9.3 Molecular gas in galaxies and its role in star formation
4 min read•august 14, 2024
Molecular gas plays a crucial role in star formation within galaxies. Primarily composed of hydrogen molecules, it's concentrated in dense regions like 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 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 ()
- Concentrated in the densest regions of the interstellar medium (giant )
- 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: (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)
- 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
- (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 (), 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)
Key Terms to Review (20)
Chemical Kinetics: Chemical kinetics is the branch of physical chemistry that studies the rates of chemical reactions and the factors that influence these rates. It plays a critical role in understanding how quickly reactions occur in various environments, which is essential for analyzing processes in space, like gas-phase reactions in the interstellar medium and chemical transformations in protoplanetary disks.
CO: Carbon monoxide (CO) is a colorless, odorless gas that plays a crucial role in the chemistry of molecular clouds and the processes of star formation. It serves as an important tracer for molecular gas, as it is produced through various chemical reactions in dense interstellar regions, helping to map out the distribution of material necessary for star formation. Additionally, CO is significant in modeling the chemical evolution of astrophysical environments and understanding the dynamics within galaxies.
Gas-phase chemistry: Gas-phase chemistry refers to the study of chemical reactions and interactions that occur in the gas phase, where molecules exist primarily as gaseous species. This area of chemistry is essential for understanding various astrophysical phenomena, including the formation and detection of interstellar molecules, the processes occurring in active galactic nuclei, and the role of molecular gas in star formation. The reactions in the gas phase can lead to complex chemistry, influencing the composition and behavior of celestial bodies and environments.
Giant molecular clouds: Giant molecular clouds (GMCs) are large regions in space, often several hundred light-years across, that are primarily composed of hydrogen molecules, dust, and other gases. These clouds serve as the primary sites for star formation, providing the necessary material and conditions for the birth of stars and planetary systems. Their high density and low temperature create an environment where gravitational forces can overcome internal pressure, leading to the collapse of regions within the cloud and ultimately forming new stars.
Gravitational collapse: Gravitational collapse is the process by which an astronomical object, such as a molecular cloud, contracts under its own gravity, leading to an increase in density and temperature. This phenomenon is essential for the formation of stars, as it initiates the process of star birth by causing material within clouds to clump together and ultimately form protostars. As gravitational collapse occurs, it also influences the chemical composition of forming stars and plays a vital role in the recycling of molecular gas in galaxies.
H2: H2, or molecular hydrogen, is the simplest and most abundant molecule in the universe, consisting of two hydrogen atoms bonded together. This diatomic molecule plays a crucial role in various cosmic processes, especially in the formation of stars and galaxies, where it acts as a primary constituent of molecular clouds and fuels the nuclear fusion that powers stars.
Herschel: Herschel refers to the astronomer William Herschel, known for his discovery of infrared radiation and for being one of the first to identify and study molecular clouds in space. His work laid foundational principles in understanding how molecular gas contributes to star formation within galaxies, revealing the complex interactions between various forms of matter in the universe.
Hubble: Hubble refers to the Hubble Space Telescope, a powerful astronomical observatory launched in 1990 that has revolutionized our understanding of the universe. By capturing high-resolution images of distant celestial objects, Hubble has provided crucial insights into the formation and evolution of galaxies, including the role of molecular gas in star formation processes. This telescope's observations have greatly enhanced our knowledge about the composition, dynamics, and interactions within galaxies, making it a cornerstone of modern astrophysics.
Hydrodynamic models: Hydrodynamic models are mathematical simulations that describe the motion of fluid substances, like gas and liquid, under the influence of various forces. In the context of molecular gas in galaxies, these models help us understand how gas interacts and moves within a galaxy, affecting star formation processes. They account for factors like gravity, pressure, and turbulence to predict how gas will condense to form stars and the overall dynamics of galactic environments.
Initial Mass Function: The initial mass function (IMF) is a mathematical distribution that describes the initial mass distribution of stars formed in a given star formation event. It indicates how many stars of various masses are produced when a molecular cloud collapses, significantly influencing the evolution of galaxies and star clusters. The IMF helps astronomers understand the relationship between star formation rates and the resulting stellar population characteristics, which are crucial for understanding the lifecycle of galaxies and the dynamics of star formation processes.
Interferometry: Interferometry is a technique that combines the signals from two or more telescopes to create a single, higher-resolution image of astronomical objects. This method takes advantage of the wave properties of light, allowing astronomers to gather more precise information about distant celestial sources. By measuring the interference patterns created when light waves from these sources combine, astronomers can determine various characteristics such as size, shape, and even the chemical composition of objects in space.
Kennicutt-Schmidt Law: The Kennicutt-Schmidt Law describes a relationship between the surface density of star formation and the surface density of gas in a galaxy. Specifically, it states that galaxies with higher gas densities tend to form stars at higher rates, suggesting that the amount of molecular gas in a galaxy plays a crucial role in determining its star formation rate.
Molecular Clouds: Molecular clouds are dense regions in the interstellar medium where gas and dust are concentrated, leading to conditions that allow for the formation of molecules, particularly hydrogen molecules (H₂). These clouds play a crucial role in the lifecycle of stars and the chemical evolution of the universe, serving as the primary sites for star formation and the birthplace of various celestial objects.
Photo-dissociation: Photo-dissociation is a chemical process where a molecule absorbs a photon and subsequently breaks apart into smaller fragments. This process is significant in astrophysical environments, as it influences the composition of molecular clouds, the chemistry of young stellar objects, and the dynamics of molecular gas in galaxies.
Protoplanetary Disks: Protoplanetary disks are rotating disks of dense gas and dust surrounding young stars, where the materials within the disk are thought to coalesce and form planets. These disks play a crucial role in the process of planet formation and provide insights into the early stages of solar system development.
Spectroscopy: Spectroscopy is a scientific technique used to analyze the interaction between matter and electromagnetic radiation. This method allows scientists to determine the composition, structure, and physical properties of substances by studying the light they emit, absorb, or scatter.
Starburst galaxies: Starburst galaxies are galaxies experiencing an exceptionally high rate of star formation, often significantly exceeding the average rate for typical galaxies. This intense activity usually occurs over a short time scale, often triggered by interactions or mergers with other galaxies, leading to a rapid consumption of the molecular gas available for star formation. The resulting burst of new stars can have profound effects on the galaxy's structure and evolution.
Stellar nucleosynthesis: Stellar nucleosynthesis is the process by which elements are formed through nuclear reactions in the interiors of stars. This process plays a crucial role in the chemical evolution of the universe, as it creates heavier elements from lighter ones, and contributes to the elemental abundance we observe in stars and galaxies today.
Thermal equilibrium: Thermal equilibrium is a state in which two or more objects in contact with each other exchange no net heat energy, resulting in a uniform temperature across those objects. This condition occurs when the energy absorbed by one body equals the energy lost by another, leading to a balance that affects various chemical and physical processes in astrophysical environments.
Thermodynamics: Thermodynamics is the branch of physical science that deals with the relationships between heat, work, temperature, and energy. It provides a framework to understand how energy is transferred and transformed in various processes. In astrophysical contexts, thermodynamics is crucial for modeling the behavior of gases in space and understanding the chemical evolution of celestial bodies and star formation processes.