arXiv subjects

Enzo Branchini

Publications and source records attributed to Enzo Branchini.

At least 37 records · Page 2Linked to original sources

arXiv2014

A new cosmological distance measure using AGN X-ray variability

We report the discovery of a luminosity distance estimator using Active Galactic Nuclei (AGN). We combine the correlation between the X-ray variability amplitude and the Black Hole (BH) mass with the single epoch spectra BH mass estimates which depend on the AGN luminosity and the line width emitted by the broad line region. We demonstrate that significant correlations do exist which allows one to predict the AGN (optical or X-ray) luminosity as a function of the AGN X-ray variability and either the HBeta or the PaBeta line widths. In the best case, when the PaBeta is used, the relationship has an intrinsic dispersion of ~0.6 dex. Although intrinsically more disperse than Supernovae Ia, this relation constitutes an alternative distance indicator potentially able to probe, in an independent way, the expansion history of the Universe. With this respect, we show that the new mission concept Athena should be able to measure the X-ray variability of hundreds of AGN and then constrain the distance modulus with uncertainties of 0.1 mag up to z~0.6. We also discuss how, using a new dedicated wide field X-ray telescope able to measure the variability of thousands of AGNs, our estimator has the prospect to become a cosmological probe even more sensitive than current Supernovae Ia samples.

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arXiv2014

On the recovery of Local Group motion from galaxy redshift surveys

There is a $\sim 150 km s^{-1}$ discrepancy between the measured motion of the Local Group of galaxies (LG) with respect to the CMB and the linear theory prediction based on the gravitational force field of the large scale structure in full-sky redshift surveys. We perform a variety of tests which show that the LG motion cannot be recovered to better than $150-200 km s^{-1}$ in amplitude and within a $\approx10^\circ$ in direction. The tests rely on catalogs of mock galaxies identified in the Millennium simulation using semi-analytic galaxy formation models. We compare these results to the $K_s=11.75$ Two-Mass Galaxy Redshift Survey, which provides the deepest, widest and most complete spatial distribution of galaxies available so far. In our analysis we use a new, concise relation for deriving the LG motion and bulk flow from the true distribution of galaxies in redshift space. Our results show that the main source of uncertainty is the small effective depth of surveys like the 2MRS that prevents a proper sampling of the large scale structure beyond $\sim100 h^{-1} Mpc$. Deeper redshift surveys are needed to reach the "convergence scale" of $\approx 250 h^{-1}Mpc $ in a $Λ$CDM universe. Deeper survey would also mitigate the impact of the "Kaiser rocket" which, in a survey like 2MRS, remains a significant source of uncertainty. Thanks to the quiet and moderate density environment of the LG, purely dynamical uncertainties of the linear predictions are subdominant at the level of $\sim 90 km s^{-1}$. Finally, we show that deviations from linear galaxy biasing and shot noise errors provide a minor contribution to the total error budget.

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arXiv2013

Cosmology with massive neutrinos I: towards a realistic modeling of the relation between matter, haloes and galaxies

By using a suite of large box-size N-body simulations that incorporate massive neutrinos as an extra set of particles, we investigate the impact of neutrino masses on the spatial distribution of dark matter haloes and galaxies. We compute the bias between the spatial distribution of dark matter haloes and the overall matter and cold dark matter distributions using statistical tools such as the power spectrum and the two-point correlation function. Overall we find a scale-dependent bias on large scales for the cosmologies with massive neutrinos. However, our results indicate that the scale-dependence in the bias is reduced if the latter is computed with respect to the cold dark matter distribution only. We find that the value of the bias on large scales is reasonably well reproduced by the Tinker fitting formula once the linear cold dark matter power spectrum is used, instead of the total matter power spectrum. We investigate whether scale-dependent bias really comes from purely neutrino's effect or from nonlinear gravitational collapse of haloes. For this purpose, we address the $Ω_ν$-$σ_8$ degeneracy and find that such degeneracy is not perfect, implying that neutrinos imprint a slight scale dependence on the large-scale bias. Finally, by using a simple halo occupation distribution (HOD) model, we investigate the impact of massive neutrinos on the distribution of galaxies within dark matter haloes. We use the main galaxy sample in the Sloan Digital Sky Survey II Data Release 7 to investigate if the small-scale galaxy clustering alone can be used to discriminate among different cosmological models with different neutrino masses. Our results suggest that different choices of the HOD parameters can reproduce the observational measurements relatively well, and we quantify the difference between the values of the HOD parameters between massless and massive neutrino cosmologies.

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arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

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arXiv2013

The Hot and Energetic Universe: The missing baryons and the warm-hot intergalactic medium

The backbone of the large-scale structure of the Universe is determined by processes on a cosmological scale and by the gravitational interaction of the dominant dark matter. However, the mobile baryon population shapes the appearance of these structures. Theory predicts that most of the baryons reside in vast unvirialized filamentary structures that connect galaxy groups and clusters, but the observational evidence is currently lacking. Because the majority of the baryons are supposed to exist in a large-scale, hot and dilute gaseous phase, X-rays provide the ideal tool to progress our understanding. Observations with the Athena+ X-ray Integral Field Unit will reveal the location, chemical composition, physical state and dynamics of the active population of baryons.

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arXiv2013

Statistical and systematic errors in redshift-space distortion measurements from large surveys

We investigate the impact of statistical and systematic errors on measurements of linear redshift-space distortions (RSD) in future cosmological surveys, analyzing large catalogues of dark-matter halos from the BASICC simulation. These allow us to estimate the dependence of errors on typical survey properties, as volume, galaxy density and mass (i.e. bias factor) of the adopted tracer. We find that measures of the specific growth rate β=f/b using the Hamilton/Kaiser harmonic expansion of the redshift-space correlation function ξ(r_p,π) on scales larger than 3/h Mpc are typically under-estimated by up to 10% for galaxy sized halos. This is significantly larger than the corresponding statistical errors, which amount to a few percent, indicating the importance of non-linear improvements to the Kaiser model to obtain accurate measurements of the growth rate. We compare the statistical errors to predictions obtained with the Fisher information matrix, based on the usual FKP prescription for the errors on the power spectrum. We show that this produces parameter errors fairly similar to the standard deviations from the halo catalogues, but only if applied to strictly linear scales in Fourier space (k<0.2 h/Mpc). Finally, we present an accurate scaling formula describing the relative error on β as a function of the survey parameters, which closely matches the simulation results in all explored regimes. This provides a handy and plausibly more realistic alternative to the Fisher matrix approach, to quickly and accurately predict RSD statistical errors expected from future surveys.

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arXiv2013

A direct probe of cosmological power spectra of the peculiar velocity field and the gravitational lensing magnification from photometric redshift surveys

The cosmological peculiar velocity field (deviations from the pure Hubble flow) of matter carries significant information on dark energy, dark matter and the underlying theory of gravity on large scales. Peculiar motions of galaxies introduce systematic deviations between the observed galaxy redshifts z and the corresponding cosmological redshifts z_cos. A novel method for estimating the angular power spectrum of the peculiar velocity field based on observations of galaxy redshifts and apparent magnitudes m (or equivalently fluxes) is presented. This method exploits the fact that a mean relation between z_cos and m of galaxies can be derived from all galaxies in a redshift-magnitude survey. Given a galaxy magnitude, it is shown that the z_cos(m) relation yields its cosmological redshift with a 1-sigma error of sigma_z~0.3 for a survey like Euclid (~10^9 galaxies at z<~2), and can be used to constrain the angular power spectrum of z-z_cos(m) with a high signal-to-noise ratio. At large angular separations corresponding to l<~15, we obtain significant constraints on the power spectrum of the peculiar velocity field. At 15<~l<~60, magnitude shifts in the z_cos(m) relation caused by gravitational lensing magnification dominate, allowing us to probe the line-of-sight integral of the gravitational potential. Effects related to the environmental dependence in the luminosity function can easily be computed and their contamination removed from the estimated power spectra. The amplitude of the combined velocity and lensing power spectra at z~1 can be measured with <~5% accuracy.

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arXiv2012

Cosmology with clustering anisotropies: disentangling dynamic and geometric distortions in galaxy redshift surveys

We investigate the impact of different observational effects affecting a precise and accurate measurement of the growth rate of fluctuations from the anisotropy of clustering in galaxy redshift surveys. We focus on redshift measurement errors, on the reconstruction of the underlying real-space clustering and on the apparent degeneracy existing with the geometrical distortions induced by the cosmology-dependent conversion of redshifts into distances. We use a suite of mock catalogues extracted from large N-body simulations, focusing on the analysis of intermediate, mildly non-linear scales and apply the standard linear dispersion model to fit the anisotropy of the observed correlation function. We verify that redshift errors up to ~0.2% have a negligible impact on the precision with which the specific growth rate beta can be measured. Larger redshift errors introduce a positive systematic error, which can be alleviated by adopting a Gaussian distribution function of pairwise velocities. This is, in any case, smaller than the systematic error of up to 10% due to the limitations of the linear dispersion model, which is studied in a separate paper. We then show that 50% of the statistical error budget on beta depends on the deprojection procedure through which the real-space correlation function is obtained. Finally, we demonstrate that the degeneracy with geometric distortions can in fact be circumvented. This is obtained through a modified version of the Alcock-Paczynski test in redshift-space, which successfully recovers the correct cosmology by searching for the solution that optimizes the description of dynamical redshift distortions. For a flat cosmology, we obtain largely independent, robust constraints on beta and OmegaM. In a volume of 2.4(Gpc/h)^3, the correct OmegaM is obtained with ~12% error and negligible bias, once the real-space correlation function is properly reconstructed.

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arXiv2012

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, σ_{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 γ-parametrization and measuring the power spectrum at least at two different redshifts. We find that in next-generation surveys like Euclid, σ_{8,0} and γcan 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.

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arXiv2012

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$z$ 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$z$ 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 $β=0.53 \pm 0.014$. For an rms density fluctuations in the PSC$z$ galaxy number density $σ_8^{\rm gal}=0.42\pm0.03$, we obtain an estimate of the growth rate of density fluctuations $fσ_{8}(z\sim0) = 0.42 \pm 0.033$, which is in excellent agreement with independent estimates based on different techniques.

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arXiv2012

Gaia: a Window to Large Scale Flows

Using redshifts as a proxy for galaxy distances, estimates of the 2D transverse peculiar velocities of distant galaxies could be obtained from future measurements of proper motions. We provide the mathematical framework for analyzing 2D transverse motions and show that they offer several advantages over traditional probes of large scale motions. They are completely independent of any intrinsic relations between galaxy properties, hence they are essentially free of selection biases. They are free from homogeneous and inhomogeneous Malmquist biases that typically plague distance indicator catalogs. They provide additional information to traditional probes which yield line-of-sight peculiar velocities only. Further, because of their 2D nature, fundamental questions regarding vorticity of large scale flows can be addressed. Gaia for example is expected to provide proper motions of at least bright galaxies with high central surface brightness, making proper motions a likely contender traditional probes based on current and future distance indicator measurements.

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arXiv2012

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).

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arXiv2011

A cross-correlation study of the Fermi-LAT $γ$-ray diffuse extragalactic signal

In this work, starting from 21 months of data from the Fermi-Large Area Telescope, we derive maps of the residual isotropic gamma-ray emission, a relevant fraction of which is expected to be contributed by the extragalactic diffuse gamma-ray background. We compute the angular two-point auto-correlation function of the residual Fermi-LAT maps at energies E>1GeV, E>3GeV and E>30GeV well above the Galactic plane and find no significant correlation signal. This is, indeed, what is expected if the EGB were contributed by BL Lacertae, Flat Spectrum Radio Quasars or star-forming galaxies, since, in this case, the predicted signal is very weak. Then, we search for the Integrated Sachs-Wolfe signature by cross-correlating the Fermi-LAT maps with the WMAP7-Cosmic Microwave Background map. We find a cross-correlation consistent with zero, even though the expected signal is larger than that of the EGB auto-correlation. Finally, in an attempt to constrain the nature of the gamma-ray background we cross-correlate the Fermi-LAT maps with the angular distributions of objects that may contribute to the EGB: QSOs in the SDSS-DR6 catalog, NVSS galaxies, 2MASS galaxies and LRG in the SDSS catalog. The cross-correlation is always consistent with zero, in agreement with theoretical expectations, but we find (with low statistical significance) some interesting features that may indicate that some specific classes of objects contribute to the EGB. A chi2 analysis confirms that the correlation properties of the 21-month data do not provide strong constraints of the EGB origin. However, the results suggest that the situation will significantly improve with the 5- and 10-year Fermi-LAT data. The future EGB analysis will then allow placing significant constraints on the nature of the EGB and might provide in addition a detection of the ISW signal. (Abridged)

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arXiv2011

A new method for the determination of the growth rate from galaxy redshift surveys

Given a redshift survey of galaxies with measurements of apparent magnitudes, we present a novel method for measuring the growth rate $f(Ω)$ of cosmological linear perturbations. We use the galaxy distribution within the survey to solve for the peculiar velocity field which depends in linear perturbation theory on $β=f(Ω)/b$, where $b$ is the bias factor of the galaxy distribution. The recovered line-of-sight peculiar velocities are subtracted from the redshifts to derive the distances, which thus allows an estimate of the absolute magnitude of each galaxy. A constraint on $β$ is then found by minimizing the spread of the estimated magnitudes from their distribution function. We apply the method to the all sky $K = 11.25$ Two-MASS Redhsift Survey (2MRS) and derive $β=0.35\pm 0.1$ at $z\sim 0$, remarkably consistent with our previous estimate from the velocity-velocity comparison. The method could easily be applied to subvolumes extracted from the SDSS survey to derive the growth rate at $z \sim 0.1$. Further, it should also be applicable to ongoing and future spectroscopic redshift surveys to trace the evolution of $f(Ω)$ to $z\sim1$. Constraints obtained from this method are entirely independent from those obtained from the two-dimensional distortion of $ξ(s)$ and provide an important check on $f(Ω)$, as alternative gravity models predict observable differences.

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arXiv2011

Bulk flows from galaxy luminosities: application to 2MASS redshift survey and forecast for next-generation datasets

We present a simple method for measuring cosmological bulk flows from large redshift surveys, based on the apparent dimming or brightening of galaxies due to their peculiar motion. It is aimed at estimating bulk flows of cosmological volumes containing large numbers of galaxies. Constraints on the bulk flow are obtained by minimizing systematic variations in galaxy luminosities with respect to a reference luminosity function measured from the whole survey. This method offers two advantages over more popular bulk flow estimators: it is independent of error-prone distance indicators and of the poorly-known galaxy bias. We apply the method to the 2MASS redshift survey (2MRS) to measure the local bulk flows of spherical shells centered on the Milky Way (MW). The result is consistent with that obtained by Nusser and Davis (2011) using the SFI++ catalogue of Tully-Fisher distance indicators. We also make an assessment of the ability of the method to constrain bulk flows at larger redshifts ($z=0.1-0.5$) from next generation datasets. As a case study we consider the planned EUCLID survey. Using this method we will be able to measure a bulk motion of $ \sim 200 \kms$ of $10^6$ galaxies with photometric redshifts, at the $3σ$ level for both $z\sim 0.15$ and $z\sim 0.5$. Thus the method will allow us to put strong constraints on dark energy models as well as alternative theories for structure formation.

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arXiv2011

Studying the Warm-Hot Intergalactic Medium in Emission

We assess the possibility to detect the warm-hot intergalactic medium (WHIM) in emission and to characterize its physical conditions and spatial distribution through spatially resolved X-ray spectroscopy, in the framework of the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are equipped with microcalorimeter-based detectors. For this purpose we analyze a large set of mock emission spectra, extracted from a cosmological hydrodynamical simulation. These mock X-ray spectra are searched for emission features showing both the OVII K alpha triplet and OVIII Ly alpha line, which constitute a typical signature of the warm hot gas. Our analysis shows that 1 Ms long exposures and energy resolution of 2.5 eV will allow us to detect about 400 such features per deg^2 with a significance >5 sigma and reveals that these emission systems are typically associated with density ~100 above the mean. The temperature can be estimated from the line ratio with a precision of ~20%. The combined effect of contamination from other lines, variation in the level of the continuum, and degradation of the energy resolution reduces these estimates. Yet, with an energy resolution of 7 eV and all these effects taken into account, one still expects about 160 detections per deg^2. These line systems are sufficient to trace the spatial distribution of the line-emitting gas, which constitute an additional information, independent from line statistics, to constrain the poorly known cosmic chemical enrichment history and the stellar feedback processes.

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arXiv2011

Expected properties of the Two-Point Autocorrelation Function of the IGM

Recent analyses of the fluctuations of the soft Diffuse X-ray Background (DXB) have provided indirect detection of a component consistent with the elusive Warm Hot Intergalactic Medium (WHIM). In this work we use theoretical predictions obtained from hydrodynamical simulations to investigate the angular correlation properties of the WHIM in emission and assess the possibility of indirect detection with next-generation X-ray missions. Our results indicate that the angular correlation signal of the WHIM is generally weak but dominates the angular correlation function of the DXB outside virialized regions. Its indirect detection is possible but requires rather long exposure times [0.1-1] Ms, large (~1° x1°) fields of view and accurate subtraction of isotropic fore/background contributions, mostly contributed by Galactic emission. The angular correlation function of the WHIM is positive for θ < 5' and provides limited information on its spatial distribution. A satisfactory characterization of the WHIM in 3D can be obtained through spatially resolved spectroscopy. 1 Ms long exposures with next generation detectors will allow to detect ~400 O VII+O VIII X-ray emission systems that we use to trace the spatial distribution of the WHIM. We predict that these observations will allow to estimate the WHIM correlation function with high statistical significance out to ~10 Mpc h^-1 and characterize its dynamical state through the analysis of redshift-space distortions. The detectable WHIM, which is typically associated with the outskirts of virialized regions rather than the filaments has a non-zero correlation function with slope γ = -1.7 \pm 0.1 and correlation length r0 = 4.0 \pm 0.1 Mpc h^-1 in the range r = [4.5, 12] Mpc h^-1. Redshift space distances can be measured to assess the dynamical properties of the gas, typically infalling onto large virialized structures.

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arXiv2011

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 $Δz=0.2$ redshift slice, while the equation of state $w$ and the index $γ$ 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.

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