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A Galaxy on the Edge

Image: This colourful stripe of stars, gas, and dust is actually a spiral galaxy named NGC 1055. Captured here by ESO’s Very Large Telescope (VLT), this big galaxy is thought to be up to 15 percent larger in diameter than the Milky Way. NGC 1055 appears to lack the whirling arms characteristic of a spiral, as it is seen edge-on. However, it displays odd twists in its structure that were probably caused by an interaction with a large neighbouring galaxy. Credit: ESO

Spiral galaxies throughout the Universe take on all manner of orientations with respect to Earth. We see some from above (as it were) or “face-on” — a good example of this being the whirlpool-shaped galaxy NGC 1232.

Image: This spectacular image of the large spiral galaxy NGC 1232 was obtained on September 21, 1998, during a period of good observing conditions. It is based on three exposures in ultra-violet, blue and red light, respectively. The colours of the different regions are well visible : the central areas contain older stars of reddish colour, while the spiral arms are populated by young, blue stars and many star-forming regions. Note the distorted companion galaxy on the left side, shaped like the greek letter "theta". Credit: ESO


Such orientations reveal a galaxy’s flowing arms and bright core in beautiful detail, but make it difficult to get any sense of a three-dimensional shape.

We see other galaxies, such as NGC 3521, at angles.

Image: This picture of the nearby galaxy NGC 3521 was taken using the FORS1 instrument on ESO’s Very Large Telescope, at the Paranal Observatory in Chile. The large spiral galaxy lies in the constellation of Leo (The Lion), and is only 35 million light-years distant. Credit: ESO/O. Maliy


While these tilted objects begin to reveal the three-dimensional structure within their spiral arms, fully understanding the overall shape of a spiral galaxy requires an edge-on view — such as this one of NGC 1055.

When seen edge-on, it is possible to get an overall view of how stars — both new patches of starbirth and older populations — are distributed throughout a galaxy, and the “heights” of the relatively flat disc and the star-loaded core become easier to measure. Material stretches away from the blinding brightness of the galactic plane itself, becoming more clearly observable against the darker background of the cosmos.

Such a perspective also allows astronomers to study the overall shape of a galaxy’s extended disc, and to study its properties. One example of this is warping, which is something we see in NGC 1055. The galaxy has regions of peculiar twisting and disarray in its disc, likely caused by interactions with the nearby galaxy Messier 77. This warping is visible here; NGC 1055’s disc is slightly bent and appears to wave across the core.

NGC 1055 is located approximately 55 million light-years away in the constellation of Cetus (The Sea Monster).


  • Text Credit: ESO
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