The evolving relations between size, mass, surface density, and star formation in 3x10^4 galaxies since z=2 (0906.4786v3)

Abstract: The presence of massive, compact, quiescent galaxies at z>2 presents a major challenge for theoretical models of galaxy formation and evolution. Using one of the deepest large public near-IR surveys to date, we investigate in detail the correlations between star formation and galaxy structural parameters (size, stellar mass, and surface density) from z=2 to the present. At all redshifts, massive quiescent galaxies (i.e. those with little or no star formation) occupy the extreme high end of the surface density distribution and follow a tight mass-size correlation, while star-forming galaxies show a broad range of both densities and sizes. Conversely, galaxies with the highest surface densities comprise a nearly-homogeneous population with little or no ongoing star formation, while less dense galaxies exhibit high star-formation rates and varying levels of dust obscuration. Both the sizes and surface densities of quiescent galaxies evolve strongly from z=2-0; we parameterize this evolution for both populations with simple power law functions and present best-fit parameters for comparison to future theoretical models. Higher-mass quiescent galaxies undergo faster structural evolution, consistent with previous results. Interestingly, star-forming galaxies' sizes and densities evolve at rates similar to those of quiescent galaxies. It is therefore possible that the same physical processes drive the structural evolution of both populations, suggesting that "dry mergers" may not be the sole culprit in this size evolution.

Evolving Relations of Size, Mass, Surface Density, and Star Formation in Galaxies since z=2

The paper investigates significant challenges in our understanding of galaxy formation and evolution by analyzing the intricate relationships between size, mass, surface density, and star formation rates among galaxies spanning from high redshift ($z=2$) to the present. This research addresses the paradox of massive, compact, quiescent galaxies observed at $z>2$, which current theoretical models struggle to fully explain.

Key Findings

Using a comprehensive dataset from some of the deepest large near-IR surveys, the authors undertake a detailed analysis involving approximately 30,000 galaxies. The principal discoveries can be summarized as follows:

1. Massive Quiescent Galaxies: Galaxies with little or no ongoing star formation consistently occupy the high-end of the surface density spectrum and exhibit a tight mass-size correlation across all redshifts. This is strikingly different from star-forming galaxies that display a wider range of both densities and sizes.
2. Size and Surface Density Evolution: The size and surface density of quiescent galaxies evolve considerably from $z=2$ to the current epoch. This evolution can be effectively described by simple power-law functions. Higher-mass quiescent galaxies undergo this transformation more rapidly, corroborating earlier findings.
3. Star-Forming Galaxies: Interestingly, the rates of size and density evolution in star-forming galaxies are analogous to those in quiescent galaxies, implying that similar physical processes could be driving the structural evolution of both populations.
4. Density and Quiescence: Quiescent galaxies at any given epoch tend to have high surface densities. Moreover, the threshold surface density above which galaxies become predominantly quiescent shifts with redshift.

Implications and Theoretical Considerations

The observations place rigorous constraints on theoretical models of galaxy formation. With a marked evolution in both physical structure and population density of massive, compact galaxies, the work suggests that mechanisms other than "dry mergers" might significantly influence these transformations.

The possibility that shared processes might affect both star-forming and quiescent galaxies indicates that models must integrate both galaxy types' evolution. The traditional view that major dry mergers drive size growth might be supplemented or replaced by processes such as minor mergers or the continuous accretion of satellite galaxies.

Future Directions in Research

The results prompt further inquiry into alternate theoretical mechanisms and the detailed processes affecting galactic evolution. As observational capabilities expand with forthcoming instruments and surveys, reevaluating theoretical models to account for the shared evolution of star-forming and quiescent galaxies will be essential.

Furthermore, extending similar analyses to even larger datasets and higher redshifts could refine our understanding of galaxy population dynamics. Such efforts will provide deeper insights into the universe's evolving conditions and the complex interplay of stellar growth and galactic structural changes.

Conclusion

This comprehensive paper offers a pivotal contribution to the field of galaxy evolution by systematically examining the interplay between size, mass, and star formation across cosmic time. It poses critical challenges and guides the future refinement of galaxy formation models, supporting a nuanced understanding of cosmic evolution patterns.