309 research outputs found
Global Star Formation Rates in Disk Galaxies and Circumnuclear Starbursts from Cloud Collisions
We invoke star formation triggered by cloud-cloud collisions to explain
global star formation rates of disk galaxies and circumnuclear starbursts.
Previous theories based on the growth rate of gravitational perturbations
ignore the dynamically important presence of magnetic fields. Theories based on
triggering by spiral density waves fail to explain star formation in systems
without such waves. Furthermore, observations suggest gas and stellar disk
instabilities are decoupled. Following Gammie, Ostriker & Jog (1991), the cloud
collision rate is set by the shear velocity of encounters with initial impact
parameters of a few tidal radii, due to differential rotation in the disk.
This, together with the effective confinement of cloud orbits to a two
dimensional plane, enhances the collision rate above that for particles in a
three dimensional box. We predict Sigma_{SFR}(R) proportional to Sigma_{gas}
Omega (1-0.7 beta). For constant circular velocity (beta = 0), this is in
agreement with recent observations (Kennicutt 1998). We predict a B-band
Tully-Fisher relation: L_{B} proportional to v_{circ}^{7/3}, also consistent
with observations. As additional tests, we predict enhanced star formation in
regions with relatively high shear rates, and lower star formation efficiencies
in clouds of higher mass.Comment: 27 pages including 3 figures and 2 tables. Accepted to ApJ. Expanded
statistical analysis of cloud SF efficiency test. Stylistic changes. Data for
figures available electronically at
http://astro.berkeley.edu/~jt/disksfr.htm
Periodic Pattern in the Residual-Velocity Field of OB Associations
An analysis of the residual-velocity field of OB associations within 3 kpc of
the Sun has revealed periodic variations in the radial residual velocities
along the Galactic radius vector with a typical scale length of
lambda=2.0(+/-0.2) kpc and a mean amplitude of fR=7(+/-1) km/s. The fact that
the radial residual velocities of almost all OB-associations in rich
stellar-gas complexes are directed toward the Galactic center suggests that the
solar neighborhood under consideration is within the corotation radius. The
azimuthal-velocity field exhibits a distinct periodic pattern in the region
0<l<180 degrees, where the mean azimuthal-velocity amplitude is ft=6(+/-2)
km/s. There is no periodic pattern of the azimuthal-velocity field in the
region 180<l<360 degrees. The locations of the Cygnus arm, as well as the
Perseus arm, inferred from an analysis of the radial- and azimuthal-velocity
fields coincide. The periodic patterns of the residual-velocity fields of
Cepheids and OB associations share many common features.Comment: 21 page
Gravitational Runaway and Turbulence Driving in Star-Gas Galactic Disks
Galactic disks consist of both stars and gas. The gas is more dynamically responsive than the stars, and strongly nonlinear structures and velocities can develop in the ISM even while stellar surface density perturbations remain fractionally small. We use 2D numerical simulations to explore formation of bound clouds and turbulence generation in the gas of two-component galactic disks. We represent the stars with collisionless particles and follow their orbits using a PM method, and treat the gas as an isothermal, unmagnetized fluid. The two components interact through a combined gravity. Using stellar parameters typical of mid-disk conditions, we find that models with gaseous Toomre parameter Q_g 1-2. The bound gaseous clouds that form have mass 6x10^7 Msun each; these represent superclouds that would subsequently fragment into GMCs. Self-gravity and sheared rotation also interact to drive turbulence in the gas when Q_g > Q_c. This turbulence is anisotropic, with more power in sheared than compressive motions. The gaseous velocity dispersion is ~ 0.6 times the thermal speed when Q_g ~ Q_c. This suggests that gravity is important in driving ISM turbulence in many spiral galaxies, since the low efficiency of star formation naturally leads to a state of marginal instability
Social disorganization and history of child sexual abuse against girls in sub-Saharan Africa : a multilevel analysis
Background:
Child sexual abuse (CSA) is a considerable public health problem. Less focus has been paid to the role of community level factors associated with CSA. The aim of this study was to examine the association between neighbourhood-level measures of social disorganization and CSA.
Methods:
We applied multiple multilevel logistic regression analysis on Demographic and Health Survey data for 6,351 adolescents from six countries in sub-Saharan Africa between 2006 and 2008.
Results:
The percentage of adolescents that had experienced CSA ranged from 1.04% to 5.84%. There was a significant variation in the odds of reporting CSA across the communities, suggesting 18% of the variation in CSA could be attributed to community level factors. Respondents currently employed were more likely to have reported CSA than those who were unemployed (odds ratio [OR] = 2.05, 95% confidence interval [CI] 1.48 to 2.83). Respondents from communities with a high family disruption rate were 57% more likely to have reported CSA (OR=1.57, 95% CI 1.14 to 2.16).
Conclusion:
We found that exposure to CSA was associated with high community level of family disruption, thus suggesting that neighbourhoods may indeed have significant important effects on exposure to CSA. Further studies are needed to explore pathways that connect the individual and neighbourhood levels, that is, means through which deleterious neighbourhood effects are transmitted to individuals
Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826
We present new high-resolution observations of the nucleus of the
counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with
the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve
0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas
disk which is truncated at an outer radius of 700pc. The total molecular gas
mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm
spirals, which develop at different radii in the disk. The distribution and
kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of
different types of m=1 perturbations in the gas. Although dominated by
rotation, the gas kinematics are perturbed by streaming motions related to the
m=1 instabilities. The non-circular motions associated with the inner m=1
perturbations agree qualitatively with the pattern expected for a trailing wave
developed outside corotation ('fast' wave). In contrast, the streaming motions
in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical
consequence is that the inner m=1 perturbations would not favour AGN feeding.
An independent confirmation that the AGN is not being generously fueled at
present is found in the low values of the gravitational torques exerted by the
stellar potential for R<530pc. The distribution of star formation in the disk
of NGC4826 is also strongly asymmetrical. Massive star formation is still
vigorous, fed by the significant molecular gas reservoir at R<700pc. There is
supporting evidence for a recent large mass inflow episode in NGC4826.
These observations have been made in the context of the NUclei of GAlaxies
(NUGA) project, aimed at the study of the different mechanisms for gas fueling
of AGN.Comment: A&A, 2003, Paper accepted (04/06/03). For a full-resolution version
of this paper see http://www.oan.es/preprint
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Galaxy Collisions - Dawn of a New Era
The study of colliding galaxies has progressed rapidly in the last few years,
driven by observations with powerful new ground and space-based instruments.
These instruments have used for detailed studies of specific nearby systems,
statistical studies of large samples of relatively nearby systems, and
increasingly large samples of high redshift systems. Following a brief summary
of the historical context, this review attempts to integrate these studies to
address the following key issues. What role do collisions play in galaxy
evolution, and how can recently discovered processes like downsizing resolve
some apparently contradictory results of high redshift studies? What is the
role of environment in galaxy collisions? How is star formation and nuclear
activity orchestrated by the large scale dynamics, before and during merger?
Are novel modes of star formation involved? What are we to make of the
association of ultraluminous X-ray sources with colliding galaxies? To what do
degree do mergers and feedback trigger long-term secular effects? How far can
we push the archaeology of individual systems to determine the nature of
precursor systems and the precise effect of the interaction? Tentative answers
to many of these questions have been suggested, and the prospects for answering
most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update
Vol.
Bulge growth through disk instabilities in high-redshift galaxies
The role of disk instabilities, such as bars and spiral arms, and the
associated resonances, in growing bulges in the inner regions of disk galaxies
have long been studied in the low-redshift nearby Universe. There it has long
been probed observationally, in particular through peanut-shaped bulges. This
secular growth of bulges in modern disk galaxies is driven by weak,
non-axisymmetric instabilities: it mostly produces pseudo-bulges at slow rates
and with long star-formation timescales. Disk instabilities at high redshift
(z>1) in moderate-mass to massive galaxies (10^10 to a few 10^11 Msun of stars)
are very different from those found in modern spiral galaxies. High-redshift
disks are globally unstable and fragment into giant clumps containing 10^8-10^9
Msun of gas and stars each, which results in highly irregular galaxy
morphologies. The clumps and other features associated to the violent
instability drive disk evolution and bulge growth through various mechanisms,
on short timescales. The giant clumps can migrate inward and coalesce into the
bulge in a few 10^8 yr. The instability in the very turbulent media drives
intense gas inflows toward the bulge and nuclear region. Thick disks and
supermassive black holes can grow concurrently as a result of the violent
instability. This chapter reviews the properties of high-redshift disk
instabilities, the evolution of giant clumps and other features associated to
the instability, and the resulting growth of bulges and associated sub-galactic
components.Comment: 37 pages, 9 figures. Invited refereed review to appear in "Galactic
Bulges", E. Laurikainen, D. Gadotti, R. Peletier (eds.), Springe
Results from PAMELA, ATIC and FERMI : Pulsars or Dark Matter ?
It is well known that the dark matter dominates the dynamics of galaxies and
clusters of galaxies. Its constituents remain a mystery despite an assiduous
search for them over the past three decades. Recent results from the
satellite-based PAMELA experiment detect an excess in the positron fraction at
energies between 10-100 GeV in the secondary cosmic ray spectrum. Other
experiments namely ATIC, HESS and FERMI show an excess in the total electron
(\ps + \el) spectrum for energies greater 100 GeV. These excesses in the
positron fraction as well as the electron spectrum could arise in local
astrophysical processes like pulsars, or can be attributed to the annihilation
of the dark matter particles. The second possibility gives clues to the
possible candidates for the dark matter in galaxies and other astrophysical
systems. In this article, we give a report of these exciting developments.Comment: 27 Pages, extensively revised and significantly extended, to appear
in Pramana as topical revie
- …