28 research outputs found
A cross-correlation-based estimate of the galaxy luminosity function
We extend existing methods for using cross-correlations to derive redshift
distributions for photometric galaxies, without using photometric redshifts.
The model presented in this paper simultaneously yields highly accurate and
unbiased redshift distributions and, for the first time, redshift-dependent
luminosity functions, using only clustering information and the apparent
magnitudes of the galaxies as input. In contrast to many existing techniques
for recovering unbiased redshift distributions, the output of our method is not
degenerate with the galaxy bias b(z), which is achieved by modelling the shape
of the luminosity bias. We successfully apply our method to a mock galaxy
survey and discuss improvements to be made before applying our model to real
data.Comment: 14 pages, 7 figures. Replaced to match the version accepted by MNRA
The contributions of matter inside and outside of haloes to the matter power spectrum
Halo-based models have been successful in predicting the clustering of
matter. However, the validity of the postulate that the clustering is fully
determined by matter inside haloes remains largely untested, and it is not
clear a priori whether non-virialised matter might contribute significantly to
the non-linear clustering signal. Here, we investigate the contribution of
haloes to the matter power spectrum as a function of both scale and halo mass
by combining a set of cosmological N-body simulations to calculate the
contributions of different spherical overdensity regions, Friends-of-Friends
(FoF) groups and matter outside haloes to the power spectrum. We find that
matter inside spherical overdensity regions of size R200,mean cannot account
for all power for 1<k<100 h/Mpc, regardless of the minimum halo mass. At most,
it accounts for 95% of the power (k>20 h/Mpc). For 2<k<10 h/Mpc, haloes with
mass M200,mean<10^11 Msun/h contribute negligibly to the power spectrum, and
our results appear to be converged with decreasing halo mass. When haloes are
taken to be regions of size R200,crit, the amount of power unaccounted for is
larger on all scales. Accounting also for matter inside FoF groups but outside
R200,mean increases the contribution of halo matter on most scales probed here
by 5-15%. Matter inside FoF groups with M200,mean>10^9 Msun/h accounts for
essentially all power for 3<k<100 h/Mpc. We therefore expect halo models that
ignore the contribution of matter outside R200,mean to overestimate the
contribution of haloes of any mass to the power on small scales (k>1 h/Mpc).Comment: 13 pages, 9 figures. Replaced to match the version accepted by MNRA
The effects of halo alignment and shape on the clustering of galaxies
We investigate the effects of halo shape and its alignment with larger scale
structure on the galaxy correlation function. We base our analysis on the
galaxy formation models of Guo et al., run on the Millennium Simulations. We
quantify the importance of these effects by randomizing the angular positions
of satellite galaxies within haloes, either coherently or individually, while
keeping the distance to their respective central galaxies fixed. We find that
the effect of disrupting the alignment with larger scale structure is a ~2 per
cent decrease in the galaxy correlation function around r=1.8 Mpc/h. We find
that sphericalizing the ellipsoidal distributions of galaxies within haloes
decreases the correlation function by up to 20 per cent for r<1 Mpc/h and
increases it slightly at somewhat larger radii. Similar results apply to power
spectra and redshift-space correlation functions. Models based on the Halo
Occupation Distribution, which place galaxies spherically within haloes
according to a mean radial profile, will therefore significantly underestimate
the clustering on sub-Mpc scales. In addition, we find that halo assembly bias,
in particular the dependence of clustering on halo shape, propagates to the
clustering of galaxies. We predict that this aspect of assembly bias should be
observable through the use of extensive group catalogues.Comment: 8 pages, 6 figures. Accepted for publication in MNRAS. Minor changes
relative to v1. Note: this is an revised and considerably extended
resubmission of http://arxiv.org/abs/1110.4888; please refer to the current
version rather than the old on
The contribution of massive haloes to the matter power spectrum in the presence of AGN feedback
The clustering of matter, as measured by the matter power spectrum, informs
us about dark matter and cosmology, as well as baryonic effects on the
distribution of matter in the universe. Using cosmological hydrodynamical
simulations from the cosmo-OWLS and BAHAMAS simulation projects, we investigate
the contribution of power in haloes with various masses, defined by particles
within some overdensity region, to the full power spectrum, as well as the
power ratio between baryonic and dark matter only (DMO) simulations for a
matched (between simulations) and an unmatched set of haloes. We find that the
presence of AGN feedback suppresses the power on all scales for haloes of all
masses examined (), by ejecting matter from within
to and potentially beyond in massive
haloes (), and likely
impeding the growth of lower-mass haloes as a consequence. A lower AGN feedback
temperature drastically changes the behaviour of high-mass haloes
(), damping the
effects of AGN feedback at small scales, .
For , group-sized haloes
() dominate the power spectrum, while on
smaller scales the combined contributions of lower-mass haloes to the full
power spectrum rise above that of the group-sized haloes. Finally, we present a
model for the power suppression due to feedback, which combines observed mean
halo baryon fractions with halo mass fractions and halo-matter cross-spectra
extracted from dark matter only simulations to predict the power suppression to
percent-level accuracy down to without
any free parameters.Comment: 20 pages, 11 figures. Submitted to MNRA
The impact of baryonic processes on the two-point correlation functions of galaxies, subhaloes and matter
The observed clustering of galaxies and the cross-correlation of galaxies and
mass provide important constraints on both cosmology and models of galaxy
formation. Even though the dissipation and feedback processes associated with
galaxy formation are thought to affect the distribution of matter, essentially
all models used to predict clustering data are based on collisionless
simulations. Here, we use large hydrodynamical simulations to investigate how
galaxy formation affects the autocorrelation functions of galaxies and
subhaloes, as well as their cross-correlation with matter. We show that the
changes due to the inclusion of baryons are not limited to small scales and are
even present in samples selected by subhalo mass. Samples selected by subhalo
mass cluster ~10% more strongly in a baryonic run on scales r > 1Mpc/h, and
this difference increases for smaller separations. While the inclusion of
baryons boosts the clustering at fixed subhalo mass on all scales, the sign of
the effect on the cross-correlation of subhaloes with matter can vary with
radius. We show that the large-scale effects are due to the change in subhalo
mass caused by the strong feedback associated with galaxy formation and may
therefore not affect samples selected by number density. However, on scales r <
r_vir significant differences remain after accounting for the change in subhalo
mass. We conclude that predictions for galaxy-galaxy and galaxy-mass clustering
from models based on collisionless simulations will have errors greater than
10% on sub-Mpc scales, unless the simulation results are modified to correctly
account for the effects of baryons on the distributions of mass and satellites.Comment: 15 pages, 9 figures. Replaced to match the version accepted by MNRA
Galaxy and Mass Assembly (GAMA): probing galaxy-group correlations in redshift space with the halo streaming model
We have studied the galaxy-group cross-correlations in redshift space for the Galaxy And Mass Assembly (GAMA) Survey. We use a set of mock GAMA galaxy and group catalogues to develop and test a novel ‘halo streaming’ model for redshift-space distortions. This treats 2-halo correlations via the streaming model, plus an empirical 1-halo term derived from the mocks, allowing accurate modelling into the non-linear regime. In order to probe the robustness of the growth rate inferred from redshift-space distortions, we divide galaxies by colour, and divide groups according to their total stellar mass, calibrated to total mass via gravitational lensing. We fit our model to correlation data, to obtain estimates of the perturbation growth rate, fσ8, validating parameter errors via the dispersion between different mock realizations. In both mocks and real data, we demonstrate that the results are closely consistent between different subsets of the group and galaxy populations, considering the use of correlation data down to some minimum projected radius, rmin. For the mock data, we can use the halo streaming model to below rmin = 5 h−1 Mpc, finding that all subsets yield growth rates within about 3 per cent of each other, and consistent with the true value. For the actual GAMA data, the results are limited by cosmic variance: fσ8 = 0.29 ± 0.10 at an effective redshift of 0.20; but there is every reason to expect that this method will yield precise constraints from larger data sets of the same type, such as the Dark Energy Spectroscopic Instrument (DESI) bright galaxy surve