Positive AGN Feedback Enhances Star Formation in Starburst Dwarf Galaxies

TL;DR

This work tackles the question of whether AGN feedback can promote or suppress star formation in dwarf galaxies. Using MACER3D to directly resolve the Bondi radius, the authors simulate starburst dwarfs with coupled AGN and SN feedback and find a net 25% enhancement in global star formation when both feedback channels operate, especially in the disk, while central regions are suppressed. The mechanism relies on AGN-driven shocks compressing gas in a regime where radiative cooling is efficient, maintaining high density after cooling and enabling rapid star formation, with the Kennicutt–Schmidt relation translating density increases into higher SFRs. The results, consistent with Henize 2-10 observations, challenge the conventional view of universal negative AGN feedback in dwarfs and suggest that positive feedback may play a meaningful role in the evolution of gas-rich, high-redshift dwarf galaxies.

Abstract

The role of active galactic nuclei (AGN) feedback in dwarf galaxies remains poorly understood, with conventional wisdom suggesting it primarily suppresses star formation. Using high-resolution MACER3D simulations that directly resolve the Bondi radius, we demonstrate that AGN feedback can significantly enhance rather than suppress star formation in starburst dwarf galaxies. Our simulations reveal that AGN feedback increases global star formation rates by approximately 25% when comparing our models with both AGN and supernova feedback to those with only supernova feedback. This enhancement occurs through AGN-driven outflows creating compressed gas regions where efficient cooling preserves the high density while quickly radiating away thermal energy, creating ideal conditions for star formation. This positive feedback mechanism operates in gas-rich starburst environments with efficient cooling and moderate AGN energy input that compresses gas without expelling it from the galaxy. Critically, it requires both AGN and supernova feedback working in concert: without SN feedback to regulate black hole activity, AGN outflows become too powerful and expel gas rather than compress it. Our results closely match observations of the starburst dwarf galaxy Henize 2-10, where similar shock-compressed regions of enhanced star formation have been observed. These findings challenge conventional understanding of AGN feedback and suggest that AGN may play a previously unrecognized role in accelerating star formation during active phases of dwarf galaxy evolution.

Figures (9)

• Figure 1: Global SFR as a function of time (left) and profiles of the mass density of new stars at $t \, = \, 500 \, {\rm Myr}$ (right) in the fullFB (inflow) (red solid), SNonly (inflow) (green solid), fullFB (red dashed), SNonly (green dashed) and AGNonly (grey) simulations. In fullFB (inflow) and fullFB, AGN feedback enhances global star formation: it suppresses star formation in the central region ($\lesssim 50\,\mathrm{pc}$) but promotes it across the wider galactic disk (from $\sim 50\,\mathrm{pc}$ to $1.5\,\mathrm{kpc}$). In the absence of SN feedback (AGNonly), the star formation is instead significantly suppressed by AGN feedback.
• Figure 2: Time-averaged radial profiles of the ratio of gas cooling timescale $t_{\rm cool}$ to its dynamical timescale $t_{\rm dyn}$. Within the star-forming region ($\lesssim 2 \, \mathrm{kpc}$), the cooling timescale is shorter than the dynamical timescale, indicating efficient cooling in our simulated dwarfs.
• Figure 3: Edge-on projections of various quantities within a 250 pc radius at different evolutionary stages in the fullFB (inflow) (top panel) and SNonly (inflow) (bottom panel) simulations: the first row shows volume-weighted gas number density; the second row displays mass-weighted gas temperature; the final row presents mass-weighted radial velocity with overplotted velocity streamlines. The AGN luminosities in the fullFB (inflow) simulation and the corresponding simulation times are shown in each column. The compression of the ISM by AGN-driven shock waves produces dense gas structures, which in turn facilitate star formation.
• Figure 4: Slice plots of gas density (first row), temperature (second row) and radial velocity (last row) on the equatorial plane within 1 kpc radius at selected evolutionary times for both the fullFB (inflow) (top panel) and SNonly (inflow) (bottom panel) simulations. Pronounced filamentary and clumpy structures are found in the fullFB (inflow) simulation, which are largely absent in the SNonly (inflow) case.
• Figure 5: Left and middle panels: time-averaged temperature-density diagrams of gas within 2 kpc for the SNonly (inflow) and fullFB (inflow) models. Right panel: the corresponding mass ratio between the fullFB (inflow) and SNonly (inflow) models. Unphysical infinite values have been removed. The black dashed rectangle represents the region containing star-forming gas. The fullFB (inflow) simulation contains substantially more high-density gas than the SNonly (inflow) simulations.