Research
CLASH: Cluster Lensing and Supernova survey with Hubble
Over a 2.5-year period from November 2010 to July 2013, one month of observing time on the Hubble Space Telescope was dedicated to CLASH. This 524-orbit Multi-Cycle Treasury program observed 25 galaxy clusters in 16 HST filters spanning the near-ultraviolet to near-infrared. We are studying dark matter halos, distant galaxies magnified by these powerful gravitational lenses, and supernovae which will further constrain the nature of dark energy.
Dark Matter Halos
The majority of matter in our universe, in galaxies, and in galaxy clusters is invisible. I help reveal this dark matter by analyzing observed gravitational lensing.
Some galaxy clusters, such as Abell 1689 (left) appear to have denser dark matter cores than expected. This may indicate they formed earlier than we thought, when the universe was more dense. This in turn may be a hint that there was more dark energy in the early universe than we thought.
Before we speculate further, we should confirm that a large sample of "regular" galaxy clusters do in fact have extra-dense cores. That is one main science goals of CLASH.
The Hubble Ultra Deep Field
Every image taken by Hubble is of a very small patch of sky, the size of a grain of sand held at arm's length. Between 2003 & 2004, Hubble stared at one of these small patches for a total of 11 days. In the resulting "Ultra Deep Field" (right), about 10,000 galaxies are revealed. The light from these galaxies has traveled billions of years to reach us. And in that time, the universe has been expanding, stretching and "redshifting" this light. By quantifying these photometric redshifts, I measured the distances to these galaxies.
Higher quality near-infrared images of the UDF have since been obtained with HST's WFC3. We obtained ultraviolet images in HST's Cycle 19 (2011-2012).
Dark Energy
Dark energy is even more abundant in our universe and more mysterious than dark matter. It gives a repulsive force to large volumes of empty space, thus accelerating the expansion of our universe. Our first step toward understanding dark energy is to better measure its repulsive strength, quantified by the letter "w".
I studied our ability to do this using one method: by measuring the amount of time light is delayed as it is bent around gravitational lenses.
Cosmological Constraints from Gravitational Lens Time Delays (Coe09 ApJ). Our estimate (TD) is compared to one estimate of the capabilities of other methods.
Photometric redshifts measured in the UDF (Coe06 AJ)
Colliding Galaxy Clusters
The Bullet Cluster gave us "direct empirical proof of the existence of dark matter". As two galaxy clusters collided, gas was stripped from galaxies and dark matter, revealing that dark matter indeed has different properties from ordinary matter and can pass right through it.
"Pandora's Cluster" (left) features four galaxy clusters in one of the most complex mergers known. Two appear have merged similarly to the Bullet Cluster, but the other two exhibit strange behavior. We find gas stripped further than ever seen before, gas perhaps slungshot around ahead of mass rather than trailing it, and dark matter perhaps offset from some galaxies.
Our analysis (Merten11) is helping to yield insights into how these most massive structures formed and perhaps into the nature of dark matter itself.
"Pandora's Cluster" unleashes "ghost", "dark", "stripped", and "bullet" clusters. Dark matter (blue) and gas (red) are overlaid on a VLT color image. Analysis of Hubble images and more presented in Merten11.
Note such active mergers are not the focus of CLASH. CLASH's primary goal is to study more relaxed, "average" clusters.
Precise new constraints on the mass profile of Abell 2261 (Coe et al. 2012a)
other Publications by the CLASH team
Overview paper (Postman et al.)
See also Richard11 and this press release
Left: MACS1206 (see press release)
CLASH: Three Strongly Lensed Images of a Candidate z ~ 11 Galaxy
MACS0647-JD (Coe et al. 2013) is a strong candidate for the most distant galaxy yet known.
I discovered MACS0647-JD in CLASH Hubble imaging + Spitzer imaging of 17 galaxy clusters (with 8 yet to be observed). This discovery at z ~ 10.8 along with MACS1149-JD at z ~ 9.6 (Zheng et al. 2012) may be expected based on lensed luminosity functions extrapolated from lower redshifts. The implication is that low luminosity galaxies could have reionized the early universe. However, due to the large uncertainties based on only two galaxies, we cannot yet rule out the dramatic evolution suggested by the paucity of z ~ 10 field galaxies (Bouwens12, Oesch12). For more info, click here. The upcoming Hubble Frontier Fields will significantly improve our understanding of the universe's first 500 Myr.
Thesis
JHU, Sept. 2007
Towards an Understanding of Dark Matter: Precise Gravitational Lensing Analysis Complemented by Robust Photometric Redshifts
Ch. 1-4: Introduction (5MB): reviews of dark matter, dark matter simulations, and gravitational lensing, plus a brief introduction to photometric redshifts
Ch. 5: UDF Photometric Redshifts (2006 AJ, 132, 926)
Ch. 6: LensPerfect (2008 ApJ, 681, 814)
Ch. 7: LensPerfect Analysis of Abell 1689 (2010 ApJ, 723, 1678)
Ch. 8: Summary and Future Work