I’ve heard that statement quite a few times In various science shows and videos. Why the “almost”? Is it just that we haven’t observed every galaxy, or are there galaxies that we know of that definitively don’t have black holes at their centers?
The reason I ask is that the same shows often suggest that a black hole (along with dark matter) is necessary for galaxy formation.
Supermassive black holes (SMBHs) are not necessary for galaxy formation; they are a consequence of interactions as the forming galaxy accrues mass or due to the mergers of two galaxies (or protogalactic clouds). Spiral galaxies without SMBHs are sometimes referred to as “pure spiral galaxies”, and often show a very distinct spiral shapes.
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Got a cite for that? Because last I knew, that was one of the big questions for which the answer wasn’t known, and none of the hypothesized mechanisms for SMBH formation were entirely consistent with observations.
And also last I knew, nobody had ever observed a galaxy without an SMBH. But scientists like to hedge our bets.
The AAS article linked in my previous post discusses this:
Further examining the brightness profiles for these pure disk galaxies, Sachdeva and Saha find that both the average central surface brightness and the average scale length are the same across different redshift bins — which means that gas isn’t being fed into the interior parts of these galaxies over time. Yet in spite of this, the total stellar mass and the size of these galaxies grows substantially from z~1 to the present day — by 40% and 60%, respectively.
How could these galaxies be growing without changing their profiles (as would happen if their growth were caused by mergers)? The authors propose that these galaxies may be isolated and protected from mergers, and they grow through smooth accretion via cosmic filaments of cold gas onto their outskirts. Additional study of this unique category of galaxies may provide further insight into different mechanisms of galaxy evolution.
The M33 (Triangulum) Galaxy is a pure disk galaxy as discussed in this article:
Most spiral galaxies, especially those as large as M33, contain a central bulge filled with densely packed stars and often a supermassive black hole. However, M33 breaks the mold as a “pure disk galaxy” — a term used to describe galaxies without a central bulge and lacking evidence of a supermassive black hole. This rarity puts M33 in the company of only about 15-18% of known galaxies, making it a compelling subject for further study.
The absence of a supermassive black hole in M33 challenges conventional theories about galaxy formation and evolution. In most cases, supermassive black holes are thought to form early in a galaxy’s life and significantly influence its development through gravitational forces and energetic feedback. However, M33’s status as a pure disk galaxy suggests that there might be alternative pathways for the evolution of spiral galaxies that do not involve a central black hole.
The formation of galaxies obviously involves a lot of speculation as we can only get snapshots galaxies at one particular state of evolution at a range of distances, and every galaxy has a unique evolution that isn’t as amenable to discrete classification as stars are, but we do know from basic principles that protogalactic clouds coalesce first and only form bulges and active galactic nuclei (AGN) indicating the presence of a supermassive black hole later. From modeling we know that a protogalactic cloud with sufficient angular momentum won’t be able to feed much mass into a central core to create a black hole or to feed a nascent one to produce an AGN, but interactions between spiral galaxies, or an elliptical galaxy with low angular momentum will allow matter to fall into the core to produce or feed a nascent SMBH.
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