2.2 Field Sample
2.2.3 FORS Configuration
The spectroscopic observations of the sample of FDF field ellipticals were performed simultane-ously with those of the late–type galaxies in
the FDF which were subject to the investiga-tion of the evoluinvestiga-tion of the Tully–Fisher rela-tion (Ziegler et al. 2002; B¨ohm et al. 2004).
A full description of the instrumental setup is given in B¨ohm (2003). Here, only the setup for the investigation of the field elliptical gal-axies is described. In addition, a second sam-ple of field early-type galaxies selected from the WHDF was observed together with a project studying the Tully–Fisher relation of late–type galaxies (B¨ohm & Ziegler 2006) under similar conditions with the VLT at Cerro Paranal. As the sample selections and instrument configura-tions were very similar the two individual data sets are discussed in combination.
In order to construct a large spectroscopic sam-ple with less amount of observing time, the Multi Object Spectroscopy (MOS) Mode was chosen to be the best option. The Mask eXchangeable Unit (MXU) mode of FORS2 was non existent and thus the FDF observations in 2001 were re-stricted to the usage of FORS1. For an effective filling of the MOS masks, the early-type galax-ies were observed simultaneously with the spiral galaxies. Both versions of FORS instruments of-fer 19 slitlets in the MOS configuration, 9 with slit lengths of 22 arcsec, 8 with slit lengths of 20 arcsec, whereas the uppermost and lowermost slits have lengths of∼11 arcsec.
FORS was operated at standard resolution setup, i.e. for the configuration of the CCDs (read out in one–port mode) and the collimator the default values of the MOS mode were used.
The spatial scale was 0.2 arcsec/pixel and in low gain mode (using port A) FORS1 offers a gain of 3.51e−/ADU and a read-out-noise (RON) of RON=7.21e−. The grism 600R was an optimal choice to achieve a medium resolution within the resulting wavelength range appropriate for spi-ral galaxies at z ≤ 1.0 that covers the spectral ranges of the [OII] 3727 doublet or the Hβ and the [OIII] 5007 emission lines. The default order separation filter GG435+81 was used. A slit po-sitioned in the center of the CCD chip covers a
Chapter 2: Sample Selection and Observations 33
wavelength range between 5200 ˚A≤λ≤7400 ˚A.
However, as the starting and the ending wave-length depend on the position of the slit on the X axis, the wavelength range can vary between two extremes of the MOS mask 6300 ˚A≤λ≤8500 ˚A (extreme left hand side) or 4100 ˚A≤λ≤6300 ˚A (extreme right hand side).
Each MOS slit can be set individually to slit widths between 0.3 arcsec and 60 arcsec. For all observations a fixed value of one arcsec-ond was chosen, yielding a spectral resolution of R≈1200 with the 600R grism. Although a smaller slit width would have increased the in-strumental resolution, a significant loss of flux would have occurred for seeing conditions above FWHM≈0.8 arcsec, which roughly corresponds to the median seeing at Paranal since 1998 ac-cording to the Differential Image Motion Moni-tor (DIMM).
Between October and November 2001 an up-grade of the FORS2 CCD system was installed at UT4 (Yepun) for testing purposes, which was available for science use in March 20025. The new FORS2 upgrade detector system consists of two 2048×4096 pixel MIT/LL CCID-20 CCD chips (15µm pixel size). The MIT CCDs pro-vide much higher response in the red wavelength range (>8000˚A), with impressively low fringe amplitudes. At default (standard) setup, the new chip has a spatial scale of 0.25 arcsec/pixel with a gain of 0.70e−/ADU (in high gain 100 kHz mode). The RON differs slightly be-tween the chips, for the “master” chip (“Thor”) RON=2.7e− and the “slave” chip (“Belenos”) RON=3.0e−. Moreover, the volume phased holographic grism 600RI for FORS2 turned out to be even more efficient than the grism 600R, particularly at redder wavelengths. Again, the default order separation filter GG435+81 was used.
In order to get an S/N∼16 over an area of 100×0.2500 at λc ≈ 6552 ˚A in the continuum
5FORS2 upgrade: www.eso.org/projects/odt/-Fors2/Fors2u.html
for a typical early-type galaxy with brightness R≤20.5m, the on–line ESO Exposure Time Cal-culator (www.eso.org/observing/etc/), sug-gested an integration time of ∼2.5 hrs with the instrumental configuration as given above.
2.2.4 Selection Criteria
After a pre–selection of possible spectroscopic field target objects, different galaxy catalogues were generated within the ESO-MIDAS environ-ment and used as input for setting up and con-structing the MOS masks with the FIMS tool.
The FIMS package output consisted of three files per mask. For example, these give definitions on the position, angle and orientation of the mask on the plane of the sky, the positions of the indi-vidual slits, coordinates of reference stars in the field–of–view (for positioning of the mask during the observations with an accuracy <0.1 arcsec), et cetera. In turn, these output files were used for the construction of the so–called Observing Blocks (OBs) which act as input to the operat-ing system of the VLT telescope in the execu-tion of the observaexecu-tions at Paranal for the field sample. For a more extensive description of the operation environment, the reader is referred to the “FORS1+2 User’s Manual” released by ESO (www.eso.org/instruments/fors/doc/).
The object selection for the FDF sub-sample was based on the deep U BgRI photometry of the FDF (Heidt et al. 2003). The 50% complete-ness limits for point sources as derived from the co-added images are 25.64, 27.69, 26.86, 26.68, 26.37 in the U, B, g, R, I filters (Vega-system), respectively. The candidates were selected ac-cording to their spectrophotometric type, their elongated structureless appearance, their lumi-nosity and photometric redshift. Main criterion was the apparent brightness. The other param-eters were taken into account to put additional constraints on the selection. For the early-type targets, this limit in total apparent magnitude wasR ≤22.0m as derived with theMag auto al-gorithm of the Source Extractor package (Bertin
& Arnouts 1996). This constraint was set to en-sure sufficient signal-to-noise of S/N≥10 in the absorption lines which is mandatory for a ro-bust determination of velocity dispersions and line strengths measurements. For this reason, faint early-type candidates with apparent mag-nitudes R > 20.5m were included in more than one MOS setup and the individual spectra com-bined after data reduction.
Spectrophotometric types and estimated red-shifts were selected from the FDF photometric redshifts catalogue of more than 3800 objects in the August 2000 release (Bender et al. 2001).
Only respective candidates with an early (E/S0) model Spectral Energy Distribution (SED) were considered and the photometric redshifts were restricted tozp ≤0.6. Based on the photometric redshifts, the elliptical candidates were spread out on different CCD positions among the spi-rals for each MOS setup in the observed spec-tral wavelength range such that either the Mgb -feature passband (λ0 ≈ 5170 ˚A) or the G-band (λ0 ≈ 4300 ˚A) was included. Both features of interest could not be observed for all candidates because of the correlation of the slit position with the available wavelength window (see also sec-tion 2.2.3 before). Moreover, for an additional star/galaxy separation criterion, targets with in-significant photometric redshift and large uncer-tainty (i.e.,zphot−dzphot ≤0) were rejected.
In contrast to spiral galaxies which require a selection upon inclinations and position angles for the derivation of rotational velocities, for the MOS targets with an early–type SED, nei-ther inclinations nor position angles have been taken into account as selection criteria. How-ever, as a small fraction of early–type galax-ies in the photometric redshift catalogues could be misclassified M–dwarf stars, it was manda-tory to use structural parameters for a ro-bust star/galaxy separation. Detections of the Source Extractor on theI-band image with 0.4900 FWHM which were most likely bonafide stellar objects exhibited a star classification parameter
star≥0.9 and a≈b≈2.5 pixel. Therefore, to avoid misclassifications targets with star≥0.9 ora≈b≤3 pixel have been rejected.
Although the two-dimensional distribution of FDF objects indicated a possible cluster of gal-axies with zp ≈ 0.33 and the primary goal was to target field elliptical galaxies, no preselec-tion against such candidates was performed. In the spectral analysis (see section 5.5), it was confirmed that 15 out of 32 observed elliptical galaxy candidates are most probably members of a cluster. Based on the the radial veloc-ity measurements for these galaxies, the lower limit for the velocity dispersion of the cluster is σc >∼ 430 km s−1. Since the cluster centre is not located on the FDF but only its outskirts, the true σc is probably larger. At a redshift of z = 0.33, the Mg 5170 absorption line is unfor-tunately corrupted due to the terrestrial absorp-tion of the B band. As an accurate measurement of the internal galaxy velocity dispersions is im-possible due to this effect these cluster galaxies were discarded from the analysis.
Apart from photometric redshifts, the target se-lection for the WHDF field early-type galaxies was performed with the same constraints as for the FDF field elliptical galaxies. As no pho-tometric redshifts were available, the redshifts of field early-type candidates were estimated using a combination of colour-colour diagrams and apparent magnitudes. Fig. 2.6 illustrates the target selection of the WHDF field ellipti-cal candidates in the (B −R)–(R−I) colour-colour diagram. The WHDF data and the final WHDF field ellipticals are compared to evolu-tionary tracks for E/S0 with different formation redshifts of zf = 2 (solid line) and zf = 4 (dot-ted line) as predic(dot-ted by the passive evolution models of Bruzual & Charlot 1993; GISSEL96 version, hereafter BC96). Note, that the BC96 models have been computed for a slightly differ-ent cosmology with H0 = 60 km s−1Mpc−1 and q0 = 0.1. In the (B−R)–(R−I) colour-colour di-agram elliptical and spiral galaxies are well
sep-Chapter 2: Sample Selection and Observations 35
Figure 2.6: (B−R)–(R−I) colour-colour diagram for the WHDF field elliptical galaxies. The WHDF spiral candidates (squares) are displayed with the fi-nal WHDF field ellipticals (circles) and compared to evolutionary tracks for E/S0 with different formation redshifts of zf = 2 (solid line) and zf = 4 (dotted line) predicted by the BC96 models. For the BC96 models a cosmology withH0= 60 km s−1Mpc−1and q0= 0.1 was adopted.
arated until z ≈ 0.5 (indicated by the arrows) and follow the predictions of the evolutionary models quite well. The apparent R magnitude was used as an additional constraint to avoid a selection of high redshift elliptical galaxies with z >1. As shown in Fig.2.6, galaxies populating a narrow range in the colour-colour diagram of 2>∼(B−R)>∼2.9 and 0.7>∼(R−I)>∼1.1 have been selected as bona fide field ellipticals and successfully verified. One object (ID # 14) with (R−I) = 0.79, (B−R) = 1.78 and R= 19.04m (z = 0.1060) is an intermediate-type Sc spiral galaxy and was therefore discarded as an early-type candidate. In fact, on the ACS images it turned out that this galaxy is a spiral with a clearly visible disc.
2.2.5 Observations
To construct a distant field elliptical galaxy sam-ple withNobj≈30 within the FDF, roughly 30 % of the reserved observing time for the Tully–
Fisher project were used. However, as half of these galaxies turned out to be possible members of a cluster at z = 0.33, an additional sample of ten elliptical galaxies was selected from the WHDF. The FDF and WHDF target selection for the observations was done by A. B¨ohm and A. Fritz. An outline of the observations of the field early-type galaxies is given in Table 2.4for the FDF sample and in Table2.5for the WHDF galaxies.
The first FDF spectroscopic observations were carried out with FORS1 in December 1999 in Visitor Mode by C. M¨ollenhoff (Heidelberg) and K. Reinsch (G¨ottingen) with a mean DIMM seeing of 0.66 arcsec. However, only one MOS setup was observed and two major drawbacks occurred. At this time the FDF imaging data re-duction was not fully completed yet and the tar-get selection was therefore based on a coadded 1200 sec R-band image (Point-Spread-Function (PSF) of 0.7200 FWHM) gained during commis-sioning time of FORS1. Furthermore, for the selection only a preliminary version of the pho-tometric catalog was available. In total, 3 early-type galaxies, 13 spiral galaxies and 3 objects without any catalog information (without SED andzphot) have been selected. The galaxies clas-sified as “anonymous” turned out to be early-type galaxies withR-band magnitudes of 19.1m, 19.4m and 20.8m. Unfortunately, after the red-shift analysis 3 early-type galaxies were identi-fied to be members of a cluster at z = 0.33.
Therefore, these galaxies were rejected for this investigation.
In September and October 2000 a total of 6 MOS setups were observed by S. Noll and D. Mehlert (both Heidelberg) with FORS2. Each setup was splitted into three single exposures of 3000 sec with a total integration time of 2.5 hours. The targets were selected from a deep FORS2
I-Table 2.4: Observations of FDF early-type galaxies
Tel./Instrument Date θrange airm. DIMM E+S0 anon. Texp [ksec]
VLT/FORS1 12/99 −30◦/ 0◦ 1.08 0.6600 3 3 9.0
P 6
-VLT/FORS2 09/27/00 −90◦/−60◦ 1.21 0.5100 5 1 9.0 09/27/00 −60◦/−30◦ 1.33 0.4300 5 - 9.0 10/04/00 −30◦/ 0◦ 1.40 0.8100 6 1 9.0 10/05/00 0◦/+30◦ 1.36 0.8000 5 2 9.0 10/05/00 +30◦/+60◦ 1.28 0.7400 8 - 9.0 10/06/00 +60◦/+90◦ 1.15 0.6600 7 2 9.0
P 28 6
VLT/FORS1 10/12/01 0◦/+30◦ 1.43 0.7600 6 1 9.0
10/14/01 +30◦/+60◦ 1.07 0.8900 5 0 9.0 10/12/01 +60◦/+90◦ 1.38 0.8200 5 3 9.0
P 10 4
band reference image consisting of 10 seeing av-eraged I-band images (between 0.47–0.50 arcsec FWHM) with a final integration time of 3000 sec and a PSF of 0.49 arcsec FWHM (see B¨ohm 2003 for the construction of the FORS1/FORS2 ref-erence images). All in all 28 E+S0 galaxies were observed from a list of 32 candidates with avail-able spectrophotometric information (see previ-ous section). Eight faint early-type candidates with apparent magnitudes R > 20.5m were ob-served with two MOS setups to increase the re-spective exposure time and the S/N in the final spectra. Two galaxies were already observed in 1999. Typically, each setup contained 2–4 el-liptical candidates. Using slit widths of 100, with the 600R grism a spectral resolution ofR= 1160 was achieved. The seeing conditions (DIMM see-ing values) were varysee-ing between 0.4300and 0.8100 FWHM (median of 0.6600FWHM) and sufficient to meet the Nyquist theorem to allow a per-fect reconstruction of the signal from the sam-ples (see section 5.1.1). The constraint on the airmass wasA≤2.0 in order to limit the correc-tions of atmospheric absorption.
Early in 2001, an additional amount of GTO time was granted by ESO for the FDF collabora-tion to compensate the high amount of time loss during the imaging phases of the FDF which was caused by the El Ni˜no phenomenon. From this GTO pool, the P.I. of the FDF collaboration, Prof. I. Appenzeller (Heidelberg), thankworthy allocated one night of dark time for the field gal-axy project of spiral and elliptical galaxies.
In October 2001 additional three different setups were acquired with with FORS1 at the VLT in Visitor Mode by J. Heidt (Heidelberg). The tar-get selection was based on a FORS1I-band ref-erence image with a PSF of 0.52 arcsec FWHM.
In the search of the photometric redshifts cat-alogues for new field elliptical candidates with R ≤ 22m and zphot < 0.6 only two objects were detected which had not been observed pre-viously. Again the setups were splitted into three separate exposures (Ttot= 9000 sec) with an in-tegration time of 2.5 hours for each setup. 15 out of 29 galaxies located in the southwestern corner of the FDF were neglected to avoid the outskirts of the cluster at z = 0.33 (see
chap-Chapter 2: Sample Selection and Observations 37
Table 2.5: Observations of WHDF early-type galaxies
Tel./Instrument Date θ range airm. DIMM E+S0 anon. Texp
[ksec]
VLT/FORS2 07/10, 10/03/02 −90◦/−60◦ 1.18 0.6400 1 1 9.0 08/07, 09/11/02 −60◦/−30◦ 1.19 0.7300 2 - 9.0 08/04, 08/07/02 −30◦/ 0◦ 1.51 0.9200 3 - 9.0 08/04, 10/02/02 0◦/+30◦ 1.13 0.8900 3 1 9.0 09/10, 09/12/02 +30◦/+60◦ 1.18 0.9100 2 - 9.0 09/12, 10/4-5/02 +60◦/+90◦ 1.14 0.8500 2 - 9.0
P 11 1
ters5.5for a further discussion of this topic). In addition, eight bonafide field E+S0 galaxies with R >20.5m which already were observed in 2000 were selected to increase the S/N in the com-bined spectra. A total of 10 early-type galaxies was targeted and thus except one bright ellipti-cal with R = 19.0m, all early–type candidates were observed two or three times including the spectroscopy in 1999 and 2000. The seeing con-ditions were slightly poorer than during autumn 2000, but still in the sub–arcsecond regime. The median DIMM seeing was 0.8200 FWHM.
Between July and October 2002 spectroscopic observations for 10 WHDF ellipticals were car-ried out with the FORS2 instrument attached to the ESO/VLT at Cerro Paranal, Chile. This observing time was not part of any GTO pro-gramme but additionally granted. A total of six different setups were observed, each splitted into three separate exposures of 3000 sec with a total integration time of 2.5 hours. Targets were selected through a combination of colour-colour diagrams and apparent magnitudes (see section 2.2.4). For the MOS mask construction with FIMS, a deep WHDF I-band reference im-age consisting of 5 seeing averim-aged I-band im-ages with a final integration time of 1500 sec and a PSF of 0.6 arcsec FWHM was used. The slit width was set to 100, and with the 600RI grism a spectral resolution of R = 1000 was achieved. The DIMM seeing values were
be-tween 0.7300 and 0.9200 FWHM with a median of 0.8900 FWHM. Two additional objects, one in the setup 0◦ ≤ θ ≤ +30◦ and one in the setup
−90◦ ≤ θ ≤ −60◦, fell by coincidence into the slit of the galaxy # 810 and # 92, respectively.
The second object in slit # 810 turned out to be a field S0 galaxy at z = 0.2118 and was named
# 810b. The other object in slit # 92 is a back-ground spiral galaxy at z = 0.5569 and thus was discarded. Therefore, the total sample of WHDF elliptical galaxies comprises eleven field early-type galaxies.
Chapter 3: Data Reduction 39
Chapter 3
Data Reduction
This thesis comprises a large amount of data from three different projects, the rich Abell 2390 cluster, the three Low–LX clusters and two field galaxy samples from the FDF and WHDF, all aiming to investigate the population of early–
type galaxies in various environments. The re-duction of the optical ground-based multi–band imaging data for the two cluster samples, from which the apparent magnitudes of the early-type galaxies were derived, was performed mainly by M. L. Balogh and I. R. Smail. Nevertheless, some aspects for this study had to be re–analysed (see chapter 4). For the FDF and WHDF field elliptical galaxies A. B¨ohm was kindly providing the measurement of ground-based magnitudes.
As a full review of the imaging reduction is be-yond the topic of this thesis, it therefore will not be included. A summary of the observations and imaging for Abell 2390 is provided in the Ta-ble2.2on page30and for the three poor clusters in Table2.3(page 31) and Table 4.2(page 56).
All the subsequent sections will focus on the special aspects of the MOSCA spectra reduc-tion. As the reduction of the FDF and WHDF VLT/FORS spectra was performed in an analo-gous manner it thus will not be described here separately. The reduction procedure was under-taken using the eso-midas1 environment with own fortran program routines and the iraf2
1ESO–MIDAS, the European Southern Observatory Munich Image Data Analysis System is developed and maintained by the European Southern Observatory.
2IRAF (Image Reduction and Analysis Facility) is
dis-data analysis software and followed the standard procedure.
The ESO-MIDAS package provides a variety of algorithms for data processing for applications of long–slit reduction, such as wavelength calibra-tion and rebinning of two–dimensional spectra.
Nevertheless, special routines had to be devel-oped, for example, to rectify the distortions of the spectra due to the focal reducer (see section 3.3).
A theoretical review of data reduction will not be given here, see, e.g., Wagner (1992) or Volume B of the ESO-MIDAS User’s Guide, Chapter 6 (available for download at www.eso.org/projects/esomidas/doc/). For a discussion of the derivation of HST structural parameters, see section 4.3). In the following, the individual stages of the reduction procedure will be described in the same order as they were applied to the data.
3.1 Bias Correction
For each observing run, bias frames were taken at the beginning and end of the night. All frames showed a very stable two–dimensional structure with spatial variations of ≤3 ADU.
Thus, all bias frames from the individual nights
tributed by the National Optical Astronomy Observato-ries (NOAO) in Tucson, Arizona, which is operated by the Association of Universities for Research in Astronomy
tributed by the National Optical Astronomy Observato-ries (NOAO) in Tucson, Arizona, which is operated by the Association of Universities for Research in Astronomy