Khee-Gan Lee

Astronomer and Cosmologist

I am Khee-Gan ("K.-G.") Lee, an assistant professor of astronomy at the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) near Tokyo, Japan. My primary research interests (see below) are the intergalactic medium and large-scale structure. primarily an observer: I led the now-completed CLAMATO spectroscopic survey on the Keck-I Telescope and am helping plan the high-redshift survey for the upcoming Prime Focus Spectrograph (PFS) on the Subaru Telescope. More recently, I am also leading the FLIMFLAM survey that will use FRB foreground mapping to constrain the cosmic baryon distribution in the Local Universe. I also have a side interest in developing new instrumentation for the next generations of spectroscopic astronomical surveys.

I grew up in Kuala Lumpur, Malaysia, did my undergraduate studies in University College London, and then obtained my Ph.D. in Astrophysical Sciences from Princeton University. Before coming to Tokyo, I was a postdoc at the Max Planck Institute for Astronomy in Heidelberg, Germany, and was also a Hubble Fellow at Lawrence Berkeley National Lab in Berkeley, California.

JOBS: I am eager to work with students and postdocs interested in the IGM, large-scale structure and cosmology, either as visitors up to several months or longer-term positions. Please contact me to discuss!

Contact details:

  • Email: kglee<at>

  • Office Phone: +81-4-7136-6567

  • Office: A57 (5th floor) at Kavli IPMU

  • Meatspace Address: Kavli IPMU (University of Tokyo), 5-1-5 Kashiwanoha, Kashiwa, 277-8583, Japan

  • Twitter: @kheegster

Scientific Interests

Lyman-alpha Forest Tomography

Currently, my primary research interest is the technique of Lyman-alpha forest tomography, which uses distant galaxies and quasars (z=2-3, or 10-11 billion light years away) to illuminate the foreground hydrogen gas tracing the underlying cosmic web. With the largest telescopes in the world, we can observe dense grids of these background sources closely-separated with each other which allows us to 'tomographically' reconstruct the full 3D gas distribution. This provides a unique probe of 3D large-scale structure on small scales at this distant epoch. I am currently leading a project called the COSMOS Lyman-Alpha Mapping And Tomography Observations (CLAMATO) Survey, which deploys this technique on the 10.3 diameter Keck-I Telescope in Hawai'i to map a region of the sky called the COSMOS Field.

Public Data: The 2017 data release can be found here, and is described by the data release paper. The data is visualized by the following video:

If you have a Google Cardboard or similar viewer, open the YouTube version to enable 360 degree viewing. These videos illustrates real data we have obtained so far, and we hope to extend the volume of this map by at least 6-8X to ~1 square degree, in order to build up a comprehensive picture of the cosmic web at this early epoch. This will yield a rich and unique set of science results: It will allow the characterization of z~2 galaxy properties (morphology, star-formation, metallicity etc) as a function of their large-scale gas environment. We also hope to identify progenitors of present-day massive galaxy clusters through intersections of the cosmic web. The data will also be powerful for Lyman-alpha forest correlation analysis: we will be able to measure the 3D Lyman-alpha forest auto-correlation at <10Mpc/h scales, which is sensitive to cosmological parameters such as neutrino mass and tilt of the primordial power spectrum. The cross-correlation between the Lyman-alpha forest with, e.g., Type-2 AGN, LAEs, sub-MM galaxies etc will yield bias estimates for these objects.

For Subaru PFS, the high-redshift component of the Subaru Strategic Program (SSP) survey will carry out IGM tomography over a much wider area of ~15 sq deg, which will allow the identification of the cosmic web as well as constraining cosmological parameters such as neutrino mass and curvature of the primordial power spectrum.

Fast Radio Bursts and the Cosmic Baryon Census

Another recent interest of mine are FRBs as a probe of the cosmic baryon distribution. The frequency dispersion of the FRB signal encodes the integrated line-of-sight electron density, which probes the IGM and CGM since most cosmic baryons in these regions are ionized. With samples of localized FRBs (i.e. host galaxy redshifts) as well as spectroscopy of foreground galaxies, me and my team have devised techniques to combine all the information to disentangle the relative distribution of the diffuse cosmic baryons in the Cosmos. See this paper for more details.

I am now leading the FRB Line-of-Sight Ionisation Measurement From Lightcone AAOmega Mapping (FLIMFLAM) Survey, which uses the 4m Anglo-Australian Telescope in Australia to map out the cosmic web in front of ~30 localized FRBs. Together with coordinated observations on Keck, Gemini and SOAR, this effort will yield the first precision constraints on the partition of CGM and IGM baryons, to within ~10-15%. FLIMFLAM is in collaboration with the Commensal Real-Time ASKAP Fast Transient (CRAFT) and the Fast and Fortunate FRB Follow-up (F^4) teams.

Instrumentation/Spectroscopic Surveys

I am on the science team for the WFOS spectrograph for the Thirty Meter Telescope.

Looking even farther ahead, I am also involved in the early discussions for a (hypothetical) hyper-multiplexed spectroscopic survey facility currently provisionally known as the Billion-Object Apparatus (BOA). Envisioned to begin operations in the 2030s, this facility will pair a 10m-class wide-field telescope with an instrument capable of simultaneously obtaining spectra for 20,000-30,000 objects per square degree. Over a 10-year survey over ~10,000 square degrees, BOA will perform a complete census of 3D galaxy redshifts down to linear scales at z<1.5, and a CLAMATO-like IGN tomography at z~2-3 over this huge area. This will be a "Stage-V" cosmology survey that will enable a quantum leap in cosmological constraints over those from LSST.

In the past, I have also been involved with the Baryon Oscillations Spectroscopic Survey (BOSS) project, which is part of the Sloan Digital Sky Survey. For my Ph.D. thesis, I used the Lyman-Alpha absorption in BOSS quasar spectra to constrain the temperature-density relationship of the IGM.