[Paper Review] Stellar- and AGN-Driven Outflows in JWST Galaxies at z=3-9: More Frequent, Wider Opening Angles, and Mostly Bounded

Yi Xu, Masami Ouchi+6 more|arXiv (Cornell University)|Oct 10, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study analyzes ionized outflows in 130 high-redshift galaxies (z=3–9) using JWST NIRSpec and NIRCam WFSS data, identifying 30 galaxies with broad Hα and [Oiii] emission lines indicating outflows. It finds that these outflows are frequent, have wide opening angles (>45°), and are mostly bounded (fountain-like), with velocities insufficient to escape galactic potentials, suggesting widespread, spherical feedback in early galaxies.

ABSTRACT

We study outflows in 130 galaxies with -221000 km s$^{-1}$ while one galaxy is identified as a Type-2 AGN by high ionization emission lines. With the velocity shift and line widths of the outflow broad lines, we obtain ~80-500 km s$^{-1}$ for the outflow velocities. We find that the outflow velocities are slower than low-z galaxies with similar SFRs, which may be explained by the low stellar masses of high-z galaxies. The outflow velocities of AGNs are large but not significantly different from the others. Interestingly, these outflow velocities are typically not high enough to escape from the galactic potentials, possibly suggesting fountain-type outflows. We estimate mass loading factors $η$ to be 0.1-1 that are not particularly large, but comparable with those of z~1 outflows. The large fraction of galaxies with outflows (30% with high resolution data) provides constraints on outflow parameters, suggesting a wide opening angle of >45 deg and a large duty-cycle of >30%, which gives a picture of more frequent and spherical outflows in high-z galaxies.

Motivation & Objective

• To investigate the prevalence and properties of ionized outflows in high-redshift galaxies (z=3–9) using JWST data.
• To determine whether stellar or AGN feedback drives outflows and how efficient it is in early galaxies.
• To constrain outflow geometry (opening angle) and duty cycle using spectroscopic signatures.
• To compare outflow velocities and mass loading factors with low-redshift galaxies to assess feedback evolution.
• To test whether outflows in high-z galaxies are capable of escaping galactic potentials or are instead confined.

Proposed method

• Analyzes rest-frame optical emission lines (Hα and [Oiii]) from JWST NIRSpec and NIRCam WFSS data in 130 galaxies with −22 < MUV < −16 at z=3–9.
• Identifies outflow signatures via broad emission-line components with FWHM ≈ 200–700 km s⁻¹ in Hα and [Oiii] lines.
• Estimates outflow velocities from the line-of-sight velocity shifts of broad components relative to systemic redshifts.
• Calculates mass loading factors (η) using outflow velocities, star formation rates (SFRs), and stellar masses.
• Uses the fraction of galaxies with detectable outflows and their kinematic properties to infer average opening angles and duty cycles.
• Compares outflow properties with low-redshift galaxies and simulations to assess escape potential and feedback efficiency.

Experimental results

Research questions

• RQ1 How frequent are ionized outflows in high-redshift galaxies (z=3–9) as revealed by JWST spectroscopy?
• RQ2 What are the typical outflow velocities, and how do they compare to escape velocities and low-redshift counterparts?
• RQ3 What is the geometry (opening angle) and duty cycle of outflows in early galaxies?
• RQ4 Are outflows driven by star formation or AGN activity, and how do their properties differ?
• RQ5 Can outflows escape galactic potentials, or are they mostly bounded (fountain-like), and what does this imply for feedback efficiency?

Key findings

• Out of 130 galaxies at z=3–9, 30 (23%) show broad emission-line components (FWHM ≈ 200–700 km s⁻¹) indicative of ionized outflows.
• Outflow velocities are measured at ∼80–500 km s⁻¹, which are slower than those in low-redshift galaxies with similar SFRs, likely due to lower stellar masses in high-z galaxies.
• AGN-driven outflows (4 Type-1, 1 Type-2) show velocities comparable to those driven by star formation, with no significant difference in bulk velocity.
• Outflow velocities are typically too low to escape galactic potentials, suggesting that most outflows are bounded and consistent with fountain-type feedback.
• The mass loading factor η is estimated to be 0.1–1, comparable to values observed at z∼1, indicating moderate feedback efficiency.
• The high fraction of outflowing galaxies (30% with high-resolution data) implies a wide opening angle of ≳45° and a large duty cycle of ≳30%, indicating frequent, spherical outflows in early galaxies.