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Herschel seeks out
galaxy ingredients

Posted: November 30, 2009

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ESA's Herschel Space Observatory has acquired new observations of galaxies that provide the best measurements yet of the chemical ingredients involved in the birth and death of stars.

SPIRE spectrum of Arp 220 showing many emission and absorption features of carbon monoxide and water. The inset is an optical image of Arp 220 made with the Hubble Space Telescope.

The data was collected using spectrometers on board all three Herschel instruments, including the UK-led SPIRE instrument, which observes in sub-millimetre wavelengths (194-672 microns).

“Some trial observations have been made during initial testing of the spectrometer, and it is clear that the data are of excellent quality, and even these initial results are very exciting scientifically, especially our ability to trace the presence of water throughout the Universe,” says SPIRE principal investigator Matt Griffin. “The spectrometer was technically very challenging to build, and the whole team is delighted that it works so well.”

Part of the SPIRE spectrum of VY Canis Majoris (VY CMa), a giant star near the end of its life. The inset is a SPIRE camera image of VY CMa, in which it appears as a bright point-source near the edge of a large extended cloud.

One of Herschel's targets was VY Canis Major, a giant star near the end of its life, which is ejecting huge amounts of gas and dust into interstellar space, including the raw ingredients for future generations of star, such as carbon, oxygen and nitrogen. More than two hundred spectral features were identified in this system, with prominent features resulting from carbon monoxide and water which suggests that the star is surrounded by hot steam.

“The exquisite sensitivity and quality of these early data reveal spectacular spectroscopic signatures that show the diversity and complexity of the birth processes common to the formation of star and planets,” says Glenn White of the Open University and STFC’s Rutherford Appleton Laboratory. “Herschel is going to help us trace the evolution and life of stars, to map the chemistry in our galactic neighborhood, and allow us to detect water and complex molecules in distant galaxies.”

Spectrum of one position on the Orion Bar. The inset shows a near infrared picture from NASA’s Spitzer Space Telescope. The SPIRE spectrum has many features from CO, appearing as the dominating narrow lines, seen here for the first time together in a single spectrum.

Part of the Orion Nebula, the Orion Bar – where gas on the edge of the nebula is partly ionized by intense radiation from nearby hot young stars – provided another target for Herschel to feast on. The spectrum revealed many features of carbon monoxide, seen in the new data set for the first time together in a single spectrum. The brightness of the spectral features will allow astronomers to estimate the temperature and density of interstellar gas. The spectrum also shows the first detection of an emission feature from the molecular ion methylidynium (CH+), a key building block for larger carbon-bearing molecules. This and similar regions are large, and the SPIRE spectrometer’s will be extremely powerful in characterizing how the gas properties vary within such sources.

Star-forming galaxy M82 is 12 million light years away. The inset shows a three-colour composite of the two galaxies made with the SPIRE camera, showing material being stripped from M81 by the gravitational interaction with M82.

Spectra of Arp 220, the result of colliding spiral galaxies located 250 million light years away, also revealed emission features of carbon monoxide and water, while data for nearby galaxy M82 showed strong emission lines from CO over the whole wavelength range, as well as emission lines from atomic carbon and ionized nitrogen. M82 is a very active star formation region and is part of an interacting group of galaxies including the large spiral M81. Both galaxies are important templates for learning how galaxies form and evolve.

“The unprecedented spectral range and the wealth of detail revealed by the SPIRE spectrometer, in a hitherto almost unexplored region of the spectrum, promises to revolutionize our understanding of the formation of molecules and dust particles during the final stages of the lives of stars,” says Mike Barlow of University College London. “These dust particles go on to play a crucial role in the formation of new stars and provide the raw material for the planetesimals and planets that form around them.”

The SPIRE FTS observations were carried out as part of the performance verification of the observatory.

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