PHOTOMETRIC AND MORPHOLOGICAL ANALYSIS OF UCM GALAXIES

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ABSTRACT A set of 212 objects of the UCM (Universidad Complutense de Madrid) survey of emission-line galaxies (ELGs), selected from objective prism plates by the presence of H-alpha emission in their spectra, has been analysed in the Thuan & Gunn r filter. From CCD images of these UCM objects we have obtained the fundamental photometric parameters (apparent and absolute magnitudes, linear diameters) and a set of characteristic morphological parameters extracted by surface photometry techniques (parameters of bulb + disk decomposition, concentration indexes and mean surface brightnesses). From this information, we have carried out intrinsecal statistical analysis of the data in the UCM sample and extrinsecal comparison with other samples of galaxies. And specially, we have simultaneously applied, for the first time in a sample of ELGs, five morphological criteria in order to obtain the Hubble type of these galaxies. As results of this work we have obtained that the UCM survey is specially effective in the detection of ELGs smaller and less luminous than normal spirals (the median values of the UCM sample are Mr = -20.6 and D24 = 19.4 kpc) and that emission arises fundamentally from the nuclear regions of the galaxies (only 22% of the sample show emitting knots outside the nucleus). Morphologically, the UCM sample is dominated by late-type galaxies, with 66% being Sb or later. This result confirms the expected tendency of the ELGs to be advanced type galaxies due to the higher mass fraction in gas available in these systems. Specially, we have found 9 candidates to BCD (blue compact dwarf galaxy), five of them previously unknown. We have also obtained that 10% of the UCM ELGs present typical parameters of S0 types, supporting the idea that not all S0 galaxies are dead systems without prominent recent star formation, and that in the subsample of 16 UCM galaxies showing obvious signs of interactions, 25% are S0 type galaxies. Finally, we have analysed the influence of the medium in the occurence and nature of the ELGs. So, we have compared the presence of ELGs and their morphologies in a subsample of galaxies in the Coma cluster respect to the subsample of field galaxies. Then, we have found that whereas 34% of the field Zwicky galaxies are ELGs, this fraction is reduced to 9% in the Coma cluster, consistent with the known efect that the fraction of galaxies that are ELGs is much lower in high-density cluster environments than in the field due to the lower fraction of star forming gas available in these dense media. And respect to the morphology, the objective prism technique permits to reveal, inside a cluster globally dominated by E-S0 type galaxies, a population of ELGs with 71% being Sb type or later. In the future, we propose to study in others filters the subset of these UCM galaxies specially peculiar, like the BCD galaxies (candidates to low metalicity objects), irregular and interacting systems, and to continue investigating the conexion between the medium and the presence of ELGs and its influence on the morphology of these galaxies.