Astronomers often describe Andromeda as a “sister galaxy” to our own Milky Way. It is relatively nearby, similarly sized, and comparably shaped. So what does it mean when we confirm that Andromeda is host to a “black hole bonanza”?

A black hole is born when a massive star collapses, resulting in a high concentration of gravity, so strong that light cannot even escape its pull. By definition, we can’t observe black holes directly, but astronomers can detect them if a close-orbiting star is pulled inside. Gravitational forces compress the star’s material, producing high-energy radiation in the process.

NASA’s Chandra X-ray observatory followed the radiation trail to identify 26 new black hole candidates, the largest number found outside of the Milky Way to date. Follow-up observations by the European Space Agency’s XMM-Newton X-ray observatory gave information useful for determining the nature of these black holes.

The first step in classifying Chandra’s findings: confirm the black hole sizes and locations. The process relies on perspective. In the same way a tall person standing far away can appear the same size as a short person much closer, objects in space can deceive us with their apparent size, so we need to look for additional clues. In the case of black holes, researchers saw bright and fast variability of X-ray emission to determine these 26 black holes are smaller “stellar mass” systems within Andromeda rather than supermassive black holes behind Andromeda.

As it turns out, neutron stars can look a lot like black holes from a distance, so researchers analyzed x-ray brightness and color. Neutron stars also emit x-ray radiation, but a black hole appears brighter—and a different color.

Eight of the black holes reside in globular clusters, concentrations of stars spherically distributed about the center of a galaxy that exist in both the Milky Way and Andromeda. However, astronomers have not yet discovered black holes in any of the Milky Way’s globular clusters.

“When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better,” said co-author Stephen Murray of Johns Hopkins University and the Harvard-Smithsonian Center for Astrophysics (CfA). “In the case of Andromeda, we have a bigger bulge and a bigger supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well.”

Perhaps the two galaxies aren’t as sisterly as we thought. The central bulge of Andromeda is larger, which explains why seven of the new candidates exist within 1,000 light years of Andromeda’s core.

Considering that we can only detect black holes when they are producing high-energy radiation, there must be more that we have not found yet—in both galaxies. Lead author Robin Barnard of CfA states, “While we are excited to find so many black holes in Andromeda, we think it’s just the tip of the iceberg, most black holes won’t have close companions and will be invisible to us.”

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA/JPL-Caltech/UCLA

Earth’s neighborhood just got a little larger.

Astronomers thought that Earth was located on a spur of an arm of the Milky Way… until data from the Very Long Baseline Array telescopes suggested that we could be located closer to the center.

While it’s simple to observe a bird’s-eye view of other galaxies, models of the Milky Way are inaccurate due to Earth’s limited vantage point. We are attempting to measure an entire galaxy using only Earth’s narrow perspective, and at the center of our galaxy is a large bulge, blocking about half of the Milky Way from view.

To make the best model possible, astronomers use a technique called parallax. Measurements are taken from locations on either side of the sun to give multiple perspectives of our location in the sky. Then, astronomers use trigonometry to calculate where we might reside in comparison to distant objects.

At the center of the Milky Way is a supermassive black hole, whose gravitational pull is capable of keeping 200–400 billion stars in orbit around the galaxy. Measurements from NASA’s Spitzer Space Telescope revealed that these stars are oriented in two arms that spiral around the black hole.

“Based on both the distances and the space motions we measured, our Local Arm is not a spur,” said Alberto Sanna, a postdoctoral fellow with the Max-Planck Institute for Radio Astronomy (MPIFR). “It is a major structure, maybe a branch of the Perseus Arm, or possibly an independent arm segment.”

Sanna and his colleagues presented their research this week at the American Astronomical Society meeting, held in Indiana.

Astronomers are creating increasingly accurate models of the Milky Way and every new finding tells us more about the entire universe.

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA

Published in the July 8 edition of the journal Nature, the finding is remarkable considering it was discovered by scientists who were looking at remnants from a supernova explosion.

Microquasars, as these objects are called, are not that uncommon. Universe Today points out that there are a dozen or so in our own Milky Way Galaxy. But the impressive size of this particular bubble, estimated at 1,000 light years across, makes it stand out from the crowd. Most microquasars in our own galaxy are less than 10 light years wide.

According to Universe Today, microquasars are formed by two objects—in this case, the small black hole and a companion star. Energy is produced

by matter falling from one component to the other, and can produce jets of high-speed particles. The fast jets slam into the surrounding interstellar gas, heating it and triggering an expanding bubble made of hot gas and ultra-fast particles colliding at different temperatures.

The jets seen in this microquasar are surprising and may alter the idea of how energy is emitted by a black hole. As New Scientist reports:

These jets are much more powerful than expected for a black hole of this size, blowing bubbles that expand faster than the speed of sound. The finding suggests that more of the energy spent by a black hole goes into accelerating matter– rather than emitting x-rays– than previously supposed.

Data in the form of x-ray emissions were gathered from both the European Southern Observatory and the Chandra X-ray Observatory in the study. Giant bubble-blowing black holes like this microquasar may help scientists better understand differences between black holes of different sizes—including the ones that lurk at the centers of most galaxies like our own.

Black holes in the centers of galaxies have millions or billions of times the mass of the Sun and draw in material at a high rate.

But does that material include dark matter?

About 23% of the Universe is made up of mysterious dark matter, invisible material only detected through its gravitational influence on its surroundings. In the early Universe, small clumps of dark matter seem to have attracted gas, which then coalesced into stars that eventually assembled the galaxies we see today.

Drs. Xavier Hernandez and William Lee of the National Autonomous University of Mexico modeled the way in which black holes absorb the dark matter.  They found that the rate at which this happens is very sensitive to the amount of dark matter found in the black holes’ vicinity.

If the concentration is large enough—more than a critical density of seven times the mass of our sun spread over each cubic light year of space—the black hole mass would increase so rapidly that it would swallow large amounts of dark matter. Soon the entire galaxy would be altered beyond recognition.

Since we don’t observe the galaxies altered in this way, the scientists concluded that the density of dark matter in the centers of galaxies must be a constant, smaller value. Or dark matter may behave in an entirely different way from what scientists expect. Especially around massive, hungry black holes.

Creative Commons image by Alain r

Scientists thought that the gamma-rays outside our galaxy were jets emitted from the large black-holes found in the center of other, distant galaxies. But, data gathered with the Fermi telescope has indicated that this is wrong… Well, not entirely wrong, but about 70 percent wrong.

“Active galaxies can explain less than 30 percent of the extragalactic gamma-ray background Fermi sees,” said Marco Ajello, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford. “That leaves a lot of room for scientific discovery as we puzzle out what else may be responsible.”

Ajello and the Fermi team analyzed data acquired by Fermi’s Large Area Telescope during the observatory’s initial year in space. The first challenge was eliminating emissions from our own galaxy.

“The extragalactic background is very faint, and it’s easily confused with the bright emission from the Milky Way,” said Markus Ackermann, another member of the Fermi team who led the measurement study. “We have done a very careful job in separating the two components to determine the background’s absolute level.”

These measurements, published online yesterday in the journal Physical Review Letters, demonstrate that active galaxies turn out to be only minor players in the gamma-ray sky.

What else may contribute to the extragalactic gamma-ray background? Particle acceleration in star-forming galaxies and merging galaxies, perhaps. Also, the ever-mysterious dark matter could be a source. According to Ajello, “Dark matter may be a type of as-yet-unknown subatomic particle. If that’s true, dark matter particles may interact with each other in a way that produces gamma rays.”

Improved analysis and continued observations will enable scientists to address these potential contributions. Meanwhile, Fermi will stay on the job, looking for more surprises in the gamma-ray sky.