HST OBSERVER SPRING 1991 SPACELINK EDITOR'S NOTE: Hard copies of this document (with photos) are available from the Space Telescope Science Institute in Baltimore. If you'd like a copy, you can contact the Institute directly: Space Telescope Science Institute Public Affairs Office Johns Hopkins Homewood Campus Baltimore, MD 21218 Or, you can leave a note when you log off Spacelink, and we'll forward it to the Institute for you. The address you gave when you registered will be attached to the note. NEW EYE ON THE RED PLANET No other world has enticed human curiosity and interest more than Mars. Astronomers of a century ago were intrigued by periodic changes in its polar caps, surface markings, and dust storms. These changes are all attributed to the fact that Mars is inclined on its axis, and so, like Earth, has seasons. In fact Mars looked so much like Earth, a few early observers thought it might be inhabited. The Martians and their legendary canals have long since disappeared from human imagination. But the romance with Mars continues with dreams of manned space flight to the red planet. U.S. and Soviet space probes led the vanguard in the 1960's and '70's. They revealed Mars as a complex and geologically diverse world with immense volcanoes, a great rift valley, and ancient river beds. Despite these spectacular findings, more Mars discoveries await NASA's Hubble Space Telescope (HST). This is because Mars is a dynamic planet with an active atmosphere which changes the appearance of martian surface. HST will allow astronomers to better understand the complex martian meteorology and climate by monitoring the red planet over many of its annual cycles. The image on the cover of the Observer is among the first high- resolution pictures of Mars sent back by HST. The sharpest picture (.2 arc second) of Mars ever taken from Earth's vicinity, it shows surface details ranging from large impact basins down to surface markings as small as 50 kilometers across. This picture inaugurates HST's long-term Mars observing program, which will allow for a better understanding of the martian climate and processes involved in surface changes. The observations may eventually allow scientists to characterize global weather patterns on Mars, which will be an important prerequisite for a manned expedition to the red planet. Imagine trying to understand Earth's meteorology by only taking a few weather satellite snapshots once every couple years. This the task astronomers have faced because they have only been able to take high resolution pictures (150-kilometer resolution) of Mars when it is closest to Earth -- a planetary alignment that occurs only once every 780 days. Most of the time Mars is too far away for ground based telescopes to resolve features any smaller than the state of Arizona, so that even major events occurring on the planet are unrecognizable. HST's Planetary Camera image is comparable to that obtainable from ground-based observatories when Mars is closest Earth. (When the second generation WF/PC is installed in 1993, HST will be able to detect martian features as small as 20 kilometers.) Unlike ground-based telescopes HST can observe Mars in ultraviolet light, which is normally absorbed by ozone in the Earth's atmosphere. Planetary astronomers will be able to study absorption by ozone in the martian atmosphere, especially over the dry polar regions. Since water vapor initiates chemical reactions which remove ozone from the martian atmosphere, ultraviolet observations can be used to monitor the amount of water in the martian atmosphere as well. The HST program will hasten the day when we can understand and predict weather on the planet, such as the onset of global dust storms. This is important for planning future planetary missions the the Red Planet. HST'S SEARCH FOR BLACK HOLES The detection of super-massive black holes at the cores of galaxies is one of the great "Holy Grails" of modern astronomy. HST has begun an initial scouting mission and some of the findings are presented in this issue. Our Milky Way galaxy is a city of stars - its bright core a crowded urban hub where stars are far more tightly crammed together than they are in the galactic suburbs where we dwell. If we lived at the Galaxy's core, the night sky would be ablaze with more than 100,000 naked-eye stars where hundreds would be brighter than Venus, or even the Full Moon. We can't see directly into the Milky Way's core because vast dust and star clouds blocks our view. When astronomers look far outside or Milky Way they can peer directly into the dense cores of neighboring galaxies. Images taken with ground-based telescopes find that the stars at galactic cores are so crowded together, the starlight blurs into a brilliant spotlight. With it's extremely high resolution, HST is ideally suited for unraveling what's happening in the dense cores of galaxies. Doubtless, complex stellar dynamics occur under such crowded conditions with numerous close encounters between stars. In some cases this stellar "pinball game" might ultimately lead to the formation of one or more super-massive black holes, equal in mass to millions or even a billion suns. Though never directly observed, there is increasing evidence for the existence of these "infernal machines" at the cores of galaxies. In some galaxies extraordinarily energetic processes are detected, such as powerful radio emissions and jet features. A super-massive black hole, feeding on nearby stars, is one of the most efficient energy sources imaginable. Do super-massive black holes really exist?, are they common among galaxies? How might they be influenced by galactic evolution ? HST promises to come up with new surprises. BLACK HOLE BASICS What is a Black Hole ? When a massive star stops producing energy through nuclear fusion, its core will implode under the crushing force of gravity. Einstein's theory of general relativity predicts that the core will collapse to an infintely tiny and dense singularity. As the core radius shrinks the gravitational field rapidly grows much stronger, until light itself cannot escape. Hence, whatever falls into the deep gravitational abyss can never get out. Do Black Holes Really Exist ? Though there is increasing circumstantial evidence for black holes, they are still a theory. Perhaps other "pathological" changes happen to matter before it collapses to a singularity. What is a Super-Massive Black Hole ? A black hole that has grown from a few solar masses to thousands or even millions of solar masses. How is a Super-Massive Black Hole Made ? One possibility is that is starts out as a "seed" black hole that results from the death of a massive star that forms early in a galaxy's existence. The black hole then grows through an agglomeration process were it collides with other black holes, produced by subsequent supernovae in the core of the galaxy. What Evidence Would "Prove" The Existence of Super-Massive Black Holes ? HST images alone can't provide an answer. But when HST's optics are repaired in 1993 the velocity of in-falling stars and gas will be measured spectroscopically. The mass of the black hole can then be estimated, much as the Sun's mass can be estimated by measuring the orbital speeds of the planets. What Factors Determine How Active a Black Hole is ? This largely depends upon the black hole's mass and amount of stars, interstellar dust and gas which is available to "feed" the black hole. The energy rates seen in extragalactic jets and active galactic nuclei could be supported by back holes accreting several to dozens of solar-masses per year. How Do you Fuel the Black Hole Engine ? Several processes may "turn on" black holes. Encounters between galaxies may be close enough for gravitational tidal effects to pull out filaments of gas and stars from each other, and drive a large fraction of this material into the core. The barred structure seen in a class of spiral galaxy may steal angular momentum from gas clouds near the nucleus and cause them to go into the center. In elliptical galaxies gas may be condensing out of the X-ray corona and falling into the center. How Does an Active Black Hole Affect a Galaxy's Appearance Quasars, Seyfert galaxies, BL Lac objects and other active galaxies are all characterized by extremely bright cores which greatly outshine the host galaxy. But Don't Black Holes Trap Light and Other Radiation ? Material doesn't fall directly into the black hole but spirals down like water swirling into a drain. The gasses form a broad, flattened accretion disk that may extend for many millions of kilometers outside the hole's point of no return - the event horizon. Gas within the accretion disk heats up to millions of degrees. Perhaps as much as 10 percent of the in-falling matter is converted to energy which escapes along the black hole's spin axis in the form of radiation and high-speed charged particles. Why Don't All Active Galaxies Have Extragalactic Jets ? Extragalactic jets are found in elliptical galaxies, which have little dust and gas to block the high-speed beam(s) of energy ejected near the black hole. The black hole's tilt relative to that of the host galaxy may also determine how the outflow is manifest. If a black hole's spin axis lies within the plane of a spiral galaxy, a lot of energy will be dissepated near the nucleus and made luminescient by interaction with gas within the galactic plane. Why Do Some Galaxies Have Only One Jet ? If the black hole's spin axis more or less points in our direction, plasma in the jet aimed toward us will be highly blue- shifted or amplified because it is moving at near the speed of light, while light from the jet pointed away from us will be weakened or red-shifted and hence be much fainter. Another possibility is that there is only a single jet which flips from one side of the black hole to the other, due to some instability that chocks off the outflow. A TELLTALE SEARCHLIGHT FROM A HIDDEN NUCLEUS Astronomers are hot on the trail of a black hole in the galaxy NGC 1068. In a photograph NGC 1068 looks like a normal barred spiral galaxy. However, its core shines with the equivalent energy of one billion suns. Because the brightness of the core can fluctuate so quickly, this energy is coming from a region only a few light-days across. A super-massive black hole is the most likely "machine" to fit within such a relatively tiny region capable of producing such a prodigious output of energy. HST can't look directly at the suspected black hole because its light is blocked presumably by a ring of dust which may be somehow associated with the black hole's accretion disk. The Wide Field and Planetary Camera has resolved the inner 150 to 300 light-years of the core to show in detail how the black hole affects it's immediate neighborhood, through jets, stellar winds, and ionizing radiation. Like a beacon from a searchlight, an intense beam of radiation is being emitted by material falling into the black hole. This is somewhat reminiscent of extragalactic jets (page 8] except for several key distinctions. Extragalactic jets are much more tightly collumated and also flow unimpeded through space. The 1068 jet lies in the plane of the galaxy and plows through dust and gas in interstellar space to carve out a cavity of ionized gas, like a blow torch through butter. Current HST observations are taking spectra of clouds caught in the black hole's "searchlight." Preliminary data collected with the HST's Faint Object Spectrograph suggest that there is a dense high-speed plasma quite close to the black hole. Is this material ejected from the hole or simply nearby gas clouds which have been caught up in a rapid stellar "wind" flowing away from the black hole ? What is more, the clouds (some as small as 10 light years across) show signs of being compressed by a jet of plasma from the nucleus. Located 30 million light-years away NGC 1068, is the closest active galaxy of its kind. HST will now probe even more distant galaxies to look for similar fireworks. TWISTS, KINKS AND KNOTS IN A 10,000 LIGHT-YEAR LONG RIVER OF ENERGY Like a science fiction weapon run amok, some galaxies eject long streamers of plasma which hurtle through space at nearly the speed of light. These extragalactic jets have been detected in radio wavelengths around hundreds of active galaxies, and a few have been seen in optical light. Extragalactic jets are a bizarre and fascinating mechanism for transporting material from the heart of a galaxy into extragalactic space. These cosmic "roman candles" pose many mysteries. How do the jets originate? What is the connection between radio and optical emissions? How does the energy stream stay so narrow across tens of thousands of light-years? Astronomers do know that the blueish, highly polarized light of an optical jet is produced by electrons spiraling along magnetic fields lines at nearly the speed of light. Like a ghostly finger, the luminescient jet points back to the very core of the galaxy as the source of both the magnetic field and high speed electrons. Presumably, both the hot plasma and magnetic fields are created when stars, dust and gas swirl deep into the intense gravitational field of a super-massive black hole located at the galaxy's core. The spinning plasma in this accretion disk creates powerful electric currents which in turn generate twisted magnetic fields which align to the black hole's spin axis. This spin axis is also an escape route for the high speed electrons. As electrons spiral outward they lose energy. The electrons responsible for the producing light lose much of their energy in only a few hundred years. However, the electrons which produce radio emissions can survive in the same magnetic field for tens of thousands of years because they lose energy at a slower rate. This explains why most extragalactic jets are seen only in radio wavelengths. The European Space Agency's Faint Object Camera provides unprecedented new details of the much more rare optical jets in active galaxies PKS 05121-36 and 3C 66B. When seen with HST's new level of detail, the two jets observed so far have remarkably different structures which might explain why, in the case of optical jets, the electrons remain energetic enough to radiate light throughout their 10,000 year-long journey along the jet. PKS 0521-36 has bright knots of emission which suggest the electrons are boosted back to higher energy levels, perhaps by instabilities along the edge of the plasma flow which would produce accelerating shock waves. The jet's braided appearance in 3C 66B suggests the electron spiral on the surface of a "tube" with much weaker magnetic resistance, hence lower energy loss. Both types of features help explain why a few jets remain luminous over tens of thousand of years. As HST surveys other optical jets, astronomers may come up with yet more exotic mechanisms, or even new physics, for explaining how active galaxies create and maintain optical jets. SPACE TELESCOPE CATCHES STARS ON THE REBOUND Space Telescope's first look into the core of globular star cluster M15 offers new clues to the dynamic evolution of tightly bound swarms of stars that in some ways might offer insight into stellar dynamics at the core of distant galaxies However, unlike a vast galaxy, M15 and other globular cluster which lie far outside the plane of our Milky Way contain an aggregates of several hundred thousand stars. Globular clusters are loosely held together under the mutual pull of gravity among these stars. Like atoms in a gas, they move randomly about each other, although, unlike the atoms, the stars don't probably collide very often, but they are deflected through gravitational close encounters. During such an encounter the smaller, less massive star takes momentum from the larger star - like a small car colliding with a truck. As a result of the near encounter, the massive star loses momentum and "falls" toward the center of the cluster. The cluster's collective gravitational field can be thought of as a shallow funnel, with the bottom of the funnel being in the center of the cluster. Given enough time, massive stars should accumulate at the core of the cluster and, like fans crowding onto a stadium field. The stars may merge to form a very dense inner cluster which would ultimately implode into a massive black hole. HST observations with the Wide Field and Planetary Camera infer that nearly 7,000 stars are concentrated within a region only .8 light years across (by comparison our own extragalactic neighborhood individual stars are several light-years apart). Though this sounds impressive, ground-based observations of M15 have previously suggested that 100 times as many stars could be crammed together within less than a light-year. The absence of such a tight cluster suggests there is no massive central object, i.e. a black hole, to glue the core together. Astronomers interpret this finding as evidence that the core is actually on the rebound, like a rubber ball that has been squeezed and then relaxed; but where did then energy come from to stop the implosion ? One notion is that things are being stirred up by a few "hornets" among the "beehive swarm" of stars in the globular. These hornets are actually binary star systems where two stars orbit about each other. The binaries may have formed early in the life of the cluster or by stellar capture when the core was at its "maximum scrunch". Regardless of their origin, these binary systems are powerful reservoirs of kinetic energy, stirring up the motion of in-falling stars like a pair of "egg beaters". (see the Observer vol. 1, pg.7 for discussion of role of binaries in globular clusters) About 140 other globular clusters orbit the Milky Way, and HST may find that these systems are in a state of rebound, or collapse. Globulars may continually fluctuate in density, the ones we see today being perhaps only the survivors of a much larger earlier population, where some some globulars eventually came apart. Acknowledgments: The editor wishes to thank: Philip James U.of Toledo Steve Lee U. of Colorado Holland Ford STScI F. Duccio Macchetto ESA/STScI Todd Lauer NOAO