Co and H₂ in Galaxies

Abstract

Matter in the universe consists mainly of hydrogen (90 per cent) and helium (10 per cent) to which is added a very small amount of heavier elements, mostly carbon, nitrogen and oxygen. In galaxies the major constituent, hydrogen, is found in a variety of forms:

• a. ionized atomic (HII) with typical electron temperatures T_e = 10⁴ K and typical densities n_e = 10 - 100 cm⁻³, observed by recombination line emission at visual, near infrared and radio wavelengths. HII represents a small fraction of all hydrogen because it needs relatively rare OB stars for its excitation.

• b. neutral atomic (HI) with a typical spin temperature T_s = 100 K and a typical density n_H = 1 – 10 cm⁻³, observed by its hyperfine structure radio line at 21.1 cm. HI is pervasive in galaxies; it usually extends beyond the optical boundaries of galaxies. The HI content of galaxies is generally related to Hubble type in the sense that the fraction of total mass represented by HI increases from early type galaxies (very low in E, SO galaxies) to later type galaxies (high in Sc, Ir galaxies). Here it should be noted that the actual nature of the atomic (neutral and ionized) is more complicated than sketched here: see a recent review by Kulkarni and Heiles (1987).

• c. neutral molecular (H₂) with typical temperatures T_k = 10 – 30 K and a typical density n(H2) = 10³ cm⁻³. H₂ is pervasive in galaxies, but details of its correlation with galaxy type and its distribution in galaxies is still uncertain due to observational difficulties as will be discussed below.

• d. part of molecular compounds (e.g. OH, CH, HCN, H₂CO, NH₃ etc). Only a minute fraction of all hydrogen is locked up in these compounds which are usually closely associated with H₂ concentrations.