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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 redshiftsIs 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
HK
3.6m
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
HK
3.6m
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 detectionIf 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