17 research outputs found
Comment on ``General Relativity Resolves Galactic Rotation Without Exotic Dark Matter'' by F.I. Cooperstock & S. Tieu
The general relativistic model of Cooperstock & Tieu, which attempts to fit
rotation curves of spiral galaxies without invoking dark matter, is tested
empirically using observations of the Milky Way. In particular, predictions for
the mass density in the solar neighbourhood and the vertical density
distribution at the position of the Sun are compared with observations. It is
shown that the model of Cooperstock & Tieu, which was so constructed that it
gives an excellent fit of the observed rotation curve, singularly fails to
reproduce the observed local mass density and the vertical density profile of
the Milky Way.Comment: 4 pages, 1 figure, accepted by New Astronom
More than just halo mass: Modelling how the red galaxy fraction depends on multiscale density in a HOD framework
The fraction of galaxies with red colours depends sensitively on environment,
and on the way in which environment is measured. To distinguish competing
theories for the quenching of star formation, a robust and complete description
of environment is required, to be applied to a large sample of galaxies. The
environment of galaxies can be described using the density field of neighbours
on multiple scales - the multiscale density field. We are using the Millennium
simulation and a simple HOD prescription which describes the multiscale density
field of Sloan Digital Sky Survey DR7 galaxies to investigate the dependence of
the fraction of red galaxies on the environment. Using a volume limited sample
where we have sufficient galaxies in narrow density bins, we have more dynamic
range in halo mass and density for satellite galaxies than for central
galaxies. Therefore we model the red fraction of central galaxies as a constant
while we use a functional form to describe the red fraction of satellites as a
function of halo mass which allows us to distinguish a sharp from a gradual
transition. While it is clear that the data can only be explained by a gradual
transition, an analysis of the multiscale density field on different scales
suggests that colour segregation within the haloes is needed to explain the
results. We also rule out a sharp transition for central galaxies, within the
halo mass range sampled.Comment: 24 pages, 21 figures, accepted for publication by MNRA
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
A model of the anisotropic correlation function xi(rp, pi) in redshift space including redshift errors
With the advent of very large volume, wide-angle photometric redshift surveys
like e.g. Pan-STARRS, DES, or PAU, which aim at using the spatial distribution
of galaxies as a means to constrain the equation of state parameter of dark
energy, w_DE, it has become extremely important to understand the influence of
redshift inaccuracies on the measurement. We have developed a new model for the
anisotropic two point large-scale (r > 64 h^-1 Mpc) correlation function
xi(rp,pi), in which nonlinear structure growth and nonlinear coherent infall
velocities are taken into account, and photometric redshift errors can easily
be incorporated. In order to test its validity and investigate the effects of
photometric redshifts, we compare our model with the correlation function
computed from a suite of 50 large-volume, moderate-resolution numerical N-body
simulation boxes, where we can perform the analysis not only in real- and
redshift space, but also simulate the influence of a gaussian redshift error
distribution with an absolute rms of sigma_z= 0.015, 0.03, 0.06, and 0.12,
respectively. We conclude that for the given volume (V_box =2.4 h^-3 Gpc^3) and
number density (n ~ 1.25 10^-4) of objects the full shape of xi(rp,pi) is
modeled accurately enough to use it to derive unbiased constraints on the
equation of state parameter of dark energy w_DE and the linear bias b, even in
the presence of redshift errors of the order of sigma_z = 0.06.Comment: Accepted for publication by MNRA
Cosmological implications from the full shape of the large-scale power spectrum of the SDSS DR7 luminous red galaxies
We obtain cosmological constraints from a measurement of the spherically
averaged power spectrum (PS) of the distribution of about 90000 luminous red
galaxies (LRGs) across 7646 deg2 in the Northern Galactic Cap from the DR7 of
the SDSS. The errors and mode correlations are estimated thanks to the 160
LasDamas mock catalogues, created in order to simulate the same galaxies and to
have the same selection as the data. We apply a model, that can accurately
describe the full shape of the PS with the use of a small number of free
parameters. Using the LRG PS, in combination with the latest measurement of the
temperature and polarisation anisotropy in the cosmic microwave background
(CMB), the luminosity-distance relation from the largest available type 1a
supernovae (SNIa) dataset and a precise determination of the local Hubble
parameter, we obtain cosmological constraints for five different parameter
spaces. When all the four experiments are combined, the flat LCDM model is
characterised by Omega_M=0.259+-0.016, Omega_b=0.045+-0.001, n_s=0.963+-0.011,
sigma_8=0.802+-0.021 and h=0.712+-0.014. When we consider curvature as a free
parameter, we do not detect deviations from flatness:
Omega_k=(1.6+-5.4)*10^{-3}, when only CMB and the LRG PS are used; the
inclusion of the other two experiments do not improve this result. Considering
the dark energy equation of state w_DE as time independent, we measure
w_DE=-1.025+-0.065, for a flat geometry, w_DE=-0.981+-0.083 otherwise. When
describing w_DE through a linear function of the scale factor, our results do
not evidence any time evolution. In the next few years new experiments will
allow to measure the clustering of galaxies with a precision much higher than
achievable today. Models like the one used here will be a valuable tool in
order to achieve the full potentials of the observations and obtain unbiased
constraints on the cosmological parameters.Comment: 28 pages, 26 figures. Accepted for publication in MNRA
A new model for the full shape of the large-scale power spectrum
We present a new model for the full shape of large-scale the power spectrum
based on renormalized perturbation theory. To test the validity of this
prescription, we compare this model against power spectra measured in a suite
of 50 large volume, moderate resolution N-body simulations. Our results
indicate that this simple model provides an accurate description of the full
shape of the power spectrum taking into account the effects of non-linear
evolution, redshift-space distortions and halo bias for scales k < 0.15 h/Mpc,
making it a valuable tool for the analysis of forthcoming galaxy surveys. Even
though its application is restricted to large scales, this prescription can
provide tighter constraints on the dark energy equation of state parameter
w_{DE} than those obtained by modelling the baryonic acoustic oscillations
signal only, where the information of the broad-band shape of the power
spectrum is discarded. Our model is able to provide constraints comparable to
those obtained by applying a similar model to the full shape of the correlation
function, which is affected by different systematics. Hence, with accurate
modelling of the power spectrum, the same cosmological information can be
extracted from both statistics.Comment: Accepted for publication in MNRA
The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with
new instrumentation and new surveys focused on Galactic structure and chemical
evolution, measurements of the baryon oscillation feature in the clustering of
galaxies and the quasar Ly alpha forest, and a radial velocity search for
planets around ~8000 stars. This paper describes the first data release of
SDSS-III (and the eighth counting from the beginning of the SDSS). The release
includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap,
bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a
third of the Celestial Sphere. All the imaging data have been reprocessed with
an improved sky-subtraction algorithm and a final, self-consistent photometric
recalibration and flat-field determination. This release also includes all data
from the second phase of the Sloan Extension for Galactic Understanding and
Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars
at both high and low Galactic latitudes. All the more than half a million
stellar spectra obtained with the SDSS spectrograph have been reprocessed
through an improved stellar parameters pipeline, which has better determination
of metallicity for high metallicity stars.Comment: Astrophysical Journal Supplements, in press (minor updates from
submitted version
Erratum: “The eighth data release of the Sloan Digital Sky Survey: first data from SDSS-III” (2011, ApJS, 193, 29)
Section 3.5 of Aihara et al. (2011) described various sources of systematic error in the astrometry of the imaging data of the Sloan Digital Sky Survey (SDSS). In addition to these sources of error, there is an additional and more serious error, which introduces a large systematic shift in the astrometry over a large area around the north celestial pole. The region has irregular boundaries but in places extends as far south as declination δ ≈ 41◦. The sense of the shift is that the positions of all sources in the affected area are offset by roughly 250 mas in a northwest direction. We have updated the SDSS online documentation to reflect these errors, and to provide detailed quality information for each SDSS field
SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems
Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II),
SDSS-III is a program of four spectroscopic surveys on three scientific themes:
dark energy and cosmological parameters, the history and structure of the Milky
Way, and the population of giant planets around other stars. In keeping with
SDSS tradition, SDSS-III will provide regular public releases of all its data,
beginning with SDSS DR8 (which occurred in Jan 2011). This paper presents an
overview of the four SDSS-III surveys. BOSS will measure redshifts of 1.5
million massive galaxies and Lya forest spectra of 150,000 quasars, using the
BAO feature of large scale structure to obtain percent-level determinations of
the distance scale and Hubble expansion rate at z<0.7 and at z~2.5. SEGUE-2,
which is now completed, measured medium-resolution (R=1800) optical spectra of
118,000 stars in a variety of target categories, probing chemical evolution,
stellar kinematics and substructure, and the mass profile of the dark matter
halo from the solar neighborhood to distances of 100 kpc. APOGEE will obtain
high-resolution (R~30,000), high signal-to-noise (S/N>100 per resolution
element), H-band (1.51-1.70 micron) spectra of 10^5 evolved, late-type stars,
measuring separate abundances for ~15 elements per star and creating the first
high-precision spectroscopic survey of all Galactic stellar populations (bulge,
bar, disks, halo) with a uniform set of stellar tracers and spectral
diagnostics. MARVELS will monitor radial velocities of more than 8000 FGK stars
with the sensitivity and cadence (10-40 m/s, ~24 visits per star) needed to
detect giant planets with periods up to two years, providing an unprecedented
data set for understanding the formation and dynamical evolution of giant
planet systems. (Abridged)Comment: Revised to version published in The Astronomical Journa