Sky & Telescope - January 2025 - 17

HOT BATH: ESA / XMM-NEWTON / SDSS / J. SANDERS ET AL. 2019; WINDSOCK GALAXY: X-RAY: NASA / CXC / UAH / M.
SUN ET AL., OPTICAL: NASA / ESA / HUBBLE HERITAGE TEAM (STSCI / AURA)
individually, their cumulative effect can be significant. With
so many galaxies now in the cluster, the encounters happen
quite often. Over time, these small kicks add up, particularly
for dwarfs. Repeated kicks puff up these galaxies and strip off
many of their stars, making them more and more diffuse as
time goes by.
And it's not just these fast flybys that galaxies have to
contend with. As the cluster continues to grow in mass, the
gravitational pull of the cluster itself becomes stronger and
stronger. Near the cluster core, the pull is fierce, creating
intense gravitational tides that galaxies must navigate as they
pass through the region. The tides strip even more stars out of
the galaxies, gradually whittling galaxies down.
While massive galaxies can withstand a good bit of tidal
stripping and stay intact, the process may completely disrupt
dwarfs, leaving their stars scattered across the cluster. Some
of these low-mass galaxies have cores dense enough to survive
the whittling process. But with their outer stars now stripped
away, the nuclei are left orbiting within the cluster as bare
and compact stellar systems, transformed into ultra-compact
dwarf galaxies.
Stripped Down to the Stars
There's more than just gravity at work transforming galaxies
in these metropolises - the hot, diffuse gas that fills the
cluster also drives strong evolution. As galaxies orbit, they
continually move through this hot intracluster medium. While
the stars in galaxies pass through unimpeded, the galaxies'
star-forming gas is not as fortunate. Like wind filling a sail,
the pressure of the intracluster medium sweeps up the starforming
gas and begins to blow it out of galaxies through a
process called ram-pressure stripping.
In the outskirts, where the density of hot gas is low, this
stripping mostly affects dwarfs and the outer parts of large
galaxies. In the latter case, it leaves the rest of the star-forming
gas intact and available to continue making stars. But for
galaxies plunging through the denser cluster core, the outcome
is dramatic. Here, ram-pressure stripping is so strong
that it can remove a galaxy's gas entirely, completely shutting
down the ability to form new stars. The galaxy's stellar populations,
immune to ram-pressure stripping, remain in place,
leaving the galaxy " red and dead, " its light dominated by
aging red giant stars. This process can transform blue spirals
into red and disky lenticulars, while dwarf irregular galaxies
fade to become red and diffuse.
Meanwhile, deep in the cluster core, even more dangers
lurk. The heart of a cluster is typically home to the largest
uHOT BATH Top: This composite image of X-ray and optical data
shows the glow of hot gas in the Coma Cluster.
uWINDSOCK Bottom: This composite image shows the spiral galaxy
ESO 137-001 plowing through the hot, X-ray-emitting gas (blue) in the
Norma Cluster. Tattered threads of bright blue are the gas torn from the
galaxy itself that have now erupted in star formation. The dark brown is
dust that's also being dragged out.
sk yand tele scope .o r g * JANUARY 2025 17

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Sky & Telescope - January 2025

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