The Effect of Many Minor Mergers on the Size Growth of Compact Quiescent Galaxies

Abstract: Massive galaxies with a half-mass radius <~ 1kpc are observed in the early universe (z~>2), but not in the local universe. In the local universe similar-mass (within a factor of two) galaxies tend to be a factor of 4 to 5 larger. Dry minor mergers are known to drive the evolution of the size of a galaxy without much increasing the mass, but it is unclear if the growth in size is sufficient to explain the observations. We test the hypothesis that galaxies grow through dry minor mergers by simulating merging galaxies with mass ratios of q=1:1 (equal mass) to q=1:160. In our N-body simulations the total mass of the parent galaxy doubles. We confirm that major mergers do not cause a sufficient growth in size. The observation can be explained with mergers with a mass ratio of q=1:5--1:10. Smaller mass ratios cause a more dramatic growth in size, up to a factor of ~17 for mergers with a mass ratio of 1:80. For relatively massive minor mergers q ~> 1:20 the mass of the incoming child galaxies tend to settle in the halo of the parent galaxy. This is caused by the tidal stripping of the child galaxies by the time they enter the central portion of the parent. When the accretion of minor galaxies becomes more continuous, when q <~ 1:40, the foreign mass tends to concentrate more in the central region of the parent galaxy. We speculate that this is caused by dynamic interactions between the child galaxies inside the merger remnant and the longer merging times when the difference in mass is larger. These interactions cause dynamical heating which results in accretion of mass inside the galaxy core and a reduction of the parent's circular velocity and density.

• The paper demonstrates that minor mergers, especially with mass ratios of 1:5 to 1:10, are key drivers of significant galaxy size growth.
• It employs N-body simulations to reveal that infalling mass expands the outer regions or concentrates centrally depending on merger dynamics.
• The results support the minor merger hypothesis, offering a robust explanation for the size evolution of compact quiescent galaxies.

Overview of "The Effect of Many Minor Mergers on the Size Growth of Compact Quiescent Galaxies"

The paper by Bedorf and Portegies Zwart explores the intriguing phenomenon of size growth in compact quiescent galaxies via the mechanism of minor mergers. The research addresses the disparity observed between massive, compact galaxies in the early universe and their significantly larger counterparts at lower redshifts. The prevalent hypothesis that minor, dry mergers account for this expansion without considerable mass accretion is rigorously examined through $N$-body simulations.

Key Findings

1. Impact of Mass Ratio on Size Growth: The study investigates mass ratios ranging from 1:1 (equal-mass mergers) to 1:160. It notably concludes that major mergers (1:1) are insufficient in explaining the size growth. Size expansion becomes significant with mass ratios in the range of 1:5 to 1:10, aligning with observed trends. For extreme minor mergers (1:80), the size of galaxies can expand by a factor approximating 17, even as the total mass doubles.
2. Mechanisms of Mass Distribution: In mergers where the mass ratio is around 1:20, the mass from infalling galaxies primarily accumulates in the outer parts of the host galaxy, due to tidal stripping effects. Beyond this ratio, the mass tends to settle more centrally, suggesting dynamic interactions among merging components contribute to this central concentration, likely due to prolonged merging times and dynamical friction adjustments.
3. Numerical and Theoretical Consistency: The findings indicate significant support for the minor merger hypothesis in accounting for observed growth without significant mass increase. Variations in outcomes associated with different merger histories were observed, particularly influenced by the initial densities of minor galaxies, but these did not overshadow the overarching trends affirming the minor merger effect.

Practical and Theoretical Implications

The paper provides substantial evidence for minor mergers as a critical contributor to galaxy size evolution, challenging the adequacy of major mergers alone. These findings have crucial implications for our understanding of galaxy evolution and the hierarchical structure formation paradigm. The observed inside-out growth provides a nuanced perspective that complements existing literature, offering a reconciliation of previously contradictory studies.

Future developments in AI could further enhance simulation accuracy by incorporating machine learning techniques to predict outcomes under varying initial conditions, thereby enriching our understanding of complex astrophysical processes.

Conclusion

Bedorf and Portegies Zwart's work enriches the discourse on galaxy evolution by substantiating minor mergers as a valid mechanism for size growth in massive, compact galaxies. The insights derived from the study suggest avenues for future research, particularly in exploring the interplay between dynamical conditions and structural evolution on a cosmological scale.