X Fan

Publications (3)4.9 Total impact

• Source

  Available from: T. Roy Choudhury

  Article: Constraining cosmic reionization with quasar, gamma ray burst, and Lya emitter observations

  S. Gallerani, A. Ferrara, T. R. Choudhury, X Fan, R. Salvaterra, P. Dayal
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  ABSTRACT: We investigate the cosmic reionization history by comparing semi-analytical models of the Lya forest with observations of high-z quasars and gamma ray bursts absorption spectra. In order to constrain the reionization epoch z_rei, we consider two physically motivated scenarios in which reionization ends either early (ERM, z_rei>= 7) or late (LRM, z_rei~6). We analyze the transmitted flux in a sample of 17 quasars spectra at 5.7<z<6.4 and in the spectrum of the gamma ray burst 050904 at z=6.3, studying the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with our models, we conclude that current observational data do not require any sudden change in the ionization state of the IGM at z~6, favouring indeed a highly ionized Universe at these epochs, as predicted by the ERM. Moreover, we test the predictions of this model through Lya emitters observations, finding that the ERM provide a good fit to the evolution of the luminosity function of Lya emitting galaxies in the redshift range z=5.7-6.5. The overall result points towards an extended reionization process which starts at z>=11 and completes at z_rei>=7, in agreement with the recent WMAP5 data. Comment: 4 pages, 4 figures, to appear in the Proceedings `L'Universo quattro secoli dopo Galileo', 53rd Congresso SAIt, Pisa, May 2009
  12/2009;
• Source

  Available from: ArXiv

  Article: Glimpsing through the high‐redshift neutral hydrogen fog

  S. Gallerani, A. Ferrara, X. Fan, T. Roy Choudhury
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  ABSTRACT: We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤zem≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, xH i, at z≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that xH i evolves smoothly from 10−4.4 at z= 5.3 to 10−4.2 at z= 5.6, with a robust upper limit xH i < 0.36 at z= 6.3. The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H ii regions around faint quasars or galaxies. In one case (the intervening H ii region of the faint quasar RD J1148+5253 at z= 5.70 along the line of sight of SDSS J1148+5251 at z= 6.42) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of tQ > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects.
  Monthly Notices of the Royal Astronomical Society 04/2008; 386(1):359 - 369. · 4.90 Impact Factor
• Source

  Available from: T. Roy Choudhury

  Article: Was the Universe neutral beyond redshift six?

  S. Gallerani, A. Ferrara, X Fan, Choudhury T. Roy, R. Salvaterra
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  ABSTRACT: We provide measurements of the neutral hydrogen fraction xHI at z~6, by comparing semi-analytical models of the Lyalpha forest with observations of high-z quasars and Gamma Ray Bursts absorption spectra. We analyze the transmitted flux in a sample of 17 QSOs spectra at 5.74<zem<6.42 studying separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with our models, we conclude that xHI evolves smoothly from 10^{-4.4} at z=5.3 to 10^{-4.2} at z=5.6, with a robust upper limit xHI<0.36 at z=6.3. We show the results of the first-ever detected transverse proximity effect in the HI Lyalpha forest, produced by the HII region of the faint quasar RD J1148+5253 at z=5.70 intervening along the LOS of SDSS J1148+5251 at z=6.42. Moreover, we propose a novel method to study cosmic reionization using absorption line spectra of high-redshift GRBs afterglows. We show that the time evolution and the statistics of gaps in the observed spectra represent exquisite tools to discriminate among different reionization models. By applying our methods to GRB 050904 detected at z=6.29, we show that the observation of this burst provides strong indications of a highly ionized intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction xHI=6.4\pm 0.3\times 10^{-5} along that line of sight. Comment: 6 pages, 3 figures; to appear in the Proceedings `A Century of Cosmology', San Servolo (Venezia, Italy), August 2007, to be published in `Il Nuovo Cimento'; typos corrected
  01/2008;