Keck NGAO Science Case
Requirements
Claire Max and Liz McGrath
NGAO Team Meeting 13
January 25, 2008
Topics
• Overview of SCRD Release 2
• Discuss three science cases (the ones that have
evolved the most)
– Extrasolar planets around low-mass stars (key)
– Gravitational lensing
– Backup Science
• New version of Rainbow Chart
• Other considerations
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SCRD Release 2 Overview
• For each science case section:
– Scientific background
– Results from science simulations or calculations
– Requirements: text and tables (tables will go into SRD, FRD)
• “Key Science Drivers”: tables in good shape
• “Science Drivers”:
– New or substantially revised sections: gas giant planets, ice giant
planets, backup science, minor planet size shape and
composition, gravitational lensing
– Beginnings of section on QSO Host Galaxies
– Some cases put off till next SCRD Release or PDR
• Astrometry in sparse fields, resolved stellar populations, debris disks
and young stellar objects,
3
Key Science Case:
Extrasolar planets around low-mass stars
• General goals (unchanged)
– Direct imaging of extrasolar planets
allows measurement of colors,
temperatures, and luminosities: test
theoretical models of planetary
evolution and atmospheres.
– Spectroscopic follow-up will be an
important means to characterize the
planet atmospheres
• Discovery space for NGAO is nice
complement to Gemini GPI
– Enabled by laser guide stars: look for
planets around faint low-mass stars
4
Target sample now differentiated into
three scientific questions
• Target sample 1: Old brown dwarf stars, to 20 pc
– Old field brown dwarfs
• Target sample 2: Young brown dwarf stars, to 80 pc
– Young (<100 Myr) field brown dwarfs and low-mass stars
• Target sample 3: Young solar-mass stars, 100-150 pc
– Solar type stars in nearby star forming regions such as Taurus
and Ophiuchus, and young clusters at distance of 100 to 150 pc.
Separated samples because each has different
requirements on contrast ratio, astrometry
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Target sample
AO
requirements
Contrast
ratio
Astrometry and Instrument and Coronagraph
photometry
All: Pl anet detection

Near IR imager, J a nd H bands ,
better th an 1.5 x Nyquist sampling .
ALL:
Planet
characterization

a) R ~150 NIR IFU , sub-Nyquis t
sampled, or b) NIR IFU with
Nyquist spatial sampling, R ~ 4,000,
or c) narro w-band filters.
H-band
relative General-purpose
coronagraph ,
photometry Š 0. 1 contrast 10-6 & inner working angle
mag for companio n of 6 É…
/D. Stabil ity of static errors
mass. Astrometric ~5nm per sqrt(hr) for PS
F
relative precision 2 subtraction or ADI.
mas (~1/10 PSF )
bet. primary
&
planet.
<10 nm calib’n o f H=10 at
quasi-static
0.2”
aberrations, esp. @
Parent stars: 2MA SS
mid spatial freq’s .
Brown Dwarfs, H=14.
<30nm bw error to
avoid
scattered
light at 0.2”.
1. Field brow n dwarfs
to 20 pc.
Same as above
J=8.5 at
0.1",
J=11 at
0.2"
3. Young solar -mass 10-20nm calib’n of Goals:
Same as above
stars,
100-150
pc. quasi-static
J=9
at
Bright parent stars
aberrations at bot h 0.07",
V=14-15,
J=10-12. lowand mi
d- J=13.5 at
Might not need LGS if spatial-frequencies. 0.2”
near-IR WFS avail.
2.
Young brown
dwarfs to 8 0 pc
Same as above.
Same as above. Would benef it fro m
dua l- or mult i-channel mode fo r
rejecting speckle supp ression (not
essential).
Requires dual - or mu lti-channe l
mode for speckle su
ppression.
Alternative: IFU, Nyquist sample d
at H, FOV > 1 arc sec.
Issues regarding the planets around
low-mass stars science case(s)
• Target sets 1, 2, and 3 get increasingly hard
• Should they all be “requirements”?
• Tentative answers:
– Target Set 1: keep as “requirement”
– Target Set 3: move to “goal”
– Target Set 2: Analyze further
• Philosophy:
– GPI, SPHERE etc are specialized instruments
– Don’t torque whole NGAO design around these
science cases, but let’s see how well we can do with
reasonable level of effort
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Science Requirements:
Gravitational Lensing by Galaxies
E. McGrath and C. Max
Outline
I.
Background
a.
b.
II.
Strong lensing: Einstein rings, multiply imaged sources
Gravitational telescopes
Scientific Goals, Galaxies Lensed by Galaxies
a.
b.
Determine mass profile of foreground deflector galaxy by studying the
geometry of multiply imaged sources
Utilize magnification power of the lens to obtain detailed kinematical
and morphological information about distant galaxies
III. Scientific Goals, Galaxies Lensed by Clusters
a.
Find high-z galaxies around clusters, lensed with high magnification
IV. NGAO Derived Requirements
a.
b.
c.
Near-IR IFU
Near-IR imager
d-IFS
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Galaxies
Lensed by
Galaxies
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Galaxies Lensed by Galaxies:
Mass Profile of Deflector Galaxy
Predominantly early-type, massive elliptical galaxies
Questions of interest:
• Mass profile of dark-matter halo
• Fraction of DM as a function of radius, redshift, mass
• Substructure in DM halos?
• Combine with scaling relations (e.g., Fundamental
Plane) to disentangle their stellar-population and massassembly histories
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Galaxies Lensed by Galaxies:
Properties of Background Galaxies
Predominantly late-type, faint spirals and irregular
galaxies
Super-resolution (from lens + NGAO) allows one
to address the following issues:
• Morphology (surface brightness profile)
• Kinematics (velocity profile)
• Stellar population information
• star formation history as a function of z
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Kinematic Lensing
• Exploits achromaticity of galaxy-galaxy lensing
– (traditional lensing exploits surface brightness
preservation to construct model of lens potential)
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Galaxies Lensed by Galaxies:
NGAO vs. HST and LGS AO
Pre-NGWFC
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Cluster-scale lensing science
• Structure of cluster dark matter halos
• Detection of very high-z galaxies and determining the luminosity
function and size distribution of such objects.
– Since clusters are large, we have longer caustic lines, which means a
better chance to observe a high-z galaxy in the region of highest
magnification
• d-IFU application
– Place multiple arms along caustic
lines to find high-z targets and to
obtain spectra of multiple lensed
sources simultaneously
• Imaging?
– Can mosaic narrow fields
– Is acquisition camera useful?
– Y/z/J-band drop-outs
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Galaxies Lensed by Galaxies:
Near-IR IFU Requirements
• The shorter the wavelength the better (better resolution, better
astrometry)
– J-band (would use z-band if available, for closer sources)
• Higher-z sources require longer 
• FOV needs to be large enough to fit lens (alternatively, mosaic)
– Typical lens sizes are ~1” in radius (e.g.,
deflector galaxy at z~0.5 and background
galaxy at z~1)
– Some are up to a ~2” in radius, requiring
larger fields of view (or mosaicing).
• PSF knowledge and stability
– In order to subtract out the deflector
galaxy. Easier when observing a
specific line (H) where foreground
galaxy will be faint
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Gravitational Lensing:
More NGAO Instrument Requirements
• Near-IR imager
– Shorter wavelengths are better (z, J-band)
– FOV slightly larger
• Want to be able to dither
• For cluster-scale lensing, FOV is up to 1 arcmin (due to
anisoplanatism, will certainly want to mosaic smaller regions)
– PSF knowledge and stability even more important, since we are
not focusing on a narrow emission line (more contamination from
deflector galaxy). Need to model the surface brightness profile
of the galaxy convolved with the PSF, and subtract off
• Near-IR d-IFS (Clusters)
–
–
–
–
Longer wavelengths will likely be key, here
Used for cluster-scale lensing to obtain spectra of multiple arcs.
May need to place several IFUs along one arc
Field of regard should be at least 30” in radius in order to
observe arcs around the entire cluster
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Backup Science
Backup Science requirements largely involve science
preparation and operations, but there are some
important considerations in NGAO design that will help
ensure we get the most out of backup programs (and the
least amount of wasted shutter time)
Two main scenarios when conditions do not allow
propagation of lasers:
1. NGS observations
2. Seeing-limited observations
Also, less demanding LGS science that can be
accomplished with less laser power.
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Backup Science
• NGS observations
– Requirements driven by most demanding science cases, which
are likely to be Extragalactic backup cases
• Currently very difficult to do extragalactic science with LGS AO
• Minimal sky coverage, faint targets and faint TT guide stars
– Tentative requirement that performance and sky coverage in
NGS mode should not be worse than current LGS-mode
operations
• Ensures that current science is still do-able as a backup program in
NGS mode with NGAO
– Sky coverage ~5%
• 1/6th of the 30% sky coverage we get with LGS NGAO
• 1/6th the number of observations (e.g, only 1 IFU available for NGS)
• Requires:
– R=14 mag guide star with a 60” diameter FOR, or
– R=15 mag guide star with a 45” diameter FOR
– Fast switching between LGS and NGS mode in case conditions19
improve
Backup Science
• Seeing-limited observations
– Acquisition camera used for seeing-limited surveys of
large region of sky (~3’)?
– Could provide follow-up targets for future NGAO
observing runs
• Question
– Is the acquisition camera sensitive enough to produce
meaningful results?
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Rainbow Chart Revisions (in red)
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Other Considerations:
Role of “Science Drivers”
• Role of “Science Drivers” (as opposed to “Key
Science Drivers”)
• Many of the “Science Drivers” will take
advantage of whatever AO capabilities flow
down from the “Key Science Drivers”
– Don’t impose strong new requirements on AO
performance, for example
• But “Science Drivers” often do highlight
additional operational scenarios, coordination,
software, or simulations needed
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Examples
• Gas Giant and Ice Giant Planets
– Requirements for non-sidereal tracking (≤ 50 arc sec/hr or 14
mas/sec), field of view of NIR imager (imager field of view ≥ 30”
diam. at K)
• Backup Science
– Motivates requirement on field of regard for high-order NGS
wavefront sensor
– Sky coverage ≥ 5%
• R=14 mag guide star with 60” diameter field of regard (FOR)
• R=15 mag guide star with 45” diameter FOR
• Quasar host galaxies
– Emphasizes analysis of requirements for accurate PSF
subtraction (to be able to image host galaxy without light from
central point source). Will serve other science cases as well.
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Other Considerations:
Future Science Team Tasks
• Short term: add tables to SRD, FRD
• Rest of System Design Phase
– Include remaining “Science Drivers” in SCRD
– Iterate with Rich’s Wavefront Error spreadsheets
• At the minimum analyze for every Key Science Driver
• Also for one or more selected Science Drivers?
– Issue Release 3 of SCRD
• PDR phase:
– Develop astrometry error budget as a quantitative tool for the
Key Science Driver cases
– Analyze PSF issues quantitatively
– Science simulations that address key design choices
– Serious progress on instrument concepts, requirements
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