With all the splendor and beauty of the galaxies around us, it is easy to forget that such "normal" matter only makes up about fifteen percent of the matter in the universe.
The rest is a form of matter that does not interact with light at all, conveniently called "dark matter." Even though we cannot see it directly, it has helped to shape the universe into what we see today, and we can trace it using what we can see.
The most successful model of the universe's structure to date has dark matter forming "halos," or spherical constructs, coalescing at the junctions of a vast cosmic web, shown in blue above.
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In many of these halos, "normal" matter would clump together as well, forming the stars and galaxies with which we are so familiar. However, it has long been thought that some halos are too small to support star formation, and are thus barren of tell-tale stars and galaxies. How small is too small?
Astronomers using the ESA's Herschel Space Observatory came up with a clever way to answer that question. Herschel scans the sky from orbit in the far-infrared and sub-millimeter wavelengths, those longer than what our eyes can see. They looked at the most prolific star-forming galaxies at the most active time in the universe's history.
These starburst galaxies are aptly named, as they can form on average hundreds or thousands of stars per year, whereas our Milky Way puts out just about one per year. These galaxies are often dusty, so that the starlight is reprocessed by the dust grains and shines out with infrared light.
These galaxies are also at high redshift, falling smack into Herschel's sweet spot, and appearing as they did just a few billion years after the Big Bang, when the universe was booming with star formation.
There are SO many of these galaxies that it is difficult to distinguish one from the other in an image. However, these astronomers looked at the power spectrum of the signal from all of them across two fields of view in the sky.
A power spectrum measures the clumpiness of a signal in space, or, it tells you how large and small your typical sources are, in a statistical way. (For some reason, the spinning spheres on the Wikipedia page for spherical harmonics always helped me visualize this, if you want to get a deeper understanding.)
The bottom line for these dark matter halos is that they need to be at least 30 billion times the mass of the sun to start creating what will become a galaxy. A large number, though smaller than our own Galaxy when you include the normal and dark matter. (As galaxies go, we're pretty beefy.) This helps tune our models of galaxy formation of evolution, but what I would personally like to know is... where are all the tiny, "empty" dark matter halos? We'll need to find a way to detect them one day for the current cosmological picture to be solidified.
Image: Left – A model of the cosmic web of dark matter. Center – A simplification of that model, showing just the dark matter halos. Right – Yellow indicates the most massive dark matter halos, or those that can create stars, as determined by this work. Credit: The Virgo Consortium/Alexandre Amblard/ESA This work has been published in Nature, and a preprint is available on arxiv.org.