2015ApJ...809..153M

This paper introduces and describes the data cubes from GHIGLS, deep Green Bank Telescope (GBT) surveys of the 21 cm line emission of H i in 37 targeted fields at intermediate Galactic latitude. The GHIGLS fields together cover over 1000 deg² at spatial resolution. The H i spectra have an effective velocity resolution of about 1.0 km/s and cover at least km/s, extending to km/s for most fields. As illustrated with various visualizations of the H i data cubes, GHIGLS highlights that even at intermediate Galactic latitude the interstellar medium is very complex. Spatial structure of the H i is quantified through power spectra of maps of the integrated line emission or column density,. For our featured representative field, centered on the north ecliptic pole, the scaling exponents in power-law representations of the power spectra of maps for low-, intermediate-, and high-velocity gas components (LVC, IVC, and HVC) are,, and, respectively. After Gaussian decomposition of the line profiles, maps were also made corresponding to the narrow-line and broad-line components in the LVC range; for the narrow-line map the exponent is, reflecting more small-scale structure in the cold neutral medium (CNM). There is evidence that filamentary structure in the H i CNM is oriented parallel to the Galactic magnetic field. The power spectrum analysis also offers insight into the various contributions to uncertainty in the data, yielding values close to those obtained using diagnostics developed in our earlier independent analysis. The effect of 21 cm line opacity on the GHIGLS maps is estimated. Comparisons of the GBT data in a few of the GHIGLS fields with data from the EBHIS and GASS surveys explore potential issues in data reduction and calibration and reveal good agreement. The high quality of the GHIGLS data enables a variety of studies in directions of low Galactic column density, as already demonstrated by the Planck Collaboration. Fully reduced GHIGLS H i data cubes and other data products are available at www.cita.utoronto.ca/GHIGLS.