3,980 research outputs found
Analytic Approach to the Cloud-in-cloud Problem for Non-Gaussian Density Fluctuations
We revisit the cloud-in-cloud problem for non-Gaussian density fluctuations.
We show that the extended Press-Schechter (EPS) formalism for non-Gaussian
fluctuations has a flaw in describing mass functions regardless of type of
filtering. As an example, we consider non-Gaussian models in which density
fluctuations at a point obeys a \chi^2 distribution with \nu degrees of
freedom. We find that mass functions predicted by using an integral formula
proposed by Jedamzik, and Yano, Nagashima and Gouda, properly taking into
account correlation between objects at different scales, deviate from those
predicted by using the EPS formalism, especially for strongly non-Gaussian
fluctuations. Our results for the mass function at large mass scales are
consistent with those by Avelino and Viana obtained from numerical simulations.Comment: 10 pages, 7 EPS files, submitted to Ap
Formation of the Galactic globular clusters with He-rich stars in low-mass halos virialized at high redshift
Recent observations have reported that the Galactic globular clusters (GCs)
with unusually extended horizontal-branch (EHB) morphologies show a
significantly lower velocity dispersion compared with that of the entire
Galactic GC system. We consider that the observed distinctive kinematics of GCs
with EHB has valuable information on the formation epochs of GCs and
accordingly discuss this observational result based on cosmological N-body
simulations with a model of GC formation. We assume that GCs in galaxies were
initially formed in low-mass halos at high redshifts and we investigate final
kinematics of GCs in their host halos at . We find that GCs formed in
halos virialized at z>10 show lower velocity dispersions on average than those
formed at z>6 for halos with GCs at z=0. We thus suggest that the origin of the
observed lower velocity dispersion for the Galactic GCs with EHBs is closely
associated with earlier formation epochs (z>10) of halos initially hosting the
GCs in the course of the Galaxy formation. Considering that the origin of EHBs
can be due to the presence of helium-enhanced second-generation stars in GCs,
we discuss the longstanding second parameter problem of GCs in the context of
different degrees of chemical pollution in GC-forming gas clouds within
low-mass halos virialized at different redshifts.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
Baryon Number Violation Involving Higher Generations
Proton stability seems to constrain rather strongly any baryon number
violating process. We investigate the possibility of baryon number violating
processes involving right-handed dynamics or higher generation quarks. Our
results strongly suggest that there will be no possibility to observe baryon
number violation in tau or higher generation quark decays, at any future
machine.Comment: Improved figures, small changes in the text, added reference. To
appear in Phys. Rev.
Effects of Ram-Pressure from Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies
Using a simple model of molecular cloud evolution, we have quantitatively
estimated the change of star formation rate (SFR) of a disk galaxy falling
radially into the potential well of a cluster of galaxies. The SFR is affected
by the ram-pressure from the intracluster medium (ICM). As the galaxy
approaches the cluster center, the SFR increases to twice the initial value, at
most, in a cluster with high gas density and deep potential well, or with a
central pressure of because the ram-pressure
compresses the molecular gas of the galaxy. However, this increase does not
affect the color of the galaxy significantly. Further into the central region
of the cluster ( Mpc from the center), the SFR of the disk
component drops rapidly due to the effect of ram-pressure stripping. This makes
the color of the galaxy redder and makes the disk dark. These effects may
explain the observed color, morphology distribution and evolution of galaxies
in high-redshift clusters. By contrast, in a cluster with low gas density and
shallow potential well, or the central pressure of ,
the SFR of a radially infalling galaxy changes less significantly, because
neither ram-pressure compression nor stripping is effective. Therefore, the
color of galaxies in poor clusters is as blue as that of field galaxies, if
other environmental effects such as galaxy-galaxy interaction are not
effective. The predictions of the model are compared with observations.Comment: 19 pages, 9 figures, to appear in Ap
Infrared Spectral Energy Distribution of Galaxies in the AKARI All Sky Survey: Correlations with Galaxy Properties, and Their Physical Origin
We have studied the properties of more than 1600 low-redshift galaxies by
utilizing high-quality infrared flux measurements of the AKARI All-Sky Survey
and physical quantities based on optical and 21-cm observations. Our goal is to
understand the physics determining the infrared spectral energy distribution
(SED). The ratio of the total infrared luminosity L_TIR, to the star-formation
rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and
L_TIR is a good SFR indicator only for galaxies with the largest SSFR. We
discovered a tight linear correlation for normal galaxies between the radiation
field strength of dust heating, estimated by infrared SED fits (U_h), and that
of galactic-scale infrared emission (U_TIR ~ L_TIR/R^2), where R is the optical
size of a galaxy. The dispersion of U_h along this relation is 0.3 dex,
corresponding to 13% dispersion in the dust temperature. This scaling and the
U_h/U_TIR ratio can be explained physically by a thin layer of heating sources
embedded in a thicker, optically-thick dust screen. The data also indicate that
the heated fraction of the total dust mass is anti-correlated to the dust
column density, supporting this interpretation. In the large U_TIR limit, the
data of circumnuclear starbursts indicate the existence of an upper limit on
U_h, corresponding to the maximum SFR per gas mass of ~ 10 Gyr^{-1}. We find
that the number of galaxies sharply drops when they become optically thin
against dust-heating radiation, suggesting that a feedback process to galaxy
formation (likely by the photoelectric heating) is working when dust-heating
radiation is not self-shielded on a galactic scale. Implications are discussed
for the M_HI-size relation, the Kennicutt-Schmidt relation, and galaxy
formation in the cosmological context.Comment: 29 pages including 28 figures. matches the published version (PASJ
2011 Dec. 25 issue). The E-open option was chosen for this article, i.e., the
official version available from PASJ site
(http://pasj.asj.or.jp/v63/n6/630613/630613-frame.html) without restrictio
Pickoff and spin-conversion quenchings of ortho-positronium in oxygen
The quenching processes of the thermalized ortho-positronium(o-Ps) on an
oxygen molecule have been studied by the positron annihilation age-momentum
correlation techinique(AMOC). The Doppler broadening spectrum of the 511 keV
gamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as a
function of the o-Ps age. The rate of the quenching, consisting of the pickoff
and the spin-conversion, is estimated from the positron lifetime spectrum. The
ratio of the pickoff quenching rate to the spin-conversion rate is deduced from
the Doppler broadening of the 511 keV gamma-rays from the annihilation of the
o-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons per
molecule which contribute to the pickoff quenching, for O_2 is determined to be
0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching is
determined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4
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