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). 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).

To recover the physical satellites around the CIG galaxies, we first focused on the satellites that are within the escape speed of each CIG galaxy. The escape speed has a characteristic trumpet shape (caustic; Kaiser 1987; Diaferio & Geller 1997) under which the satellite galaxies would be captured. Above the caustic, the neighbour galaxies possess a velocity sufficient to evade the gravitational attraction of the primary galaxy and are not captured, although fly-by interactions may influence the structure and evolution of the primary galaxy. To take into account the potential effect of fly-by encounters, we develop a more conservative method to recover most of the galaxies which have interacted with the CIG galaxies: a stacked distribution of the absolute values of the radial velocity difference between the projected neighbour galaxies and the central CIG galaxies (∆v=v_neigh − v_CIG). A Gaussian distribution with σ = 105 km/s appears for velocity differences smaller than 300 km/s. We also checked that the neighbour galaxies within 3σ show a strong tendency to gather in the inner 0.3 Mpc around the CIG galaxies. Hence, the satellite galaxies selected by the Gaussian distribution are all neighbour galaxies with |∆v| ≤ 315 km/s and lie at a distance smaller than 0.3 Mpc. This method provides an upper limit on the quantification of the local environment, since more galaxies will be considered as satellites than with the escape-speed method.

We find that 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 method to identify physical satellites around the CIG galaxies leads to upper limits: out of the 386 CIG galaxies, 327 galaxies (85% of the sample) would have no physical companion within a projected distance of 0.3 Mpc.

We defined the projected number density parameter at the fifth nearest neighbour to characterise the LSS around the CIG galaxies. We find that the CIG galaxies are distributed following the LSS of the local Universe, although presenting a large heterogeneity in their degree of connection with it. To evaluate the role of the physically bound satellites with respect to the large-scale environment, we compared the magnitudes of the sum of the tidal forces produced by the physical companions to the sum of the tidal forces engendered by all the galaxies in the LSS. When at least one physical companion is present near a CIG galaxy, its effect largely dominates (typically more than 90%) the tidal forces generated by the LSS. Although 15% at most of the CIG galaxies in the sample have physically bound satellites, almost all galaxies (97%) can be directly related to an LSS. The very large scatter in the quantification of the LSS shows that the CIG includes both galaxies dominated by their immediate environment and galaxies almost free from any external influence.

To delimit the role of the environment on the physical properties of the galaxies we compared, within the CIG, the most isolated galaxies with the galaxies with companions. As it is shown in Figure 1, we found a clear segregation between CIG galaxies with companions and isolated CIG galaxies. Isolated galaxies are in general bluer, with a younger stellar population and very  high star formation with respect to the older, redder galaxies with companions. These (u − r) colours, in combination with the morphological trends and the (g − r) colours, lead to a coherent view according to which isolated star-forming galaxies are separated from older elliptical galaxies with companions.