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G'day there, Paul,
Your question about the relative sizes and luminosities of the Milky Way and the Sombrero Galaxy has had me working on this problem for a couple of hours every day, for the last week.
The Sombrero has a rich (1900 +/- 200) globular cluster population for two reasons:
(1) It is more luminous than our own galaxy
(2) It has a gigantic spheroidal bulge/halo component(s)
vs. our galaxy which has only a very small central bulge. And there are more globulars per unit of galaxy luminosity in those galaxies dominated by a bulge or spheroid.
The luminosity data on the Sombrero has been relatively easy to find, but in an odd sort of way, assessing the total luminosity of the MW has been an unpopular problem in the professional literature (only about 4 attempts to do this, in the last 20 years).
So I am going to ask a couple of Milky Way specialists if they have some recent data on the luminosity of our own Galaxy.
There is an old figure of about absolute blue magnitude -20 for the luminosity of our own galaxy which has been endlessly rehashed in the textbooks, but this figure does not really cut it any more......it is disappointing for the amateur to realize that the textbook writers do not usually utilize the most up-to-date data!
cheers,
Robert Lang
P.S.
Relative diameter of the two galaxies is going to be easier to assess. Our own galaxy is certainly no more than 80,000 light years across.
The diam. of the sombrero can be assessed from the catalog angular diam. plus there is a good recent estimate of its distance. However, M104 is very very different from our own galaxy, as our own galaxy has only a very very very faint halo! Indeed, there are strong arguments for M104 being earlier than type Sa in the orthodox Hubble Sequence of :
E - S0 - Sa - Sb - Sc - Sd - Sdm - Sm - Irr
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