GNU Astronomy Utilities

2.5 Detecting lines and extracting spectra in 3D data

3D data cubes are an increasingly common format of data products in observational astronomy. As opposed to 2D images (where each 2D “picture element” or “pixel” covers an infinitesimal area on the surface of the sky), 3D data cubes contain “volume elements” or “voxels” that are also connected in a third dimension.

The most common case of 3D data in observational astrophysics is when the first two dimensions are spatial (RA and Dec on the sky), and the third dimension is wavelength. This type of data is generically (also outside of astronomy) known as Hyperspectral imaging64. For example high-level data products of Integral Field Units (IFUs) like MUSE65 in the optical, ACIS66 in the X-ray, or in the radio where most data are 3D cubes.

In this tutorial, we’ll use a small crop of a reduced deep MUSE cube centered on the Abell 370 galaxy cluster from the Pilot-WINGS survey; see Lagattuta et al. 2022. Abell 370 has a spiral galaxy in its background that is stretched due to the cluster’s gravitational potential to create a beautiful arch. If you haven’t seen it yet, have a look at some of its images in the Wikipedia link above before continuing.

The Pilot-WINGS survey data are available in its webpage67. The cube of the core region is 10.2GBs. This can be prohibitively large to download (and later process) on many networks and smaller computers. Therefore, in this demonstration we won’t be using the full cube. We have prepared a small crop68 of the full cube that you can download with the first command below. The randomly selected crop is centered on (RA,Dec) of (39.96769,-1.58930), with a width of about 27 arcseconds.

$ mkdir tutorial-3d
$ cd tutorial-3d
$ wget    # Downloads 287 MB

In the sections below, we will first review how you can visually inspect a 3D datacube in DS9 and interactively see the spectra of any region. We will then subtract the continuum emission, detect the emission-lines within this cube and extract their spectra. We will finish with creating pseudo narrow-band images optimized for some of the emission lines.







You can download the full cube and create the crop your self with the commands below. Due to the decompression of the +10GB file that is necessary for the compressed downloaded file (note that its suffix is .fits.gz), the Crop command will take a little long.

$ wget
$ astcrop A370_PilotWINGS_CORE.fits.gz -hDATA --mode=img \
          --section=200:300,100:200 -oa370-crop.fits --metaname=DATA
$ astcrop A370_PilotWINGS_CORE.fits.gz -hSTAT --mode=img --append \
          --section=200:300,100:200 -oa370-crop.fits --metaname=STAT