Trying to find an estimate for the stellar mass of the MW galaxy, I found this paper and the estimated stellar mass is $~6.5 \times 10^{10} M_{\odot}$. I was also trying to understand the methods used to determine this number, one of them used the measured luminosity of the MW and then multiplying it by the mass-to-luminosity ratio.

Anyways, I am actually not interested in the methods now, but there is still one question that confuses me. Could this estimated stellar mass by some sort of error have a significant part of it in non-luminous matter like brown dwarfs ? Like if there exists hundreds of billions of brown dwarfs, then are their masses considered dark matter or they are included in the stellar mass ? In other words, are the methods used reliable enough to be able to count only the luminous part of the mass ?

I ask this because I know there is still missing baryonic matter, so how can we be sure that what we are calculating is actually the stars and not other dim objects ?


1 Answer 1


In another closely related question (According to the initial mass function, should there be more brown dwarfs than red dwarfs? ), I showed that the number of brown dwarfs (with $M<0.075M_{\odot}$) is a factor of five smaller than the number of red dwarf stars (stars with $0.075<M/M_{odot}<0.5$), using the widely adopted Chabrier (2005) lognormal initial mass function (IMF). The approximate correctness of this analytic IMF has been demonstrated using censuses of stars and brown dwarfs in young open clusters (e.g. Andersen et al. 2008 ).

Given that the average red dwarf in this calculation has a mass of about $0.2 M_{\odot}$ and the average brown dwarf a mass of about $0.06M_{\odot}$, you can easily see that the total mass in brown dwarfs is only about 6 percent of that in red dwarfs and a very small component of the baryonic mass of the Galaxy. Therefore brown dwarfs are not responsible for baryonic dark matter. As the luminosities of these low-mass objects scales roughly as $M^3$, you can also see that brown dwarfs contribute almost nothing to the luminosity of a Galaxy.

Turning directly to your question - how does the presence of brown dwarfs affect the mass-to-luminosity ratio? Well, they contribute a few percent to the numerator and almost nothing to the denominator.

Therefore, at most, brown dwarfs may add a few percent to the stellar mass determined from a Galaxy's luminosity.


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