1,684 research outputs found
The Dipole Observed in the COBE DMR Four-Year Data
The largest anisotropy in the cosmic microwave background (CMB) is the
mK dipole assumed to be due to our velocity with respect to the
CMB. Using the four year data set from all six channels of the COBE
Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude
mK in the direction , where the first
uncertainties are statistical and the second include calibration and combined
systematic uncertainties. This measurement is consistent with previous DMR and
FIRAS resultsComment: New and improved version; to be published in ApJ next mont
Cosmic Mach Number as a Function of Overdensity and Galaxy Age
We carry out an extensive study of the cosmic Mach number (\mach) on scales
of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We
particularly put emphasis on the environmental dependence of \mach on
overdensity, galaxy mass, and galaxy age. We start by discussing the difference
in the resulting \mach according to different definitions of \mach and
different methods of calculation. The simulated Mach numbers are slightly lower
than the linear theory predictions even when a non-linear power spectrum was
used in the calculation, reflecting the non-linear evolution in the simulation.
We find that the observed \mach is higher than the simulated mean by more than
2-standard deviations, which suggests either that the Local Group is in a
relatively low-density region or that the true value of \Omega_m is ~ 0.2,
significantly lower than the simulated value of 0.37. We show from our
simulation that the Mach number is a weakly decreasing function of overdensity.
We also investigate the correlations between galaxy age, overdensity and \mach
for two different samples of galaxies --- DWARFs and GIANTs. Older systems
cluster in higher density regions with lower \mach, while younger ones tend to
reside in lower density regions with larger \mach, as expected from the
hierarchical structure formation scenario. However, for DWARFs, the correlation
is weakened by the fact that some of the oldest DWARFs are left over in
low-density regions during the structure formation history. For giant systems,
one expects blue-selected samples to have higher \mach than red-selected ones.
We briefly comment on the effect of the warm dark matter on the expected Mach
number.Comment: 43 pages, including 15 figures. Accepted version in ApJ. Included
correlation function of different samples of galaxies, and the cumulative
number fraction distribution as a fcn. of overdensity. Reorganized figures
and added some reference
Measuring the Stellar Masses of z~7 Galaxies with Spitzer Ultrafaint Survey Program (SURFS UP)
We present Spitzer/IRAC observations of nine -band dropouts highly
magnified (2<mu<12) by the Bullet Cluster. We combine archival imaging with our
Exploratory program (SURFS UP), which results in a total integration time of
~30 hr per IRAC band. We detect (>3sigma) in both IRAC bands the brightest of
these high-redshift galaxies, with [3.6]=23.80+-0.28 mag, [4.5]=23.78+-0.25
mag, and (H-[3.6])=1.17+-0.32 mag. The remaining eight galaxies are undetected
to [3.6]~26.4 mag and [4.5]~26.0 mag with stellar masses of ~5x10^7 M_sol. The
detected galaxy has an estimated magnification of mu=12+-4, which implies this
galaxy has an ultraviolet luminosity of L_1500~0.3 L*_{z=7} --- the lowest
luminosity individual source detected in IRAC at z>7. By modeling the broadband
photometry, we estimate the galaxy has an intrinsic star-formation rate of
SFR~1.3 M_sol/yr and stellar mass of M~2x10^9 M_sol, which gives a specific
star-formation rate of sSFR~0.7 Gyr^-1. If this galaxy had sustained this
star-formation rate since z~20, it could have formed the observed stellar mass
(to within a factor of ~2), we also discuss alternate star-formation histories
and argue the exponentially-increasing model is unlikely. Finally, based on the
intrinsic star-formation rate, we estimate this galaxy has a likely [C II] flux
of = 10^{-17} erg/s/cm2.Comment: Accepted to ApJL. 6 pages, 3 figures, 2 table
A semi-parametric approach to estimate risk functions associated with multi-dimensional exposure profiles: application to smoking and lung cancer
A common characteristic of environmental epidemiology is the multi-dimensional aspect of exposure patterns, frequently reduced to a cumulative exposure for simplicity of analysis. By adopting a flexible Bayesian clustering approach, we explore the risk function linking exposure history to disease. This approach is applied here to study the relationship between different smoking characteristics and lung cancer in the framework of a population based case control study
Spectroscopic Analysis of H I Absorption Line Systems in 40 HIRES Quasars
We list and analyze H I absorption lines at redshifts 2 < z < 4 with column
density (12 < log(N_HI) < 19) in 40 high-resolutional (FWHM = 8.0 km/s) quasar
spectra obtained with the Keck+HIRES. We de-blend and fit all H I lines within
1,000 km/s of 86 strong H I lines whose column densities are log(N_HI/[cm^-2])
> 15. Unlike most prior studies, we use not only Lya but also all visible
higher Lyman series lines to improve the fitting accuracy. This reveals
components near to higher column density systems that can not be seen in Lya.
We list the Voigt profile fits to the 1339 H I components that we found. We
examined physical properties of H I lines after separating them into several
sub-samples according to their velocity separation from the quasars, their
redshift, column density and the S/N ratio of the spectrum. We found two
interesting trends for lines with 12 < log(N_HI) < 15 which are within 200-1000
km/s of systems with log(N_HI) > 15. First, their column density distribution
becomes steeper, meaning relatively fewer high column density lines, at z <
2.9. Second, their column density distribution also becomes steeper and their
line width becomes broader by about 2-3 km/s when they are within 5,000 km/s of
their quasar.Comment: 32 pages, 14 figures, accepted for publication in the Astronomical
Journal. A complete version with all tables and figures is available at
http://www.astro.psu.edu/users/misawa/pub/Paper/40hires.ps.g
Cosmological parameters sigma_8, the baryon density, and the UV background intensity from a calibrated measurement of H I Lyman-alpha absorption at z = 1.9
We identify a concordant model for the intergalactic medium (IGM) at redshift
z=1.9 that uses popular values for cosmological and astrophysical parameters
and accounts for all baryons with an uncertainty of 6%. We have measured the
amount of absorption, DA, in the Ly-alpha forest at redshift 1.9 in spectra of
77 QSO from the Kast spectrograph. We calibrated the continuum fits with
realistic artificial spectra, and we found that averaged over all 77 QSOs the
mean continuum level is within 1-2% of the correct value. Absorption from all
lines in the Ly-alpha forest at z=1.9 removes DA=15.1 +/- 0.7% of the flux
between 1070 and 1170 (rest) Angstroms. This is the first measurement using
many QSOs at this z, and the first calibrated measurement at any redshift.
Metal lines absorb 2.3 +/- 0.5%, and LLS absorb 1.0 +/- 0.4% leaving 11.8 +/-
1.0% from the lower density bulk of the IGM. Averaging over Delta z=0.1 or 154
Mpc, the dispersion is 6.1 +/- 0.3% including LLS and metal lines, or 3.9
(+0.5, -0.7)% for the lower density IGM alone, consistent with the usual
description of large scale structure. LLS and metal lines are major
contributors to the variation in the mean flux, and they make the flux field
significantly non-Gaussian. We find that a hydrodynamic simulation on a 1024
cubed grid in a 75.7 Mpc box reproduces the observed DA from the low density
IGM with parameters values H_o=71 km/s/Mpc, Omega_Lambda=0.73, Omega_m=0.27,
Omega_b=0.044, sigma_8=0.9 and a UV background that has an ionization rate that
is 1.08 +/- 0.08 times the prediction by Madau, Haardt & Rees (1999).Comment: Submitted to Ap
Wigner Crystalline Edges in nu < 1 Quantum Dots
We investigate the edge reconstruction phenomenon believed to occur in
quantum dots in the quantum Hall regime when the filling fraction is nu < 1.
Our approach involves the examination of large dots (< 40 electrons) using a
partial diagonalization technique in which the occupancies of the deep interior
orbitals are frozen. To interpret the results of this calculation, we evaluate
the overlap between the diagonalized ground state and a set of trial
wavefunctions which we call projected necklace (PN) states. A PN state is
simply the angular momentum projection of a maximum density droplet surrounded
by a ring of localized electrons. Our calculations reveal that PN states have
up to 99% overlap with the diagonalized ground states, and are lower in energy
than the states identified in Chamon and Wen's study of the edge
reconstruction.Comment: 8 pages, 8 figures, to be published in Phys. Rev.
The Temperature of the CMB at 10 GHz
We report the results of an effort to measure the low frequency portion of
the spectrum of the Cosmic Microwave Background Radiation (CMB), using a
balloon-borne instrument called ARCADE (Absolute Radiometer for Cosmology,
Astrophysics, and Diffuse Emission). These measurements are to search for
deviations from a thermal spectrum that are expected to exist in the CMB due to
various processes in the early universe. The radiometric temperature was
measured at 10 and 30 GHz using a cryogenic open-aperture instrument with no
emissive windows. An external blackbody calibrator provides an in situ
reference. A linear model is used to compare the radiometer output to a set of
thermometers on the instrument. The unmodeled residuals are less than 50 mK
peak-to-peak with a weighted RMS of 6 mK. Small corrections are made for the
residual emission from the flight train, atmosphere, and foreground Galactic
emission. The measured radiometric temperature of the CMB is 2.721 +/- 0.010 K
at 10 GHz and 2.694 +/- 0.032 K at 30 GHz.Comment: 8 pages including 5 figures. Submitted to The Astrophysical Journa
- âŠ