21 March 2008

Galaxies and Black Holes

According to the January 22, 2005 issue of Science News (subscription only), there is a very strict mass relationship between a galaxy and its central black hole.

[Karl] Gebhardt compare[d] the mass of each black hole with the average velocities of the billion-or-so stars that surround each hole out to a distance of several thousand light-years. This swarm of stars—a major component of a galaxy—is known as the bulge, and the stellar velocities provide a measure of the bulge's mass.

When Gebhardt made the comparison, he was stunned. Regardless of their size, the bulges always turned out to be 500 times as massive as the giant black holes at the hub of their galaxies.

Also notable is that the largest black hole ever observed weighs in right at the theoretical limit for the size of the largest possible black hole.

that no black hole can become heavier than about 3 billion times the mass of the sun. The largest supermassive black hole ever detected, at the center of the elliptical galaxy M87, weighs in at just this value.

This implies that there is a fundamental physical cap on the size of a galaxy. This cap might also explain why galaxy clusters form, rather than simply superlarge unitary galaxies, and in turn why galaxy clusters have quite different dark matter profiles than ordinary galaxies.

Also notable to me, according to the current issue of Science News is that galaxies seem to be fundamentally divided into two types based upon weight (citations omitted):

Astronomers have known since the 1920s that the modern-day universe consists mainly of two galaxy types—young-looking, disk-shaped spirals like the Milky Way, and elderly, football-shaped ellipticals. Ellipticals have a reddish tinge—an indication that they are old and finished forming stars long ago—while spirals have a bluish tinge, a sign of recent star formation.

A few years ago, researchers found that in the universe today, these two populations divide sharply by weight. An analysis of the Sloan Digital Sky Survey, which has recorded about 1 million nearby galaxies of the northern sky, revealed that the "red and dead" ellipticals nearly always tip the scales at masses greater than the Milky Way, while the star-forming spirals fall below that weight. Somehow, star birth was systematically and dramatically quenched in the big guys but proceeded unimpeded in the spiral small-fry.

The puzzle deepened in 2005 when Sandy Faber of the University of California, Santa Cruz, and her colleagues announced that they found the same galactic dichotomy when the universe was 7 billion years old, half its current age.

Admittedly, this isn't too stunning considering that long range astronomical phenomena are driven almost entirely by gravity according to prevailing current theory, and that gravity should be nearly Newtonian at the interstellar distances (dark matter and dark energy complicate this in reasonable well understood ways, although a small minority of scientists think that dark matter and dark energy effects are actually due to flaws in the current equation of gravity). Such simple phenomena should produce reasonable simple outcomes. But, short range stellar phenomena involve nuclear phenomena and relativistic phenomena that complicate the picture a great deal.

These data also highly constrain theory. Any theoretical physical theory that, for example, has the seemingly benign trait of producing central black holes for galaxies that are 200 times the total weight or simply do not have a characteristic galactic-black hole mass relationship, is wrong. Any theory with galaxies bigger than M87 is wrong. Any theory that allows for large spiral galaxies or small elipitical ones, in any meaningful frequency, is wrong.

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