Do boxy bulges indicate barred spirals? Some face-on spiral galaxies show barred structure in their central bulges, whereas some edge-on spiral galaxies have boxy bulges. I wonder if there is any relation between these two. In particular, do boxy bulges indicate barred spirals?
Yes, they do.
Bars in massive galaxies undergo a vertical &quot;buckling&quot; phase about one or two billion years after they form; when this settles down (after about a billion years), the result is that the inner 1/3-2/3 of the bar is vertically thick, with the outer part of the bar still as flat as the rest of the galaxy disk.
When seen side-on, the thick part of the bar looks like a peanut (double-lobed), often with a hint of an X-shape; when seen at an intermediate angle, it looks more rectangular and &quot;boxy&quot;. This is the situation for the Milky Way, in fact. (When seen with the bar end-on, the result looks very round, like a stereotypical spherical bulge.)
You can also see this when the galaxy isn't perfectly edge-on: some bars in galaxies inclined at 45 or 60 degrees (where 90 degrees = edge-on) have rectangular interiors with thinner, offset outer regions; this corresponds to the vertically thick/peanut-shaped inner part of the bar and the vertically thin outer part.
We've even seen the signature in a couple of galaxies of a bar in the middle of forming the peanut-shaped bulge (the &quot;buckling phase&quot;).
(In principle, you could get a boxy bulge from some kinds of minor mergers, where a small galaxy has merged with a larger galaxy. But if these exist in real galaxies, they're much rarer than the boxy bulges that are part of bars.)