THE 21-SPONGE H i ABSORPTION SURVEY. I. TECHNIQUES AND INITIAL RESULTS

Abstract

We present methods and results from "21-cm Spectral Line Observations of\nNeutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral\nhydrogen (HI) absorption with the Karl G. Jansky Very Large Array (VLA). With\nthe upgraded capabilities of the VLA, we reach median root-mean-square (RMS)\nnoise in optical depth of $\\sigma_{\\tau}=9\\times 10^{-4}$ per\n$0.42\\rm\\,km\\,s^{-1}$ channel for the 31 sources presented here. Upon\ncompletion, 21-SPONGE will be the largest HI absorption survey with this high\nsensitivity. We discuss the observations and data reduction strategies, as well\nas line fitting techniques. We prove that the VLA bandpass is stable enough to\ndetect broad, shallow lines associated with warm HI, and show that bandpass\nobservations can be combined in time to reduce spectral noise. In combination\nwith matching HI emission profiles from the Arecibo Observatory ($\\sim3.5'$\nangular resolution), we estimate excitation (or spin) temperatures ($\\rm T_s$)\nand column densities for Gaussian components fitted to sightlines along which\nwe detect HI absorption (30/31). We measure temperatures up to $\\rm\nT_s\\sim1500\\rm\\,K$ for individual lines, showing that we can probe the\nthermally unstable interstellar medium (ISM) directly. However, we detect fewer\nof these thermally unstable components than expected from previous\nobservational studies. We probe a wide range in column density between\n$\\sim10^{16}$ and $>10^{21}\\rm\\,cm^{-2}$ for individual HI clouds. In addition,\nwe reproduce the trend between cold gas fraction and average $\\rm T_s$ found by\nsynthetic observations of a hydrodynamic ISM simulation by Kim et al. (2014).\nFinally, we investigate methods for estimating HI $\\rm T_s$ and discuss their\nbiases.\n