Harvard Logo    Smithsonian Logo   Lincoln J. Greenhill

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I am a Senior Research Fellow at Harvard University in the Faculty of Arts and Sciences, Lecturer in the Department of Astronomy, and Radio Astronomer at the Smithsonian Astrophysical Observatory.

Curriculum vita   

My interests include: cosmology; supermassive black holes; star formation late-type variable stars; astronomical masers; interferometry at radio and infrared wavelengths; GPGPU.
(Please see pg.2 of my CV for details.)

Recent Articles (in the process of updating)

  • Goddi, Greenhill, Chandler, Humphreys, Matthews, Gray, "Maser emission from SiO isotopologues traces the innermost 100 AU around Radio Source I in Orion BN/KL," Ap.J., 698, 1165.
  • iWayth, Greenhill, Briggs "A GPU-based Real-time Software Correlation System for the Murchison Widefield Array Prototype," P.A.S.P., 121, 857.
  • Mitchell, Greenhill, Wayth, Sault, Lonsdale, Cappallo, Morales, Ord, "Real-Time Calibration of the Murchison Widefield Array," IEEE JSTSP, 2, 707
  • Goddi, Greenhill, Humphreys, Matthews, Tan, Chandler 2008, "A 42.3-43.6 GHz spectral survey of Orion BN/KL: First detection of the v=0 J=1-0 line from the isotopologues 29SiO and 30SiO," Ap.J., 691, 1254
  • Greenhill, Tilak, Madejski 2008, "Prevalence of High X-ray Obscuring Columns among AGN that Host H2O Masers," Ap.J., 686, L13
  • Tilak, Greenhill, Done, Madejski 2008, "A Deep 0.3-10 keV Spectrum of the H2O Maser Galaxy IC2560," Ap.J., 678, 701
  • Dijkstra, M., Lidz, A., Pritchard, J. R., Greenhill, L. J., Mitchell, D. A., Ord, S. M., & Wayth, R. B. 2008, "On the detectability of the hydrogen 3-cm fine-structure line from the epoch of reionization," MNRAS, 390, 1430.
  • Humphreys, Reid, Greenhill, Moran, Argon 2008, "Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure," Ap.J., 672, 800
  • Kondratko, Greenhill, Moran 2008, "The Parsec-scale Accretion Disk in NGC 3393," Ap.J., 678, 87
  • Greenhill 2007, "Masers in AGN Environments," in Astrophysical Masers and their Environments, Proceedings of the IAU Symposium 242, eds. J. Chapman & W. Baan, 381

Recent Talks (in the process of updating)

  • MWA Status Report & Paths Forward to the SKA, Aspen Winter Conference on Astrophysics, The High Redshift Universe: A Multi-Wavelength View, January 11, 2010
  • Prospects for Percent-level Estimation of H0, 3rd International Workshop on the Interconnection Between Particle Physics and Cosmology, invited talk, May 18, 2009
  • High Performance Computing as Linchpin in Next-generation Radio Telescopes, Harvard IIC Colloquium Series, April 15, 2009. Related invited talk, Harvard-RIKEN Joint Symposium: Application of GPU Computation to Brain Science, Quantum Science, Astronomy, Fluid Dynamics..., August 28, 2009
  • First Steps to the Dark Age, Research School of Astronomy and Astrophysics Colloquium, Australia National University, January 22, 2009

Recent Press

  • Interview on topics related to the Square Kilometer Array," ISKAF2010 magazine.
  • Mentions of recent work on the birth of massive stars, Universe Today, Space.com, USA Today, Ciel & Espace...
  • "Harnessing fun for serious science," Harvard Gazette, 09/17/09.

Recent White Papers

  • Backer et al. 2009, "HERA Hydrogen Epoch of Reionization Arrays," submission to ASTRO2010 Panel on Radio, Millimeter and Submillimeter from the Ground
  • Greenhill, Humphreys, Hu, Macri, Murphy, Masters, Hagiwara, Kobayashi, Murata 2009, "Estimation of the Hubble Constant and Constraint on Descriptions of Dark Energy," submission to ASTRO2010 panel on Cosmology and Fundamental Physics

e-Prints for the past 1 year

Research Group

    Epoch of Reionization
  • Dr. Gianni Bernardi (postdoc)
  • Dr. Jonathon Kocz (visiting postdoc)
    Graduates and Alumni
  • Josh Eisner, B.A., 2002; currently Assoc. Prof. University of Az
  • Paul Kondratko, Ph.D, 2007,
  • Haystack Obs.
  • Mike Clark, Research Staff; currently Nvidia
  • Richard Edgar, Research Staff; currently Microsoft Rsrch
  • Ciriaco Goddi, Postdoc; currently ESO-Garching
  • Elizabeth Humphreys, Staff Scientist; currently ESO-Garching
  • Daniel Mitchell, Postdoc/Staff; currently U. Melbourne
  • Lynn Matthews, Visiting Scientist; currently
  • Stephen Ord, Postdoc/Staff; currently Curtin University
  • Avanti Tilak, Postdoc
  • Randall Wayth, Postdoc; currently Curtin University

Public Lectures

November, 1999    CfA Observatory Night, Open House, "Black Holes of Brobdingnagian Proportions"

November, 2001    MIT Physics class 8.224, "Evidence for Supermassive Black Holes"

December, 2005    Public Lecture, University of Tasmania, Public Lecture, "When the First Stars Formed"

November, 2009    Public Lecture, University of Florida, Public Lecture, "The End of the Astronomical Dark Ages"
(hi-res .mov format; 150 MB!)

(low-res .mov format; 9 MB)

Current Research Priorities

High Mass Star Formation

The Kalypso Project


Kalypso seeks to track the 3D dynamics of gas in very close proximity to a high-mass young stellar object in Orion, month-by-month, for the first time.

What is the geometry of accetion and outflow on Solar System scales in high-mass star formation?

Are magnetic fields important to building of high mass stars?

What drives the archetypal star forming region, Orion BN/KL?

  • Goddi, Greenhill, Humphreys, Chandler, Matthews 2011, "Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia," Ap.J., 739, l13.
  • Goddi, Humphreys, Greenhill, Chandler, Matthews 2011, "A Multi-epoch Study of the Radio Continuum Emission of Orion Source. I. Constraints on the Disk Evolution of a Massive YSO and the Dynamical History of Orion BN/KL," Ap.J., 728, 15.
  • Matthews, Greenhill, Goddi, Chandler, Humphreys, Kunz 2010, "A Feature Movie of SiO Emission 20-100 AU from the Massive Young Stellar Object Orion Source I," Ap.J., 708, 80.
  • Goddi, Greenhill, Chandler, Humphreys, Matthews, Gray 2009, "Maser emission from SiO isotopologues traces the innermost 100 AU around Radio Source I in Orion BN/KL," Ap.J., 698, 1165.
  •  Goddi, Greenhill, Humphreys, Matthews, Tan, Chandler 2008, "A 42.3-43.6 GHz spectral survey of Orion BN/KL: First detection of the v=0 J=1-0 line from the isotopologues 29SiO and 30SiO," Ap.J., 691, 1254
  •   Matthews, Goddi, Greenhill, Chandler, Reid, Humphreys 2007, "A Documentary of High-Mass Star Formation: Probing the Dynamical Evolution of Orion Source I on 10-100 AU Scales using SiO Masers,"   in Astrophysical Masers and their Environments, Proceedings of IAU Symposium 242, 130, eds. J. Chapman & W. Baan
  • 0 Greenhill, Gezari, Danchi, Najita, Monnier, Tuthill 2004, "High Angular Resolution Mid-Infrared Imaging of Young Stars in Orion BN/KL," Ap.J., 605, 57
  • Current Research Priorities


    General purpose computing with Graphics Processing Units are the future for a broad range of computing problems in the sciences. Look HERE.

    The Epoch of Reionization

    How and when did the earliest generations of stars and black holes form?

    The Universe began at the Big Bang, with the birth of a hot soup of matter and radiation. The soup cooled, and eventually the matter separated out - electrons and protons recombined to make neutral Hydrogen. This marked the beginning of the cosmological Dark Ages, about 300,000 years after the Big Bang. There were no stars, no quasars, no luminous sources.

    Over time, self-gravity fragmented the matter and condensations collapsed to form the "first" stars, black holes, and quasars. Numbers grew, and over time, these luminous objects reionized the universe, leaving behind a largely transparent universe dotted with quasars and clusters of galaxies.

    There is little data to tell us what the universe looked like during Reionization and when the earliest stars formed. The Dark Ages present astronomers with a one billion year puzzle to solve!

    The best way to understand formation of early compact objects and reionization is to map directly the distribution of Hydrogen -- the dominant visible component of the early universe.

  • Bernardi, Mitchell, Ord, Greenhill, Pindor, Wayth, Wyithe 2011, "Subtraction of point sources from interferometric radio images through an algebraic forward modelling scheme," MNRAS, 413, 411
  • Ord, Mitchell, Wayth, Greenhill, Bernardi, et al. 2010, "Interferometric Imaging with the 32 Element Murchison Wide-Field Array," PASP, 122, 1353
  • Edgar, Clark, Dale, Mitchell, Ord, Wayth, Pfister, Greenhill 2010, "Enabling a High Throughput Real Time Data Pipeline for a Large Radio Telescope Array with GPUs," , 181, 1707
  • Mitchell, Greenhill, Wayth, Sault, Lonsdale, Cappallo, Morales, Ord, "Real-Time Calibration of the Murchison Widefield Array," IEEE JSTSP, 2, 707
  • Redshifted Hydrogen 21cm Emission



    The Large-Aperture Experiment to Detect the Dark Ages seeks to detect 21cm (rest wavelength) emission from neutral Hydrogen in the intergalactic medium about 100 million years after the Big Bang (redshift ~ 20). A detection would deliver the first observational constraints on models of structure formation and on the formation of the first stars and black holes in the Universe. LEDA will develop and integrate signal processing instrumentation into the new first station of the Long Wavelength Array (LWA). This comprises a large-N correlator serving all 512 dipole antennas of the LWA-1, leveraging a packetized CASPER architecture and combining FPGAs and GPUs for the F and X stages. Iterative calibration and imaging will rely on warped snapshot imaging and be drawn from a GPU-enabled library (CUWARP) that is designed specifically to support wide-field full polarization imaging with fixed dipole arrays. Calibration techniques will include correction for ionospheric refraction and direction dependent dipole gains, and exploration of pulsar data analysis to improve performance. Accurate calibration and imaging will be crucial requirements for LEDA, necessary to subtract the bright foreground sky and detect the faint neutral Hydrogen signal. From the computational standpoint, LEDA is a O(100) TeraFlop per second challenge that enables a scalable architecture looking toward development of radio arrays requiring power efficient 10 PetaFlop per second performance. Stage two of the Hydrogen Epoch of Reionization Array (HERA2) is one example.



    A pre-LEDA program to enable reionization studies at the VLA via outfitting with a 195 MHz receiver system. (Suspended in the face of RFI from TV.)

    Supermassive Black Holes and Cosmology

    How much do black holes in the centers of galaxies weigh?

    How do these black holes accrete gas?

    What is the nature of Dark Energy?

    Is the Universe precisely flat?

    Water maser emission from the accretion disks of supermassive black holes in the centers of galaxies may be used to map the geometry of regions at less than a tenth of a parsec from the event horizons and to measure the galaxy's distances via geometry. These distances may be robust enough to improve constraints on cosmological models, including the nature of Dark Energy.

  • 0 Zhu, Zaw, Blanton, Greenhill  2011, " The Host Galaxies of Extragalactic Nuclear Water Masers," Ap. J. 742, 73
  • 0 Greenhill, Kondratko, Moran, Tilak 2009,  " Discovery of Candidate H2O Disk Masers in AGN and Estimations of Centripetal Accelerations," Ap. J. 707, 787
  • 0 Greenhill, Tilaki, Madejski 2008,  " Prevalence of High X-Ray Obscuring Columns among AGNs that Host H2O Masers," Ap. J. 686, L13
  • 0 Greenhill 2004,  "Extragalactic water masers, geometric estimation of H0, and characterization of dark energy," New Astron. Rev. 48, 1079
  • NGC4258 and the calibration of
    extragalactic distances

          Introduction to NGC4258
  • 0 Herrnstein et al. 1999,  "A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk,"  Nature 400, 539
  • 0 Macri, L. M. et al. 2006,  "A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant,"  Ap.J. 652, 1133 
  • 0 Argon et al. 2007,  "Toward a New Geometric Distance to the Active Galaxy NGC 4258. I. VLBI Monitoring of Water Maser Emission,"  Ap.J. 659, 1040
  • 0 Humphreys et al. 2008,  "Toward a New Geometric Distance to the Active Galaxy NGC 4258. II. Centripetal Accelerations and Investigation of Spiral Structure," Ap.J. 672, 800.

  • Laboratory for Visual Learning

    Dyslexia in Science


    Does dyslexia offer advantages in the visual processing of information in science and engineering?

  • Research articles of interest

  • "Don't do anything that someone else can do.
    Don't undertake a project unless it is
    manifestly important and nearly impossible."

    Edwin Land, Forbes, May, 1987, p. 83

    "Don't do anything that someone else can do.
    Don't undertake a project unless it is
    manifestly impossible and nearly important."

    Edwin Ladd, Department of Astronomy, Harvard University, August, 1990

    Office: 60 Garden Street, MS-42 • tel: 1 617 495-7194 • fax: 1 617 495-7345 • email: greenhill- at-cfa.harvard.edu

    Last modified 09 Apr. 17 LJG