Kiloparsec-scale AGN Outflows and Feedback in Merger-Free Galaxies

Abstract: Recent observations and simulations have challenged the long-held paradigm that mergers are the dominant mechanism driving the growth of both galaxies and supermassive black holes (SMBH), in favour of non-merger (secular) processes. In this pilot study of merger-free SMBH and galaxy growth, we use Keck Cosmic Web Imager spectral observations to examine four low-redshift ($0.043 < z < 0.073$) disk-dominated `bulgeless' galaxies hosting luminous AGN, assumed to be merger-free. We detect blueshifted broadened [OIII] emission from outflows in all four sources, which the \oiii/\hbeta~ratios reveal are ionised by the AGN. We calculate outflow rates in the range $0.12-0.7~\rm{M}{\odot}~\rm{yr}^{-1}$, with velocities of $675-1710~\rm{km}~\rm{s}^{-1}$, large radial extents of $0.6-2.4~\rm{kpc}$, and SMBH accretion rates of $0.02-0.07~\rm{M}{\odot}~\rm{yr}^{-1}$. We find that the outflow rates, kinematics, and energy injection rates are typical of the wider population of low-redshift AGN, and have velocities exceeding the galaxy escape velocity by a factor of $\sim30$, suggesting that these outflows will have a substantial impact through AGN feedback. Therefore, if both merger-driven and non-merger-driven SMBH growth lead to co-evolution, this suggests that co-evolution is regulated by feedback in both scenarios. Simulations find that bars and spiral arms can drive inflows to galactic centres at rates an order of magnitude larger than the combined SMBH accretion and outflow rates of our four targets. This work therefore provides further evidence that non-merger processes are sufficient to fuel SMBH growth and AGN outflows in disk galaxies.

• The paper identifies blueshifted [OIII] lines as evidence of AGN-driven outflows surpassing host galaxy escape velocities.
• The study quantifies outflow rates of 0.12–0.7 solar masses per year with velocities from 675 to 1710 km/s, highlighting significant feedback effects.
• The paper demonstrates that secular processes such as bar- and spiral arm-driven inflows can fuel SMBH growth, challenging merger-dominated models.

Summary of "Kiloparsec-scale AGN Outflows and Feedback in Merger-Free Galaxies"

The study by Smethurst et al. investigates active galactic nucleus (AGN) driven outflows in disk-dominated, merger-free galaxies, challenging the conventional view that galaxy mergers are the primary drivers of supermassive black hole (SMBH) growth. The authors utilize high-resolution observations from the Keck Cosmic Web Imager to study four low-redshift disk-dominated galaxies, each hosting a luminous AGN. These galaxies are assumed to have evolved without undergoing significant mergers, rendering them ideal candidates for probing the secular processes contributing to SMBH growth.

Key Findings:

• Detection of Outflows: The study successfully identifies blueshifted [OIII] emission lines in all four galaxies, indicating the presence of AGN-driven outflows. The observed outflows have velocities significantly exceeding the escape velocities of the host galaxies, suggesting potential substantial impacts on their interstellar media.
• Outflow Rates and Characteristics: The estimated outflow rates are between 0.12 and 0.7 solar masses per year, with velocities ranging from 675 to 1710 km/s and radial extents of 0.6 to 2.4 kpc. Such characteristics are consistent with those observed in broader low-redshift AGN populations.
• Alternate SMBH Growth Mechanisms: The results suggest that non-merger processes, like bar-driven and spiral arm-driven gas inflows, are sufficient to fuel SMBH growth and drive AGN outflows, highlighting an effective alternate mechanism for galaxy evolution.

Interpretation and Implications:

This investigation aligns with theoretical models and simulations suggesting a significant role for secular processes in SMBH growth. The potential impact of these outflows implies that merger-free galaxies can sustain galaxy-SMBH co-evolution through effective feedback mechanisms. The study indicates that even in the absence of mergers, galaxies can exhibit substantial AGN activity driven by internal dynamical processes.

The findings could influence future models of galaxy evolution, particularly in clarifying the contributions of secular versus merger-driven dynamics in AGN activity and SMBH growth. They also underscore the need for further studies using high-resolution spectroscopic data to explore the scale and influence of AGN feedback in diverse galactic environments.

Future research could extend these investigations to larger samples and higher redshifts, examining the broader applicability of these findings across cosmic time. Additionally, analysis using adaptive optics-enabled instrumentation could offer deeper insights into the spatial structures and characteristics of these AGN-driven outflows.