How do galaxies accrete their mass? Quiescent and star - forming massive galaxies at high z Paola Santini Roman Young Researchers Meeting 2009 July 21 st Università “Tor Vergata” Osservatorio Astronomico di Roma Post-doc

Before starting…… REDSHIFT Increase in the wavelength of e - m radiation received by a detector compared with the wavelength emitted by the source. Cosmological redshift is due to the expansion of the Universe: the distance between the emitting source and the observer increases while the light is propagating. high z Universe = distant Universe = young Universe Recession velocity increases with distance (Hubble’s law) c is finite

Kauffman et al. 93 ; Cole et al. 94; Somerville & Primack 00; Cole et al. 00; Menci et al. 02; Wu, Fabian, Nulsen 00, etc… “ab initio” MODELS OF GALAXY FORMATION Hierarchical formation : massive galaxies were assembled recently from mergers of smaller subunits (smaller DM halos collapse earlier) supported by observations: e.g. ongoing star formation at low z, paucity of high mass galaxies at z >1 The global picture of galaxy formation Dynamical evolution of dark matter condensations

The different models differ in the description of the baryonic processes, especially the two main processes driving galaxy evolution: Dekel+09 star formation conversion of gas into stars suppression of star formation most efficient: AGN feedback introduced to reproduce passive galaxies at high z Gas uniformly distributed in the halo, star formation on a disk Gas distributed on filaments, disk fragments, star formation on blobs Radio mode Quasar mode

z ≥ 2 : major phase in the assembly of massive galaxies 1) What drives the evolution of stellar mass at z ~ 2? (SF inside galaxies? Mergers?) 2) Quenching mechanisms? 3) Are these processes reproduced by the models?  Searching observables which directly reflect these two processes Fontana+06 Redshift M> M o

The GOODS - MUSIC sample U35 U38 (MPG/ESO-WFI) U VIMOS (VLT) B V i z (HST - ACS) J H Ks (VLT - ISAAC) µm (Spitzer - IRAC) Great Observatories Origins Deep Survey - MUltiwevelength Southern Infra - red Catalog (Grazian+06, Santini+09, Photometry from 0.3 to 24 µm (15 bands) ~ 143 arcmin 2 CDF-South ~ objects z, K s and 4.5 µm selected ~ 1800 spectroscopic z + well calibrated zphot 24 µm (Spitzer - MIPS)

Multiwavelength surveys SFR Dust Z M(stars) Photometric z Multicolour surveys allow to estimate photometric redshifts and physical properties of each object of the catalog from the overall spectral shape SED fitting U 360nm B 420nm V 520nm R 650nm I 800nm J 1250nm K 2200nm

PAH features Stellar emission Absorbed UV light Dust emission MIPS 24 μ m filter M82 (ISO) Future: Herschel SFR estimate PACS SPIRE 55  210  m 250, 350, 500  m

Empirical UV - to - midIR SEDs (Polletta+07) 24 µm 4.5 µm2.2 µm0.9 µm 4.5 µm2.2 µm0.9 µm Dusty star - forming Passively evolving Quiescent – star-forming selection

Quiescent galaxies

The very quiescent tail: Red&Dead galaxies SFR/M SFR/M < yr - 1 “RED&DEAD” galaxies 24 µm undetected galaxies Combined IR emission + SED fitting analysis Fontana+09

20% of massive galaxies is already in a very quiescent phase at z ~ Sensitive observable to constrain models: quenching mechanisms K07: Kitzbichler&White07 (Millennium Simulation) M06: Menci+06 F07: MORGANA (Monaco+07) N06: Nagamine+06 The cosmic evolution of Red&Dead galaxies Galaxies with very low levels of SFR Fontana+09 M> M o Time

Star-forming galaxies

SFR/M The Specific SFR — stellar mass relation Kitzbichler&White07 (Millennium Simulation) Santini+09 What drives the evolution of stellar mass at z ~ 2 ? Total accreted mass = active x “Duty cycle” argument: 65% of M > M o galaxies is actively SF - ing at 1.5 ~ 300M o /yr assuming that the active fraction is proportional to burst duration, the stellar mass acquired in this epoch is >10 11 M o At z ~ 2 massive galaxies are rapidly forming. The SFR directly observed in massive galaxies is enough to produce the bulk of the observed stellar mass density. Intense star formation processes within massive galaxies prevail over merging events at z ~ 2. SFR=1000 M o yr M o yr M o yr M o yr - 1

Comparison with theoretical predictions Models qualitatively reproduce the observations but underestimate the SFR in massive galaxies Kitzbichler&White07 (Millennium Simulation) MORGANA (Monaco+07) Menci+06 (See also Daddi+07, Davé+08, Fontanot+09) Santini+09

1) The epoch z> ~ 2 is a major phase in the assembly of massive galaxies 2) “Red&Dead” galaxies exist up to z=3 and most likely above: need for efficient feedback/quenching of SF mechanisms at high z 3) At z ~ 2, more than 50% of massive galaxies are experiencing a major peak in their SFRH: during this process they accrete a substantial fraction of their mass (see also Daddi+07) Summary & conclusions 5) Need for a different/new physics? 4) Theoretical models fail in predicting simultaneously the SFR (typically under - predicted) and the quenching of SF