MEGATRON: The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies

Technical Deep Dive: MEGATRON - The environments of Population III stars at Cosmic Dawn and their connection to present day galaxies

Overview

This paper presents results from the MEGATRON suite of cosmological simulations, which self-consistently model the formation and evolution of Population III (Pop III) stars in the context of a Milky Way-like progenitor galaxy at Cosmic Dawn. The key findings include:

• Pop III stars primarily form in halos with masses 10^6 - 10^8 M⊙, with a constant global star formation rate of 10^-3 M⊙/yr.
• Lyman-Werner radiation from nearby star-forming regions can suppress Pop III formation in smaller halos below 10^6 M⊙, pushing it to more massive atomic cooling halos.
• On rare occasions, halos can host "Pop III starbursts" with up to ~100 Pop III stars forming in rapid succession, likely the brightest observable Pop III systems.
• Most Pop III star remnants at z=0 reside in the stellar halo of the main galaxy, while a significant fraction remain in bound subhalos.
• Detecting individual Pop III stars remains extremely challenging, but their proximity to faint, UV-bright galaxies may offer the best observational prospects.

Methodology

The MEGATRON simulations use the RAMSES-RTZ code to model a 20 cMpc^3 region centered on a Milky Way progenitor, with:

• High spatial resolution (~1.5 pc) to resolve individual Pop III star formation
• Detailed non-equilibrium chemistry and radiative transfer, including Lyman-Werner radiation
• A top-heavy Pop III initial mass function (characterized by a 100 M⊙ peak)
• Tracking of Pop III stars and their feedback (supernovae, black hole formation)
• A dark matter-only simulation to trace Pop III remnants to z=0

Results

Pop III Formation Environments

• Pop III stars form predominantly in halos with masses 10^6 - 10^8 M⊙, with a constant global SFR of 10^-3 M⊙/yr.
• The Lyman-Werner background from nearby star formation can suppress cooling in smaller halos below 10^6 M⊙, shifting Pop III formation to more massive atomic cooling halos.
• Pop III formation occurs in two distinct populations:
  • "Pop III.1" in low-ionization, H2-cooled mini-halos at early times
  • "Pop III.2" in more massive, H-cooled atomic cooling halos at later times under higher LW backgrounds.

Rare Pop III Starbursts

• On extremely rare occasions (1-3 per simulation), halos up to 3x10^8 M⊙ can host "Pop III starbursts" with up to ~100 Pop III stars forming in rapid succession.
• These starburst halos likely represent the brightest observable Pop III systems, but are very uncommon.

Spatial Distribution of Pop III Remnants

• At z=0, 75-80% of Pop III star remnants reside in the stellar halo of the main galaxy, with the remainder in bound subhalos.
• Individual subhalos can host over 100 Pop III remnants, making small dwarf galaxies promising targets to search for surviving Pop III stars.

Observational Prospects

• Pop III galaxies generally have UV magnitudes between -7 > M_UV > -12, over 2 dex fainter than the faintest JWST-observed galaxies.
• The best prospects for detecting Pop III stars may be through their proximity to faint, UV-bright galaxies, though only ~0.06% form within the virial radius of M_UV < -17 galaxies.
• Upcoming facilities like 4MOST and WEAVE may be able to identify surviving low-mass Pop III stars through their distinct chemical signatures.

Limitations & Uncertainties

• The top-heavy Pop III initial mass function used in MEGATRON may overestimate the formation of the most massive, short-lived Pop III stars.
• Neglecting the effects of baryon-dark matter streaming velocities could lead to an overestimate of Pop III formation in mini-halos.
• The simulations do not explicitly model Lyman-α radiation pressure, which could disrupt star-forming regions and reduce the total Pop III stellar mass.
• Tracing Pop III remnants to z=0 relies on the dark matter-only simulation, which may not fully capture baryonic effects on halo structures over cosmic time.

What Comes Next

Future work should explore the impact of different Pop III IMF models, as well as include more detailed radiation physics to better constrain the environments and observational signatures of the first stars. Connecting these simulations to upcoming observational campaigns will be crucial for making progress in our understanding of Cosmic Dawn and the origins of galaxies.