ArcAdiA

Large-scale non-Gaussian mass function and halo bias: tests on N-body simulations

Wed, 31 Dec 2008 23:00:00 GMT

Large-scale non-Gaussian mass function and halo bias: tests on N-body simulations Grossi, Margherita; Verde, Licia; Carbone, Carmelita; Dolag, Klaus; Branchini, Enzo; Iannuzzi, Francesca; Matarrese, Sabino; Moscardini, Lauro 2009 Monthly Notices of the Royal Astronomical Society 398 321-332 The description of the abundance and clustering of haloes for non-Gaussian initial conditions has recently received renewed interest,motivated by the forthcoming large galaxy and cluster surveys, which can potentially yield constraints of the order of unity on the non-Gaussianity parameter f(NL). We present tests on N-body simulations of analytical formulae describing the halo abundance and clustering for non-Gaussian initial conditions. We calibrate the analytic non-Gaussian mass function of Matarrese, Verde & Jimenez and LoVerde et al. and the analytic description of clustering of haloes for non-Gaussian initial conditions on N-body simulations. We find an excellent agreement between the simulations and the analytic predictions if we make the corrections delta(c) -> delta(c)root q and delta(c) -> delta(c)q, where q similar or equal to 0.75, in the density threshold for gravitational collapse and in the non-Gaussian fractional correction to the halo bias, respectively. We discuss the implications of this correction on present and forecasted primordial non-Gaussianity constraints. We confirm that the non-Gaussian halo bias offers a robust and highly competitive test of primordial non-Gaussianity.

Anisotropy probe of galactic and extra-galactic dark matter annihilations

Tue, 30 Jun 2009 22:00:00 GMT

Anisotropy probe of galactic and extra-galactic dark matter annihilations Fornasa, Mattia; Pieri, Lidia; Bertone, Gianfranco; Branchini, Enzo 2009-07 Physical review. D 80 023518 We study the flux and the angular power spectrum of gamma rays produced by dark matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: (a) the smooth MW halo, (b) resolved and unresolved substructures in the MW, (c) external DM halos at all redshifts, including (d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we compute the angular power spectrum of gamma-ray anisotropies and show that, quite independently from the prescription used for the concentration parameters c(M,z), an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l similar to 100, assuming a particle physics scenario that does not violate gamma-ray constraints from the Galactic center. The DM contribution to the angular power spectrum is dominated by MW substructures (with mass M >= 10(4)M(circledot)), while, at least for the substructures' models that we have adopted, the contribution of extra-galactic annihilations is negligible at all scales. MW subhalos also dominate the annihilation flux in themajority of the sky, with the smooth galactic halo being brighter only in the very central region.

The spatial distribution of X-ray selected AGN in the Chandra deepfields: a theoretical perspective

Wed, 31 Dec 2008 23:00:00 GMT

The spatial distribution of X-ray selected AGN in the Chandra deepfields: a theoretical perspective Marulli, Federico; Bonoli, Silvia; Branchini, Enzo; Gilli, Roberto; Moscardini, Lauro; Springel, Volker 2009 MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 396 1404-1414 We study the spatial distribution of X-ray selected active galacticnuclei (AGN) in the framework of hierarchical coevolution ofsupermassive black holes and their host galaxies and dark matterhaloes. To this end, we have applied the theoretical model developed byCroton et al., De Lucia & Blaizot and Marulli et al. to the output ofthe Millennium Run and obtained hundreds of realizations of past lightcones from which we have extracted realistic mock AGN catalogues thatmimic the Chandra deep fields. We find that the model AGN number countsare in fair agreement with observations both in the soft and in thehard X-ray bands, except at fluxes <= 10(-15) erg cm(-2) s(-1), wherethe model systematically overestimates the observations. However, alarge fraction of these faint objects are typically excluded from thespectroscopic AGN samples of the Chandra fields. We find that thespatial two-point correlation function predicted by the model is welldescribed by a power-law relation out to 20 h(-1) Mpc, in closeagreement with observations. Our model matches the correlation lengthr(0) of AGN in the Chandra Deep Field-North but underestimates it inthe Chandra Deep Field-South. When fixing the slope to gamma = 1.4, asin Gilli et al., the statistical significance of the mismatch is 2sigma-2.5 sigma, suggesting that the predicted cosmic variance, whichdominates the error budget, may not account for the differentcorrelation length of the AGN in the two fields. However, the overallmismatch between the model and the observed correlation functiondecreases when both r(0) and gamma are allowed to vary, suggesting thatmore realistic AGN models and a full account of all observationalerrors may significantly reduce the tension between AGN clustering inthe two fields. While our results are robust to changes in the modelprescriptions for the AGN light curves, the luminosity dependence ofthe clustering is sensitive to the different light-curve modelsadopted. However, irrespective of the model considered, the luminositydependence of the AGN clustering in our mock fields seems to be weakerthan in the real Chandra fields. The significance of this mismatchneeds to be confirmed using larger data sets.

Modelling the cosmological co-evolution of supermassive black holes and galaxies - II. The clustering of quasars and their dark environment

Wed, 31 Dec 2008 23:00:00 GMT

Modelling the cosmological co-evolution of supermassive black holes and galaxies - II. The clustering of quasars and their dark environment Bonoli, Silvia; Marulli, Federico; Springel, Volker; White, Simon D. M.; Branchini, Enzo; Moscardini, Lauro 2009 Monthly notices of the Royal Astronomical Society 396 423-438 We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range 0.5 less than or similar to r less than or similar to 20 h(-1) Mpc, but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by today's quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few 10(12) h(-1) M-circle dot. Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits.Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity.

Primordial non-Gaussianities in the intergalactic medium

Wed, 31 Dec 2008 23:00:00 GMT

Primordial non-Gaussianities in the intergalactic medium Viel, Matteo; Branchini, Enzo; Dolag, Klaus; Grossi, Margherita; Matarrese, Sabino; Moscardini, Lauro 2009 Monthly notices of the Royal Astronomical Society 393 774-782 We present results from the first high-resolution hydrodynamical simulations of non-Gaussian cosmological models. We focus on the statistical properties of the transmitted Lyman-alpha flux in the high-redshift intergalactic medium. Imprints of non-Gaussianity are present and are larger at high redshifts. Differences larger than 20 per cent at z > 3 in the flux probability distribution function for high-transmissivity regions (voids) are expected for values of the non-linearity parameter f(NL) = +/- 100 when compared to a standard Lambda cold dark matter cosmology with f(NL) = 0. We also investigate the one-dimensional flux bispectrum: at the largest scales(corresponding to tens of Mpc), we expect deviations in the flux bispectrum up to 20 per cent at z similar to 4 (for f(NL) = +/- 100),significantly larger than deviations of similar to 3 per cent in the flux power spectrum. We briefly discuss possible systematic errors that can contaminate the signal. Although challenging, a detection of non-Gaussianities in the interesting regime of scales and redshifts probed by the Lyman-alpha forest could be possible with future data sets.

Studying the warm hot intergalactic medium with gamma-ray bursts

Tue, 19 May 2009 22:00:00 GMT

Studying the warm hot intergalactic medium with gamma-ray bursts Branchini, Enzo; Ursino, Eugenio; Corsi, A.; Martizzi, Davide; Amati, L.; den Herder, J. W.; Galeazzi, M.; Gendre, B.; Kaastra, J.; Moscardini, L.; Nicastro, F.; Ohashi, T.; Paerels, F.; Piro, L.; Roncarelli, M.; Takei, Y.; Viel, M. 2009-05-20 Astrophysical Journal 697 328-344 We assess the possibility of detecting and characterizing the physical state of the missing baryons at low redshift by analyzing the X-ray absorption spectra of the gamma-ray burst (GRB) afterglows, measured by a microcalorimeter-based detector with 3 eV resolution and 1000 cm2 effective area and capable of fast repointing, similar to that on board of the recently proposed X-ray satellites EDGE and XENIA. For this purpose we have analyzed mock absorption spectra extracted from different hydrodynamical simulations used to model the properties of the warm hot intergalactic medium (WHIM). These models predict the correct abundance of OVI absorption lines observed in UV and satisfy current X-ray constraints. According to these models space missions such as EDGE and XENIA should be able to detect similar to 60 WHIM absorbers per year through the OVII line. About 45% of these have at least two more detectable lines in addition to OVII that can be used to determine the density and the temperature of the gas. Systematic errors in the estimates of the gas density and temperature can be corrected for in a robust, largely model-independent fashion. The analysis of the GRB absorption spectra collected in three years would also allow to measure the cosmic mass density of the WHIM with similar to 15% accuracy, although this estimate depends on the WHIM model. Our results suggest that GRBs represent a valid, if not preferable, alternative to active galactic nuclei to study the WHIM in absorption. The analysis of the absorption spectra nicely complements the study of the WHIM in emission that the spectrometer proposed for EDGE and XENIA would be able to carry out thanks to its high sensitivity and large field of view.