Gas stars
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Gas and Star Formation in Galaxies
Molecular Gas and Star Formation Rates
The relationship between molecular gas and star formation rates (SFR) in galaxies has been a focal point of astrophysical research. Studies have shown that both low- and high-redshift star-forming galaxies (SFGs) follow similar molecular gas-star formation relations, with slopes ranging from 1.1 to 1.2 over three orders of magnitude in gas mass or surface density1. This indicates a consistent relationship between the amount of molecular gas and the rate at which new stars are formed, regardless of the galaxy's age or environment.
Gas Depletion and Star Formation Efficiency
The gas depletion timescale, which is the time it takes for a galaxy to consume its molecular gas through star formation, varies with redshift. For instance, at redshift z ≈ 2, the timescale is about 0.5 Gyr, increasing to 1.5 Gyr at z ≈ 01. This suggests that galaxies in the early universe were more efficient at forming stars compared to present-day galaxies. The star formation efficiency, defined as the fraction of gas converted into stars per unit time, is relatively low at around 2% per dynamical time1.
Impact of Galactic Dynamics on Star Formation
Galactic dynamics play a crucial role in star formation. In very luminous and ultraluminous gas-rich major mergers, the star formation rate per unit gas mass is significantly higher compared to normal disc galaxies. This is likely due to the more compact nature of merger systems, which leads to higher gas compression and shorter dynamical times, thereby accelerating star formation1.
Gas Accretion and Star Cluster Formation
Star clusters form through the accretion of gas from their surrounding environment. Simulations show that star clusters can gain mass from both dense filaments and the ambient background of the giant molecular cloud (GMC) in which they are embedded2. The mass loss from these processes is minimal compared to the total mass of the cluster, ensuring a net increase in mass over time2.
Observational Insights from Molecular Gas
Observations of molecular gas provide critical data for understanding star formation. For example, the PHANGS-ALMA and PHANGS-Hα surveys have revealed that the spatial distribution of molecular gas and star-forming regions varies with galaxy properties such as stellar mass and morphological type4. Massive and earlier-type galaxies tend to have significant reservoirs of molecular gas without active star formation, while lower-mass galaxies exhibit substantial H II regions, indicating ongoing star formation4.
Star Formation in Early-Type Galaxies
Early-type galaxies (elliptical and lenticular) with molecular gas show that star formation processes are not fundamentally different from those in other star-forming galaxies. These galaxies often contain molecular gas in central discs or rings and exhibit star formation rates that align with the Schmidt-Kennicutt law, although with some variations due to contributions from older stars to dust heating10.
Conclusion
The intricate relationship between molecular gas and star formation is influenced by various factors, including galactic dynamics, gas accretion processes, and the physical properties of the host galaxy. While the fundamental processes governing star formation appear consistent across different types of galaxies and cosmic epochs, specific conditions such as gas compression in mergers or the presence of dense filaments can significantly enhance star formation rates. Understanding these relationships is crucial for unraveling the complexities of galaxy evolution and the lifecycle of stars.
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Most relevant research papers on this topic
A study of the gas–star formation relation over cosmic time
Low- and high-z star-forming galaxies follow similar gas-to-star formation relations, with mergers potentially forming stars at slightly higher efficiencies than disc galaxies.
Highly CitedModelling Star Cluster Formation: Gas Accretion
Star cluster formation involves accretion of gas from dense filaments and the ambient background, with the star cluster losing some of its original mass while gaining mass from either source, but the total mass remains constant over time.
Using Molecular Gas Observations to Guide Initial Conditions for Star Cluster Simulations
Observations of gas in star clusters can guide initial conditions for simulations, with gas density and size impacting star formation rate and cluster assembly pace.
The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. II. Resolved Distributions of CO and Hα Emission for 49 PHANGS Galaxies
The gas-star formation cycle in galaxies varies based on stellar mass and Hubble type, with massive and earlier-type galaxies having significant reservoirs of molecular gas without star formation.
The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations
Our method quantifies the scale dependence of molecular gas and star formation in nearby galaxies, revealing a significant reservoir of quiescent molecular gas at a common scale of 140 pc.
High molecular gas fractions in normal massive star-forming galaxies in the young Universe
Distant star-forming galaxies were gas-rich, with an average fraction of cold gas relative to total galaxy baryonic mass of 44% and 34%, three to ten times higher than today's massive spiral galaxies.
Highly CitedIs molecular gas necessary for star formation
Star formation can occur just as easily in atomic gas as in molecular gas, with the most important factor controlling the rate of star formation being the gas's ability to shield itself from the interstellar radiation field.
Highly CitedStar formation and gas accretion in nearby galaxies
Gas surface density and star formation surface density are generally related, but there is no significant difference between symmetric, asymmetric, and interacting galaxies.
Gas expulsion in highly substructured embedded star clusters
High levels of substructure in embedded star clusters can survive gas expulsion better than a virialized and spherical system, making gas removal not the main culprit for their dissolution.
Molecular gas and star formation in early-type galaxies
Early-type galaxies with molecular gas show similar star formation rates to other star-forming galaxies, with potential lower star formation efficiency at a given gas surface density.
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