“So why is that? Nine years ago, I predicted this possibility from numerical simulations I had run as a PhD student. I found that in disc galaxies, the gravitational pull of the stars helps the gas to collapse to form new stars. In contrast, the gas in elliptical galaxies feels a weaker pull from the stars and does not collapse so easily. It is fascinating that the global morphology of a galaxy can control what happens at the smallest scales.”
And I think elliptical galaxies could primarily be formed by merging spiral galaxies with different axial planes. Just like what will happen between the Milky Way and Andromeda in about 3 billion years.