Guest Post: The Yin and Yang of the Visible and Invisible in Spiral Galaxies

Jayanne English, an astronomer with a visual arts background, outlines the process of making an illuminating image for the public from a rich set of research data.

Exploring how galaxies change with time requires the study of how visible light and non-visible radiation are interconnected. With the Hubble Space Telescope (HST) we can map, often in light the human eye can detect, the regions in the disk of a spiral galaxy where young stars reside and simultaneously, massive stars die. Yet invisible to our eyes, and traced by radio telescopes such as the Very Large Array (VLA), are vast halos of radiation that are intimately related to these so-called star-forming regions. The radio data inform us that the halo “light” is generated by negatively charged particles, called electrons, that are in motion at almost the speed of light. Indeed the data show that some of this motion is caused by the electrons slithering in a spiral trajectory around magnetic field lines. Some of these lines extend vertically away from the disk of a galaxy for several thousand light-years. But where do the electrons in the radio halo come from? We know that vast quantities of electrons are produced in explosive stellar deaths, called supernovae, in the disk of galaxies. In order to test the hypothesis that these electrons are dragged and pushed out of a galaxy’s disk and into the halo by winds from the star-forming regions, we need to combine visible-light observations of these regions with the invisible-to-our-eyes radio glow of the halos. Since image data is a natural format of both optical and radio telescopes, we can apply image-making techniques to create a scientifically illuminating combination of data from different parts of the electromagnetic spectrum, like this one of the galaxy NGC 5775. 

Building the Base with HST

Let’s start with the HST’s visible light data from the Advanced Camera for Surveys  (ACS) detector. There are many descriptions in this blog site about how the black and white data observed through different filters are assigned colours. The data come from the Hubble Legacy Archive. The filters are broadband F625W, which traces stellar light, and narrowband F658N which traces ionised hydrogen gas.  Since the hydrogen gas is heated by hot, young stars this filter traces the star forming regions and I assigned it a reddish colour since ionised hydrogen appears red to the human eye.  When combined with blueish-green the stellar light becomes the expected cream colour.

Combining Optical Light with Radio Light

The refurbishment of the National Radio Astronomy Observatory’s Very Large Array (NRAO VLA) inspired the state-of-the-art CHANG-ES (Continuum Halos in Nearby Galaxies) survey of galaxies, which includes NGC 5775. These rich data provide the radio halo glow that was assigned a blueish-grey colour and combined with the HST image of the galaxy’s disk.

However, radio telescopes can also determine the orientation of the electromagnetic waves from a source of radiation. As an illustration of orientation, point your thumb at your nose and your index finger upwards. Moving your thumb towards you shows you the direction the radiation is travelling while your finger shows you the orientation of the wave. Of course the finger doesn’t have to point straight up—roll your thumb and your index finger will rotate. You can make it horizontal, for example. This orientation can be plotted on the image. Scientists often plot this so-called polarisation angle as little lines, called vectors. The plot below, in a paper by Marita Krauss and collaborators (Volume 639 (July 2020) A&A, 639, A112), shows these vectors along with contour lines tracing the outer region of the radio halo. Both the length of each vector, and the colours in the plot, indicate how much of the radio “light” at that position has the same orientation. Notice that the vectors in the red region are longer—the amount of orientation, called “polarisation intensity,” in the disk is higher than in the halo.

From Vectors to Filaments

However, we do something a little different to create our image at the top of this article. My colleagues and I connect the vectors! To do this we use a computer algorithm called Linear Integral Convolution. One analogy for what the algorithm does is the act of drawing a pattern of lines in sand. We give the code a random texture (sand) and the vectors (pattern) and it draws the flow lines, creating  “filaments.”

Recall that electrons are spiraling along magnetic field lines, producing electromagnetic waves, aka radiation. So the orientation of the “filaments” actually traces the orientation of magnetic field lines. Some of these magnetic field lines align with the disk. Filaments parallel to the disk, where the polarisation intensity is highest, are assigned pink. The tips of the filaments that curve away from the disk, extending perpendicular to it, are assigned blue. This distinguishes them as regions of lower polarisation intensity. These coloured images are combined together in the same way as HST images.

Finally all the images are ready to be combined together. However, the brightness of the radio data provides a challenge. How is that mitigated so that the HST data isn’t overwhelmed?

Have you ever held your hand up to block the sun so you could see an airplane near it? You are “masking” the brightness of the sun. Masking has been used to create HST images like The Little Ghost planetary nebula. For galaxy NGC 5775 I created masks to dim down the brightness in the radio images, and you’ll notice in coloured magnetic-field images that the spatial region of the galaxy’s optical disk is darker than in the greyscale image of the filaments. Using this technique we can combine all the images—the VLA yin and HST yang—together to create the image at the top of this blog and at https://public.nrao.edu/news/2020-image-contest-winners/.  We have also used this overall image-making approach for a few other galaxies in the CHANG-ES catalogue, such as NGC 4666 by Yelena Stein.

CHANG-ES NGC 4666 by Yelena Stein

Now we can examine the star-forming regions, the extended magnetic-field lines, and spurs emanating from the radio halo all simultaneously. We can attempt to assess if there are, on average, relationships between the hot gas regions and the radio emission. In the case of NGC 5775 some of my CHANG-ES colleagues find that the star formation spread throughout the disk of this galaxy is producing a strong wind that as it flows further and further out expands wider and wider. It seems that this wind is partly responsible for the magnificent structure seen in our final image. When we use many wavelength ranges of the electromagnetic spectrum we call our endeavour multi-wavelength astronomy. And a multi-wavelength image illuminates both our discoveries and inspires us to ask more scientific questions.

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