Stars and galaxies
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Stars and Galaxies: A Comprehensive Overview
Star-Forming Galaxies in the Low-Redshift Universe
Star-forming galaxies in the low-redshift universe exhibit diverse physical properties. A study of approximately 100,000 galaxies from the Sloan Digital Sky Survey (SDSS) reveals that accurate star formation rates (SFRs) can be determined by correcting for aperture bias using resolved imaging techniques. This method allows for precise estimates of total SFRs, contributing to a better understanding of star formation density in the universe .
Early Star-Forming Galaxies and Cosmic Reionization
The earliest known galaxies, observed when the universe was about 800 million years old, played a crucial role in cosmic reionization. These galaxies emitted intense ultraviolet radiation, which reionized intergalactic hydrogen, marking a significant event in cosmic history .
Intense Star Formation in High-Redshift Galaxies
High-redshift galaxies, particularly those at z ≈ 2.3, exhibit intense star formation rates, sometimes reaching up to 1,000 solar masses per year. These starbursts are often driven by mergers between gas-rich galaxies. Observations of sub-millimeter galaxies, magnified by gravitational lensing, reveal that the star-forming regions in these galaxies are extremely dense and luminous, akin to the dense cores of giant molecular clouds in the local universe but on a much larger scale .
Elevation and Suppression of Star Formation
Star formation within galaxies can be elevated or suppressed due to various factors. Analysis of star formation rate surface density profiles in a sample of 976 star-forming galaxies shows that more massive galaxies exhibit pronounced central cores or bulges, which can lead to either elevation or suppression of star formation. This phenomenon is linked to variations in gas depletion times and accretion rates, suggesting a gas-regulator system at play .
The Oldest Stars in the Milky Way
The oldest stars in the Milky Way provide insights into the galaxy's early assembly history. Simulations indicate that these stars are less centrally concentrated and are often found in the halo, having been accreted during hierarchical assembly. These ancient stars, formed before z ≈ 5, are driven outward by bursty star formation and feedback processes, similar to those observed in dwarf galaxies .
Dusty Star-Forming Galaxies at High Redshift
Dusty star-forming galaxies (DSFGs) at high redshift are significant contributors to the assembly of stellar mass and the evolution of massive galaxies. These galaxies, often obscured in optical wavelengths, are detectable in the sub-millimeter range due to dust heated by newly forming stars. DSFGs exhibit extreme star formation rates and contribute substantially to the infrared extragalactic background light .
Molecular Gas in Young Galaxies
Young galaxies in the distant universe are rich in molecular gas, which is essential for star formation. Surveys of typical massive star-forming galaxies at redshifts of about 1.2 and 2.3 reveal that these galaxies have high fractions of cold gas relative to their total baryonic mass. This abundance of molecular gas supports the high star formation rates observed in these galaxies, indicating a need for continuous replenishment of fresh gas .
Reconstructing Galaxy Histories from Globular Clusters
Globular clusters, compact groups of stars formed early in the universe's history, offer valuable insights into galaxy formation and evolution. These clusters have survived various interactions and mergers, providing a record of tumultuous events such as cannibalism, collisions, and bursts of star formation that have shaped their parent galaxies .
Conclusion
The study of stars and galaxies, from the earliest star-forming galaxies to the ancient stars in the Milky Way, reveals a complex and dynamic history of cosmic evolution. Advances in observational techniques and simulations continue to enhance our understanding of the processes driving star formation, galaxy assembly, and the intricate interplay between gas, dust, and stellar populations across different epochs.
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