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The size evolution of early-type galaxies since z=2

The size evolution of early-type galaxies since z=2. P. Saracco 1 , M. Longhetti 1 , with the contribution of S. Andreon 1 , A. Mignano 1 , G. Feulner 2 , N. Drory 2 , U. Hopp 2 , R. Bender 2 1 INAF – Osservatorio Astronomico di Brera, Milano

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The size evolution of early-type galaxies since z=2

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  1. Thesizeevolutionofearly-typegalaxiessince 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

  2. Outlineof the talk Bologna 22.01.2009 • 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

  3. Smallsize, high-densityETGs: 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

  4. Furtherevidence Cassata et al. (2005) Trujillo et al. (2006) K20 + GOODS data Re [Kpc] Mass Re [Kpc] IR ground based observations FWHM~1.0 arcsec redshift HST-ACS observations, F850W λrest<3000 Ǻ Bologna 22.01.2009

  5. 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

  6. HST-NICMOS observations in the F160W (λ~1.6 µm) filter of a sample of 10 ETGs at 1.2<z<1.7. (Longhetti et al. 2007) 0.075 “/pixel • Effective radius re (arcsec) and mean surface brightness (SB) <>e withinre from Sersicprofile fitting • n=4de Vaucouleurs profile • n=1exponential profile • galfit(Peng et al. 2002) to perform the fitting after the convolution with the NIC2 PSFs. 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 models residuals z=1.34 z=1.40 z=1.7 n=3.2 n=4.5 n=2.7 Bologna 22.01.2009

  7. 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] The ETGs follow this tight relation with ~3 up to z~1.  is found to vary reflecting the luminosity evolution. Expected KR at z=1.5 passive luminosity evolution (maximum evolution expected for early-types). Any deviation from the KR at z=0 should reflects the evolution of <>e due luminosity evolution . Observed KR at z=0. Expected locus for z<1.5 early-type galaxies in case of luminosity evolution. Bologna 22.01.2009

  8. 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] The ETGs follow this tight relation with ~3 up to z~1.  is found to vary reflecting the luminosity evolution. Expected KR at z=1.5 passive luminosity evolution (maximum evolution expected for early-types). The SB exceeds by ~1mag the one expected in the case of PLE for constant Re, i.e. luminosity evolution does not account for the observed SB of ETGs at high-z. Observed KR at z=0. Expected locus for z<1.5 early-type galaxies in case of luminosity evolution. (Longhetti et al. 2007) Bologna 22.01.2009

  9. 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

  10. 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

  11. Literature and HST archive research • Aim – to collect a large (larger than 10…!) sample of ETGs at z>1 with • spectroscopic confimation of the spectral type; • HST-NICMOS observations in the F160W filter; • multiwavelength coverage (optical + near-IR) • in order to study the population of ETGs at 1<z<2 from an homogeneous set of data and a uniform analysis • covering a larger interval in luminosity; • defining the scaling relations at z~1.5 • (Kormendy, size-luminosity/mass relations) Sample 10 ETGs 1.2<z<1.7 from TESIS (Saracco et al. 2005; Longhetti et al. 2005) + 10 ETGs 1.4<z<1.9 from GDDS (Abraham et al. 2004; McCarthy et al. 2005) + 6 ETGs z~1.27 from RDCS 0848+4453 (Stanford et al.1997; van Dokkum et al. 2003 + 3 ETGs 1<z<1.8 from HDF-N (Stanford et al. 2004) + 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 NIC2 (0.075 ”/pixel) for 14 galaxies NIC3 (0.2 “/pixel) for 18 galaxies FWHM ~ 0.12 arcsec Bologna 22.01.2009

  12. Physical properties of ETGs • Morphological parameters • effective radius and surface brightness derived as in Longhetti et al. (2007); • Simulations done also for NIC3 images • 0.16 and 0.32 kpc at z~1.5 • Absolute magnitudes, stellar masses, ages • Fit to the observed SEDs (BVRIzJHK F160W) at fixed z • Charlot and Bruzual models (2007, CB07) • IMF=Chabrier • SFHs 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

  13. The Kormendy relation in the R-band z=0 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. Bologna 22.01.2009 Saracco et al. 2008

  14. Luminosity evolution Each ETG evolves from z=zgal to z=0 according to its own SFH. 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 ? Bologna 22.01.2009 Saracco et al. 2008

  15. Two distinct populations ? Two distinct populations ! Bologna 22.01.2009 Saracco et al. 2008

  16. 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

  17. Size-Luminosity/Mass relations SDSS Shen et al. (2003) Size-Luminosity Size- Mass Bologna 22.01.2009

  18. Size-Luminosity (S-L) relation Young Old Re of oETGs is 2.5-3 times smaller than - the local ETGs and - the yETGs with comparable luminosity. Bologna 22.01.2009 Saracco et al. 2008

  19. Size-Mass (S-M) relation 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 ! Bologna 22.01.2009 Saracco et al. 2008

  20. Constraining the formation and the evolution of ETGs Two distinct populations of ETGs at z~1-2 How did these two populations evolve from z~2 to z=0 to match the properties of the local ETGs ? Which assembly history did they follow to have the properties shown at z~1.5-2 ? Bologna 22.01.2009

  21. 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

  22. Tracing the evolution at z<2 oETGs Size evolution often used to advocate the merging processes the ETGs should experience in the hierarchical paradigm of galaxy formation. 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 Boylan-Kolchin et al. 2006-08 Khochfar and Silk 2006 Nipoti et al. 2002 Ciotti et al. 2007 Bologna 22.01.2009

  23. Tracing the evolution at z<2 oETGs - Merging would produce too much ETGs with M>1011Msun: we should observe 3 times more ETGs with M>4-5x1011Msun. - 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: interactions between galaxies (e.g. close encounters) minor or “satellite” merging (Naab et al. 2007): M1:M2 = 0.1:1 Efficiency can be constrained from simulations. Bologna 22.01.2009

  24. 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

  25. 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

  26. 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

  27. 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

  28. Mean age vs stellar mass 5% Stellar mass Bologna 22.01.2009

  29. Our sample Luminosity evolution SFH tau=0.6 Gyr, solar metallicity, Chabrier IMF Luminosity evolution + Evolution of Re The evolution of the zero point α Zero point α of the KR derived from various samples at different redshifts. The curves show the expected evolution of α for different formation redshift zf. Longhetti et al. 2007 Bologna 22.01.2009

  30. Luminosity evolution of Young and Old ETGs Bologna 22.01.2009 Saracco et al. 2008

  31. Absolute magnitudes Bologna 22.01.2009

  32. Morphological study of a sample of 10 ETGsat 1.2<z<1.7based on HST-NICMOS observations in the F160W (λ~1.6 µm) filter (Longhetti et al. 2007) NICMOSdata- NIC2 camera (0.075 “/pixel) sampling the rest-frame R-band (λrest~6500 Ǻ) at z~1.4, at a resolution <0.8 kpc (FWHM~0.12 arcsec) 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). Bologna 22.01.2009

  33. 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 K Bologna 22.01.2009

  34. Size-density and mass-density relations Bologna 22.01.2009 Saracco et al. 2008

  35. 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 reassigned 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

  36. NIC3 images (0.2 “/pixel) GDDS sample. z=1.65 z=1.73 z=1.85 NIC3 images (0.2 “/pixel) HDFS-NICMOS z=1.55 zphot=1.94 Bologna 22.01.2009

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