Cosmological Evolution of Dwarf Galaxies: The Influence of Star Formation and the Multiphase Interstellar Medium

Abstract

A model is developed to explain the cosmological evolution of dwarf galaxies. The population of small galaxies is found to evolve rapidly for z < 1, which provides a natural explanation for the evolution observed in the galaxy luminosity function. A tail is found in the redshift distribution of the faint blue excess that can extend to a redshift of 2. The star formation history is followed in detail for these objects. Constraints on the metallicity are identified for which stars are formed with much higher efficiency in a multiphase interstellar medium than in massive galaxies. Blue dwarf galaxies at the current epoch are identified with this starburst mode. The collapse of 1 and 2 σ perturbations of the initial density fluctuation spectrum is followed using the extended standard hierarchical clustering formalism. The collapse of these perturbations is normally associated with the formation of dwarf galaxies. These objects have shallow gravitational potential wells, and their evolution strongly depends upon the cooling time of the gas. The latter is determined by the ionization and chemical equilibrium of the gas in the presence of the intergalactic and local stellar radiation fields. The latter generally dominates and creates a feedback mechanism that regulates the evolutionary timescale. To improve upon previous models, essential new astrophysical ingredients are incorporated, such as a more detailed description of the physical processes regulating the multiphase structure of the interstellar medium in dwarf galaxies and the effects of evolution in the galaxy's metallicity on the formation of stars in molecular clouds. It is found that for a low star formation rate of 0.1 M☉ yr-1, the cooling time of interstellar gas is longer than the local Hubble time until z ~ 1. At this epoch, a two-phase medium makes the dwarf interstellar medium less fragile against supernova explosions, and the volume filling factor of the hot phase (107 K) becomes of order unity. The resulting X-ray luminosity is consistent with observations of nearby dwarf galaxies if exchange processes between the hot and cold phases of the interstellar medium (i.e., evaporation and ablation of clouds due the supernova blast wave) are included. The tenuous halo gas has a temperature in excess of the escape velocity and exhibits very high ionization stages. The dark matter halo potential (M/L ~ 102) influences the loss rate of metals and the star formation history. It is found that if the metallicity is between 0.01 and 0.1 Z☉ then a period of rapid star formation, ~1-3 M☉ yr-1, can be excited for z ~ 1. The physical reason for this starburst mode in our model is that the free-fall time exceeds the ambipolar diffusion timescale for clouds. Consequently, molecular clouds are not magnetically supported and star formation can proceed much more efficiently compared with more massive and metal-rich galaxies. Our results are consistent with the redshift distribution of dwarf galaxies observed in the Hubble Deep Field. In particular, a rapidly evolving dwarf population can account for the excess number counts of faint galaxies: the blue excess. Predictions are made for the color evolution of the old stellar population in these systems, observable with the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on the Hubble Space Telescope in the near future.