Black Hole Caught in a Stellar Homicide

This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Some of the gas also is being ejected at high speeds into space.

Using observations from telescopes in space and on the ground, astronomers gathered the most direct evidence yet for this violent process: a supermassive black hole shredding a star that wandered too close. NASA's orbiting Galaxy Evolution Explorer (GALEX) and the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii were used to help to identify the stellar remains.

A flare in ultraviolet and optical light revealed gas falling into the black hole as well as helium-rich gas that was expelled from the system. When the star is torn apart, some of the material falls into the black hole, while the rest is ejected at high speeds. The flare and its properties provide a signature of this scenario and give unprecedented details about the stellar victim.

To completely rule out the possibility of an active nucleus flaring up in the galaxy instead of a star being torn apart, the team used NASA's Chandra X-ray Observatory to study the hot gas. Chandra showed that the characteristics of the gas didn't match those from an active galactic nucleus.

The galaxy where the supermassive black hole ripped apart the passing star in known as PS1-10jh and is located about 2.7 billion light years from Earth. Astronomers estimate the black hole in PS1-10jh has a mass of several million suns, which is comparable to the supermassive black hole in our own Milky Way galaxy.

Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)

NASA LENDS GALAXY EVOLUTION EXPLORER TO CALTECH

J.D. Harrington

Headquarters, Washington                          

202-358-5241

j.d.harrington@nasa.gov

 

Lawren Markle

California Institute of Technology, Pasadena, Calif.

626-395-3226

lmarkle@caltech.edu

WASHINGTON -- NASA is lending the Galaxy Evolution Explorer (GALEX) to the California Institute of Technology (Caltech) in Pasadena, where the spacecraft will continue its exploration of the cosmos. In a first-of-a-kind move for NASA, a Space Act Agreement was signed May 14 so the university soon can resume spacecraft operations and data management for the mission using private funds.

"NASA sees this as an opportunity to allow the public to continue reaping the benefits from this space asset that NASA developed using federal funding," said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington. "This is an excellent example of a public/private partnership that will help further astronomy in the United States."

The Galaxy Evolution Explorer spent about nine years as a NASA mission, probing the sky with its sharp ultraviolet eyes and cataloguing hundreds of millions of galaxies spanning 10 billion years of cosmic time.

"This mission was full of surprises, and now more surprises are sure to come," said Chris Martin, who will remain the mission's principal investigator at Caltech. "It already has scanned a large fraction of the sky, improving our understanding of how galaxies grow and evolve. The astronomy community will continue those studies, in addition to spending more time on stars closer to home in our own galaxy."

The spacecraft was placed in standby mode on Feb. 7 of this year. Soon, Caltech will begin to manage and operate the satellite, working with several international research groups to continue ultraviolet studies of the universe. Projects include cataloguing more galaxies across the entire sky; watching how stars and galaxies change over time; and making deep observations of the stars being surveyed for orbiting planets by NASA's Kepler mission. Data will continue to be made available to the public.

"We're thrilled that the mission will continue on its path of discovery," said Kerry Erickson, the mission's project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "The Galaxy Evolution Explorer is like the 'little engine that could,' forging ahead into unexplored territory."

During its time at NASA, the Galaxy Evolution Explorer made many discoveries involving various types of objects that light up our sky with ultraviolet light. Perhaps the most surprising of these was the discovery of a gargantuan comet-like tail behind a speeding star called Mira. Other finds included catching black holes "red-handed" as they munch away on stars, spying giant rings of new stars around old, presumed dead galaxies, and independently confirming the nature of dark energy.

For astronomers, the most profound shift in their understanding of galaxy evolution came from the mission's findings about a "missing link" population of galaxies. These missing members helped explain how the two major types of galaxies in our universe -- the "red and dead" ellipticals and the blue spirals -- transition from one type to another.

"We were able to trace the life of a galaxy," Martin said. "With the Galaxy Evolution Explorer's ultraviolet detectors, we were able to isolate the small amounts of star formation that are the signatures of galaxies undergoing an evolutionary change. We found that galaxies don't have a single personality, but may change types many times over their lifetime."

The mission also captured a dazzling collection of snapshots, showing everything from ghostly nebulas to a spiral galaxy with huge, spidery arms. A slideshow showing some of the top images can be seen here http://www.nasa.gov/mission_pages/galex/gallery-index.html.

Under the new agreement, NASA maintains ownership and liability for the Galaxy Evolution Explorer spacecraft. When Caltech completes science activities, it will decommission the spacecraft for NASA. The mission's batteries and solar panels have an expected lifetime of 12 years or more, and the spacecraft will remain in orbit for at least 66 years, after which it will burn-up upon re-entry into Earth's atmosphere. The agreement can be renegotiated when it expires in three years.

Orbital Sciences Corporation in Dulles, Va., which built the spacecraft, will continue performing flight control functions for Caltech associated with monitoring and commanding GALEX and participating in mission planning. Universal Space Network will continue providing the ground stations for communicating with the spacecraft.

For graphics and additional information about the Galaxy Evolution Explorer, visit http://www.nasa.gov/galex.

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Black Hole Caught Red-Handed in a Stellar Homicide

J.D. Harrington

Headquarters, Washington

202-358-0321

j.d.harrington@nasa.gov

WASHINGTON -- Astronomers have gathered the most direct evidence yet of a supermassive black hole shredding a star that wandered too close. NASA's Galaxy Evolution Explorer, a space-based observatory, and the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii, were among the first to help identify the stellar remains.

Supermassive black holes, weighing millions to billions times more than the sun, lurk in the centers of most galaxies. These hefty monsters lay quietly until an unsuspecting victim, such as a star, wanders close enough to get ripped apart by their powerful gravitational clutches.

Astronomers have spotted these stellar homicides before, but this is the first time they identified the victim. Using several ground- and space-based telescopes, a team of astronomers led by Suvi Gezari of the Johns Hopkins University in Baltimore identified the victim as a star rich in helium gas. The star resides in a galaxy 2.7 billion light-years away. The team's results will appear in today's online edition of the journal Nature.

"When the star is ripped apart by the gravitational forces of the black hole, some part of the star's remains falls into the black hole while the rest is ejected at high speeds," Gezari said. "We are seeing the glow from the stellar gas falling into the black hole over time. We're also witnessing the spectral signature of the ejected gas, which we find to be mostly helium. It is like we are gathering evidence from a crime scene. Because there is very little hydrogen and mostly helium in the gas, we detect from the carnage that the slaughtered star had to have been the helium-rich core of a stripped star."

This observation yields insights about the harsh environment around black holes and the types of stars swirling around them. It is not the first time the unlucky star had a brush with the behemoth black hole.

The team believes the star's hydrogen-filled envelope surrounding the core was lifted off a long time ago by the same black hole. The star may have been near the end of its life. After consuming most of its hydrogen fuel, it had probably ballooned in size, becoming a red giant. Astronomers think the bloated star was looping around the black hole in a highly elliptical orbit, similar to a comet's elongated orbit around the sun. On one of its close approaches, the star was stripped of its puffed-up atmosphere by the black hole's powerful gravity. The stellar remains continued its journey around the center, until it ventured even closer to the black hole to face its ultimate demise.

Astronomers predict stripped stars circle the central black hole of our Milky Way galaxy. These close encounters are rare, occurring roughly every 100,000 years. To find this event, Gezari's team monitored hundreds of thousands of galaxies in ultraviolet light with the Galaxy Evolution Explorer, and in visible light with Pan-STARRS1. Pan-STARRS, short for Panoramic Survey Telescope and Rapid Response System, scans the entire night sky for all kinds of transient phenomena, including supernovae.

The team was looking for a bright flare in ultraviolet light from the nucleus of a galaxy with a previously dormant black hole. Both telescopes spotted one in June 2010. Astronomers continued to monitor the flare as it reached peak brightness a month later and slowly faded during the next 12 months. The brightening event was similar to the explosive energy unleashed by a supernova, but the rise to the peak was much slower, taking nearly one and a half months.

"The longer the event lasted, the more excited we got, because we realized this is either a very unusual supernova or an entirely different type of event, such as a star being ripped apart by a black hole," said team member Armin Rest of the Space Telescope Science Institute in Baltimore.

By measuring the increase in brightness, the astronomers calculated the black hole's mass to be several million suns, which is comparable to the size of our Milky Way's black hole.

Spectroscopic observations with the Multiple Meter Telescope Observatory located on Mount Hopkins in Arizona showed the black hole was swallowing lots of helium. Spectroscopy divides light into its rainbow colors, which yields an object's characteristics, such as its temperature and gaseous makeup.

To completely rule out the possibility of an active nucleus flaring up in the galaxy, the team used NASA's Chandra X-ray Observatory to study the hot gas. Chandra showed that the characteristics of the gas didn't match those from an active galactic nucleus.

For images, video and more information about this study, visit http://hubblesite.org/news/2012/18.

For graphics and information about the Galaxy Evolution Explorer, visit http://www.nasa.gov/galex and http://www.galex.caltech.edu.

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Filigree and Shadow

Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. The Cygnus Loop extends more than three times the size of the full moon in the night sky, and is tucked next to one of the 'swan’s wings' in the constellation of Cygnus.

The filaments of gas and dust visible here in ultraviolet light were heated by the shockwave from the supernova, which is still spreading outward from the original explosion. The original supernova would have been bright enough to be seen clearly from Earth with the naked eye.

Image Credit: NASA/JPL-Caltech

Planets Under a Red Sun

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This artist's concept illustrates a young, red dwarf star surrounded by three planets. Such stars are dimmer and smaller than yellow stars like our sun, which makes them ideal targets for astronomers wishing to take images of planets outside our solar system, called exoplanets. NASA's Galaxy Evolution Explorer is helping to identify young, red dwarf stars that are close to us by detecting their ultraviolet light (stars give off a lot of ultraviolet light in their youth).

 
Image Credit: NASA/JPL-Caltech

A Cosmic Exclamation Point

VV 340, also known as Arp 302, provides a textbook example of colliding galaxies seen in the early stages of their interaction. The edge-on galaxy near the top of the image is VV 340 North and the face-on galaxy at the bottom of the image is VV 340 South. Millions of years later these two spirals will merge -- much like the Milky Way and Andromeda will likely do billions of years from now. Data from NASA's Chandra X-ray Observatory (purple) are shown here along with optical data from the Hubble Space Telescope (red, green, blue). VV 340 is located about 450 million light years from Earth.

Because it is bright in infrared light, VV 340 is classified as a Luminous Infrared Galaxy (LIRG). These observations are part of the Great Observatories All-Sky LIRG Survey (GOALS) combining data from Chandra, Hubble, NASA's Spitzer Space Telescope and Galaxy Evolution Explorer (GALEX) and ground-based telescopes. The survey includes over two hundred LIRGs in the local Universe. A chief motivation of this study is to understand why LIRGs emit so much infrared radiation. These galaxies generate energy at a rate this is tens to hundreds of times larger than that emitted by a typical galaxy. An actively growing supermassive black hole or an intense burst of star formation is often invoked as the most likely source of the energy.

Work on the full GOALS survey is ongoing, but preliminary analysis of data for VV 340 provides a good demonstration of the power of observing with multiple observatories. The Chandra data show that the center of VV 340 North likely contains a rapidly growing supermassive black hole that is heavily obscured by dust and gas. The infared emission of the galaxy pair, as observed by Spitzer, is dominated by VV 340 North, and also provides evidence for a growing supermassive black hole. However, only a small fraction of the infrared emission is generated by this black hole.

By contrast most of the ultraviolet and short wavelength optical emission in the galaxy pair -- as observed by GALEX and HST -- comes from VV 340 South. This shows that VV 340 South contains a much higher level of star formation. (The Spitzer and GALEX images are not shown here because they strongly overlap with the optical and X-ray images, but they are shown in a separate composite image.) VV 340 appears to be an excellent example of a pair of interacting galaxies evolving at different rates.

These results on VV 340 were published in the June 2009 issue of the Publications of the Astronomical Society of the Pacific. The lead author was Lee Armus from the Spitzer Science Center in Pasadena, CA.

Image credit: X-ray NASA/CXC/IfA/D.Sanders et al; Optical NASA/STScI/NRAO/A.Evans et all