My research focuses on
giant galaxy clusters and the galaxies they contain. I am
interested in answering the following questions:
- When did the first
galaxy clusters form?
- How does the
cluster environment affect galaxy evolution?
- When did the first
galaxies appear?
- What are the
properties of dark matter in galaxies and clusters?
To answer these questions
I am leading or acting as a senior member of several successful
research projects.
- Galaxy Evolution in
X-ray Clusters.
- Galaxy clusters
are vast collections of galaxies. The galaxies we see are
only the tip of a great gravitational iceberg, making up
only about 1% of the total mass of any given cluster. About
16% of the mass is in the form of a hot cloud of protons and
electrons. At a temperature of some 10 million Kelvin this
cloud of ions produces luminous X-ray photons. The rest of
the iceberg is dark matter. We can be certain of two things
concerning the dark matter: yes, it really is there and no,
it doesn't emit any light. To detect galaxy clusters you can
perform one of the following observations: 1) take an
optical image and look for dense collections of galaxies, 2)
take an X-ray image and look for cluster-sized clouds of hot
gas, 3) take an optical image and look for evidence of
gravitational lensing where the unseen dark matter creates a
giant magnifying glass in the sky.
- I am a member of
the X-ray Xtra Large (XXL) survey. This is a
multi-wavelength survey primarily based upon the X-ray
detection of distant galaxy clusters using the XMM
satellite.
- XXL covers 50
square degrees of the sky and constitutes the largest single
project attempted with XMM (XXL
project website).
- XXL has
discovered over 500 new galaxy clusters and is supported by
a ground-based large observing program at the European
Southern Observatory to determine cluster redshifts.
- My own work has
focused on the search for distant galaxy clusters in XXL and
it predecessor, the XMM-LSS survey (see Willis
et al. 2013).
- One highlight
from this work is the discovery of a galaxy cluster at a
redshift of z=1.98 (Willis
et al. 2020). This cluster appears to be relatively
massive and mature yet is observed only 3.4 Gyr after the
Big Bang.
- My current
graduate student, Ariane Trudeau, is performing new searches
for distant galaxy clusters (Trudeau
et al. 2020) and attempting to describe the star
formation histories of their member galaxies.
- My previous
graduate students have studied topics such as the properties
of the brightest cluster galaxies (BCGs) and how they depend
upon the cluster they inhabit (Sebastien Lavoie, see Lavoie
et al. 2016), in addition to studies of the variation
in the red/blue galaxy mix in X-ray clusters in an attempt
to isolate and understand the effect of the cluster
environment on galaxy evolution (Sheona Urquhart, see Urquhart
et al. 2010).
- The XXL survey
continues to be a rich source of new projects - whether at
the graduate or undergraduate level. Please contact me for
more details.
- ZEN: Ultra-deep
narrow band searches for Lyman alpha emission at redshifts z
> 7.
- When did the
first galaxies appear? The most distant galaxies currently
identified lie at redshifts z ~ 7 - within 1 billion years
of the big bang. Detecting galaxies at earlier cosmic times
represents a considerable observational challenge. However,
the scientific motivation for doing so is very great: the
observation of galaxies at z > 7 provides a direct view
of the very first galaxies in the act of formation. In
addition, complementary observations of galaxies and quasars
at z = 6 indicate that at this time the universe underwent a
fundamental change - the end of the epoch of reionisation -
when the energy input from young stars and AGN converted the
baryonic contents of the universe to a largely ionised
state. Galaxies at z > 7 exist before this universal
change was complete and the line-of-sight they illuminate
provides an important probe of the physical state of the
universe. In order to detect the first galaxies at z > 7,
I initiated the ZEN (z equals nine) project:
- ZEN1 consisted of
a 32 hour Very Large Telescope (VLT) ISAAC (Infrared
Spectrometer And Array Camera) image of the Hubble Deep
Field (HDF) South taken in a narrow-band filter
(NB119). By combining this new data with existing,
ultra-deep images taken at other wavebands we performed a
sensitive test for the presence of star-forming galaxies at
z ~ 9. We discovered no such galaxies in this (admittedly
small) field - see Willis
and Courbin (2005) for details.
- ZEN2 is an
extension of the ZEN project to look at three massive,
lensing clusters with the same combination of narrow and
broad band filters used in ZEN1. The foreground lens
clusters (A1835, AC114 and A1689) form a magnified view of
the high-redshift universe and provide a boost to the light
levels from distant star-forming galaxies. No clear ZEN
galaxies were detected - see Willis
et al. (2008) for details.
- ZEN3 used the
Canada France Hawaii Telescope's new wide-field infrared
camera WIRCam to search for high redshift galaxies. Although
CFHT is a 4-metre telescope, compared to the 8m VLT, the
efficiency of the telescope plus instrument combination
ensures that our 40 hour NB image reached a significant
level of the sensitivity of the previous ultra-deep images.
The advantage of using WIRCam is the very large field of
view: its 20x20 arcminute field of view is approximately 60
times larger than VLT/ISAAC. Therefore, though ZEN3 is a
marginally shallower survey than ZEN1 and ZEN2, we accessed
a much larger volume of the universe. The survey generated a
number of candidate high-redshift galaxies. However, the
candidates are of marginal quality and to date none have
been confirmed by spectroscopy. See Hibon
et al. (2010) for details.
- I have continued
to obtain further narrow-band observations with the aim of
detecting z>7 galaxies, both as part of a large
international collaboration using the HAWK-I camera at the
VLT and using CFHT/WIRCam to conduct a wide field survey for
very bright Lya emitting galaxies.
- These surveys
have also generated very large samples of faint line
emitting (i.e. star forming) galaxies at redshift z<7.
These samples are some of the largest and deepest compiled
to date and I have a number of potential graduate student
projects to work on these samples.
- Discovering and
studying gravitational lenses in the SDSS and CFHLS surveys:
- What are the
properties of dark matter in galaxies? Gravitational lensing
provides a direct probe of the dark matter distribution in a
particular lens system. The difficulty is to find examples
of foreground galaxies lensing spatially extended background
galaxies. The key point is that extended background galaxies
provide an incredibly sensitive probe of both the amount and
distribution of dark matter in the foreground galaxy.
Finding such systems has proven difficult but if a sample of
several tens of such "galaxy-galaxy" lenses can be
identified then a detailed dark matter map can be created
for the lensing galaxies - a key ingredient in understanding
both the physical nature of dark matter and the physics of
galaxy formation.
- The Optimal
Line-of-Sight (OLS) Lens survey used the Sloan Digital
Sky Survey (SDSS) to discover luminous red galaxies (LRGs)
lensing background star-forming galaxies. The trick was to
look at every LRG spectrum in Sloan to discover the presence
of unexpected (background) emission line galaxies. This
project discovered 9 new gravitational lenses - see Willis
et al. (2005) and Willis
et al. (2006) for details.
- Searching for new
gravitational lenses. One of my past graduate students
- Karun Thanjavur - completed a search for new, bright
cluster lenses in the Canada France Hawaii Telescope Legacy
Survey (CFHTLS) Megacam images. His aim is to discover
lensed star-forming galaxies that are bright enough to
perform spatially resolved spectroscopy on using 8-10m class
telescopes. His work generated one of the largest catalogues
of galaxy clusters drawn from the CFHTLS - see Thanjavur
et al. (2009) - in addition to using a combination of
gravitational lensing and cluster galaxy dynamics to
determine the dark matter mass properties of the two of the
clusters identified as lensing background galaxies - see Thanjavur
et al. (2010).
- I have a number
of innovative lensing projects that would be well suited to
new graduate students.
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