The zCOSMOS 3D density field

Katarina Kovač1,

Simon Lilly1,

C. Porciani1, O. Cucciati2, A. Iovino2, C. Knobel1, C.M. Carollo1, P. Oesch1, A. Finoguenov3 and the zCOSMOS

team**The zCOSMOS team comprises over 50 scientists; the main institutes involved are 1ETH Zürich, LAM Marseille, 2INAF Milan, Univ. Bologna,

3MPE Garching and LAOMP Toulouse

zCOSMOS (600 hrs on VLT, started April 2005):

- in the COSMOS field

- about 20,000 spectra 0.1 < z < 1.4 in “-bright”: IAB < 22.5 over 1.7 deg2

- about 10,000 spectra 1.4 < z < 3.5 in “-deep”: colour-selection, B < 25, over 0.9 deg2

- designed for high success rate (~ 90% in bright, ~ 80% in deep)

- and high sampling rate (~ 70%) with multiple passes (8 in bright, 4 in deep)

- with velocity accuracy of 100 kms-1 in bright, 300 kms-1 in deep

- duplication in spectral data reduction, redshift identification and other measurements

2’

6.5’

7.5’

Despite already covering most of the COSMOS field, the N(z) is very highly structured to z ~ 1:

zCOSMOS - observations so far

Currently, about 10000 spectra (10k “observed” sample) of the zCOSMOS-bright survey are reduced, from the underlying sample of about 40000 galaxies with IAB < 22.5 mag (40k “parent” sample), or about 110000 galaxies with IAB < 24 mag (100k sample).

10k spectroscopic verification and photo-z consistency

(1) 593 repeated spectra probabilistic verification of spectroscopic Confidence Classes plus measurement of velocity accuracy v = 108 kms-1

(2) Comparison with photo-z (ZEBRA: Capak 2006 catalogue (old K)+IRAC, calibrated on old 1k sample): z = 0.03(1+z).

Class % of sample

spectral verification

photo-z consistency

4 28% 99.6% 96.4%

3 34% 99.9% 95.5%

2 15% 92% 93%

one line 6.4% 90% 95%

1 9.4% 72% 73%

0 7.8% - -

Redshift reliability

=> 85% reliable redshifts

Reconstruction of the galaxy density field:

● One of the major goals of the zCOSMOS survey is to study galaxy environments, ranging from the scales of 100 kpc (galaxy groups) to the scales of 100Mpc.

We use both galaxies with the spectroscopic and photometric redshifts to reliable reconstruct the broad range of environments.

● galaxies are correlated!

● ZADE approach: use the nearby spectro-z objects to modify photo-z

P(z) of the rest of the population:

PZ(z)dz = Nspec (R<RZADE)* P(z)dz / ∫P(z)dz

IAB < 22.5 mag

red:

ZEBRA output

blue:

RZADE <= 1 Mpc/h

black:

RZADE <= 5 Mpc/h

magenta:

RZADE <= 10 Mpc/h

ZADE in work

• Density estimate: on a grid, counting the n-nearest neighbours

• all (parts of) galaxies within +/- 1000 km/s

• n = n(+1)/Sn ; Sn = (Dn)2

RZADE = 5 Mpc/h

Testing the method

Testing the method – cont.

Testing the method – cont.

zCOSMOS (over)density field

zCOSMOS (over)density field

Density-colour relation

Complete sample: MB > -20-z

Density-morphology relation

Complete sample: MB > -20-z

ZEST morphologies (Scarlata et al. 2007)Also Tasca et al. in prep.

Conclusions

● zCOSMOS so far: 10000 spectra, high reliability 85%

● ZADE approach able to reproduce structures on a large range of scales

● ZADE advantage over 10k: smaller N, no need to model the complicated 10k sampling and z-success functions

● ZADE works also with the fixed apertures; and using the fainter galaxies

● Density-colour and density-morphology relations hold up-to higher redshifts (z~0.5 at least)

● Exploration:

1) properties of galaxies as a function of environment

2) galaxy – dark matter bias

3) structure extraction