Sky and Telescope - March 2017 - 17

One of the joys of observing

DIAGR AM: SHAPLE Y, HARLOW. HARVAR D COLLEGE OBSERVATORY BULLE TIN NO. 9 0 8, M A RCH 1938.; IM AG E: ESO

from a high
mountaintop in Chile is the panoramic view of the southern
sky. As your eyes adapt to the darkness, the brightest stars
appear ﬁrst, then the fainter ones. The Milky Way arches
overhead, and eventually you see two faint fuzzy patches
of light near the south celestial pole - the Large and Small
Magellanic Clouds. The LMC and SMC are the two largest
galactic companions of the Milky Way, and the only two that
are visible to the unaided eye.
Over the past century, we have discovered several dozen
dwarf galaxies around the Milky Way. The largest of these
contain several billion stars, modest collections compared to
the roughly 300 billion stars in our own galaxy. At the other
end of the spectrum, smaller dwarfs known as ultra-faint
dwarf galaxies may have only a few hundred ancient stars - a
single O star can outshine the faintest examples. Because
they give off so little light, we have only been able to spot
such dim galaxies within a few million light-years of Earth.
In 2015 astronomers doubled the number of previously
known Milky Way companions using data from new sky
surveys. If all the newfound stellar systems are conﬁrmed as
galaxies, they will bring the total number of known satellites
orbiting the Milky Way to more than 50.
Ultra-faint galaxies are the most common type of galaxy;
though they are hard to ﬁnd, there are potentially hundreds
of them for every giant like the Milky Way. Despite their
unassuming appearance, our tiny galactic neighbors are helping us address big questions about the nature of dark matter,
the universe's ﬁrst stars, and the process of galaxy formation.

The Galaxy-Dark Matter Connection
The close connection between galaxies and dark matter is
one of the most active research topics in astrophysics. In fact,
galaxies owe their very existence to dark matter.
Even if we don't know yet what dark matter is (see box on
page 18), we know a lot about what it does. Dark matter and
"normal" (baryonic) matter attract each other through the
force of gravity, clumping together to form cosmic structures
such as galaxies and clusters of galaxies. A galaxy's stars and
gas are embedded in a dark matter cloud - or halo - that
extends far beyond visible matter. Without this halo's gravitational pull, galaxies would have grown much more slowly,
and the universe as a whole would look vastly different.
One challenge to understanding how galaxies form and
evolve is that astronomers can only observe snapshots of
individual galaxies, each one representing a single moment
in history. By analogy, imagine an alien who visits Earth for
only one day and sees humans of all ages but isn't able to
watch a baby grow into a mature adult. What could the alien
learn about the human species?
For this reason, computer simulations that follow galaxy
assembly over billions of years are a vital tool, complementary to observations. Simulations also help us visualize
invisible dark matter. To test our ideas of galaxy formation,
we create mock universes with different laws of physics and
compare their predictions to what we see in sky surveys.
Such comparisons between theory and data lead to several
important conclusions. First, galaxies have grown in a hierarchical "bottom-up" process. Small protogalaxies of dark

p SCULPTOR GALAXY Harlow Shapley spotted Sculptor (along with its cousin Fornax, on the opposite page), using photographic plates taken at
the Boyden Observatory in South Africa. What was initially presented as a star count diagram (left) is now seen as a full-fledged galaxy thanks to the
Wide-Field Imager at the La Silla Observatory (right). Fornax and Sculptor were the first of the Milky Way's companions to be found using telescopes.
s k y a n d t e l e s c o p e .c o m

* MARCH 2017

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Sky and Telescope - March 2017

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