Transcript Near-field cosmology: formation of the stellar halo

Unveiling the formation of the
Galactic disks and Andromeda halo
with WFMOS
Masashi Chiba
(Tohoku University, Sendai)
Galactic Archaeology
thick disk
thin disk
bulge
stellar halo
Fossil records in Galaxy formation
Near-field Cosmology
Galaxy formation: tracing assembly history
Building blocks
Fossil (DNA) records
in ancient stars
Spatial distributions
• Global distribution
• Localized structures
Kinematics
• Rotational velocity
• Integral of motions
(phase space
distribution)
Chemical abundance
• [Fe/H], [α/Fe] etc.
Issues addressed here
1. Milky Way halo
 Global and local structures deduced from
kinematics and chemical abundance
2. Thick disk
 How did it form?
3. Andromeda halo
 Is it different from the Milky Way halo?
1. Milky Way halo
kinematics
SDSS
Vφ
Thick disk
inner halo
Halo
outer halo
[Fe/H]
metallicity
Mean rotation velocity of the halo
Vφ
Inner halo
Outer halo
Zmax (max. Z distance)
• Assembly process
is at work (monolithic
collapse is unlikely).
• star formation history
of each halo comp.
is yet unknown.
Formation of a stellar halo based on
CDM models
(Johnston+08)

[/Fe]
[Fe/H]
Vlos
Distribution in the sky
(Bullock & Johnston 2005)
Outer halo
(SDSS)
Halo realization 1
Galactic Halo Survey
 Chemical tagging of the
stellar halo with high-res
survey
 inner/outer halo
(Ishigaki-san’s talk)
 halo substructure
 Mapping halo substructure
patterns with low-res survey
 Vlos, [Fe/H], [/Fe]
 group finder (Sharma &
Johnston 2009)
Halo: Mtot = 109 Msun
Munit=105-6Msun
N = 10×Mtot / Munit
~ 104-5 halo stars
2. Thick disk
Vertical velocity dispersion
 Milky Way thick disk
(km/s)
 distinct kinematics,
chemistry, and age:
independent Galactic
component
 dynamically hot, large
scale height, [Fe/H]~ -0.6,
old age (~10Gyr)
log Age (Gyr)
Lthick/Lthin vs. Vcirc
in external galaxies
 Extra-galactic thick disks
 common in disk galaxies
 relatively old and metal
rich
V
Formation scenario of a thick disk
 Dissipative collapse
 metallicity gradient, no gradient in kinematics
 homogeneous age distribution
 Direct accretion of thick-disk material (satellites)
 no gradient in chemistry and kinematics
 contamination of young, low-[/Fe] stars
 Dynamical heating of a pre-existing thin disk by
sub-galactic dark halos (subhalos)
 no gradient in chemistry, gradient in kinematics ( V
as |z| )
 asymmetry and substructures in kinematics but not in
chemistry
Numerical simulation of disk heating by subhalos
(Hayashi & Chiba 2006)
Distribution of dark halos in a galactic scale
(by Moore)
young disk
Vlos distribution
Asymmetric Vlos distribution
+ kinematic substructures
⇒ evidence of disk heating
Model F
Model S
Model I
Model F
|Vlos|↓as |b|↑
i.e. |Vrot|↓as |z|↑
⇒ evidence of disk heating
Model S
Model I
Galactic Thick-Disk Survey
 Kinematics distribution
with low-res survey
 mapping of Vlos
 [Fe/H] for each
substructure + age
 Chemical tagging with
high-res survey
 , Fe-peak, s-process
elements
 Aoki-san’s talk
Thick disk: Mtot = 3
×109 Msun
Munit=105-6Msun
N = 10×Mtot / Munit
~ 104-5 disk stars
3. Andromeda halo
 How typical is the
Milky Way?
 metallicity, age,
kinematics, global
structure
 External view of a
stellar halo
 substructure,
metallicity gradient,
age gradient
Keck/DEIMOS observation
(Koch+08)
DEIMOS target fields
Spectroscopic metallicity is
more reliable.
Metallicity distribution
(Koch+08)
metal-poor halo?
Too small FOV with DEIMOS
• ~20 RGB / pointing
 Susceptible to substructure contamination
• distinguish local and global structures

Andromeda Halo
Survey
 Metallicity and
Kinematics of the
Andromeda Halo
with low-res survey
 RGB with 20.5 < I <
21.25 mag
 larger  coverage &
much wider FOV than
DEIMOS
 ~ 6900 sec exposure
for ~ 200 deg2, 220
hours
Using S-Cam (Tanaka+ 2007)
Current survey design
 Key Science Program
 High-res survey
•
•
•
•
R=30,000, 16<V<17
=628-659.3nm
~ 5×105 stars (disk and halo)
~1000 deg2, ~280 nights
 Low-res survey
•
•
•
•
R=1,800, 18<V<21.5, B-V<1
=390-900nm
~ 106 stars (halo and disk)
~ 1000 deg2, ~250 nights
 PI Science Programs
 Galactic bulge, M31/M33 halo, dwarf galaxies
b=20
l=0
Conclusions
 WFMOS GA survey will provide legacyvalue datasets, which no other
observatories enable to do over decades.
 Subaru/Gemini communities will be
benefit from these datasets and resulting
science achievements.
Thank you
high-z universe
(snapshots of
various galaxies)
complementary
stellar system
in local universe
(tracing evolution
of a galaxy)
Bekki & Chiba 2001
WFMOS survey of halo and disk stars
Munit=105-6Msun
Total halo or disk mass Mtot
Mtot = 109-10 Msun
N = 10×Mtot / Munit
~ 104-5 halo stars
~ 105-6 thick disk stars
RVs, metallicities,
ages (turn-off/subgiants),
distances (giants)
Mtot
Original plan with WFMOS
1. Dark energy survey (determination of w)
2. Galactic archaeology survey
~4500 targets in a FOV~1.5deg,
R~2000, 40000 (3000, 1500 fibers)
Operation 2012? ~
Original plan :
• Low resolution mode R ~ 2000, 17<V<22
radial velocity & abundance
0.5 million stars, 500 deg2, 140 nights
• High resolution mode R ~ 40000, V<17
abundance patterns
1.5 million stars, 3000 deg2, 490 nights
~1400 stars
@V~17
RAVE
GAIA
WFMOS
1.2m UK-Schmidt,
AAO
Astrometry satellite,
ESA
Wide-field fiber-fed
mos
Optical, 8400 ~ 8750A
Ca triplet
Optical, 5 to 11 band
Optical,
photometry + Ca triplet ~4500 targets in a field
Sp: V<12 mag
R=5000~10000
2 km/s
Sp: V<17 mag
R=11500
1~10 km/s, 10^8 stars
Sp: R=2000~30000
Hi res. V<17 mag
Low res. 17<V<22 mag
Southern hemisphere
All sky
Northern hemisphere
2003~2010
2012? ~ 2019?
2012~?
40000
R
30000
WFMOS
Inner halo
(1 million stars)
20000
GAIA
10000
RAVE
12
Photometry
to V=20
Outer halo
(0.5 million stars)
WFMOS
17
V (mag)
22