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Galactic Science andMOS on the WHT

Amina Helmi

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ESA-ESO report on Galactic populations(Turon, Primas, Binney, Chiappini, Drew, AH, Robin, Ryan)

GCDS: Gaia chemo-dynamical Survey•First meeting: Paris 26 April 2010

GREAT WG3 on Chemical Tagging

Brown, Feltzing, AH, Korn, Walton

thick disk

stellar halo

bulgethin disk

The Galaxy

How did the Galaxy come to be like this ?

What is the origin/formation epoch/mechanism and relation between the various components?

ESA-ESO Galaxy WG ESA-ESO meeting, ESTEC, 10 October ESA-ESO 2008 4

The Gaia era

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Gaia• 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

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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 WayVelocity distributions along the disk; resonance maps; coupling of dark

halo, bar and diskHalo shape, density and granularityStreams as tracers of mass distribution and evolution

• Structure and history of the disksCharacterization of star formation and chemistry as f(R)Models of the formation of thick disk Inter-relation between various components

• Metal-poor componentsStreams in the halo to trace merger historyConstrain the IMF, and star formation in the early Universe (1) not

exhaustive

A great amount of dark substructure

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CDM strong predictions on density profile, shape and granularity

Milky Way Dynamics

Narrow streams

Thin long streams better probes (more reliable tracers of underlying potential; Eyre & Binney 2009)

Internal velocity dispersions are few km/s

GD-1 stream in SDSS: dissolved cluster

Koposov et al. 2009

•Halo granularity: need very accurate radial velocities

•Distant streams preferred (d ~ 10 – 40 kpc) to isolate other effects-> 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

Chemistry

• Elemental abundances track ISM at formation

• Different elements are produced on different timescales -> their ratio is a clocke.g. -> SNII (short-lived massive stars); Fe: mainly SNI -> [/Fe] enhanced implies fast star formation

r, s processes

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-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-redshifte.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

Halo metallicity distribution function

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?)

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Cayrel et al. 2004

Chemistry of metal-poor components

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

Belokurov et al. 2007

Outer halo:•Clear evidence of substructure•Limited to high-surface brightness features (progenitors/time of events)

Font et al. 2006

Abundance substructure also expected

Characterize the properties of the building blocks of the halo500 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); #fib < 1000s R ~ 20,000 for metal-weak thick disk, halo studies; #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 2009Lamost?

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

Disentangle histories and relations between thin, thick, bulge and halo

-> abundances as f(R) and f(z)

Samples: • 105 stars with 0.1 dex precision ->

RGB• Local study with MS stars within 2

kpc, with 0.05 dex

Only for Solar neighbourhood

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