…and Their Black Holes will Follow

In the last entry I talked about black holes and where some of them come from.  In particular, we took a quick look at the origins of supermassive black holes at the hearts of some galaxies.  For a long time astronomers have talked about how some galaxies, such as the giant elliptical NGC 4621 (which has a supermassive black hole hidden in its stormy core) end up with such behemoth black holes.

Galaxies don’t wheel through the universe alone — they travel in clusters. Eventually, even in a universe as large and limitless as ours is, galaxies come together, summoned by gravity and the expansion of the universe. Given enough time and collisions, some galaxies can grow very large.  And when that happens, everything in the two galaxies gets meshed together, including the black holes.

Two scientists at the University of Texas and the Max-Planck-Institute for Extraterrestrial Physics have been studying galaxies like NGC 4622 using HST and several other instruments. The pair — John Kormendy and Ralf Bender — have been teasing out the evidence that points to the births of ever-larger black holes through mergers. Their conclusion: that giant elliptical galaxies and their massive black holes evolved together through many galaxy mergers.

When galaxies collide, their black holes end up revolving around each other.  But this isn’t just any two big things orbiting in lockstep. These are HUGELY massive objects with their own magnetic fields and gravitational effects, each whipping around the other. And they’re not doing this in some forgotten corner of the galaxy.  They’re sinking to the core of the newly merged galaxy remnant.

These big boys do their thing in the middle of what is essentially a crowded metropolis and they don’t much care about bystanders as they do their battle dance around each other.  Their actions are violent!  They stir up the galaxy center with their incredibly strong gravity, and fling stars out from the core.  As the black hole pair sinks to the center of the new galaxy merger remnant, they end up tossing many of the stars from the core.  And, when stars get tossed out, there’s less star light to be measured at the core.  Rather than grabbing nearby stars and holding them tight in the heat of battle, the black holes are having the opposite effect.

How did Kormendy and Bender think to make a correlation between big black holes and the loss of stars from galaxy cores? They (and other astronomers) had long noted that the biggest galaxies seem to have fluffy, low-density centers. They expected to see many stars clustered around the central supermassive black holes. So, they measured the resulting dimming of galaxy cores, tracking their so-called “light deficits.”

Kormendy and Bender studied 11 massive galaxies in the Virgo Cluster, using the wide field of view of the Prime Focus Camera on McDonald Observatory’s 0.8-meter Telescope, Hubble Space Telescope, and data from many other telescopes to connect the  Hubble data about the cores of galaxies with the outer data from galaxy observations made by the McDonald telescope. Their precision measurements of the brightnesses — that is, the number of stars — at various distances from the centers of elliptical galaxies allowed them to calculate the masses of stars that seem to be  “missing” from the centers of the biggest elliptical galaxies.

When they did that, they got a surprise: the missing mass increases right along with the measured mass of the galaxy’s central black hole. Now, astronomers knew that the two factors were related, but they didn’t know until now that the relationship was so tight as to be a perfect fit.

The missing mass also increases in lockstep with another galaxy property that is known to be tied directly to black holes, namely the speeds at which stars move far out to regions of the galaxy where they don’t “feel” the black hole’s gravity.  It’s all tied together.

These are the kinds of details that help astronomers track the formation and evolution of galaxies — and their black holes. And, since black holes power quasars (the active galactic nuclei that shine out brightly across huge distances of space), they may well be on the track to making the study of quasars and the studies of galaxies to be two pieces of an incredibly complex subject.  Stay tuned!