Cosmic Star Formation History

Abstract: Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

• The paper identifies a peak in the star-formation rate at z≈1.9 (around 3.5 Gyr post-Big Bang) followed by an exponential decline with a 3.9 Gyr e-folding timescale.
• It synthesizes multiwavelength photometric and spectroscopic data from Hubble, Spitzer, and submillimeter telescopes to refine SFR calibrations and address dust extinction.
• The findings reveal a synchronized evolution between star formation and black hole growth, offering key insights into reionization and galaxy evolution across cosmic time.

An Analysis of Cosmic Star-Formation History

The paper "Cosmic Star-Formation History" by Piero Madau and Mark Dickinson offers a comprehensive review of the current understanding of the Universe's star-formation rate (SFR) from its inception to the present epoch. Over recent decades, advances in multiwavelength imaging and spectroscopic surveys have significantly refined our understanding of galaxy formation and evolution. The paper synthesizes these studies to outline a coherent narrative of how stellar material accumulation, heavy element production, and the reionization process have evolved through cosmic time.

Overview

Key highlights from the review include the staggering finding that the cosmic SFR density reached its zenith approximately 3.5 Gyr post-Big Bang at a redshift of $z \approx 1.9$. Following this peak, the SFR density has experienced an exponential decline characterized by a 3.9 Gyr e-folding timescale. Notably, half of the stellar mass observed today was formed by $z = 1.3$, with significant star formation occurring earlier and later during the expansion of the Universe. Throughout the epochs, less than 1% of contemporary stars emerged during the earlier reionization period.

The paper utilizes photometric surveys, including data from the Hubble and Spitzer space telescopes, complemented by observations from submillimeter telescopic instrumentation. Findings suggest a synchronization between the SFR and central black hole accretion rates, hinting at a broader co-evolution between supermassive black holes and their host galaxies.

Methodological Insights and Implications

The widespread use of multiwavelength photometric data is foundational in tracing stellar mass and SFR histories. Challenges remain in accounting for dust extinction, particularly in ultraviolet regimes, necessitating corrections based on a variety of assumptions about dust properties. The empirical calibration of SFR indicators across different wavelengths is pivotal, contingent on stellar population synthesis models that estimate mass-to-light ratios from observed spectral energy distributions.

Determining the past SFRs has implications for understanding larger scale cosmic phenomena, such as the reionization of the Universe and the buildup of heavy elements over cosmic time. Furthermore, these findings bear relevance to theoretical work on the initial mass function (IMF) and on the conditions necessary to achieve cosmological reionization.

Future Directions

This study calls for more refined datamodels and simulations to better track uncharted cosmic phases and the evolution of galaxy subpopulations. Deep-field surveys and next-generation space telescopes like the James Webb Space Telescope (JWST) are expected to yield unprecedented insights, especially for high-redshift galaxies where current understanding and data are less comprehensive.

Conclusion

Madau and Dickinson's synthesis underscores the interconnected development of galaxies, stars, and black holes through cosmic history, outlining both the successes and challenges in comprehensively mapping the star-formation chronicles of the Universe. As astronomical techniques and data accuracy continue to improve, these findings will undoubtedly evolve, providing clearer insights into both the early and late developmental phases of the cosmos.