12,316 research outputs found
The signature of dissipation in the mass-size relation: are bulges simply spheroids wrapped in a disc?
The relation between the stellar mass and size of a galaxy's structural
subcomponents, such as discs and spheroids, is a powerful way to understand the
processes involved in their formation. Using very large catalogues of
photometric bulge+disc structural decompositions and stellar masses from the
Sloan Digital Sky Survey Data Release Seven, we carefully define two large
subsamples of spheroids in a quantitative manner such that both samples share
similar characteristics with one important exception: the 'bulges' are embedded
in a disc and the 'pure spheroids' are galaxies with a single structural
component. Our bulge and pure spheroid subsample sizes are 76,012 and 171,243
respectively. Above a stellar mass of ~ M, the mass-size
relations of both subsamples are parallel to one another and are close to lines
of constant surface mass density. However, the relations are offset by a factor
of 1.4, which may be explained by the dominance of dissipation in their
formation processes. Whereas the size-mass relation of bulges in discs is
consistent with gas-rich mergers, pure spheroids appear to have been formed via
a combination of 'dry' and 'wet' mergers.Comment: Accepted for publication in MNRAS, 6 pages, 3 figure
New tests of the pp-wave correspondence.
The pp-wave/SYM correspondence is an equivalence relation, H string = Δ-J , between the hamiltonian H string of string field theory in the pp-wave background and the dilatation operator Δ in = 4 Super Yang-Mills in the double scaling limit. We calculate matrix elements of these operators in string field theory and in gauge theory. In the string theory Hilbert space we use the natural string basis, and in the gauge theory we use the basis which is isomorphic to it. States in this basis are specific linear combinations of the original BMN operators, and were constructed previously for the case of two scalar impurities. We extend this construction to incorporate BMN operators with vector and mixed impurities. This enables us to verify from the gauge theory perspective two key properties of the three-string interaction vertex of Spradlin and Volovich: (1) the vanishing of the three-string amplitude for string states with one vector and one scalar impurity; and (2) the relative minus sign in the string amplitude involving states with two vector impurities compared to that with two scalar impurities. This implies a spontaneous breaking of the 2 symmetry of the string field theory in the pp-wave background. Furthermore, we calculate the gauge theory matrix elements of Δ-J for states with an arbitrary number of scalar impurities. In all cases we find perfect agreement with the corresponding string amplitudes derived from the three-string vertex
Core-collapse model of broadband emission from SNR RX J1713.7-3946 with thermal X-rays and Gamma-rays from escaping cosmic rays
We present a spherically symmetric, core-collapse model of SNR RX
J1713.7-3946 that includes a hydrodynamic simulation of the remnant evolution
coupled to the efficient production of cosmic rays (CRs) by nonlinear diffusive
shock acceleration (DSA). High-energy CRs that escape from the forward shock
(FS) are propagated in surrounding dense material that simulates either a
swept-up, pre-supernova shell or a nearby molecular cloud. The continuum
emission from trapped and escaping CRs, along with the thermal X-ray emission
from the shocked heated ISM behind the FS, integrated over the remnant, is
compared against broadband observations. Our results show conclusively that,
overall, the GeV-TeV emission is dominated by inverse-Compton from CR electrons
if the supernova is isolated regardless of its type, i.e., not interacting with
a >>100 Msun shell or cloud. If the SNR is interacting with a much larger mass
>10^4 Msun, pion-decay from the escaping CRs may dominate the TeV emission,
although a precise fit at high energy will depend on the still uncertain
details of how the highest energy CRs are accelerated by, and escape from, the
FS. Based on morphological and other constraints, we consider the 10^4 Msun
pion-decay scenario highly unlikely for SNR RX J1713.7-3946 regardless of the
details of CR escape. Importantly, even though CR electrons dominate the
GeV-TeV emission, the efficient production of CR ions is an essential part of
our leptonic model.Comment: 11 pages with 7 figures. Submitted to Ap
Entropy-driven formation of the gyroid cubic phase
We show, by computer simulation, that tapered or pear-shaped particles, interacting through purely repulsive interactions, can freely self-assemble to form the three-dimensionally periodic, gyroid cubic phase. The Ia3d gyroid cubic phase is formed by these particles both on compression of an isotropic configuration and on expansion of a smectic A bilayer arrangement. For the latter case, it is possible identify the steps by which the topological transformation from non-intersecting planes to fully interpenetrating, periodic networks takes place</p
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