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.

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