Galaxy Formation

In many ways, galaxies act as if they were alive. Early in their life cycle, galaxies are vibrant spirals that serve as the birthplaces of stars, planets, black holes, and nebulae. Massive streams of gas flow into and out of galaxies as they take cosmic-scale breaths to fuel the birth of billions of stars. Eventually, all galaxies “die,” either after running out of the gas used to fuel their growth or after they are shredded by the gravity of another, larger galaxy. Scientists at KIPAC study this entire lifecycle: how galaxies were born in the darkness of the early Universe, how their different components interact as they live and grow, and how they die.

When the first galaxies were born, they fought back against a process which had been marching forward since the Big Bang: the cooling of the Universe’s gas. All the Universe’s gas started out as a high-energy soup of photons and fluctuating particles. As the Universe expanded, this chaos eventually cooled into plasma and then again into neutral Hydrogen. However, the stars and black holes in the first galaxies pumped out enough high-energy light that they were able to reverse this process and convert most of the Universe’s gas back into a plasma. Scientists at KIPAC study these early galaxies with computer simulations, allowing them to dissect the many intertwining processes that are important for understanding this period of the Universe’s life.

Pictures of galaxies typically focus on the central clump of stars, gas, and dust, but there are other harder-to-see components which are equally lively. Almost all galaxies have at least one supermassive black hole lurking in their centers. Although tiny in size, these black holes can be billions of times the mass of the sun. Their intense gravity launches beams of light and particles away from the galaxy whenever the black hole consumes nearby gas or stars. These jets are so energetic that they serve as beacons that can be seen across great cosmic distances, a fact which KIPAC researchers have taken advantage of to better understand the cosmic environments of their host galaxies. The energy of these jets also disrupts the inflow of gas into their host galaxies, causing the growth of the central black hole and its galaxy to be strongly intertwined. KIPAC researchers study this process through both X-ray observations and novel computer models.

Another difficult-to-see component of galaxies are the diffuse “haloes” of invisible dark matter which surround almost all galaxies. These dark matter haloes matter are far heavier than all the visible stars and gas within them and control the movement and growth of their galaxies. Because of this, studies of galaxies face a conundrum: dark matter is far easier to model theoretically than visible matter is, but is much harder to constrain with observations. This makes studying the “galaxy-halo connection” critical for many fields of study. KIPAC researchers have been at the forefront of modeling the galaxy-halo connection, developing many of the field’s leading techniques and pushing those techniques to their limits to study the smallest galaxies in the Universe.

All galaxies are surrounded by a swarm of smaller “satellite” galaxies. These satellite galaxies are ripped apart by the gravity of their hosts after a few orbits and are just as rapidly replaced with new satellites pulled in from the local Universe. KIPAC scientists study these satellite galaxies with a combination of observational surveys -- such as DES, DESI, and SAGA -- computer simulations, and carefully tuned models. Researchers have the opportunity to study the process of star formation shutting down in these satellites as they lose their gas to and can use their orbits to pin down the mass and shape of the host galaxy’s dark matter halo. Additionally, the durability of satellites and their dark matter is closely connected to gravity and the properties of the dark matter particle. Because of this, some KIPAC researchers use satellite galaxies as laboratories to test the nature of dark matter and other cosmological theories.