Selection Criteria

A pre–selection of possible targets for spec-troscopy for all data sets was performed within the ESO-MIDAS (Munich Image Data Analy-sis System) environment, whereas the final con-struction of the setups for the field galaxies (sec-tion2.2) was carried out with the FORS Instru-mental Mask Simulator (FIMS). For construct-ing the MOSCA masks, special MIDAS pro-grams, kindly provided by Prof. J. W. Fried, were utilised. Different catalogues containing

4LX between 0.1–2.4 keV, H0 = 100 km s⁻¹Mpc⁻¹, see Table2.3.

Chapter 2: Sample Selection and Observations 25

the positions, magnitudes, structural parame-ters, morphologies et cetera of the target gal-axies were created within MIDAS and used as input for setting up of the masks and visualising the slits.

The aim of the A 2390 project is to investigate the stellar populations of a large sample of early-type cluster galaxies spanning a wide range in luminosity. However, in order to enable a good sky subtraction which requires long slit lengths (with an average length of 22⁰⁰), each mask was constrained to only about 20 galaxies (with typi-cal galaxy sizes of 4⁰⁰) in total. For this reason, we were very careful to select only galaxies which were likely to be cluster members based upon their U BI broad-band colours. Target objects were selected on the basis of the ground-basedI -band images and a combination of defined colour regions. The selection procedure was very sim-ilar to the cluster galaxies in A 2218 (Ziegler et al. 2001a). Using the existing catalogue for the field (Smail et al. 1998), the distribution of gal-axies on the (U −B) −(B −I) colour-colour plane was investigated, after a correction for blue field galaxy contamination by∼28% atI = 20^m with a deepU exposure in a high-latitude blank field was performed. The colour-colour diagram was compared to the locus of colours expected for non-evolved Spectral Energy Distributions (SEDs) representative for the spectral types of galaxies with E/S0, Sab, Sbc, Scd and Sdm mor-phologies in the local Universe, as they would be observed at z = 0.23 (see Smail et al. 1998 for further details). A region in U BI colour space was defined which was most likely occupied by cluster members. Galaxies falling outside a colour region, defined as 2.60 <(B −I) <3.50 and −0.2<(U −B)<0.40, were rejected. The width of this colour range puts negligible restric-tions on the stellar popularestric-tions of the selected objects, but eliminates the majority of back-ground galaxies. Combined with the richness of the cluster this approach ensured a high rate of success in targeting cluster members in our

spec-Figure 2.1: (B −I)–I colour magnitude diagram from the Hale imaging for bright galaxies lying in a 9.7⁰×9.7⁰ (2.12 Mpc) region centered on A 2390. The sequence of red cluster members is readily seen ex-tending down to I ∼21^m (M_B ∼ −19^m). The box encompasses galaxies matching the selection criteria, while squares denote those galaxies which were even-tually observed spectroscopically.

troscopic sample (4 non–members out of 52 tar-gets). The colour-selected sample of galaxies in-cludes 122 galaxies with 15.53^m< I_tot<19.14^m within the 9.7⁰ ×9.7⁰ area covered by the Hale images. Fig. 2.1 illustrates the selection in the (B−I)−I colour-magnitude diagram for galax-ies brighter than I = 23.5 mag within the Hale field–of–view. The box encompasses all galaxies matching the selection criteria, while those gal-axies which were actually observed are indicated as squares. The magnitude limit for the selection of spectroscopic targets was therefore adopted as Itot = 19.14^m.

A catalogue of the colour-selected galaxy sam-ple was utilised as an input for the mask de-sign preparation. Two masks contained gal-axies with an average I-band surface bright-ness within the spectroscopic slit brighter than µ_I = 21.0 mag arcsec⁻². In a third mask fainter

galaxies with surface brightnesses as faint as µ_I = 21.5 mag arcsec⁻² were included. In split-ting the galaxies between the individual masks in this way, enabled to vary the cumulative ex-posure times of the masks to ensure that a similar signal–to–noise of about 30 per ˚A (in the continuum at λ_obs = 6300 ˚A) was ob-tained in each galaxy spectrum. Particular care was taken to maximise the number of galax-ies with early-type morphologgalax-ies (E–S0–S0/Sa) within the HST field. For this reason, the po-sition angles of the masks were aligned accord-ingly to fulfill this requirement. With a total of 56 selected galaxies, a sub-population of 25%

will have sufficient members to construct an in-dependent FP and Mg–σ relation for different galaxy populations. This required number of galaxies was fulfilled by selecting galaxies down to V = 20.1^m (0.5 mag fainter than L^∗) which show typical averageR-band surface brightness-es of hµ_Ri ≈20.6 mag arcsec⁻², assuming an in-tegration over a spatial region of 1.5⁰⁰. In order to study the evolution of early-type galaxies out to large clustercentric distances, target galaxies were selected covering the whole MOSCA field–

of–view of ∼10⁰ ×10⁰ (see Fig. 4.1). Each slit was allocated to a galaxy of interest manually.

An average slit length of 22⁰⁰ was used (for a few exceptions it was set to a minimum length of 15 arcsec) and the position of the slit was also chosen so that fainter companion galaxies did not obscure the sky and an accurate sky subtraction was possible.

A first target selection and determination of world coordinates of the sample of poor clusters was performed using the ESO skycat tool. In a second step, the MOS masks for the MOSCA in-strument were constructed with special MIDAS programs. The central coordinates of the field of interest (in α and δ), target positions with precision <0.3 arcsec, the slit positions and the length of slits were computed. The slit widths were set to 1.5 arcsec. Afterwards these defini-tions were transformed to α and δ coordinates

of the same equinox on the plane sky (including a rotation of the coordinates according to a given position angle) and for a spectral coverage of the used grism and visualised via a graphic version of the mask. Examples of final MOSCA masks are illustrated in Fig.2.2.

Finally, individual configuration setups for com-puting a mechanical MOS plate mask for MOSCA were created. These program files were sent to the mechanical shop at the MPIA (Hei-delberg) which manufactured the final MOSCA plates. For the construction of configuration files for the mechanical plates special CNC machine programs were used. Before the observations, these masks are supplied by each observer and put into the aperture unit. A maximum of two masks are accessible via the GUI interface but one plate can be changed during the night if necessary. The orientation of the mask in the MOSCA instrument is unique and thus fixed.

To align the mask and targets, relatively bright stars (V ≈ 19^m) were selected and distributed over the whole MOSCA field. Their positions can be measured precisely even on short expo-sures (∼10 sec). A small but important improve-ment in the programs was the use of three (in-stead of usually at least two) reference stars for the mask alignment. An additional star warrants an even higher accurate positioning and align-ment also for lower horizontal distances. The holes for the stars have a radius of 5 arcsec (see Figs. 2.2 and 2.3). In taking short exposures through these holes and measuring the positions of the stars, Cassegrain flange angle and tele-scope position can be corrected which results in an improvement of the mask alignment. This tricky alignment of the MOS mask is visualised in Fig.2.3. For each mask and exposure, the po-sition of the galaxies and alignment stars had to be verified to avoid loss of galaxy fluxes. Since the positions of holes with 10 arcsec diameter can not be measured precisely enough on images, ad-ditional reference holes with 1 arcsec radius are slightly offset put from the large holes in the

Chapter 2: Sample Selection and Observations 27

Figure 2.2: MOSCA masks for the poor clusters Cl0849 and Cl1701/Cl1702 seen from below (i.e., the upper surface is the focal plane). The images visualise the whole MOS mask (solid line) with all slits and the field of MOSCA is indicated as a dotted square. The position angle PA of MOSCA is defined in the usual way (i.e. zero degrees, which is normally used for direct imaging, gives direct images in the standard orientation. The values of Cassegrain flange angle have to be rotated by 90^◦, i.e. Cl0849 has an angle of PA=0^◦ and Cl1701 has PA=+90^◦. Note that for Cl1701 the slits are in East-West direction.

Figure 2.3: Mask alignment image for Cl0849. An exposure of ∼90 sec was taken through the slits to verify that all galaxies are accurately positioned onto their slits and the stars positioned in their holes. The lengths for two slits are indicated. The PSF of this image is 1.1⁰⁰ FWHM. The vertical line are six bad columns located on the CCD chip between 1540–1551 pixel.

mask. The positions of these smaller reference holes can be measured precisely.

The basic selection criteria for the poor clus-ters was different to the rich clusclus-ters. Thanks to our previous study of the poor clusters based on MOSCA and LDSS-2 low–resolution spec-troscopy using the grism green 500 and the medium-blue grism (Balogh et al. 2002b), for a total of 317 galaxies spectroscopic redshifts were available. By using these measurements and the catalog of 172 Low–LX cluster members, the target selection for the follow–up spectroscopy could be performed very efficiently.

In order to fill each mask, several catalogues of early-type candidates were constructed. One catalogue contained all objects (spiral and early-type galaxies) with low–resolution spectroscopy, another one held galaxies with absorption lines at cluster redshift (i.e. passive early–type spec-tra) and another list only galaxies with struc-tural information (i.e. residing in the HST field).

Furthermore, galaxy catalogues suitable for fill–

up objects (at different redshifts than the cluster or without any spectral information) were cre-ated. All catalogues were loaded into the ESO skycat software simultaneously and marked with different symbols (see Figs 2.4 and 2.5). For this reason, the final object selection could be performed time–efficiently. As the clusters com-prise relatively few members in comparison to a rich cluster and only one mask was constructed for each cluster, particular care was taken to se-lect only cluster members which show early–type spectral features in their low–resolution spectra.

Furthermore, the position angles of the masks were chosen in such a way that the number of galaxies with early–type morphologies (E–S0–

S0/Sa) selected within the HST field was maxi-mum. For the outermost slits located at the mask edges, the coverage of the catalogues was poor or even null. Therefore, anonymous objects had to be selected as fill–up objects. In this situ-ation, preference was given to galaxies which fea-tured similar apparent luminosities as the cluster galaxies. However, for all setups the number of anonymous galaxies was not larger than three.