Quantification of isolation based on POSS I & II images
Quantification of isolation
In Verley et al. 20007a we performed a careful reevaluation of the degree of isolation of the CIG. The presence of candidate neighbours, in a different number and with different sizes and magnitudes in the environments of CIG galaxies, leads us to go a step further. We have quantified the degree of isolation of CIG galaxies according to different and complementary parameters, that produce a well characterised picture of their environment.
We have used two complementary parameters in order to quantify the degree of isolation of the CIG galaxies: the local number density of neighbour galaxies and the tidal strength that these latter produce on the candidate isolated galaxy. Both parameters were calculated for all the 950 CIG galaxies considered in AMIGAIV (when excluding the 100 nearby galaxies with V < 1500 kms−1). The local number density provides a good description of the environment in the vicinity of the primary galaxy but with the disadvantage of not taking into account the mass (or size) of the perturbers. In order to provide an estimation of the degree of isolation taking also into account the masses of the neighbours, we calculated the tidal strength affecting the primary CIG galaxies. The two isolation parameters give consistent results, as shown in Fig. 1 (local number density vs. tidal strength). When a galaxy shows low values for both the local number density and the tidal strength estimation, this galaxy is very isolated from any sort of external influence. On the contrary, when the two values are high, the evolution of the galaxy can be perturbed by the environment and this kind of galaxy is not suitable to represent the normal features of isolated galaxies.
Fig. 1 Comparison between the local number density and tidal strength (QKar) parameters for the CIG and the comparison samples. The CIG galaxies are represented by grey pluses. The KTG are depicted by green triangles, the HCG by magenta squares and the ACO by red dots. The mean values of each sample are shown by horizontal and vertical lines, following the same colour code.
The two isolation parameters are also complementary between each other. For instance, a neighbour very close to a CIG galaxy would be counted as one object by the local number density estimation but will drastically increase the value of the tidal strength affecting the CIG galaxy. On the other hand, if the tidal strength is low but the local number density high, we can conclude that the environment consists of relatively small neighbours, present in a high number near the primary galaxy. This latter case excludes, for example, major interactions. The use of the combination of these various parameters allows us to have a clear picture of the environment around the candidate isolated galaxies. The CIG galaxies flagged by Sulentic et al. (2006) as optically distorted galaxies also show the highest values of the tidal strength estimation. The presence of small neighbours near the primary galaxy has a notable effect in the visual identification of distorted galaxies. Their contribution has to be taken into account in order to be sure to select galaxies isolated not only from similar size neighbours (as done by Karachentseva 1973) but also from dwarfs.
In order to compare the isolation degree of the CIG galaxies with galaxies in denser environments, we have selected three samples for comparison: triplets from the Karachentseva’s catalogue (KTG, Karachentseva et al. 1979), compact groups from the Hickson catalogue (HCG, Hickson 1982) and Abell clusters (ACO, Abell 1958; Abell et al. 1989). The KTG and HCG catalogues complement the CIG since they were visually compiled using also an isolation criterion. The two isolation parameters are very well suited to discriminate the isolated galaxies from galaxies more in interaction with their environments.
A revised catalogue of isolated AMIGA galxies
The main aim of this study was to produce a catalogue of isolated galaxies and to have, for each, one clear picture of its environment. Starting from the CIG, we will remove all the galaxies whose evolution could have been affected by their environment. We saw that perturbations could arise when the tidal forces affecting a galaxy amounted to at least 1% of the internal binding forces (Athanassoula 1984; Varela et al. 2004). For the local number density, this translates to a value of 2.4. This value is given by the final fitted bisector correlation, calculated including all the neighbours (even the small, not considered by Karachentseva but for which Sulentic et al. (2006) detected major, obvious role in the evolution of the galaxies). This two values characterise best the limits for selecting a sample of isolated galaxies, using the complementarity between the local number density and the tidal strength estimation. One hundred twenty five CIG galaxies are affected by tidal forces that could perturb their evolution (Q ≥ −2), sixteen galaxies are lying in relatively high number density environment (ηk ≥ 2.4) and eighteen galaxies cumulate the two conditions. Consequently, a total of 159 CIG galaxies are lying in environments that could affect their evolution and are not suited to be included in a sample of isolated galaxies. We remove these galaxies and the remaining 791 galaxies define the AMIGA sample of isolated galaxies. This is still a numerous enough sample allowing statistical significance, even for subsamples of galaxies (e.g.: sorted by morphologies).
The 791 galaxies show a continuous gradient of interaction with their environment but all have their evolution dominated by their intrinsic properties. Further AMIGA studies will consider this sample as a reference and study the properties of the ISM of these galaxies in various wavelengths. The comparison with the properties of galaxies lying in denser environments or in interaction, will permit to quantify the effects added by the environment on the intrinsic evolution of galaxies.