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Extragalactic Astronomy

The observable universe is home to more than a hundred billion galaxies of many shapes and sizes. The most distant galaxies are relics of an earlier era, so observing them is a glimpse at history. Smaller galaxies merge to make larger ones, while interactions between galaxies can drive star formation and change the galaxies’ shapes. In addition, many galaxies clump together in groups or galaxy clusters. By studying both individual and populations of galaxies, extragalactic astronomers seek to understand their formation, history, and evolution.

Our Work

Center for Astrophysics | Harvard & Smithsonian extragalactic astronomers study galaxies in a wide variety of ways:

  • Using the Event Horizon Telescope (EHT) to capture an image of the supermassive black hole at the center of the galaxy M87. The EHT consists of multiple observatories spanning the planet, powerful enough to create the first image of the material right at the boundary of a black hole.
    CfA Plays Central Role In Capturing Landmark Black Hole Image

  • Mapping stars, gas, and dust within galactic disks to understand how galaxies grow and make new stars. Since Sun-like stars commonly live in the disks of galaxies, some astronomers think there may be a connection between habitable star systems and location within a galaxy.
    New Insights on How Spiral Galaxies Get Their Arms

  • Looking for the earliest galaxies in the universe. These are very hard to find, since they are far away. However, researchers use the Smithsonian’s Submillimeter Array (SMA) and other observatories to identify hundreds of these faint objects formed in the first few billion years after the Big Bang.
    Galaxies in the Early Universe

  • Describing the interactions between supermassive black holes and their galaxies, using computer simulations. These black holes somehow got very large very quickly early in the history of the universe, based on observations of young galaxies, but astronomers still haven’t figured out why. Theoretical models help explain the origins of these common objects, and the interplay between their formation and that of their host galaxies.
    Dark Matter Guides Growth of Supermassive Black Holes

  • Studying populations of stars within galaxies to understand their history. While individual stars aren’t bright enough to be picked out even in relatively nearby galaxies, astronomers realized they could see the effects of many pulsating stars together. These stars are close to the end of their lives, but their fluctuations provide a way to study the populations of stars in other galaxies.
    Discovery Measures "Heartbeats" of a Distant Galaxy's Stars

  • Observing the dynamics of galaxies in clusters. With thousands of galaxies relatively close together, they inevitably interact with each other, sometimes merging or colliding. In a few cases, astronomers identified small elliptical galaxies that were flung out of their host cluster entirely, which implies an intense three-way gravitational interaction at some point.
    Astronomers Find Runaway Galaxies

  • Studying the hot plasma that fills the space between galaxies, to understand the structure of galaxy clusters. Using NASA’s Chandra X-ray Observatory and other instruments, astronomers map the internal temperature variations of this intercluster gas. This can reveal information about the history of the cluster, as well as the way black holes in the individual galaxies affect the entire cluster.
    Astronomers Discover Powerful Cosmic Double Whammy

A Garden of Galaxies

The cosmos contains galaxies of a wide range of sizes and shapes, from tiny dwarf galaxies to giant ellipticals. They are home to most of the stars in the universe, containing anywhere from millions to trillions of stars, depending on the galaxy size. Some types of galaxies also contain large amounts of gas and dust, which are the raw ingredients for new stars. Whatever size or shape they have, most large galaxies appear to have at least one supermassive black hole, and they are all dominated by invisible dark matter, which helps hold everything together by its gravity.

Astronomers classify galaxy shapes or morphologies broadly along these lines:

  • The spiral and lenticular galaxies have a rotating disk structure containing stars and dust, and a central bulge of stars that varies in size greatly between galaxies. Spiral galaxies are distinct from the lenticulars because of their  “arms” marked out by bright stars and dust, which seems to mean that star formation is less active in the lenticulars. Further distinctions are drawn if the galaxy has “bar”: a lane of stars running through the galactic center. For example, the Milky Way is a barred spiral galaxy. Spirals make up most of the large galaxies in the universe.

  • The elliptical galaxies lack the distinctive disks and bulges, instead appearing as smoother, roughly egg-shaped masses of stars, which follow complex orbits around the galactic center. They have very little gas and dust, and their stars are generally older and redder than the spirals. Many elliptical galaxies are formed through galaxy mergers, particularly the giant elliptical galaxies, which are the biggest in the universe. Often the evidence of these mergers is visible in faint shell-like structures within the galaxies.

  • True to their name, the irregular galaxies don’t have a clear shape. Many of these galaxies, such as the Large and Small Magellanic Clouds that orbit the Milky Way, carry some remnant of structure, but don’t have clear rotation pattern. Like the Magellanic clouds, a number of irregular galaxies are close to other galaxies, which seem to have disrupted their shape.

In addition, many galaxies interact gravitationally with each other, pulling each other out of shape or even colliding. These interacting and merging galaxies often have wild shapes that don’t correspond to the above categories.

The galaxies NGC 2623 in the final stages of their merger

The galaxies NGC 2623 in the final stages of their titanic merger. Astronomers have learned that such galactic collisions produce incredible bursts of new star formation.

Credit: Hubble Legacy Archive, ESA, NASA, APOD; Processing - Martin Pugh


Galaxy Clusters

Many galaxies live inside galaxy clusters, which are the largest structures in the universe held together by their own gravity. Clusters contain hundreds to thousands of galaxies of all sizes, along with an even larger mass of hot plasma and a larger amount still of dark matter. The hot gas makes galaxy clusters bright in X-ray light. Meanwhile the huge dark matter mass bends the path of light to magnify more distant galaxies; this effect is known as gravitational lensing.


Surveying Deep Space

The observable universe contains hundreds of billions of galaxies and perhaps more. The uncertainty comes because many of those are difficult to observe: too small, faint, far away, or combinations of these. However, those we do see provide a wealth of knowledge about the universe, as well as how galaxies grow, merge, and evolve.

  • For spiral galaxies like the Milky Way, the spiral arms are the most striking features when seen in visible light. When observed using other types of light, though, we see those arms are embedded in disks, which also have a lot of stars, gas, and dust. The stars in a galactic disk are typically younger and hotter than those in other parts of the galaxy. Elliptical galaxies, which don’t have spiral arms or disks, are made of mostly older stars as well, suggesting connections between the age of stars and the structure of the galaxy hosting them. Astronomers compare the structure and dynamics of galaxies to see how   some of them galaxies form and maintain spiral arms.

  • Most — maybe all — big galaxies harbor at least one supermassive black hole. In many cases, these black holes pull large amounts of interstellar gas into orbit. The result is a complicated swirl of hot matter, some of which can be channeled into a jet or blown out into the galaxy as “wind”. This material can strongly affect the host galaxy, suppressing star formation and otherwise influence the galactic environment. The interaction of supermassive black holes with their galaxies is an important area of research.

  • Big galaxies are made from the merger of smaller galaxies. That’s a long, slow process, but researchers use observational data of galaxies at various stages of merging along with computer models to understand how it happens.

  • Astronomers study galaxy clusters for similar reasons, since big clusters are formed from smaller ones. In addition, supermassive black holes in cluster galaxies can influence the birth of stars in their neighboring galaxies. Clusters provide a fascinating environment for understanding the interactions of galaxies with each other, and their hot plasma itself is a complex system for study.

  • Galaxies and galaxy clusters are also essential for cosmology: the study of the universe as a whole. Rather than being scattered randomly, galaxies and clusters tend to chain together into a giant cosmic web, which astronomers study to understand the structure of the universe on the largest scales.