The analysis of the distant scaling relations of the FJR, KR and FP for distant early–type gal-axies at z ∼ 0.2 is in several ways consistent with previous studies. Early–type galaxies in all environments obey tight relationships similar to the local Universe. Assuming the local slope holds valid with increasing redshift, on average a modest evolution in the Gunn r and John-son B-band luminosities of the galaxies is de-rived. There is no significant increase of scatter for the distant FJR. However, elliptical galax-ies in poor clusters display a slightly larger scat-ter in the FP. In contrast to the local counscat-ter- counter-parts, the distant FJR have a shallower slope at the 2σ significance level, which implies a mass–
dependent luminosity evolution. Lower-mass gal-axies in all cluster environments are on average brighter by∼0.6 mag in rest–frame Gunnr-band at z ≈ 0.2 than locally, whereas the distant early-types with masses of M ≈ 3 ×1011M display only a mild evolution and have similar absolute magnitudes as their present–day coun-terparts.
The sub–classes of early–type galaxies, ellipti-cal and S0 galaxies, appear not to represent a homogenous group but follow different evolu-tionary tracks. Lenticular galaxies in high–mass clusters evolve faster than ellipticals, whereas el-liptical galaxies in low-mass clusters evolve as fast as S0 galaxies in rich clusters.
Assuming a typical galaxy in our sample with a size of Re = 0.34 kpc (which corresponds to 0.600) and a measured velocity dispersion of σ = 200 km s−1 gives a galaxy mass of M ≈ 1.0× 1011M. This represents a typi-cal mass of an early-type galaxy within the mass range covered by the cluster samples at z ≈ 0.2. For galaxy masses lower than this limit, the M/L evolution is faster than for gal-axies with masses of M > 2×1011M. This trend is clearly visible for all environments, from
the highest mass densities in rich clusters over lower-mass densities in poor clusters down to the field environment. For the less–massive field galaxies a faster evolution of the M/L ratios of ∆ ln(M/LB) = −(1.69 ± 0.26)z is found, whereas the more–massive ones evolve slower as ∆ ln(M/LB) = −(1.46±0.59)z. This evo-lution in M/L ratios can be translated in to a formation redshift for the lower–mass galax-ies (M < 2×1011M) of zform = 1.7 ±0.9, whereas for massive galaxies (M >2×1011M) zform = 3.2 ±1.0 is derived. The FDF and WHDF field early–type galaxies are consistent with the mass dependence of theM/L ratios as found in a recent study of E and S0 field galax-ies up to redshifts of z = 1 (van der Wel 2005).
These authors derive from a comparison with low–redshift data for the whole field galaxy sam-ple ∆ ln(M/LB) =−(1.75±0.16)z whereas gal-axies with massesM >2×1011Mshow aM/L evolution of ∆ ln(M/LB) =−(1.20±0.17)z.
These results suggests that the evolution corre-lates with the galaxy mass rather than the lumi-nosity of the galaxies. The lack of age difference found for field and cluster early-type galaxies and the dependence of the evolution on galaxy mass suggests that environmental effects play a rather minor role in the formation of early–type galaxies. More important to the evolutionary history of a galaxy are its internal properties, such as mass, size, velocity dispersion or chemi-cal composition.
The evolution of the M/L ratio in all environ-ments favours a down–sizing formation scenario (Kodama et al. 2004). In this picture, massive galaxies are predominantly dominated by red old stellar populations which evolve on longer timescales and passively. Less–massive systems, however, exhibit more extended star formation histories. In the down–sizing approach, the mass of galaxies hosting star formation processes decreases with increasing age of the Universe.
Both, the mass assembly on the one hand and the star formation on the other hand are
accelerated in massive stellar systems located in the high density environments. In contrast, in less–massive (smaller) systems, these physical processes work on longer timescales. Support for this formation picture is endorsed by several other observational studies, such as the lack of star formation in massive galaxies at lower redshifts z ≤ 1 (De Lucia et al. 2004), a fossil record of star formation in early-type galaxies detected in the local Universe (Thomas et al.
2005) or the evidence for a mass-dependent evolution in the Tully–Fisher scaling relation of spiral galaxies up to z = 1 (Ziegler et al. 2002;
B¨ohm et al. 2004).
It is difficult to anticipate on specific improve-ments of our knowledge of galaxy formation and evolution for the upcoming future. More obser-vational evidence is necessary to strengthen the results of this thesis and other studies. With the continuous rapid increase of computational power, a detailed numerical modelling on higher–
spatial resolution scales will be possible to put tighter constraints on the involved physical pro-cesses as well as the stellar populations proper-ties and chemical compositions of galaxies.
In the near future, the project described in this thesis is going to be continued and the size of the sample will be enlarged by additional high qual-ity spectra of a further ∼50 distant early-type galaxies. Thanks to the intermediate–resolution of the observed galaxy spectra, absorption line-strengths can be measured. By comparing ages, metallicities and chemical element abundance ra-tios to the calibrated library of local early-type galaxies of the Lick/IDS index system, deeper in-sights on the involved physical mechanisms and stellar population properties of early-type galax-ies will be provided. A short outline of this plan will be presented in the outlook at the end of the following summary.
Chapter 8: Summary and Outlook 175
Chapter 8
Summary and Outlook
Using high signal–to–noise intermediate–resolu-tion spectra taken with the ESO Very Large Telescope (VLT) and Calar Alto (CAHA) 3.5 m telescope complemented by deep ground-based and Hubble Space Telescope (HST) imaging, a study of early–type galaxies at redshifts 0.2 ≤ z ≤ 0.4 has been performed. An exten-sive sample of 121 early–type galaxies in vari-ous densities ranging from galaxy clusters with highest richness class, rich clusters, over poor richness class, poor Low–LX clusters, down to the isolated field population was constructed to investigate the evolution over the last ∼3 Gyrs, corresponding to 20% of the age of the Universe.
The motivation of the project was to look how the physical properties of early–type galaxies de-pend on their environment. The evolution of gal-axies in luminosity, size, mass and their stellar populations were intercompared to test possible environmental effects and to constrain the the-oretical formation and evolution model predic-tions.
Target objects were selected based on multi–
band ground-based imaging data. For the early–
type galaxies in the rich cluster Abell 2390,U BI broad-band colours were used, whereas early–
type candidates for three poor clusters were se-lected by a combination of a spectroscopic red-shift catalog and ground-based BV RI photo-metry. These Low–LX clusters display 1.0 dex lower X-ray luminosities than typical rich clus-ters and are, as a difference in LX corresponds directly to a difference in mass via the correlation
LX ∝M2, therefore considered to be low–mass galaxy clusters. Early–type field galaxies were selected over a redshift range of 0.2 < z <0.75 from the FORS Deep Field (FDF) and William Herschel Deep Field (WHDF), two multi–band imaging studies located in the southern and northern hemisphere respectively, with optical limiting magnitudes comparable to the Hubble Deep Fields and each covering a field-of-view of
∼70×70.
Spectroscopic candidates were selected accord-ing to their elongated structureless appearance, their luminosity, photometric redshift and an ad-ditional constraint of spectrophotometric type for the field targets. Main criterion was the to-tal apparentI orR-band brightness with a faint limit of I ≤ 20 mag for the cluster galaxies and R≤22 mag in case of the early–type field galax-ies. To study the evolution of early–type galaxies out to large clustercentric distances of ∼1 virial radii, the spectroscopic targets of the cluster en-vironments were distributed over a wide field-of-view of ∼100×100, which corresponds to a phys-ical field-of-view of∼1.4 Abell radii (∼2.7 Mpc) atz= 0.23.
In total, 142 high signal–to–noise spectra of 121 different early–type galaxies and additional 28 stellar template spectra were taken between Sept. 1999 to Oct. 2002 in Multi Object Spec-troscopy mode with the MOSCA spectrograph at the CAHA 3.5 m telescope and the FORS 1+2 instruments mounted on the VLT. The seeing conditions ranged between 1.2 to 1.7 arcseconds
for the CAHA observations and 0.4 to 1.0 arc-seconds for the VLT Paranal observations.
Spectroscopic redshifts could be derived for 121 early-type galaxies and 9 spiral galaxies and 12 secondary objects. No galaxy spectra of the tar-gets had too low signal–to–noise to determine a redshift.
The multi–band photometry data provided the possibility to measure accurate luminosities of the galaxies. Apparent brightnesses for the clus-ter galaxies were transformed to the rest–frame Gunn r-band. For the field galaxies in the FDF and WHDF apparent magnitudes were derived in either of the B, g, R or I passband which, depending on the redshift of the galaxy, best matched the rest–frame B-band. k-corrections were computed via synthetic photometry with an observed spectral template and a spectral temp-late from chemically consistent evolutionary syn-thesis models. Both independent approaches yielded within their uncertainties very similar re-sults. For the cluster galaxies the uncertainties in the k-corrections were δkr ≤0.05 mag and in case of the field objects δkB ≤0.1 mag over the complete redshift range.
Structural parameters for 1/4 of all early–type galaxies were analysed on the HST/WFPC2 (Wide Field and Planetary Camera 2) and HST/ACS (Advanced Camera for Surveys) im-ages using the surface brightness profile fitting algorithm developed by Saglia et al. (1997a), which searchs for the best combination of seeing-convolved, sky-corrected r1/4 and exponential laws. The total HST/F702W or F814W-band magnitude, effective (half-light) radius, central surface brightness and the mean surface bright-ness within the effective radius, were derived for the entire galaxy as well as for its bulge and disc component separately. This approach accounts for various types of observed surface brightness profiles, such as elliptical galaxies with a flat core, discy S0 galaxies or the extended profiles of central cD galaxies. The two-dimensional surface brightness distribution of the early–type
field galaxies was fitted with a combination of r1/n plus exponential disc profiles using the GALFIT package. Both algorithms gave within their errors consistent results. The complete set of structural parameters was subject to a robust and careful error treatment.
Elliptical and lenticular (S0) galaxies were morphologically classified in three inde-pendent methods using the high–resolution HST/WFPC2 and HST/ACS images. A visual inspection was executed for a sub–set of ellip-tical and S0 galaxies. Results from the surface brightness profile fitting provided a second verification and a third quantitative analysis was conducted using the bulge–to–total (B/T) fractions of the galaxies. A weak correlation of the measured bulge fraction with visual morphological Hubble type was found.
Radial velocities and velocity dispersions (σ) of the early–type galaxies were computed from the G4300-band, Hβ or Mgb absorption lines using the Fourier Correlation Quotient (FCQ) method (Bender 1990). To derive the kinematic proper-ties of the distant galaxies several modifications on the parameter settings were implemented, such as the subtraction of the continuum level in the spectra or the instrumental resolution. To ensure a reliable error treatment for the veloc-ity dispersions and radial velocveloc-ity measurements, Monte Carlo simulations for a set of different stellar templates over a range of input veloc-ity dispersions and S/N ratios were performed.
Synthetic galaxy spectra with artificial noise and for a specific instrumental setup and resolution were generated and the velocity dispersions re-covered by the FCQ software.
Velocity dispersions based on high signal–to–
noise (hS/Ni >∼ 30 per ˚A) spectra could be de-rived for 110 early–type galaxies regardless of their environment. For six galaxies with very low signal (S/N <∼ 8 per ˚A), no σ measure-ment was possible. A visual inspection of theσ values was conducted for each galaxy spectrum to assess the overall quality and reliability of
Chapter 8: Summary and Outlook 177
the measurement. Extensive consistency checks were performed by comparing σ determinations from different absorption lines, repeat observa-tions, different extraction procedures and stellar libraries.
The total sample analysed in this study com-prises 147 early–type galaxies. 48 early–type galaxies are members of the rich cluster A 2390 (z = 0.23), 27 galaxies are associated to one of the three Low–LX cluster CL 0849, CL 1701 and CL 1702 (hzi = 0.23), and 24 are field galaxies in the FDF and WHDF (hzi = 0.4). In addi-tion, 48 early–type galaxies of a previous study of the rich A 2218 cluster were combined with the A 2390 data to study a total of 96 early–type galaxies in the densest environments.
In the further analysis, the scaling relations of the Faber–Jackson (FJR) and Kormendy rela-tion (KR) as well as the Fundamental Plane (FP) for distant early–type galaxies at z ∼ 0.2 obey tight relationships similar to the local Uni-verse. Assuming the local slope holds valid with increasing redshift, on average a modest evo-lution in the Gunn r and Johnson B-band lu-minosities of the galaxies is derived, regardless of their environment locus. There is no signif-icant increase of scatter for the distant FJR, whereas elliptical galaxies in poor clusters indi-cate a slightly larger scatter in the FP. In con-trast to the local counterparts, the distant FJR have shallower slopes at the 2σ significance level, which implies amass–dependent luminosity evo-lution. Lower–mass galaxies in all cluster envi-ronments are on average brighter by∼0.6 mag in rest–frame Gunnr-band atz ≈ 0.2 than locally, whereas the distant early-types with masses of M ≈3×1011M display only a mild evolution and have similar absolute magnitudes as their present–day counterparts. Evidence for this re-sult is even found for the early–typefield galax-ies. Their mass–to–light ratios (M/L) are con-sistent with the trend detected within a recent study of E and S0 field galaxies in the literature.
For early–type cluster galaxies, a slight radial
de-pendence is found. Galaxies in the outer districts of clusters at ∼0.25 virial radii (∼0.46 Mpc) show a stronger brightening by ∼0.2 mag than their counterparts in the central cluster regions.
A difference between the morphological sub-classes of early–type galaxies, elliptical and S0 galaxies, was revealed. Lenticular galaxies in rich clusters exhibit a stronger luminosity evo-lution compared to the local Coma FP than the ellipticals. The stellar populations of these sys-tems could be more diverse or comprise more complex star formation histories. Support of this assumption is accumulated by an analysis of the rest–frame galaxy colours. The bluer galaxy population ((B−I) <2.287) shows larger FJR residuals from the local FJR, whereas the redder galaxies of the red–sequence of early–type clus-ter galaxies are less offset. Inclus-terestingly, the S0 galaxies in both colour ranges show a stronger lu-minosity evolution (brighter by∼0.7 mag for the bluer and ∼0.45 mag for the redder group), but the blue ellipticals have on average only a mild brightening of ∼0.3 mag and the redder even a
“positive” evolution, i.e. they are fainter than the local reference.
The lack of an age difference found for field and cluster early-type galaxies and the dependence of the evolution on galaxy mass suggests that en-vironmental effects are not the dominant factors which drive the formation of early–type galax-ies. Internal properties of the galaxies, such as mass, size, velocity dispersion or chemical com-position, are the main contributors to the evolu-tionary history of elliptical and lenticular galax-ies.
The evolution of the M/L ratio in all environ-ments favours a down–sizing formation scenario (Kodama et al. 2004). Massive galaxies are predominantly dominated by red old passively evolving stellar populations, whereas less mas-sive systems have more extended star formation histories. The mass of galaxies hosting star for-mation processes thereby decreases with the age of the Universe. Both mass assembly and star
formation are accelerated in massive stellar sys-tems located in the high density environments, whereas the processes work on longer timescales in less–massive (smaller) systems.
The results of this thesis therefore strengthen the need for more realistic implementations of stellar population properties into the numerical codes of N-body simulations. In particular, the fun-damental physics of the star formation processes and their involved timescales still are not well understood. Progress in the near future seems promising, looking at the most recent analytical approaches and their improvements. Recently a detailed simulation of the formation of individual early–type galaxies in a fully cosmological con-text was achieved (Meza et al. 2003). Neverthe-less, crucial mechanisms such as star formation processes were still treated based on simplified assumptions.
It is hard to predict whether a more realistic modelling of the stellar content of the galaxies will lead to a consistency between theory and observations. Some modifications on shorter scales of the hierarchical scenario would prob-ably be needed to reach a better consensus.
Further research in the upcoming years is going to be very important for our understanding of the formation and evolution of early–type galaxies to achieve a complete picture of the fundamental processes of structure growth from the early universe up to the present.
The project on distant early–type galaxies de-scribed in this thesis will be continued in the next years by the author. Spectroscopy of the integrated stellar light of a galaxy is a powerful tool for deriving the age, metallicity and grav-ity or temperature of their stellar population.
To quantify these intrinsic properties, spectral indices have been introduced which characterise the strength of a specific absorption or emission line. Thanks to the intermediate–resolution of the observed early–type galaxy spectra,
absorp-tion line-strengths can be measured and com-pared to the Lick/IDS index system. Light-averaged ages, metallicities and chemical ele-ment abundance ratios can be derived and di-rectly confronted to the predictions of stellar population models. In combination with the lu-minosity evolution measured via the scaling re-lations, results will give strong constraints on the formation redshift and the involved physical mechanisms of early–type galaxies.
To extent the sample even more, observations of
∼50 early–type galaxies in three Low–LX clus-ters at 0.2 < z < 3.0 have been acquired using MOSCA spectra. This data will allow to enlarge the statistical significance of the results for the poor clusters presented in this thesis.
Furthermore, a joint research project together with the MPE in Munich to study the colour gradients in cluster galaxies for different environ-ments was already initiated by the author. A de-tailed analysis of HST/WFPC2 and HST/ACS structural parameters in different filter pass-bands and with independent algorithms for mod-elling the galaxy surface brightness profiles will reveal how the colour gradients and the shape of galaxies depend on the computed structural parameters.
A combination of all spectroscopic data and the HST images will make up one of the largest kinematic samples of early–type galaxies at red-shift z = 0.2. Together with the analysis of the stellar populations and chemical abundances of these galaxies deeper observational insights will be achieved into how early–type galaxies, in par-ticular S0 galaxies, form and evolve with red-shift.