Recent results from the third data release (DR3) from Gaia has led to a flurry of papers. Some are good, some are great, some are neither of those. It is apparent from the comments last time that while I’ve kept my pledge to never dumb it down, I have perhaps been assuming more background knowledge on the part of readers than is adequate. I can’t cram a graduate education in astronomy into one web page, but will try to provide a little relevant context.
Galactic Astronomy is an ancient field, dating back at least to the Herschels. There is a lot that is known in the field. There have also been a lot of misleading observations, going back just as far to the Herschel’s map of the Milky Way, which was severely limited by extinction from interstellar dust. That’s easy to say now, but Herschel’s map was the standard for over a century – longer than our modern map has persisted.
So a lot has changed, including a lot that seemed certain, so I try to keep an open mind. The astronomers working with the Gaia data – the ones deriving the rotation curve – are simply following where those data take them, as they should. There are others using their analyses to less credible ends. A lot of context is required to distinguish the two.The total mass of the Milky Way
There are a lot of constraints on the mass of the Milky Way that predate Gaia; it’s not like these are the first data that address the issue. Indeed, there are lots and lots and lots of other applicable data acquired using different methods over the course of many decades. Here is a summary plot of determinations of the mass of the Milky Way compiled by Wang et al. (2019).
This is an admirable compilation, and yet no such compilation can be complete. There are just so many determinations by lots of independent authors. Still, this is nice for listing multiple results from many distinct methodologies. They all consistently give numbers around 1012 solar masses. (Cast in these terms, my own estimate is 1.4 x 1012 albeit with a substantial systematic uncertainty.) I’ve added a point for the total mass according to the alleged Keplerian downturn seen in the Gaia data, 2 x 1011 solar masses. One of these things is not like the others.
The difference from the bulk of the data has nearly every astronomer rolling our collective eyes. Most of us straight up don’t believe it. That’s not to say the Gaia data are wrong, but the interpretation of those data as indicative of such a small, finite total mass seems unlikely in the light of all other results.
As I discussed briefly last time, it is conceivable that previous results are wrong or misleading due to some systematic effect or bad assumption. For example, mass estimates based on “satellite phenomenon” require the assumption that the satellite galaxies are indeed satellites of the Milky Way on bound orbits. That seems like a really good assumption, as without it, their presence is an instantaneous coincidence particular to the most recent few percent of a Hubble time: they wouldn’t have been nearby more than a billion years ago, and won’t be around another for even a few hundred million more. That sounds like a long time to you and me, but it is not that long on a cosmic scale. Maybe they’re raining down all the time to give the appearance of a steady state? Where have I heard that before?
Even if we’re willing to dismiss satellite constraints, that doesn’t suffice. It isn’t good enough to find flaw with one set of determinations; one must question all distinct methods. I could probably do that; there’s always a systematic uncertainty that might be bigger than expected or an assumption that could go badly wrong. But it is asking a lot for all of them to conspire to be wrong at the same time by the same amount. (The assumption of Newtonian gravity is a catch-all.)
Some constraints are more difficult to dodge than others. For example, the escape velocity method merely notes that there are fast moving stars in the solar neighborhood. Those stars are many billions of years old, and wouldn’t be here if the gravitational potential couldn’t contain them. The mass implied by the Gaia quasi-Keplerian downturn doesn’t suffice.
That said, the total mass of the Milky Way as expressed above is a rather notional quantity. M200 occurs roughly 200 kpc out for the Milky Way, give or take a lot. And the “200” in the subscript has nothing to do with that radius being 200 kpc for reasons too technical and silly to delve into. So my biggest concern about the compilation above is not that the data are wrong so much as they are being extrapolated to an idealized radius that we don’t directly observe. This extrapolation is usually done by assuming the potential of an NFW halo, which makes perfect sense in terms of LCDM but none whatsoever empirically, since NFW predicts the wrong density profile at small, intermediate, and large radii: where the density profile ρ ∝ r-α is predicted to have α = (1,2,3), it is persistently observed to be more like (0,1,2). While the latter profile is empirically more realistic, it also fails to converge to a finite total mass, rendering the concept meaningless.
Rather than indulge yet again in a discussion of the virtues and vices of different dark matter halo profiles, let’s look at an observationally more robust quantity: the enclosed mass. Wang et al. also provide a tabulation of this quantity from many sources, as depicted here:
Not all of the enclosed mass data are consistent with one another. The bulk of them are consistent with the RAR model Milky Way (blue line). None of them are consistent with the small mass indicated by recent Gaia analyses (dotted line). Hence the collective unwillingness of most astronomers to accept the low-mass interpretation.
An important thing to note when considering data at large radii, especially those beyond 50 kpc, is that 50 kpc is the current Galactocentric radius of the Large Magellanic Cloud. The LMC brings with it its own dark matter halo, which perturbs the outer regions of the Milky Way. This effect is surprisingly strong*, and leads to the inference that the mass ratio of the two is only 4 or 5:1 even though the luminosity ratio is more like 20:1. This makes the interpretation of the data beyond 50 kpc problematic. If we use that as a pretext to ignore it, then we infer that our low mass Milky Way is no more massive then the LMC – an apparently absurd situation.
There are many rabbit holes we could dig down here, but the basic message is that a small Milky Way mass violates a gazillion well-established constraints. That doesn’t mean the Gaia data are wrong, but it does call into question their interpretation. So next time we’ll look more closely at the data.
*This is not surprising in MOND. The LMC is in the right place at the right time to cause the Galactic warp. The LMC as a candidate perturber to excite the Galactic warp was recognized early, but the conventional mass was thought to be much too small to do the job. The small baryonic mass of the LMC in MOND is not a problem as the long range nature of the force law makes tidal effects more pronounced: it works out about right.
Triton Station 
I have a few questions concerning the LMC in MOND. The mass ratio is also constrained from the recoil velocity induced by the LMC on the MW:
https://arxiv.org/abs/2304.09136
The Milgromian recoil of the MW due to the LMC should be a lot smaller because the mass ratio of 1:20 plus conservation of linear momentum implies that the MW should only recoil by about 16 km/s given a relative velocity of about 320 km/s. This seems too slow compared to the actually reported recoil from observations, though no doubt that is hard to measure. Still, there were some interesting features of the Sagittarius tidal stream which rely on the recoil of the Galactic disc due to the LMC:
https://doi.org/10.1093/mnras/staa3673
Another thing about the MOND scenario is that the LMC is bound to the MW, so it should have had a close encounter about 2 or 3 Gyr ago similar to its most recent one – there is no dynamical friction. Is there anything in the history of the Galactic disc suggesting something as major as a pericentre passage of the LMC at that time? I know in LCDM the LMC is supposed to be on a first infall, and perhaps its ongoing star formation suggests that it did not experience too long exposed to the Galactic corona. But maybe it makes more sense for the LMC to have undergone more pericentre passages than its most recent one. (In MOND, the LMC should have formed out of tidal debris expelled from a past MW-M31 flyby about 8 or 9 Gyr ago: https://doi.org/10.1093/mnras/stac722).
As for the Newtonian dynamical mass of the MW, I worked this out using the Local Group timing argument with a 3D code by Nobel laureate Peebles and got about 1.8e12 M_Sun. This is in line with the other estimates, which all seem to give values between about 1e12 M_Sun and 2e12 M_Sun. See table 3 of:
https://doi.org/10.1093/mnras/stx151
Indeed, the value reported based on the supposed Keplerian decline is too low. The MW cannot have a Newtonian dynamical mass of 2e11 M_Sun. The rapidly declining rotation curve is due to systematics with the Gaia data or some deficiency with their analysis, not because the rotation curve of the galaxy is Keplerian beyond 20 kpc. However, I suspect that the career interests of the authors involved are better served by publishing a spectacularly wrong result that gets a lot of media attention, compared to doing astrophysics properly.
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Yes, the recoil velocity is becoming an interesting constraint. More generally, I expect the LMC plays some role in causing the vectors of motion of stars well out not to point directly at the center of the Milky Way. It will take years to explore all this, both observationally and theoretically.
Yes, the LMC can be bound to the MW in MOND, and may have had a previous pericenter passage. That would help with forming the leading arm of the Magellanic Stream. I don’t know what sort of mark it would leave a on the Milky Way, or whether it would remain distinct.
Jim Peebles’s NAM code has been extensively applied by Ed Shaya, among others like yourself. It gives some interesting results, and some puzzling ones. They (https://arxiv.org/abs/1710.08935) got 2/3E12 for M2/M31, if I recall right. They also found that M33 couldn’t have had a close encounter with M31, despite the clear hints of a debris bridge connecting them. Like most methods that probe the group scale, they got a low conventional cosmic mass density, and have to invoke an “orphan” dark matter component to make up the difference with larger scale constraints. The orphan component has to be uniformly distributed to not mess up the orbits, which looks like no simulation ever.
I can’t speak to the motivations of the various and many authors involved. I think most are earnest about what they say, and aren’t just doing it for a cheap press release. Francoise Hammer, for example.
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I do not think my timing argument analysis suffers the same problem – you can add up all the masses in the table and see what it comes out to. I think I mention that the total mass divided by the volume gives about the cosmic mean density. Though the low density may be related to a local void. Note the study by Shaya seems to go further out.
The somewhat contrived scenario of Hammer is probably a genuine conviction that the scenario may work, and after all, who knows? I think it is contrived, but clearly that is somewhat subjective. But arguing the MW has a Keplerian decline beyond 20 kpc makes little sense I am afraid.
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Hi Stacy,
We have been recently aware about this series of blogs about our paper, and have discussed them in our team. While we are happy and interested by a thorough discussion about these results (and we have found some interesting comments), we would have been delighted to be invited to answer some of them.
I sent you the paper a week before its publication to arXiv, as well as to some other members of the SOC of the nice Potsdam Conference IAU 379 “Dynamical Masses of Local group galaxies”. After a single exchange, the blog appeared, but without responding to my answer or quoting it in the blog.
There are two aspects we’d like to mention, as they may help to clarify part of the debate:
1- When saying “Gaia is great, but has its limits. It is really optimized for nearby stars (within a few kpc). Outside of that, the statistics… leave something to be desired. Is it safe to push out beyond 20 kpc?”, one may wonder whether the significance of Gaia data has been really understood.
In the Eilers et al. 2019 DR2 rotation curve, you may see points with small error bar up to 21-22 kpc. Gaia DR3 provides proper motion (systematics) uncertainties that are 2 times smaller than from Gaia DR2, so it can easily goes to 25 kpc or more.
The gain in quality for parallaxes is indeed smaller (30% gain). However, our results cannot be affected by distance estimates, since the large number of stars with parallax estimates in Wang et al. (2023) is giving the same rotation curve than that from (a lower number of) RGB stars with spectrophotometric distances (Ou et al. 2023), i.e., following Eilers et al. 2019. And both show a Keplerian decline, which was already noticeable with DR2 results from Eilers et al 2019. The latter authors said in their conclusions: “We do see a mild but significant deviation from the straightly declining circular velocity curve at R≈19–21 kpc of Δv≈15 km s−1.” Our work using Gaia DR3 is nothing else than having a factor 2 better in accounting for systematics, and then being able to resolve what looks like a Keplerian decrease of the rotation curve.
We may also mention here that one of us participated to an unprecedented study of the kinematics LMC (Gaia Collaboration 2021, Luri’s paper), which is at 50 kpc. Unless one proves everything that people has done about the LMC and MW is wrong, and that the data are too uncertain to conclude anything about what happens at R=17-25 kpc, the above clarifications about Gaia accuracy are truly necessary for people reading your blog.
2- The argument that the result “violates a gazillion well-established constraints.” has to be taken with some caution, since otherwise, no one can do any progress in the field. In fact, the problem with many probes (so-called “satellites”) in the MW halo, is the fact that one cannot guarantee whether or not their orbits are at equilibrium with the MW potential. This is the reverse for the MW disk, for which stars are rotating in the disk, and, e.g., at 25 kpc, they have likely experience 7-8 orbits since the last merger (Gaia-Sausage-Enceladus), about 9 billion years go. In other words, the mass provided by a system mostly at equilibrium, likely supersedes masses provided by systems that equilibrium conditions are not secured. An interesting example of this is given by globular clusters (GCs). If taken as an ensemble of 156 GCs (from Baumgardt catalog), just by removing Pyxis and Terzan 8, the MW mass inside 50 kpc passes from 5.5 to 2.1 10^11 Msun. This is likely because these two GCs may have come quite recently, meaning that their initial kinetic energy is still contributing to their total energy. A similar mass overestimate could happen if one accounts the LMC or Leo I as MW satellites at equilibrium with the MW potential.
So we agree that near 25 kpc the disk of the MW may show signs of less-equilibrium, or sign of slightly less circular orbits due to different phenomenas discussed in the blog. However, why taking into account objects for which there is no proof they are at equilibrium as being the true measurements?
In our work, we have considerably focused in understanding and expanding the whole contribution of systematics, which may comes from Gaia data, but also from assumptions about stellar profile (i.e., deviations from exponential profiles), from the Sun distance and proper motion and so on. You may find a description in Ou et al.’s Figure 5 and Jiao et al.’s Figure 4, both showing that systematics cannot gives much more than 10% error on circular velocity estimates. This is an area where we are considered by the Local Group community as being quite conservative, and following Gaia specialists with who we have worked to deliver the EDR3 catalog of dwarf galaxy motions (Li, Hammer, Babusiaux et al 2021) up to about 150 kpc. Jiao et al. paper main contribution is the fair accounting of systematics, which analysis shows error bars that are much larger than those from other sources of errors especially in MW outskirts (see Fig. 2).
Finally, we noticed a ad hominem attacks in the blog, which might be linked to the nature of a blog for which discussion does not include authors of the concerned paper (hopefully not anymore after this message). One may read “However, I suspect that the career interests of the authors involved are better served by publishing a spectacularly wrong result that gets a lot of media attention, compared to doing astrophysics properly.”, which we find very unfair to us. Indeed, we may have a Press Release on this paper because it is also our duty to let aware the grand public about scientific results. Our work is more a methodological work on data analysis and then just a comparison of our results to model expectations. We would be happy to pursue discussions on topics we’re all interested, without receiving very unjustified comments.
Regards,
Francois Hammer
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The Newtonian dynamical mass must be at least 4e11 M_Sun by 100 kpc to match the Sagittarius tidal stream:
https://academic.oup.com/mnras/article/445/4/3788/1077608
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The pick of which interpretation (Wang, Ou or Zhou) and as transformation the derivative at only the range 17-23 kpc hurt us. I think it misrepresents the data. One could also try the 2nd derivative, or the third or any transformation on a seemingly neutral set of outliers. I suppose you have transformed sigma accordingly?
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Indeed, with Gaia DR2 Eilers et al. had been not able to go beyond 23 kpc. However, with Gaia DR3 and PM systematics reduced by a factor 2, we (as well as Ou et al and Wang et al) have been able to go up to 26.5 kpc, with error bars that are slightly overestimated to avoid controversy (not sure it has worked, though).
It results that the sampled range of distances is 17 to 26.5 kpc, and the significance of the Keplerian decline is given in Figure 10 of Jiao et al. We never claim for a very high significance for the result (it is just a first detection), although whatever the data analyses of Gaia, they all pointed out to a further decrease of the RC slope beyond 17-19 kpc, and they are consistent with a Keplerian decrease.
Thanks for suggesting to play with the second derivative, it is a good idea. However, the change of slope can be directly seen in Eilers et al 2019, Ou et al. 2023 and our work. Indeed it always happened between 17 and 19 kpc, i.e., quite close from the external radius of the Milky Way. By the way we have tried to sample 17-26.5 kpc and 19-26.5 kpc, and the results are very similar, i.e., they systematically favor V ~ R^(-1/2).
Yongjun Jiao have also made a further test, about the slope of -1.7 km/s/kpc that fits Gaia RC from 7 to 17 kpc (Eilers et al, Ou et al., Jiao et al). Beyond 19 kpc it is rejected at 1.5 sigma level.
Yes, evolution of sigma has been accounted for. Please have a look to Figure 5 of Wang et al. 2023, in which you may see that it is not a factor able to modify the rotation curve.
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So instead of apologizing for your pick of dataset and of transformation with at least some excuse, you proceed as if my illustration (of how sought-after the analysis was) is actually a suggestion to make it worse? If you pick all the data (0-23 kpc and from Ou, Zhou and Wang), does it correspond better to the RAR or to a keplerian decline? That would be a fair analysis, you haven’t checked the keplerian decline hypothesis with that overall picture.
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While it’s true that there is a deviation from the RAR starting around 20 kpc distance, I’d attribute it as follows:
At 20 kpc the distance to the LMC center is around 30 kpc and it’s mass is around 5% of the MW. Using the inverse of radius law in the MOND regime this means the gravity from the LMC is 30 times less than that of the MW at 20 kpc. This means 29/30th times the RAR should be found there. The RAR gives V=220 km /s and then the data should give around 213 km/s. At 24 kpc the same reasoning should give 19/20th of the RAR. That is 19/20th of V=210 km/s which is around V=200 km/s.
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It is a bit more complicated in practice – first of all, the LMC is dominated by the Galactic gravity, so the gravity from the LMC is really G*nu*M_LMC/r^2, where r is distance from the LMC and nu is the interpolating function with argument g_MW at the LMC position, so about sqrt(G*M_MW*a_0)/d(MW-LMC). It also depends where the observations are. Certainly the LMC could be pulling outwards and reducing the rotation curve amplitude. It is not so far from the Galactic disc plane.
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The issue with the paper is that it makes extraordinary claims (basically that the majority of the other tracers is wrong) without providing extraordinary proofs.
Regarding the error bars associated with the effective measurements – they may be indeed quite tight, but you need to be very sure and be able to prove that what was measured is indeed what you intended.
Just for the sake of an analogy – you place some drifting buoys on a still water lake and you can measure the drift speed with very high accuracy – but if you want to use the buoys data as a proxy for the overall water speed… well, that will not work – the net overall speed is zero for a still water lake.
Some other thoughts – may I ask why the paper specifically rules out only MoND? What not also LCDM? I mean, if the data shows already a keplerian decline at the investigated distances – the DM halo must be substantially smaller than what is typically expected in the DM paradigm.
I find it rather questionable that a press release may be in the plans for this paper as, honestly, I personally see only one side as true beneficiary for such a move. What will the press release say?
– the new data is in contradiction with the vast majority of the other tracers and because the new data comes from direct measurements and has tight error bars, all other tracers must be wrong? What do you expect from the community after such a claim? I don’t want to be rude or to get accused of ad hominem attacks – this is just the reality and I believe you and nobody can deny it… such a claim will result in many citations.
– that MoND is ruled out by the new data (with no mention of LCDM issues)? How’s that going with letting the public know about scientific results in an unbiased way. I know your comment did not include this last qualifier about an unbiased way – but from my point of view, that is a must and should be the go-to assumption when reading what you wrote.
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I have to correct part of the above comment and to apologize for the incorrect assertions that I made, based on a wrong assumption (by mistake, I associated the paper under discussion with another one that was discussed here some time ago: arXiv:2309.05252 [astro-ph.GA]).
So – because I cannot edit the comment, I’ll re-post it redacted.
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The issue with the paper is that it makes extraordinary claims (basically that the majority of the other tracers is wrong) without providing extraordinary proofs.
Regarding the error bars associated with the effective measurements – they may be indeed quite tight, but you need to be very sure and be able to prove that what was measured is indeed what you intended.
Just for the sake of an analogy – you place some drifting buoys on a still water lake and you can measure the drift speed with very high accuracy – but if you want to use the buoys data as a proxy for the overall water speed… well, that will not work – the net overall speed is zero for a still water lake.
Some other thoughts – I find it rather questionable that a press release may be in the plans for this paper as, honestly, I personally see only one side as true beneficiary for such a move. What will the press release say?
– the new data is in contradiction with the vast majority of the other tracers and because the new data comes from direct measurements and has tight error bars, all other tracers must be wrong? What do you expect from the community after such a claim? I don’t want to be rude or to get accused of ad hominem attacks – this is just the reality and I believe you and nobody can deny it… such a claim will result in many citations.
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So, again, apologies for the incorrect parts, but my main objection still stands.
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hi,
to clarify is MOND able to correctly model the Milky Way based solely on visible matter and out to the edges of the galaxy ? btw how many kpc from the Milky Way center for MOND a0 to occur ?
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The MOND radius of the Milky Way is about 9 kpc. MOND works fairly well with its rotation curve, but Stacy might be able to address that question in more detail.
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I think you’ve seen by now that the following posts answers this question.
In this MW model, a0 is reached at about R = 13 kpc.
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Good day all – I’d would like to continue the discussion focusing on the R-V plot provided by Dr. McGaugh – it is very informative. I’d also like to borrow it to illustrate the low mass estimate recently obtained from Jiao’s stellar disk RC (mass discrepancy D~3) as it relates to other tracer cohort data located at more distant radii
Taking the macro-perspective view for this R-V figure, we find the bulk of data generally trends concavely downward with radius, not horizontally to the right at constant rotation velocity. Perhaps even more apparent than the stellar disk break is the obvious Keplerian decline at the edge of the HI gas disk at 40 kpc (D~6) where parameters associated with this dynamic mass are used to position the Galaxy on the BTFR.
Just as we find for the stellar and HI gas disks (and baryons at these distances), a strong Keplerian signature is also observed in the global system escape velocity profile. This can be visualized in the above figure by mentally tracing a Keplerian D~12 curve (use 325 km/s at 40 kpc as a reference point) roughly parallel to Jiao’s dashed result, also Keplerian. You will find D~12 curve coincides with the upper limit of the data sets including uncertainty. Assuming equilibrium, this profile defines the maximal velocities allowed as a function of Galactic radius. This new interpretation makes understanding disk galaxy dynamics even simpler than originally thought.
The perceived velocity support in this outer region (20 to 200 kpc) is not due to DM or MOND, but a ‘shift’ in tracer motion into a higher energy/acceleration space (D~6->12). These tracers are loosely bound, higher D and leans toward stochasticity (the tracers fill the acceleration space including a few data points that appear to substantiate the flat RC. These “higher energy” tracer cohorts typically provide relatively greater kinematic ‘mass’ estimates than disk counterparts which has naively been construed as evidence of missing matter and/or the action of non-Newtonian dynamics.
It is evident these various works show kinematic mass estimates are dependent on tracer selection/cut criteria – even when the entire enclosed mass is highly constrained by a Keplerian break. This is no cause for alarm. In general, there is nothing inherently wrong with the low stellar disk mass estimate returned by Jiao. This particular tracer cohort selected just happens to be tightly bound, low D and with low attendant total mass – consistent with expectation. I wished Jiao would have provided more context and introspection in light of this finding because I was left wanting for more. This discussion is zeroing in on some important implications with regard to NFW DM halo fits (mass estimates) as they are currently obtained. Regards, Jeff
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Dear Dr. McGaugh and esteemed audience, I would like add a thought or two regarding the success of the RAR in reproducing the inner disk circular velocity profile, but also from the perspective at the terminal gas radius (the observed ‘edge’ of the galactic disk). Allowing for some leeway and moving ahead with the (not so) crazy assumption that the BTFR dynamic is a necessary and sufficient condition for long-term galactic quiescent equilibrium, I attempt to explain Jiao’s RC findings in this context.
For physical consistency, the RAR’s observed minimum acceleration (terminal data point) typically coincides with the global acceleration value used to place the Galaxy on the BTFR, although any point along the RAR (interior to the disk’s edge) also satisfies the BFTR. These terminal accelerations cannot exceed system escape velocity which effectively limits mass discrepancy to no more than D~12. Although this confines these relations to a fraction of available acceleration space, there still exists an infinity of potential RCs that are physically permissible given the wide variety of internal structure and external factors observed.
Conceptually, RARs can be perceived as a kinematic ‘paths’ linking two defined “end points.” With conserved fields and forces in play within the disk, one could make the argument that RCs should be inherently path independent. However, one does not find this to be the case at all. Individual galaxy RARs are not stochastically distributed in acceleration space but follow highly constrained radial acceleration profile obeying M~V4 dynamics. Adherence to this relation is so strong that it has a universal, law-like nature that motivates the MOND phenomenology.
The above discussion offers some context why the action of the virial theorem may better explain the tight correlations observed between RC and baryon accelerations than LCDM or MOND. Although many paths (RC profiles) lead to the same terminal destination, all galactic RCs presently observed are those associated with systems that have settled into quiescent equilibrium. The fact that all galaxies obey the BTFR makes this almost a precondition. What the RAR is fundamentally telling me is that there exists a preferred dynamic operating within galaxies crucial for their long-term survival.
Which is more fundamental, the RAR or BTFR? Perhaps neither as both are outcomes of the second law of thermodynamics at it plays out in the cosmic setting. Under this scenario, both relations spontaneously coevolve to satisfy the total galactic mass-energy budget. Also, neither relation is truly emergent as both are physical responses dependent on structure, ongoing secular processes, and strict regulatory oversight.
It is now evident that the a priori assumption of a highly extended and flat stellar RC is not appropriate nor consistent with our Galaxy’s dynamic requirements. From Jiao’s result, we can roughly estimate the stellar disk contribution to total dynamic mass. We find the stellar disk is not the dominant contributor first thought, only accounting for 40% of the Galaxy’s dynamic mass even though this component is 80% of its total baryonic mass. In the balance, the remaining 20% of the baryons account for 60% of the dynamic mass, an interesting stat on its own. Jiao’s meager disk contribution is all that’s needed to dynamically stabilize the Galaxy. Ultimately, all RC profiles are determined by the need to satisfy the equilibrium condition.
And yes, every bump and wiggle in the RC tells us something that is summarily discounted in nearly all current dark matter halo models. Not lost in this analysis is that Jiao’s stellar break coincides with the abrupt drop in density in the stellar halo first recognized by Deason. It also coincides with the inner cut-off radius for the dwarf satellite galaxy population observed by Fritz. Perhaps these features can be integrated with important clues from the disk RC, spiral arms, and central bars to better understand and appreciate their individual roles ensuring these objects “are present and accounted for” in the empirical (MOND) scaling relations.
There is much to be gained using the stellar disk RC as a great learning tool. Dr. McGaugh’s 4-part discussion and detail knits recent findings and controversy nicely together and is gracious in allowing off-mainstream ideas to be presented to the very best in the field. Note, these are only ideas and food for thought. Any feedback is always a gift! Best regards, Jeff
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