9.3 Molecular gas in galaxies and its role in star formation

Molecular gas plays a crucial role in star formation within galaxies. Primarily composed of hydrogen molecules, it's concentrated in dense regions like giant molecular clouds and spiral arms. Understanding its distribution and properties is key to unraveling galactic evolution.

Astronomers use various tracers to study molecular gas, with carbon monoxide being the most common. The Kennicutt-Schmidt law relates molecular gas density to star formation rates, helping us grasp how galaxies evolve over cosmic time.

Molecular gas distribution in galaxies

Composition and location of molecular gas

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• Molecular gas in galaxies is primarily composed of molecular hydrogen (H2)
• Concentrated in the densest regions of the interstellar medium (giant molecular clouds)
• Distribution is often concentrated in the central regions and along spiral arms
  • Regions where gas density and pressure are highest
  • Examples: central bulges, spiral arms of Milky Way and Andromeda galaxies

Properties of molecular gas clouds

• Temperatures ranging from 10-50 Kelvin
  • Coldest component of the interstellar medium
• Densities of 10^2 to 10^6 particles per cubic centimeter
  • Densest component of the interstellar medium
• Total mass of molecular gas in a galaxy can vary widely
  • Ranges from 10^7 to 10^10 solar masses
  • Often a small fraction of the total gas mass in a galaxy (atomic and ionized gas)

Factors influencing molecular gas content

• Galaxy morphology (spiral, elliptical, irregular)
• Star formation history (active, quiescent)
• Interactions with other galaxies (mergers, tidal interactions)
• Examples: starburst galaxies (high molecular gas content), early-type galaxies (low molecular gas content)

Molecular gas in star formation

Role of molecular gas in star formation

• Primary fuel for star formation in galaxies
  • Provides raw material from which stars are born
• Star formation occurs within the densest regions of molecular gas clouds (prestellar cores)
  • Gravitational collapse can overcome internal pressure and trigger formation of a protostar
• Rate of star formation directly related to amount and density of molecular gas available
  • Galaxies with higher molecular gas fractions tend to have higher star formation rates

Feedback processes and star formation efficiency

• Newly formed stars can disrupt and disperse surrounding molecular gas
  • Stellar winds and supernovae
• Feedback processes regulate efficiency and rate of star formation
  • Prevents all molecular gas from being converted into stars at once
• Initial mass function (IMF) influenced by properties of parent molecular cloud
  • Density and turbulence affect distribution of stellar masses formed

Astrochemical tracers of molecular gas

Challenges in detecting molecular hydrogen

• Direct detection of H2 is difficult
  • Lacks a permanent electric dipole moment
• Astronomers rely on other molecular species to trace distribution and properties of molecular gas
  • Examples: carbon monoxide (CO), HCN, HCO+, CS

Carbon monoxide as a tracer

• Most commonly used tracer of molecular gas
  • Second most abundant molecule after H2
  • Easily observable emission lines in millimeter and submillimeter wavelengths
• CO-to-H2 conversion factor (X-factor) used to estimate total molecular gas mass from CO observations
  • Can vary depending on metallicity and physical conditions of the gas

Other tracers and techniques

• HCN, HCO+, and CS used to trace densest regions of molecular gas clouds
  • Regions where star formation is most likely to occur
• Dust continuum emission in far-infrared and submillimeter wavelengths
  • Estimates total molecular gas mass
  • Dust is often well-mixed with gas in molecular clouds

Molecular gas vs star formation rates

Kennicutt-Schmidt law

• Empirical relationship between surface density of molecular gas and surface density of star formation in galaxies
• Star formation rate surface density is proportional to molecular gas surface density raised to a power of ~1.4
• Normalization and slope can vary depending on galaxy type, redshift, and physical conditions of gas
  • Examples: starburst galaxies (higher normalization), low surface brightness galaxies (lower normalization)

Deviations and implications

• Deviations from Kennicutt-Schmidt law provide insights into star formation efficiency and feedback processes
  • Galaxies above the relation: more efficient star formation
  • Galaxies below the relation: less efficient star formation or strong feedback effects
• Molecular gas depletion time: ratio of molecular gas mass to star formation rate
  • Timescale over which current molecular gas reservoir would be consumed by star formation
  • Typically a few billion years in nearby galaxies

Studying the relationship across cosmic time

• Investigating molecular gas content and star formation rates across different galaxy types and redshifts
  • Provides insights into galaxy evolution and cosmic star formation history
• Examples: high-redshift galaxies (higher molecular gas fractions and star formation rates), early-type galaxies (lower molecular gas fractions and star formation rates)