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Signs of ancient interactions in isolated dwarf galaxies

In the currently accepted paradigm for galaxy formation, present-day dwarf galaxies are the leftover of a larger population of small galaxies that was cannibalised during the hierarchical build-up of larger systems like the Milky Way.

Today we observe that dwarf satellites in the surroundings of large galaxies tipically have elliptical shape, are devoid of gas and are dominated by intermediate-old age stars, while gas-rich and star-forming disc-like dwarfs are normally found at large distances. This is one of the main observational evidence supporting the idea that the evolution of dwarf galaxies is strongly shaped by the interaction with the large galaxy they are orbiting around; for instance, strong tides and ram-pressure stripping can remove the gas reservoir of dwarfs on close orbits, thus quenching their star formation. If we want to try to recover some information on the original conditions of dwarf galaxies we have to look for those dwarfs that evolved in isolation, far from the influence of large galaxies. To this aim we are carrying on a programme to study the structure and kinematics of both the stellar and gaseous body of isolated dwarf galaxies that can be resolved into stars.

Within this programme we have recently considered the two gas-rich dwarfs Sextans A and Sextans B, lying in the outskirts of the Local Group, more than 1~Mpc away from both the Milky Way and M31. Our new deep and wide-field photometry, obtained with the LBC camera  mounted on the Large Binocular Telescope, revealed that the main body of Sextans A is surrounded by an extended and elongated stellar halo resembling a tidal tail. The structure and kinematics of the gaseous component of the dwarf also show signs of disturbance thus suggesting that, in spite of the current isolation, Sextans A had some kind of interaction with another galaxy in the past. The interaction should have been not too strong, as Sextans A managed to retain a significant amount of gas. Following up this result we found that Sextans A and B are tightly aligned in space with the two other galaxies belonging to the same loose group of dwarfs (NGC3109 and Antlia) and with the recently discovered dwarf Leo P, that is likely a new member of the group.

Our observations – when combined with theoretical results from other teams – strongly suggest that the whole group was accreted by the Local Group in a filamentary configuration and had a relatively close encounter with the Milky Way a few billion years ago. During this phase the members of the group were much close together than today and a dwarf-dwarf interaction was relatively likely to occur: this can be at the origin of the tidal tail we observe in Sextans A.   These studies, that shed light on the past history of the Local Group and on the impact of dwarf-dwarf interactions, have been recently published on Astronomy & Astrophysics:  Bellazzini et al. 2013 and Bellazzini et al. 2014.

Stellar density map of Sextans A, revealing the extended and elongated halo surrounding the round-shaped main body. Our new photometry allowed us to trace the structure of the dwarf down to a surface brightness of 31 mag/arcsec^2.

Galaxies within 3~Mpc from the Milky Way in a galacto-centric reference system. Note the very tight alignment of the NGC3109 group, including NGC3109, Antlia, Sextans A, Sextans B and Leo P.