Local data samples of early-type galaxies cover a broad range in velocity dispersions as the kinematics of high– and low–luminosity galax-ies can be resolved. Going to higher redshifts, a magnitude–limited sample of galaxies gets af-fected by the so-called “Malmquist bias of the 2nd kind” (Malmquist 1920; Teerikorpi 1997), where the magnitude limit cuts away parts of the fainter wing of the luminosity function. Due to this “selection effect”, the detection rate of low–
luminous galaxies becomes significantly lower and therefore the overall data set relys only on the most luminous objects, thereby resulting in a higher average luminosity for an intermediate redshift sample than for the local counterpart sample.
For this reason, it is important to check whether the velocity dispersions of the distant E+S0 gal-axies show a distribution different from that of local elliptical and S0 galaxies. If indications
Figure 5.17: Distribution of velocity dispersions for the combined Low–LX clusters CL 0849, CL 1701 and CL 1702 in comparison to the local samples of Jørgensen, Franx & Kjærgaard 1995 and Jørgensen 1999 (J99). The complete histograms comprise 15 E+S0 galaxies in CL 0849, five E+S0s in CL 1701, seven E+S0s in CL 1702 and 115 E+S0s in Coma (J99). The vertical dotted line indicates the com-pleteness limit of the combined distant cluster sam-ples.
of an evolution in the galaxy kinematics are de-tected, than these would have to be taken into account in the attempts to quantify the evolu-tion in luminosity within the scaling relation of the FP and its projections (FJR and KR).
Two different large sets of local comparison sam-ples were used for this purpose. For the clusters, a sample of local early-type Coma galaxies in the Gunn r-band was chosen, whereas for the field galaxies a reference of Coma galaxies and a data set comprising of Virgo and Coma galaxies in JohnsonB-band was used.
5.4.1 Cluster Samples
Jørgensen, Franx & Kjærgaard 1995 and Jørgensen 1999 (hereafter collectively J99) per-formed a detailed study of 115 early-type Coma galaxies in the Gunn r-band. The combined sample is divided into subclasses of 35 E, 55 S0 and 25 intermediate types (E/S0). Absolute
Chapter 5: Kinematic Analysis 115
Figure 5.18: Left panel: Distribution of velocity dispersions for the two rich clusters A 2218 and A 2390 in comparison to the local samples of Jørgensen, Franx & Kjærgaard 1995 and Jørgensen 1999 (J99). Sub-sample of 17 ellipticals (E) in A 2218 and A 2390 (shaded histogram) and 35 Coma ellipticals (J99). Right panel: Distribution of velocity dispersions for the two rich clusters A 2218 and A 2390 in comparison to the local samples of Jørgensen, Franx & Kjærgaard 1995 and Jørgensen 1999 (J99). Sub-sample of 17 lenticular (S0/Sa) galaxies in A 2218 and A 2390 (shaded histogram) and 66 Coma S0/Sa galaxies (J99). The vertical dotted line indicates the completeness limit of the combined distant cluster samples.
magnitudes cover a range down toMr<−20.02, which corresponds according to the authors to a completeness level of 93%. 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 orI filters. At z= 0.2, the observed I and I814 passbands are very close to rest-frame Gunn r. Therefore, the advantages of using the Gunn r-band instead of the bluer JohnsonV or B bands are the smaller k-corrections and the lower galactic extinction corrections.
The distribution of velocity dispersions for the complete cluster samples of A 2218 and A 2390 is compared to the local reference in Fig. 5.16.
Fig. 5.18 shows the distributions in σ for the sub-samples of ellipticals (E) and lenticular (S0) galaxies in comparison to the local J99 sam-ples of E and S0 galaxies. The binning was set to 15 km s−1 in order to give a representative view for both data sets. Overall, both distri-butions show a similar spread in σ. However, the distant samples failed to reproduce the
gal-axies with low velocity dispersions due to the selection upon apparent magnitudes. For this reason, the Coma galaxies were restricted to Mr < −20.42 and log σ > 2.02 (105 km s−1), which represent the completeness limit of the combined A 2218 and A 2390 data. Both the distant clusters and the local sample are fit-ted only within this region shown by a verti-cal dotted line in Fig. 5.16 and the subsequent plots. The velocity dispersions for the galaxies are equally distributed. In particular, the σ dis-tribution of the unrestricted 115 local galaxies has a median value of hσiJ99u = 154 km s−1 and the restricted 97 early-type galaxies a me-dian of hσiJ99r = 168 km s−1. The distant galaxies for A 2218 and for A 2390 show medians of hσiA2218 = 179 km s−1 and hσiA2390 = 165 km s−1, respectively. The com-bined cluster sample of 96 galaxies indicates a median in σ of hσiRich = 170 km s−1, which is in very good agreement (differs by only 1%) with the restricted local comparison data. As the velocity dispersion is an indicator for the mass of an object and the measured σ values
exhibit similar ranges, it can also be concluded that there are no significant differences in mean galaxy masses between the two samples.
A one-dimensional Kolmogorov-Smirnov (KS) test (Press et al. 1992) gives a maximum devia-tion between the cumulative distribudevia-tion func-tions of the combined distant and the local J99 sample of D = 0.14 with a probability of P = 0.34 for a larger value. The KS result yields a fairly good agreement, although the probabil-ity value is quite high.
In Fig. 5.17 the distribution of velocity disper-sions for the complete Low–LX cluster samples of CL 0849, CL 1701 and CL 1702 is shown.
As a local comparison, again the J99 data set is applied. To account for the completeness limit of the combined poor cluster galaxies, the Coma galaxies were again restricted to Mr < −20.42 and σ > 105 km s−1. Spectro-scopically confirmed cluster members comprise 15, five and seven galaxies for CL 0849, CL 1701 and CL 1702, respectively. Due to the low number of galaxies per cluster, a separation into elliptical and S0 is not provided for each individual cluster. The velocity dispersions for the poor clusters show a very homogenous distribution. In particular, the σ distribution for CL 0849, CL 1701 and CL 1702 feature me-dian values of hσiCL 0849 = 195 km s−1,
hσiCL 1701 = 155 km s−1 and
hσiCL 1702 = 164 km s−1, respectively. The combined cluster sample of 27 galaxies indicates a median in σ of hσiPoor = 164 km s−1, which is lower by 4 km s−1 than the restricted J99 sample and differs only by 2% from the local counterpart. This is in very good agreement although the Low-LX clusters show a lack of galaxies around ∼200 km s−1. In addition, the σ distribution of the Low–LX cluster galaxies is similar to the distribution covered by the rich cluster sample. Their median σ values differ by 6 km s−1 or by 4%.
For a comparison of the Low–LX sample with the J99 data set in terms of statistics, the J99 was
reduced to the same number of 27 poor cluster galaxies, thereby accounting for the original J99 coverage. A KS test yields a maximum devia-tion between the cumulative distribudevia-tion func-tions of the combined poor cluster and the local J99 sample of D = 0.15 with a probability of P = 0.93 for a larger value. This result is of less confidence as the distant sample contains only 27 objects and depends on the selection of the local counterpart data set. Due to the small numbers for the sub–samples of elliptical and lenticular galaxies, a quantitative interpretation of these distributions in terms of a KS test is not appro-priate.
The sub-samples of ellipticals (E) and lenticular (S0) galaxies for the two rich clusters are similar distributed to the local E and S0 galaxies of J99, which is shown in Fig. 5.18. In total, the local sample comprises 35 E, 5 E/S0, 55 S0, 6 S0/Sa.
The distant sub-sample with accurate structural parameters provided by HST comprises in total 34 E+S0 cluster galaxies. The clusters of A 2218 and A 2390 are divided into sub-classes of 9 E, 1 E/S0, 5 S0, 3 SB0/a, 1 Sa and 1 Sab spiral bulge and 8 E, 1 E/S0, 4 S0, 1 Sa, respectively. A detailed morphological classification is presented in section 4.4. For the ellipticals, only morpho-logically classified galaxies as E have been in-cluded. Intermediate types of E/S0, S0/Sa and Sa galaxies are accounted for in the discy sub-sample of S0/Sa types. In particular, five E/S0 are within the local J99 sample and only two in the distant sample which have a negligible ef-fect on the overall distributions if the they are considered as elliptical types. Moreover, the his-tograms are not biased when excluding the sub–
classes of S0/Sa galaxies. There are only six lo-cal S0/a galaxies and 3 Sa(b) types in the distant clusters. This is due to the fact, that actually the bulge of Sa galaxies is measured which is domi-nated by the chaotic random motion of the stars (i.e. by σ) rather than their rotational velocity.
Therefore, the notation of Sa bulge is a much more appropriate term and will be used in the
Chapter 5: Kinematic Analysis 117
Figure 5.19: Distribution of velocity dispersions for the FDF and WHDF early–type galaxies in compar-ison to the local sample of Saglia, Bender & Dressler 1993 (SBD93) and Dressler et al. 1987 (7S). The com-plete histograms comprise 24 E+S0 field galaxies in the FDF, WHDF (shaded histogram) and 39 Coma E+S0 galaxies of SBD93 (solid line) and 59 Virgo and Coma E+S0 galaxies of 7S (dotted histogram).
following. The poor cluster sample comprises 9 E but only one S0 galaxy. Therefore, a comparison of sub–types cannot be conducted.
Median σ values of the 17 E and 17 S0 galaxies in the two rich clusters and the 35 E and 66 S0 Coma galaxies are hσiRich E = 208 km s−1, hσiRich S0= 170 km s−1,hσiJ99 E = 170 km s−1, hσiJ99 S0 = 154 km s−1, respectively. Overall, there is a good agreement between the distant and local sub–classes of elliptical and lenticular galaxies. Elliptical galaxies in the rich clusters appear to have slightly larger velocity disper-sions. However, the J99 sample comprises no elliptical galaxies withσ >260 km s−1.
5.4.2 Field Sample
For the 24 FDF and WHDF early–typefield gal-axies encompassing on average higher redshifts over a broader range in redshift than the clus-ters, the rest-frame Gunn r-band is not an ap-propriate choice and thus the magnitudes were transformed to rest-frame JohnsonB. For a local
reference, a well studied sample of 39 early-type galaxies (splitted into 25 E and 14 S0s) in the Coma cluster (Saglia, Bender & Dressler 1993, hereafter SBD93) of the “7 Samurai” data set (Faber et al. 1989), and the original “7 Samu-rai” sample (Dressler et al. 1987, hereafter 7S) comprising 59 early-type galaxies in the Virgo and Coma cluster were chosen. No morphologi-cal information is available for the latter.
Fig. 5.19 displays the distribution of velocity dispersions for the complete FDF and WHDF field ellipticals compared to the local reference of SBD93 and 7S, respectively. Overall, there is a good agreement between the two data set.
The 13 FDF ellipticals feature a median in σ of hσiFDF = 178 km s−1, the 11 WHDF ellipticals exhibit a median of hσiWHDF = 150 km s−1, whereas the combined field sample shows a me-dian in σ of hσiFDF+WHDF = 178 km s−1. The median velocity dispersion in the local SBD93 sample ishσiSBD93 = 162 km s−1 and for the 7S sample hσi7S= 191 km s−1.
The 24 FDF and WHDF early–type field galax-ies are divided into 10 E and 12 S0. Two galaxgalax-ies are not visible on the ACS images and therefore not included in either group. Analogous as for the cluster galaxies, only morphologically classified field elliptical galaxies were regarded as E type (cf. section 4.4). Fig. 5.20 gives a detailed look on the sub-samples of ellipticals and lenticular field galaxies compared to the local E and S0 sample of SBD93, which com-prises 25 E and 14 S0/SB0 galaxies. Both, the field elliptical and lenticular FDF and WHDF galaxies show a similar distribution inσ to their local counterparts. Median values for the distant field E and S0 galaxies and the E and S0s of the SBD93 samples are hσiField E= 200 km s−1,
hσiField S0 = 170 km s−1,
hσiSBD93 E = 198 km s−1,
hσiSBD93 S0 = 150 km s−1, respectively.
The agreement between the sub–classes is good. Moreover, differences between the FDF and WHDF ellipticals and S0s are negligible.
Figure 5.20: Left panel: Distribution of velocity dispersions for the FDF and WHDF early–type galaxies in comparison to the local sample of Saglia, Bender & Dressler 1993 (SBD93). Sub-sample of 10 ellipticals (E) in the FDF and WHDF (shaded histogram) and 25 Coma ellipticals (SBD93). Right panel: Distribution of velocity dispersions for the FDF and WHDF early–type galaxies in comparison to the local sample of Saglia, Bender & Dressler 1993 (SBD93). Sub-sample of 12 lenticular (S0/Sa) galaxies in the FDF and WHDF (shaded histogram) and 14 Coma S0/SB0 galaxies (SBD93).
However, for this comparison it has to be taken into account that observations of distant field galaxies at higher z tend to miss low-luminous galaxies which lower velocity dispersions due to limitations in luminosity. Therefore, part of slightly differentσdistributions is not attributed to deviations between the samples but due to a selection effect.
On the basis of the complete cluster and field samples as well as the sub–samples for ellipticals and lenticulars it therefore can be concluded that the distant early-type galaxies do not indicate an evolution in the velocity dispersions. Note that it is particularly important when comparing the evolution of theM/Lratios for distant and local early–type galaxies, to reduce differences in the overall mass distribution of the samples. This point will be re–addressed in section 6.5. From the comparison given above it is strengthened that the distant cluster and field galaxies con-stitute representative early–type galaxy samples and deviations between local and distant scaling relations will mainly have to be attributed to an evolution in theluminosity, but not to differences
in the kinematics.