1. Figure: M51, the Whirlpool Nebula, NGC5194/5195.
This image of M51 (NGC5194/5195), was made by combining three CCD frames, taken at the Kitt Peak 0.9m telescope in 1991. By using different filters in front of the monochrome detector, corresponding approximately to the primary colors red, green and blue, it is possible to recreate a true color picture. Each image was processed to correct for detector sensitivity variations and to remove incorrect regions caused by manufacturing defects and by the arrival of cosmic rays at the telescope.
Note that this image was reworked in July 2000 to provide a better looking and scientifically more accurate color balance.
This picture was made using the `drift scan' technique, in which the telescope is held fixed, not tracking against the Earth's rotation in the usual manner. As the sky passes across the detector, each row of the array is `clocked' along to the next row in step with the apparent motion of the astronomical image. This makes it possible to take a picture of an arbitrarily long strip of the sky, and specialized telescopes exist solely to take advantage of the simplicity of a fixed, non-tracking mounting. The large size of the M51 system, famous as the first clearly recognized spiral nebula, made it necessary to use the drift scan technique.
Orientation: N to the left, E down.
Note that smaller telescopes with a larger field of view can take such pictures without drift techniques, and can often approach the same quality, especially with clever processing, as this picture from the Kitt Peak Visitor Center's Advanced Observing Program demonstrates.
About this object
M51 (also known as Arp~85 and VV~1) comprises the large spiral galaxy NGC5194 and its smaller, barred and more amorphous companion NGC5195. Some features are better seen in a color picture: note, for example, how NGC5195 takes on a reddish tinge due to the fact that it is behind the dust-filled arm connecting it to NGC5194. M51 was the first astronomical object in which spiral structure was discerned, by the Third Earl of Rosse in 1845. The spiral arms are perhaps the most perfect `textbook' example in any nearby galaxy, and their very perfection points to the presence of a long-lasting confining mechanism. This may be provided by the tidal pull of NGC5195, whose gravitational effects can generate the necessary spiral density waves. This pattern also shows up in radio emission, suggesting that the magnetic fields in the Whirlpool are also compressed by the density wave. The innermost core of NGC5194 contains a bright ultraviolet source, as well as one of the brightest known compact radio sources. Although smaller and less massive than our own Galaxy, M51 is considerably brighter, due to recent star formation and the resultant dominance by young, hot, bright stars of types O and B.
Location: 13 27 50 +47 29 (1950.0), constellation of Canes Venatici.
Distance: approximately 31 million light-years.
Size: over 65000 light-years across.
Credit: Todd Boroson/NOAO/AURA/NSF
Downloadable pictures at various resolutions (see NOAO Conditions of Use) at:
http://www.noao.edu/image_gallery/html/im0063.html
2. Figure: The Heart of the Whirlpool Galaxy with HST
New images from NASA's Hubble Space Telescope are helping researchers view in unprecedented detail the spiral arms and dust clouds of a nearby galaxy, which are the birth sites of massive and luminous stars.
The Whirlpool galaxy, M51, has been one of the most photogenic galaxies in amateur and professional astronomy. Easily photographed and viewed by smaller telescopes, this celestial beauty is studied extensively in a range of wavelengths by large ground- and space-based observatories. This Hubble composite image shows visible starlight as well as light from the emission of glowing hydrogen, which is associated with the most luminous young stars in the spiral arms.
M51, also known as NGC 5194, is having a close encounter with a nearby companion galaxy, NGC 5195, just off the upper edge of this image. The companion's gravitational pull is triggering star formation in the main galaxy, as seen in brilliant detail by numerous, luminous clusters of young and energetic stars. The bright clusters are highlighted in red by their associated emission from glowing hydrogen gas.
This Wide Field Planetary Camera 2 image enables a research group, led by Nick Scoville (Caltech), to clearly define the structure of both the cold dust clouds and the hot hydrogen and link individual clusters to their parent dust clouds. Team members include M. Polletta (U. Geneva); S. Ewald and S. Stolovy (Caltech); R. Thompson and M. Rieke (U. of Arizona).
Intricate structure is also seen for the first time in the dust clouds. Along the spiral arms, dust "spurs" are seen branching out almost perpendicular to the main spiral arms. The regularity and large number of these features suggests to astronomers that previous models of "two-arm" spiral galaxies may need to be revisited. The new images also reveal a dust disk in the nucleus, which may provide fuel for a nuclear black hole.
The team is also studying this galaxy at near-infrared wavelengths with the NICMOS instrument onboard Hubble. At these wavelengths, the dusty clouds are more transparent and the true distribution of stars is more easily seen. In addition, regions of star formation that are obscured in the optical images are newly revealed in the near-infrared images.
This image was composed by the Hubble Heritage Team from Hubble archival data of M51 and is superimposed onto ground-based data taken by Travis Rector (NOAO) at the 0.9-meter telescope at the National Science Foundation's Kitt Peak National Observatory (NOAO/AURA) in Tucson, AZ.
Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: N. Scoville (Caltech) and T. Rector (NOAO)
http://hubblesite.org/newscenter/archive/releases/2001/10/image/a/
3. Figure: The spiral galaxy M51:
Left, as seen with the Hubble Space Telescope;
Right, radio image showing location of Carbon Monoxide gas.
Credit: STScI, OVRO, IRAM
Gas Clouds in Whirlpool Galaxy Yield Important Clues Supporting Theory on Spiral Arms.
Astronomers studying gas clouds in the famous Whirlpool Galaxy have found important clues supporting a theory that seeks to explain how the spectacular spiral arms of galaxies can persist for billions of years. The astronomers applied techniques used to study similar gas clouds in our own Milky Way to those in the spiral arms of a neighbor galaxy for the first time, and their results bolster a theory first proposed in 1964.
The Whirlpool Galaxy, about 31 million light-years distant, is a beautiful spiral in the constellation Canes Venatici. Also known as M51, it is seen nearly face-on from Earth and is familiar to amateur astronomers and has been featured in countless posters, books and magazine articles.
"This galaxy made a great target for our study of spiral arms and how star formation works along them," said Eva Schinnerer, of the National Radio Astronomy Observatory in Socorro, NM. "It was ideal for us because it's one of the closest face-on spirals in the sky," she added.
Schinnerer worked with Axel Weiss of the Institute for Millimeter Radio Astronomy (IRAM) in Spain, Susanne Aalto of the Onsala Space Observatory in Sweden, and Nick Scoville of Caltech. The astronomers presented their findings to the American Astronomical Society's meeting in Denver, Colorado.
The scientists analyzed radio emission from Carbon Monoxide (CO) molecules in giant gas clouds along M51's spiral arms. Using telescopes at Caltech's Owens Valley Radio Observatory and the 30-meter radio telescope of IRAM, they were able to determine the temperatures and amounts of turbulence within the clouds. Their results provide strong support for a theory that "density waves" explain how spiral arms can persist in a galaxy without winding themselves so tightly that, in effect, they disappear.
The density-wave theory, proposed by Frank Shu and C.C. Lin in 1964, says that a galaxy's spiral pattern is a wave of higher density, or compression, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. Schinnerer and her colleagues studied a region in one of M51's spiral arms that presumably has just overtaken and passed through the density wave.
Their data indicate that gas on the trailing edge of the spiral arm, which has most recently passed through the density wave, is both warmer and more turbulent than gas in the forward edge of the arm, which would have passed through the density wave longer ago.
"This is what we would expect from the density-wave theory," Schinnerer said. "The gas that passed through the density wave earlier has had time to cool and lose the turbulence caused by the passage," she added.
"Our results show, for the first time, how the density wave operates on a cloud-cloud scale, and how it promotes and prevents star formation in spiral arms," Aalto said.
The next step, the scientists say, is to look at other spiral galaxies to see if a similar pattern is present. That will have to wait, Schinnerer said, because the radio emission from CO molecules that provides the information on temperature and turbulence is very faint.
"When the Atacama Large Millimeter Array (ALMA) comes on line, it will have the ability to extend this type of study to other galaxies. We look forward to using ALMA to test the density-wave model more thoroughly," Schinnerer said. ALMA is a millimeter-wave observatory that will use 64, 12-meter-diameter dish antennas on the Atacama Desert of northern Chile. Now under construction, ALMA will provide astronomers with an unprecedented capability to study the Universe at millimeter wavelengths.
The Whirlpool Galaxy was discovered by the French comet-hunter Charles Messier on October 13, 1773. He included it as object number 51 in his now-famous catalog of astronomical objects that, in a small telescope, might be mistaken for a comet. In 1845, the British astronomer Lord Rosse discovered the spiral structure in the galaxy. For amateur astronomers using telescopes in dark-sky locations, M51 is a showpiece object.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Credit: NRAO
http://www.nrao.edu/pr/2004/m51co/
Optical and CO image superimposed, original image
