1,219 research outputs found
Electroweak Sudakov at two loop level
We investigate the Sudakov double logarithmic corrections to the form factor
of fermion in the SU(2)XU(1) electroweak theory. We adopt the familiar Feynman
gauge and present explicit calculations at the two loop level. We show that the
leading logarithmic corrections coming from the infrared singularities are
consistent with the "postulated" exponentiated electroweak Sudakov form factor.
The similarities and differences in the "soft" physics between the electroweak
theory and the unbroken non-abelian gauge theory (QCD) will be clarified.Comment: 8 pages, 14 figure
Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background
We examine the constraints on high-redshift star formation, ultraviolet and
X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred
from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron
scattering optical depth of the cosmic microwave background (CMB). Half of this
scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a
fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with
Gunn-Peterson absorption in neutral hydrogen. The required additional optical
depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing
contributions of first light sources. WMAP-5 also measured a significant
increase in small-scale power, which lowers the required efficiency of star
formation and ionization from mini-halos. Early massive stars (UV radiation)
and black holes (X-rays) can produce a partially ionized IGM, adding to the
residual electrons left from incomplete recombination. Inaccuracies in
computing the ionization history, x_e(z), and degeneracies in cosmological
parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the
measurement and modeling of . From the additional optical depth from
sources at z > z_GP, we limit the star-formation efficiency, the rate of
ionizing photon production for Pop III and Pop II stars, and the photon escape
fraction, using standard histories of baryon collapse, minihalo star formation,
and black-hole X-ray preionization.Comment: Greatly revised version, based on WMAP-5 results and new models.
Accepted for ApJ (2008
Classifying bases for 6D F-theory models
We classify six-dimensional F-theory compactifications in terms of simple
features of the divisor structure of the base surface of the elliptic
fibration. This structure controls the minimal spectrum of the theory. We
determine all irreducible configurations of divisors ("clusters") that are
required to carry nonabelian gauge group factors based on the intersections of
the divisors with one another and with the canonical class of the base. All 6D
F-theory models are built from combinations of these irreducible
configurations. Physically, this geometric structure characterizes the gauge
algebra and matter that can remain in a 6D theory after maximal Higgsing. These
results suggest that all 6D supergravity theories realized in F-theory have a
maximally Higgsed phase in which the gauge algebra is built out of summands of
the types su(3), so(8), f_4, e_6, e_8, e_7, (g_2 + su(2)), and su(2) + so(7) +
su(2), with minimal matter content charged only under the last three types of
summands, corresponding to the non-Higgsable cluster types identified through
F-theory geometry. Although we have identified all such geometric clusters, we
have not proven that there cannot be an obstruction to Higgsing to the minimal
gauge and matter configuration for any possible F-theory model. We also
identify bounds on the number of tensor fields allowed in a theory with any
fixed gauge algebra; we use this to bound the size of the gauge group (or
algebra) in a simple class of F-theory bases.Comment: 28 pages, 2 figures, 1 appendix. v2: added references, minor changes;
v3: two signs correcte
Luminosity functions of Lyman-alpha emitters at z=6.5, and z=5.7: evidence against reionization at z=6
Lyman-alpha emission from galaxies should be suppressed completely or
partially at redshifts beyond reionization. Without knowing the instrinsic
properties of galaxies at z = 6.5, this attenuation is hard to infer in any one
source, but can be infered from a comparison of luminosity functions of
lyman-alpha emitters at redshifts just before and after reionization. We
combine published surveys of widely varying depths and areas to construct
luminosity functions at z=6.5 and 5.7, where the characteristic luminosity
L_star and density phi_star are well constrained while the faint-end slope of
the luminosity function is essentially unconstrained. Excellent consistency is
seen in all but one published result. We then calculate the likelihood of
obtaining the z=6.5 observations given the z=5.7 luminosity function with (A)
no evolution and (B) an attenuation of a factor of three. Hypothesis (A) gives
an acceptable likelihood while (B) does not. This indicates that the z=6.5
lyman-alpha lines are not strongly suppressed by a neutral intergalactic medium
and that reionization was largely complete at z = 6.5.Comment: Submitted to Astrophysical Journal Letter
Deformations of Oka manifolds
We investigate the behaviour of the Oka property with respect to deformations
of compact complex manifolds. We show that in a family of compact complex
manifolds, the set of Oka fibres corresponds to a G-delta subset of the base.
We give a necessary and sufficient condition for the limit fibre of a sequence
of Oka fibres to be Oka in terms of a new uniform Oka property. We show that if
the fibres are tori, then the projection is an Oka map. Finally, we consider
holomorphic submersions with noncompact fibres
Unusually Large Fluctuations in the Statistics of Galaxy Formation at High Redshift
We show that various milestones of high-redshift galaxy formation, such as
the formation of the first stars or the complete reionization of the
intergalactic medium, occurred at different times in different regions of the
universe. The predicted spread in redshift, caused by large-scale fluctuations
in the number density of galaxies, is at least an order of magnitude larger
than previous expectations that argued for a sharp end to reionization. This
cosmic scatter in the abundance of galaxies introduces new features that affect
the nature of reionization and the expectations for future probes of
reionization, and may help explain the present properties of dwarf galaxies in
different environments. The predictions can be tested by future numerical
simulations and may be verified by upcoming observations. Current simulations,
limited to relatively small volumes and periodic boundary conditions, largely
omit cosmic scatter and its consequences. In particular, they artificially
produce a sudden end to reionization, and they underestimate the number of
galaxies by up to an order of magnitude at redshift 20.Comment: 8 ApJ pages, 4 figures, ApJ. Minor changes in revised version.
Originally first submitted for publication on Aug. 29, 200
Shakhbazian compact galaxy groups. II. Photometric and spectroscopic study of ShCG 376
The results of the redshift measurements and of the detailed surface
photometry in BVR of the compact group ShCG 376 are presented. The radial
velocity dispersion, the virial mass, the total luminosity, the M/L ratio, and
the crossing time of the group are estimated. The group consists of eight
accordant redshift spiral galaxies. Four (or possibly five) of the group
members have emission-line spectra. Such morphological content and the number
of emission-line galaxies are very atypical for compact galaxy groups. There
are signs of interaction between some members of the group. It is suggested
that the irregular shape of the brightest galaxy No. 4 is probably due to
interaction with other members of the group, particularly, the emission line
galaxy No. 6 with a discordant redshift (Delta v = 2600 km/s). It is speculated
that the latter galaxy may be a infalling intruder to the group.Comment: accepted A&A, 7 pages, 6 figures are in separate file
An Overdensity of Lyman-alpha Emitters at Redshift z=5.7 near the Hubble Ultra Deep Field
We have identified an obvious and strong large scale structure at redshift
z=5.75 in a wide (31 by 33 arcminute) field, narrowband survey of the Chandra
Deep Field South region. This structure is traced by 17 candidate Lyman alpha
emitters, among which 12 are found in an 823nm filter (corresponding to Lyman
alpha at z=5.77 +- 0.03) and 5 in an 815nm image (z=5.70 +- 0.03). The Lyman
alpha emitters in both redshift bins are concentrated in one quadrant of the
field. The Hubble Ultra Deep Field, Chandra Deep Field South, and GOODS-South
fields all lie near the edge of this overdense region. Our results are
consistent with reports of an overdensity in the UDF region at z=5.9. This
structure is the highest redshift overdensity found so far.Comment: 12 pages, AASTeX. Submitted to ApJ Letters, and revised in response
to referee's comment
Extension of holomorphic functions and cohomology classes from non reduced analytic subvarieties
The goal of this survey is to describe some recent results concerning the L 2
extension of holomorphic sections or cohomology classes with values in vector
bundles satisfying weak semi-positivity properties. The results presented here
are generalized versions of the Ohsawa-Takegoshi extension theorem, and borrow
many techniques from the long series of papers by T. Ohsawa. The recent
achievement that we want to point out is that the surjectivity property holds
true for restriction morphisms to non necessarily reduced subvarieties,
provided these are defined as zero varieties of multiplier ideal sheaves. The
new idea involved to approach the existence problem is to make use of L 2
approximation in the Bochner-Kodaira technique. The extension results hold
under curvature conditions that look pretty optimal. However, a major unsolved
problem is to obtain natural (and hopefully best possible) L 2 estimates for
the extension in the case of non reduced subvarieties -- the case when Y has
singularities or several irreducible components is also a substantial issue.Comment: arXiv admin note: text overlap with arXiv:1703.00292,
arXiv:1510.0523
Massive galaxies in cosmological simulations: UV-selected sample at redshift z=2
We study the properties of galaxies at z=2 in a Lambda CDM universe, using
two different types of hydrodynamic simulation methods (Eulerian TVD and SPH)
and a spectrophotometric analysis in the Un, G, R filter set. The simulated
galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et
al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that
the number density of simulated galaxies brighter than R<25.5 at z=2 is about
2e-2 h^3/Mpc^3, roughly one order of magnitude larger than that of Lyman break
galaxies at z=3. The most massive galaxies at z=2 have stellar masses >~1e11
Msun, and their observed-frame G-R colors lie in the range 0.0<G-R<1.0. They
typically have been continuously forming stars with a rate exceeding 30 Msun/yr
over a few Gyrs from z=10 to z=2, although the TVD simulation indicates a more
sporadic star formation history than the SPH simulations. Of order half of
their stellar mass was already assembled by z~4. The reddest massive galaxies
at z=2 with G-R >= 1.0 and Mstar>1e10 Msun/h finished the build-up of their
stellar mass by z~3. Interestingly, our study suggests that the majority of the
most massive galaxies at z=2 should be detectable at rest-frame UV wavelengths,
contrary to some recent claims made on the basis of near-IR studies of galaxies
at the same epoch, provided the median extinction is less than E(B-V)<0.3.
However, our results also suggest that the fraction of stellar mass contained
in galaxies that pass the color-selection criteria could be as low as 50% of
the total stellar mass in the Universe at z=2. Our simulations suggest that the
missing stellar mass is contained in fainter (R>25.5) and intrinsically redder
galaxies. Our results do not suggest that hierarchical galaxy formation fails
to account for the massive galaxies at z>=1. (abridged)Comment: 35 pages, 11 figures. Submitted to ApJ. Error in AB magnitude
calculation corrected. Higher resolution version available at
http://cfa-www.harvard.edu/~knagamine/redgal.ps.g
- âŠ