Fingerprinting the Circumgalactic Medium

A new study finds the space surrounding dwarf galaxies in the nearby universe to be shockingly pristine.

NGC 5477, one of the dwarf galaxies circling Messier 101 in the constellation of Ursa Major.  ESA / Hubble / NASA

NGC 5477, one of the dwarf galaxies circling Messier 101 in the constellation of Ursa Major.
ESA / Hubble / NASA

The vacuum of space is not, technically speaking, a vacuum. Both interstellar and intergalactic space are far from empty. They’re awash with neutral and charged particles, dust, gas, and magnetic fields.

Astronomers expect the intergalactic medium surrounding galaxies to be further enriched with gas and quantities of heavy elements. Strong galactic winds generated by supernova explosions can easily push material out of small galaxies into the circumgalactic medium, tainting space with the heavier elements generated in stellar fusion. Observations suggest that, even when the universe was only 15% of its current age, the intergalactic medium was already polluted with heavy elements.

How did galaxies enrich the intergalactic medium so rapidly? Astronomers have long speculated that dwarf galaxies — galaxies each totaling a mass less than 10% of the Milky Way's 200 to 400 billion stars — are the culprit.

Dwarf galaxies, while considerably less spectacular than the massive spiral galaxies we see in so many Hubble images, are far more abundant throughout the universe. And being less massive, these galaxies have weaker gravitational holds on the matter inside, meaning that it’s far easier to push gas out of them to large distances and therefore pollute the intergalactic medium.

But now, two astronomers from the University of Chicago have tested this theory for the first time. And the results defy expectations.

Cameron Jia Liang and Hsiao-Wen Chen searched public data archives to find nearby dwarf galaxies paired with faraway quasars. Despite the vast distance separating the two objects, their apparent separation on the sky is small because, from Earth's perspective, the distant quasar happens to lie behind the intergalactic medium surrounding the nearby dwarf galaxy. Like distant beacons blazing through the vast cosmic haze, quasars reveal the otherwise invisible matter surrounding the nearby galaxies.

The team found 111 lucky matches in their hunt for dwarf-quasar pairs.

“Because of the large sample, we are able to place unprecedented limits on the possible presence of heavy elements in the low-redshift intergalactic medium,” says coauthor Chen. But they do not see the expected massive outflows of heavy elements. In fact, metals really only get about half way to the edge of the halo, marking a crucial boundary.

The team analyzed the covering fractions — how much light is absorbed at specific wavelengths and therefore the abundances of particular elements — visible in the quasar’s spectrum for every dwarf-quasar pair. On average the team finds that within this boundary neutral hydrogen gas (HI) has a covering fraction of 94%. It’s extremely abundant. Singly ionized carbon (C II) has a covering fraction of 20% and triply ionized carbon (C IV) 67%. The first two ions of silicon come in at 10% and 45%.

Beyond this boundary all covering fractions drop significantly. “Heavy elements are simply not observed at distances far from these dwarf galaxies,” says Chen. “The intergalactic medium appears to be ‘chemically pristine’ with lots of hydrogen.”

The result implies that the intergalactic medium not far from dwarf galaxies is extremely metal poor, defying expectations. It appears that outflows are not energetic enough to escape the weak gravitational holds of these dwarf galaxies.

This work “has profound implications for the physical processes giving rise to the circumgalactic medium around these systems,” says Jean-René Gauthier (Caltech), whose PhD thesis focused on the complex interplay between the intergalactic medium and galaxy formation.

One such implication is that the heavy elements seen in the early universe may not reside in the circumgalactic medium at all. Instead they may be signposts of faint star-forming regions that have yet to be detected.

However, before astronomers head back to the drawing board, the team notes that heavy elements may still reside in hotter phases that their study did not probe. In a hot tenuous plasma (even in one as low-density as the intergalactic medium) we expect many elements to be stripped of their electrons. The team’s work did not probe these high-ionization species.

“It will be critical to push the study to higher ionization states to reach the final conclusion,” says Chen.

Cameron Jia Liang, Hsiao-Wen Chen “Mining Circumgalactic Baryons in the Low-Redshift Universe” Monthly Notices of the Royal Astronomical Society, 2014

Astronomy News, Milky Way
Shannon Hall

About Shannon Hall

Shannon Hall, a freelance science journalist, has two bachelors in physics-astronomy and philosophy and two masters in astronomy and science journalism. Eight years of higher education explains why she's hooked on the smell of freshly ground coffee almost as much as the wonders of the universe.

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