by Ann » Sun Jul 30, 2017 7:59 pm
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.
Nuclear ring and pseudobulge(?) of NGC 1512.
NASA, ESA, Hubble, LEGUS. Acknowledgement: Judy Schmidt.
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
[quote="RJN"]Noted galactic astronomer [url=http://chandra.as.utexas.edu/]John Kormendy[/url] 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: [quote]
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 [url=https://apod.nasa.gov/apod/ap150526.html]NGC 4736[/url]), "inner rings" that usually encircle the bar (another example appears in [url=https://inspirehep.net/record/1264887/plots]NGC 2523[/url]), and "nuclear rings" near the center [url=https://apod.nasa.gov/apod/ap150526.html](NGC 4736 has a prominent one, too)[/url]. 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 [url=https://apod.nasa.gov/apod/ap130626.html]M31[/url] and in [url=https://apod.nasa.gov/apod/ap151017.html]M81[/url]. In NGC 1512, the dust lanes in the bar are [url=http://adsabs.harvard.edu/abs/1992MNRAS.259..345A]understood[/url] 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. [url=http://adsabs.harvard.edu/abs/2004ARA%26A..42..603K]This secular galaxy evolution picture[/url] 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.
[/quote][/quote]
Thanks, RJN, very interesting!
[quote]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. [/quote]
[float=left][img2]http://68.media.tumblr.com/1ae2b5efb60d81a5a8c31734fc9bb15f/tumblr_osv4s71sEd1qz9agho1_500.jpg[/img2][c][size=85]Nuclear ring and pseudobulge(?) of NGC 1512.
NASA, ESA, Hubble, LEGUS. Acknowledgement: Judy Schmidt.[/size][/c][/float]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 [url=https://apod.nasa.gov/apod/ap141119.html]this APOD showing M81[/url]. 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 [url=https://apod.nasa.gov/apod/ap140730.html]a picture of M31[/url] 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