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4 Sample characteristics

The coordinates, apparent magnitudes and redshifts of the extragalactic objects are listed in Table 1. The projected impact parameters (Col. 6) range from 0.8 to about 4.7 h-1 Mpc, and therefore probe the cluster from the inner Abell radius all the way out to the low-density environment and large-scale filaments. The wavelength and flux calibrated 1-D spectra are displayed in Fig. 1. The mean signal-to-noise ratio has been computed as a function of wavelength from the photon statistics after subtraction of the spectral lines in the normalized spectrum.

  
\begin{figure}
\includegraphics [height=20cm,clip]{H1345f1.1.ps}\end{figure} Figure 1: Optical spectra of the extragalactic objects at a resolution FWHM of 1.4 nm: the calibrated flux $F_\lambda$ in units of 10-17 W m-2 nm-1 is plotted against heliocentric wavelength $\lambda$ in nm. The curve below each spectrum is the associated $1\sigma$ standard deviation of the noise in the same flux units

 
\begin{figure}
\includegraphics [height=20cm,clip]{H1345f1.2.ps}
\end{figure} Figure 1: continued

 
\begin{figure}
\includegraphics [width=6.5cm,clip]{H1345f1.3.ps}
\end{figure} Figure 1: continued
Besides the stellar objects (which are dealt with in a separate paper), we discovered 16 new emission-line objects, a BL Lac candidate and an elliptical galaxy. We confirmed the redshift of the QSO US 370. The emission-line objects consist of 9 quasars, 3 Seyfert 1 and 4 HII-region galaxies (see below). We included HS 1309+2605 as a BL Lac candidate because none of the usual stellar absorption features could be detected in the slit spectrum (see Fig. 1). However there has been no radio detection ([Kim etal. 1994]; [Kim 1994]) and the signal-to-noise ratio in our optical spectrum is insufficient for a final decision. The normal galaxy HS 1312+2651 was originally classified as a "Narrow Emission" object according to the objective-prism spectrum, but the emission feature could not be confirmed.

Twelve emission-line objects show broad permitted emission lines and should be quasars or Seyfert 1 galaxies. We adopt the definition by [Véron-Cetty & Véron (1998)] of a quasar as a star-like object, or an object with a star-like nucleus, brighter than the absolute B-band magnitude of -21.5 $+5\log h$. We have made a distinction between QSOs and Seyfert 1 galaxies on the basis of this definition, and calculated the absolute B-band magnitude MB of all the objects in Table 1 (Col. 10). Note that no K-correction was applied since it is usually of the same order as the uncertainties in the photometry and/or it does not affect the classification.


  
Table 1: The sample of confirmed extragalactic sources

\begin{table}
$^{\rm a}$\space Adopting $<$$z_{\rm Coma}$$\gt$\space $=6853$\spa...
 ...}$\space No K-correction is applied.\\ $^{\rm d}$\space Usher (1981).\end{table}

Among the Seyfert galaxies, we find several sub-types. RXJ1252.6+3002 belongs to the class of narrow-line Seyfert 1 galaxies [(Boller etal. 1996)]. The intrinsic FWHM of H$\beta$, around 1150 km s-1, is typical for such objects, and it is only slightly larger than that of [OIII]$\lambda$500.7. Moreover, the flux ratio of [OIII]$\lambda$500.7 to H$\beta$ is less than unity which rules out the possibility that it is a Seyfert 2 [(Shuder & Osterbrock 1981)]. Additional support for this classification comes from the detection of strong blends of FeII lines in the regions around 457.0 nm and 519.0 nm [(Koski 1978)]. HS 1302+2629 also shows relatively narrow lines, but given its FWHM at H$\beta$ of $\simeq 2350$ km s-1, it has to be classified as a classical Seyfert 1 galaxy. Although HS 1312+2735 is obviously a Seyfert 1 galaxy, a Seyfert 1.5 type [(Netzer 1990)] is still possible given the shape of the H$\beta$ line.

The intrinsically faintest objects HS 1249+2620, HS 1257+2732, HS 1303+2908 and HS 1312+2644 only show narrow permitted emission lines. In Sect. 5 below, we show that these narrow emission-line galaxies (NELGs) are indeed HII region (i.e. starburst) galaxies.


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