Did the Milky Way lose its black hole?

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      And yet, despite how large, massive, and impressive this central black hole is, that’s only in comparison to the other black holes within our galaxy. When we look out at other large, massive galaxies — ones that are comparable to the Milky Way in size — we actually find that our supermassive black hole is on the rather small, low-mass side. While it’s possible that we’re simply a bit below average in the black hole department, there’s another potential explanation: perhaps the Milky Way once had a larger, truly supermassive black hole at its core, but it was ejected entirely a long time ago. What remains might be nothing more than an in-progress rebuilding project at the center of the Milky Way. Here’s the science of why we should seriously consider that our central, supermassive black hole might not be our galaxy’s original one.

      When we take a look around at the galaxies in our vicinity, we find that they come in a wide variety of sizes, masses and shapes. As far as spiral galaxies go, the Milky Way is fairly typical of large, modern spirals, with an estimated 400 billion stars, a diameter that’s a little bit over 100,000 light-years, and populations of stars that date back more than 13 billion years: just shortly after the time of the Big Bang itself.

      While the largest black holes of all, often exceeding billions or even tens of billions of solar masses are found overwhelmingly in the most massive galaxies we know of — giant elliptical galaxies — other comparable spirals generally have larger, more massive black holes than our own. For example:
      –The Sombrero galaxy, about 30% of the Milky Way’s diameter, has a ~1 billion solar mass black hole.
      –Andromeda, the closest large galaxy to the Milky Way and only somewhat larger, has a ~230 million solar mass black hole.
      –NGC 5548, with an active nucleus but bright spiral arms, has a mass of around 70 million solar masses, comparable to that of nearby spirals Messier 81 and also Messier 58.
      –And even Messier 82, much smaller and lower in mass than our own Milky Way (and interacting neighbor of Messier 81) has a black hole of 30 million solar masses

      We’re still learning, of course, how supermassive black holes form, grow, and evolve in the Universe. We’re still attempting to figure out all of the steps for how, when galaxies merge, their supermassive black holes can successfully inspiral and merge on short enough timescales to match what we observe. We’ve only recently just discovered the first object in the process of transitioning from a galaxy into a quasar, an important step in the evolution of supermassive black holes. And from observing the earliest galaxies and quasars of all, we find that these supermassive black holes can grow up remarkably fast: reaching masses of around ~1 billion solar masses in just the first 700 million years of cosmic evolution.

      Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction

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