938 research outputs found
Central Mass Concentration and Bar Dissolution in Nearby Spiral Galaxies
We use data from the BIMA Survey of Nearby Galaxies (SONG) to investigate the
relationship between ellipticity and central mass concentration in barred
spirals. Existing simulations predict that bar ellipticity decreases as
inflowing mass driven by the bar accumulates in the central regions, ultimately
destroying the bar. Using the ratio of the bulge mass to the mass within the
bar radius as an estimate of the central mass concentration, we obtain
dynamical mass estimates from SONG CO 1-0 rotation curve data. We find an
inverse correlation between bar ellipticity and central mass concentration,
consistent with simulations of bar dissolution.Comment: 10 pages, 2 figures and 2 tables, accepted for publication in the
Astrophysical Journa
The Distribution of Bar and Spiral Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamental property
of the galaxy population that has only begun to be explored. We have applied
the bar/spiral separation method of Buta, Block, and Knapen to derive the
distribution of maximum relative gravitational bar torques, Q_b, for 147 spiral
galaxies in the statistically well-defined Ohio State University Bright Galaxy
Survey (OSUBGS) sample. Our goal is to examine the properties of bars as
independently as possible of their associated spirals. We find that the
distribution of bar strength declines smoothly with increasing Q_b, with more
than 40% of the sample having Q_b <= 0.1. In the context of recurrent bar
formation, this suggests that strongly-barred states are relatively short-lived
compared to weakly-barred or non-barred states. We do not find compelling
evidence for a bimodal distribution of bar strengths. Instead, the distribution
is fairly smooth in the range 0.0 <= Q_b < 0.8. Our analysis also provides a
first look at spiral strengths Q_s in the OSU sample, based on the same torque
indicator. We are able to verify a possible weak correlation between Q_s and
Q_b, in the sense that galaxies with the strongest bars tend also to have
strong spirals.Comment: Accepted for publication in the Astronomical Journal, August 2005
issue (LaTex, 23 pages + 11 figures, uses aastex.cls
A Connection between Star Formation in Nuclear Rings and their Host Galaxies
We present results from a photometric H-alpha survey of 22 nuclear rings,
aiming to provide insight into their star formation properties, including age
distribution, dynamical timescales, star formation rates, and galactic bar
influence. We find a clear relationship between the position angles and
ellipticities of the rings and those of their host galaxies, which indicates
the rings are in the same plane as the disk and circular. We use population
synthesis models to estimate ages of each H-alpha emitting HII region, which
range from 1 Myr to 10 Myrs throughout the rings. We find that approximately
half of the rings contain azimuthal age gradients that encompass at least 25%
of the ring, although there is no apparent relationship between the presence or
absence of age gradients and the morphology of the rings or their host
galaxies. NGC1343, NGC1530, and NGC4321 show clear bipolar age gradients, where
the youngest HII regions are located near the two contact points of the bar and
ring. We speculate in these cases that the gradients are related to an
increased mass inflow rate and/or an overall higher gas density in the ring,
which would allow for massive star formation to occur on short timescales,
after which the galactic rotation would transport the HII regions around the
ring as they age. Two-thirds of the barred galaxies show correlation between
the locations of the youngest HII region(s) in the ring and the location of the
contact points, which is consistent with predictions from numerical modeling.Comment: 23 pages, 10 figures (7 color), 23 tables, accepted for publication
in ApJS (Feb 08); NASA-GSFC, IAC, University of Maryland, STSc
Comparison of bar strengths in active and non-active galaxies
Bar strengths are compared between active and non-active galaxies for a
sample of 43 barred galaxies. The relative bar torques are determined using a
new technique (Buta and Block 2001), where maximum tangential forces are
calculated in the bar region, normalized to the axisymmetric radial force
field. We use JHK images of the 2 Micron All Sky Survey. We show a first clear
empirical indication that the ellipticies of bars are correlated with the
non-axisymmetric forces in the bar regions. We found that nuclear activity
appears preferentially in those early type galaxies in which the maximum bar
torques are weak and appear at quite large distances from the galactic center.
Most suprisingly the galaxies with the strongest bars are non-active. Our
results imply that the bulges may be important for the onset of nuclear
activity, but that the correlation between the nuclear activity and the early
type galaxies is not straightforward.Comment: MNRAS macro in tex format, 9 pages, 10 figure
Boundary Limitation of Wavenumbers in Taylor-Vortex Flow
We report experimental results for a boundary-mediated wavenumber-adjustment
mechanism and for a boundary-limited wavenumber-band of Taylor-vortex flow
(TVF). The system consists of fluid contained between two concentric cylinders
with the inner one rotating at an angular frequency . As observed
previously, the Eckhaus instability (a bulk instability) is observed and limits
the stable wavenumber band when the system is terminated axially by two rigid,
non-rotating plates. The band width is then of order at small
() and agrees well with
calculations based on the equations of motion over a wide -range.
When the cylinder axis is vertical and the upper liquid surface is free (i.e.
an air-liquid interface), vortices can be generated or expelled at the free
surface because there the phase of the structure is only weakly pinned. The
band of wavenumbers over which Taylor-vortex flow exists is then more narrow
than the stable band limited by the Eckhaus instability. At small
the boundary-mediated band-width is linear in . These results are
qualitatively consistent with theoretical predictions, but to our knowledge a
quantitative calculation for TVF with a free surface does not exist.Comment: 8 pages incl. 9 eps figures bitmap version of Fig
Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383
We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha,
broadband optical and near-infrared) observations of the classic barred spiral
NGC 5383. We compare the observed central gas and dust morphology to the
predictions of recent hydrodynamic simulations. In the nuclear region, our
observations reveal three peaks lying along a S-shaped gas and dust
distribution. In contrast, the model predicts a circumnuclear ring, not the
observed S-shaped distribution; moreover, the predicted surface density
contrast between the central gas accumulation and the bar dust lanes is an
order of magnitude larger than observed. The discrepancies are not due to
unexplored model parameter space or a nuclear bar but are probably due to the
vigorous (7 solar masses per year) star formation activity in the center.
As is common in similar bars, the star formation rate in the bar between the
bar ends and the central region is low (~0.5 solar masses per yr), despite the
high gas column density in the bar dust lanes; this is generally attributed to
shear and shocks. We note a tendency for the HII regions to be associated with
the spurs feeding the main bar dust lanes, but these are located on the leading
side of the bar. We propose that stars form in the spurs, which provide a high
column density but low shear environment. HII regions can therefore be found
even on the leading side of the bar because the ionizing stars pass
ballistically through the dust laneComment: Accepted for publication in The Astrophysical Journal, 33 pages
(includes 10 figures
Corotation: its influence on the chemical abundance pattern of the Galaxy
A simple theory for the chemical enrichment of the Galaxy which takes into
account the effects of spiral arms on heavy elements output was developed. In
the framework of the model with the corotation close to the position of the Sun
in the Galaxy the observed abundance features are explained.Comment: LaTeX, 6 pages, 5 jpg figures, uses aastex.sty, submitted to ApJ Let
Multiproxy bioarchaeological data reveals interplay between growth, diet and population dynamics across the transition to farming in the central Mediterranean
The transition to farming brought on a series of important changes in human society, lifestyle, diet and health. The human bioarchaeology of the agricultural transition has received much attention, however, relatively few studies have directly tested the interrelationship between individual lifestyle factors and their implications for understanding life history changes among the first farmers. We investigate the interplay between skeletal growth, diet, physical activity and population size across 30,000 years in the central Mediterranean through a ‘big data’ cross-analysis of osteological data related to stature (n = 361), body mass (n = 334) and long bone biomechanics (n = 481), carbon (δ 13C) and nitrogen (δ 15N) stable isotopes (n = 1986 human, n = 475 animal) and radiocarbon dates (n = 5263). We present the observed trends on a continuous timescale in order to avoid grouping our data into assigned ‘time periods’, thus achieving greater resolution and chronological control over our analysis. The results identify important changes in human life history strategies associated with the first farmers, but also highlight the long-term nature of these trends in the millennia either side of the agricultural transition. The integration of these different data is an important step towards disentangling the complex relationship between demography, diet and health, and reconstruct life history changes within a southern European context. We believe the methodological approach adopted here has broader global implications for bioarchaeological studies of human adaptation more generally
Two spectroscopically confirmed galaxy structures at z=0.61 and 0.74 in the CFHTLS Deep~3 field
Adami et al. (2010) have detected several cluster candidates at z>0.5 as part
of a systematic search for clusters in the Canada France Hawaii Telescope
Legacy Survey, based on photometric redshifts. We focus here on two of them,
located in the D3 field: D3-6 and D3-43. We have obtained spectroscopy with
Gemini/GMOS and measured redshifts for 23 and 14 galaxies in the two
structures. These redshifts were combined with those available in the
literature. A dynamical and a weak lensing analysis were also performed,
together with the study of X-ray Chandra archive data. Cluster D3-6 is found to
be a single structure of 8 spectroscopically confirmed members at an average
redshift z=0.607, with a velocity dispersion of 423 km/s. It appears to be a
relatively low mass cluster. D3-43-S3 has 46 spectroscopically confirmed
members at an average redshift z=0.739. It can be decomposed into two main
substructures, having a velocity dispersion of about 600 and 350 km/s. An
explanation to the fact that D3-43-S3 is detected through weak lensing (only
marginally, at the ~3sigma level) but not in X-rays could be that the two
substructures are just beginning to merge more or less along the line of sight.
We also show that D3-6 and D3-43-S3 have similar global galaxy luminosity
functions, stellar mass functions, and star formation rate (SFR) distributions.
The only differences are that D3-6 exhibits a lack of faint early type
galaxies, a deficit of extremely high stellar mass galaxies compared to
D3-43-S3, and an excess of very high SFR galaxies. This study shows the power
of techniques based on photometric redshifts to detect low to moderately
massive structures, even at z~0.75.Comment: Accepted in A&A, final version, shortened abstrac
Systematic study of the low-lying electric dipole strength in Sn isotopes and its astrophysical implications
The -ray strength functions (GSF) and nuclear level densities (NLD)
below the neutron threshold have been extracted for Sn
from particle- coincidence data with the Oslo method. The evolution of
bulk properties of the low-lying electric dipole response has been investigated
on the basis of the Oslo GSF data and results of a recent systematic study of
electric and magnetic dipole strengths in even-even Sn isotopes with
relativistic Coulomb excitation. The obtained GSFs reveal a resonance-like peak
on top of the tail of the isovector giant dipole resonance, centered at
8 MeV and exhausting 2\% of the classical Thomas-Reiche-Kuhn
(TRK) sum. In contrast to predictions of the relativistic quasiparticle
random-phase and time-blocking approximation calculations (RQRPA and RQTBA), no
monotonous increase in the total low-lying strength was observed in the
experimental data from Sn to Sn, demonstrating rather similar
strength distributions in these nuclei. The Oslo GSFs and NLDs were further
used as inputs to constrain the cross sections and Maxwellian-averaged cross
sections of reactions in the Sn isotopic chain using TALYS. The
obtained results agree well with other available experimental data and the
recommended values from the JINA REACLIB, BRUSLIB, and KADoNiS libraries.
Despite relatively small exhausted fractions of the TRK sum rule, the low-lying
electric dipole strength makes a noticeable impact on the radiative
neutron-capture cross sections in stable Sn isotopes. Moreover, the
experimental Oslo inputs for the SnSn
reactions were found to affect the production of Sb in the astrophysical
-process, providing new constraints on the uncertainties of the resulting
chemical abundances from multi-zone low-metallicity Asymptotic Giant Branch
stellar models.Comment: 27 pages, 14 pages. Submitted to Physical Review C journal on 13
November 202
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