4,345 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

### Limitation of the Press-Schechter Formalism

The Press-Schechter(PS) formalism for the mass function of the collapsed
objects are reanalyzed. The factor of two in the Press-Schechter formalism is
argued to be correct in the sharp $k$-space filter even when we use the another
approach proposed by Jedamzik(1994) in the cloud-in-cloud problem, which is
different from the previous approach by Peacock & Heavens(1990) and Bond et
al.(1991). The spatial correlation of the density fluctuations, however, had
been neglected in the cloud-in-cloud problem. The effects of this spatial
correlation is analyzed by using the Jedamzik formalism and it is found that
this effect alter the PS mass function especially on larger mass scales.
Furthermore the exact formula of deriving mass function is shown. We also find
that the probability of the overlap of the collapsed objects can be neglected
on very small mass scales while it might not be neglected on other mass scales.Comment: 23 pages, uuencoded compressed Postscrip

### Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?

In the six billion years between redshifts z=1 and z=0.1, galaxies change due
to the aging of their stellar populations, the formation of new stars, and
mergers with other galaxies. Here I explore the relative importance of these
various effects, finding that while mergers are likely to be important for the
red galaxy sequence they are unlikely to affect more than 10% of the blue
galaxy sequence. I compare the galaxy population at redshift z=0.1 from the
Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic
Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3
mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I
evaluate the change in color and in the luminosity functions of the two
sequences using some simplistic stellar population synthesis models. These
models indicate that the luminous end of the red sequence fades less than
passive evolution allows by about 0.2 mag. Due to a lack of luminous blue
progenitors, ``dry'' mergers betweeen red galaxies then must create the
luminous red population at z=0.1, if stellar population models are correct. The
blue sequence colors and luminosity function are consistent with a reduction in
the star-formation rate since redshift z=1 by a factor of about three, with no
change in the number density to within 10%. These results restrict the number
of blue galaxies that can fall onto the red sequence by any process, and in
particular suggest that if mergers are catastrophic events they must be rare
for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at
http://cosmo.nyu.edu/blanton/deep2sdss

### Large Time-dependent CP Violation in B_s^0 System and Finite D^0-D^0bar Mass Difference in Four Generation Standard Mode

Combining the measured B_s mixing with b \to s\ell^+\ell^- rate data, we find
a sizable 4 generation t' quark effect is allowed, for example with m_{t'} \sim
300 GeV and V_{t's}^*V_{t'b} \sim 0.025 e^{\pm i 70^\circ}, which could underly
the new physics indications in CP violation studies of b \to s qbar q
transitions. With positive phase, large and negative mixing-dependent CP
violation in B_s system is predicted, \sin 2\Phi_{B_s} \sim -0.5 to -0.7. This
can also be probed via width difference methods. As a corollary, the short
distance generated D^0-D^0bar mass difference is found to be consistent with,
if not slightly higher than, recent B factory measurements, while CP violation
is subdued with \sin 2\Phi_D \sim -0.2.Comment: 5 pages, 3 figures added (to total of 11 figures), revision to
clarify b -> s photon contrast with Bs and D mixin

### 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 $z=0$. 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

### Seismic Halos Around Active Regions: An MHD Theory

Comprehending the manner in which magnetic fields affect propagating waves is
a first step toward constructing accurate helioseismic models of active region
sub-surface structure and dynamics. Here, we present a numerical method to
compute the linear interaction of waves with magnetic fields embedded in a
solar-like stratified background. The ideal Magneto-Hydrodynamic (MHD)
equations are solved in a 3-dimensional box that straddles the solar
photosphere, extending from 35 Mm within to 1.2 Mm into the atmosphere. One of
the challenges in performing these simulations involves generating a
Magneto-Hydro-Static (MHS) state wherein the stratification assumes horizontal
inhomogeneity in addition to the strong vertical stratification associated with
the near-surface layers. Keeping in mind that the aim of this effort is to
understand and characterize linear MHD interactions, we discuss a means of
computing statically consistent background states. Power maps computed from
simulations of waves interacting with thick flux tubes of peak photospheric
field strengths 600 G and 3000 G are presented. Strong modal power reduction in
the `umbral' regions of the flux tube enveloped by a halo of increased wave
power are seen in the simulations with the thick flux tubes. These enhancements
are also seen in Doppler velocity power maps of active regions observed in the
Sun, leading us to propose that the halo has MHD underpinnings.Comment: submitted to Ap

### Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT

We report a statistical study of flares observed with the Soft X-ray
Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical
parameters of 77 flares, such as the temporal scale, the size, and the magnetic
flux density and find that the sizes of flares tend to be distributed more
broadly as the GOES class becomes weaker and that there is a lower limit of
magnetic flux density that depends on the GOES class. We also examine the
relationship between these parameters and find weak correlation between
temporal and spatial scales of flares. We estimate reconnection inflow
velocity, coronal Alfven velocity, and reconnection rate using above observed
values. The inflow velocities are distributed from a few km/s to several tens
km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4
km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the
reconnection rate in a flare tends to decrease as the GOES class of the flare
increases. This value is within one order of magnitude from the theoretical
maximum value predicted by the Petschek model, although the dependence of the
reconnection rate on the magnetic Reynolds number tends to be stronger than
that in the Petschek model.Comment: 21 pages, 8 figures, accepted for publication in Ap

### Application of Complex-Valued FXLMS Adaptive Filter to Fourier Basis Control of Adaptive Optics

In this paper, the Filtered-X Least Mean Square (FXLMS) adaptive filter with bias integration technique is applied to an adaptive optics system where the Discrete Fourier Transform is used to project the measured phase onto the Fourier basis for modal control. The control law is applied in the complex-valued coefficient space and the FXLMS algorithm is modified accordingly for the complex-valued control. Numerical analysis is conducted for a feedback loop of a single Fourier mode in the presence of a disturbance representing a frozen flow atmospheric turbulence. The performance is compared with a Kalman estimator based control law proposed in the literature called Predictive Fourier Control (PFC). The proposed method demonstrated a similar performance for a stationary disturbance and improved performance for a slowly drifting disturbance. Whereas the performance of the PFC is very sensitive to the accuracy of the identification of the disturbance, the proposed method does not require such an explicit identification and produces minimum error for the given disturbance

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