Jordi Miralda-Escudé

I like to work on theoretical astrophysics. The problems I have worked on since I started my research activities, when I was a graduate student, are: gravitational lensing by clusters of galaxies and large-scale structure, the intergalactic medium and its ionization, extragalactic absorption lines as a probe of the intergalactic medium, formation of galaxies, accreting neutron stars, microlensing in our Galaxy and in external galaxies, high-energy cosmic rays, and the stellar dynamics in the Galactic center.

At present, most of my work is in cosmology and related to the formation of galaxies, the distribution of gas in space and the epoch of reionization of matter in the universe. The intergalactic medium was at one time in atomic form, and it must have been ionized later as the first stars and quasars were formed in the universe. I am interested in this process of ``reionization'', and the interpretation of observations of absorption spectra of distant sources caused by the intervening intergalactic medium, as well as denser gas in the process of forming galaxies. My other main interest is the nature of the dark matter and the primordial fluctuations that gave rise to the formation of galaxies and larger structures in the universe, and the observational tools we have available to study this: gravitational lensing by galaxies, clusters of galaxies and large-scale structure, microlensing of stars or quasars by any kind of compact object, the spatial distribution of galaxies, and the structure of dark matter halos studied through the dynamics of galaxies in clusters.

The universe has evolved from an initially homogenous state to the present highly inhomogeneous state, where some matter has collapsed into galaxies and clusters of galaxies, but other matter is left in the intergalactic medium, which is the gas between galaxies. The intergalactic gas can be observed through the absorption lines it produces on background sources. The most important of these absorption lines is the Lyman alpha line of hydrogen, which is observed in luminous quasars at high redshift. My work in this area has concentrated on studying the interplay between the evolution of the intergalactic medium and the formation of galaxies. The questions that I am investigating at present include the following:

  • The formation of the very first galaxies in the ``dark era''. The most distant galaxies discovered so far are near a redshift of 6. Some of the outstanding questions we have are: when did the first galaxies form? In what respects were they different from present galaxies?

  • The reionization of the intergalactic medium. The gas in the universe was in atomic form after the microwave background radiation was emitted. But at some later stage it must have been ionized by the first stars and quasars that formed in the very first galaxies. In fact, we know that as far as we have observed any galaxies, up to a redshift of 6, the medium had already been ionized. How did the reionization take place, and how did it affect the formation of galaxies?

  • Galaxies at high redshift are recently being discovered in large numbers, thanks to new techniques that have been developed to identify them among faint galaxies in the sky. What can be learned from the spatial correlations of these galaxies? What can we learn also from the spatial correlation of the gas detected in absorption lines on the spectra of bright quasars, out of which the galaxies are forming?

    The other main area where I have worked is gravitational lensing. The light from distant sources in the universe is deflected when propagating toward us owing to the gravitational field of the matter along the line-of-sight. The deflection of light can be studied from the distortions created on the images of these distant sources, and sometimes multiple images of a source are observed. This allows us to investigate the distribution of the masses responsible for the light deflection. Most of the mass in the universe is dark matter, meaning that it is not observed directly, but only through the gravitational influence it exerts on other matter. Therefore, gravitational lensing is a unique observational tool to investigate the distribution of dark matter around galaxies and clusters of galaxies.

    Selected Publications

    1. J. Miralda-Escudé 1991, ``The Correlation Function of Galaxy Ellipticities Produced by Gravitational Lensing'', Ap. J., 380, 1.
    2. J. Miralda-Escudé and M. J. Rees 1994, ``Reionization and Thermal Evolution of the Intergalactic Medium'', MNRAS, 266, 343.
    3. M. Rauch, J. Miralda-Escudé, W. L. W. Sargent, J. Barlow, D. H. Weinberg, L. Hernquist, N. Katz, R. Cen, and J. P. Ostriker 1997, ``The Opacity of the Lyman Alpha Forest and Implications for Omega_b and the Ionizing Background'', Ap. J., 489, 7.
    4. J. Miralda-Escudé 1998, ``Reionization of the Intergalactic Medium and the Damping Wing of the Gunn-Peterson Trough'', Ap. J., 501, 15.
    5. J. Miralda-Escudé, M. Haehnelt, and M. J. Rees 2000, ``Reionization of the Inhomogeneous Universe'', ApJ, 530, 1.
    6. J. Miralda-Escudé and A. Gould 2000, ``A Cluster of Black Holes at the Galactic Center'', ApJ, 545, 847.
    7. P. McDonald and J. Miralda-Escudé 2001, `` The Lyman-alpha Forest Flux Distribution at z ~ 5.2 and the Evolution of the Ionizing Background'', ApJL, 549, L11.
    8. X. Chen and J. Miralda-Escudé 2003, ``The Spin-Kinetic Temperature Coupling and the Heating Rate due to Lyman Alpha Scattering before Reionization: Predictions for 21 cm Emission and Absorption'', submitted to ApJ (astro-ph/0303395).

      I have also written a review article in Science on The Dark Age of the Universe, describing the period of time between the emission of the Cosmic Microwave Background and the formation of the first stars, the physics of how the first stars formed, and our present knowledge of the epoch of reionization. You can access the paper here for the Abstract and for the Full Text of the article.
      J. Miralda-Escudé 2003, ``The Dark Age of the Universe'', Science, 300, 1904.

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