One of the challenges of ground based astronomy is that Earth’s atmosphere is between us and the stars. Our atmosphere not only leads to the twinkling effect we see with stars, but it also absorbs many of the wavelengths of light we’d like to observe. One solution is to put telescopes in space. Another is to locate them at extremely high altitudes. But for some wavelengths such as infrared a high plateau isn’t high enough, but launching a telescope into space is excessively expensive. A compromise is to put a telescope on an airplane.

SOFIA, or the Stratospheric Observatory for Infrared Astronomy, is a modified 747 with a 2.5 meter infrared telescope. It operates at an altitude of 12,500 meters (41,000 feet). At that altitude the atmosphere absorbs only about 15% of infrared light. So SOFIA is able to get infrared images that simply aren’t possible from the ground.

The main mission of SOFIA is to look at things like interstellar gas and the atmospheres of planets, but what’s interesting about the project is how it is strongly tied to educational and outreach efforts. Integrated into SOFIA’s 20-year mission is an Airborne Astronomy Ambassadors Program, which involves K-12 teachers in scientific research.

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Euphrosyne is the 5th most massive asteroid in the solar system. It has the highest density of any asteroid, so it’s only the 12th largest in terms of diameter. Despite its size, however, we actually don’t know that much about Euphrosyne.

One of the reasons for this is the fact that Euphrosyne is quite dark. It’s about the color of asphalt, which makes it difficult to observe in visible light. But like many objects, the asteroid is much brighter in the infrared. That’s because objects give off heat that can be seen in the infrared. That’s what makes the NEOWISE spacecraft so useful. It scans the sky at infrared wavelengths, so it’s able to see dark objects like Euphrosyne.

From the NEOWISE data a team was able to locate and track about 1,400 smaller asteroids that follow a similar orbit to Euphrosyne. From their orbits and characteristics, these are part of the same asteroid family, and likely originated from a large impact with Euphrosyne. Because of the asteroid’s unique orbit (being rather inclined relative to other asteroids) it is easier to trace the orbits of these asteroids back to the original collision.

Studying a family of asteroids such as this is important, because gravitational interactions with planets can cause the orbits of some of the smaller asteroids to shift so that they cross the orbit of Earth. Such “near Earth objects” or NEOs could pose an impact threat to our planet. By studying Euphrosyne and its family, we can get a better understanding of the orbital dynamics that can make an asteroid a potential threat.

There’s still a great deal to learn about Euphrosyne, but with infrared observations we’re making progress.

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Although we think of deep space as being dark, that isn’t entirely true. The universe is filled with a background glow of radiation. The most famous is the cosmic microwave background, which is the remnant glow of the big bang. There is also the x-ray background, caused by things like active galactic nuclei, the radio background.  This week new research on the infrared background has been published in Science, and the results are somewhat surprising.

One of the challenges of measuring the infrared background is that lots of things in the universe emit infrared light. Stars, dust clouds, even distant galaxies all emit light in the infrared. To determine the infrared background the team had to remove all the known infrared sources such as local stars and galaxies. When they did this, they found the remaining background was brighter than expected.

One possible source for this extra light would be distant light from the earliest stars and galaxies. Because of their tremendous distance, their light is redshifted into infrared wavelengths. An excess of infrared background light would then indicate that there were more early galaxies than previously observed.

Spectrum of the observed infrared background compared with the galaxy model. Credit: NASA/JPL-Caltech

But in this case the team noticed that the infrared background did something rather strange. Rather than getting dimmer as you move toward visible wavelengths, as you would expect with distant galaxies, the background got brighter. So it can’t be an effect of the first galaxies.

It’s not clear what the cause of this brightening is, but one idea is that it’s due to rogue stars in intergalactic space. While most stars exist within galaxies, some can be flung out through gravitational interactions with other stars, a process known as evaporation. These rogue stars are too dim to be seen individually, but their combined light would affect the overall infrared background.

If that’s the case, then there are far more rogue stars than we thought. A lot of stars to be all alone in the night.

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Usually in astronomy we study objects by the amount of light they emit.  Most regular matter gives of light in some form or another.  Even the cold interstellar medium will emit some light at infrared or radio wavelengths. But one downside of this is that the light generally comes from the surface regions of an object.  To study the interior of an object we generally have to use aspects of emitted light from the surface to determine properties of the interior.  For bright objects like stars this works pretty well, but for dim objects like dark interstellar clouds this is more of a challenge.  

Recently a team with the Spitzer space telescope has used a different method.  Spitzer is a sensitive infrared telescope, and the team has been using it to observe cold, dense interstellar clouds by the infrared light that passes through them.  There is a great deal of infrared light in the universe, and when that ambient background light passes through a dark cloud we can determine things like its density and composition from the light they absorb.  Basically it is a way to study dark clouds by the shadows they cast.

Some of the team’s results were recently published in the Astrophysical Journal Letters.  One of the things they’ve announced is the darkest and densest interstellar cloud ever discovered.  It has a mass of about 70,000 Suns, and is only 50 light years across.  This cloud is probably in the earliest stages of collapsing into a cluster of large and bright stars (O-type stars).  Gaining a better understanding of dense clouds like this one will help us understand just how such large stars form.

Paper: Michael J. Butler et al. The Darkest Shadows: Deep Mid-infrared Extinction Mapping of a Massive Protocluster. ApJ 782 L30. (2014) doi:10.1088/2041-8205/782/2/L30

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In the center of most galaxies (including our own) is a supermassive black hole.  These black holes can have masses of hundreds of millions of Suns.  Some are more than a billion solar masses.  Active supermassive black holes can be extraordinarily bright.  When active, these black holes are surrounded by an accretion disk, which generates tremendous heat.  Matter streams from their polar regions, creating huge jets of material that races away at nearly the speed of light.  
How that energy is seen depends on how the galaxy (and hence the black hole) is oriented relative to us.  If we view the galaxy edge on, then we mainly see the jets streaming outward, which produces intense radio energy, and we see them as radio galaxies.  If the galaxy is tilted a bit toward us then we can see some of the accretion disk, which is so hot it gives off x-rays.  These then appear to us as quasars.  If our view is right above the pole of the black hole, then a jet is pointed in our direction and we see it as a blazar.

But this assumes we can actually have a clear view of things.  Some galaxies are incredibly dusty, which means our view of the black hole and its accretion disk is obscured.  Even if the black hole is active it would be hard to see it through all the dust of the galaxy.  This is where infrared astronomy comes in handy.

Dust obscures shorter wavelengths of light, such as visible light and x-rays, but it doesn’t obscure longer wavelengths like infrared.  When an active black hole is in a dusty galaxy, the energy it produces heats the surrounding dust, causing the dust to radiate in the infrared.  As a result, the galaxies are somewhat hot, which is why they are known as hot Dust Obscured Galaxies, or hot DOGs (who said astronomers can’t have a sense of humor).  These galaxies are not seen in the visible spectrum, but are very bright in the infrared.

You can see this in the image above, which shows a small region of sky surveyed by the WISE space telescope.  The circles indicate where these hidden black holes have been detected.  The images on the right show a close up of the center circle at different infrared wavelengths, going shorter to longer from top to bottom.  You can see that even in shorter infrared the galaxy is not very visible, but moving to longer wavelengths the galaxy soon appears quite bright.

When WISE completed its full sky survey, about 1.6 million “hidden” black holes were discovered.  Some of these are billions of light years away, which will help give us a better understanding of how these supermassive black holes evolve within galaxies.

Hot dog!  More data!

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Asteroids come in a range of sizes, from hundreds of kilometers in diameter down to a meter wide and smaller.  Determining just how many asteroids there are is a challenge, because the smaller an asteroid’s size, the more difficult it is to observe.  Additionally, smaller asteroids are far more numerous than larger ones.

The size distribution of asteroids roughly follows a power law distribution.  For every 1 kilometer asteroid, there might be a hundred 100-meter ones, ten thousand 10-meter ones, and a million 1-meter ones.  Of course this relationship isn’t exact, and variations in this distribution can significantly change the numbers, particularly for smaller asteroids.  To really get a handle on the size distribution of asteroids you need to make a large survey of mid to small-sized asteroids.

For smaller asteroids, the only way to efficiently determine their size is to estimate it from their brightness.  The idea is that larger asteroids reflect more light, therefore would appear brighter.  The problem with this trick is that some asteroids reflect much of sunlight that strikes it (have a high albedo) while others don’t reflect much light at all (low albedo).  If you imagine looking at a snowball versus a charcoal briquette, you can imagine the difference.  Of course there’s also every range in between as well.  The brightness of an asteroid depends on its albedo and its size.  So a small, high albedo asteroid can be as bright as a large, low albedo one.

So you do you determine their size by brightness, if it also depends on albedo?  You make observations in the infrared instead of visible.  Asteroids don’t reflect all the sunlight that strikes them.  Some of that light is absorbed, which warms the asteroid a bit.  The asteroid radiates this heat as infrared light.  Just how much it radiates depends on its temperature a bit, but depends on its size even more.  A larger asteroid has more surface area, and therefore gives off more infrared light.  So in the infrared, the brighter asteroids are larger, and the dimmer asteroids are smaller.  You can see this effect in the image below.

Credit: NASA/JPL-Caltech

Given that our atmosphere absorbs much of the infrared light, as I discussed yesterday, an survey of asteroids really needs to be done from space.  Sending a satellite up just to survey asteroids is cost prohibitive, but fortunately satellite data can be used for multiple purposes.

In 2009, NASA launched the Wide-field Infrared Survey Explorer (WISE), with a mission to make a complete infrared survey of the sky.  Since it observes the sky in infrared, it can see small dim stars, galaxy clusters, black holes, and a range of other interesting things.  It also allows us to search the entire sky for smaller asteroids.

What we found was that there are actually less small asteroids than we originally thought.  From earlier observations of larger asteroids it was estimated that there were about 35,000 mid-sized asteroids (100 meters – 1 kilometer in size) in our solar system.  What we found was there are less than 20,000.  So the asteroid belt is considerably less crowded than we had thought.

Of course we still don’t know that much about the number even smaller asteroids.  We can use the power law model to estimate their numbers, but more data is needed to be sure.  Some of that information could come from NASA’s capture of a 25-meter asteroid, if the rumors are true.  But that’s a whole other story.

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