from the shorter-wavelength studies will be too low, in general, since the dense 

 clouds, where many of the interstellar heavy elements reside, are impenetrable to 

 visible or ultraviolet light. However, these regions are transparent to longer- 

 wavelength radiation; thus smaller extinction corrections are necessary in the 

 infrared. Infrared line radiation can be used to measure the abundances in both 

 neutral and ionized regions. The lines of interest have wavelengths ranging from 

 4 to 200 jum, with the different lines being sensitive to different excitation 

 temperatures and densities. Some of the more important lines include the 

 157-jum C + line in neutral regions as well as the 01 1 1 lines at 88 and 52 pm and 

 the Nelll line at 16 jum in the ionized regions. 



The rotational transitions of many of the molecules containing biogenic ele- 

 ments lie in the microwave region of the electromagnetic spectrum. Observations 

 at these wavelengths will complete the biogenic element surveys by probing the 

 very densest and coldest molecular clouds. 



Measurements of the biogenic elements residing in the dust component of 

 external galaxies require different instruments and observing approaches than do 

 gas-phase studies. The dust itself can be seen either in absorption in the ultravio- 

 let through near-infrared or in emission at longer wavelengths. High-resolution 

 spectrometers are not advantageous in determining the dust composition, since 

 the solid-phase absorption features are broad in comparison with the gas-phase 

 features. Resolving powers of X/AX ~ 100 or less are best since they are sensi- 

 tive. The total quantity of dust in galaxies is best measured through broad-band 

 infrared photometry; telescopes with cooled optics are desirable because of their 

 high inherent sensitivity. 



The planned Earth-orbiting space observatories (Appendix B) will be of tre- 

 mendous utility in measuring biogenic element abundances in other galaxies. 

 The elimination of atmospheric effects will increase sensitivity so that objects, 

 which from the ground can be studied only in the Milky Way, will be routinely 

 observed in nearby galaxies. The sensitivity gain will also allow detailed studies 

 of even more distant objects for the first time, so that cosmological evolution 

 effects on the availability of biogenic elements can be studied. 



The three major observatory-class orbiting telescopes— HST, SI RTF, and 

 LDR-will be the most useful of all the planned astronomical spacecraft since 

 they alone have the fine angular resolution and pointing ability needed to work 

 on extragalactic objects. 



For ultraviolet, visible, and near-infrared studies, the HST will be the most 

 useful observing platform. The advertised angular resolution of better than 

 0.1 arcsec is sufficient to study individual stars in selected regions of nearby 

 galaxies such as M31. In addition, the first generation of science instruments 

 includes two spectrometers operating between 1150 and 8000 A with resolving 

 powers ranging from X/AX = 10 2 to 10 5 . This is more than adequate for visible 

 gas-phase elemental abundance studies in external galaxies. 



73 



