Posted: 05 November, 2008
European astronomers using the James Clark Maxwell Telescope (JCMT) have gained important information on what are known as Ultraluminous Infrared Galaxies (ULIRGs), galaxies with a huge energy output but which are obscured by their massive dust and gas clouds.
Astronomers believe that this class of galaxy was much more common in the younger Universe than it is now, with their impressive energy output attributed to extremely rapid conversion of gas into young, luminous stars and to energetic processes associated with supermassive black holes. Using the HARP receiver on the JCMT, a team of astronomers from Wales, The Netherlands and Germany directly probed the physical conditions in the active inner regions of a number of ULIRGs by penetrating the thick dusty veil surrounding the galaxies and observing the submillimetre radiation.
Spectrum of hydrogen cyanide in a ULIRG obtained with the JCMT and its HARP receiver. The background image shows UGC5101 as observed with the ACS on board the Hubble Space Telescope and shows dust clouds obscuring the most luminous parts of the galaxy which can be seen as a red-brown band. Image: NASA, ESA, the Hubble Heritage STScI/AURA-ESA/Hubble Collaboration, and A. Evans, University of Virginia, Charlottesville/NRAO/Stony Brook University.
"The submillimetre radiation observed by the JCMT can penetrate the dust shroud obscuring the nuclear regions of the ULIRGs, but the spectral lines emitted from these regions are still very faint," says Dr Papadopoulos from Bonn University. "Therefore, we had to use the JCMT and its sensitive HARP receiver for up to 12 hours under very good atmospheric conditions, to detect just a single line in a single galaxy."
Among the molecular fingerprints that the team has observed are spectral lines of warm and dense carbon monoxide (CO) and of the formyl ion (HCO+). However, the most important spectral line detected is hydrogen cyanide (HCN), which originates from warm, dense and highly toxic hydrogen cyanide gas in the most active regions of the ULIRGs. These are the first spectra of this type from a substantial set of ULIRGs, and are surprisingly difficult to detect in many of these extreme objects. When interpreted together with the rest of the data, it becomes obvious that this spectral line probes the most extremely dense gas, the very immediate fuel of the massive star formation in these objects.
"Unlike other spectral lines which probe more remote gaseous regions in these galaxies which may not be actively forming stars, the hydrogen cyanide intensity changes dramatically from galaxy to galaxy," says Paul van der Werf of Leiden University in The Netherlands. "This depends on, and reveals, the intense gravitational tides and their effects on the densest of the gas phases in the centres of the ULIRGs."
The team is continuing its study of ULIRGs with the JCMT by observing the hot dense gas associated with the formation of young stars in these galactic powerhouses. "Even future satellites will not be able to supply us with all the information we need to probe the conditions within these galaxies: the JCMT with its large collecting area provides essential pieces in the puzzle," adds Kate Isaak of Cardiff University.