What does it mean to have a Black Hole Spike?

[majormuss]

Hi all,
I am currently reading a paper and they have this statement in the abstract:

"If the supermassive black hole Sgr A* at the center of the Milky Way grew adiabatically from an initial seed embedded in an NFW dark matter (DM) halo, then the DM profile near the hole has steepened into a spike. We calculate the dramatic enhancement to the gamma ray flux from the Galactic center (GC) from such a spike if the 1-3 GeV excess observed in Fermi data is due to DM annihilations. We find that for the parameter values favored in recent fits, the point source-like flux from the spike is 35 times greater than the flux from the inner 1∘ of the halo, far exceeding all Fermi point source detections near the GC. We consider the dependence of the spike signal on astrophysical and particle parameters and conclude that if the GC excess is due to DM, then a canonical adiabatic spike is disfavored by the data. We discuss alternative Galactic histories that predict different spike signals, including: (i) the nonadiabatic growth of the black hole, possibly associated with halo and/or black hole mergers, (ii) gravitational interaction of DM with baryons in the dense core, such as heating by stars, or (iii) DM self-interactions. We emphasize that the spike signal is sensitive to a different combination of particle parameters than the halo signal, and that the inclusion of a spike component to any DM signal in future analyses would provide novel information about both the history of the GC and the particle physics of DM annihilations."
I am not sure what they mean by 'spike.' Does it mean a characteristic of a graph they plotted? If so what kind of graph? If anyone can paraphrase the abstract in simpler language, especially the first the sentence, I will be very grateful!

 

[e.bar.goum]

My understanding of the abstract (without reading the paper - it'd be good if you could link to the paper in future) is that they mean this:

If Sgr A* grew adiabatically from a small black hole that was sitting in a dark matter distribution that looks like this https://en.wikipedia.org/wiki/Navarro–Frenk–White_profile, then the density of dark matter near Sgr A* increases very rapidly ("a spike") as you get closer to Sgr A*.

This rapid increase of dark matter density means that you should see a lot more gamma rays from dark matter near Sgr A* than what we actually see looking towards Sgr A*, which suggests that the idea that Sgr A* grew adiabatically from a NFW dark matter distribution isn't correct. Instead, the authors consider alternative scenarios. They also suggest that by considering the signals from a rapid increase of dark matter density near Sgr A* is a good test for figuring out how the galactic centre formed, and how dark matter annihilates.