Radiative feedback and the low efficiency of galaxy formation in low-mass haloes at high redshift

TL;DR

This study demonstrates that local radiative feedback from massive stars—implemented as radiation pressure, photoionization, and photoheating in high-resolution cosmological zoom-in simulations—significantly suppresses star formation in ~10^11 M_⊙ halos at z≈3. Radiation pressure disperses dense gas, lowers the central stellar mass, and yields an extended, prolate stellar distribution with rising circular velocity curves, bringing the stellar-to-virial mass ratios in line with abundance matching (~0.6%). Infrared trapping provides secondary reductions in star-forming gas, while local heating shifts some dense gas to warmer phases; halo gas remains largely intact, indicating that feedback primarily prevents overcooling rather than ejecting most baryons. Together, these results show radiative feedback as a robust mechanism to establish the observed low-efficiency galaxy formation in low-mass halos at high redshift. The approach advances modeling by incorporating physically motivated, locally resolved radiative processes into cosmological simulations, informing interpretations of early galaxy growth and the connection to present-day systems.

Abstract

Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions, suggested by current abundance matching models. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modeled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. We also include photoionization and photoheating by massive stars. The full photoionization of hydrogen reduces the radiative cooling in the $10^{4-4.5}$ K regime. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. This maintains a low star formation rate of $\sim 1 \ {\rm M_\odot} \ {\rm yr}^{-1}$ in halos with masses about $10^{11} \ {M_\odot}$ at $z\simeq3$. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The galaxies residing in these low-mass halos contain only $\sim0.6\%$ of the total virial mass in stars, roughly consistent with abundance matching. Radiative feedback maintains an extended galaxy with a rising circular velocity profile.

Figures (11)

• Figure 1: Stellar spectra of a single stellar population with $M_*=10^3 \ {\rm M}_\odot$, solar composition and different ages, computed using starburst99. The spectrum gets significantly harder at 5 Myr due to the contribution of WR stars. The ionizing radiation, below 912 Ȧ is significantly suppressed after 10 Myr.
• Figure 2: cloudy cooling (blue solid curve) and heating (red dashed curve) rates for gas density $n=10 \,{\rm cm}^{-3}$ and gas metallicity $Z=10^{-3} Z_{\odot}$, illuminated by a star cluster of $M_*=10^6 \ {\rm M}_\odot$. The four panels correspond to different ages of the stellar population. For comparison, the dotted blue curves correspond to the cooling of UV shielded gas. Photoionization drastically reduces the cooling around T$=10^4-10^{4.5}$ K, if the number of ionizing photons is enough to ionize almost all hydrogen atoms. Photoheating significantly increases the temperature and thermal pressure of the HII region surrounding the star cluster by a factor of about 100.
• Figure 3: Same as Figure 2, with a stellar age of 5 Myr, where each panel corresponds to a different gas density. Higher densities correspond to higher column densities, which are able to attenuate the incident radiation. This changes the ionization conditions and the corresponding cooling curves.
• Figure 4: Projected mass density maps of the NoRadPre run at z=3. Gas (left) and stars (right), viewed face-on (top) and edge-on (bottom). Each panel is 20 kpc. The horizontal bar represents 4 kpc and the small dot marks the galaxy centre. The color scale represents the surface density in units of $\log ( {\rm M}_\odot \,{\rm pc}^{-2})$.
• Figure 5: Same as Figure \ref{['fig:SFNEW']} for the RadPre_LS run.