37 research outputs found
Modelling the Power Spectrum of Density Fluctuations: A Phenomenological Approach
We show how, based on considerations on the observed form of the galaxy
2-point spatial correlation function xi(r), a very simplified -- yet
surprisingly effective -- model for the linear density fluctuations power
spectrum can be constructed. We first relate the observed large-scale shape of
xi(r) to a power-law form for the power spectrum, P(k)\propto k^{-2.2}. For a
plausible value of the bias parameter b = 1/sigma_8 ~ 1.8, one has (delta_rho /
rho)_{rms} ~ 1 r ~ 3.5/h Mpc, suggesting that the change of slope observed in
xi(r) around this scale marks the transition between the linear and nonlinear
gravitational regimes. Under this working hypothesis, we use a simple
analytical form to fit the large-scale correlations constraints together with
the COBE CMB anisotropy measurement, thus constructing a simple
phenomenological model for the linear power spectrum. Despite its simplicity,
the model fits remarkably well directly estimated power spectra from different
optical galaxy samples, and when evolved through an N-body simulation it
provides a good match to the observed galaxy correlations. One of the most
interesting features of the model is the small-scale one-dimensional velocity
dispersion produced: sigma_{1d} = 450 Km s^{-1} at 0.5/h Mpc and sigma_{1d} =
350 Km s^{-1} for separations larger than ~ 2/h Mpc.Comment: ApJL in press, 10 pages in plain TeX, 3 figures available from
[email protected], SISSA 110/93/
The velocity field of 2MRS Ks=11.75 galaxies: constraints on beta and bulk flow from the luminosity function
Using the nearly full sky Ks=11.75 2MASS Redshift Survey [2MRS]of ~45,000
galaxies we reconstruct the underlying peculiar velocity field and constrain
the cosmological bulk flow within ~100. These results are obtained by
maximizing the probability to estimate the absolute magnitude of a galaxy given
its observed apparent magnitude and redshift. At a depth of ~60 Mpc/h we find a
bulk flow Vb=(90\pm65,-230\pm65,50\pm65) km/s in agreement with the theoretical
predictions of the LCDM model. The reconstructed peculiar velocity field that
maximizes the likelihood is characterized by the parameter beta=0.323 +/- 0.08.
Both results are in agreement with those obtained previously using the ~23,000
galaxies of the shallower Ks=11.25 2MRS survey. In our analysis we find that
the luminosity function of 2MRS galaxies is poorly fitted by the Schechter form
and that luminosity evolves such that objects become fainter with increasing
redshift according to L(z)=L(z=0)(1+z)^(+2.7 +/-0.15).Comment: 10 pages, 6 figure
A comparison of the galaxy peculiar velocity field with the PSCz gravity field-- A Bayesian hyper-parameter method
We constructed a Bayesian hyper-parameter statistical method to quantify the
difference between predicted velocities derived from the observed galaxy
distribution in the \textit{IRAS}-PSC redshift survey and peculiar
velocities measured using different distance indicators. In our analysis we
find that the model--data comparison becomes unreliable beyond 70 \hmpc
because of the inadequate sampling by \textit{IRAS} survey of prominent,
distant superclusters, like the Shapley Concentration. On the other hand, the
analysis of the velocity residuals show that the PSC gravity field provides
an adequate model to the local, \le 70 \hmpc, peculiar velocity field. The
hyper-parameter combination of ENEAR, SN, A1SN and SFI++ catalogues in the
Bayesian framework constrains the amplitude of the linear flow to be
. For an rms density fluctuations in the PSC galaxy
number density , we obtain an estimate of the
growth rate of density fluctuations ,
which is in excellent agreement with independent estimates based on different
techniques.Comment: 14 pages, 32 figures, MNRAS in press, matched the MNRAS published
versio
Simultaneous constraints on bias, normalization and growth index through power spectrum measurements
In this Letter we point out that redshift surveys can break the degeneracy
between the galaxy bias, the power spectrum normalization, \sigma_{8,0} and the
growth factor, without the need for external information by using a simple and
rather general parametrization for the growth rate, the well known
\gamma-parametrization and measuring the power spectrum at least at two
different redshifts. We find that in next-generation surveys like Euclid,
\sigma_{8,0} and \gamma can be measured to within 1% and 5%, respectively,
while the bias b(z) can be measured to within 1-2% in each of 14 equal-width
redshift bins spanning 0.7<=z<=2.Comment: 6 pages, 4 figures, version matching the one published by MNRAS
Letter
Growth factor and galaxy bias from future redshift surveys: a study on parametrizations
Many experiments in the near future will test dark energy through its effects
on the linear growth of matter perturbations. In this paper we discuss the
constraints that future large-scale redshift surveys can put on three different
parameterizations of the linear growth factor and how these constraints will
help ruling out different classes of dark energy and modified gravity models.
We show that a scale-independent bias can be estimated to a few percent per
redshift slice by combining redshift distortions with power spectrum amplitude,
without the need of an external estimation. We find that the growth rate can be
constrained to within 2-4% for each redshift slice, while the
equation of state and the index can be simultaneously estimated
both to within 0.02. We also find that a constant dimensionless coupling
between dark energy and dark matter can be constrained to be smaller than 0.14.Comment: 22 pages, 11 figure
Euclid: Constraining ensemble photometric redshift distributions with stacked spectroscopy
The ESA Euclid mission will produce photometric galaxy samples over 15000 square degrees of the sky that will be rich for clustering and weak lensing statistics. The accuracy of the cosmological constraints derived from these measurements will depend on the knowledge of the underlying redshift distributions based on photometric redshift calibrations. A new approach is proposed to use the stacked spectra from Euclid slitless spectroscopy to augment broad-band photometric information to constrain the redshift distribution with spectral energy distribution fitting. The high spectral resolution available in the stacked spectra complements the photometry and helps to break the colour-redshift degeneracy and constrain the redshift distribution of galaxy samples. We modelled the stacked spectra as a linear mixture of spectral templates. The mixture may be inverted to infer the underlying redshift distribution using constrained regression algorithms. We demonstrate the method on simulated Vera C. Rubin Observatory and Euclid mock survey data sets based on the Euclid Flagship mock galaxy catalogue. We assess the accuracy of the reconstruction by considering the inference of the baryon acoustic scale from angular two-point correlation function measurements. We selected mock photometric galaxy samples at redshift z>1 using the self-organising map algorithm. Considering the idealised case without dust attenuation, we find that the redshift distributions of these samples can be recovered with 0.5% accuracy on the baryon acoustic scale. The estimates are not significantly degraded by the spectroscopic measurement noise due to the large sample size. However, the error degrades to 2% when the dust attenuation model is left free. We find that the colour degeneracies introduced by attenuation limit the accuracy considering the wavelength coverage of Euclid near-infrared spectroscopy