My research interests lie in theoretical astrophysics and UV/X-ray space astronomy.
Past theoretical studies include studies of promordial gas cooling, galaxy formation,
thermal phases of the intergalactic medium (IGM), and the
critical star formation
rate required to maintain reionization of intergalactic hydrogen at redshifts
z ~ 6-8.
With previous students and postdocs, I have developed models of the ionizing radiation
produced by massive stars
and quasars and their
contributions to the
metagalactic ionizing
background. Our group has used
N-body hydrodynamical
cosmological simulations to understand the structure of thermal phases of the IGM,
particularly the Lyman-alpha forest and "warm-hot IGM" (WHIM) both at high redshift
and "more locally" (redshifts z < 1.5). Space observational work uses UV spectroscopic
data from the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST)
for surveys of the
missing cosmological baryons
and the
cosmic metal evolution
in the low-redshift IGM. A recent
IGM paper
analyzes the pulse dispersion of fast radio bursts (FRBs) at cosmological redshifts
by a structured IGM.
Our group has also used UV spectroscopic observations with HST/COS to produce
a composite spectrum of the
ionizing radiation
of quasars in their (rest-frame) Lyman continuum (wavelengths from 912 A down
to 400 A).
Another recent HST/COS paper presents an ultraviolet survey of strong H I absorbers
in the spectra of distant quasars. This survey provides the best data on the spatial
density of low-redshift
Partial Lyman Limit Systems
-- gas likely associated with the halos of intervening galaxies like the Milky Way and
Andromeda (M31).
Another long-term observational program uses the Wide-Field Camera-3 aboard Hubble
to search for distant galaxies at redshifts z > 8. I am working with former CU postdoc
Michele Trenti (now at Univ of Melbourne), Tommaso Treu (UCLA), and others on the BoRG
Team ("Brightest of Reionizing Galaxies). Using pure-parallel HST observations with the
optical and near-IR channels on HST/WFC, the BoRG survey has discovered 97 Y-band
"flux-dropout galaxies" at the bright end of the luminosity function at
redshift z ~ 8.
Our latest results have extended the BoRG survey to Hubble's redshift frontier at
redshifts z ~ 9-10.
The next observations in the BoRG project (500 parallel orbits in Hubble Cycle 25)
will search for galaxies out to z = 11, formed in the first 700 Myr after the Big Bang.
We will survey 580 square arcmin over 125 sightlines in five WFC3 bands (0.35 to 1.7 microns).
In the future, we will propose deep infrared observations with the James Webb Space Telescope,
scheduled for launch in early 2019.
Three press releases from the Space Telescope Science Institute describe
some of our group's past discoveries on a "Galaxy Protocluster at Redshift z ~ 8",
on UV-spectroscopic searches for "Intergalactic Missing Baryons", and on
defining the end of the the "Helium Reionization Epoch" at redshift z ~ 2.7.
Hubble Pinpoints Farthest Protocluster of Galaxies Ever Seen
Hubble Survey finds Missing Matter and Probes the Cosmic Web of
Intergalactic Matter
Hubble Astronomers Uncover an Overheated Early Universe
Given the enormous cosmological distances to these galaxies, quasars, and intergalactic
gas, it worth asking the question: What is our place in the universe?
The four images below show:
(Top Panels): Aerial view of the CU-Boulder campus and Andromeda/M31, the nearest
large galaxy (2.5 million light years distant).
(Bottom Panels): Hubble Image of galaxies in the "Ultra-Deep Field" and
a schematic time line of the "History of the Universe" from the Big Bang to the present.
This time-line follows the first galaxies and quasars (11-13 billion years ago) through
the "Epochs of Reionization" of hydrogen and helium.
I teach a variety of classes, at both undergraduate and graduate level, in astronomy
and astrophysics. In recent years (fall 2011, 2014, 2017) I have taught our graduate
core astrophysics course on
Atomic & Molecular Processes
(ASTR 5110). In Spring 2008, 2009, 2015, and 2018 I taught a large Core Curriculum
class on Modern Cosmology
(ASTR 2010) dealing with the Big Bang Theory for the origin and evolution
of the universe. In fall 2009, spring 2012,and fall 2016 I taught our graduate course on
Galaxies (ASTR 5720).
In fall 2008, fall 2010, and fall 2012, I taught the graduate course on
Cosmology (ASTR 5770).
I have also taught graduate astrophysics seminars (ASTR 6000) on topics:
The First Stars in fall 2008 (with postdoc Britton Smith);
High-Redshift Galaxies in fall 2010 (with postdoc Michele Trenti);
Feedback from Galaxies and Quasars in fall 2012;
Supernovae in spring 2013 (with postdoc Emily Levesque);
and most recently
Precision Cosmology in fall 2016
Over the last 40 years at CU, I have worked with 21 undergraduate research students,
18 PhD thesis students, and 31 postdoctoral researchers.
Undergraduate research projects include 6 senior honors theses in theoretical
astrophysics and space astronomy. Undergraduate research student
Kristin Gillmon
was recognized as the Outstanding Graduate of the College of Arts & Sciences
for her 2005 Honors Thesis on
Infrared and Ultraviolet
Studies of Molecular Hydrogen in the Galactic Infrared Cirrus.
Two recent graduate students (Evan Tilton, Joshua Moloney) completed their PhDs
in April 2017. I also continue collaborations with previous students (Devin Silvia,
Michael Topping, Matt Stevans) and postdocs (Charles Danforth, Britton Smith,
Michele Trenti, Aparna Venkatesan).
Between(2000-2014) I was active with the AURA
(Association of Universities for Research in Astronomy),
which manages a number of national observatories:
the National Optical Astronomy
Observatories (NOAO), the
Space Telescope Science Institute (STScI) ,
and the
National Solar Observatory (NSO) .
AURA's current activities are focused on working with NASA and NSF to
develop powerful new observational facilities for ground-based and space-borne
astronomy, such as:
Research in my group is supported by theory grants from the Space Telescope
Science Institute, and space-observational grants for the Hubble Space Telescope.
My research includes both theoretical and observational studies
of the interstellar medium (ISM) and intergalactic medium (IGM), quasars
and black holes, galaxy formation, heavy elements and molecules in space.
My specific research areas include:
Theoretical studies include models of the high-redshift IGM, including its
cosmological reionization
by the first hot stars and the
radiative feedback
from star formation in the first galaxies. The computer simulation images
shown below illustrate (left) active regions of star formation at
redshift z = 12.5, triggered by H2 cooling shown in blue
(Ricotti, Gnedin, & Shull 2002a,b; 2008). On the right is a density
projection showing the hot, shock-heated filaments of IGM
(work with Britton Smith and Eric Hallman).
My group's theoretical studies of the First Stars include
Nucleosynthesis, Reionization, and Mass Function of stars
formed at redshifts z = 10-30, and thermodynamic studies of the
cooling of primordial gas clouds, to determine
Critical Metallicities for Second-Generation Stars.
Recent projects include numerical simulations of the
Fate of the First Galaxies.
with Massimo Ricotti and Nick Gnedin.
We use supercomputers to model how these Dwarf Primordial Galaxies form
and evolve, and assess the impact of their radiation, gaseous outflows,
and newly synthesized heavy elements. Related work with Aparna Venkatesan
investigates
First-Light Ionizing Sources
and the Optical Depth of the Cosmic Microwave Background.
These studies constrain the epoch of reionization of the high-redshift
intergalactic medium by the earliest massive stars and black holes,
at redshifts z > 7.
My recent space-observational work includes a
Hubble Survey of Missing Matter between the Galaxies.
Charles Danforth and I used ultraviolet spectrographs on the
Hubble and FUSE satellites to observe the structure, ionization state,
and chemical composition of
Intergalactic Matter at Redshifts z < 0.4.
Our studies also produced a census of ordinary matter (baryons) in the
local Universe. The pie-chart below shows their distribution:
less than 10% resides in galaxies, and most of the matter lies in
intergalactic space, observable through absorption-line spectra in
the ultraviolet and X-ray wavelength bands.
Shull and collaborators (Joe Collins, Jennifer Westbrook, Mark Giroux)
have used ultraviolet spectra to probe the mass and chemical abundances
of Galactic halo gas seen in
High-Velocity Clouds (Collins, Shull, & Giroux 2007).
Many of these High-Velocity Clouds, such as the well-studied
Complex C (Collins, Shull, & Giroux 2003, 2007),
are falling onto the Milky Way disk. In a Hubble survey of the
high-latitude Galactic sky in the strong UV absorption
line of ionized silicon (Si III), we have found a major
reservoir of ionized gas in the
Galactic Halo that could bring a fresh
supply (about 1 solar mass per year) of matter to replenish ongoing star
formation. Other space-observational projects involve a FUSE study of
He II Reionization
at z=3 in the Intergalactic Medium,
and a FUSE survey of
High-latitude interstellar molecular hydrogen.
I have been involved in advocacy and planning for a powerful 4-8 meter UV/Optical
Space Telescope, used to study galaxies, stars, planets, and
The Emergence of the Modern Universe.
My current space-astronomy interests center around a powerful new
instrument for the Hubble Space Telescope, the
Cosmic Origins Spectrograph (COS),
which was installed during the recent Hubble Servicing Mission
(May 2009). COS delivers 10-15 times the light throughput of
the previous (STIS) spectrograph aboard Hubble. Our science team
will study the origins and evolution of the intergalactic gas, quasars,
and galaxies through ultraviolet spectroscopic studies of D/H, He II
reionization, and chemical abundances in the IGM and galactic halos.
In particular, COS (see image below) will study the spatial distribution
of primordial and shock-heated intergalactic gas and heavy elements expelled
into intergalactic space by massive stars and supernovae. The expected
topology of the intergalactic gas (shock-heated filaments in
the "Cosmic Web") is shown in the simulation figure below.
Between Sept 2013-Aug 2014, I was on sabbatical leave at the Institute of Astronomy
(Cambridge University) as the Sackler Lecturer. I continued Hubble spectroscopic surveys
of intergalactic matter (missing baryons, heavy element abundances). I also worked on o
theoretical models of galactic halos, early galaxy formation, reionization of the IGM in
hydrogen and helium, and effects of feedback from galaxies and quasars.
I served as APS Department Chair (1998-2004),
Associate Dean of the College of Arts & Sciences (1988-1990),
and Faculty Associate Vice Chancellor (1995-1997).
I led campus curricular development and strategic
planning, including the Campus Strategic Plan (1995-1997).
and the design and implementation of the previous Core Curriculum
(1989-2017) for the College of Arts & Sciences.
Nationally, I served as Chair of the Space Telescope Institute Council
(STIC) and Chair of the AURA Board of Directors (2005-2008) for the
Association of Universities for Research in Astronomy. AURA
manages the US National Optical Astronomy Observatories (for NSF)
and the Space Telescope Science Institute (for NASA).
For the APS department, I chaired the committees that wrote the last
two (2002-03 and 2009-10) strategic plans and headed the development of our
Undergraduate Degree Program in Astronomy & Astrophysics,
which now has over 300 majors and graduates ~50 students
per year. As Department Chair, I raised substantial private funds to
support graduate and undergraduate scholarships and fellowships, academic
and planetarium support, and our $2M (12.5%) capital share in the 3.5-meter
telescope at the Apache Point Observatory.
We also joined the fourth Sloan Digital Sky Survey
Fourth Sloan Digital Sky Survey (SDSS-IV).
Both telescopes are managed by the
Astrophysical
Research Consortium (ARC).
In Jan 2001 we joined the ARC consortium as a 1/16 partner,
and in Jan. 2006 we doubled our observing share to 1/8 (80 nights/year),
thanks to several generous private donations. As part of this
new agreement, CU/CASA built the $1.3M Near Infrared Camera and
Fabry-Perot Spectrograph
(NIC-FPS) for this telescope,
installed November 2004 under the supervision of Dr. James Green,
with help from CASA staff (Stephane Beland), faculty
(John Bally, John Stocke), graduate students (Fred Hearty, Meredith
Drosback, Nathaniel Cunningham) and APS undergraduate majors.
Last decade, I headed the UVOWG (Ultraviolet-Optical Working Group)
charged by NASA with studying the scientific and technology issues
for the next generation of space missions after the Hubble Space
Telescope -- the Space Ultraviolet Observatory, or SUVO.
Our efforts included a conference (Ultraviolet-Optical Space Astronomy
Beyond the Hubble Space Telescope, ASP Conf. Series, Vol 164) and a
White Paper entitled
The Emergence of the Modern Universe: Tracing the Cosmic Web.
I served on the "UV, Optical, and Infrared Astronomy from Space"
panel of the NRC
Astronomy and Astrophysics Decade Survey.
In 2009-2010 I served on the
Astro2010 (Astronomy and Astrophysics) Decadal Survey
as Chair of the Science Frontier Panel examining
The Galactic Neighborhood .
Overview:
I am a Professor of Astrophysics in the
Department of Astrophysical and Planetary Sciences (APS)
at the University of Colorado at Boulder.
I am also a Fellow in the
Center for Astrophysics
and Space Astronomy and an Affiliated Faculty Member of the
Department of Applied Mathematics.
I was educated at Caltech
(BS 1972, Physics) and Princeton
(PhD 1976, Physics). Following a postdoc at the University of California,
Berkeley (1976-77), I arrived at the University of Colorado
in fall 1977, where I have worked for the past 40 years.
I have received College awards for teaching (1986) and research (1996),
CU Distinguished Research Lecturer (2001), President's Teaching Scholar
(1994), and College Professor of Distinction (2008). The following pages give
more detail on my recent activities and interests in teaching, research, and
service to department, university, and profession.
Below are links for recent (2008-2018) classes and graduate seminars,
and lists of past students and postdocs.
Teaching and Research Activities:
Academic Information:
Over the past 38 years at CU, I have taught courses in Physics, Astrophysics,
and Space Policy at both the undergraduate and graduate level.
I wrote an undergraduate Physics textbook (with Ted Snow)
and have been co-editor of 7 Conference Procedings, including
three of the well-known "Tetons Conferences on Astrophysics".
In 2005, I was elected Fellow of the
AAAS
(American Association for the Advancement of Science).
Undergraduate Teaching:
Graduate Teaching: