arXiv subjects

Licia Verde

Publications and source records attributed to Licia Verde.

233 records · Page 13Linked to original sources

arXiv2003

Fourier analysis of luminosity-dependent galaxy clustering

We extend the Fourier transform based method for the analysis of galaxy redshift surveys of Feldman, Kaiser & Peacock (1994: FKP) to model luminosity-dependent clustering. In a magnitude limited survey, galaxies at high redshift are more luminous on average than galaxies at low redshift. Galaxy clustering is observed to increase with luminosity, so the inferred density field is effectively multiplied by an increasing function of radius. This has the potential to distort the shape of the recovered power spectrum. In this paper we present an extension of the FKP analysis method to incorporate this effect, and present revised optimal weights to maximize the precision of such an analysis. The method is tested and its accuracy assessed using mock catalogues of the 2-degree field galaxy redshift survey (2dFGRS). We also show that the systematic effect caused by ignoring luminosity-dependent bias was negligible for the initial analysis of the 2dFGRS of Percival et al. (2001). However, future surveys, sensitive to larger scales, or covering a wider range of galaxy luminosities will benefit from this refined method.

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arXiv2003

Constraints on the equation of state of dark energy and the Hubble constant from stellar ages and the CMB

We place tight constraints on the redshift-averaged, effective value of the equation of state of dark energy, w, using only the absolute ages of Galactic stars and the observed position of the first peak in the angular power spectrum of the CMB. We find w<-0.8 at the 68% confidence level. If we further consider that w > -1, this finding suggests that within our uncertainties, dark energy is indistinguishable from a classical vacuum energy term. We detect a correlation between the ages of the oldest galaxies and their redshift. This opens up the possibility of measuring w(z) by computing the relative ages of the oldest galaxies in the universe as a function of redshift, dz/dt. We show that this is a realistic possibility by computing dz/dt at z~0 from SDSS galaxies and obtain an independent estimate for the Hubble constant, H_0 = 69 \pm 12 km s-1 Mpc-1. The small number of galaxies considered at z>0.2 does not yield, currently, a precise determination of w(z), but shows that the age--redshift relation is consistent with a Standard LCDM universe with $w=-1$.

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arXiv2002

The bias of galaxies and the density of the universe from the 2dF galaxy redshift survey

By studying the bispectrum of the galaxy distribution in the 2dF galaxy redhsift survey (2dFGRS), we have shown that 2dFGRS galaxies are unbiased tracers of the mass distribution. This allows us the break the degeneracy intrinsic to power spectrum studies, between the matter density parameter Omega_m and the bias parameter b, and to obtain an accurate measurement of Omega_m: Omega_{m,z_{eff}}=0.27 \pm 0.06, a measurement obtained from the 2dFGRS alone, independently from other data sets. This result has to be interpreted as Omega_m at the effective redshift of the survey z_{eff}=0.17. Extrapolated at z=0 we obtain Omega_m=0.2 \pm 0.06 for the LCDM model. This constraint on the matter density parameter when combined with cosmic microwave background constraints on the flatness of the Universe, show firm evidence that the Universe is dominated by a vacuum energy component.

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arXiv2002

Dark Matter Spikes and Annihilation Radiation from the Galactic Center

The annihilation rate of weakly interacting cold dark matter particles at the galactic center could be greatly enhanced by the growth of a density spike around the central supermassive black hole (SBH). Here we discuss the effects of hierarchical mergers on the central spike. Mergers between halos containing SBHs lead to the formation of SBH binaries which transfer energy to the dark matter particles, lowering their density. The predicted flux of annihiliation photons from the galactic center is several orders of magnitude smaller than in models that ignore the effects of SBHs and mergers. Measurement of the annihilation radiation could in principle be used to constrain the merger history of the galaxy.

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arXiv2002

The 2dF Galaxy Redshift Survey: The bias of galaxies and the density of the Universe

We compute the bispectrum of the 2dF Galaxy Redshift Survey (2dFGRS) and use it to measure the bias parameter of the galaxies. This parameter quantifies the strength of clustering of the galaxies relative to the mass in the Universe. By analysing 80 million triangle configurations in the wavenumber range 0.1 < k < 0.5 h/Mpc (i.e. on scales roughly between 5 and 30 Mpc/h) we find that the linear bias parameter is consistent with unity: b_1=1.04 pm 0.11, and the quadratic (nonlinear) bias is consistent with zero: b_2=-0.054 pm 0.08. Thus, at least on large scales, optically-selected galaxies do indeed trace the underlying mass distribution. The bias parameter can be combined with the 2dFGRS measurement of the redshift distortion parameter beta = Omega_m^{0.6}/b_1, to yield Omega_m = 0.27 pm 0.06 for the matter density of the Universe, a result which is determined entirely from this survey, independently of other datasets. Our measurement of the matter density of the Universe should be interpreted as Omega_m at the effective redshift of the survey (z=0.17).

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arXiv2002

The abundance of dark galaxies

We show that gas in a large fraction of low mass dark matter halos may form Toomre stable disks, if angular momentum is conserved when the gas contracts. Such halos would be stable to star formation and therefore remain dark. This may potentially explain the discrepancy between the predicted and observed number of dwarf satellites in the Local Group, as well as the deviation between the predicted and the observed faint end slope of the luminosity function. The above mechanism does not require a strong variation of the baryon fraction with the virial mass of the dark halo. We show that model fits to rotation curves are also consistent with this hypothesis: none of the observed galaxies lie in the region of parameter space forbidden by the Toomre stability criterion.

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arXiv2002

Dark energy and cosmic microwave background bispectrum

We compute the cosmic microwave background bispectrum arising from the cross-correlation of primordial, lensing and Rees-Sciama signals. The amplitude of the bispectrum signal is sensitive to the matter density parameter, Omega_0, and the equation of state of the dark energy, which we parameterize by w_Q. We conclude that the dataset of the Atacama Cosmology Telescope, combined with MAP 2-year data or the Planck data set alone will allow us to break the degeneracy between Omega_0 and w_Q that arises from the analysis of CMB power spectrum. In particular a joint measurement of Omega_0 and w_Q with 10% and 30% error on the two parameters respectively, at the 90% confidence level can realistically be achieved.

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arXiv2002

Dark halo properties from rotation curves

We study a large set of high spatial resolution optical rotation curves of galaxies with the goal of determining the model parameters for a disk embedded within a CDM halo that we model either with a NFW profile or pseudo-isothermal profile. We show that parameter degeneracies present in lower resolution data are lifted at these higher resolutions. 34% of the galaxies do not have a meaningful fit when using the NFW profile and 32% when using the pseudoisothermal profile, however only 14% do not have a meaningful fit in either model. In both models we find correlations between the disk baryon fraction $f_d$ and the spin parameter of the halo $λ'$, between $f_d$ and the dark halo mass $M_{200}$, and between $M_{200}$ and the concentration parameter $c$. We show that the distribution of the concentration parameters $c$, for a NFW halo, is in good agreement with CDM predictions; no significant galaxy population is found with very low values of $c$. The overall distribution of $λ'$ is in good agreement with theoretical predictions from hierarchical tidal torque theory. The whole sample is also well fitted by a pseudo-isothermal dark halo with a core, but the size of the core is rather small (for 70% of the sample the core size is less than 2 kpc). Thus we conclude that the profile of dark matter is steep ($r^{-1}$ or steeper) down to this radius; large dark matter cores (and therefore very low dark matter central densities) seem to be excluded. LSBs tend to have higher values of $λ'$ for a given $f_d$ and lower values of $c$ for a given mass than HSBs. In an appendix we give some useful formula for pseudo-isothermal profile halos and discuss in detail the issue of parameter degeneracies.

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arXiv2001

On Galaxy-Cluster Sizes and Temperatures

We show that the distribution of the sizes and temperatures of clusters can be used to constrain cosmological models. The size-temperature (ST) distribution predicted in a flat Gaussian cluster-abundance-normalized Omega_0=0.3 model agrees well with the fairly tight ST relation observed. A larger power-spectrum amplitude sigma_8 would give rise to a larger scatter about the ST relation as would a larger value of Omega_0 and/or long non-Gaussian high-density tails in the probability density function. For Gaussian initial conditions, the ST distribution suggests a constraint sigma_8 Omega_0^{0.26} \simeq 0.76. The ST relation is expected to get tighter at high redshifts. In the process, we derive a simple formula for the halo formation-redshift distribution for non-Gaussian models. We also suggest that the discrepancy between the naive zero-redshift ST relation and that observed may be due, at least in part, to the fact that lower-mass clusters form over a wider range of redshifts. An Appendix derives an equation for the formation-redshift distribution of halos.

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arXiv2001

Are Clusters Standard Candles? Galaxy Cluster Scaling Relations With the Sunyaev-Zeldovich Effect

An extensive sample of galaxy clusters will be available in the coming years, detected through their Sunyaev-Zeldovich effect (SZE). We use a semi-analytic model to study the scientific yield of combining SZE data with X-ray and optical follow-up observations. If clusters at a given redshift z_o can be identified with virialized, spherical halos, they populate a well-defined "fundamental plane" (FP) in the parameter space of the three observables: virial temperature (T), total Sunyaev-Zeldovich flux decrement (DS), and angular size (theta). The location and orientation of the FP, as well as its redshift-evolution, are sensitive to both the internal evolution of clusters, and to the underlying cosmological parameters. We show that if clusters are not standard candles (e.g. due to feedback, or energy injection), then this can be inferred from the FP. Likewise, we study the dependence of the FP on the cosmological parameters h, sigma_8, and Omega_0, and quantify constraints on these parameters. We also show that in the absence of any non-gravitational effects, the scatter in the the (DS-T) plane is significantly smaller than in either the (theta-T) or the (theta-DS) planes. As a result, the (DS-T) relation can be an exceptionally sensitive probe of both cluster physics and cosmological parameters, and a comparison of the amount of scatter in these three scaling relations will serve as a test of the origin (cosmological vs. stochastic) of the scatter.

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arXiv2001

Tests for primordial non-Gaussianity

We investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure (LSS), high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness.

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arXiv2001

On the trispectrum as a gaussian test for cosmology

In the standard model for structure formation, bound objects originate from the gravitational collapse of small perturbations arising from quantum fluctuations with random phases. In other scenarios, based on defects, structures are seeded by localized energy density. In principle, it is possible to differentiate between these models on the basis of their statistical properties; only in the former case is the initial density field an almost-perfect random gaussian field. In this paper, we investigate the use of the trispectrum of the galaxy density field, which is the connected four-point function in Fourier space, as a discriminant between gaussian and non-gaussian models. It has the advantage of having only weak non-linear growth. We define a related statistic $τ$ which, as a test of the gaussian hypothesis, is independent of cosmology, the power spectrum and biasing, in real space, and which is, in principle, a measure of the departure from gaussian statistics. For galaxy redshift surveys, the statistic depends on cosmology and bias only through the potentially observable parameter $β$. We compute the expected errors on the estimate of $τ$, and demonstrate with numerical simulations that it can be a useful discriminant of models, with the important proviso that any bias is linear on large scales. Whether it is the most effective method is uncertain and depends on the nature of the departure from gaussianity.

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arXiv2000

Projected bispectrum in spherical harmonics and its application to angular galaxy catalogues

We present a theoretical and exact analysis of the bispectrum of projected galaxy catalogues. The result can be generalized to evaluate the projection in spherical harmonics of any 3D bispectrum and therefore has applications to cosmic microwave background and gravitational lensing studies. By expanding the 2D distribution of galaxies on the sky in spherical harmonics, we show how the 3-point function of the coefficients can be used in principle to determine the bias parameter of the galaxy sample. If this can be achieved, it would allow a lifting of the degeneracy between the bias and the matter density parameter of the Universe which occurs in linear analysis of 3D galaxy catalogues. In previous papers we have shown how a similar analysis can be done in three dimensions, and we show here, through an error analysis and by implementing the method on a simulated projected catalogue, that ongoing three-dimensional galaxy redshift surveys (even with all the additional uncertainties introduced by partial sky coverage, redshift-space distortions and smaller numbers) will do far better than all-sky projected catalogues with similar selection function.

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arXiv2000

The abundance of high-redshift objects as a probe of non-Gaussian initial conditions

The observed abundance of high-redshift galaxies and clusters contains precious information about the properties of the initial perturbations. We present a method to compute analytically the number density of objects as a function of mass and redshift for a range of physically motivated non-Gaussian models. In these models the non-Gaussianity can be dialed from zero and is assumed to be small. We compute the probability density function for the smoothed dark matter density field and we extend the Press and Schechter approach to mildly non-Gaussian density fields. The abundance of high-redshift objects can be directly related to the non-Gaussianity parameter and thus to the physical processes that generated deviations from the Gaussian behaviour. Even a skewness parameter of order 0.1 implies a dramatic change in the predicted abundance of $z\gap 1$ objects. Observations from NGST and X-ray satellites (XMM) can be used to accurately measure the amount of non-Gaussianity in the primordial density field.

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arXiv2000

Non-Gaussianity vs. non-linearity of cosmological perturbations

Following the discovery of the CMB, the hot big-bang model has become the standard cosmological model. In this theory, small primordial fluctuations are subsequently amplified by gravity to form the large-scale structure seen today. Different theories for unified models of particle physics, lead to different predictions for the statistical properties of the primordial fluctuations, that can be divided in two classes: gaussian and non-gaussian. Convincing evidence against or for gaussian initial conditions would rule out many scenarios and point us towards a physical theory for the origin of structures. The statistical distribution of cosmological perturbations, as we observe them, can deviate from the gaussian distribution in several different ways. Even if perturbations start off gaussian, non-linear gravitational evolution can introduce non-gaussian features. Additionally, our knowledge of the Universe comes principally from the study of luminous material such as galaxies, but these might not be faithful tracers of the underlying mass distribution. The relationship between fluctuations in the mass and in the galaxies distribution (bias), is often assumed to be local, but could well be non-linear. Moreover, galaxy catalogues use the redshift as third spatial coordinate: the resulting redshift-space map is non-linearly distorted by peculiar velocities. Non-linear gravitational evolution, biasing, and redshift-space distortion introduce non-gaussianity, even in an initially gaussian fluctuation field. I will investigate the statistical tools that allow us, in principle, to disentangle the above different effects, and the observational datasets we require to do so in practice.

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arXiv1999

Large-scale structure, the cosmic microwave background, and primordial non-gaussianity

Since cosmic-microwave-background (CMB) and large-scale-structure (LSS) data will shortly improve dramatically with the Microwave Anisotropy Probe (MAP) and Planck Surveyor, and the Anglo-Australian 2-Degree Field (2dF) and Sloan Digital Sky Survey (SDSS), respectively, it is timely to ask which of the CMB or LSS will provide a better probe of primordial non-gaussianity. In this paper we consider this question, using the bispectrum as a discriminating statistic. We consider several non-gaussian models and find that in each case the CMB will provide a better probe of primordial non-gaussianity. Since the bispectrum is the lowest-order statistic expected to arise in a generic non-gaussian model, our results suggest that if CMB maps appear gaussian, then apparent deviations from gaussian initial conditions in galaxy surveys can be attributed with confidence to the effects of biasing. We demonstrate this precisely for the spatial bispectrum induced by local non-linear biasing.

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arXiv1998

Large-scale bias in the Universe II: redshift space bispectrum

The determination of the density parameter $Ω_0$ from the large-scale distribution of galaxies is one of the major goals of modern cosmology. However, if galaxies are biased tracers of the underlying mass distribution, linear perturbation theory leads to a degeneracy between $Ω_0$ and the linear bias parameter $b$, and the density parameter cannot be estimated. In Matarrese, Verde & Heavens (1997) we developed a method based on second-order perturbation theory to use the bispectrum to lift this degeneracy by measuring the bias parameter in an $Ω_0$-independent way. The formalism was developed assuming that one has perfect information on the positions of galaxies in three dimensions. In galaxy redshift surveys, the three-dimensional information is imperfect, because of the contaminating effects of peculiar velocities, and the resulting clustering pattern in redshift space is distorted. In this paper, we combine second-order perturbation theory with a model for collapsed, virialised structures, to extend the method to redshift space, and demonstrate that the method should be successful in determining with reasonable accuracy the bias parameter from state-of-the-art surveys such as the Anglo-Australian 2 degree field survey and the Sloan digital sky survey.

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