THE RELATIONSHIP BETWEEN GAS, STARS, AND STAR-FORMATION IN IRREGULAR GALAXIES - A TEST OF SIMPLE-MODELS

Irregular galaxies are a unique test of models for the physical laws r egulating star formation because of their lack of spiral density waves and rotational shear. Here we explore various instability models for the onset of star formation in irregular galaxies. If the gas is unsta ble, clouds and eventually stars can form, and so these models should predict where star formation occurs. Critical gas densities were calcu lated for gravitational instabilities in two models, one with a thin, pure-gas disk (Sigma(c)) and another with a thick disk composed of gas and a starlike fluid (Sigma(c,2f)). We also calculated the stability properties of three-dimensional systems including dark matter, conside red the thermal state of the gas, and used a modified threshold column density written in terms of the local rate of shear instead of the ep icyclic frequency. The model predictions were compared to the azimutha lly averaged present-day star formation activity traced by the H alpha surface brightness and to the 1 Gyr integrated star formation activit y represented by the stellar surface brightness. We find that the rati o of the observed gas density to the critical gas density, Sigma(g)/Si gma(c), is lower by a factor of similar to 2 in most of the Im galaxie s than it is in spiral galaxies, both at the intermediate radii where Sigma(g)/Sigma(c) is highest and at the outer radii where star formati on ends. We also find that although star formation in irregulars usual ly occurs at intermediate radii where Sigma(g)/Sigma(c) is highest, th is activity often ends before Sigma(g)/Sigma(c) drops significantly in the outer regions, and it remains high in the inner regions where Sig ma(g)/Sigma(c) is often low. There are also no correlations between th e peak, average, or edge values of Sigma(g)/Sigma(c) and the integrate d star formation rates in irregulars. These results suggest that Sigma (g)/Sigma(c) does not trace star formation with the same detail in irr egular galaxies as it appears to trace it in giant spiral galaxies. Th e low value of a also implies that either the gas in irregulars is mor e stable than it is in spirals, or Sigma(c) is not a good threshold fo r star-forming instabilities. Dark matter in the disks of irregulars m akes the gas more unstable, but stars do the same for the disks of spi rals, which leaves the ratio of the two a-values about the same. Moreo ver, the instability parameter with dark matter still does not follow the star formation activity in irregulars. The thermal model suggests that irregulars have difficulty in sustaining a cool, dense gas phase, and it also fails to predict where star formation occurs. An alternat ive model in which cloud formation involves a competition between self -gravity and shear, rather than an instability in the usual sense, is more successful in defining the threshold for star formation, but it d oes not predict where star formation ends either. The failure of these models suggests that processes other than spontaneous instabilities a re important for star formation in irregular galaxies. The role of Sig ma(g)/Sigma(c) in spiral galaxies is also questioned. The observed sen sitivity of the star formation rate to Sigma(g)/Sigma(c) may be strong ly dependent on instabilities specific to spiral arms and not on gener al instabilities of the type for which Sigma(g)/Sigma(c) was originall y derived. In that case, large-scale star formation may end in the out er disks of spirals because the stellar density waves end there, at th e outer Lindblad resonance. The only azimuthally averaged quantity tha t correlates with the current star formation activity in irregulars is the stellar surface density. A causal connection is possible if stell ar energy input to the interstellar medium acts as a feedback process to star formation. If this process played a key role in initiating sta r formation in irregulars from the beginning, then it could explain wh y irregular galaxies began their evolution slowly compared to larger d isk systems with spiral arms.