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I am having trouble finding the mass of the central black hole in the Whirlpool Galaxy M51a/NGC5194, it is stated that its companion galaxy NGC5195 has a supermassive black hole at its centre that is 19 million solar masses but I can't find details on the Whirlpool Galaxy's central black hole, is its mass known?

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    $\begingroup$ Maybe it's hard to make a good mass estimate with all the dust obscuring the centre of NGC 5194. $\endgroup$ – PM 2Ring Jul 26 '20 at 17:30
  • $\begingroup$ Using the HyperLEDA mean central stellar velocity dispersion for M51a ($\sigma = 88$ km/s) and the $M_{\rm BH}$-$\sigma$ relation from Saglia et al. (2016), one would predict $M_{\rm BH} \sim 3 \times 10^{6} M_{\odot}$. That is not, of course, an actual measurement.... $\endgroup$ – Peter Erwin Dec 28 '20 at 15:46
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It is not well known, but a paper by M. Brightman et al gives a value of $10^{6.3\pm0.4}$ or between 8 hundred thousand and 5 million solar masses, while noting that this estimate is lower than previous estimates which had $10^{6.95}$ or about 9 million masses.

It seems that, although the galaxy is face-on to us, its black hole is viewed from the edge. This means that the black hole itself is hidden behind a torus of relatively cool dust and gas. [ESA] which is visible as a dark streak in the image of the central part of the galaxy

There is much that we don't understand about the M51a black hole. It is active, but not as active as would be expected, given the interaction between the two galaxies in M51. There is a second dark streak making an "X" the cause of which is unknown, and, as suggested by the wide error margins above, the mass of the black hole is not well constrained.

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