The Dual Origin of Stellar Halos (0904.3333v2)

Abstract: We investigate the formation of the stellar halos of four simulated disk galaxies using high resolution, cosmological SPH + N-Body simulations. These simulations include a self-consistent treatment of all the major physical processes involved in galaxy formation. The simulated galaxies presented here each have a total mass of ~10^12 M_sun, but span a range of merger histories. These simulations allow us to study the competing importance of in-situ star formation (stars formed in the primary galaxy) and accretion of stars from subhalos in the building of stellar halos in a LambdaCDM universe. All four simulated galaxies are surrounded by a stellar halo, whose inner regions (r < 20 kpc) contain both accreted stars, and an in-situ stellar population. The outer regions of the galaxies' halos were assembled through pure accretion and disruption of satellites. Most of the in-situ halo stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies' potential wells, possibly as part of their primordial disks. These stars were displaced from their central locations into the halos through a succession of major mergers. We find that the two galaxies with recently quiescent merger histories have a higher fraction of in-situ stars (~20-50%) in their inner halos than the two galaxies with many recent mergers (~5-10% in-situ fraction). Observational studies concentrating on stellar populations in the inner halo of the Milky Way will be the most affected by the presence of in-situ stars with halo kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.

• The paper demonstrates that stellar halos have a dual origin, with inner regions enriched by in-situ star formation and outer regions dominated by accreted stars.
• It employs high-resolution SPH and N-Body simulations to quantify halo composition variations, revealing in-situ fractions of 20–50% in quiescent merger histories versus 5–10% in active ones.
• The analysis highlights how major mergers and feedback mechanisms displace in-situ stars into halo orbits, underscoring their critical role in galactic evolution.

The Dual Origin of Stellar Halos in Simulated Disk Galaxies

In "The Dual Origin of Stellar Halos," Zolotov et al. present an examination of the formation processes of stellar halos surrounding disk galaxies, leveraging high-resolution cosmological SPH (Smoothed Particle Hydrodynamics) and N-Body simulations. The authors simulate four individual disk galaxies, each with a mass approximating $10^{12} M_{\odot}$, to delve into the contribution of in-situ star formation and the accretion of stars from subhalos in a $\Lambda$CDM universe.

Methodology and Simulation Framework

The research utilizes simulations that incorporate detailed treatments of major physical processes important to galaxy formation. By examining several galaxies with varying merger histories, the authors ascertain the distinct roles played by in-situ star formation—occurring within the primary galaxy—and the accretion of stellar materials from disrupted satellites. This dichotomy is critical in analyzing the spatial and formation-time characteristics of each stellar population within the halos.

Findings on Stellar Halo Composition

It is evident from the findings that all simulated galaxies have halos composed of both accreted and in-situ stars, primarily differentiated by their spatial locations:

• Inner Halo (r < 20 kpc): This region hosts both accreted and in-situ populations. Interestingly, the presence of in-situ stars is markedly higher in galaxies with dormant recent merger histories.
• Outer Halo: Composed primarily of accreted stars, this region extends through the disruption of satellite galaxies. This finding aligns with previous studies suggesting that accreted stars dominate beyond certain radial thresholds in galactic halos.

Implications of Merger Histories

A significant observation is that galaxies with recently quiescent merger histories have higher fractions of in-situ stars within their inner halos (approximately 20-50%) as opposed to those with more active merger histories and lower in-situ fractions (5-10%). This correlation suggests that the frequency and timing of merger events crucially affect the structural and compositional qualities of the stellar halo.

In-Situ Star Formation Mechanism

The authors discovered that in-situ halo stars often form in high redshift environments from cold, smooth accreted gas in the central regions and are displaced into halo orbits due to major mergers. This insight provides a nuanced understanding of how dissipative processes contribute to the development of complex halo structures.

Comparisons and Theoretical Implications

This paper’s results are consistent with the theoretical underpinnings set forth by prior studies such as those by Bullock & Johnston (2005), providing a robust framework for accretion-dominated halo formation models. The paper contributes to ongoing research efforts aimed at deciphering the dual origin of stellar halos, emphasizing the need for inclusion of in-situ star formation phenomena in galactic evolution models.

Feedback and Numerical Considerations

Further, the analysis highlights the critical role of feedback mechanisms in regulating galactic evolution. Different feedback models notably influence the prominence of in-situ versus accreted stars, underscoring the need for nuanced simulation frameworks to predict realistic halo features.

Conclusion and Future Directions

Overall, the dual origin framework for stellar halos urges the astronomical community to consider both in-situ and accretion processes in quantitative characterizations of halo structures. Moving forward, integrations of chemical abundance analyses, alongside kinematic and spatial datasets, could provide enhanced resolutions in differentiating between these two components in observationally dense regions of stellar halos. This integrated approach may pave the way for more accurate reconstructions of galactic assembly histories in larger sample sizes, including extragalactic observations akin to those of the Milky Way and M31.