Reconstructing the evolutionary path of galaxies: a study of the main properties of early- and late-type galaxies over the redshift range 0.6 Tamburri, Sonia 2014-01-01 Abstract The aim of the analysis carried out in this thesis work is twofold. On the one hand we are interested in addressing whether a sample of morphologically selected early-type galaxies (ETGs) differs from a sample of passive galaxies in terms of galaxy statistics. On the other hand we study how the relative abundance of galaxies, the number density, and, the stellar mass density of different morphological types change over the redshift range 0.6 ≤ z ≤ 2:5. Furthermore, we investigated if these galaxies and their properties can provide evidences on the possible variation of the abundance of low-to-high mass stars in galaxies, i.e. the variation of the IMF, with redshift, stellar mass and colours. From the 1302 galaxies brighter than Ks(AB)=22 selected from the GOODS-MUSIC catalogue, we classified the ETGs, i.e. elliptical (E) and spheroidal galaxies (E/S0), on the basis of their morphology and the passive galaxies on the basis of their specific star formation rate (sSFR≤10-11 yr-1). Since the definition of a passive galaxy depends on the model parameters assumed to fit the spectral energy distribution of the galaxy, in addition to the assumed sSFR threshold, we probed the dependence of this definition and selection on the stellar initial mass function (IMF). We find that spheroidal galaxies cannot be distinguished from the other morphological classes on the basis of their low star formation rate, irrespective of the IMF adopted in the models. In particular, we find that a large fraction of passive galaxies (> 30 %) are disc-shaped objects and that the passive selection misses a significant fraction (~ 26 %) of morphologically classified ETGs. Using the sample of 1302 galaxies morphologically classified into spheroidal galaxies (ETGs) and non-spheroidal galaxies (LTGs), we find that the fraction of these two morphological classes is constant over the redshift range 0:6 ≤ z ≤ 2:5, being 20-30 % the fraction of ETGs and 70-80 % the fraction of LTGs. However, at z < 1 these fractions change among the population of the most massive (M*≥ 1011M) galaxies, with the fraction of massive ETGs rising up to 40 % and the fraction of massive LTGs decreasing to 60 %. Parallel to this trend, we find that the number density and the stellar mass density of the whole population of massive galaxies increase by almost a factor of ~ 10 between 0:6 ≤ z ≤ 2:5, with a faster increase of these densities for the ETGs than for the LTGs. Finally, we find that the number density of the highest-mass galaxies both ETGs and LTGs (M* > 3 - 4 X 1011M) does not increase from z ~ 2:5, contrary to the lower mass galaxies. This suggests that the most massive galaxies formed at z > 2:5 - 3 and that the assembly of such high-mass galaxies is not effective at lower redshift. From the analysis f the galaxy SEDs, reproduced with different IMFs, we found that the bulk (> 80%) of the 1302 galaxies are best-fitted with IMFs that contain a greater relative number of low-mass stars compared to a Salpeter IMF. This trend is constant with redshift and galaxy mass, testifying that from our data we do not observe an IMF variation as function of the cosmic time, galaxy mass and galaxy morphology.

The aim of the analysis carried out in this thesis work is twofold. On the one hand we are interested in addressing whether a sample of morphologically selected early-type galaxies (ETGs) differs from a sample of passive galaxies in terms of galaxy statistics. On the other hand we study how the relative abundance of galaxies, the number density, and, the stellar mass density of different morphological types change over the redshift range 0.6 ≤ z ≤ 2:5. Furthermore, we investigated if these galaxies and their properties can provide evidences on the possible variation of the abundance of low-to-high mass stars in galaxies, i.e. the variation of the IMF, with redshift, stellar mass and colours. From the 1302 galaxies brighter than Ks(AB)=22 selected from the GOODS-MUSIC catalogue, we classified the ETGs, i.e. elliptical (E) and spheroidal galaxies (E/S0), on the basis of their morphology and the passive galaxies on the basis of their specific star formation rate (sSFR≤10-11 yr-1). Since the definition of a passive galaxy depends on the model parameters assumed to fit the spectral energy distribution of the galaxy, in addition to the assumed sSFR threshold, we probed the dependence of this definition and selection on the stellar initial mass function (IMF). We find that spheroidal galaxies cannot be distinguished from the other morphological classes on the basis of their low star formation rate, irrespective of the IMF adopted in the models. In particular, we find that a large fraction of passive galaxies (> 30 %) are disc-shaped objects and that the passive selection misses a significant fraction (~ 26 %) of morphologically classified ETGs. Using the sample of 1302 galaxies morphologically classified into spheroidal galaxies (ETGs) and non-spheroidal galaxies (LTGs), we find that the fraction of these two morphological classes is constant over the redshift range 0:6 ≤ z ≤ 2:5, being 20-30 % the fraction of ETGs and 70-80 % the fraction of LTGs. However, at z < 1 these fractions change among the population of the most massive (M*≥ 1011M) galaxies, with the fraction of massive ETGs rising up to 40 % and the fraction of massive LTGs decreasing to 60 %. Parallel to this trend, we find that the number density and the stellar mass density of the whole population of massive galaxies increase by almost a factor of ~ 10 between 0:6 ≤ z ≤ 2:5, with a faster increase of these densities for the ETGs than for the LTGs. Finally, we find that the number density of the highest-mass galaxies both ETGs and LTGs (M* > 3 - 4 X 1011M) does not increase from z ~ 2:5, contrary to the lower mass galaxies. This suggests that the most massive galaxies formed at z > 2:5 - 3 and that the assembly of such high-mass galaxies is not effective at lower redshift. From the analysis f the galaxy SEDs, reproduced with different IMFs, we found that the bulk (> 80%) of the 1302 galaxies are best-fitted with IMFs that contain a greater relative number of low-mass stars compared to a Salpeter IMF. This trend is constant with redshift and galaxy mass, testifying that from our data we do not observe an IMF variation as function of the cosmic time, galaxy mass and galaxy morphology.