The AMIGA sample of isolated galaxies

Abstract

Context. To understand the evolution of galaxies, it is necessary to have a reference sample where the effect of the environment is minimized and quantified. In the framework of the AMIGA project (Analysis of the interstellar Medium of Isolated GAlaxies), we present a revision of the environment for galaxies in the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973, Astrof. Issledovaniia Byu. Spec. Ast. Obs., 8, 3) using the ninth data release of the Sloan Digital Sky Survey (SDSS-DR9). Aims. The aims of this study are to refine the photometric-based AMIGA sample of isolated galaxies and to provide an improvement of the quantification of the isolation degree with respect to previous works, using both photometry and spectroscopy. Methods. We developed an automatic method to search for neighbours within a projected area of 1 Mpc radius centred on each primary galaxy to revise the CIG isolation criteria introduced by Karachentseva (1973). The local number density at the fifth nearest neighbour and the tidal strength affecting the CIG galaxy were estimated to quantify the isolation degree. Results. Of the 636 CIG galaxies considered in the photometric study, 426 galaxies fulfil the CIG isolation criteria within 1 Mpc, taking into account projected neighbours. Of the 411 CIG galaxies considered in the spectroscopic study, 347 galaxies fulfil the CIG isolation criteria when a criterion about redshift difference is added. The available redshifts allow us to reject background neighbours and thus improve the photometric assessment. On average, galaxies in the AMIGA sample show lower values in the local number density and the tidal strength parameters than galaxies in denser environments such as pairs, triplets, compact groups, and clusters. Conclusions. For the first time, the environment and the isolation degree of AMIGA galaxies are quantified using digital data. The use of the SDSS database permits one to identify fainter and smaller-size satellites than in previous AMIGA works. The AMIGA sample is improved by this study, because we reduced the sample of isolated galaxies used in previous AMIGA works by about 20%. The availability of the spectroscopic data allows us to check the validity of the CIG isolation criteria, which is not fully efficient. About 50% of the neighbours considered as potential companions in the photometric study are in fact background objects. We also find that about 92% of the neighbour galaxies that show recession velocities similar to the corresponding CIG galaxy are not considered by the CIG isolation criteria as potential companions, which may have a considerable influence on the evolution of the central CIG galaxy.

The AMIGA sample of isolated galaxies

“…main -Fernández et al (2013) found that 84% of the sources with good spectroscopic coverage were found to be isolated when neighbours separated by more than 500 km s −1 from the central object were excluded, which may suggest, contrary to the photometric findings, that the full CIG sample could contain slightly more well isolated galaxies than found by Verley et al (2007a).…”

“…A point to note is that the intrinsic scatter estimates for these fits are smaller by about 0.05 dex which could indicate that the Argudo-Fernández et al (2013) criteria have removed the non-isolated galaxies more reliably and thus reduced scatter due to contamination by non-isolated objects. However, considerable caution is required in interpreting this result because there are many possible explanations.…”

“…Criteria for both of these parameters were then chosen with the goal of removing any galaxies from the sample that could have their evolution impacted by the presence of neighbours. The isolation criteria were revised again in Argudo-Fernández et al (2013) based on SDSS DR9 images and spectroscopy. However, because AMIGA is an all sky sample, restricting it to the SDSS footprint excludes much of the collected HI data.…”

“…Appendix D: Scaling relations for SDSS-based isolation revision Argudo-Fernández et al (2013) revised the AMIGA isolation criteria for those sources which fall within the SDSS footprint. Their photometric analysis produced a sample of well isolated galaxies that was 67% of the input sample of CIG objects.…”

Effects of the environment on galaxies in the Catalogue of Isolated Galaxies: physical satellites and large scale structure

“…To evaluate the effects of the large-scale environment, we followed a methodology similar to that of Argudo-Fernández et al (2013), searching for neighbours around 386 CIG fields completely covered by the SDSS within a physical projected radius of 3 Mpc.…”