Missing Halo Baryons and Galactic Outflows

We present predictions for galactic halo baryon fractions from cosmological hydrodynamic simulations with a well-constrained model for galactic outflows. Without outflows, halos contain roughly the cosmic fraction of baryons, slightly lowered at high masses owing to pressure support from hot gas. The star formation efficiency is large and increases monotonically to low masses, in disagreement with data. With outflows, the baryon fraction is increasingly suppressed in halos to lower masses. A Milky Way-sized halo at z=0 has about 60% of the cosmic fraction of baryons, so “missing” halo baryons have largely been evacuated, rather than existing in some hidden form. Large halos (>10^13 Mo) contain 85% of their cosmic share of baryons, which explains the mild missing baryon problem seen in clusters. By comparing results at z=3 and z=0, we show that most of the baryon removal occurs at early epochs in larger halos, while smaller halos lose baryons more recently. Star formation efficiency is maximized in halos of ~10^13 Mo, dropping significantly to lower masses, which helps reconcile the sub-L* slope of the observed stellar and halo mass functions. These trends are predominantly driven by differential wind recycling, namely, that wind material takes longer to return to low-mass galaxies than high-mass galaxies. The hot gas content of halos is mostly unaffected by outflows, showing that outflows tend to blow holes and escape rather than deposit their energy into halo gas.

💡 Research Summary

The authors use cosmological hydrodynamic simulations that incorporate a well‑constrained galactic outflow model to investigate how baryons are distributed within dark‑matter halos of different masses and at different epochs. Two simulation suites are compared: one without any winds (the “no‑outflow” case) and one that includes momentum‑driven winds calibrated to reproduce observed galaxy scaling relations. In the no‑outflow runs, halos retain essentially the universal baryon fraction (≈16 % of the total mass) across the entire mass range, with only a modest reduction in the most massive systems due to pressure support from hot gas. However, the star‑formation efficiency in this scenario rises monotonically toward lower halo masses, dramatically over‑producing stars in dwarf‑scale halos and conflicting with the observed stellar‑to‑halo mass relation.

When winds are turned on, the picture changes dramatically. Outflows preferentially expel gas from low‑mass halos, lowering their baryon fractions to 30–50 % of the cosmic value for halos of 10¹¹–10¹² M☉ at z = 0. Milky Way‑mass halos (≈10¹² M☉) retain about 60 % of the universal baryon budget, implying that the “missing” baryons have been largely ejected rather than hidden in an unseen phase. Massive halos (>10¹³ M☉) still hold ≈85 % of their expected baryons, which naturally explains why galaxy clusters exhibit only a mild missing‑baryon problem.

The authors trace the temporal evolution of these trends by comparing snapshots at z ≈ 3 and z = 0. They find that most of the baryon loss in large halos occurs early, during the peak of cosmic star formation, whereas smaller halos continue to lose baryons at later times. This differential behavior is driven by wind recycling: wind material returns to massive galaxies on relatively short timescales, while in low‑mass systems the recycling time is much longer, effectively keeping the expelled gas out of the halo for a Hubble time. Consequently, star‑formation efficiency peaks in halos of ∼10¹³ M☉ and declines sharply toward lower masses, bringing the simulated stellar mass function into agreement with observations.

Importantly, the simulations show that the hot gas component (T ≈ 10⁶–10⁷ K) of halos is largely unaffected by the presence of winds. Outflows tend to carve low‑density channels and escape rather than depositing a substantial amount of thermal energy into the halo gas. This result aligns with X‑ray observations of galaxy groups and clusters, which indicate that the intracluster medium retains its thermal structure despite vigorous galactic activity.

Overall, the study demonstrates that galactic outflows, especially when modeled with realistic mass‑loading and velocity scalings, are essential for reproducing the observed baryon fractions, stellar efficiencies, and the modest missing‑baryon problem in massive halos. The work underscores the importance of differential wind recycling as a key physical mechanism governing galaxy formation across cosmic time.