English: The Two Micron All Sky Survey provides our first global look at galaxies in infrared light. This image presents the 30 largest galaxies seen in the infrared sky.

It is important to note that this list will not exactly match the list of the 30 largest galaxies as measured in visible light. One key reason is that the millions of stars whose combined light form the nebulous shape of a galaxy are brighter in the visible part of the spectrum. Galaxies that appear faint in visible light may appear much smaller in the infrared, or indeed may not be detected at all by 2MASS.

However, infrared light provides a valuable insight into the structures of galaxies. Dust clouds that can extend through galaxies can hide millions of stars from sight at visible wavelengths. In the infrared, however, they become more and more transparent allowing us to see the actual distribution of stars more clearly.

Also, in large galaxies, the visible light can be strongly dominated by a relative handful of the most massive stars. For instance, a star that is only five times the mass of the sun can put out about 1000 times as much light. In the infrared this effect is greatly reduced and the observed light is a better tracer of the distribution of star mass.

An interesting feature of this catalog of largest galaxies is that so many of them are edge-on. This is a result of the faintness of galaxies at these wavelengths. Since spiral galaxies are pancake-shaped, only the very brightest can be seen when viewed face-on because they are so thin. However, when seen from the edge, the increased thickness makes even fainter galaxies more visible. Therefore, edge-on galaxies are more likely to be detected and will tend to appear larger.