Sky and Telescope - July 2017 - 23
Red galaxies have been synonymous with galaxy clusters
for as long as there have been galaxy clusters. "Ellipticals
seem to be in place in clusters quite early in the history of
the universe," says James Geach (University of Hertfordshire,
UK). "But lenticulars only appear more recently, over the last
ﬁve billion years."
Lenticular galaxies might have evolved from spiral galaxies. Although they're disk-shaped, they have little, if any,
spiral structure. While they've lost most of their gas, they're
obscured by plenty of dust and are rich in old, red stars. They
also seem to congregate mostly in and around galaxy clusters.
Jeffrey Kenney (Yale University) thinks there's a connection. He spends his time researching the relationship
between galaxies and their environments. When the Hubble
Space Telescope imaged the face-on galaxy NGC 4921, which
features ghostly spiral arms, Kenney noticed something odd
on one side of the galaxy's disk. A prominent ridge of gas and
dust, thousands of light-years long, appears to be fraying like
loose threads on a sweater.
NGC 4921 is currently plunging deep into the Coma
Cluster, 310 million light-years distant. Its journey takes it
through the hot intracluster medium, a thick soup of X-rayemitting plasma where temperatures soar to more than 10
million kelvin. This plasma acts like a harsh wind, scouring
the leading edge of the infalling galaxy and stripping gas
into long tails. The frayed ﬁlaments are denser, magnetically
bound clumps of gas that more strongly resist the stripping
pressure as NGC 4921 rams into the intracluster medium.
Right now, this ram-pressure stripping is operating tens of
thousands of light-years from the galaxy's center, but it won't
stop there. "It will eventually eat its way in and probably
remove all the gas from the galaxy," says Kenney. "We see
lots of disk galaxies in the Coma Cluster that have almost no
gas, no star formation. Most of these were likely spiral galax-
p RED & DEAD Both elliptical (left) and lenticular (right) galaxies largely
contain red, aging stars. Although lenticulars are dusty and disk-shaped,
like many spiral galaxies, they're more like ellipticals in terms of their star
formation. Ellipticals are often found in galaxy clusters' centers, while
lenticulars more often turn up in clusters' outer regions.
ies that were completely ram-pressure stripped. [NGC 4921]
is just the one we've caught in the act; for most of them the
action is already over."
Ram pressure's effect depends heavily on the cluster's
mass. The Virgo Cluster, 54 million light-years away, is less
massive than Coma. Since its galaxies orbit at a slower pace
and its intracluster medium is less dense, its ram pressure is
consequently lower. Virgo can completely strip its dwarf galaxies of their star-forming gas, but larger spiral galaxies only
lose gas on the outer edge. "In Coma," Kenney says, by way
of comparison, "almost all spiral galaxies will be completely
stripped on their ﬁrst passage towards the cluster center."
While ram-pressure stripping may produce lenticular galaxies in massive clusters, it cannot satisfactorily explain why
every cluster, big or small, contains an ancient, red elliptical
galaxy at its hub. "It's not entirely clear what quenches star
formation in elliptical galaxies," admits Kenney.
RED & DE A D: ESA / HUBBLE & N ASA; FACING DISSOLU TION: HUBBLE LEG ACY
A RCHIV E / ESA / N ASA
Green Valley Galaxies
º Amidst fertile blue spirals and elderly red ellipticals exist a small group of inbetween galaxies that aren't quite one or the other. These galaxies lie within
the "green valley" on a color-magnitude diagram, which, a bit like a Hertzsprung-Russell diagram, plots galaxies' colors against their luminosities. The
green valley is nestled between the swarm of blue galaxies and the curve rich
with red-and-dead galaxies, implying that the green galaxies represent a brief
"The green valley galaxies are where feedback is happening now," Geach
says. Something is quenching these galaxies, whether it be black hole feedback, stellar outﬂows, or ram-pressure stripping. This stage of a galaxy's life
might be relatively brief, which explains why green galaxies are so rare. "If you
can map out where the green valley galaxies are and what their properties are,
it will help pin down that evolutionary transition," says Geach.
u FACING DISSOLUTION As NGC 4921, a typical "green" galaxy, plunges deep into the
Coma Cluster, it rams into thin, hot gas. Gradually, this ram-pressure stripping will tear
the gas from its star-forming spiral arms, transforming it into a red-and-dead galaxy.
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