8,266 research outputs found
On the limits of measuring the bulge and disk properties of local and high-redshift massive galaxies
A considerable fraction of the massive quiescent galaxies at \emph{z}
2, which are known to be much more compact than galaxies of
comparable mass today, appear to have a disk. How well can we measure the bulge
and disk properties of these systems? We simulate two-component model galaxies
in order to systematically quantify the effects of non-homology in structures
and the methods employed. We employ empirical scaling relations to produce
realistic-looking local galaxies with a uniform and wide range of
bulge-to-total ratios (), and then rescale them to mimic the
signal-to-noise ratios and sizes of observed galaxies at \emph{z} 2.
This provides the most complete set of simulations to date for which we can
examine the robustness of two-component decomposition of compact disk galaxies
at different . We confirm that the size of these massive, compact galaxies
can be measured robustly using a single S\'{e}rsic fit. We can measure
accurately without imposing any constraints on the light profile shape of the
bulge, but, due to the small angular sizes of bulges at high redshift, their
detailed properties can only be recovered for galaxies with \gax\ 0.2.
The disk component, by contrast, can be measured with little difficulty
Systematic {\em ab initio} study of the phase diagram of epitaxially strained SrTiO
We use density-functional theory with the local-density approximation to
study the structural and ferroelectric properties of SrTiO under misfit
strains. Both the antiferrodistortive (AFD) and ferroelectric (FE)
instabilities are considered. The rotation of the oxygen octahedra and the
movement of the atoms are fully relaxed within the constraint of a fixed
in-plane lattice constant. We find a rich misfit strain-induced phase
transition sequence and is obtained only when the AFD distortion is taken into
account. We also find that compressive misfit strains induce ferroelectricity
in the tetragonal low temperature phase only whilst tensile strains induce
ferroelectricity in the orthorhombic phases only. The calculated FE
polarization for both the tetragonal and orthorhombic phases increases
monotonically with the magnitude of the strains. The AFD rotation angle of the
oxygen octahedra in the tetragonal phase increases dramatically as the misfit
strain goes from the tensile to compressive strain region whilst it decreases
slightly in the orthorhombic (FO4) phase. This reveals why the polarization in
the epitaxially strained SrTiO would be larger when the tensile strain is
applied, since the AFD distortion is found to reduce the FE instability and
even to completely suppress it in the small strain region. Finally, our
analysis of the average polar distortion and the charge density distribution
suggests that both the Ti-O and Sr-O layers contribute significantly to the FE
polarization
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