9,994 research outputs found
The Scalar Sector in 331 Models
We calculate the exact tree-level scalar mass matrices resulting from
symmetry breaking using the most general gauge-invariant scalar potential of
the 331 model, both with and without the condition that lepton number is
conserved. Physical masses are also obtained in some cases, as well as
couplings to standard and exotic gauge bosons.Comment: LaTex, 15 page
Dialogues in the making: Collaborative archaeology in Sudan
In this paper we introduce the Mograt Island Collaborative Project. Launched in 2014, this project aims to share archaeological narratives with members of a local community in Sudan, and with other stakeholders engaged with the area’s past, building on a collaborative process which investigates how archaeological outputs can be explored together in a meaningful way. We discuss the wider context in which the project takes place, its individual steps, the factual outcome of its first phase, and its evaluation in spring 2018. We share this case study to promote collaborative practice in postcolonial, present-day majority Muslim contexts, such as the Nile valley countries, where the approach is thus far underrepresented. Reflecting on the project’s trajectory and its results, we also present critical thoughts on its potentials and challenges which can be helpful for readers working in similar contexts
Nonlocal Gravity: Modified Poisson's Equation
The recent nonlocal generalization of Einstein's theory of gravitation
reduces in the Newtonian regime to a nonlocal and nonlinear modification of
Poisson's equation of Newtonian gravity. The nonlocally modified Poisson
equation implies that nonlocality can simulate dark matter. Observational data
regarding dark matter provide limited information about the functional form of
the reciprocal kernel, from which the original nonlocal kernel of the theory
must be determined. We study this inverse problem of nonlocal gravity in the
linear domain, where the applicability of the Fourier transform method is
critically examined and the conditions for the existence of the nonlocal kernel
are discussed. This approach is illustrated via simple explicit examples for
which the kernels are numerically evaluated. We then turn to a general
discussion of the modified Poisson equation and present a formal solution of
this equation via a successive approximation scheme. The treatment is
specialized to the gravitational potential of a point mass, where in the linear
regime we recover the Tohline-Kuhn approach to modified gravity.Comment: 27 pages, 4 figures; v2: minor improvements, accepted for publication
in J. Math. Phy
Ultralight Scalars and Spiral Galaxies
We study some possible astrophysical implications of a very weakly coupled
ultralight dilaton-type scalar field. Such a field may develop an
(approximately stable) network of domain walls. The domain wall thickness is
assumed to be comparable with the thickness of the luminous part of the spiral
galaxies. The walls provide trapping for galactic matter. This is used to
motivate the very existence of the spiral galaxies. A zero mode existing on the
domain wall is a massless scalar particle confined to 1+2 dimensions. At
distances much larger than the galaxy/wall thickness, the zero-mode exchange
generates a logarithmic potential, acting as an additional term with respect to
Newton's gravity. The logarithmic term naturally leads to constant rotational
velocities at the periphery. We estimate the scalar field coupling to the
matter energy-momentum tensor needed to fit the observable flat rotational
curves of the spiral galaxies. The value of this coupling turns out to be
reasonable -- we find no contradiction with the existing data.Comment: 19 pages, 2 eps figures; extra references and two important Comments
adde
Forming Young Bulges within Existing Disks: Statistical Evidence for External Drivers
Contrary to traditional models of galaxy formation, recent observations
suggest that some bulges form within preexisting disk galaxies. Such late-epoch
bulge formation within disks seems to be linked to disk gas inflow and central
star formation, caused by either internal secular processes or galaxy mergers
and interactions. We identify a population of galaxies likely to be
experiencing active bulge growth within disks, using the criterion that the
color within the half-light radius is bluer than the outer disk color. Such
blue-centered galaxies make up >10% of star-forming disk galaxies within the
Nearby Field Galaxy Survey, a broad survey designed to represent the natural
diversity of the low-z galaxy population over a wide range of luminosities and
environments. Blue-centered galaxies correlate at 99% confidence with
morphological peculiarities suggestive of minor mergers and interactions. From
this and other evidence, we argue that external drivers rather than internal
secular processes probably account for the majority of blue-centered galaxies.
We go on to discuss quantitative plausibility arguments indicating that
blue-centered evolutionary phases may represent an important mode of bulge
growth for most disk galaxies, leading to significant changes in bulge-to-disk
ratio without destroying disks. If this view is correct, bulge growth within
disks may be a natural consequence of the repeated galaxy mergers and
interactions inherent in hierarchical galaxy formation.Comment: 18 pages including 12 figures, AJ, accepte
A New H I Survey of Active Galaxies
We have conducted a new Arecibo survey for H I emission for 113 galaxies with
broad-line (type 1) active galactic nuclei (AGNs) out to recession velocities
as high as 35,000 km/s. The primary aim of the study is to obtain sensitive H I
spectra for a well-defined, uniformly selected sample of active galaxies that
have estimates of their black hole masses in order to investigate correlations
between H I properties and the characteristics of the AGNs. H I emission was
detected in 66 out of the 101 (65%) objects with spectra uncorrupted by radio
frequency interference, among which 45 (68%) have line profiles with adequate
signal-to-noise ratio and sufficiently reliable inclination corrections to
yield robust deprojected rotational velocities. This paper presents the basic
survey products, including an atlas of H I spectra, measurements of H I flux,
line width, profile asymmetry, optical images, optical spectroscopic
parameters, as well as a summary of a number of derived properties pertaining
to the host galaxies. To enlarge our primary sample, we also assemble all
previously published H I measurements of type 1 AGNs for which can can estimate
black hole masses, which total an additional 53 objects. The final
comprehensive compilation of 154 broad-line active galaxies, by far the largest
sample ever studied, forms the basis of our companion paper, which uses the H I
database to explore a number of properties of the AGN host galaxies.Comment: To appear in ApJS; 31 pages. Preprint will full-resolution figures
can be downloaded from http://www.ociw.edu/~lho/preprints/ms1.pd
Receptor-like tyrosine phosphatase PTP10D is required for long-term memory in Drosophila
Tyrosine phosphorylation mediates multiple signal transduction pathways that play key roles in developmental processes and behavioral plasticity. The level of tyrosine phosphorylation is regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs). Extensive studies have investigated the roles of tyrosine kinases in memory formation. However, there were few studies on PTPs. To date, learning has been shown to be defective only for mouse knock-outs of PTPα, leukocyte common antigen-related, or PTPδ. A major limitation of these studies arises from their inability to distinguish an acute (biochemical) impairment ofmemoryformation from a more chronic abnormality in neurodevelopment. From a behavioral screen for defective long-term memory, we found chi mutants to disrupt expression of the PTP10D protein tyrosine phosphatase gene. We show that chi mutants are normal for learning, early memory, and anesthesia-resistant memory, whereas long-term memory specifically is abolished. Significantly, induction of a heat shock-PTP10D+ transgene before training fully rescues the memory defect of chi mutants, thereby demonstrating an acute role for PTP10D in behavioral plasticity. We show that PTP10D is widely expressed in the embryonic CNS and in the adult brain. Transgenic expression of upstream activating sequence-PTP10D+ in mushroom bodies is sufficient to rescue the memory defect of chi mutants. Our data clearly demonstrate that signaling through PTP10D in mushroom bodies is critical for the formation of long-term memory. Copyright © 2007 Society for Neuroscience
Self-consistent nonspherical isothermal halos embedding zero-thickness disks
Disk-halo decompositions of galaxy rotation curves are generally performed in
a parametric way. We construct self-consistent models of nonspherical
isothermal halos embedding a zero-thickness disk, by assuming that the halo
distribution function is a Maxwellian. The method developed here can be used to
study other physically-based choices for the halo distribution function and the
case of a disk accompanied by a bulge. In a preliminary investigation we note
the existence of a fine tuning between the scalelengths R_{\Omega} and h,
respectively characterizing the rise of the rotation curve and the luminosity
profile of the disk, which surprisingly applies to both high surface brightness
and low surface brightness galaxies. This empirical correlation identifies a
much stronger conspiracy than the one required by the smoothness and flatness
of the rotation curve (disk-halo conspiracy). The self-consistent models are
characterized by smooth and flat rotation curves for very different
disk-to-halo mass ratios, hence suggesting that conspiracy is not as dramatic
as often imagined. For a typical rotation curve, with asymptotically flat
rotation curve at V_{\infty} (the precise value of which can also be treated as
a free parameter), and a typical density profile of the disk, self-consistent
models are characterized by two dimensionless parameters, which correspond to
the dimensional scales (the disk mass-to-light ratio M/L and the halo central
density) of standard disk-halo decompositions. We show that if the rotation
curve is decomposed by means of our self-consistent models, the disk-halo
degeneracy is removed and typical rotation curves are fitted by models that are
below the maximum-disk prescription. Similar results are obtained from a study
of NGC 3198. Finally, we quantify the flattening of the spheroidal halo, which
is significant, especially on the scale of the visible disk.Comment: accepted for publication in A&
Semi-Analytical Models for the Formation of Disk Galaxies: I. Constraints from the Tully-Fisher Relation
We present new semi-analytical models for the formation of disk galaxies with
the purpose of investigating the origin of the near-infrared Tully-Fisher (TF)
relation. The models assume that disks are formed by cooling of the baryons
inside dark halos with realistic density profiles, and that the baryons
conserve their specific angular momentum. Only gas with densities above the
critical density given by Toomre's stability criterion is considered eligible
for star formation, and a simple recipe for supernovae feedback is included. We
emphasize the importance of extracting the proper luminosity and velocity
measures from the models, something that has often been ignored in the past.
The observed K-band TF relation has a slope that is steeper than simple
predictions based on dynamical arguments suggest. Taking the stability related
star formation threshold densities into account steepens the TF relation,
decreases its scatter, and yields gas mass fractions that are in excellent
agreement with observations. In order for the TF slope to be as steep as
observed, further physics are required. We argue that the characteristics of
the observed near-infrared TF relation do not reflect systematic variations in
stellar populations, or cosmological initial conditions, but are governed by
feedback. Finally we show that our models provide a natural explanation for the
small amount of scatter that makes the TF relation useful as a cosmological
distance indicator.Comment: 20 pages, 10 figures. Accepted for publication in Ap
The Baryonic Tully-Fisher Relation
We explore the Tully-Fisher relation over five decades in stellar mass in
galaxies with circular velocities ranging over 30 < Vc < 300 km/s. We find a
clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90
km/s fall below the relation defined by brighter galaxies. These faint galaxies
are however very gas rich; adding in the gas mass and plotting baryonic disk
mass Md = M* + Mg in place of luminosity restores a single linear relation. The
Tully-Fisher relation thus appears fundamentally to be a relation between
rotation velocity and total baryonic mass of the form Md = A Vc^4.Comment: 10 pages including 1 color figure. Accepted for publication in ApJ
Letter
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