zCOSMOS 10k: The role of group environment on the morphological

transformation of galaxies

Katarina Kovač1 and the zCOSMOS team*

*The zCOSMOS team comprises over 50 scientists; the main institutes involved are 1ETH Zürich, LAM Marseille, INAF Milan,

Univ. Bologna, MPE Garching and LATT Toulouse

Data: ● 10k zCOSMOS spectroscopy ● 10k zCOSMOS groups (Knobel et al. 2009)● ZEST morphologies (Scarlata et al. 2007)● Isolated galaxies (Iovino et al. submitted)

Completeness correction (sampling)● 10k-high CC vs 10k-total

● w = N10kCC (prop) / N10k (prop)

● prop: AB = 0.25 mag, (U-V)0= 0.25 mag, z=0.1 and

morph=0

● NP = i (1/wi)

● For the group richness 10k-high CC vs 40k

Fraction of early type galaxies: luminosity complete samples

•Fraction of early type galaxies is constantly increasing from higher to lower redshift in the group and field environments. For the isolated galaxies the build-up of early type galaxies is detected only for the faintest galaxies.

•At a given MB, fraction of early type galaxies is ordered as a f(environment)

•The morphological transformation of galaxies from late to early type MB dependenst

MB<-19-z in z<0.5

MB<-19.5-z in z<0.7

MB<-20-z in z<0.9

MB<-20.5-z in z<1

Fraction of early type galaxies: luminosity complete samples

•Indication that the fraction of early type galaxies in each of the luminosity complete samples is higher in the groups with larger effective richness, i.e. in the groups with larger masses

•Our result indicates that the fractions of early type galaxies are larger in the groups with larger velocity dispersion for MB<-19.5-z galaxies

Fraction of early type galaxies: mass complete samples

Bolzonella et al. (TBS): stellar mass function strongly dependent on environment (at least up to z ~ 0.7)

Relative contribution of galaxies of different type (late/early; blue/red) dependent on environment

Mass complete samples: 85% completeness for 1.0+2.0 ZEST types (Pozzetti et al. TBS)

log(M*/Mo) > 9.88 in z<0.45

log(M*/Mo) > 10.21 in z<0.6

log(M*/Mo) > 10.68 in z<0.8

log(M*/Mo) > 10.96 in z<0.95

Fraction of early type galaxies: mass complete samples

•Increase with cosmic time of the fraction of early type galaxies in all types of environment; for the highest mass no evolution in groups

•At a given stellar mass, the fraction of early type galaxies depends on environment

Mass bins:

Δlog(M*/Mo)=0.7

Rate of the morphological transformation (work in progress)

Rate=-(1/fL) (dfL/dt)

In the group environment, rate~0.12, in the isolated (no group) sample rate~0.5 for log(M*/Mo) < 11.2

Fraction of early type galaxies: mass complete samples

•Fraction of early type galaxies is increasing with the stellar mass up to z~0.8 (z~0.6 for the isolated galaxies)

•Fractions of early type galaxies environment dependent for lower stellar masses and z<0.6

Fraction of early type galaxies: mass complete samples

•Morphological crossing mass is decreasing from higher to lower redshift (downsizing)

•Indication for the faster shift in the morphological transformation towards lower masses in the group environment than in any other environment

Build-up of galaxies: sequence of events

Crossing mass: galaxies (on average) first shut-down SF, then turn red and after some additional time (~Gyr) their morphological transformation is complete; for galaxies of a given mass this happen first for the group galaxies and later for the isolated galaxies

Morphology-density relation in the groups (~Dressler)

•Morphology-density relation present in the groups up to z~0.6 in the mass complete samples

•Does this tell us something about the physical processes responsible for the morphological transformation?

Conclusions

Luminosity complete samples:

1) at a given MB, fractions of early type galaxies highest in the group environment

2) fractions of early type galaxies constantly increase in the group environment since z~1; the fastest increase for the least bright galaxies

3) Indication for the higher early type fractions in the more ~massive groups

4) Our data suggest that the morphological transformation of galaxies from late to early type has happened earlier for the brightest galaxies residing in the groups than for galaxies in the field.

ConclusionsMass complete samples:

• At a given stellar mass, the fraction of early type galaxies is highest in the group environment, then comes the fraction of early type galaxies and the fraction is lowest for the isolated galaxies at every redshift probed by that stellar mass

• The rate of morphological transformation ~2 times faster in the groups than for the isolated galaxies

• At every redshift, the fraction of early type galaxies is increasing with the stellar mass for galaxies residing in the groups and in the field

• The morphological crossing mass is decreasing from higher to lower redshift, indicating that the morphological transformation from late to early types starts in galaxies with higher stellar masses and shifts to galaxies with lower stellar masses over cosmic time

• The morphological crossing mass is decreasing more rapid for galaxies residing in the groups, than for the field and isolated galaxies, indicating faster shift in the morphological transformation towards lower masses in the group environment than in any other environment

• Sequence of events: first shut down of SF->colour transformation->morphological transformation : environment dependent