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

Abstract

Context. We present a study of the 3D environment for a sample of 386 galaxies in the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973) using the Ninth Data Release of the Sloan Digital Sky Survey (SDSS-DR9). Aims. We aim to identify and quantify the effects of the satellite distribution around a sample of galaxies in the CIG, as well as the effects of the large-scale structure (LSS). Methods. To recover the physically bound galaxies we first focused on the satellites that are within the escape speed of each CIG galaxy. We also propose a more conservative method using the stacked Gaussian distribution of the velocity difference of the neighbours. The tidal strengths affecting the primary galaxy were estimated to quantify the effects of the local and LSS environments. We also defined the projected number density parameter at the fifth nearest neighbour to characterise the LSS around the CIG galaxies. Results. Out of the 386 CIG galaxies considered in this study, at least 340 (88% of the sample) have no physically linked satellite. Following the more conservative Gaussian distribution of physical satellites around the CIG galaxies leads to upper limits. Out of the 386 CIG galaxies, 327 (85% of the sample) have no physical companion within a projected distance of 0.3 Mpc. The CIG galaxies are distributed following the LSS of the local Universe, although presenting a large heterogeneity in their degree of connection with it. When present around a CIG galaxy, the effect of physically bound galaxies largely dominates (typically by more than 90%) the tidal strengths generated by the LSS. Conclusions. The CIG samples a variety of environments, from galaxies with physical satellites to galaxies without neighbours within 3 Mpc. A clear segregation appears between early-type CIG galaxies with companions and isolated late-type CIG galaxies. Isolated galaxies are in general bluer, with probably younger stellar populations and very high star formation compared with older, redder CIG galaxies with companions. Reciprocally, the satellites are redder and with an older stellar populations around massive early-type CIG galaxies, while they have a younger stellar content around massive late-type CIG galaxies. This suggests that the CIG is composed of a heterogeneous population of galaxies, sampling from old to more recent, dynamical systems of galaxies. CIG galaxies with companions might have a mild tendency (0.3−0.4 dex) to be more massive, and may indicate a higher frequency of having suffered a merger in the past.

Catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe

“…After compiling a first sample of primary galaxies using these conditions, we proceeded to clean the sample following a pipeline similar to the one developed in Argudo-Fernández et al (2014): (a) we selected all galaxies within at least 1 Mpc radius completely sampled by the SDSS-DR10 photometric footprint; (b) we performed a star-galaxy separation in order to reject stars misclassified as galaxies in the automated SDSS pipelines; and (c) we applied an automated cleaning process to discard multiple identifications of the same galaxy.…”

“…Star-galaxy separation and cleaning for multiple identifications as in Argudo-Fernández et al (2014) were also performed to avoid duplication in the neighbour sample.…”

“…For the final pair and triplet catalogues, we followed a similar criteria to Argudo-Fernández et al (2014) and considered that the over-density of similar redshift galaxies found in the lower panel of Fig. 1 is due to the nature of the physically bound systems.…”

The effect of local and large-scale environments on nuclear activity and star formation

“…The systems are isolated with no neighbours in a volume of 1 Mpc projected distance within a line-of-sight velocity difference of ∆ ≤ 500 km s −1 . After defining isolation, they followed a similar method as in Argudo-Fernández et al (2014) to identify the physically bound isolated pairs. For the central galaxy in isolated pairs, Argudo-Fernández et al (2015b) found an over-density of neighbours in the two-dimensional (2D) distribution of ∆ and distance d. This over-density indicates that those neighbours are likely to be physically connected.…”

Asymmetric Star Formation Efficiency Due to Ram Pressure Stripping

“…Galaxy clusters have been extensively used to study galaxy evolution because they allow us to discern between nature and nurture galaxy evolutionary processes [1,2]. For simplicity, hereafter, we refer to the intra-cluster medium (ICM) as the hot medium of both groups and clusters of galaxies.…”