2 Spectral observations and data reduction

2.1 Observations

The results presented here were obtained in a snap-shot observing mode during two runs with the Calar Alto 2.2 m and one run with the KPNO 4 m telescopes (see Table 2).

2.2 Observations with the KPNO 4m telescope

The observations were carried out with the Ritchey-Chretien Spectrograph attached to a Tektronix 2K$\times$2K CCD detector. We used a 2''$\times$205'' slit with a KPC-10A grating (316 grooves mm^-1) in its first order, and a GG 375 order separation filter cutting off second-order contamination for wavelengths blueward of 7400 Å. This instrumental setup allows a spatial scale along the slit of $0\hbox{$.\!\!^{\prime\prime}$ }69$ pixel^-1, a scale perpendicular to the slit of 2.7 Å pixel^-1, a spectral range of 3700-8300 Å and a spectral resolution of $\sim$7 Å (FWHM). Short exposures (3-5 min.) were used in order to detect strong emission lines to allow measurement of redshifts and a crude classification. No orientation of the slit along the parallactic angle was done because of the snap-shot observing mode. Reference spectra of an Ar-Ne-He lamp were recorded to provide wavelength calibration. Spectrophotometric standard stars from Oke (1990) and Bohlin (1996) were observed at the beginning and at the end of each night for flux calibration. The dome flats, bias, dark and twilight sky frames were accumulated each night. The weather conditions were photometric, with seeing variations between 2 $.\!\!^{\prime\prime}$5 and 3 $^{\prime\prime}$ (FWHM).

 

 

Table 1: Summary of the samples observed and breakdown of the classifications after follow-up spectroscopy.

Candidate Sample		N	BCG	Other	QSO	Galaxies	Stars	Not
			&	ELGs		without		Classified
			BCG?	& SA		emission
First priority	new	141	64	43	5	3	18	8
	already known	28	21	7	-	-	-	-
	total	169	85	50	5	3	18	8
Random sample	new	16	3	1	-	1	2	9
	already known	1	-	1	-	-	-	-
APM selected sample	new	22	10	10	-	-	1	1
	already known	1	-	1	-	-	-	-
Second priority	total	40	13	13	-	1	3	10
Objects presented in this paper		209	98	63	5	4	21	18

   

Table 2: Journal of observations.

Date	Telescope	Instrument	Grating,	Wavelength	Dispersion	Observed
			grism	range [Å]	[Å/pixel]	number
(1)	(2)	(3)	(4)	(5)	(6)	(7)
09.06-18.06.1999	2.2 m CAHA	CAFOS	G-200	3700-9500	4.5	117
--	--	--	B-200	3500-7400	4.7	18*
17.06-20.06.1999	4 m KPNO	R.C.Sp.	KPC-10A	3700-8300	2.8	46
08.12-11.12.1999	2.2 m CAHA	CAFOS	G-200	3700-9500	9.0	46

^* - Objects reobserved.

2.3 Calar Alto 2.2 m telescope observations

Follow-up spectroscopy with the CAHA 2.2m telescope was carried out during two runs (June and December, 1999, see Table 2), using the Calar Alto Faint Object Spectrograph (CAFOS) and Cassegrain focal reducer. During these runs a long slit of $300\hbox{$^{\prime\prime}$ }\times (2\hbox{$^{\prime\prime}$ }-3\hbox{$^{\prime\prime}$ })$ and a G-200 grism (187 Å mm^-1, first order) were used. The B-200 grism (185 Å mm^-1, first order) was also used to reobserve 18 objects in order to improve the [O II]$\lambda$3727 Å value. There were no order separation filters applied. The spatial scale along the slit was $0\hbox{$.\!\!^{\prime\prime}$ }53$ pixel^-1. A SITE 15 2K$\times$2K CCD was operated in a 2$\times$1 binned mode during the December run (binning only along the dispersion direction), while in the June run there was no binning applied. The wavelength coverages were $\lambda\,3700{-}\lambda\,9500$ Å with maximum sensivity at $\sim$6000 Å for the G-200 grism and $\lambda\,3500{-}\lambda\,7400$ Å with maximum sensivity at $\sim$4000 Å for the B-200 grism. The spectral resolution was $\sim$10 Å (FWHM) in the June run and due to CCD binning $\sim$20 Å (FWHM) in the December run. The slit orientation was not aligned with the parallactic angle because of the snap-shot observing mode. The exposure times varied within $2\!-\!15$ min depending on the object brightness. The observations were complemented by standard star flux measurements (Oke 1990; Bohlin 1996), reference spectra (Hg-Cd lamp) for wavelength calibration, dome flat, bias and dark frames. Most of the time the weather conditions were photometric with a seeing $\approx$1.5 $^{\prime\prime}$ (FWHM). Only during two nights the weather conditions were variable with a seeing of $3\hbox{$^{\prime\prime}$ }{-}4\hbox{$^{\prime\prime}$ }$ (FWHM). The measurements of these nights are marked by ``*'' in Table 4.

2.4 Data reduction

Reduction of the CAHA and KPNO spectral data was performed at the SAO using the standard reduction systems MIDAS and IRAF.

The MIDAS command FILTER/COSMIC was found to be a quite successful way to remove automatically all cosmic ray hits from the images. After that we applied the IRAF package CCDRED for bad pixel removal, trimming, bias-dark subtraction, slit profile and flat-field corrections.

To do accurate wavelength calibration, correction for distortion and tilt for each frame, sky substraction and correction for atmospheric extinction, the IRAF package LONGSLIT was used with invoking the IDENTIFY, REIDENTIFY, FITCOORD, TRANSFORM, BACKGROUND and EXTINCTION tasks.

To obtain an instrumental response function from observed spectrophotometric flux standards, the APSUM procedure from the APEXTRACT package was used first to extract apertures of standard stars. Then the sensitivity curve determined by the STANDARD and SENSFUNC procedures was applied by the CALIBRATE task to perform flux calibration for all object images. Finally the APSUM task was used to extract one-dimensional spectra from the flux calibrated images. In case that more than one exposure was obtained with the same setup for an object, the extracted spectra were co-added and a mean vector was calculated. In case of several observations with different setups (telescopes or grisms) for the same object, the data were reduced and measured independently and the more reliable values were taken.

To speed-up and facilitate the line measurements we employed dedicated command files created at the SAO using the FIT context and MIDAS command language. The procedures for the measurement of line parameters and redshifts applied were described in detail in Papers III and IV.