Stellar halos point to similar galactic pedigree

photo:The Large Magellanic Cloud is in the southern constellation Dorado the Goldfish. It and the neighboring Small Magellanic Cloud are named after Ferdinand Magellan who observed both irregular galaxies in 1519. Anglo-Australian Observatory / Royal Observatory, Edinburgh, photo by David Malin

September 22, 2003

Big or small, we all come into this world the same way. This now also appears to apply to the birth of galaxies. Peering out at one of our neighboring galactic satellites, astronomers have discovered fresh evidence that it may share a similar evolutionary past with the Milky Way.

Using the European Southern Observatory's (ESO) Very Large Telescope (VLT) in northern Chile in January 2003, an international group of scientists has found signs of an ancient stellar halo enveloping the Large Magellanic Cloud (LMC), similar to that observed around our own galaxy.

Forming a spherical halo of giant elderly stars around the Milky Way, RR Lyrae stars can be used as a marker to find a similar population of these stellar seniors in the LMC. RR Lyrae stars are classified as old, giant, pulsating variables residing mostly in globular clusters and in galactic halos. They are used as a reliable measuring stick for up to 650,000 light-years distance.

By combing through the data of over 8,000 RR Lyrae collected for the MACHO (Massive Compact Halo Objects) microlensing project in only a 10-square-degree region, the study team was able to harvest 43 individual RR Lyrae stars located in the LMC. Kem Cook, team member from Lawrence Livermore National Laboratory in California and original discoverer of these LMC stars says that RR Lyrae are an easily identifiable tracer of old, metal-poor stellar populations.

By studying the rapid, random movements of these sample stars, the researchers were able to determine the existence of an ancient stellar halo in the LMC. This new discovery shows that the LMC and other smaller galaxies like it may contain halos similar to our own galaxy, suggesting a comparable formation history.

Clues to the formation theories for our Milky Way and other large spiral galaxies have been derived from studying these same types of stars (population II) on the outskirts of the galaxy. Amounting only to 2 percent of the entire mass of the LMC, the RR Lyrae count in the LMC matches that surveyed in our galaxy.

Astronomers suggest that these findings will narrow the possible methods by which galaxies similar to our own are formed to two scenarios. Either satellite galaxies with existing old stars are gobbled up by larger galaxies creating a stellar halo (hierarchical accretion), or before the nascent disk of a spiral galaxy collapses there is a gigantic expulsion of its elderly stellar residents into a spherical halo (rapid collapse).

"The bottom line is that the Large Magellanic Cloud seems to have had a similar early formation history as the Milky Way," says Cook.

Looted gas

photo:As shown in this illustration, gas stripped from the Magellanic Clouds follows in front and behind the two galaxies. Daisuke Kawata, Chris Fluke, Sarah Maddison, Brad Gibson (Swinburne University of Technology)

May 14, 2003

Forty years ago astronomers discovered clouds of hydrogen gas moving with high velocities near the Milky Way Galaxy. It had been unclear, until now, if these clouds were gravitationally bound to the Milky Way or were simply passing near our galaxy as they traveled through the Local Group. Recent studies presented by Mary Putman of the University of Colorado show that these clouds are probably material stripped from small galaxies and that they are now companions of the Milky Way.

Using the Commonwealth Scientific and Industrial Research Organisation's Parkes Radio Telescope in eastern Australia, Putman and her collaborators made highly detailed maps of hydrogen gas in and near the orbital paths of the Magellanic Clouds. Along the orbits of these two galaxies is a trail of material — including hydrogen gas — called the Magellanic Stream. Our galaxy ripped this material from the Magellanic Clouds. Putman and her team believe the high-velocity clouds have similarly destructive origins and were created when the Milky Way looted material from additional small galaxies.It's also possible that the clouds were created by some sort of a fountain of material from our Milky Way or via interaction with the Magellanic Clouds. Whatever their origins, the high velocity clouds are local to our galaxy.

"We now have pretty conclusive evidence that the high-velocity clouds are not scattered throughout our Local Group of galaxies, but are within the extended halo of the Milky Way," says Putman.

photo:Hydrogen maps of the Magellanic Clouds (the two white spots to the left) show streams of material trailing behind the galaxies. Called the Magellanic Stream, this material was stripped off the two small galaxies by our Milky Way. M. Putman (U. Colorado)/L. Staveley-Smith (CSIRO)/K. Freeman (ANU)/B. Gibson, D. Barnes (Swinburne)


These results fit well with related observations of other groups of galaxies. "Previous studies of other galaxy groups haven't found any counterparts of the high-velocity clouds," said team member Lister Staveley-Smith of the Australia Telescope National Facility. "If the clouds are there, they must lie close to the big galaxies rather than scattered throughout galaxy groups." If large galaxies create the clouds by stripping material from small, nearby galaxies, the clouds will naturally lay close to large galaxies.This has important implications for the dark-matter distribution in groups of galaxies like our own Local Group. Some galaxy formation models predict that there are small lumps or "halos" of cold dark matter scattered throughout the Local Group. Astronomers wondered whether high-velocity clouds represented these otherwise-invisible halos of dark matter.

photo:The 64-meter Parkes Radio Telescope was built in 1961. It is located near Alectown, 25 kilometers (16 miles) north of the town of Parkes, which is approximately 365 kilometers (227 miles) west of Sydney. CSIRO

If the high-velocity clouds traced the locations of these dark matter halos, you'd expect the clouds to be strewn throughout the Local Group. But because the gas clouds are only found close to the Milky Way they cannot be associated with the dark matter halos. While dark matter halos may still roam the Local Group, they aren't associated with these streaming clouds of hydrogen.