Chapter 33:

Galaxies

Note: Gamma Ray Bursts are covered in Chapter 32

The Clearest View of the Core of the Andromeda Galaxy

[press release from Ohio University, Athens, Ohio]

ATHENS, Ohio -- An Ohio University astronomer and an international team of collaborators have obtained the clearest view yet of the center of the Andromeda galaxy, the nearest large galactic neighbor to our Milky Way. What they've learned may confirm a theory for the mystery surrounding the heart of this massive collection of stars, gas and dust.

From the first observations in the early 1970s and later views with the Hubble Space Telescope in the 1990s, astronomers have noted that Andromeda has a double nucleus -- or two points of brightness -- unlike the single nuclei observed in almost all other galaxies. It is a matter that has confounded astronomers for years. But Thomas Statler, Ohio University assistant professor of physics and astronomy, and his colleagues have studied new Hubble images of the center of Andromeda that may offer a simple explanation for the double cores.

Most galaxies, including the Milky Way, are believed to have a core that contains a massive black hole. Black holes have gravitational fields so strong that light cannot escape them. Even though a black hole is invisible, astronomers can detect black holes in the centers of galaxies from their gravitational effect on the millions of stars that orbit closely around them. If they are orbiting in a circular disk, as astronomers presume, the stars often look like one point of bright light.

But as past observations have suggested, Andromeda has two points of brightness at its center. Statler and his colleagues used the Hubble's Faint Object Camera to record the spectrum of the two nuclei. A spectrum, unlike a direct picture, splits the light into its component colors. Using this information, the astronomers were able to map the orbital motions of the stars around the center.

Their findings, published in the February issue of The Astronomical Journal, support a model that suggests stars in Andromeda are orbiting the galaxy's black hole in a lopsided path and are piling up -- sort of a cosmic traffic jam -- at the section of the orbit that is farthest away from the black hole.

"When stars swing closer to the center, they go faster. When they move away from it, they go slower. It's almost like you're getting a traffic jam at the slow section of the orbit," Statler says. "One of the bright spots in the nucleus would be the area where the stars are piling up, and the other marks where they rush through on their closest approach to the black hole."

The theory for Andromeda's center first was offered in 1995 by Scott Tremaine of the Canadian Institute of Theoretical Astrophysics, now at Princeton University.

Statler isn't sure how this arrangement of orbits around Andromeda's black hole could have developed. One possibility, he says, is that the powerful gravity from the black hole disrupted a passing cluster of stars. However, this concept conflicts with other aspects of his data, he says. New spectra taken by Hubble of the center of Andromeda later this year might clear up some of the inconsistencies, Statler says.

While researchers aren't clear why Andromeda is such an anomaly, this close look at its nucleus tells astronomers more about the nature of galaxies, Statler says, including our own. Even though Earth is part of the Milky Way galaxy, astronomers can't examine the center of the Milky Way as clearly as that of Andromeda's.

"In our galaxy, we're not able to see the forest for the trees," Statler says. "There's too much stuff in the way for us to see the nucleus."

Although Andromeda, known as M31, is the galaxy nearest to ours, it is so distant that it takes about 2 million years for light from Andromeda to reach us. It is faintly visible to the naked eye from Earth and is the largest member of the Local Group, a loose cluster of about 30 galaxies that includes the Milky Way. Like ours, it is a spiral galaxy, but it contains more stars, estimated in the hundreds of billions.

Astronomers first detected something strange in Andromeda's core in 1971 from images taken by a balloon-borne telescope. The double structure of the nucleus was established by Hubble in 1993, but it wasn't until December 1995 that the observations with Hubble's spectrograph, first planned by Ivan King of the University of California at Berkeley, were done. Statler led the project to analyze the spectrum shots.

Statler and his colleagues' research was supported by grants from NASA and the National Science Foundation. His research was co-authored by King, Philippe Crane of the European Southern Observatory and Dartmouth College and Robert I. Jedrzejewski of the Space Telescope Science Institute. Statler holds an appointment in the College of Arts and Sciences.

GALAXIES CAME BEFORE CLUSTERS

Astronomers at the University of Birmingham, using observations from the orbiting ROSAT X-ray telescope, have discovered evidence that the afterglow of galaxy formation can be seen today in the hot gas trapped in groups of galaxies. The high temperatures seen in this gas can be explained by the energy released from the supernova explosions accompanying galaxy formation, but only if galaxies formed *before* they grouped together into galaxy clusters. This resolves a long-standing debate in cosmology over whether structures in the Universe have formed "bottom-up" (i.e. small objects forming first and then clustering) or "top-down" (large objects form first and then fragment). The results are published in the Jan.14, 1999, issue of Nature.

The Universe has a hierarchical structure: stars are grouped into galaxies, and most galaxies (including the Milky Way) congregate in small groups or large clusters which may contain up to thousands of galaxies. The space between the galaxies in these groups and clusters is not empty, but is filled with gas at a temperature of many millions of degrees Celsius, which radiates X-rays (see Figures).

A team led by Dr. Trevor Ponman of Birmingham University has used the X-ray telescope on board the German/UK/US ROSAT satellite to image these X-rays, and to compare the hot gas in small groups to that in large clusters. Most current cosmological theories predict that as the Universe expands, galaxies clump together to form groups, which in turn merge together to form clusters. In this picture the hot gas in small groups should look just like a scaled-down version of that in large clusters. In practice, the ROSAT images show that this is not the case. The gas in galaxy groups is hotter than expected, and is spread out more thinly than that in clusters, so that they shine less brightly in X-rays than would be expected. These differences in the hot gas are almost certainly due to the energy injected into it when the galaxies formed.

The formation of galaxies should be a spectacular process. As large numbers of stars are formed, the most massive of them live only a short time before exploding as supernovae, releasing large bursts of energy. This should heat the gas within the young galaxies to tens of millions of degrees, giving it enough energy to escape altogether, as high speed `galaxy winds'. These stream out of galaxies, heating the gas which surrounds them. This extra heat has little effect in large clusters (where the gas gets very hot anyway, due to the large amount of energy released as gas falls in the strong gravitational field), but in small groups it is very significant, and pushes the gas outward, causing the differences in X-ray brightness which are seen in the ROSAT images.

Calculations by the team, Drs. Trevor Ponman and Damian Cannon from the University of Birmingham, and Dr. Julio Navarro from the University of Victoria in Canada, show that energy has a bigger effect if it is released into the gas before the galaxy groups formed. (It is harder to push gas out of a group than it is to stop it falling in in the first place.) In fact, the amount of energy released when galaxies form could only account for the observations if it had heated the gas before the group was present. In other words, galaxies must have formed before groups and clusters.

This epoch of violent heating also explains why most galaxy formation stopped billions of years ago - there is plenty of gas left, but this does not generally form more galaxies, since most of it is now too hot to collapse under gravity and form stars. Most of the normal matter in the Universe has therefore been left in the form of hot gas filling intergalactic space.

CAPTIONS TO ILLUSTRATIONS
(illustrations available on http://www.sr.bham.ac.uk/public

An optical image of a nearby group of galaxies, and the same group seen in X-rays with the ROSAT X-ray telescope. The scale of the two images is the same. The very hot gas imaged by ROSAT is invisible with optical telescopes, but actually contains more material than all the galaxies combined. In this very hot (10 million degree) gas, we see the afterglow of the violent events which accompanied the birth of the galaxies.

Powerful Event Challenges Gammy Ray Burst Theories

A recently detected cosmic gamma ray burst released a hundred times more energy than previously theorized, making it the most powerful explosion since the creation of the universe in the Big Bang.

"For about one or two seconds, this burst was as luminous as all the rest of the entire universe," said Caltech professor George Djorgovski, one of the two principal investigators on the team from the California Institute of Technology, Pasadena, CA.

The team measured the distance to a faint galaxy from which the burst originated at about 12 billion light years from the Earth. The observed brightness of the burst despite this great distance implies an enormous energy release. The team's findings appear in the May 7 issue of the journal Nature.

The burst was detected on Dec. 14, 1997, by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The Compton observatory provided detailed measurements of the total brightness of the burst, designated GRB 971214, while BeppoSAX provided its precise location, enabling follow-up observations with ground-based telescopes and NASA's Hubble Space Telescope.

"The energy released by this burst in its first few seconds staggers the imagination," said Caltech professor Shrinivas Kulkarni, the other principal investigator on the team.

The burst appears to have released several hundred times more energy than an exploding star, called a supernova, until now the most energetic known phenomenon in the universe. Finding such a large energy release over such a brief period of time is unprecedented in astronomy, except for the Big Bang itself.

"In a region about a hundred miles across, the burst created conditions like those in the early universe, about one millisecond (1/1,000 of a second) after the Big Bang," said Djorgovski.

This large amount of energy was a surprise to astronomers. "Most of the theoretical models proposed to explain these bursts cannot explain this much energy," said Kulkarni. "However, there are recent models, involving rotating black holes, which can work. On the other hand, this is such an extreme phenomenon that it is possible we are dealing with something completely unanticipated and even more exotic."

Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered by U.S. Air Force Vela satellites in the 1960s. Since then, numerous theories of their origin have been proposed, but the causes of gamma-ray bursts remain unknown. The Compton observatory has detected several thousand bursts so far.

The principal limitation in understanding the bursts was the difficulty in pinpointing their direction on the sky. Unlike visible light, gamma rays are exceedingly difficult to observe with a telescope, and the bursts' short duration exacerbates the problem. With BeppoSAX, scientists now have a tool to localize the bursts on the celestial sphere with sufficient precision to permit follow-up observations with the world's most powerful ground-based telescopes.

This breakthrough led to the discovery of long-lived "afterglows" of bursts in X-rays, visible and infrared light, and radio waves. While gamma-ray bursts last only a few seconds, their afterglows can be studied for several months. Study of the afterglows indicated that the bursts do not originate within our own galaxy, the Milky Way, but rather are associated with extremely distant galaxies.

Both BeppoSAX and NASA's Rossi X-ray Timing Explorer spacecraft detected an X-ray afterglow. BeppoSAX precision led to the detection of a visible light afterglow, found by a team from Columbia University, New York, NY, and Dartmouth College, Hanover, NH, including Professors Jules Halpern, David Helfand, John Torstensen, and their collaborators, using a 2.4-meter telescope at Kitt Peak, AZ, but no distance could be measured from these observations.

As the visible light from the burst afterglow faded, the Caltech team detected an extremely faint galaxy at its location, using one of the world's largest telescopes, the 10-meter Keck II telescope at Mauna Kea, Hawaii. The galaxy is about as faint as an ordinary 100 watt light bulb would be as seen from a distance of a million miles.

Subsequent images taken with the Hubble Space Telescope confirmed the association of the burst afterglow with this faint galaxy and provided a more detailed image of the host galaxy.

The Caltech team succeeded in measuring the distance to this galaxy, using the light-gathering power of the Keck II telescope. The galaxy is at a redshift of z=3.4, or about 12 billion light years distant (assuming the universe to be about 14 billion years old).

From the distance and the observed brightness of the burst, astronomers derived the amount of energy released in the flash. Although the burst lasted approximately 50 seconds, the energy released was hundreds of times larger than the energy given out in supernova explosions, and it is about equal to the amount of energy radiated by our entire Galaxy over a period of a couple of centuries. Scientists say it is possible that other forms of radiation from the burst, such as neutrinos or gravity waves, which are extremely difficult to detect, carried a hundred times more energy than that.

NASA is planning two missions to further investigate gamma-ray bursts: the High Energy Transient Experiment II (HETE II), scheduled to launch in the fall of 1999, and the Gamma Ray Large Area Space Telescope (GLAST), scheduled to launch in 2005. HETE II will be able to precisely locate gamma-ray bursts in near real-time and quickly transmit their locations to ground-based observatories, permitting rapid follow-up studies. GLAST will detect only those gamma-ray bursts that emit the highest energy gamma rays, and will be able to locate them with sufficient precision to permit coordinated observations from the ground. Because not much is known about the bursts at these high energies, the observations may permit researchers to choose among competing theories for the origin of gamma-ray bursts.

NOTE: Images of the GRB 971214 field are available at:

FTP://PAO.GSFC.NASA.GOV/newsmedia /GRB/

Information on the BeppoSAX spacecraft is available at:

http://www.sdc.asi.it/

Information on the Compton Gamma Ray Observatory is available at:

http://cossc.gsfc.nasa.gov/cossc/descriptions/cgro.html

Information on Gamma Ray Bursts is available at:

http://cossc.gsfc.nasa.gov/cossc/nasm/VU/overview/bursts/bursts.html

Astronomers Find Galaxy, Most Distant Known

Shattering a record established just 6 weeks ago, astronomers have discovered the most distant object ever seen, an infant galaxy that lies some 12.3 billion light-years from Earth. That immense distance means that the light now reaching Earth left the galaxy when it was less than 800 million years old. Details about the finding appear in the May 2 Science News.

Astronomers observed the object, along with several other galaxies that are nearly as distant, with one of the twin Keck telescopes, the world's largest visible-light telescopes. To search for distant galaxies, Lennox L. Cowie and Esther M. Hu of the University of Hawaii in Honolulu and Richard G. McMahon of the University of Cambridge, England, resurrected an old strategy_detecting a particular wavelength of light emitted by hydrogen atoms_that had not been successful with smaller telescopes. The researchers say the method promises to reveal some of the very first galaxies in the cosmos, those that were in existence when the universe was only about 500 million years old. Astronomers often express cosmic distance in terms of redshift, the amount by which the expanding universe has shifted the light emitted by a galaxy to redder, or longer wavelengths. The more distant the galaxy, the greater the redshift. The previous record holder, discovered by another team in March, has a redshift of 5.34. The newly found galaxy has a redshift of 5.64 and hails from about 60 million years earlier in cosmic history.

That difference in time may not seem like much, but a small interval may have made a substantial difference in the properties of the universe when it was very young. "As any mother could tell you, a year's growth makes a much bigger difference in appearance and character in a toddler than in someone age 20," Hu says. The team will describe their work in an upcoming Astrophysical Journal Letters. The May 2 Science News article follows.

Science News, May 2, 1998

Searching for the First Light Long ago and far away By RON COWEN

When astronomers look through a telescope, it's as if they have entered a time machine. As the telescope peers deeper into space, it delves further back in time. The newest generation of instruments is transporting astronomers farther into the past than ever before--to an era when the first glimmers of starlight set galaxies aflame. In March, researchers reported the discovery of the most distant galaxy then known, lying 12.2 billion light-years from Earth (SN: 3/21/98, p. 182). That immense distance means that the light now reaching Earth left the galaxy when the cosmos was only about 1 billion years old.

Now, a team of scientists reports having bested that record. A galaxy found by Esther M. Hu and Lennox L. Cowie of the University of Hawaii in Honolulu and Richard G. McMahon of the University of Cambridge in England lies slightly farther from Earth and hails from about 60 million years earlier in cosmic history. That difference in time may not seem like much, but a small interval may have been substantial when the universe was very young. "As any mother could tell you, a year's growth makes a much bigger difference in appearance and character in a toddler than in someone age 20," says Hu.

Astronomers often express cosmic distance in terms of redshift, the amount by which the expanding universe has shifted the light emitted by a galaxy to redder, or longer, wavelengths. The more distant the galaxy, the greater the redshift. The galaxy reported in March has a redshift of 5.34, while the new record holder, to be described in an upcoming Astrophysical Journal Letters, has a redshift of 5.64. The researchers have also found several other galaxies that are nearly as distant.

The new find appears to be a harbinger of many more, says McMahon. He asserts that astronomers are on the verge of detecting the very first galaxies--those that were in existence when the universe was only about 500 million years old. The possibility of finding galaxies so soon after their formation flies in the face of conventional wisdom, which holds that astronomers would have to wait until the next decade--and the launch of large, space-based observatories (SN: 4/26/97, p. 262)--before they could find such youthful collections of stars and gas.

"Our paper demonstrates that we have a technique for searching for distant galaxies up to a redshift of 5.6, and we're currently doing work where we think we can find them up to a redshift of 6.5," says McMahon. "We're in new territory here."

"It's quite plausible that some of these [galaxies] are young objects that are going off for the first time and making stars," says Mark Dickinson of the Space Telescope Science Institute and Johns Hopkins University in Baltimore.

Astronomers have been searching for the denizens of the early universe for decades, but until recently they faced a major limitation--the size of their telescopes. With small telescopes, says Hu, researchers were forced to look at highly luminous objects such as quasars, the brilliant powerhouses that lie at the center of some galaxies. As bright as a trillion suns, quasars are relatively easy to spot. Indeed, McMahon and a group of British colleagues have specialized in finding distant quasars--those with redshifts between 4 and 5 (SN: 9/17/94, p. 188).

Quasars are rare, however. Many researchers view them as cosmic oddballs, fueled by galactic black holes, that drown out the light from the stars in a galaxy.

"It's a lot easier to look at something blazingly bright, even if it's extremely rare--perhaps 1 [galaxy] in 10 million houses a quasar--and unrepresentative of the typical galaxy," says Hu.

Researchers who eschewed quasars and insisted on searching for primeval galaxies lacking these powerhouses encountered an additional problem.

Ten years ago, says Hu, astronomers believed that distant galaxies--those from the early universe--would have about the same mass and size as those in today's universe. This supposition, however, "just ain't so," says Hu.

Several recent sky surveys have shown that the typical distant galaxy is surprisingly small, its mass just a few percent that of a present-day galaxy like the Milky Way. Correspondingly, the light emitted by one of these faraway galaxies is much less than had been estimated.

Little wonder, then, that searches in the 1980s and early 1990s came up empty-handed. Finding distant galaxies was much harder than most astronomers had bargained for.

Seekers of distant galaxies were saddled with one additional complication. In their hunt, researchers sought out a particular wavelength of light emitted by hydrogen atoms. Theory suggests that as the first massive stars formed, they emitted high-energy radiation that would easily excite hydrogen, the most common gas in a galaxy. The jazzed hydrogen atoms would radiate much of the absorbed energy at an ultraviolet wavelength of 121.6 nanometers. This radiation is known as the Lyman-alpha emission.

Alas, Lyman-alpha emissions from distant galaxies have remained elusive. Because of their small size, distant galaxies just can't produce much of this radiation. In addition, astronomers have difficulty seeing them because they are obscured by dust--bits of material, heavier than hydrogen and helium, that are produced as a galaxy ages.

Dust readily absorbs ultraviolet light, including Lyman-alpha radiation. Moreover, even small amounts of dust can dim this radiation. Lyman-alpha photons are easily scattered by gas molecules and so take a meandering, zigzag route out of their home galaxy. The zigzag path increases the likelihood of repeated encounters with bits of dust.

Absorption of a distant galaxy's light may have foiled the Lyman-alpha method of detection, but it proved the key to another. Hydrogen gas, ubiquitous throughout the universe, absorbs ultraviolet light efficiently. The more distant a galaxy, the more hydrogen lies between it and Earth and the more its ultraviolet emission is dimmed. Researchers, including Dickinson and Charles C. Steidel of the California Institute of Technology in Pasadena, have looked for galaxies that shine brightly over a broad spectrum of colors in visible light but vanish in the ultraviolet. To date, about 440 of 1,400 ultraviolet dropouts have turned out be faraway galaxies with redshifts between 2.2 and 4 (SN: 2/7/98, p. 92).

With such an efficient method of ferreting out distant galaxies, why would anyone resurrect the search for Lyman-alpha emissions? Hu, Cowie, and McMahon have hearkened back to the older method because they believe it can reach further back in time than the dropout method. Furthermore, they have a new tool--the twin Keck Telescopes atop Hawaii's Mauna Kea. Each of these 10-meter telescopes can gather more visible light than any other instrument. A survey that took 20 hours on the University of Hawaii's 2.2-meter telescope takes only 1 hour on one of the Keck instruments, and it can discern galaxies that are one-fifth as bright.

"What we're doing now, we can only do with Keck," notes McMahon. "Previous researchers were not pushing deep enough over a large enough volume to find [distant] Lyman-alpha emitters," says Dickinson. "Now people are, and as usual, it's thanks to a large aperture like the Keck Telescopes."

McMahon and his collaborators have two other reasons for carrying the torch for Lyman-alpha light. First and foremost, says McMahon, the very youngest galaxies in the universe, those that had just begun making stars, would not have had enough time to make dust. In these galaxies, Lyman-alpha emissions might not amount to much, but they would not be extinguished.

"The key thing is that we're talking about very short times" after the birth of the universe, McMahon says. "We're preferentially picking out very young objects."

In contrast, the ultraviolet dropout technique relies on emissions over a broad range of colors. These emissions may be muted in newborn galaxies, rendering them invisible. "If these objects are indeed so faint [that the dropout technique] can't find them, then Lyman-alpha may be a better reflection of star formation rates" in the early universe, Dickinson says.

It also turns out that the dropout technique becomes more difficult when applied to extremely distant galaxies. As the light from remote galaxies is increasingly stretched out by cosmic expansion, the wavelength at which they disappear from view moves from the ultraviolet range of the spectrum into the visible range. Astronomers must then search for more distant galaxies in a part of the spectrum that includes long-wavelength emissions from molecules in Earth's atmosphere, making detection more difficult.

Those spurious sources of radiation are much less troublesome for searches that rely on Lyman-alpha emissions, McMahon says. To detect a galaxy at a given distance from Earth, Lyman-alpha surveys require the detection of but a single redshifted wavelength of light emitted by hydrogen atoms. By limiting their search to the many redshifted wavelengths at which Earth's atmosphere does not radiate, the team avoids interference from the atmosphere altogether.

"There are gaps in between the emission lines that come from the sky," says McMahon. "We look [for distant galaxies] in these gaps, where the sky is dark."

One caveat, says Dickinson, is that such studies require highly selective filters, each of which detects Lyman-alpha radiation from galaxies that reside within an extremely narrow range of distances from Earth. Recent observations suggest that distant galaxies tend to clump together. If galaxies in a particular patch of sky don't happen to cluster within the necessarily narrow range of distances employed in the study, researchers could miss them, he says. In contrast, the dropout technique can find galaxies over a larger range of distances. McMahon agrees that the two search strategies complement each other. "The key questions we're [all] asking is when did star formation begin, and at what speeds are galaxies forming? "Only time will tell which method reveals the answers."

Bill Keel's Images

Sets of images of galaxies and other objects (comets, etc.), including a rotation curve (under his "educational materials" catalogue), taken by Prof. William Keel of the University of Alabama at Tuscaloosa, are available on the Web.

Errata

Chapter 33, p. 557, last two sentences (which are identical) of caption for Fig. 33-27: reference to Fig. 33-16 should be to p. 542, Part 7 Opener, for the STIS spectrum of M84 revealing a central black hole.