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Jordi Miralda-Escude

Publications and source records attributed to Jordi Miralda-Escude.

At least 19 recordsLinked to original sources
arXiv2018

A metal-line strength indicator for Damped Lyman Alpha (DLA) systems at low signal-to-noise

The Baryon Oscillation Spectroscopic Survey of SDSS-III has provided an unprecedentedly large sample of Damped \lya systems (DLAs), the largest repositories of neutral hydrogen in the Universe. This DLA sample has been used to determine the DLA bias factor from their cross-correlation with the \lya forest absorption in \cite{FontRibera2012,Perez2018}, showing that DLAs are associated with relatively massive halos. However, the low resolution and signal-to-noise of BOSS spectra do not allow precise measurements of the DLA metal lines. We define a metal strength parameter, $S$, based on combining equivalent widths of 17 metal lines, that can be measured with an optimal signal-to-noise ratio for individual DLAs in BOSS spectra, allowing for the classification of the DLA population into subgroups of different $S$. We present the distribution of this DLA metal strength and the dependence of its mean value on $N_{\rm HI}$ and redshift. We search for systematic effects and variations in the catalogue purity by examining the dependence of the $S$ distribution on the spectral signal-to-noise and the estimated error on $S$. A catalogue of DLAs with measured equivalent widths for the selected 17 metal lines and the value of $S$ is made publicly available, which will be used to measure the dependence of the DLA bias factor on the $S$ parameter. The relation of the metal strength on the gas metal abundances and velocity dispersion can be constrained by studying the stacked metal absorption spectra of DLAs as a function of $S$, allowing for future determinations of the dependence of the bias factor on the metallicity and velocity dispersion of DLAs.

astro-ph.GA↗
arXiv2014

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: weighing the neutrino mass using the galaxy power spectrum of the CMASS sample

We measure the sum of the neutrino particle masses using the three-dimensional galaxy power spectrum of the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 (DR9) CMASS galaxy sample. Combined with the cosmic microwave background (CMB), supernova (SN) and additional baryonic acoustic oscillation (BAO) data, we find upper 95 percent confidence limits of the neutrino mass $Σm_ν<0.340$ eV within a flat $Λ$CDM background, and $Σm_ν<0.821$ eV, assuming a more general background cosmological model. The number of neutrino species is measured to be $N_{\rm eff}=4.308\pm0.794$ and $N_{\rm eff}=4.032^{+0.870}_{-0.894}$ for these two cases respectively. We study and quantify the effect of several factors on the neutrino measurements, including the galaxy power spectrum bias model, the effect of redshift-space distortion, the cutoff scale of the power spectrum, and the choice of additional data. The impact of neutrinos with unknown masses on other cosmological parameter measurements is investigated. The fractional matter density and the Hubble parameter are measured to be $Ω_M=0.2796\pm0.0097$, $H_0=69.72^{+0.90}_{-0.91}$ km/s/Mpc (flat $Λ$CDM) and $Ω_M=0.2798^{+0.0132}_{-0.0136}$, $H_0=73.78^{+3.16}_{-3.17}$ km/s/Mpc (more general background model). Based on a Chevallier-Polarski-Linder (CPL) parametrisation of the equation-of-state $w$ of dark energy, we find that $w=-1$ is consistent with observations, even allowing for neutrinos. Similarly, the curvature Ω_K and the running of the spectral index $α_s$ are both consistent with zero. The tensor-to-scaler ratio is constrained down to $r<0.198$ (95 percent CL, flat $Λ$ CDM) and $r<0.440$ (95 percent CL, more general background model).

astro-ph.CO↗
arXiv2012

The BOSS Lyman-alpha Forest Sample from SDSS Data Release 9

We present the BOSS Lyman-alpha (Lya) Forest Sample from SDSS Data Release 9, comprising 54,468 quasar spectra with zqso > 2.15 suitable for Lya forest analysis. This data set probes the intergalactic medium with absorption redshifts 2.0 < z_alpha < 5.7 over an area of 3275 square degrees, and encompasses an approximate comoving volume of 20 h^-3 Gpc^3. With each spectrum, we have included several products designed to aid in Lya forest analysis: improved sky masks that flag pixels where data may be unreliable, corrections for known biases in the pipeline estimated noise, masks for the cores of damped Lya systems and corrections for their wings, and estimates of the unabsorbed continua so that the observed flux can be converted to a fractional transmission. The continua are derived using a principal component fit to the quasar spectrum redwards of restframe Lya (lambda > 1216 Ang), extrapolated into the forest region and normalized by a linear function to fit the expected evolution of the Lya forest mean-flux. The estimated continuum errors are ~5% rms. We also discuss possible systematics arising from uncertain spectrophotometry and artifacts in the flux calibration; global corrections for the latter are provided. Our sample provides a convenient starting point for users to analyze clustering in BOSS Lya forest data, and it provides a fiducial data set that can be used to compare results from different analyses of baryon acoustic oscillations in the Lya forest. The full data set is available from the SDSS-III DR9 web site.

astro-ph.CO↗
arXiv2012

A star disrupted by a stellar black hole as the origin of the cloud falling toward the Galactic center

We propose that the cloud moving on a highly eccentric orbit near the central black hole in our Galaxy, reported by Gillessen et al., is formed by a photoevaporation wind originating in a disk around a star that is tidally perturbed and shocked at every peribothron passage. The disk is proposed to have formed when a stellar black hole flew by the star, tidally disrupted its envelope, and placed the star on its present orbit with some of the tidal debris forming a disk. A disrupting encounter at the location of the observed cloud is most likely to be caused by a stellar black hole because of the expected dynamical mass segregation; the rate of these disk-forming encounters may be as high as $\sim 10^{-6}$ per year. The star should also be spun up by the encounter, so the disk may subsequently expand by absorbing angular momentum from the star. Once the disk expands up to the tidal truncation radius, the tidal perturbation of the outer disk edge at every peribothron may place gas streams on larger orbits which can give rise to a photoevaporation wind that forms the cloud at every orbit. This model predicts that, after the cloud is disrupted at the next peribothron passage in 2013, a smaller unresolved cloud will gradually grow around the star on the same present orbit. An increased infrared luminosity from the disk may also be detectable when the peribothron is reached. We also note that this model revives the encounter theory for planet formation.

astro-ph.GA↗
arXiv2011

Extended Lyman-Alpha Emission around Star-forming Galaxies

Lyman-alpha (Lya) photons that escape the interstellar medium of star-forming galaxies may be resonantly scattered by neutral hydrogen atoms in the circumgalactic and intergalactic media, thereby increasing the angular extent of the galaxy's Lya emission. We present predictions of this extended, low surface brightness Lya emission based on radiative transfer modeling in a cosmological reionization simulation. The extended emission can be detected from stacked narrowband images of Lya emitters (LAEs) or of Lyman break galaxies (LBGs). Its average surface brightness profile has a central cusp, then flattens to an approximate plateau beginning at an inner characteristic scale below ~0.2 Mpc (comoving), then steepens again beyond an outer characteristic scale of ~1 Mpc. The inner scale marks the transition from scattered light of the central source to emission from clustered sources, while the outer scale marks the spatial extent of scattered emission from these clustered sources. Both scales tend to increase with halo mass, UV luminosity, and observed Lya luminosity. The extended emission predicted by our simulation is already within reach of deep narrowband photometry using large ground-based telescopes. Such observations would test radiative transfer models of emission from LAEs and LBGs, and they would open a new window on the circumgalactic environment of high-redshift star-forming galaxies.

astro-ph.CO↗
arXiv2011

The SDSS-III Baryon Oscillation Spectroscopic Survey: Quasar Target Selection for Data Release Nine

The SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), a five-year spectroscopic survey of 10,000 deg^2, achieved first light in late 2009. One of the key goals of BOSS is to measure the signature of baryon acoustic oscillations in the distribution of Ly-alpha absorption from the spectra of a sample of ~150,000 z>2.2 quasars. Along with measuring the angular diameter distance at z\approx2.5, BOSS will provide the first direct measurement of the expansion rate of the Universe at z > 2. One of the biggest challenges in achieving this goal is an efficient target selection algorithm for quasars over 2.2 < z < 3.5, where their colors overlap those of stars. During the first year of the BOSS survey, quasar target selection methods were developed and tested to meet the requirement of delivering at least 15 quasars deg^-2 in this redshift range, out of 40 targets deg^-2. To achieve these surface densities, the magnitude limit of the quasar targets was set at g <= 22.0 or r<=21.85. While detection of the BAO signature in the Ly-alpha absorption in quasar spectra does not require a uniform target selection, many other astrophysical studies do. We therefore defined a uniformly-selected subsample of 20 targets deg^-2, for which the selection efficiency is just over 50%. This "CORE" subsample will be fixed for Years Two through Five of the survey. In this paper we describe the evolution and implementation of the BOSS quasar target selection algorithms during the first two years of BOSS operations. We analyze the spectra obtained during the first year. 11,263 new z>2.2 quasars were spectroscopically confirmed by BOSS. Our current algorithms select an average of 15 z > 2.2 quasars deg^-2 from 40 targets deg^-2 using single-epoch SDSS imaging. Multi-epoch optical data and data at other wavelengths can further improve the efficiency and completeness of BOSS quasar target selection. [Abridged]

astro-ph.CO↗
arXiv2010

Radiative Transfer Modeling of Lyman Alpha Emitters. II. New Effects in Galaxy Clustering

We study the clustering properties of z~5.7 Lyman-alpha emitters (LAEs) in a cosmological reionization simulation with a full Lya radiative transfer calculation. Lya radiative transfer substantially modifies the intrinsic Lya emission properties, compared to observed ones, depending on the density and velocity structure environment around the Lya emitting galaxy. This environment-dependent Lya selection introduces new features in LAE clustering, suppressing (enhancing) the line-of-sight (transverse) density fluctuations and giving rise to scale-dependent galaxy bias. In real space, the contours of the three-dimensional two-point correlation function of LAEs appear to be prominently elongated along the line of sight on large scales, an effect that is opposite to and much stronger than the linear redshift-space distortion effect. The projected two-point correlation function is greatly enhanced in amplitude by a factor of up to a few, compared to the case without the environment dependent selection effect. The new features in LAE clustering can be understood with a simple, physically motivated model, where Lya selection depends on matter density, velocity, and their gradients. We discuss the implications and consequences of the effects on galaxy clustering from Lya selection in interpreting clustering measurements and in constraining cosmology and reionization from LAEs.

astro-ph.CO↗
arXiv2010

Radiative Transfer Modeling of Lyman Alpha Emitters. I. Statistics of Spectra and Luminosity

We combine a cosmological reionization simulation with box size of 100Mpc/h on a side and a Monte Carlo Lyman-alpha (Lya) radiative transfer code to model Lyman Alpha Emitters (LAEs) at z~5.7. The model introduces Lya radiative transfer as the single factor for transforming the intrinsic Lya emission properties into the observed ones. Spatial diffusion of Lya photons from radiative transfer results in extended Lya emission and only the central part with high surface brightness can be observed. Because of radiative transfer, the appearance of LAEs depends on density and velocity structures in circumgalactic and intergalactic media as well as the viewing angle, which leads to a broad distribution of apparent (observed) Lya luminosity for a given intrinsic Lya luminosity. Radiative transfer also causes frequency diffusion of Lya photons. The resultant Lya line is asymmetric with a red tail. The peak of the Lya line shifts towards longer wavelength and the shift is anti-correlated with the apparent to intrinsic Lya luminosity ratio. The simple radiative transfer model provides a new framework for studying LAEs. It is able to explain an array of observed properties of z~5.7 LAEs in Ouchi et al. (2008), producing Lya spectra, morphology, and apparent Lya luminosity function (LF) similar to those seen in observation. The broad distribution of apparent Lya luminosity at fixed UV luminosity provides a natural explanation for the observed UV LF, especially the turnover towards the low luminosity end. The model also reproduces the observed distribution of Lya equivalent width (EW) and explains the deficit of UV bright, high EW sources. Because of the broad distribution of the apparent to intrinsic Lya luminosity ratio, the model predicts effective duty cycles and Lya escape fractions for LAEs.

astro-ph.CO↗
arXiv2009

Photo-z optimization for measurements of the BAO radial direction

Baryon Acoustic Oscillations (BAO) in the radial direction offer a method to directly measure the Universe expansion history, and to set limits to space curvature when combined to the angular BAO signal. In addition to spectroscopic surveys, radial BAO might be measured from accurate enough photometric redshifts obtained with narrow-band filters. We explore the requirements for a photometric survey using Luminous Red Galaxies (LRG) to competitively measure the radial BAO signal and discuss the possible systematic errors of this approach. If LRG were a highly homogeneous population, we show that the photo-z accuracy would not substantially improve by increasing the number of filters beyond $\sim 10$, except for a small fraction of the sources detected at high signal-to-noise, and broad-band filters would suffice to achieve the target $σ_z = 0.003 (1+z)$ for measuring radial BAO. Using the LRG spectra obtained from SDSS, we find that the spectral variability of LRG substantially worsens the achievable photometric redshift errors, and that the optimal system consists of $\sim$ 30 filters of width $Δλ/ λ\sim 0.02$. A $S/N > 20$ is generally necessary at the filters on the red side of the $Hα$ break to reach the target photometric accuracy. We estimate that a 5-year survey in a dedicated telescope with etendue in excess of 60 ${\rm m}^2 {\rm deg}^2$ would be necessary to obtain a high enough density of galaxies to measure radial BAO with sufficiently low shot noise up to $z= 0.85$. We conclude that spectroscopic surveys have a superior performance than photometric ones for measuring BAO in the radial direction.

astro-ph↗
arXiv2009

Comment on the claimed radial BAO detection by Gaztanaga et al

Gaztanaga et al. have recently claimed to measure the Baryon Acoustic Oscillation (BAO) scale in the radial direction from the publicly available SDSS DR6 data. They focus on the correlation function of Luminous Red Galaxies (LRG) close to the line-of-sight direction to find a feature that they identify as the BAO peak, arguing that a magnification bias effect from gravitational lensing increases the amplitude of the BAO peak, facilitating its detection. In this Comment, we clarify that lensing has a negligible impact on the measurement of the BAO peak, and that the interpretation by Gaztanaga et al. is incorrect. The feature they identify in the LRG correlation function near the line-of-sight cannot be explained by any known physical effect and is in fact consistent with noise.

astro-ph.CO↗
arXiv2007

The 21cm Signature of the First Stars

We predict the 21-cm signature of the first metal-free stars. The soft X-rays emitted by these stars penetrate the atomic medium around their host halos, generating Lyman alpha photons that couple the spin and kinetic temperatures. These creates a region we call the Lyman alpha sphere, visible in 21-cm against the CMB, which is much larger than the HII region produced by the same star. The spin and kinetic temperatures are strongly coupled before the X-rays can substantially heat the medium, implying that a strong 21-cm absorption signal from the adiabatically cooled gas in Hubble expansion around the star is expected when the medium has not been heated previously. A central region of emission from the gas heated by the soft X-rays is also present although with a weaker signal than the absorption. The Lyman alpha sphere is a universal signature that should be observed around any first star illuminating its vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere can be calculated as a function of the luminosity, spectrum and age of the star. For a star of a few hundred solar masses and zero metallicity (as expected for the first stars), the physical radius of the Lyman alpha sphere can reach tens of kiloparsecs. The first metal-free stars should be strongly clustered because of high cosmic biasing; this implies that the regions producing a 21-cm absorption signal may contain more than one star and will generally be irregular and not spherical, because of the complex distribution of the gas. We discuss the feasiblity of detecting these Lyman alpha spheres, which would be present at redshifts $z\sim 30$ in the Cold Dark Matter model. Their observation would represent a direct proof of the detection of a first star.

astro-ph↗
arXiv2007

The Most Massive Black Holes in the Universe: Effects of Mergers in Massive Galaxy Clusters

Recent observations support the idea that nuclear black holes grew by gas accretion while shining as luminous quasars at high redshift, and they establish a relation of the black hole mass with the host galaxy's spheroidal stellar system. We develop an analytic model to calculate the expected impact of mergers on the masses of black holes in massive clusters of galaxies. We use the extended Press-Schechter formalism to generate Monte Carlo merger histories of halos with a mass 10^{15} h^{-1} Msun. We assume that the black hole mass function at z=2 is similar to that inferred from observations at z=0 (since quasar activity declines markedly at z<2), and we assign black holes to the progenitor halos assuming a monotonic relation between halo mass and black hole mass. We follow the dynamical evolution of subhalos within larger halos, allowing for tidal stripping, the loss of orbital energy by dynamical friction, and random orbital perturbations in gravitational encounters with subhalos, and we assume that mergers of subhalos are followed by mergers of their central black holes. Our analytic model reproduces numerical estimates of the subhalo mass function. We find that the most massive black holes in massive clusters typically grow by a factor ~ 2 by mergers after gas accretion has stopped. In our ten realizations of 10^{15} h^{-1} Msun clusters, the highest initial (z=2) black hole masses are 5-7 x 10^9 Msun, but four of the clusters contain black holes in the range 1-1.5 x 10^{10} Msun at z=0. Satellite galaxies may host black holes whose mass is comparable to, or even greater than, that of the central galaxy. Thus, black hole mergers can significantly extend the very high end of the black hole mass function.

astro-ph↗
arXiv2007

The Stryngbohtyk Model of the Universe: a Solution to the Problem of the Cosmological Constant

Astronomical observations have shown that the expansion of the universe is at present accelerating, in a way consistent with the presence of a positive cosmological constant. This is a major puzzle, because we do not understand: why the cosmological constant is so small; why, being so small, it is not exactly zero; and why it has precisely the value it must have to make the expansion start accelerating just at the epoch when we are observing the universe. We present a new model of cosmology, which we call the stryngbohtyk model, that solves all these problems and predicts exactly the value that the cosmological constant must have. The predicted value agrees with the observed one within the measurement error. We show that in the stryngbohtyk model, the fact the cosmological constant starts being important at the present epoch is not a coincidence at all, but a necessity implied by our origin in a planet orbiting a star that formed when the age of the universe was of the same order as the lifetime of the star.

astro-ph↗
arXiv2007

The Eccentricity-Mass Distribution of Exoplanets: Signatures of Different Formation Mechanisms?

We examine the distributions of eccentricity and host star metallicity of exoplanets as a function of their mass. Planets with M sin i >~ 4 M_J have an eccentricity distribution consistent with that of binary stars, while planets with M sin i <~ 4 M_J are less eccentric than binary stars and more massive planets. In addition, host star metallicities decrease with planet mass. The statistical significance of both of these trends is only marginal with the present sample of exoplanets. To account for these trends, we hypothesize that there are two populations of gaseous planets: the low-mass population forms by gas accretion onto a rock-ice core in a circumstellar disk and is more abundant at high metalliticities, and the high-mass population forms directly by fragmentation of a pre-stellar cloud. Planets of the first population form in initially circular orbits and grow their eccentricities later, and may have a mass upper limit from the total mass of the disk that can be accreted by the core. The second population may have a mass lower limit resulting from opacity-limited fragmentation. This would roughly divide the two populations in mass, although they would likely overlap over some mass range. If most objects in the second population form before the pre-stellar cloud becomes highly opaque, they would have to be initially located in orbits larger than ~30 AU, and would need to migrate to the much smaller orbits in which they are observed. The higher mean orbital eccentricity of the second population might be caused by the larger required intervals of radial migration, and the brown dwarf desert might be due to the inability of high-mass brown dwarfs to migrate inwards sufficiently in radius.

astro-ph↗
arXiv2006

Probing the Shape of the Galactic Halo with Hyper-Velocity Stars

Precise proper motion measurements (sigma_mu ~ 10 mkas/yr) of the recently discovered hyper-velocity star (HVS) SDSS J090745.0+024507 would yield significant constraints on the axis ratios and orientation of a triaxial model for the Galactic halo. Triaxiality of dark matter halos is predicted by Cold Dark Matter models of galaxy formation and may be used to probe the nature of dark matter. However, unless the distance to this star is determined to better than 10%, these constraints suffer from one-dimensional degeneracies, which we quantify. We show how proper motion measurements of several HVSs could simultaneously resolve the distance degeneracies of all such stars and produce a detailed picture of the triaxial halo. Additional HVSs may be found from radial velocity surveys or from parallax/proper-motion data derived from GAIA. High-precision proper-motion measurements of these stars using the Space Interferometry Mission (SIM PlanetQuest) would substantially tighten the constraints they yield on the Galactic potential.

astro-ph↗
arXiv2005

The maximum contribution to reionization from metal-free stars

We estimate the maximum contribution to reionization from the first generation of massive stars, with zero metallicity, under the assumption that one of these stars forms with a fixed mass in every collapsed halo in which metal-free gas is able to cool. We assume that any halo that has already had stars previously formed in one of their halo progenitors will form only stars with metals, which are assigned an emissivity of ionizing radiation equal to that determined at z=4 from the measured intensity of the ionizing background. We examine the impact of molecular hydrogen photodissociation (which tends to reduce cooling when a photodissociating background is produced by the first stars) and X-Ray photoheating (which heats the atomic medium, raising the entropy of the gas before it collapses into halos). We find that in the CDM$Λ$ model supported by present observations, and even assuming no negative feedbacks for the formation of metal-free stars, a reionized mass fraction of 50% is not reached until redshift $z\sim 12$. The combination of ordinary metal-enriched stars and early metal-free stars can yield a CMB optical depth to electron scattering of at most 0.13. The contribution of metal-free stars to the present Cosmic Infrarred Background is negligibly small.

astro-ph↗
arXiv2005

Galactic Wind Effects on the Lyman-alpha Absorption in the Vicinity of Galaxies

We present predictions of Ly-alpha forest-galaxy correlations at z=3 from Eulerian simulations that include the effects of galactic winds, driven primarily by supernova explosions. Galactic winds produce expanding bubbles of shock-heated gas within ~0.5 comoving Mpc/h of luminous galaxies in the simulation, which have space density similar to that of observed Lyman break galaxies (LBGs). However, most of the low-density intergalactic gas that determines the observed properties of the Ly-alpha forest is unaffected by winds. The impact of winds on the Ly-alpha optical depth near galaxies is less dramatic than their impact on gas temperature because winds heat only a small fraction of the gas present in the turnaround regions surrounding galaxies. Hence, Ly-alpha absorption from gas outside the wind bubbles is spread out over the same velocity range occupied by the wind-heated gas. In general, Ly-alpha absorption is expected to be stronger than average near galaxies because of the high gas density. Winds result in a modest reduction of this expected increase of Ly-alpha absorption. Our predictions can be compared to future observations to detect the wind effects and infer their strength, although with the caveat that the results are still dependent on the correspondence of simulated galaxies and observed LBGs. We find that wind effects in our simulations are not strong enough to reproduce the high Ly-alpha transmission within 0.5 Mpc/h comoving of galaxies that has been suggested by recent observations; powerful galactic explosions or ejecta with hyper-escape velocities would be required, but these are unlikely to be produced by ordinary star formation and supernovae alone

astro-ph↗