DSpace Angular :: Browsing by Author "Padilla, Nelson D."

Browsing by Author "Padilla, Nelson D."

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A deep learning approach to halo merger tree construction
(2022) Robles, Sandra; Gomez, Jonathan S.; Ramirez Rivera, Adin; Padilla, Nelson D.; Dujovne, Diego
A key ingredient for semi-analytic models of galaxy formation is the mass assembly history of haloes, encoded in a tree structure. The most commonly used method to construct halo merger histories is based on the outcomes of high-resolution, computationally intensive N-body simulations. We show that machine learning (ML) techniques, in particular Generative Adversarial Networks (GANs), are a promising new tool to tackle this problem with a modest computational cost and retaining the best features of merger trees from simulations. We train our GAN model with a limited sample of merger trees from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation suite, constructed using two halo finders-tree builder algorithms: SUBFIND - D-TREES and ROCKSTAR - ConsistentTrees. Our GAN model successfully learns to generate well-constructed merger tree structures with high temporal resolution, and to reproduce the statistical features of the sample of merger trees used for training, when considering up to three variables in the training process. These inputs, whose representations are also learned by our GAN model, are mass of the halo progenitors and the final descendant, progenitor type (main halo or satellite), and distance of a progenitor to that in the main branch. The inclusion of the latter two inputs greatly improves the final learned representation of the halo mass growth history, especially for SUBFIND-like ML trees. When comparing equally sized samples of ML merger trees with those of the EAGLE simulation, we find better agreement for SUBFIND-like ML trees. Finally, our GAN-based framework can be utilized to construct merger histories of low- and intermediate-mass haloes, the most abundant in cosmological simulations.
Angular momentum-large-scale structure alignments in ΛCDM models and the SDSS
(2008) Paz, Dante J.; Stasyszyn, Federico; Padilla, Nelson D.
We study the alignments between the angular momentum of individual objects and the large-scale structure in cosmological numerical simulations and real data from the Sloan Digital Sky Survey, Data Release 6 (SDSS-DR6). To this end, we measure anisotropies in the two point cross-correlation function around simulated haloes and observed galaxies, studying separately the one- and two-halo regimes. The alignment of the angular momentum of dark-matter haloes in Lambda cold dark matter (Lambda CDM) simulations is found to be dependent on scale and halo mass. At large distances (two-halo regime), the spins of high-mass haloes are preferentially oriented in the direction perpendicular to the distribution of matter; lower mass systems show a weaker trend that may even reverse to show an angular momentum in the plane of the matter distribution. In the one-halo term regime, the angular momentum is aligned in the direction perpendicular to the matter distribution; the effect is stronger than for the one-halo term and increases for higher mass systems.
Evolution mapping: a new approach to describe matter clustering in the non-linear regime
(2022) Sanchez, Ariel G.; Ruiz, Andres N.; Jara, Jenny Gonzalez; Padilla, Nelson D.
We present a new approach to describe statistics of the non-linear matter density field that exploits a degeneracy in the impact of different cosmological parameters on the linear dimensionless matter power spectrum, Delta(2)(L)(k). We classify all cosmological parameters into two groups, shape parameters, which determine the shape of Delta(2)(L)(k), and evolution parameters, which only affect its amplitude at any given redshift. With this definition, the time evolution of Delta(2)(L)(k) in models with identical shape parameters but different evolution parameters can be mapped from one to the other by relabelling the redshifts that correspond to the same clustering amplitude, which we characterize by the linear mass fluctuation in spheres of radius 12 Mpc, sigma(12) (z). We use N-body simulations to show that the same evolution-mapping relation gives a good description of the non-linear power spectrum, the halo mass function, or the full density field. The deviations from the exact degeneracy are the result of the different structure formation histories experienced by each model to reach the same clustering amplitude and can be accurately described in terms of differences in the suppression factor g(a) = D(a)/a. These relations can be used to drastically reduce the number of parameters required to describe the cosmology dependence of the power spectrum. We show how this can help to speed up the inference of parameter constraints from cosmological observations. We also present a new design of an emulator of the non-linear power spectrum whose predictions can be adapted to an arbitrary choice of evolution parameters and redshift.
Not hydro: using neural networks to estimate galaxy properties on a dark-matter-only simulation
(2023) Hernandez, Cristian A.; Gonzalez, Roberto E.; Padilla, Nelson D.
Using data from TNG300-2, we train a neural network (NN) to recreate the stellar mass (M*) and star formation rate (SFR) of central Galaxies in a dark-matter-only simulation. We conider 12 input properties from the halo and sub-halo hosting the galaxy and the near environment. M* predictions are robust, but the machine does not fully reproduce its scatter. The same happens for SFR, but the predictions are not as good as for M*. We chained NNs, improving the predictions on SFR to some extent. For SFR, we time-averaged this value between z = 0 and z = 0.1, which improved results for z = 0. Predictions of both variables have trouble reproducing values at lower and higher ends. We also study the impact of each input variable in the performance of the predictions using a leave-one-covariate-out approach, which led to insights about the physical and statistical relation between input variables. In terms of metrics, our machine outperforms similar studies, but the main discoveries in this work are not linked with the quality of the predictions themselves, but to how the predictions relate to the input variables. We find that previously studied relations between physical variables are meaningful to the machine. We also find that some merger tree properties strongly impact the performance of the machine. We conclude that machine learning models are useful tools to understand the significance of physical different properties and their impact on target characteristics, as well as strong candidates for potential simulation methods.
Power spectrum of density fluctuations, halo abundances, and clustering with primordial black holes
(2021) Padilla, Nelson D.; Magana, Juan; Sureda, Joaquin; Araya, Ignacio J.
We study the effect of dark matter (DM) being encapsulated in primordial black holes (PBHs) on the power spectrum of density fluctuations P(k); we also look at its effect on the abundance of haloes and their clustering. We allow the growth of Poisson fluctuations since matter and radiation equality and study both monochromatic and extended PBH mass distributions. We present updated monochromatic black hole mass constraints by demanding < 10 per cent deviations from the Lambda cold dark matter (CDM) power spectrum at a scale of k = 1 hMpc(-1). Our results show that PBHs with masses > 10(4) h(-1) M-circle dot are excluded from conforming all of the DM in the Universe. We also apply this condition to our extended Press-Schechter (PS) mass functions, and find that the Poisson power is scale dependent even before applying evolution. We find that characteristic masses M* <= 10(2) h(-1) M-circle dot are allowed, leaving only two characteristic PBH mass windows of PS mass functions when combining with previous constraints, M* similar to 10(2) h(-1) M-circle dot and similar to 10(-8) h(-1) M-circle dot where all of the DM can be in PBHs. The resulting DM halo mass functions within these windows are similar to those resulting from CDM made of fundamental particles. However, as soon as the parameters produce unrealistic P(k), the resulting halo mass functions and their bias as a function of halo mass deviate strongly from the behaviour measured in the real Universe.
Press-Schechter primordial black hole mass functions and their observational constraints
(2021) Sureda, Joaquin; Magana, Juan; Araya, Ignacio J.; Padilla, Nelson D.
We present a modification of the Press-Schechter (PS) formalism to derive general mass functions for primordial black holes (PBHs), considering their formation as being associated with the amplitude of linear energy density fluctuations. To accommodate a wide range of physical relations between the linear and non-linear conditions for collapse, we introduce an additional parameter to the PS mechanism, and that the collapse occurs at either a given cosmic time, or as fluctuations enter the horizon. We study the case where fluctuations obey Gaussian statistics and follow a primordial power spectrum of broken power-law form with a blue spectral index for small scales. We use the observed abundance of supermassive black holes (SMBH) to constrain the extended mass functions taking into account dynamical friction. We further constrain the modified PS by developing a method for converting existing constraints on the PBH mass fraction, derived assuming monochromatic mass distributions for PBHs, into constraints applicable for extended PBH mass functions. We find that when considering well-established monochromatic constraints, there are regions in parameter space where all the dark matter can be made of PBHs. Of special interest is the region for the characteristic mass of the distribution similar to 10(2) M-circle dot, for a wide range of blue spectral indices in the scenario where PBHs form as they enter the horizon, where the linear threshold for collapse is of the order of the typical overdensities, as this is close to the black hole masses detected by LIGO, which are difficult to explain by stellar collapse.
Redshift-space effects in voids and their impact on cosmological tests - II. The void-galaxy cross-correlation function
(2022) Correa, Carlos M.; Paz, Dante J.; Padilla, Nelson D.; Sanchez, Ariel G.; Ruiz, Andres N.; Angulo, Raul E.
This is the second part of a thorough investigation of the redshift-space effects that affect void properties and the impact they have on cosmological tests. Here, we focus on the void-galaxy cross-correlation function, specifically, on the projected versions that we developed in a previous work. The pillar of the analysis is the one-to-one relationship between real and redshift-space voids above the shot-noise level identified with a spherical void finder. Under this mapping, void properties are affected by three effects: (i) a systematic expansion as a consequence of the distortions induced by galaxy dynamics, (ii) the Alcock-Paczynski volume effect, which manifests as an overall expansion or contraction depending on the fiducial cosmology, and (iii) a systematic off-centring along the line of sight as a consequence of the distortions induced by void dynamics. We found that correlations are also affected by an additional source of distortions: the ellipticity of voids. This is the first time that distortions due to the off-centring and ellipticity effects are detected and quantified. With a simplified test, we verified that the Gaussian streaming model is still robust provided all these effects are taken into account, laying the foundations for improvements in current models in order to obtain unbiased cosmological constraints from spectroscopic surveys. Besides this practical importance, this analysis also encodes key information about the structure and dynamics of the Universe at the largest scales. Furthermore, some of the effects constitute cosmological probes by themselves, as is the case of the void ellipticity.
Redshift-space effects in voids and their impact on cosmological tests. Part I: the void size function
(2021) Correa, Carlos M.; Paz, Dante J.; Sanchez, Ariel G.; Ruiz, Andres N.; Padilla, Nelson D.; Angulo, Raul E.
Voids are promising cosmological probes. Nevertheless, every cosmological test based on voidsmust necessarily employ methods to identify them in redshift space. Therefore, redshift-space distortions (RSD) and the Alcock-Paczynski effect (AP) have an impact on the void identification process itself generating distortion patterns in observations. Using a spherical void finder, we developed a statistical and theoretical framework to describe physically the connection between the identification in real and redshift space. We found that redshift-space voids above the shot noise level have a unique real-space counterpart spanning the same region of space, they are systematically bigger and their centres are preferentially shifted along the line of sight. The expansion effect is a by-product of RSD induced by tracer dynamics at scales around the void radius, whereas the off-centring effect constitutes a different class of RSD induced at larger scales by the global dynamics of the whole region containing the void. The volume of voids is also altered by the fiducial cosmology assumed to measure distances, this is the AP change of volume. These three systematics have an impact on cosmological statistics. In this work, we focus on the void size function. We developed a theoretical framework to model these effects and tested it with a numerical simulation, recovering the statistical properties of the abundance of voids in real space. This description depends strongly on cosmology. Hence, we lay the foundations for improvements in current models of the abundance of voids in order to obtain unbiased cosmological constraints from redshift surveys.
Simultaneous Estimation of Large-scale Structure and Milky Way Dust Extinction from Galaxy Surveys
(2021) Bravo, Matias; Gawiser, Eric; Padilla, Nelson D.; DeRose, Joseph; Wechsler, Risa H.
The high cosmological precision offered by the next generation of galaxy surveys hinges on improved corrections for Galactic dust extinction. We explore the possibility of estimating both the dust extinction and large-scale structure from a single photometric galaxy survey, making use of the predictable manner in which Milky Way dust affects the measured brightness and colors of galaxies in a given sky location in several redshift bins. To test our method, we use a synthetic catalog from a cosmological simulation designed to model the Vera C. Rubin Observatory Legacy Survey of Space and Time. At high Galactic latitude (vertical bar b vertical bar greater than or similar to 20 degrees) and a resolution of 1 degrees (7'), we predict the uncertainty in the measurement of dust extinction, E(B - V), to be 0.005 mag (0.015 mag). This is similar to the uncertainty of existing dust maps, illustrating the feasibility of our method. Simultaneous estimation of large-scale structure is predicted to recover the galaxy overdensity delta with a precision of similar to 0.01 (similar to 0.05) at 1 degrees (7') resolution. We also introduce a Bayesian formalism that combines prior information from existing dust maps with the likelihood of Galactic dust extinction determined from the excursion of observed galaxy properties.
The origin of star-gas misalignments in simulated galaxies
(2022) Casanueva, Catalina, I; Lagos, Claudia del P.; Padilla, Nelson D.; Davison, Thomas A.
We study the origin of misalignments between the stellar and star-forming gas components of simulated galaxies in the eagle simulations. We focus on galaxies with stellar masses >= 10(9) M-circle dot at 0 <= z <= 1. We compare the frequency of misalignments with observational results from the SAMI survey and find that overall, eagle can reproduce the incidence of misalignments in the field and clusters, as well as the dependence on stellar mass and optical colour within the uncertainties. We study the dependence on kinematic misalignments with internal galaxy properties and different processes related to galaxy mergers and sudden changes in stellar and star-forming gas mass. We find that galaxy mergers happen in similar frequency in mis- and aligned galaxies, with the main difference being misaligned galaxies showing a higher tidal field strength and fraction of ex situ stars. We find that despite the environment being relevant in setting the conditions to misalign the star-forming gas, the properties internal to galaxies play a crucial role in determining whether the gas quickly aligns with the stellar component or not. Hence, galaxies that are more triaxial and more dispersion dominated display more misalignments because they are inefficient at realigning the star-forming gas towards the stellar angular momentum vector.
Too dense to go through: the role of low-mass clusters in the pre-processing of satellite galaxies
(2022) Pallero, Diego; Gomez, Facundo A.; Padilla, Nelson D.; Bahe, Yannick M.; Vega-Martinez, Cristian A.; Torres-Flores, S.
We study the evolution of satellite galaxies in clusters of the c-eagle simulations, a suite of 30 high-resolution cosmological hydrodynamical zoom-in simulations based on the eagle code. We find that the majority of galaxies that are quenched at z = 0 (greater than or similar to 80per cent) reached this state in a dense environment (log(10)M(200)[M-circle dot] >= 13.5). At low redshift, regardless of the final cluster mass, galaxies appear to reach their quenching state in low-mass clusters. Moreover, galaxies quenched inside the cluster that they reside in at z = 0 are the dominant population in low-mass clusters, while galaxies quenched in a different halo dominate in the most massive clusters. When looking at clusters at z > 0.5, their in situ quenched population dominates at all cluster masses. This suggests that galaxies are quenched inside the first cluster they fall into. After galaxies cross the cluster's r(200) they rapidly become quenched (less than or similar to 1 Gyr). Just a small fraction of galaxies (less than or similar to 15 per cent) is capable of retaining their gas for a longer period of time, but after 4 Gyr, almost all galaxies are quenched. This phenomenon is related to ram pressure stripping and is produced when the density of the intracluster medium reaches a threshold of rho(ICM) similar to 3 x10(-5) n(H) (cm(-3)). These results suggest that galaxies start a rapid-quenching phase shortly after their first infall inside r(200) and that, by the time they reach r(500), most of them are already quenched.

Browsing by Author "Padilla, Nelson D."

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