Resumen de Characterisation of an isolated galaxy sample: astrophysical implications
Maria del Carmen Argudo Fernández
In order to understand the evolution of galaxies, it is necessary to have a reference sample where the effects of the environment are minimised and quantified. Recent advances in large redshift galaxy surveys, such as the Sloan Digital Sky Survey (SDSS), allow to reach a 3-dimensional picture of the environment. In the first two parts of the thesis, we present, in the framework of the AMIGA project (Analysis of the interstellar Medium of Isolated GAlaxies Verdes-Montenegro et al., 2005), a revision of the isolation degree and a study of the 3-dimensional environment for galaxies in the Catalogue of Isolated Galaxies (CIG; Karachentseva, 1973). Using the 3-dimensional information, new catalogues of isolated galaxies, isolated pairs, and isolated triplets are assembled in the third part of the thesis.
The main aims of this thesis are:
- to refine the photographic-based CIG and to provide an improvement of the quantification of the isolation degree with respect to previous works (Verley et al., 2007a,b), using both photometry and spectroscopy;
- to identify and quantify the effects of the physical satellite distribution around galaxies in the CIG, as well as the effects of the Large Scale Structure (LSS);
- to construct a catalogue of galaxies isolated in 3-dimension, and build catalogues of physically associated isolated pairs and isolated triplets.
We develop an automatic method to search for neighbours around each CIG galaxy in the SDSS (Ninth Data Release, DR9), within a projected area up to 3 Mpc. To recover the physically bound neighbour galaxies we focus on the satellites which are within the escape speed of each CIG galaxy. The local number density, at the 5th nearest neighbour, and the tidal strength affecting the CIG galaxy are estimated to quantify the local and LSS isolation degrees.
For the first time, the environment and the isolation degree of CIG galaxies are quantified using digital data. Besides, the availability of the spectroscopic data allows us to check the validity of the CIG isolation criterion, and shows that it is not fully efficient. Indeed, 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 neighbour galaxies showing recession velocities similar to the corresponding CIG galaxy are not considered by the CIG isolation criterion as potential companions, and may have a non negligible influence on the evolution of the central CIG galaxy. The CIG actually samples a variety of environments, from galaxies in interaction with physical satellites to galaxies with no neighbours within 3 Mpc.
A clear segregation appears between early-type CIG galaxies in weak interaction and isolated late-type CIG galaxies. Isolated galaxies are in general bluer, with likely younger stellar populations and a rather high star formation rate with respect to older, redder interacting CIG galaxies. Reciprocally, the satellites are redder and with 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 systems of galaxies to more recent, dynamical systems of galaxies. Interacting CIG galaxies might have a mild tendency (0.3-0.4 dex) to be more massive, which may indicate a higher frequency of having suffered a merger in the past.
In light of the above findings, the construction of catalogues of galaxies in relation to their environments should take into account the redshift information to distinguish small, faint, interacting satellites from a background projected galaxy population, and reach a more comprehensive 3-dimensional picture of the surroundings. Therefore, in the last part of the thesis work, we present new catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe (z ¿ 0.080), homogeneously selected using the 3-dimensional information contained in the SDSS-DR9. We provide catalogues of 4,191 isolated galaxies, 1,270 isolated pairs, and 300 isolated triplets and characterise their relation to the LSS using the projected number density and the tidal strength. The physical companions in the pair and triplet samples usually account for more than 98% of the total tidal strength affecting the central galaxy. We find no difference in their relation with the LSS, which may suggest that these isolated systems have a common origin in their formation and evolution. Most of the isolated galaxies, isolated pairs, and isolated triplets, belong to the outer parts of filaments, walls, and clusters, and generally differ from the void population of galaxies. The three catalogues are publicly available to the scientific community. For galaxies in the catalogues, we provide their positions, redshifts, and degrees of relation with their physical and LSS environments.
