Galaxies cover



The keen eye of hubble has revealed intricate details of the shapes, structures and histories of galaxies — whether alone, as part of small groups or within vast clusters. From supermassive black holes at galactic centers to giant bursts of star formation to titanic collisions between galaxies, these discoveries allow astronomers to probe the current properties of galaxies as well as examine how they formed and developed.

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The keen eye of Hubble has revealed intricate details of the shapes, structures and histories of galaxies — whether alone, as part of small groups or within vast clusters.

From supermassive black holes at galactic centers to giant bursts of star formation to titanic collisions between galaxies, these discoveries allow astronomers to probe the current properties of galaxies as well as examine how they formed and developed.

The Space Between Galaxies

A galaxy is a collection of stars, gas, dust, and likely a supermassive black hole in its center, all held together by their mutual gravitational pull.

The region outside but near a galaxy is referred to as “circumgalactic space.” The region between galaxies is known as “intergalactic space.” What, if anything, can be found in these spaces between these cosmic islands of stuff?

Hubble’s Observations

There is the enigmatic dark matter which makes up the majority of matter in the universe and provides the scaffolding onto which the stars, gas, dust, and other normal matter are gravitationally anchored.

But is there normal matter between galaxies, too? Hubble’s observations, over the course of 25 years, have given us a more complete picture of the environments around galaxies.

A galaxy is not a static lone oasis in the dark, but is rather continually influencing and being shaped by its surrounding environment. This is true for both isolated galaxies and for galaxies that live amongst other galaxies in large groups and clusters.

Hubble Probes The Invisible Halo Of A Galaxy

Hubble Probes The Invisible Halo Of A Galaxy

The light of a distant quasar shines through the invisible gaseous halo of a foreground galaxy. Elements in the halo absorb certain frequencies of light. They become detectable, and can be used to measure the halo’s mass.

Image by: NASA, ESA, and A. Field (STScI).

Science Credit: NASA, ESA, N. Lehner (University of Notre Dame), T. Tripp (University of Massachusetts, Amherst), and J. Tumlinson (STScI)

Hubble's Cosmic Origins Spectrograph

Distant quasars serve as lighthouse beacons that shine through the gas-rich "fog" of hot plasma enveloping galaxies.

At ultraviolet wavelengths, Hubble's Cosmic Origins Spectrograph (COS) is sensitive to absorption from hot gas, which shows up as reduced brightnesses at specific colors in a spectrum.

In this result from 2011, astronomers found that gas blown out of galaxies from energetic newborn stars often will fall back into the galaxy, providing additional fuel for new star formation.

Gas In Intergalactic Space

Even up until Hubble’s launch in 1990, scientists were unsure whether or not gas existed in intergalactic space.

In 1956, astronomer Lyman Spitzer Jr. proposed that there was a significant amount of hot gas outside of galaxies that could be detected in high-energy ultraviolet light. It would take an orbiting space telescope, first proposed by Spitzer in 1946, to detect ultraviolet light that is mostly blocked from reaching Earth-based telescopes due to Earth’s atmosphere. Finding this gas was one of Hubble’s primary science goals when it launched in 1990.


How does one go about finding this gas?

Astronomers could not simply take an image of the gas. The gas was thought to be hot, but it was also expected to be very tenuous, and thus it would not produce a lot of light. Astronomers would need to use other instruments in Hubble’s toolkit, called spectrographs. Three spectrographs have been particularly important in this endeavor – the Faint Object Spectrograph (FOS), the Space Telescope Imaging Spectrograph (STIS), and the Cosmic Origins Spectrograph (COS).

Quasar Light

Gas does not just emit light, it can also absorb light.

Astronomers using Hubble observed the intense light from distant quasars. Quasars are galaxies that have bright central regions due to the presence of supermassive black holes. As this quasar light travels for as long as billions of years through intergalactic space, it passes through filaments of intergalactic gas which absorb certain colors of the light. Astronomers can use a spectrograph to break up the incoming light into its individual colors and search for the missing colors of light that were absorbed by the intergalactic gas.

Analyzing The Gas

Analyzing The Gas

Identifying the colors that are missing, or absorbed, in spectra gives astronomers a great deal of information about the intergalactic gas, including its composition and temperature. The spectra also allow astronomers to make estimates of the total amount of matter that exists in the form of intergalactic gas.


The Cosmic Web

Hubble, being a space-based observatory with state-of-the-art spectrographs, can search for the missing colors of light absorbed by the intergalactic gas, particularly in the high-energy ultraviolet colors where the hot intergalactic gas does a significant amount of absorption.

Hubble’s spectrographs – partially fulfilling the promise of a large space telescope envisioned by Spitzer all those years ago – were critical in allowing astronomers to map the web-like structure of intergalactic gas in the universe, now termed the “cosmic web.” However, as is often the case in science, these new discoveries opened all new questions – questions which Hubble was primed to explore.

Where Did This Gas Go?

Hubble found that much of the intergalactic gas in the distant universe was being heated by stars and regions around supermassive black holes to about 10,000 degrees.

But astronomers were not finding the same quantity of 10,000-degree intergalactic gas in the local universe. Where did this gas go? With the installation of COS on Hubble during the 2009 servicing mission, astronomers obtained an incredibly sensitive, ultraviolet-detecting spectrograph to probe for this missing gas.

Many recent results from COS suggest that much of this “missing” gas in the local universe did not disappear, but instead was superheated to millions of degrees after falling into dense regions of the cosmic web.

Searching The Space

Searching The Space

Hubble has much work still to do. Astronomers have not detected the conditions and locations of all of the normal matter in the universe. This effort remains a critical mission of Hubble decades after Lyman Spitzer Jr. proposed searching the space between galaxies for gas.

Origins Of Intergalactic Matter

So, how did this matter get into intergalactic space? Much of it is likely left over from the formation of the early universe. However, some of the matter we find between galaxies came from the galaxies. There are several ways this can happen:

Galactic Gravitational Interactions

Galaxies are constantly feeling the pull of gravity from neighboring galaxies and groups of galaxies.

When galaxies get too close, their mutual gravitational interaction can distort the shapes of the interacting galaxies and send stars, gas, and dust off into intergalactic space. Hubble has been imaging these interactions for decades.

Galactic Jets

Galactic Jets

One of the more spectacular ways in which a galaxy can directly inject energy and matter into circumgalactic and intergalactic space is via jets emanating from the center of the galaxy due to the effects of a supermassive black hole. These jets can propel massive amounts of gas and dust into circumgalactic and intergalactic space. Many of Hubble’s first observations were of jets being emitted from the centers of galaxies.

Galactic Outflows

Galaxies also eject gas and dust in outflows from stellar explosions called supernovae – the end-state of very massive stars.

Much of this gas can fall back into the galaxy, where it becomes available to form new stars. Some galaxies, undergoing immense bursts of star formation and subsequent supernovae, can exhibit tremendous outflows of gas.

This gas can be expelled so far from these “starburst galaxies” that the material is lost forever, thus removing the gas needed for new star formation. Galactic outflows from supernovae, it turns out, plays a significant role in how galaxies regulate their star formation.

Key Evidence

In 2011, astronomers using Hubble data provided key evidence that furthered our understanding of these galactic outflows.

The astronomers used the COS spectrograph to study gas in the outer confines, or halos, of more than 40 galaxies. Astronomers were surprised to find a large quantity of previously undetected gas in the far outskirts of the galaxies’ halos, enough to provide the material needed for new star formation for billions of years as it slowly falls back into the galaxy’s depths. Once again COS was used to help discover some of the missing matter in the universe that was first proposed by Lyman Spitzer Jr. all those years ago.

What Happens When Galaxies Collide?

In addition to the familiar spiral and elliptical shapes, astronomers have found a small population of galaxies with peculiar appearances.

Many of these unusual galaxies exhibit long "tails" of stars, gas and dust. A combination of ground-based observations and computer simulations show that interactions and collisions between galaxies explain the strange structures. Moreover, astronomers deduce that big elliptical galaxies form through the merging of smaller galaxies.

Examining Interacting Galaxies

Examining Interacting Galaxies

When Hubble examined interacting galaxies, new details emerged. Observations immediately uncovered a new class of exceptionally large and bright star clusters that form during these interactions. This result helps explain how galaxy mergers create the larger numbers of big star clusters seen in elliptical galaxies. Hubble also probed the cores of collisions, showing that interactions fuel supermassive black holes at the centers of galaxies.

Studies Of Galaxy Collisions

Studies across a wide variety of galaxy collisions displayed their diversity, interconnections and unexpected abundance.

Hubble's collection of galaxy collision images vividly illustrates the progression of a collision from approach to interaction, through tidal tail development, and ending in the merger of the galaxies. Astronomers were also surprised by the large number of galaxy interactions occurring in clusters of galaxies and in very distant — and thus very young — galaxies.

These Hubble discoveries helped firmly establish a larger role for collisions and mergers in galaxy development.

Merging Galaxies

Closer to home, Hubble was the first to establish the eventual fate of our own Milky Way galaxy.

Astronomers had long known that the Andromeda galaxy is approaching us, but were unsure if a collision was in our future. Hubble's keen eye was used to measure the sideways motion of Andromeda, discovering that it was consistent with a head-on collision between the two galaxies in about 4 billion years. If humans survive that long, our distant descendants will one day see two galaxies stretched across the night sky. The Milky Way and Andromeda will merge to become one large elliptical galaxy about 6 billion years from now.

Hubble's Discoveries

Hubble's extraordinary resolution has provided details of interacting galaxies both near and far.

It has studied the often-unusual structures produced in these gravitational encounters, investigated the huge bursts of star formation that are induced, and documented that galaxy evolution relies on interactions and mergers more than previously expected. Future high-resolution infrared observations from the James Webb Space Telescope will complement these discoveries by providing greater detail of the dynamics of cool stars, gas and dust during galaxy collisions.

Simulation vs. Observations

This scientific visualization presents a galaxy collision supercomputer simulation and compares the different stages of the collision to different interacting galaxy pairs observed by Hubble. Two spiral galaxies distort, stretch and merge together, matching different images at different times and different viewing angles.

Visualization Credit: F. Summers; Simulation Credit: C. Mihos and L. Hernquist