The Spatially Resolved Star Formation Law In Nearby Galaxies

This thesis presents a comprehensive analysis of the relationship between gas and star formation (SF) at sub-kpc resolution in a large sample of nearby galaxies. The analysis is based on recent very high quality radio, infrared and UV data. Key to this thesis are new, sensitive and high resolution atomic gas maps from "The HI Nearby Galaxy Survey" (THINGS). A combination of these multiwavelength data are used to study the gas-SF relation across the H2–dominated centers of the spirals as well as their HI–dominated outskirts and HI–rich late type/dwarf galaxies. For the spiral galaxies, a Schmidt-type power law with index N = 1.0 ± 0.2 relates star formation rate and H2. This implies that H2 forms stars at a constant efficiency, i.e. star formation rate per unit gas, in spirals. Most galaxies show little or no correlation between the star formation rate and HI. The star formation efficiency is observed to decrease with increasing radius in the spirals, while the dwarf galaxies in our sample display star formation efficiencies similar to those found in the outer optical disks of the spirals. There is a sharp saturation of HI at a certain column density in both the spiral and dwarf galaxies. In the case of spirals, gas in excess of this limit is observed to be molecular. The decreasing star formation efficiency is observed to extend smoothly from the optical disk into the outskirts of galaxies. In this outer regime, SF is observed to decline 4 times more quickly than HI. As a result, the time that it takes SF to consume the gas reservoir is ~10 times longer in the outer disks, corresponding to about a Hubble time, than in the centers of spiral galaxies. For very low HI columns, which are typically found at large radii, the depletion time is even longer, suggesting that SF at such low HI columns may be suppressed by inhospitable conditions in the interstellar medium