👽Galaxies and the Universe Unit 5 – Interstellar Medium & Star Formation

What's the Interstellar Medium?

• Consists of gas and dust that fills the space between stars within a galaxy
• Mostly composed of hydrogen and helium, with traces of heavier elements
• Plays a crucial role in the cycle of star formation and galactic evolution
• Exhibits a wide range of temperatures, densities, and ionization states
• Shaped by various physical processes, such as gravity, radiation, and magnetic fields
• Serves as a reservoir for future star formation and enrichment of the galaxy
• Interacts with stellar winds, supernovae, and other energetic events in the galaxy

Key Components of the ISM

• Atomic hydrogen (HI) is the most abundant component, found in neutral gas clouds
  • Emits 21-cm radio waves due to hyperfine transition
• Molecular hydrogen (H2) is the second most abundant, concentrated in dense molecular clouds
  • Difficult to observe directly, often traced by carbon monoxide (CO) emissions
• Ionized hydrogen (HII) regions surrounding hot, young stars
  • Emit characteristic hydrogen recombination lines (Hα, Hβ)
• Dust grains, composed of silicates, graphites, and ices
  • Absorb and scatter light, causing interstellar extinction and reddening
• Cosmic rays, high-energy charged particles that permeate the ISM
• Magnetic fields that thread through the gas and dust, influencing dynamics and structure

Physical Processes in the ISM

• Gravitational collapse of dense regions leads to star formation
• Thermal processes, such as heating by stellar radiation and cooling by line emission and dust
  • Determine the temperature and ionization state of the gas
• Shocks from supernovae and stellar winds compress and heat the gas
  • Can trigger star formation or disrupt molecular clouds
• Cosmic rays ionize and heat the gas, driving chemical reactions
• Magnetic fields provide support against gravity and guide the motion of charged particles
• Turbulence, driven by various energy sources, creates complex structures and mixing
• Dust grain surface chemistry enables the formation of complex molecules

Molecular Clouds: Stellar Nurseries

• Dense, cold regions ($T \sim 10-20$ K) where star formation occurs
• Composed primarily of molecular hydrogen (H2) and dust
• Exhibit complex filamentary and clumpy structures, with cores and filaments
• Gravitational instability leads to the collapse of dense cores into protostars
• Contain a wide range of masses, from small globules to giant molecular clouds (GMCs)
  • GMCs can have masses up to $10^6$ solar masses and span hundreds of light-years
• Observed through various tracers, such as CO line emissions and infrared dust continuum
• Efficiency of star formation is relatively low, with only a small fraction of gas converted into stars

Stages of Star Formation

• Prestellar core: dense, gravitationally bound region within a molecular cloud
  • Supported by thermal pressure, turbulence, and magnetic fields
• Protostellar phase: collapse of the prestellar core forms a central protostar and disk
  • Characterized by accretion of material onto the protostar and bipolar outflows
• Pre-main-sequence stage: protostar becomes visible as it contracts and heats up
  • Includes T Tauri stars (low-mass) and Herbig Ae/Be stars (intermediate-mass)
• Main sequence: the star reaches hydrostatic equilibrium and begins hydrogen fusion in its core
  • Stellar properties depend on the initial mass and composition
• Post-main-sequence evolution: the star expands, sheds mass, and undergoes nucleosynthesis
  • Culminates in planetary nebula (low-mass) or supernova (high-mass) events

Feedback and Regulation

• Stellar feedback processes impact the surrounding ISM and regulate star formation
• UV radiation from massive stars ionizes and heats the gas, creating HII regions
  • Can trigger star formation in the periphery or disrupt molecular clouds
• Stellar winds from young, massive stars inject energy and momentum into the ISM
  • Can sweep up gas and dust, forming shells and bubbles
• Supernovae explosions shock and enrich the ISM with heavy elements
  • Drive turbulence and can trigger or quench star formation
• Protostellar outflows and jets remove angular momentum and clear the surrounding material
  • Limit the efficiency of star formation in molecular clouds
• Cosmic rays accelerated by supernovae and stellar winds ionize and heat the gas
• The balance between these feedback processes and gravity regulates the overall star formation rate in galaxies

Observational Techniques

• Radio observations (e.g., 21-cm HI line, CO rotational lines) trace neutral and molecular gas
  • Reveal the distribution, kinematics, and temperature of the gas
• Infrared observations (e.g., dust continuum, PAH features) probe dust and embedded star formation
  • Measure the dust temperature, mass, and star formation rates
• Optical and UV observations (e.g., Hα, absorption lines) trace ionized gas and hot stars
  • Provide information on the ionization state, abundances, and stellar populations
• X-ray observations detect hot gas, supernova remnants, and young stellar objects
• Polarization measurements reveal the orientation and strength of magnetic fields
• Spectroscopic surveys (e.g., Gaia, APOGEE) provide detailed chemical and kinematic information
• Multi-wavelength studies are essential to understand the complex interplay of processes in the ISM

Galactic Impact and Evolution

• The ISM plays a crucial role in the evolution and appearance of galaxies
• Star formation history is regulated by the availability and properties of the ISM
  • Gas content, density, and turbulence determine the star formation efficiency
• Stellar feedback processes shape the structure and dynamics of the ISM
  • Drive galactic winds, outflows, and fountains that enrich the circumgalactic medium
• Chemical enrichment of the ISM by supernovae and stellar winds affects subsequent star formation
  • Governs the metallicity and dust content of galaxies over cosmic time
• Interactions and mergers between galaxies can trigger starbursts and alter the ISM
• The ISM is a key component in the baryon cycle of galaxies
  • Inflows from the intergalactic medium replenish the gas reservoir
  • Outflows and stripping processes remove gas from galaxies
• Understanding the ISM is essential for constraining models of galaxy formation and evolution