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Using a realistic model for line emission from the broad emission line regions of quasars, we are able to reproduce the previously observed correlations of emission-line ratios with the shape of the spectral energy distribution (SED). In agreement with previous studies, we find that the primary driving force behind the Baldwin Effect (EW ~ L^beta, beta < 0) is a global change in the SED with quasar luminosity, in that more luminous quasars must have characteristically softer ionizing continua. This is completely consistent with observations that show correlations between L_uv, alpha_ox, alpha_uvx, line ratios and EWs. However, to explain the complete lack of a correlation in the EW(NV)--L_uv diagram we propose that the more luminous quasars have characteristically larger gas metallicities (Z). As a secondary element, nitrogen's rapid...
Comment: 35 pages, 8 figures, LaTex, aaspp4. Accepted by ApJ. Minor changes to match accepted version
Comment: 22 pages, Latex, 21 figures, now accepted in ApJ, improved arguments relating to possible systematic effects and more discussion on mean line shapes
I Zw 18 is the most metal poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival HST WFPC2 imaging and FOS spectroscopy we were able to improve previous studies. We constructed a continuum free HeII map, which was used to identify Wolf-Rayet (WR) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the HeII emission is associated with the NW stellar cluster, clearly displaced from the surrounding shell-like [OIII] and Halpha emission. We found evidence for HeII sources, compatible with 5--9 WNL stars and/or compact nebular HeII emission, as well as residual diffuse emission. Only one of them is outside the NW cluster. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate met...
Comment: revised version, 6 pages, 1 figures in MNRAS LaTex style
Comment: very strongly revised analysis of GRB971214 and discussion, submitted to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included)
Comment: 19 pages, 24 figures, submitted to MNRAS, also available in http://www.ast.cam.ac.uk/~ciliegi/elais/papers
Comment: submitted to MNRAS, Feb. 1998, 10 pages, 3 figures in MNRAS LaTex style
Comment: 16 pages, 5 figures, submitted to AJ
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