How much denser is it in the galactic bar than the "normal" density at the same radius?
Is it just a few percent? or is it, say, "three times" as dense?
Or are other factors at play: star brightness, gasses?
Or do we really not know?
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$\begingroup$ I think observational constraints on this are sparse, if any, but from numerical simulations I think 10-20% overdensity would be a realistic guess. Can't seem to find any references though, so I wouldn't feel comfortable posting an answer. $\endgroup$– pelaDec 20, 2018 at 13:46
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1$\begingroup$ It's definitely quiet mysterious, right @pela ? Furthermore: some references will say that the only overdensity is an overdensity of brighter stars (ie, young ones); there's actually no overdensity at all. It does seem to be highly unknown. There is no real literature review of the issue it seems. $\endgroup$– FattieDec 20, 2018 at 13:49
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$\begingroup$ In the spiral arms, that's at least the case. Here the overdensity is roughly 10%, but you have a larger amount of newly formed stars (because of pressure waves initiating star formation), and since the brightest stars die fast, they are predominantly found in the spiral arms, making them more visible. Something similar probably is the cases in the bar, but I'm not sure if it's to the same extent, because of the redder color of the bar. $\endgroup$– pelaDec 20, 2018 at 14:21
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1$\begingroup$ The stellar population is older, so the massive, blue stars have died. The metallicity is also generally higher in the center, leading to redder colors. $\endgroup$– pelaDec 20, 2018 at 15:19
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2$\begingroup$ @pela -- also, the density contrast for spiral arms can much more than 10% -- it can be factors of 2 or 3. E.g., from this classic study by Rix & Rieke (1993) of M51: "In M51 we find the surface mass density contrast (arm/interarm) to range from 1.8 to 3, comparable to results from N-body simulations of the galaxy's tidal encounter with NGC 5195." $\endgroup$– Peter ErwinDec 21, 2018 at 10:09
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2 Answers
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In other barred galaxies which are vaguely similar to the Milky Way, the contrast in (projected) stellar surface density between the bar and the inter-bar region at the same radius (e.g., along the bar's minor axis, perpendicular to the bar) is typically a factor of at least two; in particularly strong bars it can be as high as six (see, e.g., Figure 5 in Ohta et al. 1990). Similar contrasts are seen in N-body models of disk galaxies that form bars.
It's much harder to figure this out for the Milky Way, because we're not looking down it from above. The best attempt to derive a model of the bar's 3D stellar density from star counts and distance estimates that I know of is Wegg et al. (2015). From the face-on projected view of their model (their Figure 14), I would guesstimate the maximum contrast as a factor of 4 or so.
Figure 14 of Wegg et al.: projected face-on view of stellar density for the Milky Way (full model in right-hand panel).
The 3D density (which is maybe what you're really asking about) in the inner part of the bar is not quite as great as this suggests, because the inner part of the bar is vertically thick, forming a "boxy/peanut-shaped" bulge (this would correspond to the red region in the figure above). So the contrast would be a little less compared to the (less thickened) inter-bar region. But the outer part of the bar is roughly as thin as the rest of the disk, so the projected surface density contrast would mean a similar contrast in 3D stellar density.
answered Dec 21, 2018 at 9:52
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$\begingroup$ Incredible. BTW Surely GAIA data will blow away existing data, on precisely this issue, no?? $\endgroup$– FattieDec 21, 2018 at 11:55
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$\begingroup$ " .. because the inner part of the bar is vertically thick, forming a "boxy/peanut-shaped" bulge .." AHHHHHH that's a great point! Of course, it may just be simply thicker, NOT more dense!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I did not think of that! $\endgroup$– FattieDec 21, 2018 at 11:56
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1$\begingroup$ GAIA data will undoubtedly help a lot, although much of this analysis is based on infrared data that allows stars to be seen at large distances, including the far side of the bar; since GAIA is optical, I don't think it can get that kind of data. $\endgroup$ Dec 21, 2018 at 22:16
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In the Milky Way, the density in the bar seems to roughly 5 times larger than "next to the bar".
The most recent model of the Galactic bar I could find is Portail et al. (2017), whose model is constructed to match a range of observational surveys (VVV, UKIDSS, 2MASS, BRAVA, OGLE, and ARGOS). The figure below from this paper shows the density profile of the bar/bulge (left panel), the disk (middle panel), and the combined mass (right panel).
The red curve shows the density along the bar (i.e. the major axis), and the blue curve shows it perpendicular hereto (the minor axis). The central bump in the blue curve thus is inside the bar, but after roughly 2 kpc (i.e. 6-7000 lightyears), it flattens out. Here the mass (surface) density is roughly $10^9\,M_\odot\,\mathrm{kpc}^{-2}$. Off-bar, however, the blue line shows that the density is only $2\times10^8\,M_\odot\,\mathrm{kpc}^{-2}$, i.e. 5 times lower.
