Introduction
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.
I am also a member of the Harvard University Initiative for Innovative Computing and Project Scientist for the
Murchison Wide-field Array radio telescope.
Curriculum vita 
My interests include:
cosmology;
supermassive black holes;
star formation
late-type variable stars;
astronomical masers;
interferometry at radio and infrared wavelengths.
(Please see pg.2 of my CV for details.)
Recent Journal Articles
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 Tilak, Greenhill, Done, Madejski, G. 2008, "A Deep 0.3-10 keV Spectrum of the H2O Maser Galaxy IC2560," Ap.J., in press
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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
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Kondratko, Greenhill, Moran 2008, "The Parsec-scale Accretion Disk in NGC 3393," Ap.J., in press
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Greenhill 2007, "Masers in AGN Environments," in Astrophysical Masers and their Environments, Proceedings of the IAU Symposium 242, 381, eds. J. Chapman & W. Baan
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Macri, Stanek, Bersier, Greenhill, Reid 2006, "A New Cepheid Distance to the Maser-Host Galaxy NGC4258 and its
Implications for the Hubble Constant," Ap.J., 652, 1133
Research Group
Epoch of Reionization
- Daniel Mitchell (postdoc)
- Steven Ord (postdoc)
- Randall Wayth (postdoc)
- Ben Maruca (graduate student, Harvard University)
- Robert Harris (graduate student, Harvard University)
Active Galactic Nuclei
- Avanti Tilak (postdoc)
High-mass Star Formation
- Lynn Matthews (visiting scientist)
- Ciriaco Goddi (postdoc)
Graduates
- Paul Kondratko, Ph.D, 2007
- Josh Eisner, B.A., 2002
Lectures available for download
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"
Current Research Priorities
High Mass Star Formation
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?
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
Greenhill, Gezari, Danchi, Najita, Monnier, Tuthill 2004, "High Angular Resolution Mid-Infrared Imaging of Young Stars in Orion BN/KL," Ap.J., 605, 57
Laboratory for Visual Learning
Does dyslexia offer advantages in the visual processing of information in science and engineering?
Research articles of interest
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Current Research Priorities
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.
Redshifted Hydrogen 21cm Emission
The CfA instrumentation group leads development of the MWA all-sky
survey and the real-time
calibration and imaging pipeline, which will take in an amazing 128
gigabits per second from the MWA correlator. At these rates, storing
raw data is not an option!
Mitchell, Greenhill, Wayth, Sault, Morales, and Ord 2008,“Real-time Calibration of the Murchison Wide-field Array,” IEEE JETSP, in review.
An SAO-led, pre-MWA program to enable reionization studies at the VLA
via outfitting with a 195 MHz receiver system. (Suspended - for now? -
in the face of RFI from TV and data processing constraints
prior to arrival of the EVLA correlator.)
2s-2p Hydrogen Fine Structure Transition
Serious difficulties in detection the 21 cm transition of Hydrogen include
the need to build suitable observatories from scratch (e.g., MWA,
LOFAR) and the brightness
of the night's sky at VHF radio frequencies (e.g., from the Milky Way).
Sethi et al. (2007)
analyze the possibility of detecting Hydrogen in the early universe
in absorption against the
Cosmic Microwave Background, via the 2s-2p fine-structure transition. A rest
frequency of 10 GHz would enable use of existing radio astronomical facilities and
optimized receiver systems - a major plus.
A small group of observers is evaluating feasibility for several
observatories..
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.
Greenhill 2004, "Extragalactic water masers, geometric
estimation of H0, and characterization of dark energy," New Astron. Rev. 48, 1079
Herrnstein et al. 1999, "A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk," Nature 400, 539
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
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
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.
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