Damped Lyα absorbers from dwarf galaxy ejecta

It is argued that the formation of a dwarf galaxy causes a massive burst of star formation, resulting in the ejection of most of the available gas from the galaxy as a weakly collimated wind. The ejected gas can give rise to a damped Lyα absorber (DLA). Weakly collimated outflows naturally explain the asymmetric profiles seen in low-ionization absorption lines caused by heavy elements associated with DLAs, where absorption is strongest at one edge of the absorption feature. The shape of the distribution of column densities in the model agrees reasonably well with observations. In particular, the break in slope is caused by external photoionization of the wind. A semi-analytical model for galaxy formation is used to show that, for currently acceptable cosmological parameters, dwarf galaxy outflows can account for the majority of DLA systems and their distribution with redshift. This model also predicts a correlation between velocity structure and metallicity of DLA systems, in qualitative agreement with observations. DLAs do not require many large, rapidly rotating disc galaxies to have formed early on, as in other models for their origin.     

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.     

Encounters between spherical galaxies – II. Systems with a dark halo

We perform N -body simulations of encounters between spherical systems surrounded by a spherical halo. Following a preceding paper with a similar aim, the initial systems include a spherical Jaffe model for the luminous matter and a Hernquist model for the halo. The merger remnants from this sample are mainly slowly rotating, prolate spheroids with a radially anisotropic velocity distribution. The results are compared with real-life ellipticals and with the models without halo in Paper I. We argue that elliptical galaxies with evidence of dark matter could be formed in the field via a merger of spheroids surrounded by a dark matter halo, while ellipticals with no evidence of dark matter might be formed via a merger of two spheroids in a cluster.