Transcript Li-cai Deng

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 facts
Aperture: ~4 m
Type: Schmidt, Alt-Az
Focal length: 20m
Relative aperture: f/5
Field of view: 5 degree diameter
Size of focal plane: 1.75 m
Sky coverage: Dec>-10 degrees, 1.5 hours around meridian
Wavelength range: 370 nm to 900 nm, R=1000/2000
Number 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 effective
aperture as a function
of declination
δ°
-10
φ(0)
4.88 4.84 4.79 4.71 4.62 4.51 4.37 4.22 4.04 3.83 3.60
0
10
20
30
40
50
60
70
80
90
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
δ = -10°
δ = 40°
δ = 90°
δ = -10°
δ = 40°
δ = 90°
with atmosphere
with atmosphere
with atmosphere
Spot sizes (80% of light) for central 3° field
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.
3.15
arc minutes
4.7 arc 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
New CCD chip
Sky brightness measured by BATC in dark photometric nights
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, or
Virgo Stellar Stream
Monoceros stream,
Stream in the Galactic Plane,
Galactic Anticenter Stellar Stream,
Canis Major Stream,
Argo Navis Stream
Pal 5
Stellar Spheroid?
Sagittarius Dwarf Tidal Stream
Belokurov et al. 2007
Hercules-Aquila
Cloud
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-Aquila
Cloud
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).
Blue – model Milky Way
Pink – model planar stream
Monoceros, stream
in the Galactic plane,
Galactic Anti-center
Stellar Stream (GASS)
Canis Major or
Argo Navis
Sun
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
Tidal Stream in the Plane of the Milky Way
If it’s within 30° of the Galactic plane, it is tentatively
Press release, November 4, 2003 assigned to this structure
Summary of Spheroid Substructure
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
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?
Sino-Western collaborations on
spheroid substructure:
Xue, X.X., Rix, H.-W., Zhao, G.,
et al. 2008, ApJ, 684, 1143
Liu, C., Hu, J.Y. Newberg, H.J.,
& Zhao, Y.H. 2008, A&A, 477,
139
density
selected
sky density of SDSS spectra
velocity
selected
velocity selected Sgr stream
The Cetus Polar Stream
Following Helmi et al. 1999
Stars within 1 kpc of the Sun, with Hipparcos proper motions
Tidal streams separate in angular momentum
– need 3D position and velocity through space.
Smith et al. 2009
Klements et al. 2009
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.
GAIA Astrometric Satellite
Magnitude limit: 20
1 billion Galactic stars
Astrometry and radial velocities
2012-2020
Will only get precise radial velocities
for 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).
t
Metal-poor stars
Metal-poor stars
Norbert
Christlieb
MPIA, January
09
See also:
Zhao, G., Butler, K., Gehren, T. 1998, A&A, 333, 219
35/37
Zhao, G. et al. 2006, ChJAA, 6, 265
Karlsson & Gustafsson (2005, IAU 238)
Number of contributing SNe
Norbert Christlieb
MPIA, January 09
MPIA, January 09
36/49
HE 05574840
HE 01075240
HE 13272326
The halo metallicity distribution function
Norbert Christlieb
MPIA, January 09
MPIA, January 09
37/49
EMP and HMP stars expected to be
found
Survey
Effective
sky
coverage
HES
6400 deg2
SEGUE
1000 deg2
LAMOST 12,200 deg2
SSS
20,000 deg2
Effective N < 3.0 N < 5.0
mag
(EMP)
(HMP)
limit
B < 16.5
200
2
B < 19
1000
10
B < 18.0
3000
30
B < 17.5
2500
25
Norbert Christlieb
Heidelberg, April 2009
MPIA, January 09
38/48
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 structure
In 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.
LAMOST Experiment for Galactic
Understanding and Exploration (LEGUE)
Science Goals
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
Discovery of spheroid substructure
Constrain Galactic potential
Disk/spheroid interface near Galactic anticenter
Search for extremely metal poor stars
Identify smooth component of spheroid
Structure of thin/thick disks, including chemical abundance and
kinematics
Search for hypervelocity stars
Survey OB stars and 3D extinction in Galaxy
Globular cluster environments
Properties of open clusters
Complete census of young stellar objects across the Galactic plane
LAMOST Experiment for Galactic
Understanding and Exploration (LEGUE)
Survey Strategy (five years)
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 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.
Deng Licai, Liu Chao
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.
7.5 M halo objects
5 M anticenter objects
3 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 all of the
time for a five year period, including moon and likely weather
conditions as a function of season.
2.5 M halo objects
3 M anticenter objects
3 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.
Deng Licai, Liu Chao
Number of spectra
winter
summer
Survey footprint covered
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.
Deng Licai, Liu Chao
Number of spectra
winter
summer
Survey footprint covered
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.
2.5 M halo objects
3 M anticenter objects
3 M disk objects
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
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!