HEIC: Don’t Trust Your Eyes (NGC 5011)

[bystander]

Don’t Trust Your Eyes
HEIC/ESA Hubble Picture of the Week | 2012 Dec 31

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The Universe loves to fool our eyes, giving the impression that celestial objects are located at the same distance from Earth. A good example can be seen in this spectacular image produced by the NASA/ESA Hubble Space Telescope. The galaxies NGC 5011B and NGC 5011C are imaged against a starry background.

Located in the constellation of Centaurus, the nature of these galaxies has puzzled astronomers. NGC 5011B (on the right) is a spiral galaxy belonging to the Centaurus Cluster of galaxies lying 156 million light-years away from the Earth. Long considered part of the faraway cluster of galaxies as well, NGC 5011C (the bluish galaxy at the centre of the image) is a peculiar object, with the faintness typical of a nearby dwarf galaxy, alongside the size of an early-type spiral.

Astronomers were curious about the appearance of NGC 5011C. If the two galaxies were at roughly the same distance from Earth, they would expect the pair to show signs of interactions between them. However, there was no visual sign of interaction between the two. How could this be possible?

To solve this problem, astronomers studied the velocity at which these galaxies are receding from the Milky Way and found that NGC 5011C is moving away far more slowly than its apparent neighbour, and its motion is more consistent with that of the nearby Centaurus A group at a distance of 13 million light-years. Thus, NGC 5011C, with only about ten million times the mass of the Sun in its stars, must indeed be a nearby dwarf galaxy rather than member of the distant Centaurus Cluster as was believed for many years.

Problem solved.

This image was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.

Credit: NASA/ESA/Hubble

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[Ann]

This is a very interesting image.

It's not for me to argue, but NGC 5011B doesn't look like a spiral galaxy to me. Instead it looks like a lenticular galaxy, with a disk but no arms. The galaxy has a huge halo and what looks like a pretty high surface brightness inner halo and disk. The galaxy is completely monocolored, at least at this resolution, which suggests that it's made up of an entirely old population. I would guess that its B-V index may be around +0.9 or possibly even 0.95.

NGC 5011C fairly screams "dwarf galaxy"! It has a very low surface brightness, it is "large and spread out", it is easily resolved into stars by Hubble, and its color - to the extent that its color can be judged by exposures through an orange and and infrared filter - suggests a mostly old and relatively metal-poor population. In all probability, this galaxy hasn't been through many major star forming episodes through its lifetime. However, there is some suggestion that this galaxy has in fact formed a few stars within the last few million years. Tiny groups or very small associations of bluish stars can be seen in the inner part of the galaxy. Its outer halo is redder, and no stars may have formed here for at least a few hundred million years. I would guess that its color index is somewhere between 0.6 and 0.8.

My software erroneously places NGC 5011B and 5011C at the same distance, so it's a good thing that Hubble has cleared up that misunderstanding!

Ann