My hypothesis is that if the ratio of dark matter/matter in galaxies with supermassive(weighing billions of suns) black holes are higher the black hole itself would be made of a large portion of dark matter.

That might have interesting implications.

Are there any data supporting this or that the composition of galaxies with supermassive black holes are different.


1 Answer 1


Supermassive black holes (SMBHs) are primarily made of baryonic matter, not dark matter (DM).

Accretion disks

In order for the black hole to grow, it needs to accrete matter. In general, the probability of a particle, dark or baryonic, just randomly passing the SMBH and falling in is small. Rather, a large accretion disk forms around the SMBH. This disk is made from baryons which may collide and lose energy; in other words, the gas loses energy due to friction, emitting it as radiation. But because DM is collisionless, it doesn't take part in this accretion disk.

However, the answer to your question (about the galactic ratio, not about the SMBH's ratio) may still be "No, it's not the same", but for another reason than you think:

Feedback from stars and AGN

The more massive the galaxy is, the more massive its SMBH tends to be, and hence the more luminous its accretion disk tends to be (at least for some periods of time). The result is an active galactic nucleus (AGN), the most luminous of which are known as quasars.

Since galaxies are all formed from the same matter with a primordial ratio of baryons-to-DM, in principle all galaxies should retain this ratio. However, comparing the stellar mass of galaxies to their total mass, the so-called stellar mass-halo mass (SMHM) relation shows a distinct trend: Baryons seem to be "missing" at both the low-mass and the high-mass end, as seen in this cartoon plot from Silk & Mason (2012):


The plot shows the number of galaxies as a function of luminosity (which correlates with mass). The higher the mass, the fewer the galaxies there are. If all galaxies shared the same baryon-to-DM fraction, they should lie on the red line. But observationally, the relation seems to follow something like the blue line.

The reason is thought to be feedback processes expelling the baryons from the galaxy, hence lowering the baryon-to-DM fraction. The more massive the galaxy, the larger the AGN feedback. Low-mass galaxies have less efficient AGN, but still have feedback from stars (both exploding supernovae and radiation pressure). So do the massive galaxies, but the low-mass galaxies have a shallower gravitational potential, making it harder for them to retain the gas.

  • $\begingroup$ During recombination, the plasma, due to acoustic oscillations, contained spherical shells of over-density due to an excess of baryons. These shells have a region of over-density at their centres due to an excess of dark matter. Both have more gravity than the average value in the universe and both act as seeds for galaxy formation. But shouldn't those galaxies that form in the outer shells have a greater baryon to dark matter ratio than those that form in the central regions? $\endgroup$ Commented Mar 15, 2023 at 14:12
  • $\begingroup$ @JohnHobson That's a good question, and I think the answer is in principle yes, but the effect is very small: Although the BAO is a wave of gas and photons, DM will tend to follow along due to gravity. There will be a delay, and if galaxies were somehow able to form much earlier, the effect you mention would be large. But at t = 100–200 Myr, where the first galaxies form, DM has "caught up" with the baryons. $\endgroup$
    – pela
    Commented Mar 15, 2023 at 16:17
  • $\begingroup$ If you take a look at Fig. 1 from Eisenstein+ 07 you can se how, by the time photons decouple, the fraction is back near the cosmic mean. $\endgroup$
    – pela
    Commented Mar 15, 2023 at 16:18

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