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 imaging⁶³. For example high-level data products of Integral Field Units (IFUs) like MUSE⁶⁴ in the optical, ACIS⁶⁵ 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 webpage⁶⁶. 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 crop⁶⁷ 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 http://akhlaghi.org/data/a370-crop.fits    # Downloads 287 MB

In the sections below, we will first review how you can visually inspect a 3D data cube 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.

• Viewing spectra and redshifted lines
• Sky lines in optical IFUs
• Continuum subtraction
• 3D detection with NoiseChisel
• 3D measurements and spectra
• Extracting a single spectrum and plotting it
• Pseudo narrow-band images