[Paper Review] Dwarf stellar haloes: a powerful probe of small-scale galaxy formation and the nature of dark matter

Sownak Bose, Azadeh Fattahi+2 more|arXiv (Cornell University)|Dec 3, 2021

Galaxies: Formation, Evolution, Phenomena95 references38 citations

TL;DR

This paper uses N-body cosmological simulations and empirical galaxy models to show that dwarf stellar haloes are highly sensitive probes of small-scale galaxy formation and dark matter physics. It finds that minor mergers dominate stellar halo growth in low-mass dwarfs, with intermediate-mass ratios (~1:5) maximizing distant halo formation; warm dark matter suppresses minor mergers, and stellar haloes could be detectable via stacking or phase-space overlapping streams in the Milky Way halo.

ABSTRACT

We use N-body cosmological simulations and empirical galaxy models to study the merger history of dwarf-mass galaxies (with Mhalo∼1010M⊙⁠). Our input galaxy models describe the stellar mass–halo mass relation, and the galaxy occupation fraction. The number of major and minor mergers depends on the type of dark matter; in particular, minor mergers are greatly suppressed in warm dark matter models. In addition, the number of mergers that bring in stars is strongly dependent on the galaxy occupation model. For example, minor mergers are negligible for stellar halo growth in models with a high mass threshold for galaxy formation (i.e. 109.3M⊙ at z = 0). Moreover, this threshold for galaxy formation can also determine the relative difference (if any) between the stellar haloes of satellite and field dwarfs. Using isolated simulations of dwarf–dwarf mergers, we show that the relative frequency of major and minor mergers predict very different stellar haloes: Typically, ‘intermediate’ dark matter merger ratios (∼1:5) maximize the growth of distant stellar haloes. We discuss the observability of dwarf stellar haloes and find that the surface brightness of these features are incredibly faint. However, when several dwarfs are stacked together, models that form particularly rich stellar haloes could be detectable. Finally, we show that stellar streams in the Galactic halo overlapping in phase space with known dwarf satellites are likely remnants of their stripped stellar haloes. The mere existence of dwarf stellar haloes can already put constraints on some small-scale models, and thus observational probes should be a high priority.

Motivation & Objective

• To investigate how dwarf stellar haloes form and evolve under different dark matter models (CDM vs. WDM).
• To assess the role of galaxy formation physics—particularly the halo mass threshold for galaxy formation—in shaping stellar halo properties.
• To evaluate the observability of faint dwarf stellar haloes and identify feasible detection strategies.
• To determine whether stellar haloes of satellite and central dwarfs differ due to environmental effects and merger histories.
• To explore the potential of stellar streams overlapping with known dwarf satellites as tracers of stripped stellar haloes.

Proposed method

• N-body cosmological simulations with CDM and WDM initial conditions to model halo and galaxy assembly at dwarf mass scales (Mhalo ~10^10 M⊙).
• Empirical galaxy models incorporating the stellar mass-halo mass (SMHM) relation and galaxy occupation fraction to assign galaxies to haloes.
• Isolated simulations of dwarf-dwarf mergers to study the dependence of stellar halo growth on merger mass ratio and binding energy.
• Phase-space and surface brightness analysis to assess detectability of stellar haloes, including stacking of multiple dwarfs to enhance signal-to-noise.
• Comparison of merger histories and halo structural properties between central and satellite dwarfs at z=0.
• Use of simulated phase-space overlap between streams and surviving dwarfs to test observational detectability.

Experimental results

Research questions

• RQ1 How does the frequency and nature of mergers (major vs. minor) depend on the dark matter model (CDM vs. WDM) in dwarf-mass haloes?
• RQ2 To what extent does the halo mass threshold for galaxy formation influence the growth and structure of dwarf stellar haloes?
• RQ3 What is the relative contribution of major versus minor mergers to the formation of extended stellar haloes at large radii (>0.1 r200)?
• RQ4 Can dwarf stellar haloes be detected observationally, and what strategies (e.g., stacking, stream detection) offer the best prospects?
• RQ5 Do stellar streams overlapping in phase space with known Milky Way dwarf satellites likely originate from stripped stellar haloes of their progenitors?

Key findings

• Minor mergers are strongly suppressed in warm dark matter (WDM) models, reducing their contribution to stellar halo growth compared to cold dark matter (CDM).
• The relative difference between stellar haloes of satellite and central dwarfs is significant only in galaxy formation models with a high halo mass threshold (e.g., 10^9.3 M⊙ at z=0), otherwise differences are modest.
• Isolated dwarf-dwarf merger simulations show that intermediate-mass ratios (~1:5) maximize the deposition of stars at large distances (>0.1 r200), balancing binding energy and stellar mass input.
• Stellar haloes in models with low halo mass thresholds and shallow SMHM relations are the most likely to produce observable surface brightness features, especially when multiple dwarfs are stacked.
• Stellar streams overlapping in phase space with known dwarf satellites are likely remnants of their stripped stellar haloes, making these streams prime observational targets.
• The mere existence of dwarf stellar haloes can already constrain galaxy formation and dark matter models, particularly by ruling out high-threshold galaxy formation models if such haloes are not observed.