Star Formation and Galactic Structure

Abstract

We recall the results of previous works which support the idea that, in spiral and irregular I galaxies, the star formation rate is governed by a law of the type: dp_* /dt ∼ K (p_g ⁿ (Schmidt 1959), with n∼2, p_g and p* being the volume density of gas and young objects, as seems established for our Galaxy. The mean rates of star formation in 15 galaxies of the Local Group (Lequeux 1978), are examined. Applicability of a universal Schmidt’s law (n∼2, K depending only on the type of young objects considered) to spirals and irregulars, cannot be ruled out within the uncertainties, the agreement being more easily obtained if the relative amount of molecular gas in late type, hydrogen-rich galaxies is significantly lower than in the solar neighbourhood in accordance with negative detections of molecules. However a decrease of K with increasing M_HI / M_tot Is suggested. Among the possible explanations are the absence of spiral compression and the deficiency in heavy elements. This factors, together with a lower volume gas density, could also be responsible for a slight enrichment of the Initial Mass Function in massive stars with respect to the local IMF. A similar Interpretation could account for the possible differences in star formation efficiency and Initial Mass Function between the solar neighbourhood and the galactic molecular 5 kpc ring.

“Let me say at first … how struck I am by the delicate symbiosis that exists between the stars and the interstellar medium, how each is nourished by the other, and how the Galaxy as we know it is entirely a consequence of that balance and interplay”

G.H. Herbig, IAU symposium no 75 on star formation

“You crunch gas, and you get plenty of light”

A. Toomre (discussion following the paper by R.B. Larsson, Yale Conference, 1977).

The purpose of this paper is not to review the different topics related to star formation (S.F.) and galactic evolution, which have been treated in recent meetings (among them: Geneva, Yale, or Torun). I rather intend to examine a few points connecting the gas density to S.F. in spirals and irregular I galaxies (Early-type galaxies are treated by C.A. Norman in the present meeting). The main development will concern the relative importance of gas density, spiral structure, and heavy element abundance as regards star formation in 15 galaxies of the Local Group as compared to the solar neighbourhood. A few words will be said about the “intergalactic clouds”. Finally, some problems concerning the central regions and the 5 kpc molecular ring of our Galaxy will be discussed.