1,435 research outputs found
Multi-Phase Gas Dynamics in a Weak Barred Potential
The structure of the interstellar medium in the central kpc region of a
galaxy with a weak bar-like potential is investigated taking into account
realistic cooling and heating processes and the self-gravity of the gas. Using
high resolution hydrodynamical simulations, it is revealed that the resonant
structures (e.g. smooth spiral shocks and a nuclear ring) are very different
from those seen in past numerical models where simple models of the ISM, i.e.
non-self-gravitating, isothermal gas were assumed. We find that the pc-scale
filaments and clumps form large scale spirals, which resemble those seen in
real galaxies. The fine structures are different between the arms and in the
nuclear region. The next generation millimeter interferometer (ALMA) may reveal
the fine structures of the cold gas in nearby galaxies. We also find a large
scale anisotropy in the gas temperature, which is caused due to non-circular
velocity field of the gas.The damped orbit model based on the epicyclic
approximation explains the distribution of the hot (> 10^4 K) and cold (< 100
K) gases appearing alternately around the galactic center. Because of the
temperature anisotropy, cold gases observed by molecular lines do not
necessarily represent the real gas distribution in galaxies. Position-Velocity
diagrams depend strongly on the viewing angles. As a result, the rotational
velocity inferred from the PV maps could be two times larger or smaller than
the true circular velocity.Comment: 7 pages, 8 figures, to appear in PASJ, vol. 56, no.6 (2001
Star formation efficiency in the Barred Spiral Galaxy NGC 4303
We present new CO(J=1-0) observations of the barred galaxy NGC 4303
using the Nobeyama 45m telescope (NRO45) and the Combined Array for Research in
Millimeter-wave Astronomy (CARMA). The H images of barred spiral
galaxies often show active star formation in spiral arms, but less so in bars.
We quantify the difference by measuring star formation rate and efficiency at a
scale where local star formation is spatially resolved. Our CO map covers the
central 2\farcm3 region of the galaxy; the combination of NRO45 and CARMA
provides a high fidelity image, enabling accurate measurements of molecular gas
surface density. We find that star formation rate and efficiency are twice as
high in the spiral arms as in the bar. We discuss this difference in the
context of the Kennicutt-Schimidt (KS) law, which indicates a constant star
formation rate at a given gas surface density. The KS law breaks down at our
native resolution ( 250 pc), and substantial smoothing (to 500 pc) is
necessary to reproduce the KS law, although with greater scatter.Comment: 17 pages, 10 figures, published by ApJ;
http://adsabs.harvard.edu/abs/2010ApJ...721..383
Magnetic ground state of pyrochlore oxides close to metal-insulator boundary probed by muon spin rotation
Magnetism of ruthernium pyrochlore oxides A2Ru2O7 (A = Hg, Cd, Ca), whose
electronic properties within a localized ion picture are characterized by
non-degenerate t2g orbitals (Ru5+, 4d3) and thereby subject to geometrical
frustration, has been investigated by muon spin rotation/relaxation (muSR)
technique. The A cation (mostly divalent) was varied to examine the effect of
covalency (Hg > Cd > Ca) on their electronic property. In a sample with A = Hg
that exhibits a clear metal-insulator (MI) transition below >> 100 K (which is
associated with a weak structural transition), a nearly commensurate magnetic
order is observed to develop in accordance with the MI transition. Meanwhile,
in the case of A = Cd where the MI transition is suppressed to the level of
small anomaly in the resistivity, the local field distribution probed by muon
indicates emergence of a certain magnetic inhomogeneity below {\guillemotright}
30 K. Moreover, in Ca2Ru2O7 that remains metallic, we find a highly
inhomogeneous local magnetism below >>25 K that comes from randomly oriented Ru
moments and thus described as a "frozen spin liquid" state. The systematic
trend of increasing randomness and itinerant character with decreasing
covalency suggests close relationship between these two characters. As a
reference for the effect of orbital degeneracy and associated Jahn-Teller
instability, we examine a tetravalent ruthernium pyrochlore, Tl2Ru2O7 (Ru4+,
4d4), where the result of muSR indicates a non-magnetic ground state that is
consistent with the formation of the Haldane chains suggested by neutron
diffraction experiment.Comment: 12 pages, 13 figure
Will Nonlinear Peculiar Velocity and Inhomogeneous Reionization Spoil 21cm Cosmology from the Epoch of Reionization?
The 21cm background from the epoch of reionization is a promising
cosmological probe: line-of-sight velocity fluctuations distort redshift, so
brightness fluctuations in Fourier space depend upon angle, which linear theory
shows can separate cosmological from astrophysical information. Nonlinear
fluctuations in ionization, density and velocity change this, however. The
validity and accuracy of the separation scheme are tested here for the first
time, by detailed reionization simulations. The scheme works reasonably well
early in reionization ( 80% ionized).Comment: 2 figures, matches published PRL versio
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