ITN: ESO's VLT Great Barred Spiral Shots

[CraigS]

In the News: Another zoom-in sequence, this time of NGC 1365 (The Great Barred Spiral Galaxy) taken by the HAWK-I infrared camera on ESO's Very Large TElescope at Paranal Observatory in Chile:

An elegant galaxy in an unusual light (w/ Video)

A new image taken with the powerful HAWK-I camera on ESO's Very Large Telescope at Paranal Observatory in Chile shows the beautiful barred spiral galaxy NGC 1365 in infrared light. NGC 1365 is a member of the Fornax cluster of galaxies, and lies about 60 million light-years from Earth.

interestingly the article (journos) say:

The huge bar disturbs the shape of the gravitational field of the galaxy and this leads to regions where gas is compressed and star formation is triggered.
…
The bar also funnels gas and dust gravitationally into the very centre of the galaxy, where astronomers have found evidence for the presence of a super-massive black hole, well hidden among myriads of intensely bright new stars.
…
Different parts of the galaxy take different times to make a full rotation around the core of the galaxy, with the outer parts of the bar completing one circuit in about 350 million years.

Some reading up to do on these latter statements.
… If I can find the paper ...

Cheers

[renormalised (Carl)]

Just go to arXiv.org and type in barred spiral galaxy formation or dynamics of bars in spiral galaxies. You can do the same in Google Scholar. Or...you can buy this book...Galactic Dynamics

[CraigS]

(This one is almost further to our previous discussions in threads on Rotation Curves)...

Found a really good paper on arXiv this morning. Reasonably recent also (April 2010):

Global disk model for galaxies NGC 1365, NGC 6946, NGC 7793, UGC 6446
Joanna Jałocha1, Łukasz Bratek1, Marek Kutschera and Piotr Skindzier.

Recently, we have analyzed mass distribution in several spiral galaxies (Jałocha et al. 2007, 2008; Bratek et al. 2008). We find that for some galaxies it cannot be spherical at larger radii. Since we expect in this case a flattened mass distribution to better approximate the gravitational potential at larger radii than the spherical one, we apply the global disk model. We then find that mass distribution of luminous matter accounts for rotation curves of the examined galaxies.

There is also increasing number of studies whose authors, using more involved galaxy models than so far, conclude, contrary to earlier findings, that luminous matter accounts for rotation in the internal galactic regions. (Sellwood & Kosowsky 2000; Evans 2001; Palunas & Williams 2000; Williams, Bureau & Cappellari 2009). There is also possibility that rotation of galaxies in the outermost regions can be driven by magnetic fields, not by dark matter (Battaner, Garrido, Membrado & Florido 1992).
These results need to be seen properly in the context of non-baryonic dark matter searches. Currently, there is strong observational evidence of non-baryonic dark matter in clusters of galaxies, in particular, from gravitational lensing by clusters and from colliding clusters, as e.g. the Bul- let cluster (Clowe et al. 2006). Also, the cosmological model provides support to the non-baryonic dark matter hypothesis, albeit at a more theoretical level. On the other hand, direct searches give null results, except for DAMA/LIBRA (Bernabei et al. 2007), however, these results are disputable and have not been confirmed by other searchers

An interesting paper. Well balanced, too. They go at it from an 'anything's possible' perspective and put all the known ideas to the test.

Also very interesting to see them considering the possibility that Magnetic fields might be counteracting gravitational forces in the outer arms.

Very interesting.

Cheers

[renormalised (Carl)]

Better hide that stuff about magnetic fields...you know who will have a field day with this if he comes back and sees this

The big problem with that is the very weakness of the magnetic fields. These fields have to drive rotational velocities that can be up to 500kms or more in some of the most massive of the galaxies and 160-220kms for most galaxies in general. You would need a more intense field than what is observed. You not only have to account for the dipole moment of the atoms within the gas/dust clouds being influenced by those fields, you also have to drive the rotation of the stars as well. Considering the magnetic fields of stars are far more intense than the fields in the ISM, I would seriously doubt that these fields would make any headway with influencing the rotation of the stars about the galaxies.

[CraigS]

More interesting stuff ..

Of course, in real galaxies, masses are neither distributed in infinitely thin disks nor in ideally spherically symmetric structures, orbits are not circular, rotation field has a very complicated structure, and mass distribution does not have axial symmetry – axial symmetry is clearly broken by bars and spiral structure.
….
The models are only slightly more accurate than would be a dimensional analysis, which states that the amount of electro- magnetic energy radiated by a galaxy should be given by L=αDc Vc2 / (Gμ), where Vc is some characteristic velocity derived from Doppler image, Dc is its radial size estimated based on the spatial range of the luminous stuff, μ is some mass-to-light ratio derived on population analysis ground, and α is some unknown dimensionless factor related to the particular geometry of mass distribution and of rotation field. All parameters apart from α can be estimated from measurements.
…
The approximation should improve when one distinguishes spherical and disk-like subcomponents, further, one can add a width to the disk, and so on. But it would be hard to treat these models seriously when dynamics and stability of a galaxy is concerned (especially when maximal disk model is used which treats on equal footing flat and spheroidal internal mass components).
…
Without realistic modeling such difficult problems as structural stability, cannot be answered.

Aside from the Electromagnetic paragraph (only included to show they have a way of modelling it), Carl, I think you've pointed out before, the importance of getting the model right before making any conclusions about Galaxy Rotation Curves. I think Steven may have, also. It certainly is complex and dependent on the shapes of the Galaxies, matter densities etc.

Lots more work to do in this area, it seems.

Cheers

[CraigS]

They go on to say, in the conclusions, that:

There is no need for non-baryonic dark matter in galaxies NGC 4736 or NGC 7793, but one cannot conclude that one does not need it in other spiral galaxies.

NGC 6946 is not so clear. There are significant discrepancies between published rotation curves, and consequently, in the obtained Mass/Light (M/L) ratios.

Galaxy UGC 6446 satisfies some of our criteria without non-baryonic dark matter, however its M/L ratio is highest and, in addition, it grows dramatically toward galaxy "edge".

There is a strong argument for absence of spherical non-baryonic dark halo. This argument does not exclude the presence of a non-spherical halo. However, the mass needed in the galaxy outskirts to account for the rotation in the disk model is practically that of gas. It would be therefore worthwhile to consider other than CDM causes of the M/L ratio growth.

So, in other words;

- some Galaxies don't need any Cold Dark Matter (CDM) to explain their RCs;
- different amounts of CDM needed to explain some Galaxies' RCs;
- different models are needed to explain other Galaxies' RCs.

Not as straightforward as the commonly heard saying: 'flat rotation curves in galaxies means Dark Matter is required to explain them'.

Cheers