3.1 1 Evolution of Elemental Abundances in Galaxies 



The biogenic elements available for the formation of planets, and perhaps 

 life, in external galaxies must be measured in order to estimate the probability 

 for planetary and life formation. To have an exobiological impact, these ele- 

 ments must be present in the interstellar medium rather than locked up in stars. 

 There, they can be in either gaseous or solid form; measurements are needed of 

 both states in order to estimate the total abundances available for planetary 

 formation. Furthermore, elemental abundance gradients within a galaxy will 

 determine where planets having the "necessary" inventory of biogenic elements 

 are most likely to form. Thus, measurements should be made of galaxies of dif- 

 ferent ages and morphological types so that the production rate of biogenic 

 elements in the universe as a whole can be accurately estimated. Is it possible 

 that there are galaxies that have not yet attained the stage of nucleosynthetic 

 evolution necessary to provide the raw materials from which geologically active 

 planets can be produced? 



Observations of the biogenic elements in the gas phase can be made only 

 spectroscopically, primarily in the ultraviolet, visible, and infrared spectral 

 regions for atoms and in the infrared, submillimeter, and microwave regions for 

 molecules formed from the biogenic elements. Spectral resolutions of 

 X/AX ~ 1000 are needed to clearly separate the spectral lines from the back- 

 ground continuum. Even higher spectral resolutions allow studies of the velocity 

 distributions within the lines, reducing the spatial resolution penalty incurred by 

 studying objects as distant as external galaxies. For evaluating Earth-orbiting 

 observatories, we split the observations into separate spectral regimes. 



Ultraviolet or visible lines must be seen either by emission from, or in absorp- 

 tion against, a high-temperature gaseous environment such as a stellar photo- 

 sphere or an HI I region. Studies of heavy-element (carbon, nitrogen, oxygen, 

 etc.) abundances inside stellar atmospheres are important, since the elements can 

 be returned to the interstellar medium in the course of stellar evolution, making 

 them available for incorporation into subsequent generations of stars and 

 planets. Outside stellar atmospheres the material in interstellar space will gener- 

 ate absorption features in the ultraviolet and visible regions. These features can 

 be strong in comparison to features at longer wavelengths because the oscillator 

 strengths are greater at the higher frequencies. Thus ultraviolet/visible spectro- 

 scopic observations are very sensitive to trace amounts of biogenic elements in 

 the diffuse interstellar medium. However, as will be noted later, these observa- 

 tions must be complemented by longer-wavelength measurements of the dense 

 molecular cloud regions. Studies of emission lines from extragalactic HII regions 

 will also be useful in surveying heavy-element abundances, although the infrared 

 fine-structure line observations (see below) require less correction for extinction. 



Infrared observations of biogenic elements in the gas phase are necessary to 

 complement the shorter-wavelength observations. Abundances derived solely 



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