Dwarfs walking in a row

The filamentary nature of the NGC 3109 association

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: michele.bellazzini@oabo.inaf.it
² Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁴ Dipartimento di Fisica e Astronomia - Università degli Studi di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany

Received: 24 September 2013
Accepted: 22 October 2013

Abstract

We re-consider the association of dwarf galaxies around NGC 3109, whose known members were NGC 3109, Antlia, Sextans A, and Sextans B, based on a new updated list of nearby galaxies and the most recent data. We find that the original members of the NGC 3109 association, together with the recently discovered and adjacent dwarf irregular Leo P, form a very tight and elongated configuration in space. All these galaxies lie within ~100 kpc of a line that is ≃1070 kpc long, from one extreme (NGC 3109) to the other (Leo P), and they show a gradient in the Local Group standard of rest velocity with a total amplitude of 43 km s^-1 Mpc^-1, and a rms scatter of just 16.8 km s^-1. It is shown that the reported configuration is exceptional given the known dwarf galaxies in the Local Group and its surroundings. We conclude that (a) Leo P is very likely an additional member of the NGC 3109 association, and (b) the association is highly ordered in space and velocity, and it is very elongated, suggesting that it was created by a tidal interaction or it was accreted as a filamentary substructure.