| dc.description.abstract |
The goal of this thesis is to reduce multidimensional space of galactic properties to
the smallest number of dimensions su cient to describe them. For this purpose, the
statistical analysis is applied over the parameters that describe fundamental galactic
properties on the morphologically representative sample of 2180 galaxies. The sample of
galaxies used in this thesis is based on the Arecibo Legacy Fast ALFA (Alfalfa) blind HI
survey.
The importance of an HI blind survey lies in the fact that galaxies are chosen on the
basis of their gas content (HI) solely, thus free of optical selection e ects. From the initial
sample counting 10000 galaxies, 2180 of them were chosen, since for this subsample
the optical spectroscopy from the Sloan Digital Sky Survey (SDSS) was available and
moreover the photometry in the UV (Galaxy Evolution Explorer, GALEX), and optical
(SDSS) to the near-infrared (Two Micron All Sky Survey, 2MASS).
Parameters are selected according to the previously established correlations between
fundamental galactic properties, relying on the previous work. They are extensively
tested and confronted between each other to be chosen from the larger parametric space.
To select parameters, we rst measured stellar kinematics using publicly available
code (pPXF), and tested both empirical and synthetic stellar libraries. In particular,
we have measured the velocity dispersion and the higher moments of the line-of-sight
velocity distribution function. This is the largest galaxy sample created so far with detailed
stellar kinematics measured including higher moments of the line-of-sight velocity
distribution function. The sample size allows statistical tests to be applied to the higher
moment of the velocity distribution function (h4), with respect to the di erent groups
of morphological galaxy types. Various tests agree with the previous indication that
elliptical and lenticular galaxies have the same origin.
Further, we have measured the line strength indices for several absorption lines (Lick
indices), since some of them are good proxies to galaxy ages and metallicity, also the
fundamental galactic properties. In the nal statistical analysis, metallicity proves to
be of no importance, but the inclusion of galaxy ages in the analysis, the results change
signi cantly.
The last step in the parameter selection is the modelling of the galaxies' surface
brightness pro les with the Sersic pro le, that is performed in this thesis with the
Gal t code. The velocity dispersion measured, along with the Sersic index and effective
radius of the Sersic pro le takes the role in the dynamical mass calculation,
being the fundamental galactic property and hence used in the nal statistical analysis.
Finally, we have taken the mass of the gas component and maximal rotational
velocity from the radio-spectroscopy and Kron magnitudes (i.e. colours) from the ultraviolet/
optical/nearinfrared photometry (GALEX/SDSS/2MASS databases). After
extensive testing, we have chosen the colour calculated from ultraviolet and optical magnitudes
(NUV r colour), for the nal statistical analysis. It is worth noting that
previous analysis of the galactic properties lack velocity dispersion, as well as the colour
with the ultraviolet component, although it is a direct proxy to the speci c star formation
rate in the galaxy. This particular colour makes correlations among analysed
parameters stronger and proves to be more important than optical colours.
Finally, when the proper parametric space of galactic properties is formed (velocity
dispersion, colour, luminosity, Petrosian radii R50 and R90, dynamical, HI and stellar
masses, maximal rotational velocity and the galaxy ages), the correlation analysis is
performed to inspect correlations between parameters. This analysis con rms relations
that are already known to hold. Then the principal component analysis is done with the
purpose of nding and identifying the smallest number of galactic properties responsible
for the nal products of galaxy evolution, as we see today, in the local Universe.
The results of the corresponding analysis are the following: there are at least three
statistically important, independent components. The rst and the most important
component cannot be identi ed with either galactic property, but presents the mixture
of several properties: dynamical mass, mass of the stellar and gas component, luminosity
and Petrosian radii R50 and R90. Relaying on the previous work, this component may be
identi ed with the "size" of the galaxies. The second component, mostly in
uenced by
the galactic colour, may be identi ed with the "aspect" of the galaxies. The colour was
not found to be important in previous work. The galaxy ages can be identi ed with the
third principal component. There is a hint on the fourth component, dominated by the
maximal rotational velocity that can be identi ed with the speci c angular momentum
of galaxies. Although not proven to be statistically important, it may become so in
the larger sample of galaxies which will provide the information of the true peak of
the galaxies' rotational curves, since the single-beam HI spectra may show the single
maximum and this may not be the true maximum. Also, the rotational velocity includes
the inclination correction, another questionable parameter in the analysis.
To conclude: there are at least three, and possibly four dimensions of the multidimensional
galactic space, as we see today. |
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