21-SPONGE: 21-cm SPectral line Observations of Neutral Gas with the EVLA

Very few measurements of the excitation temperature of the warm neutral medium (WNM) exist to date. Yet, the fraction of the thermally stable vs unstable WNM gas is crucial to constrain various models of the interstellar medium, as well as conditions for the formation of cold clouds, which are necessary steps toward star formation. We performed pilot observations with the EVLA and showed that the EVLA can successfully measure HI absorption lines with a peak optical depth of ~10-3, which is expected for a typical WNM component with an excitation temperature of ~8000 K. We then started a large EVLA survey of 58 radio continuum sources to measure, in a statistically significant way, the temperature, column density and abundance of the WNM. 26 out of 58 target sources are complete and results so far are excellent. The 21-SPONGE survey is designed to take advantage of, or "sponge away" any unused telescope time for configurations from A to C (including hybrids) especially during configuration moves.

The image below is a map of the Milky Way in HI (LAB; Kalberla et al. 2005), with our sources superimposed. The lines represent the field of view of the Arecibo radio telescope, from which we will gather matching emission line data on all of our sources.