The AMIGA sample of isolated galaxies^*,**

IV. A catalogue of neighbours around isolated galaxies

¹ LERMA – Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France e-mail: [Simon.Verley;Francoise.Combes]@obspm.fr
² Instituto de Astrofísica de Andalucía – CSIC, C/ Camino Bajo de Huetor, 50, 18008 Granada, Spain e-mail: [simon;lourdes;jsm;gilles;daniel;garcia]@iaa.es
³ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: simon@arcetri.astro.it
⁴ McDonald Observatory, University of Texas, USA e-mail: sco@astro.as.utexas.edu
⁵ Instituto de RadioAstronomía Milimétrica, Avenida Divina Pastora, 7, Núcleo Central, 18012 Granada, Spain e-mail: leon@iram.es
⁶ Department of Astronomy, University of Alabama, Tuscaloosa, USA e-mail: giacomo@merlot.astr.ua.edu
⁷ Departamento de Física Teórica y del Cosmos, Facultad de Ciencias, Universidad de Granada, Spain e-mail: ute@ugr.es

Received: 15 February 2007
Accepted: 5 April 2007

Abstract

Context.Studies of the effects of environment on galaxy properties and evolution require well defined control samples. Such isolated galaxy samples have up to now been small or poorly defined. The AMIGA project (Analysis of the interstellar Medium of Isolated GAlaxies) represents an attempt to define a statistically useful sample of the most isolated galaxies in the local (z ≤ 0.05) Universe.

Aims.A suitable large sample for the AMIGA project already exists, the Catalogue of Isolated Galaxies (CIG, Karachentseva, 1973, Astrofizicheskie Issledovaniia Izvestiya Spetsial'noj Astrofizicheskoj Observatorii, 8, 3; 1050 galaxies), and we use this sample as a starting point to refine and perform a better quantification of its isolation properties.

Methods.Digitised POSS-I E images were analysed out to a minimum projected radius R ≥ 0.5 Mpc around 950 CIG galaxies (those within V_r = 1500 km s^-1 were excluded). We identified all galaxy candidates in each field brighter than with a high degree of confidence using the LMORPHO software. We generated a catalogue of approximately 54 000 potential neighbours (redshifts exist for ≈30% of this sample).

Results.Six hundred sixty-six galaxies pass and two hundred eighty-four fail the original CIG isolation criterion. The available redshift data confirm that our catalogue involves a largely background population rather than physically associated neighbours. We find that the exclusion of neighbours within a factor of four in size around each CIG galaxy, employed in the original isolation criterion, corresponds to 18 000 km s^-1 indicating that it was a conservative limit.

Conclusions.Galaxies in the CIG have been found to show different degrees of isolation. We conclude that a quantitative measure of this is mandatory. It will be the subject of future work based on the catalogue of neighbours obtained here.