Starship Asterisk*

A Cosmic Optical Illusion
ESA/HEIC Hubble Picture of the Week | 2013 Aug 19

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At first glance, this Hubble picture appears to capture two space colossi entangled in a fierce celestial battle, with two galaxies entwined and merging to form one (ESO 252-1). But this shows just how easy it is to misinterpret the jumble of sparkling stars and get the wrong impression — as it’s all down to a trick of perspective.

By chance, these galaxies appear to be aligned from our point of view. In the foreground, the irregular dwarf galaxy PGC 16389 — seen here as a cloud of stars — covers its neighbouring galaxy APMBGC 252+125-117, which appears edge-on as a streak. This wide-field image also captures many other more distant galaxies, including a quite prominent face-on spiral towards the right of the picture.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Luca Limatola.

Credit: ESA/Hubble & NASA
Acknowledgement: Luca Limatola

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That's really fascinating!

What makes the picture particularly interesting to me is that I believe that the edge-on galaxy is another dwarf galaxy, not interacting with the foreground swarm of stars, but not too far in the background, either. An obvious graininess can be seen in the background galaxy, which suggests that it contains clusters that are really large compared with the overall size of the galaxy itself. Also, the bright disk of the galaxy is strangely truncated at its lower left side, in a manner that just isn't seen in large disk galaxies. Finally, no dust can be seen in the disk galaxy despite its edge-on perspective, even though dust can be seen in many of the distant background galaxies. All of this suggests that the edge-on galaxy is really small.

It is almost impossible to judge the color of galaxies photographed through an orange and an infrared filter. However, my software says that the colors of this "galaxy pair" are really very blue. The combined B-V index is 0.300, which is very blue for galaxies, and the combined U-B index is -0.360, also very blue.

Dwarf galaxies are typically somewhat blue. The very blue combined colors of this pair strongly suggests that both galaxies are dwarfs. The foreground galaxy appears to contain some star formation, particularly on its left side, but it may also owe its blue color to metal-poor stars. The edge-on galaxy may be just large enough to form a disk, and it contains a lot of young blue clusters.

Ann