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Galaxy mass accuracy? (Andromeda..)
"Just how big is the Andromeda galaxy?" (astronomy.com)
http://astronomy.com/news/magazine/2018/02/adromeda-is-the-same-size-as-the-milky-way If the mass of this very nearby galaxy is already difficult to measure, does this mean that the total baryonic mass in the universe is in fact also known only with, say, a factor 2 of experimental error? [And could this total error be in the other direction, thereby reducing the amount of required dark matter?] -- Jos |
#2
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Galaxy mass accuracy? (Andromeda..)
In article , Jos bergervoet
writes: "Just how big is the Andromeda galaxy?" (astronomy.com) http://astronomy.com/news/magazine/2018/02/adromeda-is-the-same-size-as-the-milky-way If the mass of this very nearby galaxy is already difficult to measure, does this mean that the total baryonic mass in the universe is in fact also known only with, say, a factor 2 of experimental error? [And could this total error be in the other direction, thereby reducing the amount of required dark matter?] The best constraints on the baryon density come from big-bang nucleosynthesis and the CMB. These are theoretically sound and based only on standard physics. These indicate that there is MORE baryonic matter than we know about, by a factor of a few. There are some indications that it might be in cold gas between galaxies. Even within the uncertainties, all of the baryons can't be closely associated with galaxies. There are many lines of evidence for non-baryonic dark matter, the density of which is many times that of baryonic matter, including baryonic dark matter. So, at best, finding more baryons in galaxies would SLIGHTLY reduce the amount of required non-baryonic dark matter, but not by much. |
#3
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Galaxy mass accuracy? (Andromeda..)
In article ,
Jos bergervoet writes: "Just how big is the Andromeda galaxy?" (astronomy.com) http://astronomy.com/news/magazine/2018/02/adromeda-is-the-same-size-as-the-milky-way A preprint of the scientific paper is at https://arxiv.org/abs/1801.03949 The authors use radial velocities of planetary nebulae to derive a _dynamical mass_. That includes both baryonic and non-baryonic matter. If the mass of this very nearby galaxy is already difficult to measure, does this mean that the total baryonic mass in the universe is in fact also known only with, say, a factor 2 of experimental error? [And could this total error be in the other direction, thereby reducing the amount of required dark matter?] As Phillip wrote, the total masses of both baryonic and non-baryonic matter are known with about 4% precision from the microwave background measurements and other cosmological data. See Table 3 of https://doi-org/10.1051/0004-6361/201525830 The same table shows baryonic mass known to about 1%. (In interpreting the table, remember that h=0.6731 so h^2 = 0.453.) Measuring masses of individual galaxies is difficult. Stellar masses depend on corrections for unseen parts of the mass function, for example. The light is emitted mostly by the most massive stars, but it's the least massive stars that constitute the bulk of the mass. The Andromeda galaxy has an additional problem because its angular extent on the sky is so large that it's difficult to measure the total light it emits. That doesn't affect dynamical measurements as the one in the OP, but as noted, the dynamical measurement includes the non-baryonic matter. As Phillip also noted, when you add up the inferred stellar masses of all the galaxies plus minor constituents such as gas in galaxies and easily observable gas in galaxy clusters, you get about half the mass indicated by the cosmological observations. This is called the "missing baryon problem." X-ray observations in the last few years seem to indicate that most of the missing baryons are in extremely hot gas associated with galaxy clusters. There is also some cold gas between galaxy clusters, but I _think_ that's not so important. I'm not sure what the limitation on the hot gas measurements is, but it might be knowing the gas temperature. An observational problem is that the gas is very diffuse and has low X-ray surface brightness, so the observations have relatively large uncertainties. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#4
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Galaxy mass accuracy? (Andromeda..)
On 2/20/18 11:41 PM, Steve Willner wrote:
In article , As Phillip wrote, the total masses of both baryonic and non-baryonic matter are known with about 4% precision from the microwave background measurements and other cosmological data. See Table 3 of https://doi-org/10.1051/0004-6361/201525830 The same table shows baryonic mass known to about 1%. (In interpreting the table, remember that h=0.6731 so h^2 = 0.453.) Note the Table 3 Heading Table 3. Parameters of the base =CE=9BCDM cosmology computed from the 2015 baseline Planck likelihoods, illustrating the consistency of parameters determined from the temperature and polarization spectra at high multipoles. The paper describing Corotating galactic satellite systems https://arxiv.org/abs/1802.00081 provides a clue that present cosmological models 'base =CE=9BCDM cosmology' are wrong. [[Mod. note -- More accurately, if correct, that paper suggests that one or more of * lambda-CDM cosmology * our understanding of satellite galaxy formation * our understanding of satellite galaxy dynamics may be wrong. -- jt]] This is like Galileo observing the Jovian moons that should not be there. |
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