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3.2 Spectroscopy of Arcs

It is important to get spectroscopic redshifts of the arcs because these ultimately confirm their true gravitational origin. Indeed, spectra give the distance of the source Ds which is necessary to have an estimate of the total mass in the innermost regions of clusters (mass scaling, see Eq. 7 and 8). Spectroscopy also gives us an idea of the redshift distribution of background sources (part 5). It can also provide very valuable information on the stellar content of very faint distant galaxies that would be unobservable without the magnification of the images. However, since the surface brightness of arcs is extremely low (µB > 25 arcsec square), only the brightest 30% have been successfully measured. There is a natural trend to obtain redshifts only from emission line sources, provided the emission lines (mainly [OII]lambda3727) fall between bright sky emission features. This explains why there are ``privileged'' redshifts at approx 0.7, approx 0.9 and approx 1.1 (see Table 1). A new spectroscopic method, the so-called ``va et vient'' spectroscopy, has been proposed to obtain low resolution spectra, with better signal-to-noise ratios of such faint objects with VLTs (Cuillandre et al. 1994).

Table 1. Summary of the present status of arc surveys in early 1994. We only report on clusters for which images or papers have been presented somewhere. The first column gives the cluster name, the second and third column are the cluster and arc redshifts, if any. The last two columns give the references for the arc announcement and for the redshift measurement of the arc.
Cluster zc zs Ref. discovery arc Ref. redshift arc
A2104 0.155 - Pierre et al. 1994 -
A2218 0.176 0.702
1.034 		Pelló et al. 1992
Pelló et al. 1992 		Pelló et al. 1992
Pelló et al. 1992
A1689 0.18 - Tyson et al. 1990 -
MS0440+02 0.19 0.53 Luppino et al. 1993 Luppino et al. 1994
A2163 0.201 0.742
0.728 		Soucail et al. 1993
Soucail et al. 1993 		Soucail et al. 1993
Soucail et al. 1993
A963 0.206 0.771 Lavery and Henry 1988 Ellis et al. 1991
A1942 0.22 - Smail et al. 1991 -
MS1006+12 0.221 - Le Fèvre et al. 1994 -
A2390 0.231 0.913 Pelló et al. 1991 Pelló et al. 1991
A2397 0.24 - Smail et al. 1991 -
MS1910+67 0.246 - Le Fèvre et al. 1994 -
MS1455+22 0.259 - Le Fèvre et al. 1994 -
Cl0017 0.272 - Infante et al. 1991 -
S295 0.299 0.93 Le Borgne et al. 1992 Mellier et al. 1994
MS1008-12 0.301 - Le Fó et al. 1994 -
AC118 0.31 - Smail et al. 1991 -
AC114 0.31 - Smail et al. 1992 -
MS2137-23 0.313 -
** 		Fort et al. 1992
Fort et al. 1992 		-
-
Cl0500-24 0.316 0.913 Giraud 1988 Giraud 1992a
GHO 2154+0508 0.32 0.721 Lavery et al. 1993 Lavery et al. 1993
Cl2244-02 0.331 2.237 Lynds and Petrosian 1986 Mellier et al. 1991
Q0957+561 0.36 - Bernstein et al. 1993 -
A370 0.375 0.724
1.305 		Soucail et al. 1987
Fort et al. 1988 		Soucail et al. 1988
Mellier et al. 1991
Cl0024+17 0.39 (1-2) Koo 1988 Mellier et al. 1991
MS0302+17 0.423 - Mathez et al. 1992 -
MS1621+27 0.426 - Luppino and Gioia 1992 -
Cl1409+52 0.46 - Tyson et al. 1990 -
MS0451-03 0.55 -
- 		Le Fèvre et al. 1994
Luppino et al. 1994 		-
Cl2236 0.56 1.11 Melnick et al. 1992 Melnick et al. 1992
MS2054-04 0.583 - Luppino and Gioia 1992 -
3C 200.1 0.620 - Dickinson 1993 -
Q2345+01 (?)* - Bonnet et al. 1993 -
* An arc without any optical rich cluster associated. ** First radial arc detected.

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