Andromeda Surrounded By Dwarf Galaxies
January 4, 2013

Dwarf Galaxies Orbit Andromeda En Masse

April Flowers for - Your Universe Online

Astronomers have discovered a group of dwarf galaxies moving in unison near the Andromeda Galaxy around the host known as Messier 31 using the MegaCam on the Canada-France-Hawaii and W. M. Keck Observatory telescopes on the summit of Mauna Kea, Hawaii. Analogous to the planets of our own Solar System, the structure of these small galaxies — nearly half that orbit Andromeda - lies in a plane. What is truly surprising to the scientists, however, is that they orbit the Andromeda galaxy en masse. This presents a serious challenge to the currently held theories for the formation and evolution of all galaxies.

While Persian astronomers first catalogued the Andromeda galaxy over 1,000 years ago, only in the last five years have we studied in exquisite detail the most distant suburbs of the Andromeda galaxy. Scientists have examined this detail via the Pan-Andromeda Archaeological Survey (PAndAS), which was active between 2008 and 2011, using the Canada-France-Hawaii Telescope and measured with the Keck Observatory, providing our first panoramic view of our closest large companion in the cosmos.

The study, published in a recent issue of Nature, is the result of many years of effort by an international team of astronomers who have discovered a large number of satellite galaxies, and developed new techniques to measure their distances. The team has also used the Keck Observatory to measure their radial velocities, or Doppler shifts - the speed of the galaxy relative to the Sun. Previous studies hinted at the existence of this structure, however, the new study has demonstrated its existence to a high level of statistical confidence (99.998%).

Scientists have long known that large galaxies like the Milky Way or Andromeda are orbited by an entourage of smaller galaxies that are individually anywhere from ten to hundreds of thousands of times fainter than their hosts. These systems were previously thought to trace a path around the larger galaxy independent of every other dwarf galaxy.

Almost 30 dwarf galaxies orbit the Andromeda galaxy in this regular, solar system-like plane, the study reveals. The scientists expected that these smaller galaxies should be buzzing around randomly, like bees around a hive.

“This was completely unexpected,” said Geraint Lewis, one of the lead authors on the Nature publication. “The chance of this happening randomly is next to nothing.”

The structure is extremely flattened out, nearly one million light years across. The fact that the scientists are now able to see a majority of these systems and map out the structure implies that this understanding is horribly incorrect. Some mechanism in how these galaxies formed, or evolved, must have led them to trace out this peculiar coherent structure.

“We know of a number of galaxies that have experienced a collision, causing some of their stars to be expelled great distances, in sheets and tails. However, it´s unlikely that kind of event explains what we are observing,” said R. Michael Rich, who led the Keck spectroscopy team.

Dwarf galaxies are the most numerous galaxy type in the universe, even though they are not massive. Understanding this assembly of small galaxies will shed new insight into the formation of galaxies at all masses.

Astronomers have used computer models to predict how dwarf galaxies should orbit large galaxies for the past several decades, and every time they found that dwarfs should be scattered randomly over the sky. These efforts have resulted in simulations in ever-increasing fidelity, powered by supercomputers. However, none of these simulations have ever generated dwarfs arranged in a revolving plane like that observed around Andromeda.

“It is very exciting for my work to reveal such a strange structure,” said PhD student, Anthony Conn. “It has left us scratching our heads as to what it means.”

Scientists have claimed that the Milky Way galaxy has an extensive plane of dwarf galaxies, with some of them claiming that the existence of such strange structures point to a failing in our understanding of the fundamental nature of the Universe.

“We don´t yet know where this is pointing us” said Rodrigo Ibata, lead author on the report. “It flies in the face of our ideas about galaxy formation, but it surely is very exciting.”

Image 2 (below):  This composite shows the alignment of the satellite galaxies of Andromeda, in relation to the view that we see from Earth (the top left panel shows a true color image of the center of the Andromeda galaxy taken with MegaCam on the Canada-France-Hawaii Telescope). New distance measurements allow us to ascertain the three-dimensional positions of the satellite galaxies, which together with new velocity measurements, reveal their true nature as part of a gigantic rotating structure (side view: bottom left panel; front view: top right panel). Credit: R. Ibata (PAndAS team)