Dwarf Galaxies and Dark Matter

Marla Geha

Today's issue of Astrophysical Journal contains a paper by Joshua Simon of Department of Astronomy, California Institute of Technology and Marla Geha of Hertzberg Institute of Astrophysics, Victoria , Canada (currently at Department of Astronomy, Yale University) reporting results of a new observation that have shed new light on the "Missing Dwarf Galaxy" puzzle--a discrepancy between the number of extremely small, faint galaxies that cosmological theories predict should exist near the Milky Way, and the number that have actually been observed.

The "Cold Dark Matter" model, which explains the growth and evolution of the universe, predicts that large galaxies like the Milky Way should be surrounded by a swarm of up to several hundred smaller galaxies, known as "dwarf galaxies" because of their diminutive size. But until recently, only 11 such companions were known to be orbiting the Milky Way. To explain why the missing dwarfs were not seen, theorists suggested that although hundreds of the galaxies indeed may exist near the Milky Way, most have few, if any, stars. If so, they would be comprised almost entirely of dark matter which does not interact with electromagnetic waves and thus cannot be directly observed but has gravitational effects on ordinary atoms.

Joshua Simon

In the past two years, researchers used images from the Sloan Digital Sky Survey to find out as many as 12 additional very faint dwarf galaxies near the Milky Way. The new systems are unusually small, even compared to other dwarf galaxies; the least massive among them contain only 1% as many stars as the most minuscule galaxies previously known. "These new dwarf galaxies are fascinating systems, not only because of their major contribution to the Missing Dwarf problem, but also as individual galaxies," says Joshua Simon, "We had no idea that such small galaxies could even exist until these objects were discovered last year."

Marla Geha added,"We thought some of them might simply be globular star clusters, or that they could be the shredded remnants of ancient galaxies torn apart by the Milky Way long ago. To test these possibilities, we needed to measure their masses." Joshua and Marla used the DEIMOS spectrograph on the 10-meter Keck II telescope at the W. M. Keck Observatory in Hawaii to study 8 of the new galaxies. The Doppler effect--a shift in the wavelength of the light coming from the galaxies caused by their motion with respect to the earth-- was closely observed to determine the speeds of stars of each dwarf galaxy, which are determined by the total mass of the galaxy.

They measured precise speeds of 18 to 214 stars in each galaxy, three times more stars per galaxy than any previous study. The speeds of the stars ranged between 4 to 7 km/s, which were much slower than the stellar velocities in any other known galaxy [For comparison, the sun orbits the center of the Milky Way at about 220 km/s]. When the speeds were coverted to masses, all these galaxies fell among the smallest ever measured, more than 10,000 times less massive than the Milky Way. Joshua and Marla conclude that the fierce ultraviolet radiation given off by the first stars, born just a few hundred million years after the Big Bang, may have blown away all of the hydrogen gas from dwarf galaxies also forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or, in many cases, completely dark. When this effect is included in theoretical models, the number of expected dwarf galaxies agrees with the number of observed dwarf galaxies.

An image showing positions of these dwarf galaxies relative to Milky Way can be accessed here: http://www.keckobservatory.org/images/article_pictures/147_308.jpg

Although the Sloan Digital Sky Survey was successful in finding a dozen ultrafaint dwarfs, it covered only about 25% of the sky. Future surveys that scan the remainder of the sky are expected to discover as many as 50 additional dark matter-dominated dwarf galaxies orbiting the Milky Way. Telescopes for one such effort, the Pan-STARRS project on Maui, are now under construction.

"Explaining how stars form inside these remarkably tiny galaxies is difficult, and so it is hard to predict exactly how many star-containing dwarfs we should find near the Milky Way", says Joshua, "Our work narrows the gap between the Cold Dark Matter theory and observations by significantly increasing the number of Milky Way dwarf galaxies and telling us more about the properties of these galaxies."

Marla says,"One implication of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way's cosmic neighborhood. If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence."

Reference:
"The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem" ,
Joshua D. Simon and Marla Geha,
The Astrophysical Journal, v670, p313-331 (2007 November 20), Abstract

[We thank Caltech Media Relations for materials used in this posting]