5,894 research outputs found
Quantifying cosmic variance
We determine an expression for the cosmic variance of any "normal" galaxy
survey based on examination of M* +/- 1 mag galaxies in the SDSS DR7 data cube.
We find that cosmic variance will depend on a number of factors principally:
total survey volume, survey aspect ratio, and whether the area surveyed is
contiguous or comprised of independent sight-lines. As a rule of thumb cosmic
variance falls below 10% once a volume of 10^7h_0.7^-3Mpc^3 is surveyed for a
single contiguous region with a 1:1 aspect ratio. Cosmic variance will be lower
for higher aspect ratios and/or non-contiguous surveys. Extrapolating outside
our test region we infer that cosmic variance in the entire SDSS DR7 main
survey region is ~7% to z < 0.1. The equation obtained from the SDSS DR7 region
can be generalised to estimate the cosmic variance for any density measurement
determined from normal galaxies (e.g., luminosity densities, stellar mass
densities and cosmic star-formation rates) within the volume range 10^3 to 10^7
h^-3_0.7Mpc^3. We apply our equation to show that 2 sightlines are required to
ensure cosmic variance is <10% in any ASKAP galaxy survey (divided into dz ~0.1
intervals, i.e., ~1 Gyr intervals for z <0.5). Likewise 10 MeerKAT sightlines
will be required to meet the same conditions. GAMA, VVDS, and zCOSMOS all
suffer less than 10% cosmic variance (~3%-8%) in dz intervals of 0.1, 0.25, and
0.5 respectively. Finally we show that cosmic variance is potentially at the
50-70% level, or greater, in the HST Ultra Deep Field depending on assumptions
as to the evolution of clustering. 100 or 10 independent sightlines will be
required to reduce cosmic variance to a manageable level (<10%) for HST ACS or
HST WFC3 surveys respectively (in dz ~ 1 intervals). Cosmic variance is
therefore a significant factor in the z>6 HST studies currently underway.Comment: Accepted for publication in MNRA
Cation composition effects on oxide conductivity in the Zr_2Y_2O_7-Y_3NbO_7 system
Realistic, first-principles-based interatomic potentials have been used in
molecular dynamics simulations to study the effect of cation composition on the
ionic conductivity in the Zr2Y2O7-Y3NbO7 system and to link the dynamical
properties to the degree of lattice disorder. Across the composition range,
this system retains a disordered fluorite crystal structure and the vacancy
concentration is constant. The observed trends of decreasing conductivity and
increasing disorder with increasing Nb5+ content were reproduced in simulations
with the cations randomly assigned to positions on the cation sublattice. The
trends were traced to the influences of the cation charges and relative sizes
and their effect on vacancy ordering by carrying out additional calculations in
which, for example, the charges of the cations were equalised. The simulations
did not, however, reproduce all the observed properties, particularly for
Y3NbO7. Its conductivity was significantly overestimated and prominent diffuse
scattering features observed in small area electron diffraction studies were
not always reproduced. Consideration of these deficiencies led to a preliminary
attempt to characterise the consequence of partially ordering the cations on
their lattice, which significantly affects the propensity for vacancy ordering.
The extent and consequences of cation ordering seem to be much less pronounced
on the Zr2Y2O7 side of the composition range.Comment: 22 pages, 8 figures, submitted to Journal of Physics: Condensed
Matte
Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?
We use measurements of the projected galaxy correlation function w_p and
galaxy void statistics to test whether the galaxy content of halos of fixed
mass is systematically different in low density environments. We present new
measurements of the void probability function (VPF) and underdensity
probability function (UPF) from Data Release Four of the Sloan Digital Sky
Survey, as well as new measurements of the VPF from the full data release of
the Two-Degree Field Galaxy Redshift Survey. We compare these measurements to
predictions calculated from models of the Halo Occupation Distribution (HOD)
that are constrained to match both w_p and the space density of galaxies. The
standard implementation of the HOD assumes that galaxy occupation depends on
halo mass only, and is independent of local environment. For luminosity-defined
samples, we find that the standard HOD prediction is a good match to the
observations, and the data exclude models in which galaxy formation efficiency
is reduced in low-density environments. More remarkably, we find that the void
statistics of red and blue galaxies (at L ~ 0.4L_*) are perfectly predicted by
standard HOD models matched to the correlation function of these samples,
ruling out "assembly bias" models in which galaxy color is correlated with
large-scale environment at fixed halo mass. We conclude that the luminosity and
color of field galaxies are determined predominantly by the mass of the halo in
which they reside and have little direct dependence on the environment in which
the host halo formed. In broader terms, our results show that the sizes and
emptiness of voids found in the distribution of L > 0.2L_* galaxies are in
excellent agreement with the predictions of a standard cosmological model with
a simple connection between galaxies and dark matter halos. (abridged)Comment: 20 emulateapj pages, 9 figures. submitted to Ap
Dynamical segregation of galaxies in groups and clusters
We have performed a systematic analysis of the dynamics of different galaxy
populations in galaxy groups from the 2dFGRS. For this purpose we have combined
all the groups into a single system, where velocities v and radius r are
expressed adimensionally. We have used several methods to compare the
distributions of relative velocities of galaxies with respect to the group
centre for samples selected according to their spectral type (as defined by
Madgwick et al., 2002), bj band luminosity and B-R colour index. We have found
strong segregation effects: spectral type I objects show a statistically
narrower velocity distribution than that of galaxies with a substantial star
formation activity (type II-IV). Similarly, the same behavior is observed for
galaxies with colour index B-R>1 compared to galaxies with B-R<1. Bright
(Mb-19) galaxies show the same segregation. It is not
important once the sample is restricted to a given spectral type. These effects
are particularly important in the central region (Rp<Rvirial/2) and do not have
a strong dependence on the mass of the parent group. These trends show a strong
correlation between the dynamics of galaxies in groups and star formation rate
reflected both by spectral type and by colour index.Comment: 7 pages, 8 figures. Accepted for publication in MNRA
The spectral-type/luminosity and the spectral type/satellite-density relations in the 2dFGRS
We examine the relative fractions of passive (Type 1), quiet-SF (Type 2) and
active-SF (Type 3+4) galaxies as a function of luminosity and number of
neighbours in several volume limited samples selected from the 2dFGRS.
Neighbours are counted within 1 Mpc projected distance, and
1000 km s depth. We apply a maximum magnitude difference criterion and
require neighbours to be fainter than the galaxy itself. We show that, whatever
the environment, passive galaxies dominate in bright samples and active-SF
galaxies in faint samples, whereas quiet-SF galaxies never dominate. We further
show that in bright samples (M -- 5 ) the fraction of passive galaxies grows steadily with fainter neighbour
density, whereas in faint samples a threshold like dependence is observed. This
suggests that the spectral-type / density ( morphology / density)
relation extends to intermediate dense environment, but only in the surrounding
of luminous galaxies and that it reflects an enhancement of the number of
satellites rather than stronger clustering among galaxies themselves. Our
analysis indicates that, in general, luminosity is a good tracer of a galaxy
halo mass and that it dominates over environment (satellite density) in setting
the spectral type mix of a population. However, minority populations exist,
such as luminous SF galaxies and faint passive galaxies, whose luminosity is an
unfair tracer of halo mass.Comment: 8 pages, 5 figur
The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence
(Abridged) Projected rotational velocities (vsini) have been measured for a
sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun)
located in the Orion star-forming complex. These measurements have been
supplemented with data from the literature for Orion stars with masses as low
as 0.1 Msun. The primary finding from analysis of these data is that the upper
envelope of the observed values of angular momentum per unit mass (J/M) varies
as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3
Msun. This power law extends smoothly into the domain of more massive stars (3
to 10 Msun), which in Orion are already on the ZAMS. This result stands in
sharp contrast to the properties of main sequence stars, which show a break in
the power law and a sharp decline in J/M with decreasing mass for stars with M
<2 Msun. A second result of our study is that this break is seen already among
the PMS stars in our Orion sample that are on radiative tracks, even though
these stars are only a few million years old. A comparison of rotation rates
seen for stars on either side of the convective-radiative boundary shows that
stars do not rotate as solid bodies during the transition from convective to
radiative tracks.Comment: to appear in Ap
Clustering properties of a type-selected volume-limited sample of galaxies in the CFHTLS
(abridged) We present an investigation of the clustering of i'AB<24.5
galaxies in the redshift interval 0.2<z<1.2. Using 100,000 precise photometric
redshifts in the four ultra-deep fields of the Canada-France Legacy Survey, we
construct a set of volume-limited galaxy catalogues. We study the dependence of
the amplitude and slope of the galaxy correlation function on absolute B-band
rest-frame luminosity, redshift and best-fitting spectral type. We find: 1. The
comoving correlation length for all galaxies decreases steadily from z~0.3 to
z~1. 2. At all redshifts and luminosities, galaxies with redder rest-frame
colours have clustering amplitudes between two and three times higher than
bluer ones. 3. For bright red and blue galaxies, the clustering amplitude is
invariant with redshift. 4. At z~0.5, less luminous galaxies have higher
clustering amplitudes of around 6 h-1 Mpc. 5. The relative bias between
galaxies with red and blue rest-frame colours increases gradually towards
fainter absolute magnitudes. One of the principal implications of these results
is that although the full galaxy population traces the underlying dark matter
distribution quite well (and is therefore quite weakly biased), redder, older
galaxies have clustering lengths which are almost invariant with redshift, and
by z~1 are quite strongly biased.Comment: 16 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
Superclusters of galaxies in the 2dF redshift survey. III. The properties of galaxies in superclusters
We use catalogues of superclusters of galaxies from the 2dF Galaxy Redshift
Survey to study the properties of galaxies in superclusters. We compare the
properties of galaxies in high and low density regions of rich superclusters,
in poor superclusters and in the field, as well as in groups, and of isolated
galaxies in superclusters of various richness. We show that in rich
superclusters the values of the luminosity density smoothed on a scale of 8
\Mpc are higher than in poor superclusters: the median density in rich
superclusters is , in poor superclusters . Rich superclusters contain high density cores with densities while in poor superclusters such high density cores are absent. The
properties of galaxies in rich and poor superclusters and in the field are
different: the fraction of early type, passive galaxies in rich superclusters
is slightly larger than in poor superclusters, and is the smallest among the
field galaxies. Most importantly, in high density cores of rich superclusters
() there is an excess of early type, passive galaxies in groups
and clusters, as well as among those which do not belong to groups or clusters.
The main galaxies of superclusters have a rather limited range of absolute
magnitudes. The main galaxies of rich superclusters have larger luminosities
than those of poor superclusters and of groups in the field. Our results show
that both the local (group/cluster) environments and global (supercluster)
environments influence galaxy morphologies and their star formation activity.Comment: 13 pages, 10 figures, submitted to Astronomy and Astrophysic
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