Sky & Telescope - January 2025 - 13

G
alaxies are found in a wide variety of environments.
Some galaxies, like people, live in rural environments
- the low-density " field " where galaxies have very
few neighbors. Others are more suburban, living in groups
like our own Milky Way Galaxy and its close companions
Andromeda (M31) and Triangulum (M33).
But massive galaxy clusters are the true cities of the universe.
These clusters are home to hundreds or even thousands
of bright galaxies, all packed within a few million light-years
of one another - quite close by astronomical standards.
All types of galaxies swarm inside these clusters: giant
ellipticals, blue star-forming spirals, disky lenticular galaxies,
faint dwarf ellipticals, and dwarf irregular galaxies. They're
joined by strange neighbors, such as ghostly ultra-diffuse
galaxies (UDGs), whose stars are spread so thinly in space
that their light is almost lost in contrast to that of the night
sky, and dense ultra-compact dwarf galaxies (UCDs) - objects
so small and point-like that they are almost indistinguishable
from foreground stars in the Milky Way.
What gives rise to such a wide variety of galaxy types
HERCULES CLUSTER Abell 2151 is a fairly young cluster, relatively
rich in spiral galaxies. It's both less populated and more spread out than
the older Coma Cluster. This color composite by Bob and Janice Fera
combines 38 exposures taken with a 17-inch Cassegrain.
inside clusters? It turns out that life as a cluster galaxy is
hard, and it leaves scars.
As galaxies plunge through the dense environment of the
cluster, they can be rocked by collisions with other galaxies,
swept clean of their star-forming gas, stripped apart by
intense gravitational forces, or swallowed whole by massive
galaxies in the cluster core. Over time, these complex forces
work together to reshape, damage, and even destroy cluster
galaxies. Compared with field and group environments,
where galaxies are predominantly spirals and irregulars, rich
galaxy clusters have a much higher fraction of elliptical and
lenticular galaxies and fewer star-forming spirals. Astronomers
call this the morphology-density relation.
These processes don't only reshape cluster galaxies. Their
effects are also written into the diffuse intracluster starlight
that fills the space between the galaxies. Together, the starlight
and the galaxies can reveal to us a cluster's history.
Groups: Distorted, Frayed, and Disrupted
To understand in detail how the cluster environment shapes
and reshapes the properties of cluster galaxies, we have to
start in the early universe, when galaxies and clusters are
starting to coalesce. Structure in the universe - galaxies,
clusters, and the vast cosmic web they're part of - forms
ANTENNAE NGC 4038 and 4039 are the textbook example of interacting galaxies. Photographer Chuhong Yu created this image by combining
forty-nine, 10-minute frames taken with the 1-meter Ritchey-Chrétien instrument at Chilescope's remote observatory.
sk yand tele scope .o r g * JANUARY 2025 13

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Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025