1,352 research outputs found
Angular momentum of free variable mass systems is partially conserved
Variable mass systems are a classic example of open systems in classical mechanics with rockets being a standard practical example. Due to the changing mass, the angular momentum of these systems is not generally conserved. Here, we show that the angular momentum vector of a free variable mass system is fixed in inertial space and, thus, is a partially conserved quantity. It is well known that such conservation rules allow simpler approaches to solving the equations of motion. This is demonstrated by using a graphical technique to obtain an analytic solution for the second Euler angle that characterizes nutation in spinning bodies
Dwarf Dark Matter Halos
We study properties of dark matter halos at high redshifts z=2-10 for a vast
range of masses with the emphasis on dwarf halos with masses 10^7-10^9 Msun/h.
We find that the density profiles of relaxed dwarf halos are well fitted by the
NFW profile and do not have cores. We compute the halo mass function and the
halo spin parameter distribution and find that the former is very well
reproduced by the Sheth & Tormen model while the latter is well fitted by a
lognormal distribution with lambda_0 = 0.042 and sigma_lambda = 0.63. We
estimate the distribution of concentrations for halos in mass range that covers
six orders of magnitude from 10^7 Msun/h to 10^13} Msun/h, and find that the
data are well reproduced by the model of Bullock et al. The extrapolation of
our results to z = 0 predicts that present-day isolated dwarf halos should have
a very large median concentration of ~ 35. We measure the subhalo circular
velocity functions for halos with masses that range from 4.6 x 10^9 Msun/h to
10^13 Msun/h and find that they are similar when normalized to the circular
velocity of the parent halo. Dwarf halos studied in this paper are many orders
of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet,
in all respects the dwarfs are just down-scaled versions of the large halos.
They are cuspy and, as expected, more concentrated. They have the same spin
parameter distribution and follow the same mass function that was measured for
large halos.Comment: Accepted to be pusblished by ApJ, 12 pages, 8 figures, LaTeX
(documentclass preprint2). Differences with respect to the previous
submission are: (i) abstract was modified slightly to make it more
transparent to the reader, (ii) an extra figure has been added, and (3) some
minor modifications to the main text were also don
Heating of the IGM
Using the cosmic virial theorem, Press-Schechter analysis and numerical
simulations, we compute the expected X-ray background (XRB) from the diffuse
IGM with the clumping factor expected from gravitational shock heating. The
predicted fluxes and temperatures are excluded from the observed XRB. The
predicted clumping can be reduced by entropy injection. The required energy is
computed from the two-point correlation function, as well as from
Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon
excludes radiative or gravitational heating as a primary energy source. We
argue that the intergalactic medium (IGM) must have been heated through violent
processes such as massive supernova bursts. If the heating proceeded through
supernova explosions, it likely proceeded in bursts which may be observable in
high redshift supernova searches. Within our model we reproduce the observed
cluster luminosity-temperature relation with energy injection of 1 keV/nucleon
if this injection is assumed to be uncorrelated with the local density. These
parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV
with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near
future.Comment: to appear in ApJ Lett., 11 pages incl 1 figur
- …