Evidence for a Population of MassiveEvolved Galaxies at z > 6.5

Bahram Mobasher

M.Dickinson NOAOH . Ferguson STScIM. Giavalisco , M. Stiavelli STScIAlvio Renzini, Piero Rosati (ESO)

T. Wiklind ESA/STScIR. S. Ellis (Caltech)D. Stark (Caltech), Nino PanagiaM. Fall, A. Koekemoer (STScI)

It is generally accepted that galaxies we see today, form by mergers of smaller sub-units

throughout the age of the UniverseGalaxies are younger and less massive at

higher redshifts

Is there NO massive and evolved system at high redshifts, when the Universe was < 1 Gyrs old ?

Only recently, with combined optical, near-Infrared and mid-Infrared observations one could

explore this.

Balmer Break Technique forSelecting Massive and Old Galaxies

at High redshifts

z = 7

no extinction

t = 50 Myrt = 100 Myrt = 300 Myrt = 500 Myrt = 600 Myrt = 800 Myr

The Balmer break is a prominent feature for stellar populations age t > 100 Myrs

H

K3.6m

JH K

3.6m

J

dust-free post-starburst z ~ 7 dusty starburst z ~ 2.5

Dusty starburst at z ~ 2.5

Post starburst at z ~ 7

old Elliptical at z ~ 2.5

H

K3.6m

JH K

3.6m

J

dust-free post-starburst z ~ 7 dusty starburst z ~ 2.5

Dusty starburst at z ~ 2.5

Post starburst at z ~ 7

old Elliptical at z ~ 2.5

Alternate way of selecting very high redshift candidates:

Color-color selection: H - 3.6m vs K - 3.6m

The slope of the short- side of the Balmer break is differentfor dusty starburst and dust-free post-starburst galaxies

Avoids using J-band - often faint

Selecting Massive Evolved Galaxies at high-z

- No detection at optical bands (BViz)

- Red J-H colors (undetected in J-band)

- Blue H-K ( i.e. flat H/K SED)

- Red K- m 3.6 (K/IRAC(3.6 m) break)

18 galaxies found with 5 < z < 7

and stellar mass 8 x 1010 – 5 x 1011 Msun

Wiklind et al (2006)

HUDF vs GOODS

GOODS CDFS – 13 orbits HUDF – 400 orbits

Discovery of an Extremely Massive and Evolved Galaxy at

z ~ 6.5

Mobasher, B. et al ApJ 2005

further constraints

- No detection at optical bands (BViz) to the HUDF limit at 2 level

- J110 - H160 > 1.3 corresponding to the observed color of an LBG at z=8, taking

into account IGM opacity

J-band dropout candidates in the HUDF

• no detection at optical bands• close to slope unit line on H-3.6 vs. K-3.6• red K-3.6 color

Selection criteria (Mobasher et al Ap.J 2005)

(Mobasher et al 2005)

ACS

ACS+NICMOS+ISAAC

ACS+NICMOS+ISAAC+IRAC

mAB 2.5log f ZPT

f 2

cf

B V iz

J HK

IRAC

B V iz

J HK

B V iz

J HK

IRAC

B V iz

J HK

Bruzual and Charlot models (BC03)Starburst99 models (SB99)

Simultaneously fitting:• Redshift z• Extinction EB-V

• Age, t• Star formation history e-folding time • Metallicity Z

Summary of fitted parameters:

• High redshift, z = 6.5• Luminous, LBOL ~ 1 1012 Lo

• No extinction, EB-V = 0.0

• No on-going star formation• ‘Old’, age ~ 1 Gyr• Very, very massive, M* ~ 5 1011 Mo

BC03 Starburst99

2 vs. EB-V and redshift

Best fit region covers : 6.0 < z < 7.7

Secondary fit for a dusty galaxy at z ~ 2.5

How stable is the solution?black : 2 minimum (1.9)white : 2 = 10

z=6.5 evolved

Massive

Single burst

zdustyStarburst

Continous SFR

zoldevolved

Formation redshift(excluding those with ages > age of the universe)

Results from Monte Carlo simulations:

• z = 6.5• EB-V = 0.0

• Age = 600 Myr• Z = 1.0 Zo

• M* = 4.6 1011 Mo

• zform ~ 10-15

median values

A massive post-starburst galaxy at z ~ 7

Most frequently asked questions:

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

z=6.5 evolved

Massive

Single burst

zdustyStarburst

Continous SFR

zoldevolved

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

Mrk 231 BL QSO ULIRG

Mrk 231 +NGC 1608 highly obscured AGN

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

Gemini GNIRS spectroscopy of the z ~ 7 candidate:

No lines detected

Ly (1216) z ~ 7 (0.97 micron)

z ~ 0.8 - 1.2 J-bandz ~ 1.3 - 1.8 H-bandz ~ 2.0 - 2.8 K-band

H (6563)

No lines detected

Ly (1216) z ~ 7 (0.97 micron)

z ~ 0.8 - 1.2 J-bandz ~ 1.3 - 1.8 H-bandz ~ 2.0 - 2.8 K-band

H (6563)

In addition:

• Gemini-S GNIRS cross-dispersed

•VLT FORS

•HST ACS Grism

•Keck NIRSPEC

Keck and VLT: sensitive to Ly emission fromgalaxies at 6.8 < z < 8.0 with SFR 3-5 Mo/yr

Keck and Gemini: sensitive to H emission fromgalaxies at 0.8 < z < 2.8 (with gaps) with linefluxes 1 10-17 - 2 10-18 erg cm-2 s-1

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

• Could it be gravitationally lensed ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

• Could it be gravitationally lensed ?

• Could it be a star ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

• Could it be gravitationally lensed ?

• Could it be a star ?

• how about other pop synthesis models ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

• Could it be gravitationally lensed ?

• Could it be a star ?

• how about other pop synthesis models ?

• different dust models ?

Most frequently asked questions:

• Could it be a dusty starburst at z~2 ?

• Could it be an evolved old galaxy at z~2-3 ?

• Does it have MIPS detection ?

• Do you have spectroscopic data ?

• Could it be gravitationally lensed ?

• Could it be a star ?

• how about other pop synthesis models ?

• different dust models ?

• Have you found more objects like this ?

Sample B J-band drop-out

z = 4.9EB-V = 0.10age = 700 MyrM* = 8 1011 Mo

z = 7.2EB-V = 0.05age = 400 MyrM* = 4 1011 Mo

z = 5.3EB-V = 0.0age = 300 MyrM* = 0.8 1011 Mo

z = 7.2EB-V = 0.0age = 300 MyrM* = 2 1011 Mo

Sample A massive post-starburst candidates

Sample A massive post-starburst candidates

Summary

• We present a technique for selecting very massive evolved galaxies at high redshift.

• This is based on combining deep optical/near-IR data with medium deep Spitzer observations

• An object is found in HUDF with an SED consistent with a post-starburst (evolved) galaxy with M=(2-5) x 1011 Msun at z=6.-7.5. The object has undergone a single burst of SF and has an age of 1 Gyr.

Cont..

• A less likely possibility is a very dusty evolved galaxy at z=2.5 with an age of ~600 Myrs. However, this produces a significantly worse fit to the observed SED.

• The low-z alternative requires the object to be detected in radio (1.4 GHz) or have spectral features indicative of SF. Our source has none of these.

Case against dusty post-starburst at low-z

No detection of diagnostic spectral features in the spectra

No radio detection

If the 24 micron flux is due to PAH feature in a star-forming galaxy at z~2, we would have expected to

detect this to the depth of our radio survey.

local galaxiesm*>2.5E10 MO

m*>1.0E11 MO

EROs

sub-mm

K20

SDSS QSOs

LBGs

Somerville 2004

z = 7

no extinction

t = 50 Myrt = 100 Myrt = 300 Myrt = 500 Myrt = 600 Myrt = 800 Myr

The Balmer break is a prominent feature for stellar populations age t > 100 Myrs

z = 3 z = 4 z = 5 z = 6 z = 7

z = 7

no extinction

At z ~ 6 - 8 the Balmer break falls between theK-band and the IRAC 3.6 micron band

At z ~ 6 - 8 the J-band is significantly fainterthan K for ages of a few 100 Myrs

t = 50 Myrt = 100 Myrt = 300 Myrt = 500 Myrt = 600 Myrt = 800 Myr

z = 7

no extinction

t = 50 Myrt = 100 Myrt = 300 Myrt = 500 Myrt = 600 Myrt = 800 Myr

At z ~ 6 - 8 the Balmer break falls between theK-band and the IRAC 3.6 micron band

At z ~ 6 - 8 the J-band is significantly fainterthan K for ages of a few 100 Myrs

Selecting galaxies based on their K - 3.6m and J - H colorswould allow isolating very high redshift post-starburst galaxies

Requires high quality near-infrared and Spitzer IRAC photometry

This exists for the GOODS South field