Lacerta I and Cassiopeia III: Two luminous and distant Andromeda satellite dwarf galaxies found in the 3{\pi} Pan-STARRS1 survey

TL;DR

This paper reports the discovery of two luminous, distant dwarf galaxies near Andromeda using Pan-STARRS1 survey data, highlighting the survey's potential to find more such satellites.

Contribution

The paper presents the first identification of Lacerta I and Cassiopeia III as new Andromeda satellite dwarf galaxies using wide-area Pan-STARRS1 imaging.

Findings

Lacerta I and Cassiopeia III are likely satellites of Andromeda.

Both galaxies are more luminous and larger than many known dwarf satellites.

The discovery demonstrates the effectiveness of the 3π Pan-STARRS1 survey for finding distant dwarf galaxies.

Abstract

We report the discovery of two new dwarf galaxies, Lacerta I/Andromeda XXXI (Lac I/And XXXI) and Cassiopeia III/Andromeda XXXII (Cas III/And XXXII), in stacked Pan-STARRS1 r_P1- and i_P1-band imaging data. Both are luminous systems (M_V ~ -12) located at projected distances of 20.3{\deg} and 10.5{\deg} from M31. Lac I and Cas III are likely satellites of the Andromeda galaxy with heliocentric distances of 756^{+44}_{-28} kpc and 772^{+61}_{-56} kpc, respectively, and corresponding M31-centric distances of 275+/-7 kpc and 144^{+6}_{-4} kpc . The brightest of recent Local Group member discoveries, these two new dwarf galaxies owe their late discovery to their large sizes (r_h = 4.2^{+0.4}_{-0.5} arcmin or 912^{+124}_{-93} pc for Lac I; r_h = 6.5^{+1.2}_{-1.0} arcmin or 1456+/-267 pc for Cas III), and consequently low surface brightness (\mu_0 ~ 26.0 mag/arcsec^2), as well as to the lack of a systematic survey of regions at large radii from M31, close to the Galactic plane. This latter limitation is now alleviated by the 3{\pi} Pan-STARRS1 survey, which could lead to the discovery of other distant Andromeda satellite dwarf galaxies.