Finding Bulgeless Galaxies With Growing Black Holes

Galaxies are often a bit of a hot mess. Not only are the stars, gas and dust within a galaxy all coalescing and expanding, heating and cooling, absorbing and emitting, but this whole system is embedded within a halo of dark matter that interacts only via gravity, a force acting on scales big and small, from that huge halo to the relatively compact central supermassive black hole. As these various parts of a galaxy combine in some proportion to drive its evolution, galaxies also merge with others to form larger galaxies, which of course changes the evolution of the resultant system.

It’s all extremely beautiful, as you know, but it makes studying them complicated. For example, the question of how supermassive black holes and galaxies seem to co-evolve is fundamental to the field of galaxy evolution, but it’s very difficult to separate how much of this co-evolution is due to mergers and how much comes from non-merger processes.

Happily, Galaxy Zoo is in an excellent position to help answer this question. Galaxy Zoo classifications have already helped us understand the role of mergers in the evolution of galaxies (also see previous blog posts related to mergers). Recently, I’ve been working on a project that approaches this question from the other side: what part of the growth of galaxies and black holes happens in the absence of mergers?

Mergers leave behind clear signatures on a galaxy’s morphology. Even long after the tidal tails and stellar streams of an ongoing or recent merger have faded away, the effect of a merger can be seen in the strength of the galactic bulge, the collection of stars on disordered orbits very different from the ordered rotation of a disk. Simulations show that at least some of the stars from a disk are re-distributed into a bulge during a merger (how many stars depends on the kind of merger). Most of those simulations show that even mergers where a big galaxy gobbles up a galaxy only a tenth of its mass will form a bulge. In other words, significant mergers inevitably lead to the formation of a bulge.

Pure disk galaxies without bulges, therefore, have had a merger-free history. And if we want to also study the growth of black holes in these galaxies, we need to find bulgeless galaxies that host active galactic nuclei, supermassive black holes that are actively being fed with surrounding material.

Chris and I started this project several months ago, and many other members of the team joined in. The first time we looked in the Sloan data from Galaxy Zoo 2, we didn’t find any galaxies that were both classified as bulgeless and had clear spectral evidence of an AGN.

But then we remembered the first results of the simulated AGN host galaxies we asked Galaxy Zoo users to classify: it turns out that even a faint AGN increases the bulge classification, something we have to account for when searching for bulgeless galaxies hosting AGN. Using the results of these simulations, we went back and looked again — and this time we found 15 candidates:

Fifteen may not seem like many, but bulgeless galaxies are thought to be quite rare, and only a small percentage of galaxies has an actively growing black hole, so it’s pretty impressive to find even that many when we’re looking for a rare subset of a rare subset of galaxies.

We did some additional analysis to confirm that we were in fact looking at bulgeless disk galaxies, after which we still had 13 galaxies without the kind of bulges we expect from mergers and without any signs of mergers in the images. (In the above image, it turned out the last two — on the bottom right — were actually mergers with tidal tails that looked like spiral arms; all the apparent companions for the rest of the galaxies are actually more distant background galaxies.)

We also have data on the growing black hole: two of them have enough information that we can calculate the black hole mass, and for the others we can use their luminosity to estimate the lowest mass that the black hole could possibly have (any lower and it would be overfeeding).

It turns out that the black holes can grow pretty big: one of the two we know the masses for is about 10 million times the mass of the Sun, and the other is almost that big. The lower limits on the other black hole masses all suggest the black holes are similarly massive: given that we believe these are systems that have evolved in the absence of significant mergers, this tells us that black holes can grow pretty large without any of the black hole feeding that occurs when two galaxies collide.

The black holes also don’t seem to be related to the bulges (or lack thereof) in the same way as most galaxies with measured black hole masses, which may hint that the observed relationship between bulges and black holes is not a fundamental correlation, but rather something that only occurs when there’s enough of a bulge present that the bulge traces the gravity of the whole (or nearly the whole) system. In other words, maybe bulges don’t have a special relationship with black holes: maybe the relationship is between the black hole and the total content of the galaxy, which is often (but not always, and not with our sample) traced by the bulge.

We have submitted the paper presenting these findings, and had our first set of comments from the referee. The comments were positive overall, as well as being very helpful and detailed, so we’re working on incorporating those suggestions and I’ll keep you posted on the paper’s progress. But the project would not have been possible without Galaxy Zoo: your classifications are helping us learn about not just galaxies but the central supermassive black holes that live within them.

This post is part of Citizen Science September at the Zooniverse.