[Paper Review] Gas and Metal Contents of Galaxies and Gaseous Halos: Preventive versus Ejective Feedback

Yu Lu, H. J. Mo+1 more|arXiv (Cornell University)|Apr 8, 2015

Spectroscopy and Laser Applications5 citations

TL;DR

This study compares ejective and preventive feedback models in galaxy formation using a semi-analytical framework to assess their predictions for baryonic masses and metallicities in galaxies and gaseous halos. It finds that only the preventive feedback model successfully matches multiple observational constraints—especially gas-phase metallicities and mass fractions—while ejective models fail to reconcile high gas fractions and low metallicities simultaneously, suggesting feedback is primarily preventative in low-mass galaxies.

ABSTRACT

Using a semi-analytical approach we investigate the characteristics of predictions for the masses and metallicities of the baryonic matter in and around galaxies made by three galaxy formation models. These models represent three different feedback scenarios: one model with purely ejective feedback, one model with ejective feedback with reincorporation of ejected gas, and one preventative model. We find that, when the model parameters are adjusted to predict the correct stellar masses for a range of halo masses between 10^{10} to 10^{12}Msun, these three scenarios have very different predictions for the masses and metallicities of the interstellar and circum-galactic media. Compared with current observational data, the model implementing preventative feedback has a large freedom to match a broad range of observational data, while the ejective models have difficulties to match a number of observational constraints simultaneously, independent of how the ejection and reincorporation are implemented. Our results suggest that the feedback process which regulates the amounts of stars and cold gas in low-mass galaxies is preventative in nature.

Motivation & Objective

• To test whether ejective or preventive feedback better explains the observed baryonic mass and metallicity distributions in galaxies and their gaseous halos.
• To assess the viability of ejective feedback models—both with and without gas reincorporation—against observational constraints on stellar mass, gas mass, and metallicity.
• To determine whether current observational data can distinguish between different feedback mechanisms in galaxy formation.
• To evaluate whether the observed stellar and gas-phase metallicity relations can be reproduced by different feedback scenarios.

Proposed method

• Adopts a semi-analytical galaxy formation model from Lu et al. (2015) as a baseline framework.
• Systematically varies only the feedback prescription—comparing three models: pure ejection, ejection with gas reincorporation, and preventive feedback.
• Tunes model parameters to match the observed stellar mass-halo mass relation across halo masses from 10^10 to 10^12 M_sun.
• Calculates baryonic mass fractions and metallicities in stars, interstellar medium (ISM), and circum-galactic medium (CGM) for each model.
• Compares model predictions against observational data, including the COS-Halos survey, stellar mass-metallicity relations, and gas-phase metallicity trends.
• Uses a ΛCDM cosmology with standard parameters (Ω_M,0=0.27, h=0.70, σ8=0.82) to simulate galaxy evolution from z=2 to present.

Experimental results

Research questions

• RQ1 Can ejective feedback models—with or without gas reincorporation—simultaneously reproduce the observed stellar mass, gas mass, and gas-phase metallicity in galaxies?
• RQ2 How do different feedback mechanisms affect the predicted metallicity evolution of stars, ISM, and gaseous halos?
• RQ3 Does the inclusion of gas reincorporation in ejective models resolve discrepancies with observational data?
• RQ4 Can the preventive feedback model, which suppresses baryon collapse into low-mass halos, match the observed baryonic mass and metallicity distributions across different phases?
• RQ5 What are the implications of the observed gas-phase metallicity-stellar mass relation for distinguishing between ejective and preventive feedback?

Key findings

• The ejective feedback model with or without gas reincorporation fails to simultaneously match the observed gas mass fraction and gas-phase metallicity, especially in Milky Way-sized galaxies.
• Even with a high mass-loading factor, ejective models predict too low gas-phase metallicities, and increasing the outflow strength further reduces metallicity, worsening the mismatch.
• Ejective models predict a nearly constant gas-phase metallicity-stellar mass relation from z=2 to present, inconsistent with observations showing strong evolution in metallicity trends.
• The ejective model with rapid gas reincorporation overpredicts the metal mass in gaseous halos, contradicting COS-Halos observations that constrain halo metallicities.
• The preventive feedback model successfully matches the observed stellar and cold gas mass fractions, sizes of disk galaxies, and star formation histories across halo masses.
• The only discrepancy in the preventive model is an overprediction of metal mass in the stellar component of low-mass galaxies, which may be resolved by considering inhomogeneous ISM metallicity or early star formation before z=2.