Seeing between the lines: first SPIRE spectra
Friday 27 November 2009
Fig 1: Part of the SPIRE spectrum of VY Canis Majoris. Click here for larger version.
The European Space Agency has released spectacular new results from the Herschel Space Observatory, including the SPIRE instrument. SPIRE was designed and built by an international team led by scientists from Cardiff University School of Physics and Astronomy. Spectrometers on board all three Hershel instruments have been used to analyse the light from objects inside our galaxy and from other galaxies, producing the best measurements yet of atoms and molecules involved in the birth and death of stars.
The SPIRE Fourier Transform Spectrometer covers the submillimetre wavelength range (194-672 microns), and provides a complete survey of the source spectrum over that whole wavelength range in a single observation, something that has never been possible with previous submillimetre instruments. This will be invaluable to astronomers in determining the composition, temperature, density and mass of interstellar material in nearby galaxies and in star-forming clouds in our own galaxy. At the same time as measuring the intensities of narrow spectral features from gas atoms and molecules, the SPIRE spectrometer also accurately measures the broadband emission from dust. With its multi-pixel detector arrays, it can also produce spectral images, allowing astronomers to measure the spatial variation in the interstellar material.
Professor Matt Griffin of Cardiff University, who is the SPIRE Principal Investigator, said: "Some trial observations have been made during initial testing of the spectrometer, and it is clear that the data are of excellent quality -- even these initial results are very exciting scientifically. With this instrument astronomers will be able to examine the composition and properties of interstellar matter in a way never before possible. The spectrometer was technically very challenging to build, and the whole team is delighted that it works so well".
Fig 2: A spectrum of one position on the Orion Bar part of the Orion nebula in which the gas on the edge of the nebula is partly ionised by intense radiation. Click here for larger version.
Professor Derek Ward-Thompson, also of Cardiff University, and an expert in the field of star formation, said: "These results are spectacular. We have never had access to spectra at these wavelengths before. They provide a fascinating insight into the interstellar medium, which will keep scientists busy for many years to come."
Figure 1 above shows part of the SPIRE spectrum of VY Canis Majoris (VY CMa), a giant star near the end of its life, which is ejecting huge amounts of gas and dust into interstellar space, including elements such as carbon, oxygen and nitrogen (which form the raw material for future planets, and eventually 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. The spectrum is amazingly rich, with prominent features from carbon monoxide (CO) and water (H2O). More than 200 other spectral features have also been identified, many due to water, showing that the star is surrounded by large quantities of hot steam. Observations like these will help to establish a detailed picture of the mass loss from stars and the complex chemistry occurring in their extended envelopes.
Fig 3: The spectrum of Messier 82 (M82), a nearby galaxy (only 12 million light years away) with very active star formation. Click here for larger version.
Figure 2 is a spectrum of one position on the Orion Bar, part of the Orion nebula in which the gas on the edge of the nebula is partly ionised by intense radiation from nearby hot young stars. 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. These mean that the entire spectrum is observed at the same time and calibrated together. They 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 will be extremely powerful in characterising how the gas properties vary within such sources.
Figure 3 shows the spectrum of Messier 82 (M82), a nearby galaxy (only 12 million light years away) with very active star formation. It is part of an interacting group of galaxies including the large spiral M81. The accompanying image (inset) is a spectacular three-colour composite picture of the two galaxies made with the SPIRE camera, showing material being stripped from M81 by the gravitational interaction with M82. The SPIRE spectrum of M82 shows strong emission lines from CO over the whole wavelength range, as well as emission lines from atomic carbon and ionized nitrogen.
Fig 4: A SPIRE spectrum of Arp 220, a galaxy 250 million light years away from Earth with very active star formation. Click here for larger version.
Figure 4 shows a SPIRE spectrum of Arp 220, a galaxy 250 million light years away from Earth with very active star formation triggered when to large spiral galaxies collided to produce the complex object we see today. Arp 220 is an important template for understanding even more distant galaxies and galaxy formation in the early universe. The spectrum shows many emission features of CO, and H2O features are seen both in emission and absorption. The inset is an optical image of Arp 220 made with the Hubble Space Telescope.
The SPIRE FTS observations were carried out as part of the performance verification of the observatory. The scientific rights of some of these observations are owned by Key Programme consortia: for Arp 220 and M82, the Nearby Galaxies consortium lead by C. Wilson; for VY CMa the MESS consortium led by M. Groenewegen; for the Orion Bar, the Evolution of Interstellar Dust consortium led by A. Abergel.
Further details of the new observations by SPIRE, and by the other two Herschel instruments, may be found at the ESA Herschel Science Centre web site (herschel.esac.esa.int) and on the Herschel UK website (herschel.cf.ac.uk).