A tidally disrupting dwarf galaxy in the halo of NGC 253

TL;DR

This paper reports the discovery of Scl-MM-Dw2, a likely tidally disrupted dwarf galaxy satellite of NGC 253, characterized by its old, metal-poor stellar population, low gas content, and unusual elongation and surface brightness.

Contribution

It presents the first identification and detailed characterization of Scl-MM-Dw2, a new dwarf galaxy in the halo of NGC 253, including its properties and evidence of tidal disruption.

Findings

Scl-MM-Dw2 is a dwarf spheroidal galaxy with old, metal-poor stars.

It shows signs of tidal disruption due to its elongation and diffuse structure.

It has a very low gas fraction and is an outlier in standard scaling relations.

Abstract

We report the discovery of Scl-MM-Dw2, a new dwarf galaxy at a projected separation of $\sim$50 kpc from NGC 253, as part of the PISCeS (Panoramic Imaging Survey of Centaurus and Sculptor) project. We measure a tip of the red giant branch distance of $3.12\pm0.30$ Mpc, suggesting that Scl-MM-Dw2 is likely a satellite of NGC 253. We qualitatively compare the distribution of red giant branch (RGB) stars in the color-magnitude diagram with theoretical isochrones and find that it is consistent with an old, $\sim$12 Gyr, and metal poor, $-2.3<$[Fe/H]$<-1.1$, stellar population. We also detect a small number of asymptotic giant branch stars consistent with a metal poor $2-3$ Gyr population in the center of the dwarf. Our non-detection of HI in a deep Green Bank Telescope spectrum implies a gas fraction $M_{HI}/L_V<0.02$ Msun/Lsun. The stellar and gaseous properties of Scl-MM-Dw2 suggest that it is a dwarf spheroidal galaxy. Scl-MM-Dw2 has a luminosity of $M_V=-12.1\pm0.5$ mag and a half-light radius of $r_h=2.94\pm0.46$ kpc which makes it moderately larger than dwarf galaxies in the Local Group of the same luminosity. However, Scl-MM-Dw2 is very elongated ($\epsilon=0.66\pm0.06$) and it has an extremely low surface brightness ($\mu_{0,V}=26.5\pm0.7$ mag arcsec$^{-2}$). Its elongation and diffuseness make it an outlier in the ellipticity-luminosity and surface brightness-luminosity scaling relations. These properties suggest that this dwarf is being tidally disrupted by NGC 253.