Maltby, David Terence
(2013)
The effect of the galaxy environment on the size and structure of galaxies.
PhD thesis, University of Nottingham.
Abstract
In this thesis, we explore the effect of the galaxy environment on the physical size and structure of the stellar distribution for relatively local galaxies (z < 0.3) using Hubble Space Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope A901/2 galaxy evolution survey (STAGES).
We determine the effect of the environment on the size of the stellar distribution (i.e. galaxy sizes) by comparing the stellar-mass-size relations in the field and cluster environments for different Hubble-type morphologies. For elliptical, lenticular, and high-mass (M* > 10^10 M_sun) spirals, we find no evidence to suggest that a galaxy's size (i.e. effective radius a_e) is dependent on the environment. This result suggests that internal drivers are responsible for any potential size evolution inherent to these galaxies. However, for intermediate-/low-mass spirals (M* < 10^10 M_sun) we do find some evidence for a possible environmental effect, with the mean galaxy size (<a_e>) being ~15-20 per cent larger in the field than in the cluster. This result is driven by a population of low-mass, large-a_e field spirals (observed to contain extended stellar discs) that are largely absent from the cluster environments. This difference implies that the fragile extended stellar discs of these spiral galaxies may not survive the environmental conditions in the cluster.
We expand on this result by investigating the effect of the environment on the structure of galactic discs in spiral and S0 galaxies. Using V-band radial surface brightness mu(r) profiles, we identify break features in the stellar disc (down-bending break - truncation; up-bending break - antitruncation) and evaluate their dependence on the galaxy environment. For both spiral and S0 galaxies, we find no evidence to suggest an environmental dependence on the frequency of these break features. We also find no evidence to suggest an environmental dependence on the scalelength h of pure exponential discs, or the break strength T (outer-to-inner scalelength ratio) of broken exponential discs. These results indicate that the stellar distribution in the outer regions of spiral/S0 galaxies is not significantly influenced by the galaxy environment.
In our structural analyses, one interesting observation was that truncated mu(r) profiles (down-bending breaks) are very rare in S0s; whereas in spiral galaxies they are commonplace. We expand on this result by comparing the structural properties of the disc (scalelength h, break strength T, break surface brightness mu_brk) in spiral and S0 galaxies. In these comparisons, we find no evidence to suggest that the scalelength h of pure exponential discs or the break surface brightness mu_brk of broken exponentials is dependent on the galaxy morphology. However, we do find some evidence to suggest that the break strength T is smaller (weaker) in S0s compared to spiral galaxies. This result suggests that some process inherent to the morphological transformation of spiral galaxies into S0s does affect the structure of the stellar disc causing a weakening of mu(r) breaks and may even eliminate truncations from S0 galaxies. In additional structural comparisons, we also find that the fraction of exponential bulges is the same (~20 per cent) in both spiral and S0 galaxies, suggesting that major mergers are not driving this transformation.
Finally, we complement our structural analyses with an assessment of whether the excess light in the outer regions of antitruncated (up-bending) mu(r) profiles is caused by an outer exponential disc or an extended spheroidal component: we use bulge-disc decomposition in order to achieve this. For spiral galaxies, in the vast majority of cases, evidence indicates that the excess light at large radii is related to an outer shallow disc. We thus conclude that in the majority of spiral galaxies, antitruncated outer stellar discs cannot be explained by bulge light and thus remain a pure disc phenomenon. However, for S0s, bulge light can have a significant effect in the outer regions of the mu(r) profile. In approximately half of S0 antitruncations, the excess light at large radii can be entirely accounted for by light from an extended spheroidal component. These results suggest that as a galaxy evolves from a spiral into an S0, the galaxy naturally evolves into a more bulge-dominated system. We suggest a fading stellar disc (e.g. caused by gas stripping and the termination of star formation) is consistent with this result.
In conclusion, our environmental studies indicate that the environment has little direct affect on the size and structure of a galaxy's stellar distribution. This result implies that physical processes directly affecting the structure of the stellar distribution (e.g. mergers or harassment), are not driving the observed morphology-density relation. With respect to both our environmental and morphological studies, we can conclude that more subtle processes acting on the gaseous component of a galaxy (e.g. ram-pressure stripping) are more likely to play an important role in the origin of the morphology-density relation and the transformation of spirals into S0s.
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