HEIC: NGC 634: A Perfect Spiral with an Explosive Secret

[bystander]

NGC 634: A Perfect Spiral with an Explosive Secret
ESA/HEIC Hubble Picture of the Week | 2011 May 30

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The NASA/ESA Hubble Space Telescope is renowned for its breathtaking images and this snapshot of NGC 634 is definitely that — the fine detail and exceptionally perfect spiral structure of the galaxy make it hard to believe that this is a real observation and not an artist’s impression or a screenshot taken straight from Star Wars.

This spiral galaxy was discovered back in the nineteenth century by French astronomer Édouard Jean-Marie Stephan, but in 2008 it became a prime target for observations thanks to the violent demise of a white dwarf star. The type Ia supernova known as SN2008a was spotted in the galaxy and briefly rivalled the brilliance of its entire host galaxy but, despite the energy of the explosion, it can no longer be seen this Hubble image, which was taken around a year and a half later.

White dwarfs are thought to be the endpoint of evolution for stars between 0.07 to 8 solar masses, which equates to 97% of the stars in the Milky Way. However, there are exceptions to the rule; in a binary system it is possible for a white dwarf to accrete material from the companion star and gradually put on weight. Like a person gorging on junk food, the star can eventually grow too full — when it exceeds 1.38 solar masses nuclear reactions ignite that produce enormous amounts of energy and the star explodes as a type Ia supernova.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a yellow filter (F555W, coloured blue) have been combined with images through red (F625W, coloured green) and near-infrared (F775W, coloured red) filters. The total exposure times per filter were 3750 s, 3530 s and 2484 s, respectively and the field of view is 2.5 x 1.5 arcminutes.

Credit: NASA/ESA/Hubble

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[bystander]

The dusty depths of a spectacular spiral galaxy
Discover Blogs | Bad Astronomy | 2011 Aug 01

[Click to galactinate]

In something I’m considering making a tradition here at BA Central, here is your Monday morning jaw-dropping spiral galaxy: NGC 634 as seen by Hubble:

Isn’t that something? This galaxy is a gorgeous nearly edge-on spiral, about 120,000 light years across or so — slightly bigger than the Milky Way — and 220 million light years away. The press release (at the link above) for this spiral talks about a supernova that blew up in this galaxy back in 2008, and I was going to write about that, but then something else tickled my brain.

Look at the picture. The disk of the galaxy, like in most spirals, is ribboned with dark dust lanes, huge clouds of complex organic molecules expelled by stars being born and stars dying. It’s pretty common to see them, but what struck me is the asymmetry of the lanes: they are darker on the bottom than at the top. The overwhelming impression is that we’re looking down on the spiral, so the dust lanes are more obvious on the near side than the far side.

This cannot be a physical effect of having dust only on one side of the galaxy. If it were, then random chance would make it pretty unlikely to have it on the side tipped toward us. Plus, I realized that I’ve seen this before! For example, in this image here of NGC 7049 you see the exact same thing. It’s also clear in images of M 64, where the feature is so obvious it gave the object its nickname of the Black Eyed Galaxy.

[Click to embiggen]

After mulling it over for a moment, I suddenly realized what it was: each of these galaxies must have an extended halo of stars surrounding the central bulge and disk. If the galaxy were just a disk these features would be very difficult to explain, but if there are a few billion stars that exist above and below the plane of the disk, they would "fill in" the dark lanes, making them less dark. And that explains why we see the dust better on the galaxy’s near side: we’re not looking through as many stars. It’s like we’re looking through a fog; stuff nearby is clearer than stuff farther away because we’re looking through less mist. Except in this case, the fog isn’t obscuring anything. It’s stars, adding their light to the overall picture.

That’s pretty cool, and something I never thought of before. I don’t expect this to be a very common feature of spirals, since most angled or nearly edge-on galaxies don’t show this effect very strongly. That might make an interesting project for an undergrad. You could take a sample of a few hundred spirals at various tilts and then compare the brightness of the dust lanes on one side versus the other. You should be able to get a census of the stars in the halo as well. Having spectra would help too.

I’m not soliciting research here; for me a grant is when I go to the ATM. But if you’re an undergrad in astronomy, ask your professor. Maybe this has been done already… and if so, I’d be interested in reading about it! It’s a lot of fun to get a flash of insight on something, and even better to find out it’s attackable by science.

Image Credits: NASA/ESA/Hubble

[Ann]

After mulling it over for a moment, I suddenly realized what it was: each of these galaxies must have an extended halo of stars surrounding the central bulge and disk. If the galaxy were just a disk these features would be very difficult to explain, but if there are a few billion stars that exist above and below the plane of the disk, they would "fill in" the dark lanes, making them less dark. And that explains why we see the dust better on the galaxy’s near side: we’re not looking through as many stars. It’s like we’re looking through a fog; stuff nearby is clearer than stuff farther away because we’re looking through less mist. Except in this case, the fog isn’t obscuring anything. It’s stars, adding their light to the overall picture.

That is an astute observation. It is obvious from the Hubble that NGC 634 has a large central bulge, which may indeed function as a sort of "light fog", making the dust lanes on the far side look less dark and less distinct than the dust lanes on the near side.

However, it should be noted that dust lanes on the far side of the bulge may reflect the light from the bulge, thus acting as reflection nebulae. That will make them look quite bright. By contrast, the light from the bulge will be filtered through the dust lanes on the near side, making those dust lanes look quite dark.

Click to view full size image

This is an interesting example, a Hubble picture of NGC 2841. Not only do the dust lanes look much brighter on the far side than on the near side, but young blue clusters stand out much more clearly on the far side than on the near side. My guess is that this is more of a "reflection nebulae versus dark nebula" effect than the effect of the bright bulge and halo of NGC 2841.








Ann

EDIT: I see that the ESA/Hubble picture of NGC 634 was taken through relatively red filters. That would dampen the "reflection nebulae" effect, since reflection nebulae primarily reflect blue light. The Hubble image of NGC 634 does not detect blue light, and the blue color in the image is just "mapped blue", which explains why the far dust lanes don't look particularly blue. On the other hand, the faint stars of the central bulge will consist mostly of old red stars, which show up very well through red and infrared images. This bulge will therefore look brighter through red and infrared filters than through standard RGB filters, which explains why there may indeed appear to be a "light fog" in front of the far dust lanes, making them look not exactly bluer, but less distinct.