quasar beams to understand the composition of
CGMs on average.
“We’ve been like the three blind people grabbing
at the elephant,” says John O’Meara, an observa-tional astronomer at Saint Michael’s College in
Teams using two new spectrographs — KCWI,
the Keck Cosmic Web Imager on the Keck
telescope in Hawaii, and MUSE, the Multi Unit
Spectroscopic Explorer on the Very Large Telescope in Chile — are racing to change that. These
instruments, called integral field spectrographs,
can read spectra across a full galaxy all at once.
Given enough background light, astronomers can
now examine a single galaxy’s entire CGM. Finally,
astronomers have a way to test theories of how gas
circulates into and out of a galaxy.
A Chilean team, led by astronomer Sebastian
Lopez of the University of Chile in Santiago and
colleagues, used MUSE to observe a small dim
galaxy that happens to be sandwiched between a
bright, distant galaxy and a massive galaxy cluster
closer to Earth. The cluster acts as a gravitational
lens, distorting the image of the distant galaxy
into a long bright arc (SN: 3/10/12, p. 4). The light
from that arc filtered through the CGM of the
sandwiched galaxy, which the team called G1, at
56 different points.
Surprisingly, G1’s CGM was lumpy, not smooth
as expected, the team reported in the Feb. 22
Nature. “The assumption has been that that gas
is distributed homogeneously around every
system,” Lopez says. “This is not the case.”
O’Meara is leading a group that is hot on Lopez’s
trail. Last year, while KCWI was being installed,
O’Meara got an hour of observing time and was
able to see hydrogen — which is associated with
cool, star-forming gas — in the CGM of another
galaxy backlit by a bright lensed arc. He’s not
ready to discuss the results in detail yet, but the
team is submitting a paper to Science.
Meanwhile, Peeples’ team is revisiting how
computers render CGMs. “The resolution of the
circumgalactic medium in simulations is, um,
bad,” she says. Existing simulations are good at
matching the visible properties of galaxies — their
stars, the gas between the stars, and the overall
shapes and sizes. But they “utterly fail at reproducing the properties of the circumgalactic
medium,” she says.
So she’s running a new set of simulations called
FOGGIE, which focus on CGMs for the first time.
“We’re finding that it changes everything,” she says:
The shape, star formation history and even the orientation of the galaxy in space look different.
Together, the new observations and simulations suggest that the CGM’s function in the
life cycle of a galaxy has been underestimated.
Theorists like Peeples and observers like O’Meara
are working together to make new predictions
about how the CGM should look. Then the
researchers will check real galaxies to see if they
“Molly will post a really amazing new render of
a simulation on Slack, and I’ll go, ‘Holy crap, that
looks weird!’ ” O’Meara says. “I’ll go scampering
off to find a similar example in the data, and we
get into this positive feedback loop of going ‘Holy
crap! Holy crap!’ ”
While future circumgalactic studies will focus
on gathering spectra from full CGMs, Tumlinson is
hoping to squeeze more information out of Hubble
while he still can. Hubble made CGM studies pos-
sible, but the telescope is 28 years old, and probably
has less than a decade left. Hubble’s spectrograph
is still the best at observing certain atoms in CGMs
to help reveal the gaseous halos’ secrets. “It’s some-
thing we definitely want to do,” he says, “before
Hubble ends up in the ocean.” s
s Jason Tumlinson, Molly S. Peeples and Jessica
K. Werk. “The Circumgalactic Medium.”
Annual Review of Astronomy and Astrophysics.
MUSE makes a mark Light from a source galaxy is deflected and magnified by
an intervening galaxy cluster to form the bright arc seen in the projected image at far
right. Unlike a quasar’s narrow beam of light, the extensive arc lights up a large area of
galaxy G1’s CGM, showing it is surprisingly lumpy.
CGM resolution. In
these renderings of the
same galaxy, the bottom
shows FOGGIE at work.
The galaxy’s shape and
size change dramatically.