LIGER

A Next Generation Keck AO Instrument

Liger is a near-infrared integral field spectrograph (IFS) for the W. M. Keck Observatory. It will use the upcoming Keck All-Precision Adaptive optics (KAPA) system on the Keck I telescope with performance and advancements for large format near-infrared detectors. Liger is based largely on the design of IRIS for the forthcoming Thirty Meter Telescope (TMT).    This will minimize costs and fabrication and construction timescales.

The Liger instrument uses the latest near-infrared detectors for an imaging camera that operates simultaneously with an innovative IFS that uses both a slicer and lenslet array for spatial sampling. It will help explore a wide range of science cases including   dark matter substructure, distant galaxies, supermassive black holes, nearby star forming regions, the Galactic Center, and Solar System bodies. It will enable new science by offering enhanced capabilities such as higher spectral resolving power (R∼8000-10000), access to shorter wavelengths (0.84 - 2.4 µm), and larger fields of view compared to any current and future ground- and space-based IFS systems.

Liger is a crucial stepping stone both technologically and scientifically for the next decade in astronomy.

Science Drivers for New Capabilities of Liger

New Capability of Liger	Main Science Driver
Extending to higher spectral resolution (R > 8000-10,000)	Supermassive and intermediate-mass black hole measurements; high-z galaxy kinematics; planetary and stellar atmospheres and abundance patterns; radial velocity measurements
Extending to visible wavelengths (λ < 1 μm)	Dynamics and abundances in a range of systems; solar system bodies; exoplanet metallicities; accretion diagnostics in young stars and planets; low-z galaxies; black hole mass measurements
Larger IFS field of view	Crowded field spectroscopy; galactic nuclei; star forming regions; solar system; strong gravitational lens systems
Larger imager field of view	Star forming regions; astrometric accuracy for proper motion measurements; Jupiter and Saturn
Parallel imaging	Many science cases benefit from parallel imaging to provide astrometric reference and PSF characterization

Liger Subsystem Overview

Opto-mechanical layout of the Liger imaging camera (green) and the lenslet re-imaging optics (red) and the slicer re-imaging optics (blue). The Keck AO f/15 beam is fed into Liger at the top left. The filter wheel and rotating lyot stop feeds both the imager and integral field spectrograph. There are two pick-off mirrors that can select between the slicer and lenslet IFS modes. The slicer and lenslet IFS share all complex optics like the gratings, common TMA camera, and detector (bottom right). The nominal field of view of the Liger optical system is in the top right with imaging camera (green), two spatial scales slicer IFS (blue), two spatial scales lenslet (red).

Liger Cryostat

A rendering of the cylindrical Liger cryostat. The red beam from the Keck Adaptive Optics system enters on the left side. The beam is picked off at two spots on the imaging plane, one for the IFS-Slicer and one for the IFS-Lenslet. The lower white structure is the existing cart on a solid model of the Keck rail system located at the output port location of Keck AO. The electronics cabinet is depicted in yellow.

Liger Modes

Liger Field of View Compared with OSIRIS

IFS observations of a z=2.01 lensed galaxy (SDSS J1206+5142, magnified ~28x by a galaxy group at z=0.42). Liger’s field of view will allow the entire arc and central lens to be observed in one pointing, whereas OSIRIS requires multiple positions even at the largest scale.

Metallicity gradient (O/H) of the lensed galaxy measured from OSIRIS data in the central R < 1 kpc, where the requisite faint emission lines are detected (Jones et al., 2013). Liger’s slicer field of view and considerably higher sensitivity will map low surface brightness emission lines at larger radii, improving gradient measurements and enabling diagnostics of shocked outflows and active galactic nuclei. Observations of similar lensed quasar systems will enable sensitive probes of dark matter substructure and dark energy, utilizing Liger’s field of view and higher spectral resolution.

Simulated Galactic Center Observations

Using Liger (right) compared to OSIRIS (left). Liger’s increased field of view and sensitivity yields a dramatic increase in the number of stars with precise radial velocity measurements (indicated with green circles), including several with short-period orbits (white ellipses).

Liger Technical Team

Name	Affiliation	Expertise
Aaron Brown	UCSD	Project Manager
Maren Cosens	UCSD	Graduate Student, Opto-mechanical Design and Testing
Michael Fitzgerald	UCLA	UCLA Co-Lead
Chris Johnson	UCLA	Systems Administrator
Tucker Jones	UCSD	Project Scientist
Marc Kassis	WMKO	Instrument WMKO Program Manager
Renate Kupke	UCSC	UCSC Lead, Optical Design of Imager and IFS
Evan Kress	UCLA	Mechanical Engineer
James Larkin	UCLA	UCLA Lead and Co-PI
Ken Magnone	UCLA	Electronics
Rosalie McGurk	WMKO	Staff Astronomer, Onsite Integration and Commissioning
Jerome Maire	UCSD	Opto-Mechanical Design, Detector Testing, DRS
Nils-Erik Rundquist	UCSD	Software Engineer, DRS Design
Arun Surya	UCSD	DRS, Readout Processing, Optical Design Characterization
Eric Wang	UCLA	Opto-Mechanical Design
James Wiley	UCSD	Graduate Student, Opto-Mechanical Design and Testing
Peter Wizinowich	WMKO	KAPA PI, Liger Performance
Shelley Wright	UCSD	Principal Investigator (PI)
Sherry Yeh	WMKO	Staff Astronomer, Onsite Integration and Commissioning
Andrea Zonca	UCSD	Software Engineer, DRS Design

Liger Science Team

Name	Affiliation	Expertise
Lee Armus	Caltech/IPAC
Devin Chu	UCLA
Maren Cosens	UCSD	Nearby galaxies and HII regions
Tuan Do	UCLA	Galactic center; supermassive black holes
Chris Fassnacht	UC Davis	Cosmology; dark energy and Hubble constant
Deanne Fisher	Swinburne	Galaxy formation and evolution
Michael Fitzgerald	UCLA
Andrea Ghez	UCLA	Galactic center
Jenny Greene	Princeton
Aurelien Hees	Paris Observatory
Jessie Hirtenstein	UC Davis
Tucker Jones	UC Davis	Project Scientist, Galaxy formation and evolution
Theodora Karalidi	UCF
Jacqueline Keane	Hawaii	Comets; solar system
Pat Kelly	Minnesota
Evan Kirby	Caltech	Nearby galaxies; stellar populations
Quinn Konopacky	UCSD	Exoplanets; star formation
James Larkin	UCLA
Jessica Lu	UC Berkeley	Star formation; nearby galaxies
Mark Marley	LPL U Arizona
Anne Medling	U Toledo
Max Millar-Blanchaer	UCSB
Quinn Minor	BMCC
Anna Nierenberg	UC Merced
Naveen Reddy	UC Riverside	Galaxy formation and evolution
Michael Rich	UCLA
Jean-Baptiste Ruffio	Caltech
David Sand	U Arizona
Ryan Sanders	UC Davis	Galaxy formation and evolution
Karin Sandstrom	UCSD	Nearby galaxies; interstellar medium
Alice Shapley	UCLA	Galaxy formation and evolution
Arun Surya	UCSD	Instrumentation
Tommaso Treu	UCLA	Cosmology; dark matter substructure
Mike Wong	UC Berkeley	Solar system; planetary science
Shelley Wright	UCSD