arXiv subjects

Jordi Miralda-Escude

Publications and source records attributed to Jordi Miralda-Escude.

At least 55 records · Page 3Linked to original sources

arXiv2000

A Cluster of Black Holes at the Galactic Center

If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M \ga 30 M_{\odot}, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii), or after losing orbital energy through gravitational waves. The overall depletion timescale is ~ 30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster due to relaxation, implying a core in the profile of solar-mass red giants with a radius of ~ 2 pc (i.e., 1'). The observed central density cusp (which has a core radius of only a few arc seconds) should be composed primarily of young (\la 1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (\la 2 Gyr) stars found at distances ~ 3 - 10 pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution $K$-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary (``planet-like'') perturbations on the main event.

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arXiv2000

Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach

Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters which govern them but are currently limited to small fields and subject to selection biases. We show how the expected signal from large-scale structure arises from the contributions from and correlations between individual halos. We determine the convergence power spectrum as a function of the maximum halo mass and so provide the means to interpret results from surveys that lack high mass halos either through selection criteria or small fields. Since shot noise from rare massive halos is mainly responsible for the sample variance below 10', our method should aid our ability to extract cosmological information from small fields.

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arXiv1999

The first stars: Where did they form?

Several emerging links between high-redshift observational cosmology and the Galactic fossil evidence found in the kinematics, metallicities and ages of Milky Way stars are discussed. In a flat Cold Dark Matter model with $Ω\simeq 0.3$ that agrees with present large-scale structure observations, the oldest stars in the Milky Way should have formed in the first halos where gas was able to cool, at $z\simeq 20$. These earliest, weakly bound dwarf galaxies probably turned only a small fraction of their gas to stars, which should be metal-poor. However, the merging rate in the early universe was much faster than the present one, so massive halos with more efficient star formation and metallicities up to the highest values present today in the bulge could have formed less than $10^9$ years after the oldest stars. The mean metallicity produced in the universe by a given redshift is related to the mean surface brightness of star-forming galaxies above this redshift, and also to the reionization epoch if galaxies were the dominant sources of ionizing radiation. The biased distribution of the early dwarf galaxies where the first stars formed should result in an age gradient with radius of the low-metallicity stars in the Milky Way, with the oldest ones concentrated in the bulge and the youngest in the outer halo.

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arXiv1999

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyman-alpha Forest

A sample of eight quasars observed at high resolution and signal-to-noise is used to determine the probability distribution function (PDF), the power spectrum, and the correlation function of the transmitted flux in the \lya forest, in three redshift bins centered at z=2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the \lya forest of a Lambda-CDM model with Omega=0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the PDF, if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted PDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln(k) of $Δ^2_ρ(k)=0.72\pm0.09$ at k=0.04(km/s)^{-1} and z=3, and the slope of the power spectrum near the same k to be $n_p=-2.55\pm0.10$ (statistical error bars). The results are statistically consistent with Croft et. al. (1999), although our value for the rms fluctuation is lower by a factor 0.75. For the Lambda-CDM model we use, the implied primordial slope is $n=0.93\pm0.10$, and the normalization is $σ_8=0.68+1.16(0.95-n)\pm0.04$.

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arXiv1999

Soft X-ray Absorption by High-Redshift Intergalactic Helium

The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has been measured with HST in four quasars over the last few years, over the redshift range 2.4 < z < 3.2. These observations have indicated that the HeII reionization may not have been completed until z\simeq 2.8, and that large fluctuations in the intensity of the HeII-ionizing background were present before this epoch. The detailed history of HeII reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, because the IGM can be completely optically thick to Lya photons when only a small fraction of the helium remains as HeII. In addition, finding quasars in which the HeII Lya absorption can be observed becomes increasingly difficult at higher redshift, owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that HeII in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was HeII should recover from the HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic HeII absorption can be detected as an excess over the expected Galactic absorption from the 21cm HI column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift, if the measurement is done on a large sample of high-redshift sources over a range of redshift.

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arXiv1999

La frontera a alto corrimiento al rojo: historia de la formacio'n de las galaxias

This review presents a brief discussion of the theory of Cold Dark Matter of structure formation in the universe, describing the main processes determining the power spectrum, the non-linear gravitational collapse, the formation of galaxies and the evolution of the intergalactic medium. Recent advances in the observations of high redshift galaxies, and their interpretation in the context of this theory, are then summarized. Este art\'ıculo de revisión presenta una breve exposición de la teor\'ıa de la Materia Invisible Fr\'ıa de la formación de estructura en el universo, describiendo los procesos más importantes que determinan el espectro de potencia, el colapso gravitatorio no lineal, la formación de galaxias y la evolución del medio intergaláctico. Resumimos también los avances recientes en las observaciones de galaxias a alto corrimiento al rojo, y la interpretación en el contexto de esa teor\'ıa.

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arXiv1999

Reionization of the Inhomogeneous Universe

A model of the density distribution in the intergalactic medium, motivated by that found in numerical simulations, is used to demonstrate the effect of a clumpy IGM and discrete sources on the reionization of the universe. In an inhomogeneous universe reionization occurs outside-in, starting in voids and gradually penetrating into overdense regions. Reionization should not be sudden but gradual, with a continuous rise of the photon mean free path over a fair fraction of the Hubble time as the emissivity increases. We show that a hydrogen Gunn-Peterson trough should be present at $z\simeq 6$ unless the emissivity increases with redshift at $z>4$. However, the epoch of overlap of cosmological \hii regions could have occurred at a higher redshift if sources of low luminosity reionized the IGM; the Gunn-Peterson trough at $z\sim 6$ would then appear because even the most underdense voids have a large enough neutral fraction in ionization equilibrium to be optically thick to \lya photons. Cosmological \hii regions near the epoch of overlap can produce gaps of transmitted flux only if luminous quasars contributed to the reionization. Despite the clumpiness of the matter distribution, recombinations are not very important during the reionization of hydrogen because the high density gas is not ionized until a late time. We show that the \heii reionization was most likely delayed relative to the hydrogen reionization, but should be completed by $z\sim 3$, the redshift where observations are available. The reported large optical depth fluctuations of \heii are probably not due to an incomplete \heii reionization, but arise from a combination of density fluctuations and the variations in the intensity of the ionizing background due to luminous QSO's.

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arXiv1998

Galaxy Formation and the Kinematics of Damped Lyman Alpha Systems

A model of damped lyman alpha systems is presented based on randomly moving clouds in spherical halos. We use the Press-Schechter model for the abundance of halos, and assume that each halo has a similar population of clouds, with total mass and spatial distribution constrained to fit observations of the column density distribution. We show that the kinematics of the multiple absorbing components revealed in absorption profiles of the low-ionization lines, presented by Prochaska & Wolfe, are consistent with our spherical halo model. The presence of multiple absorbing components with a large covering factor, combined with the small impact parameters of the systems predicted in our analytical model and in numerical simulations, implies a high rate of energy dissipation in cloud collisions. We calculate the rate of energy dissipation in our model, and show that it is far greater than the rate at which energy can be supplied by gravitational mergers of halos. This poses a problem for the model of merging protogalactic clumps of Haehnelt et al., based on numerical simulations. We also present new constraints on the amplitude of the power spectrum in hierarchical theories required to account for the observed velocity dispersion in the absorbers. We find that the linearly extrapolated rms fluctuation on spheres of radius HR = 100 km/s at z=4 must be greater than 0.75. Although this limit is obtained only for our specific model of the absorbing components, it should not be highly model-dependent because the velocity dispersion of the absorbers is essentially determined by the velocity dispersion of the halos where the gas is moving.

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arXiv1998

Small-Angle Scattering of X-Rays from Extragalactic Sources by Dust in Intervening Galaxies

Gamma-ray bursts are now known to be a cosmological population of objects, which are often accompanied by X-ray and optical afterglows. The total energy emitted in the afterglow can be similar to the energy radiated in the gamma-ray burst itself. If a galaxy containing a large column density of dust is near the line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to diffraction by the dust grains will give rise to an X-ray echo of the afterglow. A measurement of the angular size of the echo at a certain time after the afterglow is observed yields a combination of the angular diameter distances to the scattering galaxy and the gamma-ray burst that can be used to constrain cosmological models in the same way as a time delay in a gravitational lens. The scattering galaxy will generally cause gravitational lensing as well, and this should modify the shape of the X-ray echo from a circular ring. The main difficulty in detecting this phenomenon is the very low flux expected for the echo. The flux can be increased when the gamma-ray burst is highly magnified by gravitational lensing, or when the deflecting galaxy is at low redshift. X-ray echos of continuous (but variable) sources, such as quasars, may also be detectable with high-resolution instruments and would allow similar measurements.

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arXiv1998

Measuring the Cosmological Geometry from the Lyman Alpha Forest along Parallel Lines of Sight

We discuss the feasibility of measuring the cosmological metric using the redshift space correlation function of the Lya forest in multiple lines of sight, as a function of angular and velocity separation. The geometric parameter that is measured is f(z) = H(z) D(z)/c, where H(z) is the Hubble constant and D(z) the angular diameter distance at redshift z. The correlation function is computed in linear theory. We describe a method to measure it from observations with the Gaussianization procedure of Croft et al (1998) to map the Lya forest transmitted flux to an approximation of the linear density field. The effect of peculiar velocities on the shape of the recovered power spectrum is pointed out. We estimate the error in recovering the f(z) factor from observations due to the variance in the Lya absorbers. We show that ~ 20 pairs of quasars (separations < 3') are needed to distinguish a flat Ω_0=1 universe from a universe with Ω_0=0.2, Ω_Λ=0.8. A second parameter that is obtained from the correlation function of the Lya forest is β\simeq Ω(z)^{0.6}/b (affecting the magnitude of the peculiar velocities), where b is a linear theory bias of the Lya forest. The statistical error of f(z) is reduced if b can be determined independently from numerical simulations, reducing the number of quasar pairs needed for constraining cosmology to approximately six. On small scales, where the correlation function is higher, f(z) should be measurable with fewer quasars, but non-linear effects must then be taken into account. The anisotropy of the non-linear redshift space correlation function as a function of scale should also provide a precise quantitative test of the gravitational instability theory of the Lya forest.

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arXiv1998

The X-Ray Forest: A New Prediction of Hierarchical Structure Formation Models

We use numerical simulations of structure formation in a Cold Dark Matter model to predict the absorption lines in the soft X-rays produced by heavy elements in the shock-heated intergalactic medium at low redshift. The simulation incorporates a model for metal production in galaxies and the subsequent dispersion of the metals to the intergalactic medium. We analyze in particular absorption lines produced by oxygen, and calculate the ionization stage taking into account the observed X-ray background at the present time. We find that oxygen is fully ionized by the X-ray background in low-density voids, and is mostly in the form of OVII and OVIII in the sheets and filamentary regions. Strong absorption lines of OVII and OVIII with equivalent widths W ~ 100 km/s are produced in filamentary regions of overdensities ~ 100 and temperatures ~ 10^6 K, located in the outskirts of groups and clusters of galaxies. The OVII line at E = 574 eV is generally the strongest one in these systems. Our model predicts that any X-ray source (such as a quasar) should typically show about one OVII absorption line with W > 100 km/s in the interval from z=0 to z=0.3. These lines could be detected with the upcoming generation of X-ray telescopes, and their origin in intervening systems could be confirmed by the association with groups of galaxies and X-ray emitting halos near the line-of-sight at the same redshift. The hot intergalactic medium may be one of the main reservoirs of baryons in the present universe, and the heavy element X-ray absorption lines offer a promising possibility of detecting this new component in the near future.

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arXiv1997

Searching for the Earliest Galaxies using the Gunn-Peterson Trough and the Lyman Alpha Emission Line

If the universe was reionized by O and B stars in an early population of galaxies, the associated supernovae should have enriched the universe to a mean metallicity $\bar Z = 10^{-5} (1+ n_{rec})$, where $n_{rec}$ is the mean number of times that each baryon recombined during the reionization era. This is consistent with recent observations of the metallicity in the Lya forest at $z\simeq 3$. The mean surface brightness observable at present from the galaxies that produced these heavy elements, in the rest-frame wavelengths $1216Å< λ\lesssim 2500Å$, should be $\sim 10^{-6} (\bar Z/10^{-4}) (Ω_b h^2/0.02) ~ photons \cm^{-2} s^{-1} arcsec^{-2}$. Most of this radiation was emitted at $z>5$, before reionization was complete. These high-redshift galaxies may be detectable in near-infrared photometric surveys, identifying them via the Gunn-Peterson trough (analogous to the use of the Lyman limit cutoff to search for galaxies at $z\sim 3$, where the Lya forest blanketing is smaller). Their spectrum may also be characterized by a strong Lya emission line. However, the spectra of galaxies seen behind intervening gas that is still neutral should show the red damping wing of the Gunn-Peterson trough, with a predictable profile that obstructs part of the Lya emission. The low-mass galaxies formed before reionization might constitute a distinctive population; we discuss the signature that this population could have in the faint number counts. Although most of these galaxies should have merged into larger ones, those that survived to the present could be dwarf spheroidals.

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arXiv1997

Reionization of the Intergalactic Medium and the Damping Wing of the Gunn-Peterson Trough

Observations of high-redshift quasars show that the IGM must have been reionized at some redshift $z>5$. If a source of radiation could be observed at the rest-frame Lya wavelength, at a sufficiently high redshift where some of the IGM in the line-of-sight was not yet reionized, the Gunn-Peterson trough should be present. Longward of the Lya wavelength, a damping wing should be observed caused by the neutral IGM whose absorption profile can be predicted. Measuring the shape of this damping wing would provide irrefutable evidence of the observation of the IGM before reionization, and a determination of the density of the neutral IGM. This measurement might be hindered by the possible presence of a dense absorption system associated with the source. Shortward of the \lya wavelength, absorption should be seen from the patchy structure of the IGM in the process of reionization. We show that a complete Gunn-Peterson trough is most likely to continue to be observed through the epoch where the IGM is partially ionized. The damping wings of the neutral patches should overlap if the proper pathlength through an ionized region is less than 1 h^{-1} Mpc; even in larger ionized regions, the characteristic background intensity should be low enough to yield a very high optical depth due to the residual neutral fraction, although occasionally some flux may be transmitted through large, underdense voids within an ionized region. The case of the HeII reionization is also discussed, and we argue that helium was already doubly ionized by z=3 throughout the IGM. The recently discovered afterglows of gamma-ray bursts might soon be observed at very high redshifts. Their featureless continuum spectrum and high luminosities make them ideal sources for studying absorption by the IGM.

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arXiv1997

High-Redshift Supernovae and the Metal-Poor Halo Stars: Signatures of the First Generation of Galaxies

Recent evidence on the metal content of the high-redshift Lyman alpha forest seen in quasar spectra suggests that an early generation of galaxies enriched the intergalactic medium (IGM) at z ~ 5. We calculate the number of supernovae that need to have taken place to produce the observed metallicity. The progenitor stars of the supernovae should have emitted $\sim 20$ ionizing photons for each baryon in the universe, i.e., more than enough to ionize the IGM. We calculate that the rate of these supernovae is such that about one of them should be observable at any time per square arc minute. Their fluxes are, of course, extremely faint: at z=5, the peak magnitude should be K=27 with a duration of ~ 1 year. However, these supernovae should still be the brightest objects in the universe beyond some redshift, because the earliest galaxies should form before quasars and they should have very low mass, so their luminosities should be much lower than that of a supernova. We also show that, under the assumption of a standard initial mass function, a significant fraction of the stars in the Galactic halo should have formed in the early galaxies that reionized and enriched the IGM, and which later must have merged with our Galaxy. These stars should have a more extended radial distribution than the observed halo stars.

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arXiv1997

A Lower Bound on the Cosmic Baryon Density

We derive analytic lower bounds on the cosmic baryon density by requiring that the high-redshift IGM contain enough neutral hydrogen to produce the observed \lya absorption in quasar spectra. The key theoretical assumption is that absorbing structures are no more extended in redshift space than in real space. This assumption might not hold if \lya clouds are highly overdense and thermally broadened, but it is likely to hold in the gravitational instability picture for the \lya forest suggested by cosmological simulations, independently of the details of the cosmological model. The other ingredients in these bounds are an estimate of the UV background from quasars, a temperature $T~10^4K$ for the "warm" photoionized IGM that produces most of the \lya absorption, a value of the Hubble constant, and observational estimates of the mean \lya flux decrement $\Dbar$ or, for a more restrictive bound, the distribution function $P(τ)$ of \lya optical depths. With plausible parameter values, the mean decrement bound implies a baryon density parameter $\Omb \ga 0.0125/h^2$. With conservative values, the bound weakens to $\Omb \ga 0.005/h^2$, but the required clustering of the IGM is then incompatible with other properties of quasar spectra. A recent observational determination of $P(τ)$ implies $\Omb \ga 0.0125/h^2$ even for a conservative estimate of the UV background, and $\Omb \ga 0.018/h^2$ for a more reasonable estimate. These bounds are consistent with recent low estimates of the primordial deuterium-to-hydrogen ratio $\dtoh$, which imply $\Omb \approx 0.025/h^2$, but the $P(τ)$ bound can only be reconciled with high $\dtoh$ estimates by abandoning standard big bang nucleosynthesis or the gravitational instability picture for the origin of the \lya forest. (Shortened abstract.)

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arXiv1996

Signatures of Accretion Disks in Quasar Microlensing

We propose that relative variability on short time-scales of the multiple images of a lensed quasar, after removal of the time delay, may be caused by hot spots or other moving structures in the accretion disk crossing microlens caustics caused by stellar mass objects in the lensing galaxy. Such variability has been reported in the two images of 0957+561. The short durations would be due to the high rotation speed of the disk (v/c ~ 0.1), rather than planetary mass objects in the slowly moving (v/c ~ 0.001) lens. This interpretation could be confirmed by finding periodicity, or correlations of the spectral and flux variations due to the Doppler effect in the disk. We also propose another signature of stationary accretion disks (with no intrinsic variability): the gradient of the magnification over the accretion disk should cause a relative color change between the images whose sign and amplitude are correlated with the time derivative of the flux difference between the images. Other color terms induced by the radial variation of disk colors are of second order in the magnification gradient. The methods proposed here can be used first to verify that accretion disks near supermassive black holes are the source of the continuum radiation from quasars, and then to study them.

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arXiv1996

Images of Bursting Sources of High-Energy Cosmic Rays. I: Effects of Magnetic Fields

It has recently been shown that the highest energy cosmic rays (CRs) may originate in the same cosmological objects producing $γ$-ray bursts. This model requires the presence of intergalactic magnetic fields (IGMF) to delay the arrival times of $\sim 10^{20}$ eV CRs by 50 years or longer relative to the $γ$-rays, of an amplitude that is consistent with other observational constraints. Sources of CRs coming from individual bursts should be resolved with the planned ``Auger'' experiment, with as many as hundreds of CRs for the brightest sources. We analyze here the apparent angular and energy distribution of CRs from bright sources below the pion production threshold (in the energy range $10^{19}{\rm eV} < E < 4\times10^{19}{\rm eV}$) expected in this model. This observable distribution depends on the structure of the IGMF: the apparent spectral width $ΔE$ is small, $ΔE/E\lesssim1\%$, if the intergalactic field correlation length $λ$ is much larger than $1{\rm Mpc}$, and large, $ΔE/E=0.3$, in the opposite limit $λ\ll 1{\rm Mpc}$. The apparent angular size is also larger for smaller $λ$. If the sources of CRs we predict are found, they will corroborate the bursting model and they will provide us with a technique to investigate the structure of the IGMF.

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arXiv1996

Microlensing Events from Measurements of the Deflection Angle

Microlensing events are now regularly being detected by monitoring the flux of a large number of potential sources and measuring the combined magnification of the images. This phenomenon could also be detected directly from the gravitational deflection, by means of high precision astrometry using interferometry. Relative astrometry at the level of $10\muas$ may become possible in the near future. The gravitational deflection can be measured by astrometric monitoring of a bright star having a background star within a small angular separation. This type of monitoring program will be carried out for the independent reasons of discovering planets from the angular motion they induce on the nearby star around which they are orbiting, and for measuring parallaxes, proper motions and orbits of binary stars. We discuss three applications of the measurement of gravitational deflections by astrometric monitoring: measuring the mass of the bright stars that are monitored, measuring the mass of brown dwarfs or giant planets around the bright stars, and detecting microlensing events by unrelated objects near the line of sight to the two stars. We discuss the number of stars whose mass could be measured by this procedure. We also give expressions for the number of expected microlensing events by unrelated objects, which could be stars, brown dwarfs, or other compact objects accounting for dark matter in the halo or in the disk.

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