Overview

(1) Constraints and suitability of the LAMOST Telescope

(2) Scientific justification for a Galactic survey of millions of stars

(3) LEGUE Survey Strategy - a survey of 2.5 million spheroid stars, and 5 million disk stars

LAMOST factsAperture: ~4 mType: Schmidt, Alt-AzFocal length: 20mRelative aperture: f/5Field of view: 5 degree diameterSize of focal plane: 1.75 mSky coverage: Dec>-10 degrees, 1.5 hours around meridianWavelength range: 370 nm to 900 nm, R=1000/2000Number of fibers: 4000, 16 spectrographs with 250 fibers each>10,000 spectra per night (>2 million spectra/year), 2-3 gigabytes/night

LAMOST: 4000 fibers in 20 square degrees (200 fibers/sq. deg.)UKST: 250 fibers in 28 square degrees (5 fibers/sq. deg.)SDSS: 640 fibers in 7 square degrees (90 fibers/sq. deg.)

LAMOST Constraints

(1) Pointing - Light lost with distance from the meridian, declinations away from the optical axis of the telescope.

(2) Fibers - must be uniformly distributed.

(3) Weather – poor summer weather limits view of Galactic center.

Optical System

MA is the Schmidt corrector, 5.72m x 4.40m, with 24 hexagonal plane sub-mirrors, each with 1.1m diagonal and 2.5 cm thickness.

MB is the spherical primary, 6.67m x 6.05m, with a radius of curvature of 40m, 37 hexagonal spherical sub-mirrors, each with 1.1m diagonal and 7.4 cm thickness.

Active control for aspheric shape of corrector (34 force actuators plus 3 mount points per submirror). Optimal shape changes with declination and hour angle.

Active control for MB is just 3 mount points plus three actuators per submirror.

Optical axis is 25° from horizontal.The focal plane has a radius of curvature of 20m.

LAMOST effectiveaperture as a functionof declination

δ° -10 0 10 20 30 40 50 60 70 80 90

φ(0) 4.88 4.84 4.79 4.71 4.62 4.51 4.37 4.22 4.04 3.83 3.60

φ(0.75h) 4.87 4.83 4.78 4.70 4.61 4.50 4.36 4.21 4.03 3.83 3.60

Spot sizes (80% of light) for central 3° field

δ = -10° δ = 40° δ = 90°

δ = -10° δ = 40° δ = 90°

with atmosphere with atmosphere with atmosphere

1.5 hours of tracking atmosphere included

The plots above show the largest linear extent of the spot size containing 80% of the light, as a function of declination. Above declination of 60°, the field of view has been reduced from 5 degrees to 3 degrees in diameter, which is the reason for the apparent sudden reduction in spot size. The spot size at the edge of the field (5°) at δ=60° is the same as the spot size at δ=90° at the edge of the 3° field.

4.7 arc minutes

3.15arc minutes

In one pointing, fewer than 20 LAMOST fibers can be placed inside the 10’ tidal radius of a globular cluster. Fewer than 50 LAMOST fibers can be placed within 20’ of the center of an open cluster.

The combination of site weather patterns and the need to look at the meridian puts strong constraints on the footprint of the LAMOST survey.

Potential Worries

• Sky brightness

• Scattered light

• Dust/pollution

• Temperature

• Calibration

But note that we already have a spectrum of similar quality to SDSS.

6660 Å, bandwidth 480 Å

Sky brightness measured by BATC in dark photometric nights

New

CC

D c

hip

Comments on sky brightness

• BATC measurements avoid sky emissions (as of KPNO) therefore somewhat fainter than true values;

• There is a scatter ~1mag in each night. The scatter has a weak dependence on direction.

The LAMOST spectrum (top) is comparable to the SEGUE spectrum (bottom) of the same star, with similar exposure time. The LAMOST sky subtraction and response function still need more work (note O2 line at 6880Å and 7600Å), but it can already achieve simlar S/N as SDSS.

Newberg et al. 2002

Vivas overdensity, orVirgo Stellar Stream

Sagittarius Dwarf Tidal Stream

Stellar Spheroid?

Monoceros stream,Stream in the Galactic Plane,Galactic Anticenter Stellar Stream,Canis Major Stream,Argo Navis Stream

Pal 5

Hercules-AquilaCloud

Areal density of SDSS stars with 0.1<g-i<0.7 and 20<i<22.5 in Galactic coordinates. The color plot is an RGB composite with colors representing regions of the CMD as shown in the inset. The estimated distance to the cloud is 10-20 kpc.

Belokurov et al. 2007

Hercules-AquilaCloud

The large scale lumpiness of the stellar halo density has made it difficult to determine whether the outer parts of the Galaxy are axisymmetric (Xu, Deng & Hu 2006, 2007).

Belokurov et al. 2007

Press release, November 4, 2003

Blue – model Milky WayPink – model planar stream

Tidal Stream in the Plane of the Milky Way

Sun

Canis Major orArgo Navis

Monoceros, stream in the Galactic plane, Galactic Anti-center Stellar Stream (GASS)

If it’s within 30° of the Galactic plane, it is tentatively assigned to this structure

TriAnd,TriAnd2

Explanations:(1) One or more

pieces of tidal debris; could have puffed up, or have become the thick disk.

(2) Disk warp or flare(3) Dark matter

caustic deflects orbits into ring

Summary of Spheroid Substructure

Globular cluster streams:

(1) Pal 5: Odenkirchen et al. 2003

(2) GD-1 Grillmair & Dionatos 2006

(3) NGC 5466: Grillmair & Johnson 2006(4-6) Acheron, Cocytos, and Lethe:

Grillmair 2009 Other:

(1) Hercules-Aquila Cloud(2) Virgo Overdensity?

Dwarf galaxy streams:

(1) Sagittarius: Ibata et al. 2001a, Ibata et al. 2001b, Yanny et al. 2000

(2) Canis Major/Argo Navis? Monoceros (Newberg et al. 2002, Yanny et al. 2003), GASS (Frinchaboy et al. 2004), TriAnd (Majewski et al. 2004), TriAnd2 (Martin, Ibata & Irwin 2007), tributaries (Grillmair 2006)

(3) ?? Orphan stream, Grillmair 2006, Belokurov et al. 2006

(4) ?? Virgo Stellar Stream, Vivas et al. 2001, Newberg et al. 2002, Zinn et al. 2004, Juric et al. 2005, Duffau et al. 2006, Newberg et al. 2007(5) Styx, Grillmair 2009(6) Cetus Polar Stream, Newberg, Yanny & Willett 2009

Sino-Western collaborations on spheroid substructure:Xue, X.X., Rix, H.-W., Zhao, G., et al. 2008, ApJ, 684, 1143Liu, C., Hu, J.Y. Newberg, H.J., & Zhao, Y.H. 2008, A&A, 477, 139

The Cetus Polar Stream

velocity selected Sgr stream

velocityselected

densityselected

sky density of SDSS spectra

Fol

low

ing

Hel

mi e

t al.

1999 Tidal streams separate in angular momentum

– need 3D position and velocity through space.

Stars within 1 kpc of the Sun, with Hipparcos proper motions

Moving groups found from 22,321 low metallicity ([Fe/H]<-0.5) SDSS/SEGUE stars within 2 kpc of the Sun, from SEGUE data (above). Moving groups from SDSS /Segue stars within 5 kpc of the Sun (right). Both analyses show significant velocity substructure in the Solar neighborhood.

Klements et al. 2009Smith et al. 2009

GAIA Astrometric SatelliteMagnitude limit: 201 billion Galactic starsAstrometry and radial velocities2012-2020

Will only get precise radial velocitiesfor stars brighter than 15th magnitude!

With LAMOST, radial velocities can be obtained for the most interesting magnitude range of 15<V<20

Other large spectroscopic surveys of stars include RAVE (I<12), SDSS III/ SEGUE II (400,000 stars), APOGEE (infrared bulge), HERMES (V<14, in fabrication), and WFMOS (in planning stages).

MPIA, January 09 35/37

Metal-poor stars

Metal-poor starst

Norbert Christlieb

See also:Zhao, G., Butler, K., Gehren, T. 1998, A&A, 333, 219Zhao, G. et al. 2006, ChJAA, 6, 265

Number of contributing SNe

MPIA, January 09 36/49

Kar

lsso

n &

Gus

tafs

son

(200

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38)

MPIA, January 09

Norbert Christlieb

MPIA, January 09

The halo metallicity distribution function

HE

010

752

40

HE

132

723

26

37/49

HE

055

748

40

MPIA, January 09

Norbert Christlieb

Heidelberg, April 2009

EMP and HMP stars expected to be found

Survey Effective sky

coverage

Effective mag limit

N < 3.0 (EMP)

N < 5.0 (HMP)

HES 6400 deg2 B < 16.5 200 2

SEGUE 1000 deg2 B < 19 1000 10

LAMOST 12,200 deg2 B < 18.0 3000 30

SSS 20,000 deg2 B < 17.5 2500 25

38/48MPIA, January 09

Norbert Christlieb

Outstanding Problems• Describe the chemical evolution of the Galactic disk(s), and

especially the first generation of stars.• What is the detailed structure of the Milky Way’s disk? How is

it related to Monoceros/ Canis Major? • What does the dark matter potential of the Milky way look like?

We have yet to successfully extract information about the Galactic potential from tidal streams.

• How many stellar components are there in the Milky Way, and how do we describe them?

• Galactic stellar data in all Galactic components is more complex than the models in structure and in dynamics. How do we compare them?

• How many small galaxies merged to create the Milky Way, and when?

• So far, advances have primarily come from reducing the data size to analyze very clean samples. How do we utilize all of the partial chemical, kinematic, and spatial information at the same time?

The Future of Galactic structureIn the Milky Way, we have the opportunity to learn the whole history

of one galaxy instead of comparing snapshots of many. It is only now that we have large surveys of the whole sky that we are able to comprehend the Milky Way as a whole. Unlike external galaxies, the picture we are building is in three dimensions of position and velocity, with much higher accuracy information for each star.

Many surveys currently in progress will provide multi-color imaging of the sky. However, there is a great need for spectroscopic surveys of millions of stars.

Twenty years ago, when the idea for the SDSS was born, large scale structures of galaxies had just been discovered. But there was structure on all scales of the largest surveys of the day. There was a pressing need for a larger spectroscopic survey.

We are at the same place now in the study of the Milky Way. Spatial substructure and moving groups are found in every spectroscopic sample of spheroid stars that is well constrained in position and stellar type. It is guaranteed that a larger survey will reveal more substructure.

Science Goals(1) Discovery of spheroid substructure(2) Constrain Galactic potential(3) Disk/spheroid interface near Galactic anticenter(4) Search for extremely metal poor stars(5) Identify smooth component of spheroid(6) Structure of thin/thick disks, including chemical abundance and

kinematics(7) Search for hypervelocity stars(8) Survey OB stars and 3D extinction in Galaxy(9) Globular cluster environments(10) Properties of open clusters(11) Complete census of young stellar objects across the Galactic plane

LAMOST Experiment for Galactic Understanding and Exploration (LEGUE)

Three subsets:

(1) Spheroid (|b|>20°) portion will survey at least 2.5 million objects at R=2000, with 90 minute exposures, during dark/grey time, reaching g0=20 with S/N=10.

(2) Anticenter (|b|<30°, 150°<l<210°) portion will survey about 3 million objects at R=2000 with 40 minute exposures, during bright time (and some dark/grey time), reaching J=15.8 with S/N=20.

(3) Disk (|b|<20°, 20°<l<230°) and will survey about 3 million objects at R=2000 and R=5000, with 10 and 30 minute exposures, respectively, during bright time, reaching g0=16 with S/N=20

Survey Strategy (five years)

LAMOST Experiment for Galactic Understanding and Exploration (LEGUE)

Survey footprint, shown as an Aitoff projection in Galactic coordinates. The region with open circles will be observed at R=2000. The region with filled circles at low Galactic latitude will be surveyed with shorter, bright time exposures including R=5000 and R=2000. There is a small region that is part of both the anticenter and speroid surveys. The Celestial Equator is shown as a solid line. This illustrates the footprint, but not the exact placement of the survey plates.

Spheroid Survey

Use SDSS, PanSTARRS, or SuperCOSMOS photometry, in that order, as available. A u-band survey is planned on the 2.3 meter Bok telescope of Arizona’s Steward Observatory; a camera is currently being fabricated. This survey can be used in conjunction with PanSTARRS or SuperCOSMOS

(1) Select as many 0.1<(g-r)0<1.0, g0<17 stars as possible (a nearly complete sample where surveyed, except below b=40°, randomly sample to g0<18

(2) Randomly sample stars with (g-r)0<0.4 in the magnitude range 17<g0<20

If u-band photometry is available, we will deselect QSOs. The subsampling will be about one in two or one in three at higher latitudes.

Additional Criteria for spheroid

• We will observe a sample of high proper motion stars with colors of M dwarfs in the magnitude range 16<g0<20 (local spheroid stars)

• If u-band available, subsample K and M giant candidates with 17<g0<20

• Within 3 tidal radii of 40 selected GCs, we will use a completely different selection algorithm to select stars with color/magnitude of the GC stars

• We will include bright (V<12) K and M stars from the Tycho-2 catalog, without regard to their proper motion.

Simulation of LAMOST stellar spectral density in a five year survey, using an Aitoff projection in Galactic coordinates, using *all* of the clear weather. The result is 7.5 million halo objects, 5 million anticenter objects, and 3 million disk objects.

Deng Licai, Liu Chao

Sample survey coverage in fibers per square degree, shown as an Aitoff projection in Equatorial coordinates (Galactic coordinates shown in blue). The survey simulation was done assuming all of the time for a five year period, including moon and likely weather conditions as a function of season.

7.5 M halo objects5 M anticenter objects3 M disk objects

Deng Licai, Liu Chao

Sample survey coverage in fibers per square degree, shown as an Aitoff projection in Equatorial coordinates (Galactic coordinates shown in blue). The survey simulation was done assuming 1/3 of the dark/grey time and all of the bright time for a five year period, including moon and likely weather conditions as a function of season.

2.5 M halo objects3 M anticenter objects3 M disk objects

Deng Licai, Liu Chao

Accumulation of data as a function of time assuming all of the dark and grey time for a five year survey. After the first year, we begin to re-observe parts of the sky that have already been covered.

Survey footprint covered

Number of spectra

summerwinter

Deng Licai, Liu Chao

Accumulation of data as a function of time assuming 1/3 of the dark/grey time and all of the bright time for a five year survey. After the first year, we begin to re-observe parts of the sky that have already been covered.

Survey footprint covered

Number of spectra

summerwinter

Deng Licai, Liu Chao

LEGUE Science Working Group: DENG Licai, HOU Jinliang, NEWBERG Heidi, CHRISTLIEB Norbert, LIU Xiaowei, HAN Zhanwen, CHEN Yuqin, ZHU Zi., PAN Kaike, LEE Hsu-Tai, WANG Hongchi

2.5 M halo objects3 M anticenter objects3 M disk objects

Recent development

• We learnt last week that the telescope can point 2 hour away from the meridian without loosing much efficiency;

• The survey plan may change a lot.

• The expected data size for LEGUE will not change.

• Site quality decays quickly, if we want to do the survey, we should act fast!