The mass distribution is driven by a combination of the initial mass function (IMF) and the remnant mass prescription. The peak at 3 M$_\odot$ is peak lower-mass stars are more common in the IMF, and the minimum BH mass in our simulation is 3 M$_\odot$. The remnant mass prescription maps carbon-oxygen core masses to BH masses.

Although every black hole is born in a binary in our simulations, these pairings can often end in disaster for the binary. We have three categories in the plot:

Isolated (disruption) — the supernova natal kick disrupted the binary, ejecting the BH. This is the most common outcome.

Isolated (merger) — the two stars merged before either reached core collapse, leaving a single object that later formed a BH.

Binary at present day — the BH is still bound to a companion (another compact object or a star).

So the reason that 91% of BHs are isolated is a combination of two things: supernova kicks often disrupt a binary, and many binaries merge before there's even a chance to have a disruption.

Massive black holes get weaker kicks! In the fiducial model we scale natal kicks by the fraction of ejecta that falls back onto the collapsing core following Fryer+2012. High-mass BHs have more fallback, so their kicks are weak and they rarely disrupt their binaries. You can see the scaling between BH mass and natal kicks in the plot below that's from the paper.

You can drag the interactive histogram to zoom in on the area above 30 M$_\odot$ and see how much more prevalent BHs in binaries become.

The further from the plane you look, the lower mass the BHs are on average. This trend is also driven by the mass-dependent natal kicks, which lead to low-mass BHs being kicked far from the plane, and high-mass BHs retaining similar orbits to their formation.

You can test this out with the slider bar, move the left switch to increasingly higher $|z|$ values to see how the mass distribution shifts!

Only a small fraction (~3% in the fiducial model) of BHs manage to escape the Milky Way's Galactic potential. They tend to be very low-mass BHs, since the natal kick required to escape must be high, which disfavours the BH having any sizeable fallback fraction.

You can try this out by toggling between the "Bound to MW" or "Escaped" BH populations and seeing how the $y$-axis scale and mass distribution shift.

When BH natal kicks are assumed to follow the same distribution as neutron stars (i.e. no fallback scaling), the rate of BHs in binaries plummets. Conversely, if you assume that BHs do not receive natal kicks, the rate of BHs in binaries increases dramatically.

These changes are driven simply because a stronger natal kick is more likely to unbind a binary. You can learn more about this on another of my interactive pages

Yes! In the fiducial model, the further that you are from the plane, the lower the average mass of BHs. However, when you remove fallback scaling from BH natal kicks, you also remove the mass dependence, and so the distribution doesn't really change shape as you move further from the plane.

On the other hand, if you disable BH natal kicks, then the mass-dependent shift in the distribution also vanishes. However, there are also many fewer BHs at large heights because none of them receive sufficient kicks to reach those heights.

Overall, this means that if observations find a correlation between BH mass and distance from the plane, we may be able to constrain how much BH natal kick models.

Extremely!! Switching between the fiducial model and the other red buttons, you'll see that the shape of the distribution changes significantly, in terms of average masses, extra features, and the total number of BHs itself. These prescriptions make strong predictions for the expected BH mass distribution.

With the upcoming datasets of BHs (e.g. from Roman or Gaia), we will hopefully be able to rule out some of these models with the most extreme predictions. For example, the Maltsev prescription predicts that almost no BHs in the Milky Way have masses between 10 and 18 solar masses.

Our model for a time-evolving potential predicts that 2.5x more BHs will escape the Milky Way. Moreover, those BHs are slightly more massive on average.

The model we implement accounts for the expected mass growth of the Milky Way and as such has a shallower Galactic potential at early times. This weaker potential allows BHs to more easily escape early in the Galaxy's history.

Most changes to the initial distributions have only a very small effect on the overall shape of the total distribution. You may note that the relative rate of different channels varies with some choices (for example, making initial orbital periods tighter leads to a larger fraction of BHs from mergers), but the total remains fairly consistent.

The rates are mostly unchanged, except for the variation that changes the stellar initial mass function (IMF). As one may expect, a top-heavy IMF produces more massive stars, and thus more BHs.

The BH mass distribution is robust to fairly large variations in binary mass transfer physics! The relative rates of each channel are also fairly steady, except for the variations in which we force case B mass transfer to all be stable or unstable, which significantly changes the merger rates.