g- and R_c-band photometry, we detect a red giant branch consistent with an old, metal-poor stellar population at a distance of ~ 3.5 Mpc. From the distribution of likely member stars, we infer a highly elongated shape with a semi-major axis half-light radius of (2 +/- 0.4) kpc. Star counts also yield a luminosity estimate of ~ 2x10^6 L_Sun,V (M_V ~ -10.7). The morphological properties of NGC 253-dw2 mark it as distinct from normal dSphs and imply ongoing disruption at a projected distance of ~ 50 kpc from the main galaxy. Our observations support the hierarchical paradigm wherein massive galaxies continously accrete less massive ones, and provide a new case study for dSph infall and dissolution dynamics. We also note the continued efficacy of small telescopes for making big discoveries.