Black holes are places where ordinary gravity has become so extreme that it overwhelms all other forces in the universe. Once inside, nothing can escape a black hole's gravity — not even light.
Massive stars, although relatively rare, end their short lives in spectacular explosions called supernovae. The most massive of these dying stars leave behind a remnant known as a black hole. Black holes created by supernovae can be about ten to twenty times the mass of the Sun, but these stellar black holes are miniscule in comparison to the beasts that astronomers think lie at the centers of most ordinary galaxies. These black holes are supermassive – millions to billions of times the mass of our Sun. Thanks to Hubble and other observatories, we now know that supermassive black holes are intricately tied to the evolution of the galaxies in which they reside.
Hubble’s early contributions:
Prior to Hubble, astronomers did not have conclusive evidence that supermassive black holes existed in the universe. Many of Hubble’s first observations showed the effects of supermassive black holes on their immediate galactic environment.
In 1990, shortly after deployment, Hubble imaged a 30,000-light-year-long jet emanating from a galaxy known to be a prodigious emitter of radio light. With Hubble’s observations, astronomers had the data they needed to determine that these jets emanate from very small regions in the centers of galaxies and are likely powered by supermassive black holes.
Hubble’s fine resolution – the ability to see tiny details – helped propel the case for supermassive black holes even further in 1994, when astronomers took spectra of the gas in the center of the elliptical galaxy M87. Spectra, or the breaking up of light into component colors, can give astronomers a great deal of information about the gas, including its velocity. Astronomers noted that in M87, the central gas was circling in a disk at very high speeds around a small but massive object. The only type of object that can be that massive and yet very small in size is a black hole. These observations by Hubble helped confirm nearly two centuries of theories and conjectures about the existence of black holes.
Astronomers continue to use Hubble to explore the physics of supermassive black holes via observations of jets and spectra of orbiting hot gas. The elliptical galaxy M87 has been observed so many times with Hubble for over a decade that astronomers have even made a movie of M87’s jets changing with time.
The surprising connections between galaxies and their supermassive black holes:
Hubble observations have helped astronomers conclusively find supermassive black holes in some host galaxies, but even more surprising was the observed connection between the mass of a supermassive black hole and the size of its host galaxy. In 2000, astronomers released an exciting discovery using Hubble spectroscopic data of gas and stars swirling around supermassive black holes in more than 30 galaxies. The results of the study indicate that larger galaxies have more massive black holes. This indicates that supermassive black holes are not primordial relics, but they are growing with their host galaxies.
From the wealth of Hubble data, astronomers now understand that black holes can have profound influences on the galaxy as a whole. For example, the jets from supermassive black holes can propel massive amounts of gas and dust into intergalactic space, thus ridding the galaxy of the much-needed fuel for ongoing star formation. Or in the case of a 2015 result using Hubble observations of large elliptical galaxies, jets from supermassive black holes may regulate star formation in such a way that it keeps going, albeit at a slower rate.
Perhaps the most surprising result from Hubble’s observations is that supermassive black holes must reside in the majority of all normal galaxies. Once the objects of extreme speculation, supermassive black holes are now considered integral components of galaxies and crucial to the study of how galaxies evolve with time.
Over the past 25 years, Hubble has given us incredible, detailed views of galaxies, surpassing anything we’d seen before. But the grand spiral galaxies and enormous elliptical galaxies we see in those images were not born that way. Hubble has looked into the far reaches of the universe and found that the earliest galaxies were mostly small and clumpy. How did those modest galaxies grow into the magnificent structures we see around us today?
To find out, we need to search for galaxies even farther away and study them in ways we currently cannot do.
When Hubble and other telescopes peer into the universe, they look back in time. Hubble has seen back to a time when the universe was young and galaxies were newly born. The most distant galaxies Hubble has spied are more than 13 billion light-years away. That means the light Hubble captures left those galaxies over 13 billion years ago. It views them as they were back then. However, Hubble can only see so far.
As the universe expands, light gets stretched into longer and longer wavelengths, turning visible light into infrared light. By the time visible light from extremely distant galaxies reaches us, it appears as infrared light. Hubble can detect some infrared light — the wavelengths closest to the red end of the visible spectrum. But infrared light will be the James Webb Space Telescope’s specialty. This new, bigger telescope will probe far deeper into the infrared realm and see infrared features with much more resolution, or clarity, than Hubble possibly can.
While Hubble sees infant galaxies, Webb will show us newborns. Webb will see more of the farthest galaxies, showing us the early stages of galaxy formation. It could even reveal when galaxies first started forming in the universe. Webb could prove whether small galaxies in the early universe merged to form larger galaxies. And by seeing more distant galaxies than ever before, Webb will expand our map of the universe’s overall structure.
Webb’s infrared prowess will also allow it to see inside dust-cloaked regions of galaxies that visible light cannot escape from and find out what’s happening within them. For many different types and ages of galaxies, Webb will expose how stars are forming, how many stars are forming, and how star formation is affected by the surrounding environment. Webb will study star-birth regions in merging galaxies, revealing how these galactic encounters trigger and alter the course of star formation. Webb will analyze how elements are produced and distributed in galaxies, and also examine the exchange of material between galaxies and the space between them.
Webb will also investigate the relationship between the evolution of galaxies and the development of the supermassive black holes at their centers — perhaps solving a cosmic chicken-and-egg problem: did black holes come first and galaxies form around them, did galaxies form first and develop black holes, or did the galaxies and black holes develop together?
In its 25 years of service so far, Hubble has taught us much about galaxies, and clever astronomers are pushing the venerable telescope to show us as much as it possibly can. But after its launch in 2018, Webb will be able to show us galaxies in ways we’ve never seen before.
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?
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.
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).
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. 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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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 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.
Watch throughout the year for more articles on Hubble's 25 years of discovery.