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Search Results: 1 - 10 of 198397 matches for " Nelson D. Padilla "
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Dark Matter annihilation energy output and its effects on the high-z IGM
Ignacio J. Araya,Nelson D. Padilla
Physics , 2013, DOI: 10.1093/mnras/stu1780
Abstract: We study the case of DM self annihilation, in order to assess its importance as an energy injection mechanism, to the IGM in general, and to the medium within particular DM haloes. We consider thermal relic WIMP particles with masses of 10GeV and 1TeV and we analyse in detail the clustering properties of DM in a $\Lambda$CDM cosmology, on all hierarchy levels, from haloes and their mass function, to subhaloes and the DM density profiles within them, considering adiabatic contraction by the presence of a SMBH. We then compute the corresponding energy output, concluding that DM annihilation does not constitute an important feedback mechanism. We also calculate the effects that DM annihilation has on the IGM temperature and ionization fraction, and we find that assuming maximal energy absorption, at z ~ 10, for the case of a 1TeV WIMP, the ionization fraction could be raised to $6 \times 10^{-4}$ and the temperature to 10K, and in the case of a 10GeV WIMP, the IGM temperature could be raised to 200K and the ionization fraction to $8 \times 10^{-3}$. We conclude that DM annihilations cannot be regarded as an alternative reionization scenario. Regarding the detectability of the WIMP through the modifications to the 21 cm differential brightness temperature signal ($\delta$Tb), we conclude that a thermal relic WIMP with mass of 1TeV is not likely to be detected from the global signal alone, except perhaps at the 1-3mK level in the frequency range 30MHz < $\nu$ < 35MHz corresponding to 40 < z < 46. However, a 10GeV mass WIMP may be detectable at the 1-3mK level in the frequency range 55MHz < $\nu$ < 119MHz corresponding to 11 < z < 25, and at the 1-10mK level in the frequency range 30MHz < $\nu$ < 40MHz corresponding to 35 < z < 46.
The Intrinsic Shape of Galaxies in SDSS/Galaxy Zoo
Silvio Rodríguez,Nelson D. Padilla
Physics , 2013, DOI: 10.1093/mnras/stt1168
Abstract: By modelling the axis ratio distribution of SDSS DR8 galaxies we find the intrinsic 3D shapes of spirals and ellipticals. We use morphological information from the Galaxy Zoo project and assume a non-parametric distribution intrinsic of shapes, while taking into account dust extinction. We measure the dust extinction of the full sample of spiral galaxies and find a smaller value than previous estimations, with an edge-on extinction of $E_0 = 0.284^{+0.015}_{-0.026}$ in the SDSS r band. We also find that the distribution of minor to major axis ratio has a mean value of $ 0.267 \pm 0.009 $, slightly larger than previous estimates mainly due to the lower extinction used; the same affects the circularity of galactic discs, which are found to be less round in shape than in previous studies, with a mean ellipticity of $0.215 \pm 0.013$. For elliptical galaxies, we find that the minor to major axis ratio, with a mean value of $0.584 \pm 0.006$, is larger than previous estimations due to the removal of spiral interlopers present in samples with morphological information from photometric profiles. These interlopers are removed when selecting ellipticals using Galaxy Zoo data. We find that the intrinsic shapes of galaxies and their dust extinction vary with absolute magnitude, colour and physical size. We find that bright elliptical galaxies are more spherical than faint ones, a trend that is also present with galaxy size, and that there is no dependence of elliptical galaxy shape with colour. For spiral galaxies we find that the reddest ones have higher dust extinction as expected, due to the fact that this reddening is mainly due to dust. We also find that the thickness of discs increases with luminosity and size, and that brighter, smaller and redder galaxies have less round discs.
Cluster correlations in redshift space
Nelson D. Padilla,Carlton M. Baugh
Physics , 2001, DOI: 10.1046/j.1365-8711.2002.04992.x
Abstract: We test an analytic model for the two-point correlations of galaxy clusters in redshift space using the Hubble Volume N-body simulations. The correlation function of clusters shows no enhancement along the line of sight, due to the lack of any virialised structures in the cluster distribution. However, the distortion of the clustering pattern due to coherent bulk motions is clearly visible. The distribution of cluster peculiar motions is well described by a Gaussian, except in the extreme high velocity tails. The simulations produce a small but significant number of clusters with large peculiar motions. The form of the redshift space power spectrum is strongly influenced by errors in measured cluster redshifts in extant surveys. When these errors are taken into account, the model reproduces the power spectrum recovered from the simulation to an accuracy of 15% or better over a decade in wavenumber. We compare our analytic predictions with the power spectrum measured from t! he! APM cluster redshift survey. The cluster power spectrum constrains the amplitude of density fluctuations, as measured by the linear rms variance in spheres of radius 8 h^(-1) Mpc, denoted by sigma_8. When combined with the constraints on sigma_8 and the density parameter Omega derived from the local abundance of clusters, we find a best fitting cold dark matter model with sigma_8~1.25 and Omega~0.2, for a power spectrum shape that matches that measured for galaxies. However, for the best fitting value of Omega and given the value of Hubble's constant from recent measurements, the assumed shape of the power spectrum is incompatible with the most readily motivated predictions from the cold dark matter paradigm.
Black hole spin and radio loudness in a LCDM universe
Claudia del P. Lagos,Nelson D. Padilla,Sofia A. Cora
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.14451.x
Abstract: We use a combination of a cosmological N-body simulation of the concordance Lambda cold dark matter (LCDM) paradigm and a semi-analytic model of galaxy formation to investigate the spin development of central supermassive black holes (BHs) and its relation to the BH host galaxy properties. In order to compute BH spins, we use the alpha-model of Shakura & Sunyaev and consider the King et al. warped disc alignment criterion. The orientation of the accretion disc is inferred from the angular momentum of the source of accreted material, which bears a close relationship to the large-scale structure in the simulation. We find that the final BH spin depends almost exclusively on the accretion history and only weakly on the warped disc alignment. The main mechanisms of BH spin-up are found to be gas cooling processes and disc instabilities, a result that is only partially compatible with Monte-Carlo models where the main spin-up mechanisms are major mergers and disc instabilities; the latter results are reproduced when implementing randomly oriented accretion discs in our model. Regarding the BH population, we find that more massive BHs, which are hosted by massive ellipticals, have higher spin values than less-massive BHs, hosted by spiral galaxies. We analyse whether gas accretion rates and BH spins can be used as tracers of the radio loudness of active galactic nuclei (AGN). We find that the current observational indications of an increasing trend of radio-loud AGN fractions with stellar and BH mass can be easily obtained when placing lower limits on the BH spin, with a minimum influence from limits on the accretion rates; a model with random accretion disc orientations is unable to reproduce this trend. (ABRIDGED)
Cosmic queuing: galaxy satellites, building blocks and the hierarchical clustering paradigm
Claudia del P. Lagos,Nelson D. Padilla,Sofia A. Cora
Physics , 2009, DOI: 10.1111/j.1745-3933.2009.00678.x
Abstract: We study the properties of building blocks (BBs, i.e. accreted satellites) and surviving satellites of present-day galaxies using the SAG semi-analytic model of galaxy formation in the context of a concordance Lambda Cold Dark Matter (LCDM) cosmology. We consider large numbers of DM halo merger trees spanning a wide range of masses (~1x10^10 - 2.14x10^15 Msun). We find higher metallicities for BBs with respect to surviving satellites, an effect produced by the same processes behind the build-up of the mass-metallicity relation. We prove that these metallicity differences arise from the higher peak height in the density fluctuation field occupied by BBs and central galaxies which have collapsed into a single object earlier than surviving satellites. BBs start to form stars earlier, during the peak of the merger activity in LCDM, and build-up half of their final stellar mass (measured at the moment of disruption) up to four times faster than surviving satellites. Surviving satellites keep increasing their stellar masses rather quiescently down to z~1. The difference between the metallicities of satellites, BBs and central galaxies depends on the host DM halo mass, in a way that can be used as a further test for the concordance cosmology.
Galaxy groups in the 2dF galaxy redshift survey: Large Scale Structure with Groups
Ariel Zandivarez,Manuel E. Merchan,Nelson D. Padilla
Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06827.x
Abstract: We use the 2dF Galaxy Group Catalogue constructed by Merch\'an & Zandivarez to study the large scale structure of the Universe traced by galaxy groups. We concentrate on the computation of the power spectrum and the two point correlation function. The resulting group power spectrum shows a similar shape to the galaxy power spectrum obtained from the 2dF Galaxy Redshift Survey by Percival et al., but with a higher amplitude quantified by a relative bias in redshift space of $b_s(k) \sim 1.5$ . The group two point correlation function for the total sample is well described by a power law with correlation length $s_0=8.9 \pm 0.3 \mpc$ and slope $\gamma=-1.6 \pm 0.1$ on scales $s < 20 \mpc$. In order to study the dependence of the clustering properties on group mass we split the catalogue in four subsamples defined by different ranges of group virial masses. These computations allow a fair estimate of the relation described by the correlation length $s_0$ and the mean intergroup separation $d_c$ for galaxy systems of low mass. We also extend our study to the redshift space distortions of galaxy groups, where we find that the anisotropies in the clustering pattern of the 2dF group catalogue are consistent with gravitational instability, with a flattening of the redshift-space correlation function contours in the direction of the line of sight.
The effects of environment on the intrinsic shape of galaxies
Silvio Rodríguez,Nelson D. Padilla,Diego García Lambas
Physics , 2015,
Abstract: We measure the effect of the environment on the intrinsic shapes of spiral and elliptical galaxies by finding the 3D shape distribution and dust extinction that fits better the projected shape of galaxies in different environment. We find that spiral galaxies in groups are very similar to field spirals with similar intrinsic properties (magnitudes, sizes and colours). But for spirals in groups, those in denser environments or closer to the centre of the group tend to have a more circular disc than similar galaxies in less dense environments or far from the group centres. Also we find that central spiral galaxies in their groups tend to be thinner than other similar spirals. For ellipticals, we do not find any important dependence of their shape on their position in a group or on the local density. However, we find that elliptical galaxies in groups tend to be more spherical than field ellipticals with similar intrinsic properties. We find that, once in groups, the shape of member galaxies do not depend on group mass, regardless of their morphological type.
Clustering tomography: measuring cosmological distances through angular clustering in thin redshift shells
Salvador Salazar-Albornoz,Ariel G. Sánchez,Nelson D. Padilla,Carlton M. Baugh
Physics , 2014, DOI: 10.1093/mnras/stu1428
Abstract: We test the cosmological implications of studying galaxy clustering using a tomographic approach, by computing the galaxy two-point angular correlation function $\omega(\theta)$ in thin redshift shells using a spectroscopic-redshift galaxy survey. The advantages of this procedure are that it is not necessary to assume a fiducial cosmology in order to convert measured angular positions and redshifts into distances, and that it gives several (less accurate) measurements of the angular diameter distance $D_\rm{A}(z)$ instead of only one (more precise) measurement of the effective average distance $D_\rm{V}(z)$, which results in better constraints on the expansion history of the Universe. We test our model for $\omega(\theta)$ and its covariance matrix against a set of mock galaxy catalogues and show that this technique is able to extract unbiased cosmological constraints. Also, assuming the best-fit $\Lambda$CDM cosmology from the cosmic microwave background measurements from the Planck satellite, we forecast the result of applying this tomographic approach to the final Baryon Oscillation Spectroscopic Survey catalogue in combination with Planck for three flat cosmological models, and compare them with the expected results of the isotropic baryon acoustic oscillation (BAO) measurements post-reconstruction on the same galaxy catalogue combined with Planck. While BAOs are more accurate for constraining cosmological parameters for the standard $\Lambda$CDM model, the tomographic technique gives better results when we allow the dark energy equation of state $w_{DE}$ to deviate from $-1$, resulting in a performance similar to BAOs in the case of a constant value of $w_{DE}$, and a moderate improvement in the case of a time-dependent value of $w_{DE}$, increasing the value of the Figure-of-Merit in the $w_0-w_a$ plane up to $15\%$.
How accurate is it to update the cosmology of your halo catalogues?
Andrés N. Ruiz,Nelson D. Padilla,Mariano J. Domínguez,Sofía A. Cora
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19635.x
Abstract: We test and present the application of the full rescaling method by Angulo & White (2010) to change the cosmology of halo catalogues in numerical simulations for cosmological parameter search using semi-analytic galaxy properties. We show that a reduced form of the method can be applied in small simulations with box side of ~50/h Mpc. We perform statistical tests on the accuracy of the properties of rescaled individual haloes, and also on the rescaled population as a whole. We find that individual positions and velocities are recovered with almost no detectable biases. The dispersion in the recovered halo mass does not seem to depend on the resolution of the simulation. Regardless of the halo mass, the individual accretion histories, spin parameter evolution and fraction of mass in substructures are well recovered. The mass of rescaled haloes can be underestimated (overestimated) for negative (positive) variations of either sigma_8 or Omega_m, in a way that does not depend on the halo mass. Statistics of abundances and correlation functions of haloes show also small biases of <10 percent when moving away from the base simulation by up to 2 times the uncertainty in the WMAP7 cosmological parameters. The merger tree properties related to the final galaxy population in haloes also show small biases; the time since the last major merger, the assembly time-scale, and a time-scale related to the stellar ages show correlated biases which indicate that the spectral shapes of galaxies would only be affected by global age changes of ~150 Myr. We show some of these biases for different separations in the cosmological parameters with respect to the desired cosmology so that these can be used to estimate the expected accuracy of the resulting halo population. We also present a way to construct grids of simulations to provide stable accuracy across the Omega_m vs sigma_8 parameter space.
Host galaxy-active galactic nucleus alignments in the SDSS-DR7
Claudia del P. Lagos,Nelson D. Padilla,Michael A. Strauss,Sofia A. Cora,Lei Hao
Physics , 2010, DOI: 10.1111/j.1365-2966.2011.18531.x
Abstract: We determine the intrinsic shapes and orientations of 27,450 type I and II active galactic nucleus (AGN) galaxies in the spectroscopic sample of the SDSS DR7, by studying the distribution of projected axis ratios of AGN hosts. Our aim is to study possible alignments between the AGN and host galaxy systems (e.g. the accretion disc and the galaxy angular momentum) and the effect of dust obscuration geometry on the AGN type. We define control samples of non-AGN galaxies that mimic the morphology, colour, luminosity and concentration distributions of the AGN population, taking into account the effects of dust extinction and reddening. Assuming that AGN galaxies have the same underlying three-dimensional shape distribution as their corresponding control samples, we find that the spiral and elliptical type I AGN populations are strongly skewed toward face-on galaxies, while ellipticals and spirals type II AGN are skewed toward edge-on orientations. These findings rule out random orientations for AGN hosts at high confidence for type I spirals (delta chi^2~230) and type II ellipticals (delta chi^2~15), while the signal for type I ellipticals and type II spirals is weaker (delta chi^2~3 and delta chi^2~6, respectively). We obtain a much stronger tendency for the type II spirals to be edge-on when just high [OIII] equivalent width (EW) AGN are considered, suggesting that >20% of low [OIII] EW edge-on type II AGN may be missing from the optical sample. Galactic dust absorption of the broad-line region alone cannot explain the observed inclination angle and projected axis ratio distributions of type I and II Seyfert types, implying that obscuration by a small-scale circumnuclear torus is necessary. These results favour a scenario in which the angular momentum of the material which feeds the black hole retains a memory of its original gas source at least to some small, non-negligible degree.
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