994 research outputs found
Redshift-Independent Distances to Type Ia Supernovae
We describe a procedure for accurately determining luminosity distances to
Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure,
which may be used as an extension of any of the various distance determination
methods currently in use, is based on marginalizing over redshift, removing the
requirement of knowing a priori. We demonstrate that the Hubble diagram
scatter of distances measured with this technique is approximately equal to
that of distances derived from conventional redshift-specific methods for a set
of 60 nearby SNe Ia. This indicates that accurate distances for cosmological
SNe Ia may be determined without the requirement of spectroscopic redshifts,
which are typically the limiting factor for the number of SNe that modern
surveys can collect. Removing this limitation would greatly increase the number
of SNe for which current and future SN surveys will be able to accurately
measure distance. The method may also be able to be used for high- SNe Ia to
determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical
Journal Letter
Accelerated Expansion of the Universe in Gauss-Bonnet Gravity
We show that in Gauss-Bonnet gravity with negative Gauss-Bonnet coefficient
and without a cosmological constant, one can explain the acceleration of the
expanding Universe. We first introduce a solution of the Gauss-Bonnet gravity
with negative Gauss-Bonnet coefficient and no cosmological constant term in an
empty -dimensional bulk. This solution can generate a de Sitter
spacetime with curvature . We show that an
-dimensional brane embedded in this bulk can have an expanding feature
with acceleration. We also considered a 4-dimensional brane world in a
5-dimensional empty space with zero cosmological constant and obtain the
modified Friedmann equations. The solution of these modified equations in
matter-dominated era presents an expanding Universe with negative deceleration
and positive jerk which is consistent with the recent cosmological data. We
also find that for this solution, the derivative of the scale factor
with respect to time can be expressed only in terms of Hubble and deceleration
parameters.Comment: 12 pages, no figure, references added, typos corrected, Section 4
ammended, an appndix added, version to be appeared in Phys. Rev.
Asymptotically (anti)-de Sitter solutions in Gauss-Bonnet gravity without a cosmological constant
In this paper we show that one can have asymptotically de Sitter (dS),
anti-de Sitter (AdS) and flat solutions in Gauss-Bonnet gravity without any
need to a cosmological constant term in field equations. First, we introduce
static solutions whose 3-surfaces at fixed and have constant positive
(), negative (), or zero () curvature. We show that for
, one can have asymptotically dS, AdS and flat spacetimes, while for
the case of , one has only asymptotically AdS solutions. Some of these
solutions present naked singularities, while some others are black hole or
topological black hole solutions. We also find that the geometrical mass of
these 5-dimensional spacetimes is , which is different from
the geometrical mass, , of the solutions of Einstein gravity. This feature
occurs only for the 5-dimensional solutions, and is not repeated for the
solutions of Gauss-Bonnet gravity in higher dimensions. We also add angular
momentum to the static solutions with , and introduce the asymptotically
AdS charged rotating solutions of Gauss-Bonnet gravity. Finally, we introduce a
class of solutions which yields an asymptotically AdS spacetime with a
longitudinal magnetic field which presents a naked singularity, and generalize
it to the case of magnetic rotating solutions with two rotation parameters.Comment: 13 pages, no figur
Dark Matter Relic Abundance and Scalar-Tensor Dark Energy
Scalar-tensor theories of gravity provide a consistent framework to
accommodate an ultra-light quintessence scalar field. While the equivalence
principle is respected by construction, deviations from General Relativity and
standard cosmology may show up at nucleosynthesis, CMB, and solar system tests
of gravity. After imposing all the bounds coming from these observations, we
consider the expansion rate of the universe at WIMP decoupling, showing that it
can lead to an enhancement of the dark matter relic density up to few orders of
magnitude with respect to the standard case. This effect can have an impact on
supersymmetric candidates for dark matter.Comment: 12 pages, 13 figures; V2: references added, matches published versio
The mass-metallicity gradient relation of early-type galaxies
We present a newly observed relation between galaxy mass and radial
metallicity gradients of early-type galaxies. Our sample of 51 early-type
galaxies encompasses a comprehensive mass range from dwarf to brightest cluster
galaxies. The metallicity gradients are measured out to one effective radius by
comparing nearly all of the Lick absorption-line indices to recent models of
single stellar populations. The relation shows very different behaviour at low
and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10
M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a
tight relation with mass, such that metallicity gradients become shallower with
decreasing mass and positive at the very low-mass end. Above the mass
transition point several massive galaxies have steeper gradients, but a clear
downturn is visible marked by a broad scatter. The results are interpreted in
comparison with competing model predictions. We find that an early star-forming
collapse could have acted as the main mechanism for the formation of low-mass
galaxies, with star formation efficiency increasing with galactic mass. The
high-mass downturn could be a consequence of merging and the observed larger
scatter a natural result of different merger properties. These results suggest
that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed
initially by mergers of gas-rich disc galaxies and then subsequently evolved
via dry merger events. The varying efficiency of the dissipative merger-induced
starburst and feedback processes have shaped the radial metallicity gradients
in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette
Low-mass X-ray binaries and globular clusters streamers and ARCS in NGC 4278
We report significant inhomogeneities in the projected two-dimensional spatial distributions of low-mass X-ray binaries (LMXBs) and globular clusters (GCs) of the intermediate mass elliptical galaxy NGC 4278. In the inner region of NGC 4278, a significant arc-like excess of LMXBs extending south of the center at ∼50″ in the western side of the galaxy can be associated with a similar overdensity of the spatial distribution of red GCs from Brassington et al. Using a recent catalog of GCs produced by Usher et al. and covering the whole field of the NGC 4278 galaxy, we have discovered two other significant density structures outside the D 25 isophote to the W and E of the center of NGC 4278, associated with an overdensity and an underdensity, respectively. We discuss the nature of these structures in the context of the similar spatial inhomogeneities discovered in the LMXBs and GCs populations of NGC 4649 and NGC 4261, respectively. These features suggest streamers from disrupted and accreted dwarf companions.Peer reviewe
Core-collapse supernovae in low-metallicity environments and future all-sky transient surveys
Aims: Massive stars in low-metallicity environments may produce exotic
explosions such as long-duration gamma-ray bursts and pair-instability
supernovae when they die as core-collapse supernovae (CCSNe). Here we determine
the feasibility of searching for these CCSNe in metal-poor dwarf galaxies using
various survey strategies. Methods: We determine oxygen abundances and
star-formation rates for all spectroscopically typed star-forming galaxies in
the Sloan Digital Sky Survey, Data Release 5, within z = 0.04. We then estimate
the CCSN rates for sub-samples of galaxies with differing upper-metallicity
limits. Using Monte-Carlo simulations we then predict the fraction of these
CCSNe that we can expect to detect using different survey strategies. Results:
Using a single 2m telescope (with a standard CCD camera) search we predict a
detection rate of ~1.3 CCSNe/yr in galaxies with metallicities below 12 +
log(O/H) < 8.2 which are within a volume that will allow detailed follow-up
with 4m and 8m telescopes (z = 0.04). With a network of seven 2m telescopes we
estimate ~9.3 CCSNe/yr could be found, although this would require more than
1,000 hrs of telescope time allocated to the network. Within the same radial
distance, a volume-limited search in the future Pan-STARRS PS1 all-sky survey
should uncover 12.5 CCSNe/yr in low-metallicity galaxies. Over a period of a
few years this would allow a detailed comparison of their properties. We then
extend our calculations to determine the total numbers of CCSNe that can
potentially be found in magnitude-limited surveys with PS1 (24,000/yr, within z
< 0.6), PS4 (69,000/yr, within z < 0.8) and LSST (160,000/yr, within z < 0.9)
surveys.Comment: Accepted by journal Astronomy & Astrophysic
Quintessential Inflation on the Brane and the Relic Gravity Wave Background
Quintessential inflation describes a scenario in which both inflation and
dark energy (quintessence) are described by the same scalar field. In
conventional braneworld models of quintessential inflation gravitational
particle production is used to reheat the universe. This reheating mechanism is
very inefficient and results in an excessive production of gravity waves which
violate nucleosynthesis constraints and invalidate the model. We describe a new
method of realizing quintessential inflation on the brane in which inflation is
followed by `instant preheating' (Felder, Kofman & Linde 1999). The larger
reheating temperature in this model results in a smaller amplitude of relic
gravity waves which is consistent with nucleosynthesis bounds. The relic
gravity wave background has a `blue' spectrum at high frequencies and is a
generic byproduct of successful quintessential inflation on the brane.Comment: 9 pages, 5 eps figures. Discussion and one eps figure summarizing the
GB correction to steep brane world inflation added, typos corrected and
references added; final version to appear in PR
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