Starship Asterisk*

JohnD wrote:I share Guest's confusion!

The blurb just doesn't match the image shown as today's APOD, or the galaxies that the blurb links to!
It just doesn't look like a barred galaxy - no bar! - and nothing like the example NGC 1672.
The Wiki entry for NGC 1512, given to show it "Seen in total", looks exactly like the APOD - no outer rings at all.
And a Google for other images of 1512 find it as an oval, angulated as seen from Earth, with a distinct bar, no dust lanes and wide spiral arms, not an outer ring. See: http://www.billionsandbillions.com/ngc_1512.html

Shurley shome osher Galashie?
John

It can be confusing to spot a barred spiral, I know.

A typical feature of barred spirals is that they have two dust lanes running along the bar. In the picture at right of NGC 1097, you can clearly see the two dust lanes in the bar, one running from the inner ring to the lower left until it meets the outer ring, the other dust lane running from the inner ring to the upper right to the outer ring.

If you take a look at today's APOD, you can see two dust lanes running from the inner ring, one to the upper left, the other to the lower right. These two dust lanes strongly suggest that the nuclear ring is connected to a bar.

Here is a 549 KB image of galaxy NGC 1512. I recommend it, and you can clearly see both the inner and the outer ring and the connecting bar. One nuclear dust lane is obvious, while the other one is harder to spot. Please note that the inner ring is very tiny compared with the outer one, but also brighter than the outer one, which is typical.

Ann

bystander wrote:

[img3="NGC 1512 - Credit: NASA / ESA / LEGUS / Judy Schmidt"]https://c1.staticflickr.com/1/645/30969 ... 7524_z.jpg[/img3][hr][/hr]

Thanks for posting that image, bystander, and great processing of that NASA/ESA/LEGUS picture, Geck!

Ann

Art, I come from the haunts of hoot and cern...

As a regular guy who's addicted to APOD, if the blurb sez there's a bar there, I figure the spectrum that the photo was taken in just doesn't feature it, and the blurb is pointing out other aspects of the galaxy. Since Andromeda has such 'low surface brightness' so as to keep it small in our sky at optical wavelengths, I've learned to take Hubble's (and APOD's) word for it on these things. This APOD is a Dust thing, not a Bar thing.

neufer wrote:

When two comparably sized galaxies (e.g., Andromeda & the Milky Way) collide
what prevents their dark matter from elastically escaping into space
and leave their disturbed in-elastically colliding gases behind

Nothing.

Bad original poetry by Bruce

Congratulations and thanks to Judy Schmidt for capturing one of the wonders and sharing her work with us.

BDanielMayfield wrote:

We baryonic beings and things,
attracted to dark matter we are.

But baryons don't matter,
to the darkest of matter,
no matter how dense or diffuse.
For us DM has absolutely no use.

Dark Matter just ain't into us,
'tho it's passing right through us.

Bad original poetry by Bruce

Hey, I like it, Bruce! Except, I would say,

Dark Matter just ain't us.
tho it's passing right through us.

Ann wrote:
A typical feature of barred spirals is that they have two dust lanes running along the bar. In the picture at right of NGC 1097, you can clearly see the two dust lanes in the bar, one running from the inner ring to the lower left until it meets the outer ring, the other dust lane running from the inner ring to the upper right to the outer ring.

If you take a look at today's APOD, you can see two dust lanes running from the inner ring, one to the upper left, the other to the lower right.

Sorry about quoting myself, but I was wrong here. The dust lanes are channeling gas into the inner ring, which is why there is enhanced star formation there.

Ann

It seems that compared with this arm of the Milky Way, things are a lot more "exciting" in a lot more places in the Galaxy. Exciting, as in "lethal".

Jobn

Ann wrote:

BDanielMayfield wrote:

We baryonic beings and things,
attracted to dark matter we are.

But baryons don't matter,
to the darkest of matter,
no matter how dense or diffuse.
For us DM has absolutely no use.

Dark Matter just ain't into us,
'tho it's passing right through us.

Bad original poetry by Bruce

Click to view full size image

I ain't you!
Source: https://www.spacetelescope.org/images/potw1506a/

Hey, I like it, Bruce! Except, I would say,

Dark Matter just ain't us.
tho it's passing right through us.

Thanks Ann. Both ways work.

Bruce

BDanielMayfield wrote:
Thanks Ann. Both ways work.

Bruce

They do!

Ann

Now that line of Tennyson's keeps popping back into my head:

I hope to see my Pilot face to face
When I have crost the bar.

I have come up with about 7 interpretations,
1. The literal was a reference to a ship crossing over a sand-bar
2. The original was meant metaphorically to refer to human death (as a passage) and is now sometimes used in the military for that.
More punnish-ly ...
3. I noted that it could refer to a star circling a galactic central bar
4. Surely lawyers like using it concerning their exams
5. Runners in a relay race, possibly, perhaps a better fit would be for high-jumpers and pole vaulters
6. In a cowboy movie, sometimes it is a major feat to get crost the bar
7. Wilderness hikers don't like to come across a bar (old word for "bear")
Got any more?

But, back to this APOD, both inside and outside of the featured bright ring, the dark tendrils are seen in regions that appear generally luminous and cream-colored. This is all part of the bar of NGC 1512, so the glow here is the diffuse light of many, many stars in those regions, right? In which case, although the ring stands as uniquely productive in young, bright star formation, this is actually occurring within a part of the galaxy that is well-populated with stars, probably more mature and average. I don't think there is an inherent conflict between a region having star formation and star populations on a large scale, although they tend to conflict on smaller scales (as stellar winds remove clouds locally). But I find it interesting to consider what dynamics could produce such a ring. Actually stellar winds would not eliminate gas, even though the term evaporation is sometimes misleadingly used. All they really do is push the stuff around. I guess it has to collect somewhere in the end. There may be a reason it can become concentrated into a ring-shaped region.

Undoubtedly, Tennyson was reffering to death. He wrote the Bar three years before his own death, and previous works - Ulysses, Tithonius are similarly melancholy, as are The Idylls of the King, mosly concerned with Arthur's funeral as his boat disappears over the horizon. If you are an LoR fan its influence on Tolkien is extreme, even to the "one black dot" as Boromir vanishes of the Falls.

JOhn

Noted galactic astronomer John Kormendy has emailed in some corrections to the text of this APOD. In general, John notes that in galaxy collision more commonly created rings in the early universe, whereas "secular evolution" through a galaxy's own asymmtries is more likely to account for galactic rings in the more recent (and local) universe. To help increasing understanding, John volunteered some text adaptations which I have incorporated. Also, John volunteered the following paragraph that should be considered in addition to the adapted APOD text.

JK:

All of these structures are understood by astronomers to be produced by the slow ("secular") evolution of galaxy disks as nonaxisymetric structures such as bars and global ovals rearrange the energy, the angular momentum, and therefore the orbits of disk gas. This evolution concentrates gas into three types of rings, "outer rings" at roughly 2.2 times the radius of the bar or oval (e. g., in NGC 4736), "inner rings" that usually encircle the bar (another example appears in NGC 2523), and "nuclear rings" near the center (NGC 4736 has a prominent one, too). Where gas concentrates, it bursts into stars, so nucler rings are sites of rapid star formation. In fact, they are believed to be growing "pseudobulges" -- dense central regions of galaxies that are recognizably diskier and more rapidly star-forming than elliptical-galaxy-like "classical bulges" such as the ones in M31 and in M81. In NGC 1512, the dust lanes in the bar are understood to be signatures of gas shocks; they outline where gas loses orbital energy and therefore falls toward the center. Starbursts naturally follow when the gas density gets high enough. This secular galaxy evolution picture complements the better known picture of galaxy evolution by hierarchical clustering and merging. As the Universe ages, evolution switches from being driven mostly by hierarchical clustering to slower evolution driven mostly by internal galaxy asymmetries.

RJN wrote:Noted galactic astronomer John Kormendy has emailed in some corrections to the text of this APOD. In general, John notes that in galaxy collision more commonly created rings in the early universe, whereas "secular evolution" through a galaxy's own asymmtries is more likely to account for galactic rings in the more recent (and local) universe. To help increasing understanding, John volunteered some text adaptations which I have incorporated. Also, John volunteered the following paragraph that should be considered in addition to the adapted APOD text.

JK:

All of these structures are understood by astronomers to be produced by the slow ("secular") evolution of galaxy disks as nonaxisymetric structures such as bars and global ovals rearrange the energy, the angular momentum, and therefore the orbits of disk gas. This evolution concentrates gas into three types of rings, "outer rings" at roughly 2.2 times the radius of the bar or oval (e. g., in NGC 4736), "inner rings" that usually encircle the bar (another example appears in NGC 2523), and "nuclear rings" near the center (NGC 4736 has a prominent one, too). Where gas concentrates, it bursts into stars, so nucler rings are sites of rapid star formation. In fact, they are believed to be growing "pseudobulges" -- dense central regions of galaxies that are recognizably diskier and more rapidly star-forming than elliptical-galaxy-like "classical bulges" such as the ones in M31 and in M81. In NGC 1512, the dust lanes in the bar are understood to be signatures of gas shocks; they outline where gas loses orbital energy and therefore falls toward the center. Starbursts naturally follow when the gas density gets high enough. This secular galaxy evolution picture complements the better known picture of galaxy evolution by hierarchical clustering and merging. As the Universe ages, evolution switches from being driven mostly by hierarchical clustering to slower evolution driven mostly by internal galaxy asymmetries.

Thanks, RJN, very interesting!

John Kormendy wrote:
Where gas concentrates, it bursts into stars, so nucler rings are sites of rapid star formation. In fact, they are believed to be growing "pseudobulges" -- dense central regions of galaxies that are recognizably diskier and more rapidly star-forming than elliptical-galaxy-like "classical bulges" such as the ones in M31 and in M81.

I don't get the difference between bulges and pseudobulges. How can you tell if the yellow stuff in the center of a galaxy is a bulge or a pseudobulge?

Okay, I admit that the central yellow stuff of NGC 1512 looks quite "disky" to me, mostly because of the thin elegant dust lanes winding their way all the way to the center of the galaxy. So I would guess that NGC 1512 has a pseudobulge.

Yes, but Robert Kormendy said that M81 has a classic bulge. Take a look at this APOD showing M81. You can see what looks like dust lanes winding their way towards the center of the galaxy. So how can we tell that M81 has a classic bulge and not a pseudobulge?

Here is a picture of M31 showing the dust lanes deep in the central yellow population. The dust lanes look different from the dust lanes of typical galactic bars, and they also look different from the inner dusty spiral of NGC 1512.

Even so, how can we tell the difference between pseudobulges and classic bulges?

Ann

Ann wrote:
I don't get the difference between bulges and pseudobulges.

Bulges are what everyone else my age has on their stomachs.

Pseudobulges are what I have.

Ann wrote:I don't get the difference between bulges and pseudobulges. How can you tell if the yellow stuff in the center of a galaxy is a bulge or a pseudobulge?

It may not be apparent visually (or at least, ambiguous). Active star formation is probably the best visual indicator. The disk-like kinematics may only be detectable by looking at rotation curves. Careful photometry can reveal differences by looking at the Sersic index or other measures of structure.

Chris Peterson wrote:

Ann wrote:I don't get the difference between bulges and pseudobulges. How can you tell if the yellow stuff in the center of a galaxy is a bulge or a pseudobulge?

It may not be apparent visually (or at least, ambiguous). Active star formation is probably the best visual indicator. The disk-like kinematics may only be detectable by looking at rotation curves. Careful photometry can reveal differences by looking at the Sersic index or other measures of structure.

Thanks, Chris. Take a look at these two galaxies, M95 and M100. M95 is a classic barred galaxy, with one ring encircling the bar, and one much brighter and much smaller ring encircling the nucleus. M100, by contrast, is not a classic barred galaxy at all, but it does have massive dust lanes leading to a bright inner ring.

Would you say that M95 and M100 probably have pseudobulges, and that the clearest sign of this is their bright nuclear rings?













NGC 1398, by contrast, doesn't seem to have an nuclear ring inside its bar. The GALEX ultraviolet picture of NGC 1398 shows no sign of a starforming nuclear ring. So does NGC 1398 have a classic bulge or a pseudobulge? What do you think?

Ann

Ann wrote:

Chris Peterson wrote:It may not be apparent visually (or at least, ambiguous). Active star formation is probably the best visual indicator. The disk-like kinematics may only be detectable by looking at rotation curves. Careful photometry can reveal differences by looking at the Sersic index or other measures of structure.

Thanks, Chris. Take a look at these two galaxies, M95 and M100. M95 is a classic barred galaxy, with one ring encircling the bar, and one much brighter and much smaller ring encircling the nucleus. M100, by contrast, is not a classic barred galaxy at all, but it does have massive dust lanes leading to a bright inner ring.

Would you say that M95 and M100 probably have pseudobulges, and that the clearest sign of this is their bright nuclear rings?

Honestly, I have no idea. Beyond the most obvious things, working out what's going on in galaxies from images isn't my expertise.