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Galactic Science and
MOS on the WHT
Amina Helmi
ESA-ESO report on Galactic populations
(Turon, Primas, Binney, Chiappini, Drew, AH, Robin, Ryan)
GREAT WG3 on Chemical Tagging
Brown, Feltzing, AH, Korn, Walton
GCDS: Gaia chemo-dynamical Survey
•First meeting: Paris 26 April 2010
The Galaxy
stellar halo
thick disk
thin disk
How did the Galaxy come to be like this ?
What is the origin/formation epoch/mechanism and relation
between the various components?
The Gaia era
ESA-ESO meeting, ESTEC, 10 October ESA-ESO 2008
• The volume and quality of data that Gaia will provide will
revolutionise the study of the Galaxy
•Full 6D phase-space information only available for a subset
•Not as accurate as proper motions
-> Incomplete dynamical map of the Galaxy
e.g. substructures (clusters, resonances) in disk limited to few kpc from Sun
Gaia: spectroscopy
• Only gross abundances measurable for a subset of brightest stars
-> MDF only known within few kpc from the Sun, in sections of the bulge or in dwarf
galaxies (100 x farther away)
• Detailed elemental abundances missing
-> crucial for chemical history, star formation and assembly history
Science questions1
• Dynamics of the Milky Way
 Velocity distributions along the disk; resonance maps; coupling of dark halo,
bar and disk
 Halo shape, density and granularity
 Streams as tracers of mass distribution and evolution
• Structure and history of the disks
 Characterization of star formation and chemistry as f(R)
 Models of the formation of thick disk
 Inter-relation between various components
• Metal-poor components
 Streams in the halo to trace merger history
 Constrain the IMF, and star formation in the early Universe
(1) not exhaustive
Milky Way Dynamics
CDM strong
predictions on
density profile,
shape and
Springel et al. 2008
A great amount of dark substructure
Narrow streams
Thin long streams better probes (more
reliable tracers of underlying potential;
Eyre & Binney 2009)
GD-1 stream in SDSS: dissolved cluster
Internal velocity dispersions are few
•Halo granularity: need very accurate radial velocities
et al. 2009
•Distant streams preferred (d ~ 10 – 40 kpc) to isolate other
-> faint stars
•Low surface brightness -> need to go as far down on RGB
•Need to follow stream across large area on the sky
-> Wide-field, accurate RV, faint magnitudes, multiplex ~ 100
• Elemental abundances track ISM at formation
• Different elements are produced on different timescales -> their
ratio is a clock
e.g.  -> SNII (short-lived massive stars); Fe: mainly SNI -> [/Fe] enhanced
implies fast star formation
r, s processes
Matteucci 2001
Halo metallicity distribution function
-Very small number of extremely metal-poor stars known to date:
3 with [Fe/H] < -4.5
-Direct counts provide constraints on the IMF at high-redshift
e.g. there may be a critical Z below which only very massive stars form
-Currently limited by small number statistics
Salvadori et al. 2007
Spectroscopic survey of 105 halo stars at intermediate
res. to identify candidates for follow up
-> Wide-field, deep & 100 multiplex
Chemistry of metal-poor components
Nomoto et al. 2006
Cayrel et al. 2004
Knowledge of very metal-poor stars detailed abundance patterns
•Constraints on the IMF
•On the nature of the first stars and explosions (SN or HN)
•On the early history of the Galaxy (e.g. why lack of scatter?)
Merger history and streams
Latest cosmological simulations predict much
substructure in the halo
75% of stars near Sun from 3-5 parents
 Memory in kinematics -> 100’s streams crossing
Solar neighbourhood
 Should be visible with Gaia!
Cooper et al. 2010
Helmi et al. 2010
Outer halo:
•Clear evidence of substructure
•Limited to high-surface brightness
features (progenitors/time of events)
Abundance substructure also expected
Belokurov et al. 2007
Font et al. 2006
Characterize the properties of the building blocks of the halo
500 streams -> 100*/stream -> 5x104 stars, wide-field (2-3 deg2) 100 multiplex,
V ~ 17 for a survey of 2000 deg2 (V ~ 15 for 10000 deg2); HR ~ 20,000
Global Requirements
• Wide-field: 2- 3 deg2
 Galaxy is an all-sky object
 Stars in halo and thick disk are rare -> build up large samples in reasonable time
• Spectral resolution and multiplexing
 R ~ 5,000 for radial velocities (1 – 2 km/s);
 R ~ 20,000 for metal-weak thick disk, halo studies;
#fib < 1000s
#fib ~ 100s
• Survey sizes
 LR mode for disk: 106 stars for 17< V < 20
 HR mode for field populations: 105 stars
 HR mode for stream characterization: 5 x 104 stars
• Large spectral coverage
 blue sensitive: well-known region of the spectrum;
 many useful lines; little atmospheric lines
 for metal-poor stars (halo-like): there are fewer lines -> gain
 e.g. Eu (r-process) two lines around 6500 A
The landscape
Recio-Blanco 2009
Structure and history of the disks
• Characterization of star formation and chemistry as f(R)
• Test models of the formation of thick disk
• Inter-relation between various components
star formation history in
galactic thin disk from
Solar Neighbourhood:
roughly uniform, with episodic
star bursts for ages < 10 Gyr,
but lower for ages > 10 Gyr
Rocha-Pinto et al (2000)
Age-metallicity-velocity relations
Only known for the solar neighbourhood (GCS, d < 100 pc)
Holmberg et al. 2008
Only for Solar neighbourhood
Disentangle histories and
relations between thin,
thick, bulge and halo
-> abundances as f(R) and f(z)
Bensby et al. 2003
• 105 stars with 0.1 dex precision > RGB
• Local study with MS stars within
2 kpc, with 0.05 dex