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NGC 3628: Sideways Spiral Galaxy

Explanation: What do spiral galaxies look like sideways? Featured is a sharp telescopic view of a magnificent edge-on spiral galaxy NGC 3628, a puffy galactic disk divided by dark dust lanes. Of course, this deep galactic portrait puts some astronomers in mind of its popular moniker, The Hamburger Galaxy. The tantalizing island universe is about 100,000 light-years across and 35 million light-years away in the northern springtime constellation Leo. NGC 3628 shares its neighborhood in the local Universe with two other large spirals M65 and M66 in a grouping otherwise known as the Leo Triplet. Gravitational interactions with its cosmic neighbors are likely responsible for the extended flare and warp of this spiral's disk.

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In the title, you have written NGC 3682 instead of NGC 3628.

Rogelio Bernal Andreo should do a mosaic of the whole sky!

So why are the outer parts, all around, blue? Blue surely means hot young stars? Does not seem to make sense!

.
A.k.a. "Sarah's Galaxy."

So who was Sarah (and did she live in Hamburg)

heehaw wrote: ↑Wed Sep 05, 2018 11:44 am So why are the outer parts, all around, blue? Blue surely means hot young stars? Does not seem to make sense!

I agree with you. That is why I have not commented before. The colors do look odd to me.

One possibility is that the stars in the halo of NGC 3682 are metal-poor. That might shift their color towards the blue. But in my opinion, they should not be that blue, because old metal-poor stars are still not as blue as very young metal-rich massive ones. We see very few obvious signs of star formation in NGC 3682, so any significant numbers of very young massive stars should not be there. Maybe possibly maybe there has been some significant star formation right outside the "leftmost" part of the long central dust lane, which consumed the gas and dust there and left a "naked" association of blue stars behind. Maybe.

I note that the details of the galaxy are mostly sharp, but some parts are blurry.

Ann

Ann wrote: ↑Wed Sep 05, 2018 3:13 pm

heehaw wrote: ↑Wed Sep 05, 2018 11:44 am
So why are the outer parts, all around, blue? Blue surely means hot young stars? Does not seem to make sense!

I agree with you. That is why I have not commented before. The colors do look odd to me.

One possibility is that the stars in the halo of NGC 3682 are metal-poor. That might shift their color towards the blue. But in my opinion, they should not be that blue, because old metal-poor stars are still not as blue as very young metal-rich massive ones. We see very few obvious signs of star formation in NGC 3682, so any significant numbers of very young massive stars should not be there. Maybe possibly maybe there has been some significant star formation right outside the "leftmost" part of the long central dust lane, which consumed the gas and dust there and left a "naked" association of blue stars behind. Maybe.

• Ann commented before about NGC 3628:

Ann wrote: ↑Fri Jul 06, 2012 1:37 pm

Canadain Grandma wrote:
How do you KNOW it is a spiral galaxy when we see it edge on?

NGC 7217. Photo: Adam Block.

This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.

We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We can see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.

Ann wrote: ↑Wed Sep 05, 2018 3:13 pm I agree with you. That is why I have not commented before. The colors do look odd to me.

Hi Ann,

For whatever is worth, the blueish color in the outer skirts of the galaxy came out easily. Originally it appeared more grayish than blue, but a small punch on color saturation brought it where it is. Just saying that it didn't pop out of some personal preference

Cheers,
Rogelio

The jpeg quality of the full scale image is crappy I am afraid.

neufer wrote: ↑Wed Sep 05, 2018 3:44 pm

• Ann commented before about NGC 3628:

Ann wrote: ↑Fri Jul 06, 2012 1:37 pm

Canadain Grandma wrote:
How do you KNOW it is a spiral galaxy when we see it edge on?

NGC 7217. Photo: Adam Block.

This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.

We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We can see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.

Well, Art, guilty as charged. But in that APOD that I commented on back in July 2012 (have I been here this long? Gosh), NGC 3628 only looks blue at the edges.

Let's compare NGC 3628 with a more "normal" spiral galaxy, NGC 4565. In the picture at left by ESO, NGC 4565 looks pretty much "all yellow", with a few small flecks of blue in the dust lane. When I first started looking at astroimages, NGC 4565 looked like that - all yellow.

Note that you can see in the ESO image that NGC 4565 does seem to have a faint halo. The halo is grayish to grayish-yellow in color, less yellow than the yellow stars in the bulge and the inner disk inside the dust lane. But the halo is certainly not blue.

In Éder Iván's picture from 2009, the halo around NGC 4565 is bluish-gray.













Now let's compare NGC 3628 in today's APOD with Mark Hanson's picture of NGC 4565, the picture at right.

A difference that fairly jumps out at you is that the disk of NGC 3628 doesn't bulge. While NGC 4565 has a very obvious bulge that sticks up above and below the disk in the middle of it, almost nothing sticks up in the middle of the disk of NGC 3628. (There is a hint of a yellowish "X" sticking up.) Another thing that sets NGC 3682 very much apart from NGC 4565 is that the disk of NGC 3628 flares up at the edges. The disk of NGC 4565, by contrast, just gets thinner and thinner the farther out it stretches.

A reasonable conclusion is that NGC 3628 started out as a quite flat galaxy with a very modest (or even non-existent) bulge. It did have a disk with a large population of, say, A-type stars like Sirius and Vega. It is in fact possible that NGC 3628 had experienced an extremely widespread case of star formation in the past, which more or less exhausted the galaxy's star formation ability (that happens), but left a huge young but aging population behind.

Such a widespread burst of star formation may have happened in the past because NGC 3628 is interacting with two other large spiral galaxies, M65 and M66. Later interactions with M65 and M66 may not have produced a lot of new stars in NGC 3628, but it may have stirred up the disk population, tossing it this way and that and sending much of it into the halo of NGC 3628. And if the disk population is dominated by A-type stars, the halo around NGC 3628 might look blue, particularly if its color is saturated by processing. The widening of the halo, and the mixing of different populations of stars in the halo, would be due to the same forces that cause the flaring of the edges of the disk.

As for star formation in NGC 3628: There is indeed star formation going on at the right edge of the disk of NGC 3628, because there are obvious spots of pink there. The right edge of NGC 3628 is not very blue, however. The left edge shows no obvious spots of pink, but there are little streaks of dark red there that look slightly weird. Star formation could go on there.

NGC 3628 is a weirdo.

Ann