Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31

Abstract

We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey, using observations obtained with the OSIRIS+GTC instrument. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated and no horizontal branch stars are found. The RGB is nevertheless well matched by an old (12.5 Gyr), metal-poor ([Fe/H]= - 2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of 119 kpc from M31, And XXXVI is therefore likely a satellite of Andromeda. With MV -5.9 +-0.1, half-light radius rh 64 +30-19 pc, and an ellipticity 0.015 +0.032-0.012, And XXXVI is one of the faintest of ultra-faint M31 dwarfs discovered to date. The discovery of And XXXVI adds to the faint end of M31's satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31's halo. Combined with a detailed star formation history, such data would help determine whether its old, metal-poor stellar population indicates early quenching, similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.