6.2 The Faber–Jackson Relation
6.4.1 Elliptical versus S0 galaxies
Plane have not revealed significant differences in the zero-point, slope and/or scatter between el-liptical and lenticular galaxies (Bender, Burstein
& Faber 1992; Saglia et al. 1993b; Jørgensen et al. 1996). Moreover, they behave very similar as one single group of galaxies with respect to their M/L ratios and within the FP. Going to higher redshifts, differences could be more significant if recent star formation activity plays a role for S0
Chapter 6: Galaxy Scaling Relations at z∼0.2 143
Table 6.4: Evolution of the FP in JohnsonB-band as derived for the early-type field galaxies in the FDF and WHDF.N shows the number of galaxies and ∆γindicates the mean FP zero-point offset. In the fourth and fifth column, the median FP zero-point evolution ∆hγiand the median evolution in the FP ∆hµei[in mag] are listed. The last column gives the±1σscatter of the mean offsets.
Sample N ∆γ ∆hγi ∆hµei σγ
FDF 11 0.066 0.075 −0.235 0.136
WHDF 10 0.199 0.214 −0.667 0.289
FDF+WHDF 21 0.130 0.089 −0.279 0.227 FDF+WHDFa 19 0.170 0.126 −0.393 0.197
E 9 0.075 0.075 −0.230 0.121
S0 12 0.171 0.211 −0.643 0.281
S0a 10 0.255 0.241 −0.735 0.219
S0b 9 0.214 0.211 −0.643 0.185
low lum.c 11 0.056 0.082 −0.255 0.180 high lum. 10 0.211 0.165 −0.515 0.254 high lum.d 9 0.164 0.126 −0.393 0.219 low masse 11 0.099 0.089 −0.279 0.221 high mass 10 0.163 0.126 −0.393 0.240
a Omitting # 6336 and # 508, both Sa bulges.
b Omitting # 6336 and # 508 and # 111.
c lower-luminosity: MB>−21.404, higher-luminosity: MB <−21.404.
d Omitting # 111 atz= 0.74.
eless-massive: logσ <2.275, more-massive: logσ >2.275.
galaxies. However, previous studies of samples at higher redshift did not find any differences between elliptical and S0 galaxies (van Dokkum
& Franx 1996; Kelson et al. 2000b). For ex-ample, within the large sample of 30 early-type galaxies in CL 1358+62 at z= 0.33 of Kelson et al. (2000b), the 11 ellipticals displayed identi-cal zero-points as the 13 (non–E+A) S0 galaxies with no hint for an offset between these groups at all. Moreover, the difference in the slope of
∼14 % detected between the S0 and elliptical gal-axies is not significant.
For the case of S0 galaxies, two main questions are still a matter of debate. How many are a pri-ori S0 galaxies? Which and how many are the result of galaxy transformations and account for the dominant S0 population in rich clusters
to-day? For scenarios which suggest a transforma-tion of star-forming spiral galaxies into passive S0 systems (Dressler et al. 1997), these objects would only be classified as lenticulars after their morphology has been changed (∼5 Gyrs). How-ever, after this relatively long time–scale, their star formation (SF) could already have ceased which would make these galaxies hard to detect via their SF. E+A galaxies, which are charac-terised by strong Hδ absorption lines, could rep-resent galaxies in an intermediate stage of such a transformation where the merging process of two individual galaxies just has stopped and the spheroidal remnant is indistinguishable from a regular E/S0 galaxy, but SF is still present due the result of a starburst which ended within the last 1.5–2 Gyr (Barger et al. 1996). Recently,
Figure 6.14: Edge-on view of the Fundamental Plane for A 2390 and A 2218 in rest-frame Gunn r. The distant FP is compared to the FP of the local Coma sample of J99, indicated by the principal component fit (thick solid line). Left: FP constructed using a combination of anr1/4 and exponential disc profile. Right:
Similar to the left plot except thatr1/4-law parameters were used.
Yang et al. (2004) performed a study of E+A galaxies in the local Universe and argued that these objects only show an offset in their surface brightnesses but not in their total magnitudes.
If such galaxies, which contribute about 20% to the galaxy population in clusters at z∼0.5, are also within the galaxy samples investigated here, they would have no offset in their absolute lumi-nosity and therefore would be indistinguishable from other non–E+A galaxies in the FJR which is based on total magnitude measurements. But E+A galaxies could be identified within the FP due to their high surface brightness magnitudes which makes them stand apart from the FP of early–type galaxies.
Figure 6.14 illustrates the edge-on view of the FP for 34 E+S0 galaxies of the clusters A 2218 and A 2390 in rest-frame Gunn r-band. The FP constructed for the distant clusters at intermedi-ate redshift is compared to the FP for the local Coma sample of J99. The combined sample rep-resents the most extensive investigation of early-type galaxies in clusters at z∼0.2. The Fig.6.14
has been splitted in order to visualise the small variations if different luminosity profile fits are used for deriving structural parameters. The left figure displays the FP constructed using a com-bination of anr1/4-law+exponential disc profile, the right figure is based on pure r1/4-law fits.
The variations in the structural parameters only affect the galaxies to move along the edge-on pro-jection of the FP plane, thereby maintaining the tightness of the plane. Both the r1/4 FP and r1/n FP have the same scatter and slope within their errors. A second comparison is given by the FP parameter RehIei0.8 in Fig. 6.15, where the structural parameters ofReandhIeiwere de-rived from a combination of anr1/4+exponential disc profile and by a purer1/4-law model. As the errors inReandhIeiare correlated and this cor-relation is almost parallel to the FP, the prod-uctRehIei0.8 can be established with high accu-racy. The individual measurements of Fig. 6.15 are listed in Table4.5and Table4.6(column 9).
In general, the agreement is good for both ellip-tical and S0 galaxies.
Chapter 6: Galaxy Scaling Relations at z∼0.2 145
Figure 6.15: FP parameter RehIei0.8 derived from a pure r1/4-law model compared to the RehIei0.8constructed using a combination of anr1/4 -law+exponential disc profile. The productRehIei0.8 holds for both elliptical and S0 galaxies and displays a scatter of only 4%.
In the following, possible differences between el-liptical and lenticular galaxies will be consid-ered in more detail. As a local reference the Coma FP coefficients by Jørgensen et al. (1996) for the Gunn r-band α = 1.24 ± 0.07 and β =−0.82±0.02 were adopted. For the Johnson B-band α = 1.25±0.06 and β = −0.80±0.02 were assumed which are slightly different but still within the scatter found for the Coma FP pa-rameters. The uncertainties in the coefficients of the local FP were assessed by the authors through a bootstrap fitting technique. Gener-ally, the choice of the fitting technique, the selec-tion criteria and the measurement errors which are correlated can lead to systematic uncertain-ties in the FP coefficients in the order of±0.1 dex (Jørgensen et al. 1996). Nevertheless, as the FP coefficients α and β depend only weakly on the wavelength and were fixed for the local and the distant sample, a significant change within a photometric band as a function of redshift can be ruled out.
A morphological analysis of the HST images
re-vealed that the A 2390 sub-sample splits nearly equally into elliptical (8) and lenticular (S0) gal-axies (6). Out of the 20 cluster members in the HST field of A 2218, nine and eleven were iden-tified as ellipticals and S0s, respectively. The early-type sample of the Low–LX clusters was morphologically classified as nine ellipticals and one S0 galaxy. As a comparison of sub–classes cannot be performed with these numbers, the following analysis concentrates on the rich clus-ters only. Both ellipticals and lenticular galaxies are uniformly distributed along the surface of the FP plane. An edge-on projection can therefore be taken for a robust comparison of their stellar populations. Table 6.2 lists the derived evolu-tion for the E and S0 galaxies. Fixing the slope of the local Coma reference, the zero-point offset for the ellipticals in the sample is
h∆γEc(z= 0.2)i= 0.01±0.07. (6.11) Restricting the sample to the elliptical galax-ies a negligible zero-point deviation with re-spect to the local FP is derived, which corre-sponds to an insignificant luminosity evolution of ∆MrE =−0.02±0.21m which is within their 1σ scatter. On the other hand, the ZP offset for the S0 galaxies yields
h∆γS0c (z= 0.2)i= 0.14±0.06. (6.12) This zero-point offset for lenticular galaxies cor-responds to an evolution ∆MrS0 =−0.44±0.18m with respect to the local counterparts. In com-parison to the ellipticals, the difference in evo-lution between E and S0 galaxies is significant on the 2σ level. For the combined sample of 34 E+S0 cluster galaxies a median offset of
h∆γE+S0c (z= 0.2)i= 0.10±0.06, (6.13) is derived, which corresponds to a brightening of the stellar populations by −0.31±0.18 mag.
The errors on the zero-points were individually derived as
δZP2=δFP2r+δBS2 (6.14)
Figure 6.16: Evolution of the field FP in rest-frame Johnson B-band. The early-type field galaxies in the FDF and WHDF are binned into three different redshift slices, each compared to the Coma galaxies of SBD93 (small squares) and shown along the short axis of the edge-on view. Filled symbols denote ellipticals, open symbols S0 galaxies and Sa bulges and the brackets the number of the respective morphological class.
For illustration reasons, only the mean error is shown in the left bottom. The offset of the distant field galaxies from the local FP increases with redshift, whereas the scatter appears to increase primarily for S0 galaxies with look–back time.
where δFPr denotes the total error which enters the FP in the rest-frame Gunn r-band (cf. sec-tion 4.3.3) andδBS is the uncertainty computed through an iterative bootstrap re–sampling of the data points 100 times. On average, the lenticular galaxies in the sample show a larger luminosity evolution than the ellipticals. In both interpretations of Fig. 6.14 the S0 galax-ies are predominantly located below the ellip-ticals and may indicate a different evolution-ary trend between the stellar populations of el-liptical and S0 galaxies. Dividing the sample with respect to velocity dispersion into a low mass (log (σ) < 2.283) and a high mass sub-sample (log (σ) > 2.283), a different evolution is found. The lower-mass galaxies are on aver-age with−0.47±0.24mmore luminous than their more massive counterparts with −0.03±0.21m atz∼0.2. This is also seen for the different sub-groups of elliptical and S0 galaxies, but with less significance (see Table 6.2). To investigate dif-ferences between elliptical and lenticular
galax-ies in clusters even further, the next section 6.5 concentrates on comparisons ofM/L ratios and masses of these galaxy types. Before this dis-cussion, possible variations in the morphological sub–classes of field galaxies will be explored.
Looking at differences between the morphologi-cal types offield early-type galaxies, the S0 gal-axies display a stronger evolution than the ellip-tical galaxies. Assuming that the slope of the lo-cal reference holds valid for the distant galaxies, the 9 field ellipticals show in edge-on projection of the FP a zero-point offset
h∆γEf (z= 0.4)i= 0.08±0.06, (6.15) which corresponds to a brightening in their stel-lar populations of ∆MBE = −0.23±0.18m. By comparison, the 12 field lenticulars exhibit an offset in the zero-point with respect to the local Coma galaxies
h∆γS0f (z= 0.4)i= 0.21±0.09, (6.16) which corresponds an average evolution of
∆MBS0 = −0.64± 0.27m. Errors in the
zero-Chapter 6: Galaxy Scaling Relations at z∼0.2 147
points of elliptical and S0 galaxies were analo-gously computed as in Eq.6.14with the only ex-ception that the FP error in the B-band δFPB
was utilised. Limiting the S0 galaxies to red-shifts z < 0.7 and rejecting the two Sa bulges, the same results are derived (see Table 6.4). In addition, the field ellipticals obey a tight FP with a small 1σ scatter of 0.121, whereas the S0 types have a larger dispersion of 0.219 (omitting the two Sa bulges). This gives further evidence that the discy galaxies have a larger range of different stellar populations. Even more astonishing is the fact that three elliptical galaxies in the WHDF sample feature weak [OII] 3727 emission in their spectra. In particular, the galaxies # 810, # 437 and # 810b exhibit [OII] 3727 =−3.73±0.47 ˚A,
−0.35 ±0.37 ˚A and −6.62±1.87 ˚A. Generally, a line emission is evidence for star formation activity which increases the average luminosity and thereby causing the galaxy to be offset from the local FP relation. In contrast to what is ex-pected, these ellipticals indicate no offset along the edge-on projection of the FP but a tight re-lation. One explanation could be that the star formation rates are too low to result in an visible effect within the FP. Although the evidence for the galaxy # 437 is rather poor due to its mea-surement uncertainty, it could be that the low star formation rates of the galaxies are either the relicts of a former starburst or the weak imprints of galaxy–galaxy interactions which have taken place at an earlier epoch. This suggests that galaxy–galaxy interactions which trigger the star formation activity are a common phenomenon among field galaxies.
The evolution of the FP for the early-type FDF and WHDF field galaxies in the rest-frame John-sonB-band is illustrated in Fig.6.16. Early-type galaxies were binned in redshift space to inves-tigate their location within and along the edge-on FP as a functiedge-on of redshift and to test the effects of possible outliers. Ellipticals are repre-sented as filled, S0s and Sa bulges as open sym-bols. For each morphological type the number of
galaxies within a redshift bin is indicated in the brackets. Small squares denote the Coma galax-ies by SBD93 and the straight line is a princi-pal component fit to the local sample (Ziegler et al. 2005). Fig. 6.16 clearly shows the evolution of the FP for the distant galaxies with respect to the local Coma FP. The offset of the field FP increases with redshift but the scatter ap-pears to be mainly amplified for lenticular galax-ies. The two galaxies with a “positive” evolution in the last two panels are the Sa bulges # 6336 and # 508 and show a clear disc on the ACS images. These objects can mimic on average a weaker evolution in luminosity for the whole gal-axy population (cf. Table 6.4). Regardless of the Sa bulges, the S0s exhibit a larger scatter at higher redshift 0.26< z < 0.75 than the ellipti-cals which suggests that the stellar populations of E and S0 types are different and that lentic-ulars are a more heterogeneous group. An ex-planation could be that these S0 galaxies resem-ble post–starburst galaxies as no strong emission lines were detected in their spectra.