The slightly surprising survival of spirals

Galaxy evolution is very similar to that of large, ruthless corporations : it's driven by merger after merger, until what started off as a small family shop becomes a huge international conglomerate that sells weapons specifically designed for killing babies living in third-world countries. Or something. A slightly more popular analogy is that galaxies are cannibals, but this is no less grisly as it still involves fat, bloated monster galaxies that mercilessly devour their smaller, weaker siblings.

Look, I've been stuck inside for a month. I've got to make my own entertainment. Stop judging me !

All that horror and questionable politics aside, there's a bit of a puzzle here. Merging is thought to be the most common way to turn spiral galaxies into ellipticals. But if that's so, how can there be any massive spirals around today ? Shouldn't their formation necessitate lots of mergers, making them far more likely to become ellipticals ?

The authors of this study look at one of the all-singing, all-dancing, "everything on" cosmological simulations to trace the evolution of the most massive spiral galaxies. This simulates a large cosmological volume, so unlike smaller simulations (which have to be focused on specific environments) it includes a whole range of evolutionary processes. It's also of extremely high mass resolution, enough to simulate even very modest dwarf galaxies. This means that from the perspective of the most massive spirals, which they concentrate on here, it should be complete.

They find that giant spirals are indeed rare. Of galaxies with more than 100 billion solar masses in stars, about 11% are spirals, in rough agreement with observations.

There are two different ways they can form. About 70% result from mergers... but not just any mergers. The mass ratio of the progenitors doesn't seem to matter much, but their gas content does. Since the gas is collisonal, the merger causes it to shock and collapse to a disc supported by the angular momentum imparted by the merging galaxy.

Even more persuasively, 98% of all massive spheroids which experience a gas-rich merger subsequently become discs, so gas-rich mergers appear to be a highly effective route for disc formation. And the discs tend to have had their last major merger more recently than spheroids, meaning there hasn't been much time for other, unsuitable mergers (or other processes) to destroy them. In addition, as the gas content of the simulated universe decreases over time, so does the fraction of galaxies which are massive discs. It's pretty clear that it's gas what done it.

Apart from these "rejuvenated discs", the remaining 30% are always discs. Presumably they initially form by mergers, but subsequently they get lucky and have a very quiet merger history. They mention an interesting idea was that such galaxies could be near to even more massive galaxies whose enormous gravity could protect their smaller brethren from incoming would-be mergers, but this doesn't appear to be the case for their sample : they seldom have a more massive companion. So for the most massive objects, at least, it's simply a matter of luck.

Whether this has any bearing on runaway capitalism or the culinary practises of uncontacted jungle tribes is left as an exercise for the reader.

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios.