neufer wrote: ↑
Wed Sep 05, 2018 3:44 pm
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.
NGC 3628. Data: Paul Gradner,
Processing: Rogelio Bernal Andreo.
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.