GALAH view of the Galactic disk

Gayandhi De Silva

Australian Astronomical Observatory

Ken Freeman (ANU), Joss Bland-Hawthorn (U. Sydney) and the GALAH Survey team

Dynamics of Disk Galaxies Seoul. 24 October 2013

What’s a Galah?

Target: a million stellar spectra at high resolution! 2/13

HERMES is a High Efficiency and Resolution Multi-Element Spectrograph on the 4m Anglo-Australian Telescope

• SNR = 100 @ V = 14 in 1hr

• R ~28,000 or ~45,000

• 4 channels cover maximum ‘good’ chemical element lines

• Uses 2dF fiber positioner to get 392 stars over 2degree field

Channel Wavelength (Å)

Blue 4708 - 4893

Green 5649 - 5873

Red 6481 - 6739

IR 7590 - 7890

3/13

Survey Goals

Key GALAH science questions:

(a) When and where were the major episodes of star formation in the disk and what drove them?

(b) To what extent is the Galactic disk composed of stars from merger events?

(c) How have the stars that formed in situ in the disk evolved dynamically since their birth?

(d) Where are the solar siblings that formed together with our Sun?

The major goal of GALAH is to identify chemically similar groups of stars in the Galactic disk, which are most likely ancient disrupted clusters, to decipher the star formation, migration and minor-merger history of the Milky Way.

4/13

Target Selection

Simple selection criteria: • 2MASS positions, APASS photometry, PPMXL proper motions • Galactic latitudes between ±10° and ±45° up to declination +10 • All targets from 12 < V < 14 • no colour selection

Expected contribution from Galactic components:

−Thin disk ~ 75% −Thick disk ~ 24% −Bulge ~ 1% −Halo ~ 0.1%

With HERMES ~ 350 stars per hour gives > 2000 stars per night

At 100 nights per year for ~5 years gives > 1 million stars

5/13

Data Analysis

Data reduction pipeline

“2dfrdr” developed at the

AAO handles all their fibre

based instruments and runs

real time between exposures

Automated abundance analysis

pipeline “Theremin” developed

within the GALAH collaboration

(based on MOOG) provides:

• Stellar parameters: effective temperature, gravity, rotation • Radial velocities, variability, multiplicity flags, oddities • Abundances of individual elements: Li, C, O, Na, Al, K, Mg, Si,

Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Ce, Eu

6/13

Chemical Tagging

Use the chemical information within stars to “tag” them to common formation sites (Freeman & Bland-Hawthorn 2002)

Assumes stars form in clusters and such clusters are chemically homogenous – holds for present day open clusters.

Method verified by identification of moving groups and stellar streams. Chemistry can differentiate origins of kinematical disk substructure, for example: • HR 1614, Argus Association & Wolf 360 group = homogenous clusters De Silva et al. 2007, De Silva et al. 2013, Bubar & King 2010

• Hercules = dynamical stream Bensby et al. 2007

• Hyades = both! Pompeia et al 2011 De Silva et al 2011 Tabernero et al 2011

7/13

HR 1614 group Hercules stream

Hyades stream

8/13

Blind chemical tagging of the Bensby et al. 2013 disk stars: Use group finding algorithm to tag 703 disk stars purely using chemical

info

Mitschang et al. 2013a, MNRAS

# st

ars

per

grou

p δ c

(dex

)

9/13

[Fe/

H]

[α/F

e]

Mitschang et al. 2013b, submitted

Chemical tagging could also provide ages for groups

10/13

Recover such groups in a million stars to answer key questions:

Chemo-dynamical evolution: What are the dominant nucleosynthetic processes (principle chemical components) as a function of position, velocity and stellar orbital motions. Direct evidence for a physical sequence of disk evolution. Mixing and migration: what will be the distribution of stars in chemically recovered clusters - narrow range in RGC or spread over kpcs in radius? Will depend on migration and mixing efficiencies. Merger history: what will be the frequency of chemically tagged dwarf galaxy signatures vs. homogenous clusters. Will depend on the satellite galaxy infall rate. Solar siblings: chemically tag the solar family – chemical signature of planets, aliens?

11/13

HERMES commissioning: started, continue till Nov 2013

GALAH pilot survey: Nov 2013 – Jan 2014

GALAH main survey: 2014 – 2019 (?)

Current Schedule

12/13

The GALAH team on a horizontal branch

More details at: www.aao.gov.au/HERMES/GALAH/

13/13

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