Properties of Isolated Galaxies within a Redshift Range of 0.005 < Z < 0.080
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Date
2024-05
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Kenyatta University
Abstract
In the study of galaxy formation and evolution process, a number of processes and
properties are known to influence the formation and evolution process such as
protogalactic clouds, galaxy masses and galaxy environment. Despite studies done on
different environments, the effect of environment on galaxy properties has not been
fully quantified. In this study, a sample of isolated galaxies within a redshift of 0.005
< z < 0.080, from the Sloan Digital Sky Survey, data release sixteen, (SDSS-DR 16)
was analysed. The aim of the study was to investigate the physical properties of single
isolated galaxies in low-density environment, to understand the internal processes and
mechanisms dominating galaxy evolution. The classification of galaxies into late-type
(mostly spirals) and early-type (mostly ellipticals) in the sample was first done using
the central concentration index tool. A Concentration Index, Cr , of 2.65 well classified
the sample. The automated classification was later confirmed using visual
classifications from the galaxy zoo, were the spiral galaxies dominated the sample of
isolated galaxies with about 68%. Through the analysis of colour, magnitude and
stellar mass relations, the isolated elliptical galaxies are redder, massive and more
luminous, while spiral galaxies seem biased towards the blue, less massive and less
luminous side. However, a significantly larger fraction of isolated red spiral galaxies
with larger stellar masses and highly luminous was observed. A slightly different
distribution by both the isolated blue elliptical and spiral galaxies, characterised by
lower stellar masses at lower luminosity was also noted. With the tight correlation of
the (u − r) and (r − z) colours, the galaxy sample was analysed as star-formation,
quiescent with a fraction of recently quenched elliptical galaxies (RQE). Most of the
elliptical galaxies were well selected as quiescent, with majority of the spirals being
star forming. This was expected since most elliptical galaxies are thought to be older
and have quenched their star-formation, while the spirals are younger and have star forming activities. 5% of the isolated elliptical galaxies seem to have recently
quenched their star formation and are transiting to the red sequence, with 90% being
blue. The red recently quenched elliptical galaxies are among the 20% low-mass red
elliptical galaxies, confirming that most massive elliptical galaxies quenched their star
formation long ago. The Hα luminosity star formation rates were derived and
correlated with the galaxy morphology and stellar masses. From the analysis, spiral
galaxies have higher star formation rates, indicating strong star formation activities,
while elliptical galaxies dominate the passive region. An indicated fraction of
passively evolving high-mass spiral galaxies and actively star-forming elliptical
galaxies was observed. In the emission line diagnostics, 50% of the spiral galaxies had
higher star-forming activities, while 54% of the elliptical galaxies dominated in the
Active Galactic Nuclear (AGN) powered region. A number of galaxies, both spirals
and ellipticals are in transition from star-forming to AGN and occupy the composite
region. The star forming galaxies selected in the colour - colour analysis tightly
follows the star formation sequence, with 47% having star forming emissions and only
18% have AGN emissions. Recommendations for future work includes, a detailed
study on the effect of galaxy properties on the central concentration index
morphological classification tool, and further investigation on the powering
mechanism of isolated galaxies with other methods.
Description
A Thesis Submitted in Partial Fulfillment of the Requirements for the Award of the Degree of Master of Science in Physics in the School of Pure and Applied Sciences of Kenyatta University, May 2024.
Supervisors
1. Naftali Kimani
2. Raphael Nyenge