Astron. Astrophys. 332, 514-522 (1998)

2. Observations and reduction

The spectra were taken on September 1, 1995 (UT) using the Keck 10m telescope in combination with the Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) with a TEK 2048² CCD. The night was cloudy and we were only able to observe for a few of hours at the end of the night when the clouds thinned sufficiently to warrant observing. We used LRIS in two configurations. For the observations of PKS 2338+042, we used with the 600 l m^-1 grating (blazed at 5000Å). The resulting spectrum covered the spectral range 3860 - 6430Å and had a resolution of about 4.5Å (as measured using the night sky lines). This range was sufficiently large to include the Ly , CIV 1549, and HeII 1640 emission lines. The final spectrum of PKS 2338+042 is a result of averaging three separate exposures of 1800 seconds duration. The slit for these observations was oriented along PA= and was selected to be along the most extended position angle of the Ly emission as revealed in the images of Heckman et al. 1991aand approximately along the position angle of the most extended radio emission. For the observation of PKS 0445+097, we used the 300 l m^-1 grating (blazed at 7500Å). The resulting spectrum covered the spectral range 3980 - 9040Å and had a resolution of 11Å FWHM (as measured using the night sky lines). The spectral range was sufficient to include CIV 1549, HeII 1640, CIII] 1909, and MgII 2798. We were only able to obtain one 1800 second exposure of PKS 0445+097 due to increasing cloudiness. The slit was oriented along PA= which was chosen such that the slit would pass through the nucleus and the blob of continuum emission seen in the HST images of PKS 0445+097 in Lehnert et al. 1997a. Both quasars were observed through a 1" wide slit. The scale of the CCD is 0.22" pixel^-1. The seeing during the observations was about 1".

The spectra were reduced in the standard way using IRAF. The data were bias subtracted using the values of the overscan region, flat-fielded using several exposures of the internal lamp taken immediately after the observations, and flux-calibrated (to remove the response only since the night was not spectroscopic) using observations of Hz4. The spectra were wavelength calibrated using exposures of the Ne, Hg, and Kr lamps. The parameters of the lines (center, width, and flux) were then estimated by fitting a Gaussian profile using the IRAF task SPLOT. The Ly emission in the longslit spectrum of PKS 338+042 was obviously spatially extended. We measured the properties of the extended emission in sums of 4 CCD rows which is about 0.9" in projection on the sky. Since the seeing was 1", these extractions provide independent information. Error estimates were provided by SPLOT assuming that the noise in each pixel was just the root mean square of the noise in the surrounding continuum (see the help for SPLOT). The uncertainties in the fit estimated this way probably slightly under-estimates the true uncertainty of the fit. Generally, for the analysis of the extended line emission, we excluded spatial regions of the spectrum that had strong contamination due to line emission from the nucleus. Thus none of our results rely on the rather difficult and uncertain method of subtracting off a template nuclear spectrum in order to reveal the narrow extended emission line component.

Since the night was not photometric, we have compared our emission line fluxes with those obtained through ground-based narrow-band imaging from Heckman et al. 1991a. We find that our measured Ly for PKS 2338+042 is about a factor of 4 lower than measured during photometric conditions by Heckman et al. 1991a. Along similar lines of reasoning, we have used the HST F555W (similar to "V") image of PKS 0445+097 to estimate the how much of the flux we may have missed due to the cloudy weather. We find little difference (within about 20%) between the flux estimated from the HST image and that of the Keck spectrum of PKS 0445+097 (the clouds were patchy).

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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