Galaxy collisions and ordinary dark matter

Abstract

Bertram Schwarzschild's Search and Discovery item (Physics Today, November 2006, page 21) brings back vivid memories from the early 1950s, when I was a junior faculty member at Yerkes Observatory. Such notable astrophysicists as Subrahmanyan Chandrasekhar, Gerard Kuiper, William Morgan, and Gerhard Herzberg at Yerkes and Lyman Spitzer and Martin Schwarzschild at Princeton University had ongoing discussions about the dynamics of galaxy collisions. Those notables generally agreed that only the gas in each galaxy collides and is combined and left behind. All condensed matter, stars, and failed stars down to gram-sized debris proceeded without significant collisions, affected only by the gravitational field of the galaxies and perhaps of some individual stars.Kuiper was adamant that most of the baryonic matter in the nascent galaxies never condensed into stars, remaining invisible but discrete entities. He pointed out that the optical cross section per gram of matter drops rapidly with increasing size, thus the effect of this dark matter on optical extinction would be minimal. Because of this dependence, he stated his opinion that most of the baryonic matter in the universe would be invisible to detection except by gravitational effects.Herzberg agreed that dark matter would have no spectroscopic signature in the observable spectral regions. Spitzer added that this dark matter would be heated by the stellar radiation field to a temperature of a few degrees kelvin, a function of the stellar radiation intensity distribution in each galaxy and the albedo of the matter. At that time there was no prospect on the horizon that detection of dark matter at 3–5 K would become possible.In the 1950s model that arose from the discussions, there was no need for exotic dark matter. That simple model of a collision between galaxies and of the role ordinary dark matter plays in such an event deserves to be restated, since it seems to have become eclipsed when exotic dark matter took center stage.© 2007 American Institute of Physics.