30,388 research outputs found
No Crisis for Big Bang Nucleosynthesis
Contrary to a recent claim, the inferred primordial abundances of the light
elements are quite consistent with the expectations from standard big bang
nucleosynthesis when attention is restricted to direct observations rather than
results from chemical evolution models. The number of light neutrino (or
equivalent particle) species () can be as high as 4.53 if the
nucleon-to-photon ratio () is at its lower limit of , as constrained by the upper bound on the deuterium abundance in high
redshift quasar absorption systems. Alternatively, with , can
be as high as if the deuterium abundance is bounded from
below by its interstellar value.Comment: 10 pages LaTeX (uses ReVTeX), including 3 PostScript figures (uses
epsf); Full paper available from
ftp://ftp.physics.ox.ac.uk/pub/local/users/sarkar/BBNcrisisnot.ps.gz ;
Revised to include discussion of new deuterium observations in quasar
absorption systems and helium-3 measurement in local interstellar gas,
updated Fig 1, no change in conclusions, resubmitted to Phys. Rev. Let
The constraining effect of gas and the dark matter halo on the vertical stellar distribution of the Milky Way
We study the vertical stellar distribution of the Milky Way thin disk in
detail with particular focus on the outer disk. We treat the galactic disk as a
gravitationally coupled, three-component system consisting of stars, atomic
hydrogen gas, and molecular hydrogen gas in the gravitational field of the dark
matter halo. The self-consistent vertical distribution for stars and gas in
such a realistic system is obtained for radii between 4-22 kpc. The inclusion
of an additional gravitating component constrains the vertical stellar
distribution toward the mid-plane, so that the mid-plane density is higher, the
disk thickness is reduced, and the vertical density profile is steeper than in
the one-component, isothermal, stars-alone case. We show that the stellar
distribution is constrained mainly by the gravitational field of gas and dark
matter halo in the inner and the outer Galaxy, respectively. We find that the
thickness of the stellar disk (measured as the HWHM of the vertical density
distribution) increases with radius, flaring steeply beyond R=17 kpc. The disk
thickness is reduced by a factor of 3-4 in the outer Galaxy as a result of the
gravitational field of the halo, which may help the disk resist distortion at
large radii. The disk would flare even more if the effect of dark matter halo
were not taken into account. Thus it is crucially important to include the
effect of the dark matter halo when determining the vertical structure and
dynamics of a galactic disk in the outer region.Comment: 8 pages,7 figures, Accepted for publication in A &
Flaring stellar disk in the low surface brightness galaxy UGC 7321
We theoretically study the vertical structure of the edge-on low surface
brightness (LSB) galaxy UGC 7321. This is one of the few well-observed LSBs. We
modeled it as a gravitationally coupled disk system of stars and atomic
hydrogen gas in the potential of the dark matter halo and treated the realistic
case where the rotation velocity varies with radius. We used a dense and
compact halo as implied by the observed rotation curve in this model. We
calculated the thickness of stellar and HI disks in terms of the half-width at
half-maximum of the vertical density distribution in a region of R=0 to 12 kpc
using input parameters constrained by observations. We obtain a mildly
increasing disk thickness up to R=6 kpc, in a good agreement with the observed
trend, and predict a strong flaring beyond this. To obtain this trend, the
stellar velocity dispersion has to fall exponentially at a rate of 3.2R_D ,
while the standard value of 2R_D gives a decreasing thickness with radius.
Interestingly, both stellar and HI disks show flaring in the outer disk region
although they are dynamically dominated by the dark matter halo from the very
inner radii. The resulting vertical stellar density distribution cannot be fit
by a single sech^2/n function, in agreement with observations, which show wings
at larger distances above the mid-plane. Invoking a double-disk model to
explain the vertical structure of LSBs as done in the literature may therefore
not be necessary.Comment: 10 pages, 11 figures, published in A&
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