HEIC: A Dwarf Galaxy with a Bright Nebula (NGC 2366)

[bystander]

A Dwarf Galaxy with a Bright Nebula
ESA/HEIC Hubble Photo Release | 2012 May 10

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The NASA/ESA Hubble Space Telescope has made detailed observations of the dwarf galaxy NGC 2366. While it lacks the elegant spiral arms of many larger galaxies, NGC 2366 is home to a bright, star-forming nebula and is close enough for astronomers to discern its individual stars.

The starry mist streaking across this image obtained by the NASA/ESA Hubble Space Telescope is the central part of the dwarf galaxy known as NGC 2366. The most obvious feature in this galaxy is a large nebula visible in the upper-right part of the image, an object listed just a few entries prior in the New General Catalogue as NGC 2363.

A nearby yellowish swirl is not in fact part of the nebula. It is a spiral galaxy much further away, whose light is shining right through NGC 2366. This is possible because galaxies are not solid objects. While we see the stars because they shine brightly, galaxies are overwhelmingly made up of the empty space between them. Hubble’s high-resolution image illustrates this perfectly: the stars are small points of light surrounded by the darkness of space.

The splendid interconnected objects of NGC 2366 and NGC 2363 are located about 10 million light-years away in the constellation of Camelopardalis (the Giraffe). As a dwarf galaxy, NGC 2366’s size is in the same ballpark as the two main satellite galaxies of our Milky Way, named the Large and Small Magellanic Clouds. Like the Magellanic clouds, NGC 2366's lack of well-defined structure leads astronomers to further classify it as an irregular galaxy.

Although NGC 2366 might be small by the standards of galaxies, many of its stars are not, and the galaxy is home to numerous gigantic blue stars. The blue dots scattered throughout the galaxy speak to the burst of star formation that the galaxy has undergone in recent cosmic time. A new generation of these stellar titans has lit up the nebula NGC 2363.

In gas-rich star-forming regions, the ultraviolet radiation from young, big, blue stars excites the hydrogen gas, making it glow. NGC 2363, as well as other, smaller patches seen throughout Hubble’s image, serve as the latest formation sites for stellar giants.

Imaged through green and infrared filters, these nebulae take on a blueish tinge in this image, though the actual colour is a shade of red.

This image was produced from two adjacent fields observed by Hubble’s Advanced Camera for Surveys. The field of view is approximately 5.5 arcminutes across, which is equivalent to a little over a fifth of the diameter of the full Moon. Although this is comparatively large by the standard of Hubble’s images, NGC 2366 is much too faint to observe with the naked eye.

Credit: NASA/ESA/Hubble

Zoomable Image

[Ann]

Splendid resolution. Disappointing color. Well, the ESA/HEIC images are almost always like that: they are the product of exposures through two filters, one infrared and one visual, and the colors you can tease out of a combination of such exposures are quite uninteresting. Geckzilla complained about this when she was processing Hubble images.

Fortunately, however, the ESA/HEIC page links to a picture where NGC 2366 has been imaged through two visual filters, one red and one blue. Combined with an exposure through an infrared filter, that produces an interesting image, colorwise. Check it out here. The very red star you can see at far right in the widefield image is AA Cam, a hugely infrared star which may be about 500 times as bright as the Sun in visual light. As for the galaxy NGC 2366 itself, it looks really quite blue here, as you can see.

Why is it, though, that the emission nebulae don't look red in this picture either? The reason is probably that the red filter used for the image wasn't a narrowband filter centered at 656 nm, but a much more broadband filter, which is nowhere near as good at detecting the red Ha light of an emission nebula as a narrowband filter will be. Also the emission nebulae may well contain reflected blue starlight, and they contain blue-green Hβ light, too. Considering all the blue light you find in and near a bright star formation region, and with the filters used and balanced for the image the way they were here, the emission nebulae came out blue in this picture.

Fascinatingly, you can see one orange spot in the otherwise very blue galaxy seen in the groundbased image. I wonder what that orange spot could be. There is clearly an orange foreground star at about that spot, but there is also, to the lower right of the foreground star, a fuzzy round orange area. Is it possible that the fuzzy orange area is the center, the mini-mini-bulge of NGC 2366? All things considered, I guess it's more likely that the orange spot seen in the groundbased image is the foreground star instead.

Ann