predicted satellites never form stars and consist purely of dark matter, making them invisible to even the most powerful telescopes? Radio astronomers Robert Braun and Butler Burton recently uncovered 65 gas clouds with angular sizes of one degree on the sky, four times the area covered by the full moon. They found these objects through a systematic search of the database of the Leiden-Dwingeloo Survey through their emission in the 21 cm line, a radio wavelength used to trace neutral hydrogen gas. The gas clouds have velocities exceeding 60 miles or 100 km per second relative to the Sun, similar to a group of objects known as high-velocity clouds. However, unlike the well-known high-velocity clouds, which tend to span many tens of degrees on the sky and form diffuse extended complexes, the clouds identified by Braun and Burton are compact, isolated objects. Thus Braun and Burton coined the term “compact high-velocity clouds”.

 

While most high-velocity clouds are relatively nearby at distances of 30,000 to 300,000 light years in the outskirts and immediate surroundings of our parent galaxy, the Milky Way, compact high-velocity clouds appear to be situated much farther away at distances of 1.5 to 3 million light years. These preliminary distance estimates place compact high velocity clouds close to the fringe of the ensemble of nearby galaxies known as the “Local Group”, to which our Milky Way belongs as well.

 

“We believe that compact high velocity clouds populate the entire outer regions of the Local Group and are falling in toward its center,” said Braun. His and Burton’s follow-up radio observations with the Westerbork Synthesis Radio Telescope in the Netherlands, an antenna array of 14 antennas with 25 meters (82 feet) diameter each, which is operated by the Netherlands Foundation for Research in Astronomy, and with the 305 meter (1000 feet) Arecibo radio telescope in Puerto Rico of the National Astronomy and Ionosphere Center, which is operated by Cornell University and the National Science Foundation, revealed that their gas masses exceed ten million times the mass of the sun and are comparable to the total mass of the least massive dwarf galaxies known. Their kinematic properties show that they contain ten to forty times more dark matter than gas. “We estimate that there may be more than 200 of these compact high-velocity clouds in the Local Group, which may contribute a signifiant fraction of the mass of the Local Group,” said Braun. These numbers are consistent with predictions from hierarchical structure formation scenarios, if the derived distances are correct.

 

Compact high-velocity clouds do not have known stellar counterparts. Are they pristine gas clouds that never experienced any kind of star formation, ancient proto-galaxies left over from the early times of the formation of the Universe? Or do they contain stars as well?

 

“If stars are present we expect them to be faint and not very numerous, otherwise the compact high velocity clouds would have been identified as dwarf galaxies in existing optical surveys,” said Grebel. If compact high-velocity clouds underwent low-level, but otherwise normal star formation, the most likely stellar population to expect consists of so-called red giants; old, long-lived, low-mass stars. Grebel, Braun, and Burton are therefore carrying out a targeted search for stars using the 4 meter (150 inch) telescopes of the National Science Foundation's National Optical Astronomy Observatories near Tucson, Arizona and La Serena, Chile. These telescopes are equipped with a special array of wide-field detectors that allow one to image half a square degree in one exposure, one quarter of the angular size of a compact high-velocity cloud. Furthermore, Grebel and her colleagues are using a special filter combination that makes it possible to eliminate faint foreground stars belonging to our own Milky Way.

 

The detection of stars would not only reveal the evolutionary status of the compact high-velocity clouds but also lead to improved distance determinations and, in turn, more accurate masses. “Intriguingly, our data show an increase in star-like objects at luminosities consistent with the distances predicted via radio techniques, and temperatures consistent with their being red giants,” said Grebel.

 

Encouraged by this suggestive discovery of possible stars in compact high-velocity clouds, the researchers caution that one additional source of contamination remains: Distant, redshifted starburst and irregular galaxies have similar luminosities and temperatures as the selected candidate red giants. Owing to their distance these galaxies are no longer extended but appear like stellar point sources in optical images and are thus indistinguishable. Only the determination of velocities through spectroscopy can ultimately answer the question whether the candidate red giants are indeed stars or distant galaxies. The team will carry out spectroscopic observations of the faint stellar candidates that they discovered with the 10 meter (400 inch) W.M. Keck telescopes in fall.

 

 

 

Editor’s note: You may contact Eva Grebel at (+49 6221) 528 225 or grebel@mpia-hd.mpg.de.Robert Braun can be reached at (+31 521) 595 100 or rbraun@nfra.nl, and Butler Burton at (+31 71) 527-5848 or burton@strw.leidenuniv.nl.

 

 

This release is also available on the World Wide Web at http://www.mpia-hd.mpg.de/~grebel/aas196_pressrel_2.html .