Bologna 22.01.2009

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, Milano2 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

SmallSmall size, high-density ETGs: first evidence

Daddi et al. (2005)

Hubble UDF - 7 ETGs z>1.4

HST-ACS obs., FWHM~0.12”, HST-ACS obs., FWHM~0.12”, F850W filter, F850W filter, λλrestrest<3000 Ǻ<3000 Ǻ

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Cassata et al. (2005)

K20 + GOODS data

HST-ACS observations, F850Wλrest<3000 Ǻ

Further evidenceFurther evidence

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Trujillo et al. (2006)

IR ground based observationsFWHM~1.0 arcsec

redshift

Re

[Kpc

]

Mass

Re

[Kpc

]

Are ETGs at z>1 really more compact/denser than local Are ETGs at z>1 really more compact/denser than local counterparts ?counterparts ?

These results were based on These results were based on

• HST optical observations sampling the blue and UV rest-frame of the HST optical observations sampling the blue and UV rest-frame of the galaxies sensitive to k-correction and star formation and/orgalaxies sensitive to k-correction and star formation and/or

• seeing limited ground-based observationsseeing limited ground-based observations

Doubts on the reliability of the estimate of RDoubts on the reliability of the estimate of Ree

Doubts on the reliability of the comparison high-z Doubts on the reliability of the comparison high-z vsvs low-z low-z

High-resolution near-IR obs. sampling High-resolution near-IR obs. sampling λλrestrest~~6500 Ǻ for a reliable6500 Ǻ for a reliable

comparison between high-z comparison between high-z andand low-z ETGs. low-z ETGs.

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• Effective radius re (arcsec) and mean surface brightness (SB) <>e within re from Sersic profile fitting

n=4 de Vaucouleurs profilen=1 exponential profile

• galfit (Peng et al. 2002) to perform the fitting after the convolution with the NIC2 PSFs.

]1)/[( /1

)( n

en rrbeeIrI

0.075 “/pixel

NIC2 images

models

residuals

z=1.34 z=1.40 z=1.7

n=3.2 n=4.5 n=2.7

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HST-NICMOS observations in the HST-NICMOS observations in the F160W (F160W (λλ~~1.6 1.6 µm) µm) filter of a filter of a sample of 10 ETGs at sample of 10 ETGs at 1.2<z<1.71.2<z<1.7..

(Longhetti et al. 2007)(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 “)

It is a scaling relation between the effective radius Re [Kpc] and the mean SB <>e [mag/arcsec2]

)log( ee R

Any deviation from the KR at z=0 should reflects the evolution of <>e

due luminosity evolution .

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).

Observed KR at z=0.

Expected locus for z<1.5 early-type

galaxies in case of luminosity evolution.

The Kormendy relation in the R-bandThe Kormendy relation in the R-band

57.38)log(5)( ee RzM

Bologna 22.01.2009

It is a scaling relation between the effective radius Re [Kpc] and the mean SB <>e [mag/arcsec2]

)log( ee R

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).

Observed KR at z=0.

Expected locus for z<1.5 early-type

galaxies in case of luminosity evolution.

The Kormendy relation in the R-bandThe Kormendy relation in the R-band

57.38)log(5)( ee RzM

The SB exceeds by The SB exceeds by ~1~1 mag the one mag the one expected in the expected in the case of PLE for case of PLE for constant constant RRee, i.e. , i.e.

luminosity luminosity evolution does not evolution does not account for the account for the observed SB of observed SB of ETGs at high-zETGs at high-z..

(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

GMASS sample 13 ETGs 1.4<z<2

Spectroscopic data

Morphology based on HST-ACS obs. F850W(λrest~3000 Ǻ)

(Cimatti et al. 2008)

The Kormendy relation in the B-band

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Literature and HST archive researchAim – 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

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Physical properties of ETGsMorphological 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 )

sec02.02 arcNICre

sec04.03 arcNICre

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teSFR

)log( ee R

The Kormendy relation in the R-band

)log(92.22.18 eRe R

)log(72.21.16 5.02.0

1.02.0 e

Re R

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.

Saracco et al. 2008 Bologna 22.01.2009

Luminosity evolution

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 ?

57.38)log(5)0( eRe RzMtemplategalgal

WFRLR

tAgeRAgeRzE

zEkzDWFzM

)]()([)(

)()(log5160)0( 160,

Each ETG evolves from z=zgal to z=0 according to its own SFH.

Saracco et al. 2008 Bologna 22.01.2009

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Two distinct populations ? Two distinct populations !

Saracco et al. 2008

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Two distinct populations of ETGs at 1<z<2

• Old ETGs , <Age>~3.5 Gyr, <z>=1.5 zf>5Their 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.

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Size-Luminosity/Mass relations

SDSS Shen et al. (2003)Size-Luminosity Size- Mass

02.526.0][log Re MkpcR56.0

*51047.3][

M

MkpcRe

02.526.0][log Re MkpcR

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Size-Luminosity (S-L) relation

WFRLR kzDWFzM 160,)(log5160)(

Saracco et al. 2008

Young

Old

)()(log5160)0( 160, zEkzDWFzM WFRLR

Re of oETGs is 2.5-3 times smaller than - the local ETGs and - the yETGs with comparable luminosity.

02.526.0][log Re MkpcR

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Size-Mass (S-M) relation

Saracco et al. 2008

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 !

Young - 9 out of 13 (70%) follow the S-M relation Old - 4 out of 19 (20%) follow the S-M relation

56.0

*51047.3][

M

MkpcRe

Constraining the formation and the evolution of ETGs

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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 ?

Tracing the evolution at z<2

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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.

Tracing the evolution at z<2

Bologna 22.01.2009

oETGsSize 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

*MRe 3.16.0 Boylan-Kolchin et al. 2006-08

Khochfar and Silk 2006Nipoti et al. 2002Ciotti et al. 2007

56.0

*51047.3][

M

MkpcRe

if RR 6.2 if MM 6.2 if MM /16.2 3.1if MM 1.2

Tracing the evolution at z<2

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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 = 0.1:1Efficiency can be constrained from simulations.

Tracing the evolution at z<2

Bologna 22.01.2009

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.

Constraining the path at z>2 - Toward the formation of ETGs

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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

Constraining the path at z>2 - Toward the formation of ETGs

Bologna 22.01.2009

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 progenitorsoETGs: we should see them as they are (younger) till z~3-3.5yETGs: in the phase of merging, or star forming and interacting with other galaxies at z>2.5

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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/specObservations started in November 2008…we shall see!

Summary and conclusions

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Mean age vs stellar mass

5% Stellar mass

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.

Luminosity evolution+

Evolution of Re

Our sample

Luminosity evolution

SFH tau=0.6 Gyr, solar metallicity,

Chabrier IMF

1)5.0()0()( zzRzR eeLonghetti et al. 2007

)log( ee R

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Luminosity evolution of Young and Old ETGs

templategalgal

WFRLR

tAgeRAgeRzE

zEkzDWFzM

)]()([)(

)()(log5160)( 160,

Saracco et al. 2008

Absolute magnitudes

templatezzWFR

WFRgalLR

galWFRk

kzDWFM

)160(

)](log[5160

0160,

160,

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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)

NICMOS data - 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).

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Estimating the mean age of the stellar population

5% Stellar mass0.5 Gyr old

95% stellar mass, 4 Gyr old

B V R I z J H K

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Size-density and mass-density relations

250

5.0

e

star

R

M

350 3/4

5.0

e

star

R

M

Saracco et al. 2008

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

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Simulations

Real galaxies

Simulated De Vaucouleurs profile

To assess the robustness of the results we applied the same fitting procedure to a set of simulated galaxies

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

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