Infrared Spectroscopy


Galaxies


Up to 90% of the matter in galaxies cannot be seen by optical telescopes. This "invisible" material is usually either too cold to radiate enough visible light to be detected, or is hidden behind thick interstellar dust. Infrared light, however, can detect cool objects in space and can penetrate regions of thick dust. Infrared studies have shown that a possible candidate for this "missing mass", which cannot be seen in visible light, is the cold, invisible matter which makes up massive molecular clouds in the disks of galaxies.

The image to the right shows the infrared signatures of molecular hydrogen gas (H2) in the central dust lane of the edge-on spiral galaxy NGC 891. The gas temperature determined from this spectra is about 80 degrees Kelvin. In addition, the molecular hydrogen is the amount needed to account for this galaxy's missing mass.


ISO-SWS data from Valentijn and van der Werf / SRON. Optical photograph of NGC 891: Blair Savage, Chris Howk (U. Wisconsin)/N.A.Sharp (NOAO)/AURA/NSF


Infrared spectral studies of galaxies are used to learn about galactic structure and its influence on star formation. Infrared spectroscopy has helped show that one class of galaxy - the ultraluminous infrared galaxies - are powered by intense bursts of star formation. Detailed studies of the closest ultraluminous infrared galaxy, Arp 220, have shown that intense radiation from bursts of star formation heats much of the dense molecular gas in this galaxy to surprisingly high temperatures.



Infrared spectra of the galaxy Arp 220. The emission lines indicate very active star formation.


Spectral signatures providing clues as to what powers a galaxy lie in the infrared. For example, lines like triply ionized Oxygen [OIV] or four times ionized Neon [NeV] indicate the existense of a central black hole. The absence of these lines along with the presence of lines typical for starburst galaxies, such as singly ionized Neon [NeII], indicate massive star formation as the main power source for a galaxy.


ISO spectra of a galaxy (M82) powered by star formation


ISO spectra of a galaxy (Circinus) powered by a central black hole


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