1. Gas Accretion: Nearby galaxies acquire gas primarily through two mechanisms: cold gas accretion and hot gas accretion. Cold gas accretion involves the inflow of pristine gas from the intergalactic medium, while hot gas accretion occurs when hot gas from the circumgalactic medium or a neighboring galaxy cools and falls onto the galaxy's disk.
2. Gas Cooling: Once gas enters the galaxy, it needs to cool down to form stars. Cooling mechanisms include:
- Radiative cooling: Gas emits radiation in the form of infrared light as it cools, losing energy and becoming denser.
- Collisional cooling: Gas particles collide with each other, transferring energy and causing the gas to cool down.
3. Formation of Giant Molecular Clouds: Cooling gas condenses into dense regions known as giant molecular clouds (GMCs). GMCs are characterized by high densities and are the birthplaces of stars.
4. Star Formation: Within GMCs, gravity causes the gas to collapse into dense cores, forming protostars. These protostars accrete more gas and dust from their surroundings, gradually growing in mass and temperature. As they heat up, nuclear fusion reactions ignite at their centers, marking the birth of stars.
5. Stellar Feedback: Newly formed stars emit intense radiation, stellar winds, and supernova explosions. These processes inject energy and momentum into the surrounding gas, influencing subsequent star formation. Stellar feedback can trigger new star formation or inhibit it by disrupting the stability of GMCs.
6. Galactic Environment: The overall star formation activity in a nearby galaxy is influenced by its galactic environment. Galaxies with a high abundance of gas, low levels of metallicity (indicating a younger stellar population), and a lack of strong galactic winds tend to have higher star formation rates.
7. Galaxy Interactions and Mergers: Interacting or merging galaxies experience enhanced star formation due to the compression of gas and triggering of gravitational instabilities. Collisions and interactions can lead to the formation of tidal tails, bridges, and starbursts — regions of intensely concentrated star formation.
8. Feedback from Active Galactic Nuclei: In galaxies hosting active galactic nuclei (AGN), the energy output from the AGN can affect star formation. Powerful AGN can drive outflows of gas, suppressing star formation, while weaker AGN feedback might enhance star formation by triggering gas inflows and compressing molecular gas clouds.
The balance between these processes and environmental factors determines the overall star formation rate and the characteristics of the stellar population in nearby galaxies.
