HERMES: Deciphering the Milky Way’s History Daniel Zucker with Gayandhi de Silva and the HERMES...

HERMES: Deciphering the Milky Way’s History

Daniel Zuckerwith Gayandhi de Silvaand the HERMES team

HERMES: Introduction

• HERMES: High Efficiency and Resolution Multi-Element Spectrograph

• Under construction at the AAO (as of 1 July, the Australian Astronomical Observatory)

22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA, Beijing

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HERMES Basics

• 4 arms with VPH gratings and 4k2 CCDs

• R~28,000, 200-300 Å/per arm (~1000Å total); higher res w/slitmask (R~50,000)

• For V~14, S/N ~ 100 in 1 hour

• Designed to work with 2dF top end on 3.9m AAT at Siding Springs, Australia


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HERMES and 2dF

• 2dF: prime focus robotic positioner with 392 data fibres and a 2º field of view

• 2dF currently used with AAOmega spectrograph (low- to moderate resolution, 2 arms)

• Minimum spacing between fibres: 30-40” 22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA,

Beijing4

HERMES Instrument Status

• Preliminary Design Review completed, design meets science requirements

• Final Design Review: late 2010

• Optics procurement, grating prototyping, detector testing underway

• Data simulator and reduction pipeline in testing

• On track for science operations in 2013 22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA,

Beijing5

Galactic Archaeology with HERMES

• Look for evidence from the epoch of the Galaxy’s formation to gain insight into the processes involved

• One approach: reconstruct the star-forming aggregates that built up the disk, bulge and halo of the Galaxy

• Some still recognisable kinematically (moving groups); dynamical information for others may be lost, but recognisable by compositions (“chemical tagging”)


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Galactic Archaeology with HERMES

• How important were mergers/ accretion for building the Galactic disk and bulge? (CDM: very)

• Goal: try to find dispersed groups of stars which were associated at birth because they– were born in a common Galactic

star-forming event, or– came from the same accreted

galaxy22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA, Beijing

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Chemical Tagging

• Use detailed chemical abundance patterns of individual stars to associate them with common ancient star-forming aggregates with similar patterns

• The detailed abundance pattern reflects the chemical evolution of the gas from which the aggregate formed: different supernovae provide different yields scatter in detailed abundances, especially at lower metallicities


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Freeman & Bland-Hawthorn 2002, Bland-Hawthorn & Freeman 2004, De Silva+ 2009

Chemical Tagging

• For chemical tagging to work within the Galactic disk, some things need to be true (or at least mostly true):– Stars form in large aggregates– Aggregates are chemically homogeneous– Aggregates have unique chemical signatures (defined

by several elements) which do not vary in lockstep from one aggregate to another

• With chemical tagging, we can identify disk stars that are the debris of star-formation aggregates, as well as those accreted from disrupting satellites22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA,

Beijing9

Chemical Tagging in Action


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Clusters vs. Field Stars

HyadesColl 261HR1614

De Silva 2007

HERMES: Galactic Archaeology Survey

• Stellar survey, complete down to V = 14 (~fiber density), covering ~half the southern sky (|b| > 30) ~10,000 square degrees = 3000 pointings, spectra of 1.2 x 106 (!) stars

• For V ~ 14, R ~ 28,000, with SNR ~ 100 per resolution element in 1 hour, with ~ 8 fields per night can be done in ~ 400 clear nights (bright time)


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HERMES: GA Survey

• In order to maximise chemical “resolution”, select four wavelength regions to allow abundance measurements from a range of 7 independent element groups:


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• Light elements (Na,Al)• Mg• Other alpha-elements (Ca, Si,

Ti)• Fe and Fe-peak elements• Light s-process elements

(Sr,Zr)• Heavy s-process elements

(Ba)• r-process (Eu)

Channel Wavelengths

Blue 4708 – 4893Å

Green 5649 – 5873Å

Red 6481 – 6739Å

IR 7590 – 7890Å

HERMES: GA Survey


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Expected Fractional Contribution from Galactic Components

Dwarf GiantThin disk 0.58 0.20Thick disk 0.10 0.07Halo 0.02 0.03

Old disk dwarfs will be seen out to distances of about 1 kpcDisk giants ___________________________________ 5 kpcHalo giants ___________________________________ 15 kpc

HERMES: GA Survey

• Ultimate goal of the archaeology program is unravelling the star formation history of the thin and thick disk and the halo via chemical tagging

• Interim data products include:– distribution of stars in [position, velocity,

chemical] space for a million stars– distribution of Fe/H, α/Fe and X/Fe for enormous

samples of stars from thin and thick disks, halo– detailed abundance gradients in each

component– chemical and kinematical correlations in inner

and outer thick and thin disks• 7-year timeline for survey (including

construction)22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA, Beijing

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(Some) Other Science with HERMES: The Spectrograph

• Substructure in the halo and studies of closest Milky Way satellites

• Magellanic Clouds• Skymapper Follow-up• ISM in the Milky Way + Local

Group• Globular and open clusters• Stellar astrophysics• Time Domain: Stellar binarity

and variability, planet searches, etc., etc.


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HERMES with Fainter Stars

• For V ~ 14, HERMES will achieve S/N ~ 100 in 1 hour; for V ~ 17, S/N ~ 10 in 1 hour (S/N ~ 20 in 4 hours)

• For R ~ 28,000 spectra with S/N ~ 10, [M/H] and velocities can be determined with errors of ~0.12 dex and <1 km/s, respectively22 July 2010 Daniel Zucker LAMOST Milky Way Workshop // KIAA,

Beijing16

Carney et al. 1987

HERMES and the Halo

• V~17 reaches the TRGB out to ~100 kpc, HB out to ~ 25 kpc

• HERMES will be able to directly measure [Fe/H], [α/H], etc. for these stars, as well as precision RVs


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Mochejska et al. 2001

TRGB: ~ 100 kpc

HB: ~ 25 kpc

Satellites, Streams, Substructure

• Surviving (~bound) satellites, stellar streams and substructure in general should be most apparent in galaxy halos (lowρstellar , long τdyn)

• Substructure should be detectable in multi-dimensional (spatial, kinematic, chemical) parameter space


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Bullock, Johnston, Font et al.

HERMES and the Halo

• Streams and faint satellites in the halo are typically extremely diffuse and extend over large areas of the sky

• Thanks to the FOV and ~400 fibres of 2dF, HERMES will be ideally suited for studies of halo substructure


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UMa II

Zucker+ 2006

Belokurov+ 2006

HERMES and the Magellanic Clouds

• A magnitude limit of V ~ 17 reaches the RGB in the LMC (upper RGB in the SMC)

• HERMES will make it possible to study the chemical and dynamical evolution of the MCs in unprecedented detail


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Alcock et al. 2000

LMC

HERMES and Skymapper

• Skymapper initial 5-second survey input photometry and astrometry for main HERMES survey

• Symbiosis: Skymapper targets for spectroscopy, HERMES precision abundances and radial velocities for Skymapper discoveries


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HERMES in the Gaia Era

• Gaia: astrometry and spectroscopy parallaxes, proper motions, RVs

• HERMES + Gaia: combine 6-D phase space info with detailed abundances to disentangle the formation history of the Milky Way (e.g., “real” phase-space substructures vs. artefacts)


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Helmi and de Zeeuw 2000

Brown+ 2005

HERMES Recap

• New spectrograph for AAT + 2DF: ~400 fibres, 2° FOV

• 4 arms, R (Δλ/λ)~28000, ~ 200 - 300Å per arm (total ~ 1000Å); higher res capability (R~50,000)

• S/N ~ 100 @ V ~ 14 in 1 hour (S/N ~ 10 @ V ~ 17 in 1 hour)

• First light: late 2012, full operations: mid-2013


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Summary

• High efficiency, high spectral resolution, wide field of view and massive multiplexing will make HERMES a uniquely powerful tool for disentangling the complex formation and evolution of the Milky Way and its components


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• In conjunction with data from other projects (e.g., Skymapper and Gaia) HERMES has the potential to revolutionise our understanding of the Milky Way’s history – and of galaxy formation in general


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Summary

JOBS! JOBS! JOBS!

Do you have what it takes to join Team Hermes?

Are you:• Smart?• Motivated?• Within 3 years of

receiving your PhD?


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HERMES Super Science Fellowships

• 3 x 3-year Super Science Fellowship postdocs starting 2011, at Macquarie University, in beautiful Sydney, Australia

• Very competitive salary and generous research budget

• Linked PhD scholarships with supervisory opportunities


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Science with HERMES

28 – 29 September 2010ATNF Lecture theatre

Epping, Sydney, Australia

www.aao.gov.au/HERMES

22 July 2010 28Daniel Zucker LAMOST Milky Way Workshop // KIAA, Beijing


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HERMES: Deciphering the Milky Way’s History Daniel Zucker with Gayandhi de Silva and the HERMES...

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