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Galaxy mass accuracy? (Andromeda..)



 
 
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  #1  
Old February 20th 18, 10:20 AM posted to sci.astro.research
Jos bergervoet
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Default 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  
Old February 20th 18, 03:16 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default 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  
Old February 21st 18, 06:41 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default 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  
Old February 25th 18, 10:35 PM posted to sci.astro.research
Richard D. Saam
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Posts: 240
Default 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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