July 14, 2014
Dwarf Galaxies Discovered By Astronomers Using Dragonfly Telephoto Array
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
While images of the Milky Way or Andromeda galaxies may dominate our mind’s image of what a galaxy looks like, the fact is that the largest objects in the Universe come in all shapes and sizes. In fact, most galaxies in the cosmos are known as dwarf galaxies. These systems are, as the name might suggest, smaller versions of the elliptical, spiral, and irregular galaxies that usually spring to mind.
In many cases, these galaxies may look more similar to a simple star cluster than a whole galaxy, but there is a distinct and important difference: dark matter. Dwarf galaxies swim in a dark matter halo, influenced by its gravitational pull, while star clusters are driven less by this substance.
This makes dwarf galaxies an excellent laboratory for indirect dark matter studies since dark matter collisions are thought to produce gamma rays. Given that little else in a dwarf galaxy would be able to produce such high-energy radiation, astronomers are hoping that by imaging their gamma-ray distribution, they may be able to definitively demonstrate dark matter collisions. Initial work by the Fermi Gamma-ray Telescope has been promising in this regard.
Also, dwarf galaxies are often found to orbit about a larger host. Our very own Milky Way hosts dwarf galaxies. (Two of the more famous ones are visible from the Earth’s southern hemisphere: the Small and Large Magellanic Clouds.) Studying how their motion is affected by their massive neighbor is also part of the key to solving the dark matter puzzle.
A problem exists however, because of their small size and the relatively dim, diffuse light that is produced in these systems, dwarf galaxies would need to be relatively close in order to be detected. But a new study has revealed seven new potential dwarf galaxies surrounding the large Pinwheel Galaxy, known in astronomy circles as M101. This system is a mere 21 million light-years from Earth – virtually next door in cosmological scales.
Pieter van Dokkum , chair of Yale's astronomy department, along with University of Toronto astronomer Roberto Abraham, designed a robotic telescope using eight telephoto lenses with special coatings that suppress internally scattered light. "These are the same kind of lenses that are used in sporting events like the World Cup. We decided to point them upward instead," van Dokkum said.
Interestingly, the team hit the jackpot with their first run of the new instrument. "We got an exciting result in our first images," said Allison Merritt, a Yale graduate student and lead author of the paper reporting the results. "It was very exciting. It speaks to the quality of the telescope."
Built by van Dokkum and Abraham in 2012, the instrument, known as The Dragonfly Array, is specifically designed to detect diffuse objects with very low surface brightness; in other words, it is perfectly crafted to detect nearby dwarf galaxies.
"It's a new domain. We're exploring a region of parameter space that had not been explored before," van Dokkum said. "We knew there was a whole set of science questions that could be answered if we could see diffuse objects in the sky."
While the team initially believed these objects are in fact dwarf galaxies – a reasonable thought, given their close proximity to M101 – there is the possibility that at least a couple of these objects are much closer or farther away, and is simply directionally coincident with M101. If true, this could usher in a whole new level of excitement about the discovery.
"There are predictions from galaxy formation theory about the need for a population of very diffuse, isolated galaxies in the universe," Merritt said. "It may be that these seven galaxies are the tip of the iceberg, and there are thousands of them in the sky that we haven't detected yet."
There is still significant work to do. Follow-up observations are required in order to ascertain the distances to these objects; the first step in determining if they are in fact satellite galaxies around M101, or something even more rare. Time has already been granted to the team on the Hubble Space Telescope in order to begin such studies. "I'm confident that some of them will turn out to be a new class of objects," van Dokkum said. "I'd be surprised if all seven of them are satellites of M101.”
“We are collecting new data with the Dragonfly telescope every clear night. We're all curious to see what other surprises the night sky has in store for us," Merritt said.
The results of this study are published in The Astrophysical Journal Letters.
Image 2 (below): Yale astronomers used a new telescope, the Dragonfly Telephoto Array, to detect the diffuse light of the new galaxies. Credit: Yale University