Monthly Archives: February 2009

astronomy

Summon the Galaxies…

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…and Their Black Holes will Follow

NGC 4261: a giant elliptical galaxy with a central supermassive black hole. Courtesy Sloan Digital Sky Survey/WikiSky.
NGC 4261: a giant elliptical galaxy with a central supermassive black hole. Courtesy Sloan Digital Sky Survey/WikiSky.

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!

astronomy, black holes

Mass Holes

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Where Do They Come From?

A simulated view of a 10-solar-mass black hole 600 miles (900 km) away from the observer -- and against the plane of the Milky Way Galaxy. (Courtesy Wikimedia)
A simulated view of a 10-solar-mass black hole 600 miles (900 km) away from the observer -- and against the plane of the Milky Way Galaxy. (Courtesy Ute Kraus, Physics education group Kraus, Universität Hildesheim, Space Time Travel, (background image of the milky way: Axel Mellinger) via Wikimedia at http://commons.wikimedia.org/wiki/File:Black_Hole_Milkyway.jpg; click to biggify.)

No, today’s entry is not about automobile drivers in Massachusetts. It’s about black holes — those ubiquitous conglomerations of huge amounts of mass — that have such strong gravitational influences that nothing (not even light) can escape their grasp. These are truly mass holes — or think of them as “mass sinks”, where mass (stars, gas, dust, etc.) is deposited and can never be retrieved.

Black holes have been around as theoretical constructs (i.e. an idea in somebody’s head) since at least the 18th century. As actual objects, however, they’ve been around probably since the beginning of the universe and they come in various flavors (or types, and if you want a more rigorous discussion of the physics behind a black hole, go here or here).

So, where do these mass holes come from?  How do they form?

The ones we’re most familiar with are those that form when a supermassive star collapses in on itself (in a supernova explosion) or when a pair of massive stars (a massive binary) somehow manage to merge together.  In either case, the matter in the stars is so dense and there’s so much of it that not even the individual atoms and neutrons in the star’s core can withstand the pressure to keep collapsing.  When it does, a new black hole is born — and becomes what astronomers call a “stellar mass” black hole.

The motions of stars around the black hole at the center of the Milky Way Galaxy. This is a time-lapse movie in infrared light, courtesy Astronomy Picture of the Day.

The other astronomically interesting types of black holes are the supermassive ones that lie at the hearts of galaxies and gobble up stars and clouds of gas and dust. In very active galaxies — that is, the ones with jets shooting out from their hearts — the black holes are incredibly massive, often containing the equivalent of the mass of millions or billions of stars.

Our own galaxy has at least one black hole at its heart, and although it doesn’t shoot out a jet, it does eat up material and it does influence the motions of stars in its nearby neighborhood.

The big mass holes at the hearts of galaxies probably formed when the galaxies they live in were built through the collisions of two or more older galaxies that already had central black holes. As time went by, those black holes just continued to eat up more and more stars, growing ever larger.

There is a class of black holes that probably exist called intermediate-mass. They most likely form when smaller black holes (those that are several times the mass of a stellar black hole) collide with each other.  And, at the other end of the black hole spectrum, we have the micro black holes (sometimes called “mini black holes” if you want to be cute about it).

We haven’t seen any of these forming,yet. But they could have blipping in and out of existence during the very earliest epochs of the formation of the universe, and if they have formed, the gamma-ray radiation from their evaporation could be detectable. It’s also possible that (if it gets working again) the Large Hadron Collider could create some short-lived mini black holes during its experiments. There’s nothing to worry about, though. They wouldn’t last long enough to do damage.

No matter how they form, black holes are among the most interesting creatures in the cosmic zoo of objects that astronomers study.  And, as long as galaxies keep making massive stars and/or colliding with each other, astronomers will have plenty of them to study “in the wild.”