Near-field cosmology: formation of the stellar halo

Galactic Archaeology with Subaru:
Prospects for Wide-Field Surveys
Masashi Chiba
(Tohoku University, Sendai)
with HSC and PFS science working groups
 Background
 Wide-field photometric survey with
Hyper Suprime Cam (HSC)
 Wide-field, fiber-fed multi-object
spectroscopic survey with Prime Focus
Spectrograph (PFS)
What do we want to know?
Formation of the Galaxy in CDM theory
Assembly of CDM
Dark halo in the Galaxy
By Moore
Resolved stars provide fossil records
of galaxy formation process
Bright parts are
centrally concentrated
By Moore
Then, what data do we need?
 Photometry :
 mag., color ( → color-mag. diagram)
 Spectroscopy :
 metallicity , Vrad (kinematics)
 abundance pattern ( → SFH & chemical evol.)
 Astrometry :
 proper motion, distance (→ 6d phase space)
⇒ galaxy formation and evolution
Prime focus
(Hyper Suprime Cam)
Funded since 2006
FOV: 1.77 sq deg
(1.5 deg diameter)
Pixel scale: 0”.17/pix
Filters: grizy
+ several NB
First light: Jan., 2012
International collaboration: Japan, Princeton, Taiwan
Wide-field FoV is essential for mapping stars
Subaru Strategic Program (SSP)
with HSC (to be submitted)
(dedicated to a newly commissioned instrument:
max: 60 nights/year x 5 years)
1. Wide-field survey over ~ 1,500 deg2
 cosmology: primary science driver
weak lensing, strong lensing, galaxy clusters
 other science incl. galactic archaeology
2. Deep and ultra-deep survey
 galaxy and QSO/AGN evolution
high-z galaxies, re-ionization, z~7 QSOs
HSC white paper
(July, 2011)
Planned GA survey with HSC
1. Wide-field survey of the MW outer halo
Discovery of new ultra-faint dSphs and halo streams
 3 bands (g, r, i), ~ 26 mag (3 mag deeper than SDSS)
 ~1,500 deg2 (fully utilizing weak-lensing survey data)
 color-magnitude diagram for old MS + RGB stars in the
outer halo (r = 30 – 250 kpc)
2. Deep and wide-field survey of the M31/M33 halo
Detailed mapping of stellar halos in M31/M33
 Use (DDO51-type) NB515 filter centered @ 515 nm
to distinguish halo RGBs with (g, i) + NB515 imaging
 NB515 is fully optimized for z=3 BAO as well
UFDs and substructures
revealed by SDSS
20 < r < 22
(Belokurov+ 2006)
Observed photometric properties of Galactic satellites
Tollerud +08
Single LSST:
rlim = 24.5
Co-added LSST:
rlim =27.5
(wide-f. survey):
Survey of the MW outer halo with HSC
Halo realization
(Bullock & Johnston 2005)
HSC survey over ~1,500 deg2
• ~ 20 new UFDs @ Mv<-3
• 3~4 new streams with lim~34 mag/”
Stellar halos in M31/M33
using (DDO51-type) NB515 filter
CW: 5145 A, FWHM: 80 A
To separate RGBs in M31/M33 from Galactic dwarfs
+ optimized for BAO science using z=3 LAEs
Feasibility for LG science
(with M.Tanaka)
Requirement from BAO science
(with Y. Matsuda)
Deep and wide-field survey of
stellar halos in M31/M33 with HSC
Metallicities and RVs of substructures
and satellites with PFS (+ star clusters)
130 HSC pointings
Prime focus
(Prime Focus Spectrograph)
FOV: 1.5 deg in diameter
2400 fiber positioners
λ: 380~1,300 nm
(3 channels: Blue, Red, IR)
R: ~3,000
First light: 2016
Led by IPMU (U. of Tokyo) + NAOJ/Subaru community
+ Caltech/JPL, Princeton, (JHU), LAM, UK, Brazil, Taiwan
Spectrograph design
By Jim Gunn
Key science with PFS
(Science white paper, 1st ver. in Jan 2011)
1. Cosmology: primary science driver
 BAO, lensing
2. Galaxy evolution
 Galaxies at z ~ 2 and beyond
3. Galactic archaeology
 Milky Way and Local Group
4. QSO/AGN evolution
 QSOs at z < 6
1st version of PFS white paper
(Jan, 2011)
SuMIRe=Subaru Measurement of Images and Redshifts
The project name using both HSC and PFS
GA science goals with PFS
Many fibers
Merging history
GA science goals with PFS
1. Formation of Galactic structures
Merging history of the Milky Way (MW)?
Formation of old MW components (thick disk, halo)?
Star-formation history of dwarf satellites?
Formation of M31: is it different from the MW?
2. The nature of galactic dark matter
 How is dark matter distributed in the MW?
 Dark matter of dwarf satellites: is CDM correct?
All of these information are recorded in stars
Extracting past merging history: how?
Debris of building blocks
1. Space and velocity distributions of
ancient stars
 Past collapse and merging events
 Tracer of dark matter profiles
2. Chemical abundance of ancient stars
 Star formation and chemical evolution
Freeman & Bland-Hawthorn 2002
Phase space
A building block
Astrometry with Gaia
(precise distances and
proper motions)
+ Vrad & [Fe/H] distrib.
with PFS
Helmi & de Zeeuw 2000
will resolve each of blocks
and its chemical evolution
Accretion time of a satellite can be deduced
from phase-space distribution of stars
(McMillan & Binney 2008, Gomez+ 2010)
Orbital freq.
7.9 Gyr
8.9 Gyr
7.6 Gyr
GA survey plan with PFS
1. The Milky Way survey
i. Gaia sample: ~ 106 stars with 18<V<20 (Halo
~55%, Thick disk ~40%, Thin disk ~5%)
ii. Faint sample: stars in 8-9 stellar streams and ~
105 stars in the outer halo with 18<V<21.5
Total area: 1,000 + 180 deg2 (560 + 100 pointings)
Survey time: ~30 + 20 = 50 nights
2. The M31 halo survey
Sample: RGBs, V < 21.5
Total area: ~ 230 deg2 (~130 pointings)
Survey time: ~ 30 nights
3. Dwarf galaxy survey
Sample: RGBs, V < 21.5
Survey time: ~ 8 nights (can be PI-led programs)
Summary: GA with Subaru next decade
 HSC (2012~)
• 1.5 deg FOV, gri+NB515, g<26
• Discovery of new UFDs and halo
substructures in the MW
• Detailed halo structure in M31/M33
 PFS (2016~)
• R=3,000, ~2,400 fibers,
λ=380-1300 nm, 18<V<21.5
• Discovery of kinematical substructures and
merger history in the MW
• Dark matter and abundance distribution in
Local Group galaxies
(Thirty Meter Telescope)
R~5,000 for mV<26 mag
R~50,000 for mV<21 mag
Japan will join TMT
GA with TMT
Science Working Group (Stars and Local Group)
led by Wako Aoki
1. Exploring stellar activities and explosions
2. Clarifying galaxy formation with resolved stars
Supernovae, GRB, Seismology, First stars,
Extremely metal-poor stars, Our Galaxy,
Andromeda, The Local Group and beyond …
Summarized in Science Report (2011)
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