Infrared Space Observatory Measurements of a [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies

Details Infrared Space Observatory Measurements of a [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Infrared Space Observatory Measurements of a [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...504l..11l&link_type=abstract

Astrophysical Journal Letters v.504, p.L11

Astronomy and Astrophysics

Astronomy

78

Galaxies: Active, Galaxies: Ism, Galaxies: Starburst, Infrared: Galaxies, Infrared: Ism: Lines And Bands, Ism: Atoms

Scientific paper

We report measurements of the [C II] 157.74 mu m fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) ( L_{{IR}}>1012 Lsolar) with the Long Wavelength Spectrometer on the Infrared Space Observatory. The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons ( h nu >=11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 mu m line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10-20 W cm-2) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n ( >>3x103 cm-3) or with a high ratio of incident UV flux G0 to n ( G0/n>~10 cm3), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high-z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.

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