Thursday, February 22, 2018

Triton Station

I've added the blog Triton Station to the sidebar. It is the blog of Stacy McGaugh, the astrophysicist who is best known for compiling the mountains of data the support either modified gravity along the lines of MOND or some other mechanism that achieves the same result to bring about the phenomena often attribute to dark matter.

It debuted on April 16, 2016. Before that McGaugh had a webpage with aesthetics on a par with pong, PacMan, and Usenet, that was packed with great links, but lacked a narrative and flash. It's still out there. But, the blog is updated fairly regularly and the posts are cogent and persuasive, in addition to being 21st century pretty.

Full disclosure: McGaugh and I got our graduate degrees at the same school (he graduated around the same time I arrived on campus), so I could be biased, although this is highly unlikely as I didn't learn this fact until today, and I've followed McGaugh's work for many years.

Fun fact: Papers that I tag on this blog as "dark matter", I first save in a browser bookmarks folder labeled "gravity".


Ryan said...

Any thoughts on the latest SMBH research and how it affects MOND?

andrew said...

Not sure what SMBH research you're referring to.

Ryan said...

andrew said...

The result favors MOND over dark matter. The money quote is: "This “implies that the black hole-galaxy scaling relations do not hold for these extreme objects[.]"

MOND and kindred theories don't need to be very fussy about the galaxy formation and mass assembly process, because MOND effects are entirely a function of current baryonic (i.e. ordinary) matter distributions. In contrast, in dark matter models, the only way that you get the tight link between baryonic matter distributions and inferred dark matter distributions assuming ordinary general relativity, is through a highly structured process of galaxy evolution, in which the black hole-galaxy scaling relationship is a key orchestrating mechanism.

More generally, any observation that demonstrates a lack of a lockstep process of galaxy formation and mass assembly for all galaxies weakens dark matter particle hypotheses because it leaves the manner in which inferred dark matter halos take their current shape unanswered.

Ryan said...

I read this the opposite way - that the relationship isn't nearly as tight as previously thought. How would MOND explain itself behaving differently at higher mass scales?

andrew said...

The SMBH in a galaxy makes up less than 1% or so of the total galaxy mass and in the vicinity of the SMBH and the central bulge (in galaxies where there is a bulge) there are no MOND effects. MOND affects only kick in when the gravitational field gets weak at the edges of a spiral galaxy.

In the places where MOND effects can be discerned, the difference between a SMBH that is 1.5% of total galaxy mass v. 0.75% of galaxy mass (both fake numbers for illustration purposes only) is too small to notice, so even significant lack of correlation between SMBH size and total galactic mass has a negligible impact in the parts of a galaxy where MOND effects are observed.

In contrast, a tight correlation between SMBH size and galaxy mass (which had previously been thought to be strong than it is) would suggest that through some not well understood mechanism that the SMBH very precisely controlled mass assembly in the galaxy including dark matter halo formation which mechanism, if understood, could explain why DM halos seem to be so tightly correlated with luminous matter distributions.

This insight about weak SMBH size correlation to galaxy mass is also of a piece with the fact that MOND holds equally well in bulgeless spiral galaxies and spiral galaxies that do have bulges, with bulgeless spiral galaxies being far more common that they should be in a conventional DM galaxy formation scenario that relies upon mergers of small galaxies to get them big enough which should almost always create a bulge, and would also create a characteristic merged DM halo.

A serious problem with DM models is that inferred halos in bulgeless spiral galaxies are way too similar to inferred halos in spiral galaxies with bulges, which follows naturally from MOND but makes not sense in DM particle theories where the merged halos that should arise when smaller galaxies merge and create a bulge should be very different in shape and size than in bulgeless spiral galaxies that never developed a bulge because they never experienced a major galactic merger event.

andrew said...

Not to overstate the obvious, but the key relationship in MOND is between the amount and distribution of total baryonic matter in a galaxy and the phenomena otherwise attributed to DM. This holds from dwarf galaxies to ellipical galaxies, but then in toy model original MOND understates the amount of DM phenomena in clusters by almost an order of magnitude based upon the observable baryonic matter in the cluster.

There is a separate relationship between a galaxy's SMBH size and its total mass which isn't as tight as the Tully-Fisher relationship we know now, but is still reasonably regular. But, that relationship is not part of MOND and never has been. MOND doesn't care if baryonic matter is in a SMBH or a star or interstellar gas and dust. But, the different components are important to understand in any evolutionary model of galaxy formation.

The SMBH relation is closely related to the galaxy formation main sequence discussed in the most recent post at the Triton Station blog. Basically, galaxies grow by forming new stars and need interstellar mass to suck up to feed that process and once the space between stars gets sucked into stars and black holes there is nothing else to feed it and it stops making new stars and the stars it already had go through the multibillion year process of aging into "red and dead" stars.

andrew said...

My rough estimate of the SMBH mass to galaxy mass ratio was too high. The mean value is actually about 0.3% of the total (a.k.a. 10^-2.5). See

So, even if there is a +/- 200% variation in SMBH size relative to total galaxy mass you are talking about 0.15% to 0.9% of the total mass of the galaxy, which has virtually no discernible affect on MOND phenomena at the precision with which we are able to measure them. The variation around the mean is just one part in 200 which in MOND simply has the effect of pushing the MOND transition radius plus or minus half a percent from the center of the galaxy, a discrepancy that is significantly smaller than the uncertainty in the empirically measured value of the MOND critical acceleration constant.