My first Magazine Sky & Telescope - 04.2019 | Page 21

p JELLYFISH GALAXY This composite X-ray (blue haze) and optical image reveals the hot gas trailing off the spiral galaxy ESO 137-001 as it plows
through the Norma Cluster. Bluish newborn stars bedazzle gas tendrils near the disk.
21-centimeter-long radio waves. But in 2014, Fox’s team
detected large quantities of a different type of gas, gas that
is ionized. In such gas, the hydrogen atoms have lost their
electrons. This gas emits no radio waves but does absorb
ultraviolet light from background quasars. Surprisingly,
there’s three times more ionized gas than neutral. Altogether
the Magellanic gas adds up to some 3 billion solar masses.
The amount could even be twice that if, as is likely, much of
the Magellanic Stream is more distant than the two galaxies:
The farther the gas, the more there must be to explain the
strength of the radio waves we detect.
“That helps us out enormously,” says F. Jay Lockman (Green
Bank Observatory). “It’s got a huge amount of gas. That has got
to fall down eventually. I mean, we’ll get that gas.”
Fox himself is more cautious, though, because in order to
help the Milky Way, that gas must fall into the disk. But the
galaxy’s halo harbors million-degree gas that, though tenu-
ous, could evaporate the infalling gas before it gets here, just
as hot dry air over a desert can cause rain to vanish before
reaching the ground.
The hot halo gas itself represents yet another potential
source of fuel. Because the halo is vast, spanning more than
a million light-years, all that hot, diffuse gas adds up. One
recent estimate put the total at 25 billion solar masses, more
than twice what’s in the disk. Other giant spirals also have
hot gaseous halos, which astronomers detect because the gas
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