Showing papers by "I. M. Hook published in 2004"

A survey for low‐luminosity quasars at redshift z∼ 5

Abstract: We present the results of a multicolour (VIZ) survey for low-luminosity (M B < - 23.5) quasars with z ∼ 5 using the 12K CCD mosaic camera on the Canada-France-Hawaii Telescope (CFHT). The survey covers 1.8 deg 2 to a limiting magnitude of m z = 22.5(Vega), about two magnitudes fainter than the SDSS (Sloan Digital Sky Survey) quasar survey. Twenty candidates were selected by their VIZ colours, and spectra for 15 of these were obtained with GMOS on the Gemini North telescope. A single quasar with z = 4.99 was recovered; the remaining candidates are all M stars. The detection of only a single quasar in the redshift range 4.8 < z < 5.2 accessible to the survey is indicative of a possible turnover in the luminosity function at faint quasar magnitudes, and a departure from the form observed at higher luminosities (in agreement with quasar lensing observations by Richards et al.). However, the derived space densities of quasars more luminous than M B (Vega) < - 23.5, of 2.96 x 10 -7 Mpc -3 is consistent at the 65 per cent confidence level with extrapolation of the quasar luminosity function as derived by Fan et al. at m i < 19.6(Vega).

The Gemini Deep Deep Survey: III. The Evolution of Galaxy Stellar Masses

Abstract: We present the observed evolution in the stellar mass densit y locked up in giant galaxies over 0.8 < z < 2 measured by the near-infrared selected, spectroscopicall y complete, Gemini Deep Deep Survey. This is deeper and covers more area than previous surveys and we find a great a bundance of massive, old galaxies at z >> 1. The cosmological mass density in the most massive objects ( M⋆ > 10M⊙) declines only very slowly to z = 2. In contradiction to the paradigm of standard hierarchical formation based models, the bulk of the stellar mass in large galaxies was assembled at high reds ifts rather than recently. We find no evidence for a dependence of the evolutionary rate on mass, again in co ntrast with a hierarchical assembly picture. The mass density follows an evolutionary path whose shape ag rees well with that independently determined from integration of previous measurements of cosmic star-f ormation history. This represents the first precision measurement of mass assembly over 1 < z < 2 and demonstrates that new ingredients are essential in mode ls of galaxy formation to push back the epoch of star-formation in giant galaxies to higher redshifts. Subject headings: galaxies: evolution