HEIC: Glitter galaxy — An edge-on view of ESO 318-13

[bystander]

Glitter galaxy — An edge-on view of the ESO 318-13 galaxy
ESA/HEIC Hubble Picture of the Week | 2012 Dec 03

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The brilliant cascade of stars through the middle of this image is the galaxy ESO 318-13 as seen by the NASA/ESA Hubble Space Telescope. Despite being located millions of light-years from Earth, the stars captured in this image are so bright and clear you could almost attempt to count them.

Although ESO 318-13 is the main event in this image, it is sandwiched between a vast collection of bright celestial objects. Several stars near and far dazzle in comparison to the neat dusting contained within the galaxy. One that particularly stands out is located near the centre of the image, and looks like an extremely bright star located within the galaxy. This is, however, a trick of perspective. The star is located in the Milky Way, our own galaxy, and it shines so brightly because it is so much closer to us than ESO 318-13.

There are also a number of tiny glowing discs scattered throughout the frame that are more distant galaxies. In the top right corner, an elliptical galaxy can be clearly seen, a galaxy which is much larger but more distant than ESO 318-13. More interestingly, peeking through the ESO 318-13, near the right-hand edge of the image, is a distant spiral galaxy.

Galaxies are largely made up of empty space; the stars within them only take up a small volume, and providing a galaxy is not too dusty, it can be largely transparent to light coming from the background. This makes overlapping galaxies like these quite common. One particularly dramatic example of this phenomenon is the galaxy pair NGC 3314 (heic1208).

Credit: NASA/ESA/Hubble

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[Ann]

This galaxy was imaged through two filters, one centered at 606 nm and the other at 814 nm. The 606 nm filter is either clear or orange, which means that it doesn't react strongly to blue stars. The near infrared filter reacts to red stars. In other words, there is no filter that specifically picks out blue stars.

Moreover, the caption doesn't offer any information about what kind of galaxy this is. Yes, I can see that it is oval-shaped and that it is a dwarf galaxy that contains rather few stars, since its surface brightness is obviously quite low and the brightness of the individual stars is quite high compared with the overall brightness of the galaxy. But what kind of stars are they? Are they relatively young blue stars of spectral class A and late B, or are they red giants? The caption gives us no clue, and it is very hard to tell from the picture, too.

Ann

[bystander]

Phil Plait gives a little analysis here:

Tiny Galaxy Sparkles in Our Back Yard
Slate Blogs | Bad Astronomy | 2012 Dec 03

[Ann]

Thanks for the link, bystander.

Phil Plait wrote:

I searched through the professional journals and found only one paper about it

I checked the paper, but I didn't find any B-V information about ESO-318-13.

Phil Plait wrote:

For a moment I was thrown by the stars being all blue, since only hot, young, massive stars are that color.

Exactly - I, too, think that pictures of galaxies and globulars imaged through an orange and an infrared filter and being shown as predominantly blue are really confusing.

Phil Plait wrote:

The astronomers used two filters to make this picture, one that lets through only orange/red light (shown as blue), and another that lets through only infrared light (shown as red). Those two colors are useful when you’re looking at stars

I disagree. I agree that these two filters are very useful when you look at distant galaxies that are considerably redshifted. For moderately nearby objects, however, such as ESO-318-13, a blue and a red filter would do a much better job of distinguishing between hot and cool stars.

Phil Plait wrote:

So this looks to be an ordinary dwarf galaxy, fairly flat, and otherwise unremarkable except for its beauty.

Its flatness is interesting. That suggests to me that this is a disk galaxy rather than a "blob galaxy". Many small galaxies are indeed blob-shaped. A few examples are the Sagittarius Dwarf Galaxy, IC 1613 (picture by Brian Lula), Sextans A and Sextans B. All these galaxies are classified as dwarf irregulars, and all experience some star formation. But there are also dwarf galaxies with no star formation which are typically blob-shaped, such as the Leo Dwarf, UGC 5470.

I find it interesting that ESO 318-13 is "diskier" than the small galaxies I've just linked to. So, does ESO 318-13 contain any star formation? Yes, I think it does. There are a few small blue-looking clumps of stars in the left part of the galaxy that definitely look like clusters. This suggests that ESO 318-13 might be slightly similar to a galaxy like, say, Sextans B.

I searched for a disk-shaped galaxy that might be comparable to ESO 318-13, but I found none. One galaxy I checked up is NGC 4144. But even though NGC 4144 is likely a smallish galaxy, you can see how different it is from ESO 318-13. The inner part of NGC 4144 is quite bright, and you can't pick out individual stars there. The stars in the inner part appears to be clearly redder in color than the stars in the outer disk. There are several blue clusters in the outer disk, and you can detect a thin and broken dust lane in the galaxy. ESO 318-13, by contrast, has a very low surface brightness and no central concentration of stars, and there is certainly no dust lane.

Fascinating. I still think the stellar content of "the glitter galaxy" is slightly similar to that of Sextans B, although ESO 318-13 has fewer young clusters.

Ann

[Ann]

I just remembered a few dwarf galaxies that have definite disks. One is NGC 784. NGC 784 appears to be more elongated than ESO 318-13, and it definitely contains more young stars. NGC 784 has a blue B-V index of +0.49, so ESO 318-13 must be redder. A realistic color index for it might be +0.7.

By the way, I earlier compared ESO 318-13 with Sextans B, but Sextans B has a B-V index of +0.52. That is too blue for ESO 318-13.

Ann