In a similar manner to the spectra of the galax-ies, the spectra of standard stars were reduced.
Therefore, only a summary is presented but no explicit description will be given here. During each observing run at least two spectrophoto-metric flux standards (HZ2 and HZ4) were ob-served through an acquisition star hole in one mask. Moreover, a multiplicity of kinematic template stars were gained during each observa-tion period for all described projects. In total, 12
Chapter 3: Data Reduction 45
templates in Sep. 1999, ten in July 2000, six in April 2001 and three stars in February 2002. The stars were observed through different longslit widths between 0.5–1.500. However, only the stars of the last run (Feb. 2002) had a spectral resolution which was sufficiently high to utilise them as templates for even resolving velocity dis-persions withσgal ≈90 km s−1. A more detailed description of the data for these kinematic tem-plates with an application to the Lick/IDS sys-tem is illustrated in appen.A. For the kinematic templates of the Feb. 2002 run, three K giant stars (SAO 32042 (K3III), SAO 80333 (K0III), SAO 98087 (K0III)) were observed through a 0.500 longslit using the same grism green 1000 as for the galaxies. These stars had a spectral resolution at Hβand Mgbof∼2.2 ˚A FWHM, cor-responding to σ∗ = 55 km s−1). A stellar tem-plate spectrum was split into several positions along the Y axis, which were slightly shifted from each other by <3×10−300 via a manually con-trolled zig–zag movement. To minimise the ef-fect of any possible variation (e.g., of instrumen-tal line profile, see also section5.2) in slit width, the star spectra were averaged over a small num-ber of rows only.
Figure 3.4: Example spectra (not flux-calibrated) of early–type galaxies in the A 2390 sample. The lower X axis represents the rest frame wavelengths, the upper one the observed wavelengths (both in ˚A). The ordinate gives the flux in counts (1 ADU=1.1e−1). Prominent absorption features are marked and the ID and the determined redshift are given above each panel.
Chapter 4: Photometric Analysis 47
Chapter 4
Photometric Analysis
Apart from the VLT/FORS and MOSCA intermediate–resolution spectroscopy, this study uses multi–band imaging data from different sources. The ground–based photometry pro-vides the basis for the derivation of the lu-minosities of the early–type galaxies which are utilised in the scaling laws of the Faber–Jackson relation. Hubble Space Telescope (HST) imag-ing with the Wide Field and Planetary Cam-era 2 (WFPC2) offers a spatial resolution of
∼0.15 arcsec FWHM and the Advanced Cam-era for Surveys (ACS) which is opCam-erational since July 2002 even∼0.05 arcsec FWHM. Both pho-tometric instruments are perfectly suited to anal-yse the surface brightness distribution of the dis-tant galaxies and to perform a detailed mor-phological classification into the sub–classes of early–type galaxies. Photometry in different passbands (U, B and I for Abell 2390 and B, V, R and I for the Low–LX clusters) for both cluster samples was acquired from ground–based telescopes. Additionally, each cluster centre was observed with the HST/WFPC2 instrument ei-ther in theV I (A 2390) or theR (Low–LX clus-ters) filters. Thanks to the very deep multi–band imaging of the FDF and WHDF, the basis for the derivation of the luminosities of the field early–
type galaxies is more robust than most of other distant field early–type samples. For the FDF, photometry in four filters (B,g,RandI) is avail-able, whereas for the WHDF sample three filter passbands (B,R and I) could be used.
In the next two sections4.1and 4.2, details will
be given on the ground–based and HST pho-tometry of the Abell 2390 cluster and of the three Low-LX clusters, respectively. Besides a description of the imaging data, the analysis will concentrate on the derivation of absolute mag-nitudes for the galaxies. The reduction and calibration of this large amount of photometric data is described elsewhere (for A 2390: Smail et al. 1998; for the Low–LX clusters: Balogh et al. 2002b) and was not part of this the-sis. The measurement of structural parameters by modelling the surface brightness distribution for all early–type galaxy candidates located on the HST/WFPC2 and HST/ACS images is pre-sented in section 4.3. To distinguish between the main classes of early–type galaxies, ellipti-cal and S0 galaxies, section 4.4will focus on the morphological classification. Two independent approaches will be presented, a visual type classi-fication according to the original Hubble scheme and a quantitative analysis following the de Vau-couleurs’ classification of the revised Hubble se-quence using the bulge–to–total fraction. A pos-sible correlation of the measured bulge fraction with visual morphological Hubble type is inves-tigated and performance tests between different filter passbands are discussed. The analysis steps to derive the absolute magnitudes, one basic par-ameter for the construction of the Faber–Jackson scaling relations, will be presented in section4.5.
Results on the ground–based luminosities will be outlined in section4.6. In the following analysis, it is assumed that the spectroscopic redshifts of
Figure 4.1: I-band image of the cluster Abell 2390 at z = 0.23, taken with the 5.1-m Hale telescope on Mt. Palomar. The total FOV is 9.06×8.05. North is up, east to the left. The length of the scale bar in the left corner corresponds to 10000, or 365 kpc at the distance of A 2390. Galaxies with spectra are indicated by squares together with their ID number. The cD galaxy is marked with a circle. Stars used for the mask alignment are labelled as ID = 900X. An overlay of the HST/WFPC2 field-of-view is also shown.
Chapter 4: Photometric Analysis 49
the galaxies have already been determined. The measurements are presented in chapter 5.
4.1 Photometry of Abell 2390
The cluster Abell 2390 (α2000 = 21h53m34.006, δ2000 = +17◦40010.009) at z = 0.228, rich-ness class 1, has a large velocity disper-sion of σ = 1100 ± 63 km s−1(Carlberg et al. 1996) and a high X-ray luminos-ity, LX(0.7−3.5 keV) = 4.7 × 1044 erg s−1 (Le Borgne et al. 1991). Carlberg et al.
(1996) analysed the dynamical state of the cluster and its mass distribution and found a virial radius of Rv = 3.156h−1100 Mpc and virial mass of Mv = 2.6 × 1015h−1100M, which makes A 2390 more massive than Coma (Mv = 2.1×1015h−1100M). At a mean interior density of 200ρc, A 2390 and Coma have M200 -masses of 1.2 and 1.3×1015h−1001M 1, respec-tively.
The study of the early–type galaxy population in A 2390 is based upon Multi-Object Spectroscopy (MOS) using MOSCA at the Calar Alto 3.5-m telescope on Calar Alto Observatory in Spain.
Section 2.1.6 describes these spectroscopic ob-servations. In addition, optical photometry from the 5.1-m Hale telescope at Palomar Observatory is available (section 4.1.1) and the WFPC2 im-ages taken withHSTproviding high-quality mor-phological information for a subset of the sample have been exploited (section4.1.2). Table2.2on page30gives a summary of the photometric and spectroscopic observations for this project.
Since the galaxies were distributed over the whole field-of-view (FOV) of ∼100 ×100, corre-sponding to 1.53×1.53 h−270 Mpc2,the evolution of early–type galaxies out to large clustercentric distances of ∼0.5 virial radii can be studied.
1For the virial masses a cosmology with H0 = 100 km s−1Mpc−1, q0 = 0.1 and Λ = 0 was assumed.
Figure 4.2: Zoom of the Hale I-band image of the cluster Abell 2390 atz= 0.23. Galaxies with spectra are denoted with squares and the cD galaxy is marked with a circle. The HST/WFPC2 field-of-view is over-layed, with the WF chip corresponding to 11400along one side and the PC chip with a size of 5200.
4.1.1 Ground-based U BI Imaging Abell 2390 was observed at the 5.1-m Hale telescope on Mount Palomar using COSMIC (CarnegieObservatoriesSpectroscopicMultislit and Imaging Camera) in the U (3000 sec), B (500 sec) and I-band (500 sec), allowing to se-lect early-type galaxies in the full field-of-view of MOSCA due to the nearly equally large field-of-view of COSMIC of 9.70×9.70. Seeing conditions ranged from 1.400in theU-band, 1.300in theB to 1.100 in the CousinsIC-band (Smail et al. 1998).
Hereafter the Cousins IC-band is referred to as I-band. AtI = 22.5 mag a completeness level of 80% from a comparison with deeper field counts is warranted. All frames from the ground-based imaging data were reduced in a standard manner with irafusing standard reduction packages.
Fig. 4.1 shows the I-band image of the cluster Abell 2390 at z = 0.23, gained with the 5.1-m
Figure 4.3: Left: Distribution of Abell 2390 cluster members atz = 0.228 by Yee et al. (1996). In total, 159 early–type galaxies (E) and 13 E+A galaxies are distributed over a strip of 6.470×35.430, centered on the cluster cD galaxy (square). Only early–type galaxies (asterisks) and E+A galaxies (triangles) within a redshift interval of zclus±2σare shown. Right: Comparison between Yee et al. (1996) and the catalogued A 2390 galaxies. Spectroscopically observed galaxies of this work are indicated with open circles. Ten galaxies with spectra which are included in both data sets are marked as solid circles.
Hale telescope on Mount Palomar. The total FOV is 9.06×8.05, north is up and east to the left. Galaxies with available MOSCA spectra are denoted by squares and the central cD galaxy (no spectroscopic information) is indicated with a circle. The central 15000×15000 of the cluster are covered by the HST/WFPC2 mosaic, which provides a detailed morphological and structural analysis.
As a consistency check of the ground-based pho-tometry the photometric data was compared with the results for the cluster A 2390 by Yee et al. (1996), which were derived as part of the CNOC cluster redshift survey. Through a cross-correlation, 12 galaxies with spectra were iden-tified which are included in both data sets. The selection process for these galaxies is visualised in Fig. 4.3. The left panel shows the distribu-tion of 159 early–type cluster members and 13 E+A galaxies in A 2390 by Yee et al. (1996) over a large strip of the sky of 6.470×35.430
cen-tered on the galaxy cluster. On the right hand plot, a comparison between Yee et al. (1996) and the A 2390 galaxies in this study is illus-trated. Galaxies with available spectra are in-dicated with open circles, whereas the galaxies by Yee are indicated by asterisks. In total, ten spectroscopic cluster members which were found in both samples are marked as solid circles. One object (# 5552), fell by coincidence into the slit and only a redshift was derived but no magni-tude could be measured. The galaxy (# 3038) shown with an open circle is a foreground gal-axy at z = 0.18 (cf. Table 4.1). Fig. 4.2 shows a zoom of the ground-based Hale I-band image containing these 12 early–type galaxies which are in common. After the transformation of theI to Gunnr observed magnitudes using r−I = 1.14 based on Kinney E/S0 spectra, no significant difference between the magnitudes was found,
∆(|r−rYee|) = 0.04±0.12 mag. Figure4.4 and Table4.1give a comparison of the Gunn r
mag-Chapter 4: Photometric Analysis 51
nitudes for the 11 galaxies which are in com-mon. In addition, the redshift determinations of all these objects show a very good agreement (|z−zYee|= 0.3−3±0.2−3). Note that the trans-formation fromIobs to robs for the I-band mag-nitudes are very large at a redshift of z= 0.23.
For this reason, in the subsequent analysis of the scaling relations the magnitudes were trans-formed to Gunn r rest-frame magnitudes which involves much smaller k-corrections and there-fore less uncertainties. A detailed description on the transformation of magnitudes from observed to rest–frame wavelength range can be found in section 4.5.3.
In Fig. 4.5a) the colour-magnitude diagram (CMD) (B−I) versus I from the Hale imaging is shown for all galaxies brighter thanI = 23.5m lying in a 9.70×9.70 (2.12 Mpc) region centered on A 2390. Bona fide stellar objects selected by a SExtractor based star classification par-ameter of star≥0.9 have been discarded. The 48 early-type cluster members of A 2390 with available spectroscopic information are indicated with open squares. The red sequence of early-type cluster members is readily seen extending down to I ∼ 21m (MB ∼ −19m). A least-squares fit to seven ellipticals in A 2390 gives (B−I)E=−0.011 (Itot) + 3.405 which is shown by the solid line in Fig. 4.5. One elliptical E/S0 galaxy (# 6666) is not included in the CMDs as this galaxy was not selected as a spectroscopic target but fell by coincidence into the slit dur-ing the observations. The outlier object # 2237 with the bluest colour of (B −I) = 2.77 was classified a spiral Sa galaxy. Fig. 4.5b) dis-plays the (U −B)–I colour magnitude relation for the same galaxies as in Fig. 4.5 a. Again, a least-squares fit to seven A 2390 cluster ellipti-cals yields (U −B)E=−0.020 (Itot) + 1.092.
The (B − I)–(U −B) colour–colour plane for galaxies brighter than I = 23.5m is shown in Fig.4.6. At first glance a “clump” of red galax-ies at (B−I)∼3.2 and (U−B)∼0.6 is clearly visible. This region is occupied by all E+S0
clus-Figure 4.4: Comparison between ground-based Gunn r magnitudes by Yee et al. (1996) and this study for A 2390 cluster members (solid circles).
Gunnrmagnitudes were derived from theItot mag-nitudes. The galaxy denoted with the open circle is a foreground galaxy atz= 0.18 (see text for details).
ter members of A 2390 which are indicated with open squares. The observed galaxies are com-pared to evolutionary tracks for E/S0 galaxies using the BC96 models assuming a formation redshift of zf = 2. The predicted BC96 colour for a passively evolved galaxy with an old stellar population at redshift z = 0.23 suggests colours of (B−I) = 3.06 and (U−B) = 0.57, which are shown as dashed lines in Fig.4.6. Note that for the computation of the BC96 models a slightly different cosmology withH0 = 60 km s−1Mpc−1 and q0 = 0.1 was adopted.
4.1.2 HST Photometry
Apart from the deep multi-colour ground-based imaging, detailed morphological information is provided via two HST/WFPC2 images of A 2390 for a subset of the galaxies in the rich cluster sample.
During Cycle 4 A 2390 was observed on
Decem-Table 4.1: Comparison between ground-based Gunn r magnitudes by Yee et al. (1996) and the Gunn r magnitudes of this study. The mean errors in Gunnrand (g−r) colour by Yee et al. (1996) are 0.05mand 0.07m, respectively. The uncertainties in the redshifts are listed in units of 10−5.
ID z r IDYee rYee (g−r) zYee δzYee ∆(r−rYee)
[mag] [mag] [mag] 10−5 [mag]
2120 0.2280 19.83±0.10 100873 20.15 0.78 0.2286 23 −0.316 2138 0.2465 18.58±0.04 100908 18.64 0.86 0.2470 28 −0.056 2180 0.2283 19.21±0.05 100995 19.23 0.91 0.2290 24 −0.016 2198 0.2335 19.98±0.06 100975 20.05 0.87 0.2332 29 −0.066 2438 0.2372 20.18±0.07 101197 20.09 0.85 0.2371 37 0.094 2460 0.2319 18.91±0.05 101106 19.29 0.94 0.2318 25 −0.376 2592 0.2300 18.60±0.05 101183 18.58 0.87 0.2304 22 0.024 3038 0.1798 18.90±0.04 101992 18.87 0.78 0.1796 22 0.034 3053 0.2282 19.08±0.05 101930 19.04 0.91 0.2281 28 0.044 3473 0.2228 19.19±0.06 101961 19.1 0.94 0.2221 26 0.094
5552 0.2231 101987 20.25 0.94 0.2233 29
6666 0.2240 18.82±0.11 101190 18.83 0.88 0.2244 24 −0.006
ber 10, 1994 with theHST2in the F555W (V555) and in the F814W (I814) filters as part of a large gravitational lensing survey (P.I. Prof. B. Fort (CNRS, Paris), Proposal ID 5352). In total, 5 exposures in the I814-filter and 4 in the V555 -filter each with 2100 sec resulting in a total ex-posure time of Ttot = 10.5 ks and Ttot = 8.4 ks for the I814 and V555, respectively. These expo-sure times are deep enough to determine struc-tural parameters down to rest–frame MB∼23m (Ziegler et al. 1999). The final mosaic covers a field of approximately 2.50×2.50at∼0.1500 resolu-tion in the core of the cluster. All single standard WFPC2 pipeline reduced frames were further re-duced and afterwards combined to a final mosaic image by I. Smail. Here only a brief overview is given to the reader. Further details about the general reduction process and the mosaic
align-2Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with program # 5352.
ment are given in Smail et al. (1997). In oder to allow for hot pixel rejection, individual expo-sures were grouped in sets of four single-orbit exposures each offset by 2.000. After standard pipeline reduction, the images were aligned us-ing integer pixel shifts and then combined usus-ing the IRAF/STSDAS3 task CRREJ. In the final mosaic the PC chip has the same linear scale as the WF chips and the sky background levels are equalised for all four chips. Details on the calibration will be outlined in section 4.3. The WFPC2 image of A 2390 has a 1σ surface bright-ness limit of µI ∼28 mag/pixel2, which is more than adequate to provide high-quality morpho-logical information on the brighter cluster mem-bers. A two colourV I HST image of the cluster A 2390 is shown in Fig.4.7. Thumbnail images of the 14 galaxies for which MOSCA spectra have been acquired are displayed in Fig.4.12 and the
3The Space Telescope Science Data Analysis System (STSDAS) is a data processing software pipeline for cal-ibrating and analysing data from the HST using IRAF, operated by the STScI. (http:\\www.stsci.edu\resources
\software hardware\stsdas).
Chapter 4: Photometric Analysis 53
Figure 4.5: Left: (B−I)–I colour magnitude diagram from the Hale imaging for galaxies brighter than I= 23.5mlying in a 9.70×9.70 (2.12 Mpc) region centered on A 2390. The sequence of red cluster members is readily seen extending down to I ∼21m (MB ∼ −19m). The 48 early-type cluster members of A 2390 with available spectra are denoted with squares. Filled squares represent the ellipticals in A 2390 which enter the Fundamental Plane. The solid line is a least-square fit to seven ellipticals. Right: (U −B)–I colour magnitude diagram from the Hale photometry for bright galaxies lying in a 9.70 ×9.70 (2.12 Mpc) region centered on A 2390. Symbol notations as in the left Figure.
position of the HST field is indicated on Fig.4.1.
The cD was spectroscopically not observed and thus is not shown. Several strong and faint grav-itational lensed (cuspy and folded) arcs are visi-ble next to numerous galaxies (e.g., cD, # 2592,
# 6666). Remarks on special features for individ-ual HST objects of Abell 2390 are summarised in Table 4.9. Two arcs features (a cusp arc H3 and a fold arc H5) associated with the early-type galaxy # 6666 were analysed through lens mod-elling by Pell´o et al. (1999) and identified as mul-tiple images of a high-redshift source atz= 4.05, which is in rest-frame wavelength a Lyαemitter behind the cluster A 2390. The “straight arc” as-sociated in the western part of the galaxy # 2592 was analysed and can be splitted into two com-ponents, where component “A” is located in the southern part with zA = 1.033 and object “C”
in the northern part with zC = 0.913 (Bunker, Moustakas & Davis 2000).
The analysis of structural parameters and mor-phologies as well as the derivation of rest-frame properties for the sub-sample of E+S0 galaxies falling into the HST field is presented in sec-tion 4.3.