CLASH for Everyone
The Hubble Space Telescope can sense dark matter and use it to bring the very distant universe into view
Hubble's New View of the Cosmos
In May 2009, NASA astronauts successfully installed new instruments and repaired older ones onboard the Hubble Space Telescope. Hubble's upgraded abilities include larger and sharper images in the near-ultraviolet and near-infrared. Heading into its 20th year of operation, Hubble was now better equipped than ever to tackle the mysteries of the universe. But with no further servicing missions planned, we astronomers felt a sense of urgency to exploit Hubble's new capabilities while we could before there were any further instrument failures. Happily, ten years later (2019) Hubble is still working well.
The Hubble Space Telescope as seen by the crew of the space shuttle after completion of the final HST servicing mission in 2009.
In August 2009, astronomers were asked to submit bold new proposals to use Hubble to its fullest potential. For the first time, the proposed observing programs were allowed to be very large and span multiple years. Up for grabs were a total of about 2200 orbits (~5 months) on Hubble over a 3 year timeframe. In all, 39 proposals were submitted and 4 were selected. Two of the proposals were merged into one, resulting in three large observing programs. CLASH is one of them.
We were awarded 525 orbits (about one month) of Hubble observing time over 3 observing cycles (2010 - 2013). Observations began in November 2010 and concluded in July 2013. The targets of our observations were galaxy clusters, the largest gravitationally bound structures yet to form in our universe. Galaxy clusters are made mostly of dark matter: 85% or more by mass. The galaxies that we see in clusters make up only about 5% of the cluster's total mass, with the remaining 10% consisting of very hot (10 million degree!) gas visible to X-ray telescopes.
By observing these galaxy clusters, CLASH accomplishes its four main science goals:(1) Map the dark matter in galaxy clusters using observed gravitational lensing; (2) Detect very distant supernovae (the final burst of light from a dying massive star), allowing us to test the constancy of dark energy's repulsive force over time; (3) Detect some of the most distant (if not the most distant) galaxies yet discovered, thanks to the magnifying power of these gravitational lenses; (4) Study the internal structure and the evolution of the galaxies in and behind these clusters.
The matter in galaxy clusters alters the path of lightwaves coming from objects behind it similar to the way an optical lens magnifies images. The distribution of matter in the cluster can be inferred from the way the shapes and brightnesses of the background objects are altered by this "gravitational" lensing.
Mapping the unseen dark matter that comprises 85% of the largest structures in our universe is intrinsically satisfying. But these dark matter maps may also reveal new clues about the formation of structure in our universe.
Above: The galaxy cluster MACSJ1206-0847 as imaged by the CLASH team as part of their Hubble Space Telescope observing program. This image is a combination of data obtained using the Advanced Camera for Surveys (ACS) and the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope.
What is dark matter? That is a question physicists and astronomers have been trying to answer for the past 50 years. It's a hard question to answer because dark matter, as its name suggests, emits no light and only weakly interacts with normal matter. We currently believe that dark matter is a type of sub-atomic particle (or perhaps several types of particles). Scientists are using both large atomic colliders (like the Large Hadron Collider) as well as astronomical telescopes to detect and constrain this elusive dark matter.
Computer simulations have been very successful at simulating the 13-billion-year evolution of our universe and the growth of large dark matter structures. However some evidence suggests that the galaxy clusters formed in simulations may be slightly different from those formed in real life. One inspiration for the CLASH project was that it appeared that real clusters had, on average, denser cores (higher "central concentrations") than simulated clusters of the same total mass. If CLASH confirmed this difference as real, then that might indicate that our simulations' ingredients aren't quite right, and that the universe is a bit different than we thought.
One possible explanation for this discrepancy would be "early dark energy". Dark energy is a repulsive force proposed to explain the accelerating expansion of our universe. We believe it has only become a dominant force recently on large scales. But if it had even a slight impact at early times, the growth of structure could have been altered to produce the dense cluster cores we observe today.
To confirm if this discrepancy was due to inadequate astrophysics models or just inadequate amounts of homogeneous data, we observed 25 carefully selected clusters each for 16 hours, or the time you spend awake each day. Imagine waking up, holding a dime at arm's length, and spending all day and night staring at President Roosevelt's eye until going to sleep 16 hours later. This is basically what Hubble did for each of our 25 targets (staring at that small an area for that amount of time), all while whipping around the Earth at 17,500 miles per hour! In the end, the CLASH project demonstrated that the discrepency was not a fundamental problem with physics but rather that the pre-CLASH data were not homogeneous and thus subtle biases were being introduced to create a mismatch between the theory and the observations. Results using CLASH data, combined with more rigorously performed simulations, revealed our current models do reproduce the distribution of dark matter in the cores of clusters quite well.
Above: Computer model of how dark matter distributes itself in space. A cluster of galaxies like MACSJ1206-0847 would occupy a small region at the very center of this simulation. The image shown here represents a region of space that is approximately 30 million light years across.
Dark Energy and Distant Supernova
Observations of a specific type of exploding star called a Type Ia supernova have led the way in measuring the expansion rate of the universe. Such stars emit a nearly constant amount of visible light when they explode and are therefore good "distance indicators." Since we know how intrinsically bright such supernovae are, we can use their apparent brightness to infer their distance. When this distance is compared with the speed at which the supernova is moving away from us, we can track the expansion rate of the universe as a function of time.
The biggest cosmological surprise in decades has come from observations of high-redshift (very distant) Type Ia supernovae, which provided the first evidence that the expansion of the Universe now appears to be accelerating (Riess et al. 1998; Perlmutter et al. 1999), indicating the Universe is dominated by a repulsive force that astronomers have dubbed "dark energy." The presence of dark energy has galvanized cosmologists as they seek to understand it. The goal for cosmologists now is to measure the equation of state of dark energy, w, and its time variation in the hope of discriminating between viable explanations. Not since Sir Arthur Eddington's famous eclipse observations of 1919 have telescopic observations offered the means to reform our understanding of gravity.
With the availability of HST's advanced onboard digital sensors, we were poised to take the next steps in these investigations. The CLASH survey team used these powerful cameras to detect and study distant Type Ia supernova to map the expansion rate of the universe across cosmic time. In particular, we focused on finding Type Ia supernova at distances greater than 8 billion light years from Earth. Observations at these great distances provide the unique chance to test for any time dependence to the dark energy equation as well as looking for the deleterious effects of evolution of the properties of supernova, independent of our ignorance of dark energy. The CLASH survey discovered a total of ~40 supernova over 3 years, of which ~12 are at z >1, doubling the number of known SNe Ia at such great distances. An example of some distant Type Ia supernovae that HST has detected are shown below.
Above: Five examples of distant supernovae discovered with the Hubble Space Telescope. The bottom row shows images of the supernova host galaxies BEFORE the star exploded. The top row shows the discovery images with the supernova's location indicated by the white arrow.
Distant Galaxies: Using Nature's Biggest Telescopes
One of the most important goals in observational astronomy is to find the first generation of galaxies. This will help us understand how and when galaxies formed and how they helped shape the universe we see today. Amazing progress has been made in recent years in finding very distant galaxies, with HST playing a key role because it is so sensitive to even the faintest objects (distant galaxies are very, very faint).
Gravitational lensing by clusters amplifies the flux of background sources considerably. Hence, any distant galaxy that is "lensed" by a cluster will appear much larger and much brighter than it otherwise would. This allows astronomers to see details in the galaxy that would normally be below the resolution limit of even the Hubble telescope. CLASH has now detected hundreds of such very distant galaxies that are highly magnified by the cluster between us and them.
Above: A distant galaxy that has been highly magnified by a cluster acting as a gravitational lens. The galaxy image here has been "de-lensed" to show how it would look once the cluster's distorting effects are removed. We can now see detail that would have required a telescope 8x bigger than Hubble to see! The cluster lens provides a big boost in resolution, revealing much smaller structures in the galaxy that tell us about how such distant objects assemble.
CLASH and the other large Multi-Cycle Treasury programs are among the impressive explorations of the universe enabled by the power and sensitivity of the Hubble Space Telescope. The resulting observations have produced an archive of stunning and rich legacy images that have helped to revolutionize our understanding of the universe.