79,583 research outputs found
A systematic study of the inner rotation curves of galaxies observed as part of the GASS and COLD GASS surveys
We present a systematic analysis of the rotation curves of 187 galaxies with
masses greater than 10^10 M_sol, with atomic gas masses from the GALEX Arecibo
Sloan Survey (GASS), and with follow-up long-slit spectroscopy from the MMT.
Our analysis focuses on stellar rotation curves derived by fitting stellar
template spectra to the galaxy spectra binned along the slit. In this way, we
are able to obtain accurate rotation velocity measurements for a factor of 2
more galaxies than possible with the Halpha line. Galaxies with high atomic gas
mass fractions are the most dark-matter dominated galaxies in our sample and
have dark matter halo density profiles that are well fit by Navarro, Frenk &
White profiles with an average concentration parameter of 10. The inner slopes
and of the rotation curves correlate more strongly with stellar population age
than with galaxy mass or structural parameters. At fixed stellar mass, the
rotation curves of more actively star-forming galaxies have steeper inner
slopes than less actively star-forming galaxies. The ratio between the galaxy
specific angular momentum and the total specific angular momentum of its dark
matter halo, R_j, correlates strongly with galaxy mass, structure and gas
content. Low mass, disk-dominated galaxies with atomic gas mass fractions
greater than 20% have median values of R_j of around 1, but massive,
bulge-dominated galaxies have R_j=0.2-0.3. We argue that these trends can be
understood in a picture where gas inflows triggered by disk instabilities lead
to the formation of passive, bulge-dominated galaxies with low specific angular
momentum.Comment: 10 pages, 12 figures, submitted to MNRA
The CMB and the measure of the multiverse
In the context of eternal inflation, cosmological predictions depend on the
choice of measure to regulate the diverging spacetime volume. The spectrum of
inflationary perturbations is no exception, as we demonstrate by comparing the
predictions of the fat geodesic and causal patch measures. To highlight the
effect of the measure---as opposed to any effects related to a possible
landscape of vacua---we take the cosmological model, including the model of
inflation, to be fixed. We also condition on the average CMB temperature
accompanying the measurement. Both measures predict a 1-point expectation value
for the gauge-invariant Newtonian potential, which takes the form of a
(scale-dependent) monopole, in addition to a related contribution to the
3-point correlation function, with the detailed form of these quantities
differing between the measures. However, for both measures both effects are
well within cosmic variance. Our results make clear the theoretical relevance
of the measure, and at the same time validate the standard inflationary
predictions in the context of eternal inflation.Comment: 28 pages; v2: reference added, some clarification
The Nature of the Giant Outbursts in the Bursting Pulsar GRO J 1744-28
We investigate the possible role of an accretion disk instability in
producing the giant outbursts seen in GRO J1744-28. Specifically, we study the
global, time dependent evolution of the Lightman-Eardley instability which can
develop near the inner edge of an accretion disk when the radiation pressure
becomes comparable to the gas pressure. Broadly speaking, our results are
compatible with earlier works by Taam \& Lin and by Lasota \& Pelat. The
uniqueness of GRO J1744-28 appears to be associated with the constraint that,
in order for outbursts to occur, the rate of accretion at the inner edge must
be within a narrow range just above the critical accretion rate at which
radiation pressure is beginning to become significant.Comment: 11 pages in .tex file, 4 Postscript figures, .tex file uses
aasms.sty; Ap. J. L. 1996, in pres
Evolution of a global string network in a matter dominated universe
We evolve the network of global strings in the matter-dominated universe by
means of numerical simulations. The existence of the scaling solution is
confirmed as in the radiation-dominated universe but the scaling parameter
takes a slightly smaller value, , which is
defined as with the energy density of
global strings and the string tension per unit length. The change of
from the radiation to the matter-dominated universe is consistent with
that obtained by Albrecht and Turok by use of the one-scale model. We also
study the loop distribution function and find that it can be well fitted with
that predicted by the one-scale model, where the number density of
the loop with the length is given by with and . Thus, the evolution of the
global string network in the matter-dominated universe can be well described by
the one-scale model as in the radiation-dominated universe.Comment: 10 pages, 5 figure
Dynamical Stability and Environmental Influences in Low Surface Brightness Disk Galaxies
Using analytic stability criteria, we demonstrate that, due to their low
surface mass density and large dark matter content, LSB disks are quite stable
against the growth of global nonaxisymmetric modes such as bars. However,
depending on their (poorly constrained) stellar velocity dispersions, they may
be only marginally stable against local instabilities. We simulate a collision
between an LSB and HSB galaxy and find that, while the HSB galaxy forms a
strong bar, the response of the LSB disk is milder, manifesting weaker rings
and spiral features. The lack of sufficient disk self-gravity to amplify
dynamical instabilities naturally explains the rarity of bars in LSB disks. The
stability of LSB disks may also inhibit interaction-driven gas inflow and
starburst activity in these galaxies.Comment: 13 pages, 3 figures, LaTeX using AASTeX macros 4.0, accepted for
publication in the Astrophysical Journal Letter
Local Axion Cosmic Strings from Superstrings
Axionic cosmic string solutions are investigated in a superstring motivated
model with a pseudo-anomalous U(1) gauge symmetry. The inclusion of a gauge
field and spatially varying dilaton allow local defect solutions with finite
energy per unit length to be found. Fermion zero modes (whose presence is
implied by supersymmetry) are also analysed. The corresponding fermion currents
suggest strong cosmological bounds on the model. It is shown that the unusual
form of the axion strings weakens these bounds. Other cosmological constraints
on the underlying theory are also discussed.Comment: 17 page
Cosmic Perturbations Through the Cyclic Ages
We analyze the evolution of cosmological perturbations in the cyclic model,
paying particular attention to their behavior and interplay over multiple
cycles. Our key results are: (1) galaxies and large scale structure present in
one cycle are generated by the quantum fluctuations in the preceding cycle
without interference from perturbations or structure generated in earlier
cycles and without interfering with structure generated in later cycles; (2)
the ekpyrotic phase, an epoch of gentle contraction with equation of state
preceding the hot big bang, makes the universe homogeneous, isotropic
and flat within any given observer's horizon; and, (3) although the universe is
uniform within each observer's horizon, the global structure of the cyclic
universe is more complex, owing to the effects of superhorizon length
perturbations, and cannot be described in a uniform Friedmann-Robertson-Walker
picture. In particular, we show that the ekpyrotic phase is so effective in
smoothing, flattening and isotropizing the universe within the horizon that
this phase alone suffices to solve the horizon and flatness problems even
without an extended period of dark energy domination (a kind of low energy
inflation). Instead, the cyclic model rests on a genuinely novel,
non-inflationary mechanism (ekpyrotic contraction) for resolving the classic
cosmological conundrums.Comment: 28 pages, 5 figures, 1 tabl
A Genealogy of Emancipatory Values
Analytic moral philosophers have generally failed to engage in any substantial way with the cultural history of morality. This is a shame, because a genealogy of morals can help us accomplish two important tasks. First, a genealogy can form the basis of an epistemological project, one that seeks to establish the epistemic status of our beliefs or values. Second, a genealogy can provide us with functional understanding, since a history of our beliefs, values or institutions can reveal some inherent dynamic or pattern which may be problematically obscured from our view. In this paper, I try to make good on these claims by offering a sketchy genealogy of emancipatory values, or values which call for the liberation of persons from systems of dominance and oppression. The real history of these values, I argue, is both epistemologically vindicatory and functionally enlightening
Republicanism and Markets
The republican tradition has long been ambivalent about markets and commercial society more generally: from the contrasting positions of Rousseau and Smith in the eighteenth century to recent neorepublican debates about capitalism, republicans have staked out diverse positions on fundamental issues of political economy. Rather than offering a systematic historical survey of these discussions, this chapter will instead focus on the leading neo-republican theory—that of Philip Pettit—and consider its implications for market society. As I will argue, Pettit’s theory is even friendlier to markets than most have believed: far from condemning commercial society, his theory recognizes that competitive markets and their institutional preconditions are an alternative means to limit arbitrary power across the domestic, economic, and even political spheres. While most republican theorists have focused on political means to limit such power—including both constitutional means (e.g., separation of powers, judicial review, the rule of law, federalism) and participatory ones (democratic elections and oversight)—I will examine here an economic model of republicanism that can complement, substitute for, and at times displace the standard political model. Whether we look at spousal markets, labor markets, or residential markets within federal systems, state policies that heighten competition among their participants and resource exit from abusive relationships within them can advance freedom as non-domination as effectively or even more effectively than social-democratic approaches that have recently gained enthusiasts among republicans. These conclusions suggest that democracy, be it social or political, is just one means among others for restraining arbitrary power and is consequently less central to (certain versions of) republicanism than we may have expected. So long as they counteract domination, economic inroads into notionally democratic territory are no more worrisome than constitutional ones
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