Graziano CoppaM.Mignoli, G.Zamorani, M.Bolzonella, D.VerganiS.Bardelli, E.Zucca & the zCosmos collaboration

Università di Bologna - Dipartimento di Astronomia / Osservatorio astronomico di Bologna

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

• Redshift survey in the COSMOS field using VIMOS multi-slit spectrograph on VLT (Lilly et al. 2007)

• Bright: redshifts of 20 thousand galaxies with magnitudes I<22.5 covering 1.7 deg^2 (.1<z<1.2)• Deep: redshifts of 10 thousand galaxies selected in colours within central 1 deg^2 (1.4<z<3)

Scientific goals:• Determine environment• Large scale structures maps of few 100 km/s precision;• Diagnostics on galaxies themselves (SFRs, stellar population ages, metallicities)• Calibration of photometric redshift schemes to be applied to objects with no spectra.

The DATAThe DATA

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

The ideaThe idea

Galaxies show well known bimodal distributions:•Colours•Spectral features•Morphologies•Stellar populations•...

Transition objects exist“green valley”

“Green valley” is defined in many ways, mostly in U-V colour (Brand et al. 2006), but also in other colours (Strateva et al. 2001)

Faber et al. 2007

Can “green valley” be defined as a general property?

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

PC1 PC2 PC3 PC4 PC5 PC6 PC7 PC8

SD 4.69 1.13 0.87 0.51 0.34 0.19 0.18 0.08

C.Var 0.59 0.73 0.84 0.90 0.94 0.97 0.99 1

The PCAThe PCA4874 sources in redshift range ~0.48<z< ~1.28

Chosen variables:•D4000•EW[OII]•Δcol (B-z)•Gini coefficient•M20•clumpiness•concentration•asymmetry

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

The PCAThe PCA

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

Gath-Geva (GG) algorithm

Gath-Geva is able to detect clusters of various sizes, shapes & densities

Fuzzy membership – every object is “spread over” various clusters

Number of clusters c MUST be known a priori, as well as good priors for cluster centres and initial membership values (FCM or GK algorithms)

Cluster analysisCluster analysis

11

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May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

Membership threshold = 90%

Every object with membership between10% and 90% belongs neither to “red”cluster nor to “blue” cluster.

1413 (30%) early types3035 (62%) late types426 (8%) intermediate object (“green valley”)

“Green valley” objects fall in the low-density region of the PC plane

Cluster analysisCluster analysis

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

•Virtually all of the CUBE-111 objects reside in red cluster & all CUBE-222 in blue cluster

•Some of the CUBE outliers are pretty well defined within red cluster (211s, 121s)

•Others are spread over clusters (212s, 122s, 221s)

•Green valley objects seem to be quite heterogeneous

PCA/clustering vs. CUBEPCA/clustering vs. CUBE

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

•Dashed histogram shows distribution in U-B of the whole sample Bimodality

•Green histogram shows distribution of the “green valley” sample peaked around 0.6

•Independent parameter! (not included in PCA)

•Good agreement with literature

Green valley…Green valley…

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

•PCA + cluster analysis is a reliable, powerful and flexible way to catalog galaxies•“Green valley” objects are interesting, (masses, luminosities, X properties)

Conclusions & TBDConclusions & TBD

May 26th, 2009 – SNAF Bertinoro (FC)

Bimodalities for zCOSMOS galaxies: the hunt for Green Valley

THANKS

The (other) LOC