NICMOS imaging search for high-redshift damped Lyα galaxies

We are engaged in a programme of imaging with the STIS and NICMOS (NIC2) instruments aboard the Hubble Space Telescope ( HST ), to search for the galaxy counterparts of 18 high-redshift z >1.75 damped Ly α absorption lines and five Lyman-limit systems seen in the spectra of 16 target quasars. This paper presents the results of the imaging campaign with the NIC2 camera. We describe the steps followed in reducing the data and combining in mosaics, and the methods used for subtracting the image of the quasar in each field, and for constructing error frames that include the systematic errors associated with the PSF subtraction. To identify candidate counterparts, which are either compact or diffuse, we convolved the image and variance frames with circular top-hat filters of diameter 0.45 and 0.90 arcsec respectively, to create frames of summed S /N within the aperture. For each target quasar we provide catalogues listing positions and aperture magnitudes of all sources within a square of side 7.5 arcsec centred on the quasar, detected at S / N >6 . We find a total of 41 candidates, of which three have already been confirmed spectroscopically as the counterparts. We provide the aperture magnitude detection limits as a function of impact parameter, for both detection filters, for each field. The average detection limit for compact (diffuse) sources is H _AB =25.0 (24.4) at an angular separation of 0.56 (0.79) arcsec from the quasar, improving to H _AB =25.5 (24.8) at large angular separations. For the brighter sources we have measured the half-light radius and the n parameter of the best-fitting deconvolved Sersic-law surface brightness profile, and the ellipticity and orientation.     

Damped Lyα systems at high redshift and models of protogalactic discs

We employ observationally determined intrinsic velocity widths and column densities of damped Lyman alpha (Lyα) systems at high redshift to investigate the distribution of baryons in protogalaxies within the context of a standard cold dark matter (CDM) model. We proceed under the assumption that damped Lyα systems represent a population of cold, rotationally supported, protogalactic discs, and that the abundance of dark matter haloes is well approximated by a CDM model with critical density and vanishing cosmological constant. Using conditional cross-sections to observe a damped system with a given velocity width and column density, we compare observationally inferred velocity width and column density distributions to the corresponding theoretically determined distributions for a variety of disc parameters and CDM normalizations. In general, we find that the observations cannot be reproduced by the models for most disc parameters and CDM normalizations. Whereas the column density distribution favours small discs with large neutral gas fraction, the velocity width distribution favours large and thick discs with small neutral gas fraction. The possible resolutions of this problem in the context of this CDM model may be (1) an increased contribution of rapidly rotating discs within massive dark matter haloes to damped Lyα absorption, or (2) the abandoning of simple disc models within this CDM model for damped Lyα systems at high redshift. Here the first possibility may be achieved by supposing that damped Lyα system formation occurs only in haloes with fairly large circular velocities, and the second possibility may result from a large contribution of mergers and double discs to damped Lyα absorption at high redshift.     

Broadening of QSO Lyα forest absorbers

We investigate the dependence of QSO Ly α absorption features on the temperature of the absorbing gas and on the amplitude of the underlying dark-matter fluctuations. We use high-resolution hydrodynamic simulations in cold dark matter dominated cosmological models. In models with a hotter intergalactic medium (IGM), the increased temperature enhances the pressure gradients between low- and high-density regions and this changes the spatial distribution and the velocity field of the gas. Combined with more thermal broadening, this leads to significantly wider absorption features in hotter models. Cosmological models with little small-scale power also have broader absorption features, because fluctuations on the scale of the Jeans length are still in the linear regime. Consequently, both the amplitude of dark-matter fluctuations on small scales and thermal smoothing affect the flux decrement distribution in a similar way. However, the b -parameter distribution of Voigt profile fits, obtained by deblending the absorption features into a sum of thermally broadened lines, is largely independent of the amount of small-scale power, but does depend strongly on the IGM temperature. The same is true for the two-point function of the flux and for the flux power spectrum on small scales. These three flux statistics are thus sensitive probes of the temperature of the IGM. We compare the values computed for our models and obtained from a HIRES spectrum of the quasar Q1422+231 and conclude that the IGM temperature at z ∼3.25 is fairly high, T ₀≳15 000 K. The flux decrement distribution of the observed spectrum is fitted well by that of a ΛCDM model with that temperature.     

On the small-scale clustering of the Lyα forest clouds

Recent measurements of the autocorrelation function of the Ly α clouds are analysed from the point of view of a simple model with strong clustering on the small scales. It is shown that this toy model reproduces fairly well the important linear relation between amplitude of the absorber autocorrelation function and neutral hydrogen column density. In addition, it predicts a correct evolutionary trend of correlation amplitudes. Some possible ramifications of these results are discussed.     

Molecular hydrogen in damped Lyα systems: clues to interstellar physics at high redshift

In order to interpret H₂ quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, we model their H₂ abundance as a function of dust-to-gas ratio, including H₂ self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H₂ data. Using H₂ excitation data for DLAs with H₂ detections, we derive a gas density  1.5 ≲ log n (cm⁻³) ≲ 2.5  , temperature  1.5 ≲ log T (K) ≲ 3  , and an internal ultraviolet (UV) radiation field (in units of the Galactic value)  0.5 ≲ log χ≲ 1.5  . We then find that the observed relation between the molecular fraction and the dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H₂ deficient DLAs and subDLAs with H₂ fractions less than  ∼10⁻⁶  are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H₂ fraction  (≲10⁻⁶)  observed for  κ≲ 1/30  (κ is the dust-to-gas ratio in units of the Galactic value); H₂ self-shielding causes a rapid increase in, and large variations of, H₂ abundance for  κ≳ 1/30  . We finally propose an independent method to estimate the star formation rates of DLAs from H₂ abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed.