Gas, dust and molecules in the Galaxy

Abstract

The relative abundances of cool neutral hydrogen, carbon monoxide and formaldehyde are studied using all the available observational data in the literature. The obtained mean valuesN _{H 1}/$N_{{\text{H}}_{\text{2}} {\text{CO}}}$,N _{H 1}/N _CO,N _CO/$N_{{\text{H}}_{\text{2}} {\text{CO}}}$ are approximately constant in the dark clouds of the solar neighbourhood and in the distant molecular clouds.

The observed correlationsN _CO,A _v and$N_{{\text{H}}_{\text{2}} {\text{CO}}}$,A _v show that formaldehyde can also be used as an indicator of molecular hydrogen. The ratioN H₁/A _v depends on densities and decays considerably in the ranges of visual absorptions in which the molecules become detectable (A _v ≳2 mg); an average of$N_{{\text{H}}_{\text{2}} }$/N _{H 1}∼10 is calculated for the dense dark clouds.

Indications of systematic temperature gradiens ∂T/∂A _v are found for formaldehyde and neutral hydrogen inside the dark clouds, and qualitative comparisons are made with theoretical quantum mechanics calculations.

The observed carbon monoxide and formaldehyde abundances, the free electron layer in the Galaxy, the distribution of neutral hydrogen in different states are only compatible if an ionization rate of ξ≥10⁻¹⁶ is accepted, provided presumably by 2 MeV protons of cosmic radiation.

Three main states for neutral hydrogen and dust are identified from different kinds of observational data (21 cm line in emission, absorption in galactic radio sources and self-absorption in the hot gas background): (1) a homogeneous intercloud stratum of tenuous gas and dust with a galactic halfwidth of 350 pc and mean parametersn _H=0.2 atom cm⁻³, spin temperatureT _s ≈10000 K andn _d ≈0.3 mg kpc⁻¹; (2) cool gas and dust concentrated in spiral features with a galactic half-width of less than 100 pc, probably forming clouds with diffuse and indefinite limits, with mean parametersn _H≈2 atom cm⁻³,T _s <1100 K (probable average,T _s =135 K) andn _d ≈3 mg kpc⁻¹; (3) dense gas and dust clouds with a mean diameter of 7 pc and mean parametersn _H≈700 atom cm⁻³ (90% in a molecular state),T _s ≤63 K andn _d ≈1 mg pc⁻¹ on which molecules as CO and H₂CO are formed.

The application of the Jeans criteria for gravitational instability shows that the dense clouds are gravitationally bound while the gas in the intermediate state (2) can be protected against collapse by the total internal energy in the medium increasing due to cosmic rays and the magnetic field in the Galaxy.

The observed velocity halfwidths and galacticZ-halfwidths in states (1) and (2) are compatible with a total mass density in the galactic layer of 90M _⊙ pc⁻² (gas plus stars) according to the barometric equation.

The relative abundancesN _{H 1}/N _CO, calculated from C¹²O and C¹³O data and comparisons with studies in the 21 cm emission line, show that the antenna temperatureT ⁺_A in the 2.6 mm line of C¹²O is a good indicator of the cool gas densities in the Galaxy. The possible application of this for studies in galactic structure is discussed and hypothetical distributions of carbon monoxide in the zones outside the galactic planeB=0° are presented.

From a synthesis based on the results obtained, a cycle is postulated for the neutral hydrogen in the Galaxy: condensation and cooling of gas → molecular formation → gravitational collapse and star formation → gas dissipation and heating by cosmic rays and UV radiation.