LONE RANGER Hubble images of SECCO 1 show a smattering of stars, but the much more abundant clumps of hydrogen gas are scarcely visible (right). When viewed based on data from the MUSE spectrograph in Chile, the gas is visible (left).
M. Beccari et al/Monthly Notices of the Royal Astronomical Society 2017
A pair of dark loners wander a distant cluster of galaxies. The two small gas clouds have been roaming the Virgo cluster, some 55 million light-years away, for at least a billion years. Such small, isolated clouds of gas shouldn’t be able to form stars on their own — and yet they are doing just that.
Astronomer Michele Bellazzini of the Italian National Institute for Astrophysics in Bologna and his colleagues found the small, dim clouds in 2014 in the SECCO survey, which looks for the building blocks of galaxies. The two are moving at the same speed and have the same chemical composition, so the researchers think they have the same origin story.
Together, the clouds, called SECCO 1, have just 160,000 solar masses worth of stars, but 20 million solar masses of hydrogen gas—a lot more hydrogen than found in other small starry bodies. Dwarf galaxies typically have 10 times more hydrogen than stars; SECCO 1 has more than 100 times more. And the duo is abnormally isolated: the nearest potential parent galaxies are about 975 light-years away. “This is a novelty,” Bellazzini says.
Simulations suggest SECCO 1 was stripped from a trio of interacting dwarf galaxies. Weirdly, it started forming stars long after it wandered away, which researchers didn’t think was possible. Its latest bout of star formation started only 4 million years ago. How did the tiny clouds compress enough gas to form stars?
Bellazzini thinks the key could be the clouds’ home within the Virgo cluster. Hot gas there could surround the clouds and compress them enough to make them light up.