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The size evolution of early-type galaxies since z=2 P. Saracco1, M. Longhetti1, with the contribution of S. Andreon1, A. Mignano1, G. Feulner 2, N. Drory 2, U. Hopp 2, R. Bender 2 1 INAF – Osservatorio Astronomico di Brera, Milano 2 Max Planck Institute and University of Munchen Bologna 22.01.2009 Outline of the talk Small/compact Early-Type Galaxies (ETGs) at z>1: first evidence A morphologycal study of a sample of 10 ETGs at 1.2<z<1.7: size evolution of ETGs required The population of ETGs at 1<z<2: new clues on their formation and evolution ? Summary and conclusions Bologna 22.01.2009 Small size, high-density ETGs: first evidence Daddi et al. (2005) Hubble UDF - 7 ETGs z>1.4 HST-ACS obs., FWHM~0.12”, F850W filter, λrest<3000 Ǻ Bologna 22.01.2009 Further evidence Cassata et al. (2005) Trujillo et al. (2006) Re [Kpc] Re [Kpc] K20 + GOODS data Mass IR ground based observations FWHM~1.0 arcsec redshift HST-ACS observations, F850W λrest<3000 Ǻ Bologna 22.01.2009 Are ETGs at z>1 really more compact/denser than local counterparts ? These results were based on • HST optical observations sampling the blue and UV rest-frame of the galaxies sensitive to k-correction and star formation and/or • seeing limited ground-based observations Doubts on the reliability of the estimate of Re Doubts on the reliability of the comparison high-z vs low-z High-resolution near-IR obs. sampling λrest~6500 Ǻ for a reliable comparison between high-z and low-z ETGs. Bologna 22.01.2009 HST-NICMOS observations in the F160W (λ~1.6 µm) filter of a sample of 10 ETGs at 1.2<z<1.7. 0.075 “/pixel (Longhetti et al. 2007) Data sampling the rest-frame R-band (λrest~6500 Ǻ) at z~1.4, at a spatial resolution <0.8 kpc (FWHM~0.12 “) NIC2 images • Effective radius re (arcsec) and mean surface brightness (SB) <>e within re from Sersic profile fitting I (r ) I e e models bn [( r / re )1 / n 1] residuals n=4 de Vaucouleurs profile n=1 exponential profile • galfit (Peng et al. 2002) to perform the fitting after the convolution with the NIC2 PSFs. z=1.34 z=1.40 z=1.7 n=3.2 n=4.5 n=2.7 Bologna 22.01.2009 The Kormendy relation in the R-band It is a scaling relation between the effective radius Re [Kpc] and the mean SB <>e [mag/arcsec2] e log( Re ) The ETGs follow this tight relation with ~3 up to z~1. is found to vary reflecting the luminosity evolution. e M ( z ) 5 log( Re ) 38.57 Expected KR at z=1.5 Any deviation from the KR at z=0 should reflects the evolution of <>e due luminosity evolution . passive luminosity evolution (maximum evolution expected for early-types). Observed KR at z=0. Expected locus for z<1.5 early-type galaxies in case of luminosity evolution. Bologna 22.01.2009 The Kormendy relation in the R-band It is a scaling relation between the effective radius Re [Kpc] and the mean SB <>e [mag/arcsec2] e log( Re ) The ETGs follow this tight relation with ~3 up to z~1. is found to vary reflecting the luminosity evolution. e M ( z ) 5 log( Re ) 38.57 Expected KR at z=1.5 The SB exceeds by ~1 mag the one expected in the case of PLE for constant Re, i.e. passive luminosity evolution (maximum evolution expected for early-types). Observed KR at z=0. luminosity evolution does not account for the observed SB of ETGs at high-z. Expected locus for z<1.5 early-type galaxies in case of luminosity evolution. (Longhetti et al. 2007) Bologna 22.01.2009 Are ETGs at z>1 really more compact/denser than local counterparts ? These results are based on • HST near-IR observations sampling the red rest-frame of the galaxies NOT sensitive to k-correction and star formation and/or • NO seeing limited ground-based observations NO doubts on the reliability of the estimate of Re High-z ETGs (at least some of them) are more compact then their local counterparts. (Longhetti et al. 2007) Bologna 22.01.2009 The Kormendy relation in the B-band GMASS sample 13 ETGs 1.4<z<2 Spectroscopic data Morphology based on HST-ACS obs. F850W (λrest~3000 Ǻ) (Cimatti et al. 2008) Bologna 22.01.2009 Literature and HST archive research Aim – to collect a large (larger than 10…!) sample of ETGs at z>1 with Sample •10 spectroscopic confimation of(Saracco the spectral ETGs 1.2<z<1.7 from TESIS et al.type; 2005; Longhetti et al. 2005) +•10 ETGs 1.4<z<1.9 from GDDS in (Abraham et al.filter; 2004; McCarthy et al. 2005) HST-NICMOS observations the F160W +•6multiwavelength ETGs z~1.27 from RDCS 0848+4453 (Stanford et al.1997; van Dokkum et al. coverage (optical + near-IR) 2003 + 3 ETGs 1<z<1.8 from HDF-N (Stanford et al. 2004) in order to study the population of ETGs at 1<z<2 from an homogeneous set of data and a uniform analysis + 2 ETGs z=1.4,1.9 from GMASS H-UDF (Daddi et al. 2005; Cimatti et al. 2008) + 1 ETGs z=1.55 53W091 (Dunlop et al. 1996; Waddington et al. 2002) = 32 ETGs 1<z<2, 17.0<K<20, HST-NICMOS observations F160W • covering a larger interval in luminosity; NIC2 (0.075 ”/pixel) for 14 galaxies • defining the scaling relations at z~1.5 NIC3 (0.2 “/pixel) for 18 galaxies (Kormendy, size-luminosity/mass relations) FWHM ~ 0.12 arcsec Bologna 22.01.2009 Physical properties of ETGs Morphological parameters • effective radius and surface brightness derived as in Longhetti et al. (2007); • Simulations done also for NIC3 images rNIC 2 0.02arc sec rNIC 3 0.04arc sec 0.16 and 0.32 kpc at z~1.5 e e Absolute magnitudes, stellar masses, ages • Fit to the observed SEDs (BVRIzJHK F160W) at fixed z Charlot and Bruzual models (2007, CB07) IMF=Chabrier SFHs SFR e t τ=0.1,0.3,0.6 Gyr (best-fit τ<0.3 Gyr for 28 out of 32) Metallicity Z☼,0.4 Z ☼ (best-fit Z☼ ) AV<0.6 mag (best-fit AV<0.3 for 24 out of 32 ) Bologna 22.01.2009 The Kormendy relation in the R-band 18.2 2.92 log( Re ) R e e log( Re ) z=0 eR 16.100..12 2.72 00..52 log( Re ) z~1.5 The ETGs at z~1.5 are placed on the [<µ>e,Re] plane according to the KR. z~1.5 ETGs follow the same KR of ETGs at z=0 but with a different zero-point. Saracco et al. 2008 Bologna 22.01.2009 Luminosity evolution Each ETG evolves from z=zgal to z=0 according to its own SFH. e M R ( z 0) 5 log( Re ) 38.57 M R ( z 0) F160W 5 log DL ( z ) k R, F160W E ( z ) E ( z ) [ R( Agegal ) R( Agegal t )]template Only 40% (13 gal) of the sample occupies the KR at z=0. The remaining 60% (19 gal) does not match the local KR, the SB exceeds by 1-1.5 mag the one expected. Two distinct populations ? Saracco et al. 2008 Bologna 22.01.2009 Two distinct populations !? Saracco et al. 2008 Bologna 22.01.2009 Two distinct populations of ETGs at 1<z<2 • Old ETGs , <Age>~3.5 Gyr, <z>=1.5 zf>5 Their stellar population formed in the early universe. Pure luminosity evolution does not account for their high SB. The evolution of their size must be invoked. • Young ETGs , <Age>~1.2 Gyr, <z>~1.5 zf~2.5 Their stellar population formed much later than the stellar population of Old ETGs. Pure luminosity evolution from zgal to z=0 brings them onto the local KR. Bologna 22.01.2009 Size-Luminosity/Mass relations SDSS Shen et al. (2003) Size-Luminosity log Re [kpc] 0.26M R 5.02 Size- Mass M Re [kpc] 3.47 10 5 * M Bologna 22.01.2009 0.56 Size-Luminosity (S-L) relation log Re [kpc] 0.26M R 5.02 M RR ( z )0F z )L( zk)R, Fk160RW, F 160W E ( z ) ) 160 FW 160W5 log 5D log L (D Young Old Re of oETGs is 2.5-3 times smaller than - the local ETGs and - the yETGs with comparable luminosity. Saracco et al. 2008 Bologna 22.01.2009 Size-Mass (S-M) relation M Re [kpc] 3.47 10 * M 0.56 5 Young - 9 out of 13 (70%) follow the S-M relation Old - 4 out of 19 (20%) follow the S-M relation Re of Old ETGs is 2.5-3 times smaller than - the local ETGs and - the yETGs with comparable stellar mass. Old ETGs are 15-30 times denser ! Saracco et al. 2008 Bologna 22.01.2009 Constraining the formation and the evolution of ETGs Two distinct populations of ETGs at z~1-2 1. How did these two populations evolve from z~2 to z=0 to match the properties of the local ETGs ? 2. Which assembly history did they follow to have the properties shown at z~1.5-2 ? Bologna 22.01.2009 Tracing the evolution at z<2 oETGs Luminosity evolution DOES NOT bring them onto the local Kormendy and S-L relations. They DO NOT match the local S-M relation. They are 2.6(±0.5) times smaller than their local counterparts. They must change their structure. Size evolution from z~2 to z=0 is required to move them onto the local scaling relations. Bologna 22.01.2009 Tracing the evolution at z<2 oETGs M Size evolution often the merging processes R [kpcused ] 3.47 to 10 advocate the ETGs should experience inM the hierarchical paradigm of galaxy formation. 0.56 5 * e Dissipation-less (“dry”) merging is the most obvious and efficient mechanism to increase the size of galaxies. The size of ETGs increases according to the relation Re M * 0.6 1.3 Boylan-Kolchin et al. 2006-08 Khochfar and Silk 2006 Nipoti et al. 2002 Ciotti et al. 2007 M2i .6M 1.3 1/ R f 2.6M Rif M2f.1 M f 2.6 M i i Bologna 22.01.2009 Tracing the evolution at z<2 oETGs - Merging would produce too much ETGs with M>1011 Msun: we should observe 3 times more ETGs with M>4-5x1011 Msun . - Why α=1.3 ? Merging cannot be the mechanism with which oETGs increase their size at z<2. Alternative mechanism(s) leaving nearly unchanged the mass and relaxing the system: 1. interactions between galaxies (e.g. close encounters) 2. minor or “satellite” merging (Naab et al. 2007): M1:M2 = 0.1:1 Efficiency can be constrained from simulations. Bologna 22.01.2009 Tracing the evolution at z<2 yETGs Luminosity evolution brings them on the local Kormendy and S-L relations. They match the local S-M relation. No size evolution is required. To move them along the S-M, α~0.6 Mf~5Mi No evidence of merging at z<2. The build-up of yETGs was already completed at z~2. Bologna 22.01.2009 Constraining the path at z>2 - Toward the formation of ETGs oETGs <Age>~3.5 Gyr, <z>=1.5 zf>5 (Age Univ. 4.2 Gyr at z=1.5) To build-up 1011 Msun SFR>>100 Msun/yr Size 2.5-3 times smaller mechanism(s) acting at z>2 must be capable to produce galaxies 5-10 times more compact (15-30 times denser) than local ones Gas-rich merging with high fraction of stars formed during the merger in a violent starburst can produce highly compact ETGs (Khochfar et al. 2008; Naab et al. 2007). BUT tmerger>3 Gyr Bologna 22.01.2009 Constraining the path at z>2 - Toward the formation of ETGs yETGs <Age>~1.2 Gyr, <z>~1.5 zf>2.5 Constraints on the mechanism(s) acting at z>2 less stringent: They can increase their mass and enlarge their size by subsequent mergers (major and minor/satellite) and through starburts till z~2.5 (contrary to oETGs). Different progenitors oETGs: we should see them as they are (younger) till z~3-3.5 yETGs: in the phase of merging, or star forming and interacting with other galaxies at z>2.5 Bologna 22.01.2009 Summary and conclusions Two distinct populations of ETGs at z~1-2 whose stellar populations differ in age by about 2 Gyr Young ETGs: No size/mass evolution is required. Old ETGs: Strong size evolution is required at z<2. The system must relaxes from high to low redshift oETGs must show higher central velocity dispersion than local ETGs and than yETGs. Key observational test: measuring the velocity dispersion of oETGs. ESO-P82 VLT-FORS2: spectra of 10 oETGs, 10 hrs/spec Observations started in November 2008…we shall see! Bologna 22.01.2009 Mean age vs stellar mass 5% Stellar mass Bologna 22.01.2009 The evolution of the zero point α e log( Re ) Our sample Zero point α of the KR derived from various samples at different redshifts. Luminosity evolution SFH tau=0.6 Gyr, solar metallicity, Chabrier IMF The curves show the expected evolution of α for different formation redshift zf. Luminosity evolution + Evolution of Re Re ( z ) Re ( z 0) ( z 0.5) 1 Longhetti et al. 2007 Bologna 22.01.2009 Luminosity evolution of Young and Old ETGs M R ( z ) F160W 5 log DL ( z ) k R , F160W E ( z ) E ( z ) [ R( Agegal ) R( Agegal t )]template Saracco et al. 2008 Bologna 22.01.2009 Absolute magnitudes M R F160W 5 log[ DL ( z gal )] k R , F 160W k R , F 160W ( Rz 0 F160Wz gal ) template Bologna 22.01.2009 Morphological study of a sample of 10 ETGs at 1.2<z<1.7 based on HST-NICMOS observations in the F160W (λ~1.6 µm) filter (Longhetti et al. 2007) Sample - K<18.5, spectroscopic confirmation of the spectral type from TESIS (TNG EROs Spectroscopic Identification Survey; Saracco et al. 2003, 2005; Longhetti et al. 2005). NICMOS data - NIC2 camera (0.075 “/pixel) sampling the rest-frame Rband (λrest~6500 Ǻ) at z~1.4, at a resolution <0.8 kpc (FWHM~0.12 arcsec) Bologna 22.01.2009 Estimating the mean age of the stellar population 0.5 Gyr old 5% Stellar mass 95% stellar mass, 4 Gyr old B V R I z J H Bologna 22.01.2009 K Size-density and mass-density relations 50 0.5M star Re2 Saracco et al. 2008 0.5M star 50 4 / 3Re3 Bologna 22.01.2009 Simulations To assess the robustness of the results we applied the same fitting procedure to a set of simulated galaxies Real galaxies Simulated De Vaucouleurs profile 100 simulated galaxies • magnitudes F160W and re assigned randomly in the ranges 19<F160W<21 and 0.1< re <0.5 arcsec (1-5 Kpc at z~1.4); • axial ratio b/a and position angle PA in the ranges 0.4<b/a<1 and 0<PA<180 Bologna 22.01.2009 NIC3 images (0.2 “/pixel) NIC3 images (0.2 “/pixel) GDDS sample. HDFS-NICMOS z=1.65 z=1.73 z=1.85 z=1.55 Bologna 22.01.2009 zphot=1.94