Browsing by Author "Lewis, GF"

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    Luminous early-type field galaxies at z∼0.4 -: II.: Star formation history and space density
    (2002) Willis, JP; Hewett, PC; Warren, SJ; Lewis, GF
    We present a combined photometric and spectroscopic analysis of the star formation history and space density of a sample of 485 luminous, M-V - 5 log h < -20.5, field early-type galaxies at redshifts 0.3 ≲ z ≲ 0.6. The observed b(J)ori colours as a function of redshift, mean absorption-line strengths and [O II] λ3727 emission properties are used to constrain the star formation history of the galaxies. The mean star formation history of the early-type galaxy sample is consistent with an old (z(f) > 1), passively evolving luminosity-weighted stellar population. 21 per cent of the sample possess detectable [O II] lambda3727 emission consistent with a low level (less than or similar to1 M-circle dot yr(-1)) of on-going star formation. Parametric and non-parametric estimates of the space density of the sample are derived. The integrated luminosity density at z similar or equal to 0.4, allowing only for passive luminosity evolution, is in excellent agreement with the local, ((z) = 0.1), luminosity density of early-type galaxies. Overall, the sample properties are consistent with a galaxy formation scenario in which the majority of luminous field early-type galaxies formed at redshifts z > 1 and have largely evolved passively since the formation epoch.
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    The sizes and kinematic structure of absorption systems towards the lensed quasar APM08279+5255
    (2004) Ellison, SL; Ibata, R; Pettini, M; Lewis, GF; Aracil, B; Petitjean, P; Srianand, R
    We have obtained spatially resolved spectra of the z(em)=3.911 triply imaged QSO APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We study the line of sight equivalent width (EW) differences and velocity shear of high and low ionization absorbers (including a damped Lyman alpha [DLA] system identified in a spatially unresolved ground based spectrum) in the three lines of sight. The combination of a particularly rich spectrum and three sight-lines allow us to study 27 intervening absorption systems over a redshift range 1.180%) over kpc scales. A minimum radius for strong (EW>0.3Angstrom) Mg II systems of >1.4 h(70)(-1) kpc is inferred from absorption coincidences in all lines of sight. For weak Mg II absorbers (EW<0.3 angstrom), a maximum likelihood analysis indicates a most probable coherence scale of 2.0 h(70)(-1) kpc for a uniform spherical geometry, with 95% confidence limits ranging between 1.5 and 4.4 h(70)(-1) kpc. The weak Mg II absorbers may therefore represent a distinct population of smaller galaxies compared with the strong Mg II systems which we know to be associated with luminous galaxies whose halos extend over tens of kpc. Alternatively, the weak systems may reside in the outer parts of larger galaxies, where their filling factor may be lower. By cross-correlating spectra along different lines of sight, we infer shear velocities of typically less than 20 km s(-1) for both high and low ionization absorbers. Finally, for systems with weak absorption that can be confidently converted to column densities, we find constant N(C IV)/N(Si IV) across the three lines of sight. Similarly, the [Al/Fe] ratios in the z(abs)=2.974 DLA are consistent with solar relative abundances over a transverse distance of similar to0.35 h(70)(-1) kpc.