DHIGLS: DRAO HI Intermediate Galactic Latitude Survey

The HI (neutral hydrogen) spectral data were obtained with the Synthesis Telescope at the Dominion Radio Astophysical Observatory and Green Bank Telescope at the National Radio Astronomy Observatory. The H I spectra have an effective velocity resolution 1.32 km s-1 or 0.66 km s-1, depending on the channel spacing, 0.824 km s-1 or 0.412 km s-1, respectively. The larger channel spacing results in coverage of at least -105 < vLSR < +35 km s-1, while the smaller channel spacing results in reduced coverage of at least -53 < vLSR < +18 km s-1. The GBT data are from the GHIGLS project described in Martin et al. (2015) and are available on the GHIGLS archive.

Any use of these data should cite the DHIGLS paper by Blagrave et al. (2016) (accepted for publication in The Astrophysical Journal).

In separate sections of this "Explanatory Supplement" below we describe the fields, how to explore and access the data (preview, download, movies, "average" spectrum), and maps of column density.

Fields

DHIGLS targeted 7 fields at intermediate Galactic latitude that together cover around 150 deg2 at 1 arcmin spatial resolution. The locations of the fields are shown in the figure (Fig. 1 from Blagrave et al. 2016). The background image is the velocity-integrated H I emission from the all-sky LAB survey (Kalberla et al. 2005), expressed as column density NHI in units 1019 cm-2.

Channel spacing 0.824 km s-1
DF
DR
EN
UM
Channel spacing 0.412 km s-1
MC
MG
PO

Exploring and accessing the data

The DHIGLS data cubes can be explored by first clicking on the name of the field of interest, which opens a "data window" that provides further resources specific to the field.

Preview of the data cube

A preview movie is available at the top of the data window. The velocity extremes in the movie were selected to include all low and intermediate velocity (LVC, IVC) HI emission for the selected field. The negative velocity extreme is adjusted to include high velocity (HVC) HI emission for the one field (EN) where it is detected within the observed velocity range. The brightest emission is yellow. Each field has a scaling appropriate to the range of brightness temperature. A colourbar in K can be seen in the high-resolution movies described below.

Download the data cube

From the data window the original FITS data cube is available for download. This file has three extensions as follows:

  • Ext[0] : Brightness temperature cube (K)
  • Ext[1] : Absolute weight map output from mosaicking routine SUPERTILE, set to zero to indicate the region outside of which the data are simply the lower-resolution GHIGLS data, not the new DHIGLS product. This can be used to create a noise map (see Blagrave et al. 2016).
  • Ext[2] : Mask map flagging those spectra where baselines were not removed from the GBT data

Movies

There are high-resolution movies in both MP4 and OGG formats. These can be viewed in a browser or downloaded.

Mean, median, and standard deviation spectra

The overall velocity structure in the cube has been summarized on the data window. Mean (solid), median (dashed), and standard deviation (dotted) spectra are all shown (see Figs. 22 and 23 in Blagrave et al. 2016). The standard deviation spectrum has been used to identify HVC, IVC and LVC components (see next section).

Column density NHI

Also available for download on the data window is a FITS file containing maps of the integrated line emission, converted to column density NHI (using Ts = 80 K), for HVC, IVC, and LVC components whose velocity ranges are defined using local minima in the standard deviation spectrum (see Table 2 in Blagrave et al. 2016).

The column density maps are in units 1019 cm-2. The three planes in the NHI data cube correspond to IVC, LVC, and IVC+LVC. For EN, the five planes correspond to HVC, IVC, LVC, IVC+IVC, and IVC+LVC+HVC.