Science
Main Science Goals of VLA-ANGST
Star Formation Histories:
The color-magnitude diagrams produced by the ANGST observations will allow one to directly observe the strength and spatial distribution of past star formation in these galaxies. In particular, sequence and red- and blue-helium-burning stars are used to derive the star formation history over the last 500 Myr, with a time resolution as short as 30 Myr. The star formation histories are derived in small spatial regions with a typical angular size of 6”, corresponding to about 100 parsecs at a distance of 3 Mpc. These maps are used to measure the locations and magnitudes of all identifiable star formation events.
The Distribution of Neutral Hydrogen
VLA-ANGST has a spatial resolution of ~6" and velocity resolution of 0.6-1.3km/s. We follow mostly THINGS survey (PI Walter) in terms of array configuration and sensitivity. In addition, LITTLE THINGS (PI Hunter) and SHIELD (PI Cannon), also have very similar setups and the data of all surveys can eventually be used together as a relatively smooth sample. For VLA-ANGST, we analyze the resulting maps by measuring the gas densities and velocities with respect to the locations of individual star formation events isolated using the HST imaging (see image of IC2574 below). Although the HI distribution allows only one snapshot in time and the star formation histories derived from the HST observations allow a detailed sampling of the time domain, it will be possible to reconstruct the sequences of events that led to the present day ISM structures.
Triggers of Star Formation
Star formation is thought to arise primarily in response to regions of high gas density. However, the immediate trigger of the density increase is often poorly constrained. The VLA-ANGST HI density maps coupled to the ANGST SFH maps will allow us to determine the fraction of star formation that has been triggered directly by other nearby star formation events, rather than by stochastic processes, interactions, or density waves. We test for differences between the amplitudes of star formation events as a function of the different triggers and the properties of the cold ISM. We can also test for the robustness of gas over-densities in the presence of star formation, as suggested by the presence of HI column density maxima adjacent to ``hot spots'' of previous or repeated star formation as observed in Sextans A and GR 8 (Dohm-Palmer et al. 1997; 1998; 2003). In addition, the high spatial and velocity resolution will allow us to measure the spatial variation of the ISM turbulence, and correlate its properties with the star formation observed in the HST data.
Energy Input Into the ISM
We can quantify the impact of previous star formation on the present day ISM structure by combining the star formation history maps with the proposed high resolution HI observations. For example, for a given star formation event, under the assumption of a constant initial mass function (IMF), we can estimate how many stars have evolved off the main sequence and exploded as supernovae (SNe), giving an estimate for the total energy available from SNe as a function of time and location within the galaxy (Warren et al. 2011, Stilp et al. 2012, submitted). When these past events are found to correlate with the positions of expanding HI shells, as one might expect, then the SN energy input can be compared to the HI kinematics, yielding an estimate of to the degree to which the SN energy input couples mechanically to the ISM. This analysis can be repeated for expanding HI shells of different ages (dated by the time of the last coincident star formation event) to derive the rate of energy dissipation and slowing of the shells, as a function of the shells' environments. By comparing high metallicity spirals to low metallicity dwarfs, we test how shell propagation differs in systems with different cooling efficiencies.
Interaction with the Warm and Cold ISM
In addition to the HI observations, we constrain the warm and cold ISM phases using ancillary data. A current view of the warm phase of the ISM is available from Hα observations. These will become publicly available for all of the ANGST sample from the 11 Mpc Hα and Ultraviolet Galaxies Survey (PI: Kennicutt), which has mapped all nearby gas-rich galaxies out to 11 Mpc using ground-based Hα and GALEX UV imaging. The resulting Hα maps provide complementary data on the current star formation rate (the last 5 Myr). We derive the the dense cold phase by an analysis of the HI line width (Warren et al. 2012, submitted). This technique, in conjunction with the Hα and Spitzer data paints a fairly complete picture of the structure of the ISM from the most diffuse to the densest phases. In spiral galaxies, we trace the variation of star formation and gas density/phase in response to the propagation of spiral arms. In dwarfs, we trace the variations of gas phase and star formation rate in a more stochastic star formation regime.
