To investigate possible environmental effects in the galaxy properties and differences in the stel-lar populations of elliptical and lenticustel-lar galax-ies, a large sample of more than 50 early–type galaxies is required. In a recent study, Ziegler et al. (2001a) performed a detailed analysis of a sample of 48 early-type galaxies in the rich clus-ter A 2218 at a redshift ofz= 0.18. If this work
Chapter 5: Kinematic Analysis 123
Figure 5.27: Distribution of the field early–type galaxies within the WHDF. The bar corresponds to 600 kpc in projection at a redshift ofz = 0.33. The three galaxies located in the southern (upper mid-dle) part of the WHDF are not members of a group.
North is down and east is to the left.
could be combined with the sample ofN = 48 in A 2390, it would offer the possibility to explore the evolution of∼100 early-type galaxies over a large luminosity range and wide field-of-view at similar cosmic epochs.
Before the individual galaxies of the two clus-ters can be combined into one large sample, it has to be proven that both data sets are charac-terised by very similar properties and that pos-sible differences within the sub-samples were ac-counted for. For this reason, the cluster galax-ies were compared with respect to their luminos-ity, colour, size and velocity dispersion distribu-tions. Both clusters feature similar global cluster properties (e.g., richness class, X-ray luminos-ity). Furthermore, the sample selection and all observations, especially the spectroscopy, have been carried out in very similar manner. There-fore, it is not expected that significant differences between the galaxy samples exist.
In the subsequent analysis the cluster galaxies are investigated over the same dynamical range in their properties. Therefore, the two cD gal-axies of the cluster A 2218 were excluded as no cD galaxy is included in the A 2390 sub-sample.
A comparison of the cluster properties in ab-solute rest-frame Gunn r magnitude, effective radius, (B −I) rest-frame colour and velocity dispersion (aperture corrected) is presented in Fig. 5.28. For each set of parameters, the mean values with the resp. ±1σ scatter are indicated as overlayed Gaussian curves. Overall, the galax-ies are similarly distributed in all plots. For the galaxies in A 2218 the range of absolute Gunn r magnitudes is −20.50 ≥ Mr ≥ −23.42, for the objects in A 2390 −20.47 ≥ Mr ≥ −22.99 (up-per left panel in Fig. 5.28) The median value for A 2390 is hMri=−21.31m, 0.17 mag fainter than the median luminosity for the galaxies in A 2218. For the size distribution only the 32 objects within the HST fields are considered.
In the distributions of galaxy size (upper right panel) small (but not significant) differences are visible. The sizes of the A 2218 galaxies cover a range between 0.22 and 0.82 in logRe (kpc), with a median of loghRei = 0.46. The A 2390 sample contains more galaxies with smaller effec-tive radii 0.01 ≤ logRe ≤ 0.92, with a mean of logRe= 0.38±0.27 (median loghRei= 0.37), re-sulting in a broader distribution than for A 2218.
These galaxies are low-luminosity galaxies. This issue will be addressed in more detail in the forthcoming section 6.4. The rest-frame colour distributions show no significant differences be-tween the clusters. The galaxies’ colours in A 2390 cover a range of 1.86 < (B −I) < 2.51, in A 2218 1.95<(B−I)<2.44. For the A 2390 galaxies a median value of h(B−I)i = 2.29 is derived, for the A 2218 objects h(B−I)i= 2.28, respectively. Both are in very good agreement with the typical colour of (B −I) = 2.27 for ellipticals at z = 0, given by Fukugita et al.
(1995). The velocity dispersions for the galax-ies are equally distributed (lower right panel),
with a median value of 165 km s−1 and a mean of logσ= 2.238±0.11 for A 2390 and 178 km s−1 (logσ = 2.253±0.12) for A 2218. As the veloc-ity dispersion is an indicator for the mass of an object and the measuredσ values exhibit similar ranges, it can also be concluded that there are no significant differences in mean galaxy masses be-tween the two samples (see section6.5 for a fur-ther discussion). In addition, the scale lengths (h), disc-to-bulge ratios (D/B) and the surface brightnesses of the member galaxies were com-pared to each other and again negligible differ-ences between the distributions were found.
As a conclusion no significant offset in the dis-tribution of any galaxy parameters between the two samples was detected. The comparison as-serted that properties of the galaxies within the two clusters are very homogeneous and therefore a combination of the two data sets is adequate and warranted, which results in a final sample of 96 early-type galaxies.
The combined sample allows an extensive in-vestigation of the evolutionary status of galax-ies in rich clusters at z ∼ 0.2. A sub-sample with accurate structural parameters provided by HST comprises 34 E+S0 galaxies, splitted into 17 ellipticals (E), 2 E/S0, 9 S0, 3 SB0/a, 2 Sa bulges and 1 Sab spiral bulge that can be in-vestigated in the FP. With this large sample, possible radial and environmental dependences can be explored in detail for the galaxy proper-ties from the cluster centre to the outskirts us-ing the Faber-Jackson relation and for different sub-populations (E/S0 and S0/Sa bulges). For the subsequent analysis, the sample of early-type galaxies in A 2218 and A 2390 is referred to as the rich cluster sample.
Chapter 5: Kinematic Analysis 125
Figure 5.28: Comparison of galaxy properties. Thin lines and hashed areas represent the distribution for the members in A 2390, solid thick lines the characteristics of the galaxies in A 2218. Gaussian fits showing the mean values with ±1σ scatter are overlayed. Top left: Absolute Gunn r magnitudes for the whole sample. Top right: Size distribution of the HST sub-sample. Bottom left: (B−I) rest-frame colours of member galaxies. Bottom right: Distribution of velocity dispersions.
Chapter 6: Galaxy Scaling Relations at z∼0.2 127
Chapter 6
Galaxy Scaling Relations at z ∼ 0.2
After the descriptions of the sample construc-tion and the derivaconstruc-tions of photometric proper-ties, structural parameters and the measurement of velocity dispersions, the analysis and results of the distant scaling relations of early-type gal-axies will be presented.
In a first step, a sample of nearby early-type gal-axies has to be chosen which acts as a local com-parison reference (section 6.1). Afterwards the scaling relations of the projections of the Funda-mental Plane, the Faber–Jackson (section 6.2) and Kormendy relation (section 6.3) of the dis-tant cluster and field early-type galaxies will be compared to the local ones in the present-day universe. In a next step, the Fundamental Plane (FP) is outlined in section 6.4. After an investi-gation of the whole early–type galaxy population in the FP, the discussion concentrates on possi-ble differences between the morphological types of early-type galaxies, elliptical and S0 galaxies.
To rule out systematic errors in the interpreta-tion of the evoluinterpreta-tion within the FP, the mass ranges of the galaxies are analysed and the evo-lution of the mean mass–to–light (M/L) ratios are explored (section 6.5). A comparison with previous studies of the FP is given in section6.6.
Detailed tests on possible environmental effects on the properties of E+S0 galaxies will be de-scribed in the next chapter 7.
Some aspects of the analysis in this chapter have been presented in Fritz et al. (2005a, 2005b) and Ziegler et al. (2005).
6.1 The Local Reference
To derive the kinematic and/or the spectropho-tometric evolution of early-type galaxies at inter-mediate redshift, it is crucial to carefully choose a sample of early-type galaxies at low redshift which can be utilised as a reference for the pur-pose of comparison. As already discussed in chapter 1, a large number of local samples have been constructed based on the Virgo and Coma clusters. Recent results on the SDSS Data Re-lease 1 database encompass maybe one of the largest sample of early-type galaxies located at different redshifts and in various environments.
However, unfortunately at the time of this study, the SDSS sample was not yet electronically avail-able.
The Coma cluster at z = 0.024 is one of the best studied local rich clusters. The cluster red-shift of z = 0.024 corresponds to a look–back time of only ∼0.3 Gyrs, or 2% of the age of the Universe, and therefore galaxies within this limit can be considered to represent the same cosmic epoch, i.e., the present-day Universe. A number of previous works in the literature (Saglia et al.
1993b; van Dokkum & Franx 1996; Jørgensen et al. 1999; Kelson et al. 2000b; Treu et al.
2001b; Ziegler et al. 2001a; Holden et al. 2005) used this cluster for the purpose of comparison of the Fundamental Plane at z >0.1 and it there-fore provides a reliable and widely accepted lo-cal reference when addressing evolutionary ques-tions. To achieve a direct comparability with
the results of these authors, the Coma data by Jørgensen 1999 and Jørgensen et al. 1995 (here-after collectively J99) will be used as a local com-parison sample for the following analysis of the early-type cluster galaxies. These authors per-formed a detailed study of a large number of early-type Coma galaxies in Gunn v, g and r-bands and Johnson U and B filters. The com-bined sample comprises 115 early-type galaxies, divided into subclasses of 35 E, 55 S0 and 25 intermediate types (E/S0). Note that J99 did not find any differences between elliptical and S0 or Sa bulge galaxies. For this reason, the local early-type galaxies are not splitted into different sub-classes based on their morpholo-gies. Absolute magnitudes cover a range down to Mr < −19.58 and the sample is 93 % com-plete at absolute magnitudes Mr<−20.02, cor-responding to r = 15.08m. In oder to match the local J99 sample, the parameters of the dis-tant clusters were aperture corrected (cf. sec-tion 5.3.2). In the intermediate redshift range, observations of early-type galaxies are preferably in the R or I filters. Atz= 0.2, the observedI and I814 passbands are very close to rest-frame Gunnr. For this reason, the advantages of using the Gunn r-band instead of the bluer JohnsonV orB bands are the smallerk–corrections and the lower Galactic extinction corrections.
In all subsequent figures, large symbols denote the distant early-type galaxies, either of the two rich clusters Abell 2218 and Abell 2390 or the three Low–LX clusters Cl 0849, Cl 1701 and Cl 1702, whereas small boxes represent the lo-cal reference sample. Morphologilo-cally classified lenticular galaxies (S0) or bulges of early-type spiral galaxies within the HST field of the clus-ters are indicated by open symbols, elliptical gal-axies are denoted by filled symbols. Under as-sumption of a non changing slope, the residuals from the local relation are computed and derive a mean evolution for the distant galaxies. For a comparison of the slope of the local and dis-tant galaxies, the bisector method is used (e.g.,
Figure 6.1: The local Faber–Jackson relation in Gunnrfor 115 early-type galaxies in the Coma clus-ter from Jørgensen (1999, J99) and Jørgensen et al.
1995, J99). The individual objects of the J99 sample are denoted by the small squares. The dot-dashed line is a bisector fit to the local FJR within the se-lection boundaries (dashed lines) as defined by the distant cluster samples, together with the 1σerrors.
Ziegler et al. 2001a), which is a combination of two least-square fits with the dependent and in-dependent variables interchanged. The errors on the bisector fits were evaluated through a boot-strap re–sampling of the data 100 times.
The early–type field galaxies in the FDF and WHDF cover a broader redshift range than the clusters and on average higher redshifts. A trans-formation of the measured magnitudes to the Gunn r rest–frame band is not appropriate as it would involve large k–corrections (cf. sec-tion 4.5.3). For this reason, as a local refer-ence the well studied Coma sample in the John-sonB-band by Saglia, Bender & Dressler (1993, hereafter SBD93) comprising 39 early-type clus-ter galaxies (splitted into 25 E and 14 S0s) was chosen. This data compilation is a sub–set of the original “7 Samurai” sample (Faber et al. 1989;
Dressler et al. 1987) which contains 59 early-type galaxies in the Virgo and Coma cluster (cf.
Chapter 6: Galaxy Scaling Relations at z∼0.2 129
Figure 6.2: Faber–Jackson relation in Gunnrfor 96 early-type galaxies in A 2390 and A 2218, compared to the local Coma sample of J99. The solid lines show the±1σerrors of the 100 iteration bootstrap bisector fits to the local FJR within selection boundaries, the hashed area indicates the bisector fits to the distant sample (within±1σ).
section 5.4.2). As for the latter no morphologi-cal information is available the Coma study by SBD93 was used as a local reference.