4.2 Photometry of Low-L X Clusters
4.2.2 Ground-based Photometry
based on three different observatories (i) Calar Alto CAFOS (2.2 m telescope) and MOSCA (3.5 m telescope) imaging mode observations, (ii) Isaac Newton telescope (INT) Wide-Field Camera (WFC) imaging and (iii) Palomar 200-inch (P200) telescope observations at Palomar Observatory. Table 4.2 gives a summary of the ground-based imaging observations of all ten poor clusters. The Calar Alto CAFOS observa-tions were carried out by R. G. Bower during the 28th of March 1999 and the MOSCA obser-vations by B. L. Ziegler between the 3.–6. March 2000. The COSMIC imaging of the Palomar 200-inch telescope was performed by I. Smail
dur-Chapter 4: Photometric Analysis 55
Figure 4.7: V I colour composite HST WFPC2 image of the cluster Abell 2390, The total FOV is 2.05×2.05. Galaxies with MOSCA spectra are indicated with their ID numbers. The cD galaxy is also marked, but was not observed. The PC chip is not shown as no galaxy was selected from it.
All candidates display red colours. Object # 2933 is an Sbc background galaxy at z= 0.398.
Table 4.2: Ground–based Observations of Low–LX clusters. Total exposure times are given in [ksec].
Name Instrument Date B V R I
CL 0818+56 COSMIC 26/11/98 0.60 0.30
CAFOS 28/03/99 1.20
CL 0819+70 COSMIC 26/11/98 0.60 0.25
CAFOS 28/03/99 0.30 0.60 1.80
MOSCA 03/03/00 1.20
CL 0841+70 COSMIC 26/11/98 0.60 0.25
CAFOS 28/03/99 0.30 0.60 1.80
MOSCA 03/03/00 1.20
CL 0849+37 COSMIC 26/11/98 0.60 0.25
CAFOS 28/03/99 0.30 0.60 1.20
MOSCA 04/03/00 1.20 MOSCA 05/03/00 0.60 MOSCA 05/03/00 0.30
CL 1309+32 INT/WFC 19/06/98 1.80 2.40 0.60
CAFOS 28/03/99 0.30 0.60 0.60
MOSCA 06/03/00 5.40
CL 1444+63 INT/WFC 18/01/99 1.10 2.40 0.72
CAFOS 28/03/99 0.30 0.30 0.30
MOSCA 06/03/00 5.40
CL 1633+57 INT/WFC 10/02/99 1.80 0.60 1.20 CAFOS 28/03/99 0.30 0.30 0.60 0.30 MOSCA 04/03/00 1.20
MOSCA 05/03/00 1.20
CL 1701+64/ INT/WFC 10/02/99 2.40 0.60 0.90 CL 1702+64 CAFOS 28/03/99 0.30 0.30 0.30
Chapter 4: Photometric Analysis 57
Table 4.3: Overview of the observing conditions for ground-based photometry of four Low–LX clusters.
The mean seeing FWHM values for a night are given in each column.
Name CAFOS MOSCA P200
R B R
n2 / n3 CL 0819+70 1.900 1.100 CL 0841+70 1.900 1.500
CL 0849+37 2.500 ∼3.800/ 1.400 1.700 CL 1633+57 1.600 ∼3.800/ 1.000–1.400
ing two nights in November 26.–27. 1998. The galaxy clusters CL 1701 and CL 1702 cover the same field on the sky. For this reason, only one pointing was needed and thus are shown in com-bination in Table 4.2. Seeing conditions were not photometric and varied between values of 1.000 ≤ FWHMB ≤ 1.004 1.006 ≤ FWHMR ≤ 1.009 for the B and R-band images, respectively. Ex-posures with seeing conditions worse than 2.005 were replaced by repeat observations or by im-ages of other telescopes. Table 4.3 lists the ob-serving conditions for ground-based photometry of the CAHA B and R-band images as well as the P200 R-band exposures of the CL 0849+37 cluster. TheI-band images are not included be-cause they were not used for this study. First results of the Calar Alto B and R-band images for four clusters of the sample, CL 0819, CL 0841, CL 0849 and CL 1633 were presented by Gaztelu (2000) and Ziegler et al. (2001b). As the R-band data from Calar Alto of the cluster CL 0849 had poor seeing (2.500), the PalomarRimage was used instead. The remaining imaging data was reduced and analysed by M. L. Balogh and I. R.
Smail.
During the imaging observations the conditions were not photometric. Therefore, the images were calibrated by comparing aperture magni-tudes of several (usually 2–3) relatively isolated,
Figure 4.8: Comparison between ground-based R magnitudes by Gaztelu (2000) and theRmagnitudes for CL 0849 cluster members. Open circles denote measured R magnitudes by Gaztelu (RAG) with an aperture radius of rap = 2.6500, solid circles repre-sent re–measuredRAGmagnitudes with an aperture ofrap= 5.5700 (see text for details).
early-type galaxies with the F702W photometry on the WFPC2 images (see next section 4.2.3), and converted this to standard R magnitudes, assuming RF702W − Rc = −0.2 (Fukugita et al. 1995). The photometric calibration is accu-rate to approximately 0.1m, including blue clus-ter members (but see discussion in the following paragraph).
Based on ground-based imaging from the Palo-mar 200-inch telescope and the INT, a sample of 581 galaxies was selected for low–resolution follow–up spectroscopy of all 10 clusters (Balogh et al. 2002b). The sample selection was per-formed on the single INT WFC chip cover-ing 11.40 at 0.0033 arcsec/pixel. The Palomar COSMIC images have a field-of-view of 13.70 with a pixel scale of 0.004 arcsec/pixel. Gal-axies were selected for spectroscopic follow–up from theR−band images, except for two clusters (CL 1309+32 and CL 1444+63), where the
selec-tion was done on the I−band images because no R−band images were available. Reliable red-shifts could be derived for a total of 317 galaxies, of which 172 are cluster members (see section5.5 for more details). From this final catalog of 172 Low–LX cluster members, galaxies for follow–up spectroscopy with intermediate–resolution for a detailed kinematic analysis presented here were selected.
To check the consistency between the different data sets, the ground-based R photometry of the CL 0849 cluster galaxies was compared with the results from Gaztelu (2000). Fig. 4.8gives a comparison between the ground-basedR magni-tudes by Gaztelu (2000) and the R magnitudes used in this work for CL 0849 cluster members at z = 0.234. On average the differences be-tween the magnitudes of h∆Ri = 0.06m are sat-isfactory for fainter galaxies with R >∼ 18.4m. However, for the brightest galaxies significant de-viations arise. Note that the work of Gaztelu concentrated only on the fainter galaxy popula-tion and thus chose a f ixed aperture radius of rap = 2.6500 (5 pixel) to measure the R-band magnitudes. Furthermore, the Palomar R band image was analysed with a wrong seeing value and calibrated using the CAHA CAFOS R im-age which introduced an additional calibration offset of 0.05m to the magnitudes. Therefore, the calibrated magnitudes by Gaztelu have an accuracy of 0.08m only. The measured R mag-nitudes by Gaztelu are indicated as open cir-cles. The choice of this fixed aperture size has a dramatic effect on the brighter galaxies, show-ing fainter magnitudes with differences of about
∆R ≈ 0.45m, for the brightest object even up to 0.9 mag. Due to the inconsistent values and to check the SExtractor analysis of the galax-ies in this study, the R magnitudes of the gal-axies by Gaztelu (2000) were re–measured us-ing a nominal aperture of rap = 5.5700. The size of the aperture was examined for the whole set of brightest galaxies and yielded the best ap-proximation. The re–measured R magnitudes
of Gaztelu derived with an aperture diameter of dap = 11.1400 are represented as solid circles. Re-sults of these re–measurements were compared to the aperture magnitudes derived with vari-able elliptical apertures using the SExtractor and GIM2D algorithms on both ground–based and HST images (section 4.5.1). All three in-dependent approaches resulted in very similar R magnitudes. The differences were less than their scatter, e.g., ∆hRSEx−RGIM2Di= 0.001m. For the subsequent analysis the SExtractor magnitudes were applied. These total appar-ent magnitudes showed mean uncertainties of δR= 0.029±0.013m (median value 0.03).
Fig.4.9shows the (B−R)–RCMD from the Hale imaging for all galaxies brighter thanR= 22.5m lying in a 9.70 ×9.70 (2.17 Mpc) region centered on the cluster CL 0849. The 14 early-type cluster members with available intermediate–resolution spectroscopic information are represented with open squares. Galaxy with ID # 54 has no (B −R) colour and is therefore not included.
One galaxy (object # 3), which was previously not targeted in Balogh et al. (2002b), was revealed to be a cluster member based on its intermediate–resolution spectra. Circles denote 13 early-type cluster members, which were ver-ified by low–resolution spectroscopy (Balogh et al. 2002b). Rejecting one galaxy which is in-cluded in both setups, this yields a total sam-ple of 26 spectroscopic confirmed cluster mem-bers for CL 0849. At R ∼ 17.7m, R ∼ 19.3m and R∼20.6m the mean errors in R are 0.01m, 0.03m and 0.07m and in (B−R) 0.05m, 0.08m and 0.18m, respectively. These typical uncer-tainties are shown in the top of Fig. 4.9 as er-ror bars. A least-squares fit to three ellipticals gives (B−R)E=−0.0667 (Rtot) + 3.6889 which is shown by the solid line in Fig. 4.9. The CM sequence for the CL 0849 cluster is less well es-tablished than for the colour-magnitude relation of A 2390 (cp. Fig.4.5) and shows a larger scat-ter compared to the early–type galaxies in mas-sive clusters. To verify these results, the
ob-Chapter 4: Photometric Analysis 59
servations can be compared to theoretical SSP model predictions by Kodama & Arimoto (1997, hereafter KA97). These authors calibrate their models using the CM zero-point and slope of the observed (V−K) vs. MV colour-magnitude rela-tion (CMR) for Coma cluster ellipticals (Bower et al. 1992). The population synthesis code uses as main characteristic parameters an IMF slope ofx= 1.20, a mass range 0.10≤m/M ≤60.0, a metallicity range of 0.1−3 ≤ Z ≤ 0.05 and a burst duration of τ = 0.1 Gyr. The KA97 models assume that elliptical galaxies have been formed in a single burst event at a high formation redshift ofzf ∼2 with a following passive evolu-tion of their stellar populaevolu-tions. KA97 demon-strate that for sub-L∗ galaxies the CM slope changes are mainly caused by fainter galaxies due to variations in their mean stellar metallic-ity. The timescales for the loss of metals via galactic winds is shorter for less massive gal-axies, which results on average in lower metal-licities for these systems. The scatter around the CM relation is rather influenced by age ef-fects. Applying the KA97 models, the evolution of the slope and the zero-point for any given red sequence of galaxies as a function of redshift can be derived. Comparing our observed red sequence with SSP KA97 model assumptions, for a redshift of z = 0.234 a red sequence of (B−R)E=−0.0668 (Rtot) + 3.6500 is predicted, which is indicated in Fig.4.9by the dashed line.
This “theoretical” red cluster sequence is in ex-tremely good agreement with the observed red cluster galaxies spanned by the elliptical galaxies of CL 0849. The model CMR has almost iden-tical slope and zero-point as the observed CMR defined by the red cluster elliptical galaxies.
For the spectroscopic confirmed cluster member galaxies of CL 0849 the fraction of blue galaxies fb within the CM plane can be derived (Butcher
& Oemler 1984). Galaxy clusters at low red-shift have a very homogeneous group of galax-ies with the cluster cores essentially devoid of blue galaxies, whereas atz >0.1 the blue galaxy
Figure 4.9: (B−R)–R colour magnitude diagram from the P200 (R-band) and CAHA (B-band) imag-ing for galaxies brighter than R = 22.5m lying in a 9.70×9.70 (2.17 Mpc) region centered on CL 0849.
14 early-type cluster members with available spectra are denoted with squares. Filled squares represent ellipticals which enter the Fundamental Plane with the solid line being a least-square fit to three ellipti-cals. Circles indicate 13 early-type cluster members verified through low–resolution spectroscopy. The dashed line is the predicted CMDs using SSP models by Kodama & Arimoto (1997). The vertical dotted line, which denotesMV <−20m, and the horizontal dotted line are used to define the blue galaxy fraction.
fraction increases with redshift and the number of fb becomes significantly higher at z ∼ 0.5.
Only galaxies brighter than MV <−20m in the rest-frame (indicated as the vertical dotted line in Fig. 4.9) and those objects whose rest-frame (B−V) colors are at least 0.2 mag bluer than the ridge line of the early-type galaxies at that magnitude cut are considered. For the cluster redshift of CL 0849 MV <−20m corresponds to R = 20.34m and at z = 0 ∆(B−V) = −0.2 is approximately the difference in colour of an ellip-tical and Sab-type spiral galaxy (Fukugita et al.
1995). Atz= 0.2, the ∆(B−V) colour criterium translates into (B −R)E = −0.0668 (Rtot) +
3.2500, which is indicated as the horizontal dot-ted line in Fig. 4.9. The blue galaxy fraction for CL 0849 gives fb = 0.077±0.052. To put proba-bility limits on the galaxy fractions, the error of fb was evaluated assuming Poisson statistics.
4.2.3 HST Photometry
HST/F702W (R702) imaging during Cycle 8 (P.I.
Prof. R. L. Davies (Oxford), Proposal ID 8325) was acquired in July 2000 for 8 of the 9 clus-ters between 0.2 < z < 0.30 and with a mean redshift of z = 0.25. Each cluster was observed with three single orbit exposures in the F702W filter with exposure times ranging from 2100 to 2600 sec per orbit. Total integration times of 7.500 sec warrant an accurate measurement of the structural and photometric parameters of those galaxy candidates located within the HST fields. The three pointing positions for the ex-posures were offset by 10 pixels (Wide Field Camera (WFC) pixel size) from one another but during the reduction procedure the exposures were aligned and coadded in oder to remove cosmic rays artefacts and hot pixels. The im-ages were not drizzled or regridded because this does not accurately preserve the noise charac-teristics of the pixels. After coadding the single exposures, each WFC chip image was trimmed to the area of full sensitivity (see Balogh et al.
2002a for details). The photometry is calibrated on the Vega system and updated zero-points were taken from the current instrument man-ual. The final images reach a 3σ point source sensitivity of R702 ∼25.5m, and cover a field of 2.50 ×2.50 (or 0.57h−701Mpc at z = 0.25) with an angular resolution of 0.1700 (∼0.71h−170 kpc).
The cluster CL 1633+57 was observed in Cycle 7 (P.I. Prof. P. Rosati (ESO), Proposal ID 7374).
The data of cluster CL 1633+57 were retrieved from the CADC4 HST archive. These data are WFPC2/F702W observations, with four
expo-4Canadian Astronomy Data Centre, which is operated by the Herzberg Institute of Astrophysics, National Re-search Council of Canada.
sures of 1200 seconds. The total exposure time of 4800 seconds is therefore less than that of the other nine clusters. Table 4.4 gives a summary of the HST imaging and the X-ray properties for the rich and poor cluster samples.
For the Low–LX clusters a nominal magnitude limit of R702 = 23.0m atz = 0.25 was adopted, which corresponds toM702 =−17.6+5 logh70or approximately MR ∼ −17.4 + 5 logh70 (assum-ing a ΛCDM cosmology and includ(assum-ing small k-corrections of<∼0.1m), which is 4.2 magnitudes below MR∗ ≈ −21.60m (Blanton et al. 2001).
Galaxies brighter than this limit are luminous enough and have large enough sizes in oder to warrant a reliable and accurate classification.