Final answer:
Supermassive black holes form primarily by accreting gas and merging with other black holes, a process supported by astronomical observations and calculations that show significant growth potential when dense gas is abundant and interference from other stars is minimal.
Step-by-step explanation:
Prof. Haggard suggests that supermassive black holes form primarily by accreting gas and merging. This process begins with the collapse of massive stars, which results in the creation of smaller black holes. These black holes then grow by merging with other black holes and accreting gas from the rich early gas supply in the galaxy. Figuratively speaking, Figure 27.18 in their textbook shows colliding galaxies with two supermassive black holes, supporting the idea that black holes can indeed grow to enormous masses in the centers of galaxies by merging with other black holes from nearby galaxies. Furthermore, calculations imply that a black hole, even as small as 10 times the mass of the Sun, could grow to more than 10 billion times the mass of the Sun, given that it's surrounded by a sufficient amount of dense gas and lacks interference from nearby stars that could prevent the formation of an accretion disk.