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send material raining into the center of the bulge where it could be used to form and grow a supermassive
black hole. In contrast, the life story of low-mass galaxies is more sedate. Originating in regions of lower
density, they were only slowly supplied with gas, formed their stars gradually over the history of the
universe, suffered fewer major mergers, and retained their disk-like form to the present day. These
different life stories explain the strong dichotomy in the observed properties of the high and low mass
galaxies.
Internal processes in galaxies are complex and affect their ability to make new stars. Supernovae
from the explosive deaths of short-lived massive stars violently heat the surrounding gas (see Box 2-3). If
the rate of such supernova explosions is high enough, they can act together to expel much of the galaxy’s
gas supply (Figure 2-4-4). This will have a more severe impact on low-mass galaxies: their gravity is so
weak that material can be easily ejected from them. This may explain why dark matter halos with low
mass contain so few stars and so little gas today. The role played by the supermassive black hole is
instead important for the lives of the most massive galaxies (which contain the most massive black holes).
The energy released by the black hole during periods of intense eruptions can prevent new gas from being
captured by the galaxy, explaining why the most massive galaxies are no longer forming stars.
Understanding the details of galaxies and their interstellar gas, dust, and stars requires a
community of astronomers to study stellar populations, dynamics of galaxies and clusters, interstellar and
intergalactic gas, stars with a range of properties such as high and low metallicities, stellar streams
resulting from tidal interactions of galaxies, and studies of the wide range of galaxies around us, from the
smallest dwarf galaxies to the largest spirals and ellipticals. From the analysis of stellar populations, we
can study how the Milky Way assembled.
While we have a rather good description of the properties of galaxies in the present-day universe,
we have far less information about how these properties have changed over the 13.7 billion year history of
the universe. The galaxies we can observe in detail teach us of the complex interplay among the
components of normal and dark matter, constrained by the physical laws of the cosmos. A high priority
in the coming decade will be to undertake large and detailed surveys of galaxies as they evolve across the
wide interval of cosmic time⎯to have a movie of the lives of galaxies rather than a snapshot. See Box 2-
4.
As described above, the lives of galaxies and the supermassive black holes at their centers seem
to be inextricably linked. Two of the major goals of the coming decade are to understand the cosmic
evolution of black hole ecosystems⎯the intense interplay between the black holes and their
environments⎯and to figure out how these extremely powerful “engines” function. Black hole masses
will be measured by JWST and ground-based optical and radio telescopes. Observations of black holes in
the X-ray and gamma-ray regime offer uniquely powerful insights. For example, the Fermi Gamma-ray
Space Telescope as well as the ground-based atmospheric Čerenkov telescopes such as VERITAS are
constantly reporting new and powerful variations of emission, in both the energy and time domains, from
large numbers of these systems over the whole sky and from cosmological distances. The Chandra and
XMM-Newton X-ray observatories are being used to measure the environmental impact of energy
injection from the black hole and to also give us a glimpse of matter as it swirls inexorably inward toward
the event horizon at the very edge of the black hole. Future more powerful X-ray observatories will
provide detailed maps of these processes, so that we can directly witness the accretion of matter (by
which black holes grow) and can also understand the impact they have on the lives of their “host” galaxy.
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