6,637 research outputs found
Stars beyond Galaxies: The Origin of Extended Luminous Halos around Galaxies
(Abridged) We use numerical simulations to investigate the origin and
structure of the luminous halos that surround isolated galaxies. These stellar
structures extend out to several hundred kpc away from a galaxy, and consist of
stars shed by merging subunits during the many accretion events that
characterize the hierarchical assembly of galaxies. Such origin suggests that
outer luminous halos are ubiquitous and that they should appear as an excess of
light over extrapolations of the galaxy's inner profile beyond its traditional
luminous radius. The mass profile of the accreted stellar component is well
approximated by a model where the logarithmic slope steepens monotonically with
radius; from -3 at the luminous edge of the galaxy to -4 or steeper near the
virial radius of the system. Such spatial distribution is consistent with that
of Galactic and M31 globular clusters, suggesting that many of the globulars
were brought in by accretion events, in a manner akin to the classic
Searle-Zinn scenario. The outer stellar spheroid is supported by a velocity
dispersion tensor with a substantial and radially increasing radial anisotropy.
These properties distinguish the stellar halo from the dark matter component,
which is more isotropic in velocity space, as well as from some tracers of the
outer spheroid such as satellite galaxies. Most stars in the outer halo formed
in progenitors that have since merged with the central galaxy; very few stars
in the halo are contributed by satellites that survive as self-bound entities
at the present. These features are in reasonable agreement with recent
observations of the outer halo of the MW, of M31, and of other isolated
spirals, and suggest that all of these systems underwent an early period of
active merging, as envisioned in hierarchical models of galaxy formation.Comment: Submitted to MNRAS, 13 pages, 12 figure
A Sagittarius-Induced Origin for the Monoceros Ring
The Monoceros ring is a collection of stars in nearly-circular orbits at
roughly 18 kpc from the Galactic center. It may have originated (i) as the
response of the disc to perturbations excited by satellite companions or (ii)
from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros
stars differs from that of disc stars at comparable Galactocentric distances,
an observation that disfavours the first scenario. On the other hand, circular
orbits are difficult to accommodate in the tidal-disruption scenario, since it
requires a satellite which at the time of disruption was itself in a nearly
circular orbit. Such satellite could not have formed at the location of the
ring and, given its low mass, dynamical friction is unlikely to have played a
major role in its orbital evolution. We search cosmological simulations for
low-mass satellites in nearly-circular orbits and find that they result, almost
invariably, from orbital changes induced by collisions with more massive
satellites: the radius of the circular orbit thus traces the galactocentric
distance of the collision. Interestingly, the Sagittarius dwarf, one of the
most luminous satellites of the Milky Way, is in a polar orbit that crosses the
Galactic plane at roughly the same Galactocentric distance as Monoceros. We use
idealized simulations to demonstrate that an encounter with Sagittarius might
well have led to the circularization and subsequent tidal demise of the
progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters
(http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract
Negative Cell Cycle Regulation and DNA Damage-inducible Phosphorylation of the BRCT Protein 53BP1
In a screen designed to discover suppressors of mitotic catastrophe, we identified the Xenopus ortholog of 53BP1 (X53BP1), a BRCT protein previously identified in humans through its ability to bind the p53 tumor suppressor. X53BP1 transcripts are highly expressed in ovaries, and the protein interacts with Xp53 throughout the cell cycle in embryonic extracts. However, no interaction between X53BP1 and Xp53 can be detected in somatic cells, suggesting that the association between the two proteins may be developmentally regulated. X53BP1 is modified via phosphorylation in a DNA damage-dependent manner that correlates with the dispersal of X53BP1 into multiple foci throughout the nucleus in somatic cells. Thus, X53BP1 can be classified as a novel participant in the DNA damage response pathway. We demonstrate that X53BP1 and its human ortholog can serve as good substrates in vitro as well as in vivo for the ATM kinase. Collectively, our results reveal that 53BP1 plays an important role in the checkpoint response to DNA damage, possibly in collaboration with ATM
Remote Preparation of Single-Photon "Hybrid" Entangled and Vector-Polarization States
Quantum teleportation faces increasingly demanding requirements for
transmitting large or even entangled systems. However, knowledge of the state
to be transmitted eases its reconstruction, resulting in a protocol known as
remote state preparation. A number of experimental demonstrations to date have
been restricted to single-qubit systems. We report the remote preparation of
two-qubit "hybrid" entangled states, including a family of vector-polarization
beams. Our single-photon states are encoded in the photon spin and orbital
angular momentum. We reconstruct the states by spin-orbit state tomography and
transverse polarization tomography. The high fidelities achieved for the
vector-polarization states opens the door to optimal coupling of down-converted
photons to other physical systems, such as an atom, as required for scalable
quantum networks, or plasmons in photonic nanostructures.Comment: Letter: 4 pages, 1 figure. Supplementary material: 1 pag
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