What is meant by hierarchical (bottom-up) assembly of galaxies?

The largest galaxies formed first and then broke apart into smaller ones Small structures formed first and merged over time to build larger ones — galaxies grew through collisions and mergers All galaxies formed at exactly the same time from the same cloud of gas Stars formed before galaxies, which then assembled around the oldest stars

What role did dark matter play in galaxy formation?

Dark matter clumped under gravity first, creating halos that acted as gravitational wells for ordinary matter to fall into Dark matter provided the fuel for nuclear fusion in the first stars Dark matter emitted light that allowed astronomers to track early galaxy formation Dark matter had no role in galaxy formation — only gas and dust mattered

Approximately when did the first stars form after the Big Bang?

About 1 million years after the Big Bang About 200-400 million years after the Big Bang About 5 billion years after the Big Bang About 10 billion years after the Big Bang

What were the first galaxies like?

Large, massive spirals similar to the Milky Way Smooth, perfectly round ellipticals with no star formation Small, irregular, and very active with star formation — quite different from the large structured galaxies we see today Invisible because they were made entirely of dark matter

What is AGN feedback, and why is it important for galaxy evolution?

Stars in the galactic nucleus emit feedback that regulates how fast the black hole grows Energy released by an active supermassive black hole can heat and expel gas from a galaxy, shutting down star formation Feedback from dark matter halos prevents galaxies from merging with each other The nuclear reactions in stars produce energy that eventually expels the galaxy from its cluster

When did the universe's star formation rate peak?

Right at the moment of the Big Bang About 10 billion years ago, and it has been slowly declining ever since About 5 billion years ago, when our Sun formed Today, because the universe now has the most galaxies

Why do galaxies in dense galaxy clusters tend to be ellipticals, while isolated galaxies tend to be spirals?

Galaxy clusters have more dark matter, which causes elliptical shapes Isolated galaxies are younger and have not had time to evolve into ellipticals Galaxies in clusters have undergone many mergers with other galaxies, which transforms spirals into ellipticals over time The hot gas in galaxy clusters prevents spiral arms from forming

How many smaller galaxies has the Milky Way absorbed over its history?

None — the Milky Way formed entirely from its original gas cloud Exactly one — the Sagittarius Dwarf Galaxy Dozens — evidence of past mergers is visible in streams of stars in the Milky Way's halo Thousands — the Milky Way is the most merger-active galaxy in the Local Group

What caused the quantum fluctuations that seeded galaxy formation?

Random collisions between dark matter particles in the very early universe Cosmic inflation in the first fraction of a second after the Big Bang, which stretched tiny quantum fluctuations to cosmic scales The explosion of the first massive stars, which seeded density variations with heavy elements Magnetic fields in the early universe that concentrated gas into dense knots

Which of the following best describes how a galaxy grows more massive over cosmic time?

By absorbing energy from the cosmic microwave background Through mergers with other galaxies and by pulling in streams of fresh gas from the surrounding cosmic web By slowing down its rotation, which concentrates mass toward the center Through nuclear fusion in its central black hole, which converts energy to mass