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Study of the stellar populations in galaxies from integrated spectra a thesis submitted for the degree of Doctor of Philosophy in the faculty of science, Bangalore University, Bangalore A. K. Pati [Ph.D Thesis]

By: Contributor(s): Material type: TextTextPublication details: Bangalore Indian Institute of Astrophysics 1988Subject(s): Online resources: Dissertation note: Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1988 Summary: In the foregoing chapters we have detailed a study of the populations of stars in early type galaxies. The method used for the study is that of population synthesis, in which the observed spectrum of a galaxy is matched by the co-added spectra of stars of known types within the Galaxy. We have obtained spectroscopic data of galaxies and stars belonging to standard stellar types and carried out reduction of the data to finally obtain the energy distribution as a function of wavelength. The library of stellar spectra for use in population synthesis has to cover the entire range of stars seen and studied within the galaxy. The compilation of such a library, with each stellar group observed a suffi- cient number of times to get good representative averages, can take a lot of observing time. Since such compilations have been made earlier, it is prudent to use them, if possible. We have used an existing library after augmenting it with our own stellar observations. Three early type galaxies were studied in this work; of these, NGC 4472 in the Virgo cluster has been the subject of many earlier investigations, including the derivation of its stellar content using population synthesis The second galaxy is a typical early type galaxy, in the sense that it was not known to show any signs of activity characterized by emission lines in the spectrum, abnormal colors or morphology or radio emission. These has hardly been any earlier work on this galaxy, except for measurements of radial velocities and photometry. The third galaxy, NGC 5128, also known as Centaurus A, is in that class of elliptical galaxies classified as peculiar. It is one of the most powerful sources of radiowave radiation in the sky and had other features such as the gigantic band of dust and gas obsuring part of the light, which have attempt at attracted much attention; strangely no population synthesis using optical spectra have been made before. For the galaxy NGC 4472, we find evidence for star formation activity in the region observed (which was the optically bright centre of the galaxy) to the extent thought possible by others, from studies of strengths of spectral lines (Rose 1985). We also find that the last major star formation activity in the galaxy occurred 8 to 10 giga years ago as indicated by the main sequence turnoff point derived. One puzzling result that we get in the best constrained synthesis model is that contrary to what is believed of giant elliptical galaxies, NGC 4472 does not have a significant proportion of metal rich giant stars. We feel that the stellar libraries used in population synthesis should discriminate between different metallicities among dwarf stars, particularly of late spectral type, to resolve this problem. Very cool metal rich giant stars such as now known to exist at the centre of our Galaxy, should also be used in the stellar library.
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Thesis Supervisors J. C. Bhattacharyya and K. S. Raghavendra Rao

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1988

In the foregoing chapters we have detailed a study of the populations of stars in early type galaxies. The method used for the study is that of population synthesis, in which the observed spectrum of a galaxy is matched by the co-added spectra of stars of known types within the Galaxy.
We have obtained spectroscopic data of galaxies and stars belonging to standard stellar types and carried out reduction of the data to finally obtain the energy distribution as a function of wavelength. The library of stellar spectra for use in population synthesis has to cover the entire range of stars seen and studied within the galaxy. The compilation of such a library, with each stellar group observed a suffi- cient number of times to get good representative averages, can take a lot of observing time. Since such compilations have been made earlier, it is prudent to use them, if possible. We have used an existing library after augmenting it with our own stellar observations.
Three early type galaxies were studied in this work; of these, NGC 4472 in the Virgo cluster has been the subject of many earlier investigations, including the derivation of its stellar content using population synthesis The second galaxy is a typical early type galaxy, in the sense that it was not known to show any signs of activity characterized by emission lines in the spectrum, abnormal colors or morphology or radio emission. These has hardly been any earlier work on this galaxy, except for measurements of radial velocities and photometry. The third galaxy, NGC 5128, also known as Centaurus A, is in that class of elliptical galaxies classified as peculiar. It is one of the most powerful sources of radiowave radiation in the sky and had other features such as the gigantic band of dust and gas obsuring part of the light, which have attempt at attracted much attention; strangely no population synthesis using optical spectra have been made before.
For the galaxy NGC 4472, we find evidence for star formation activity in the region observed (which was the optically bright centre of the galaxy) to the extent thought possible by others, from studies of strengths of spectral lines (Rose 1985). We also find that the last major star formation activity in the galaxy occurred 8 to 10 giga years ago as indicated by the main sequence turnoff point derived. One puzzling result that we get in the best constrained synthesis model is that contrary to what is believed of giant elliptical galaxies, NGC 4472 does not have a significant proportion of metal rich giant stars. We feel that the stellar libraries used in population synthesis should discriminate between different metallicities among dwarf stars, particularly of late spectral type, to resolve this problem. Very cool metal rich giant stars such as now known to exist at the centre of our Galaxy, should also be used in the stellar library.

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