The Rise of Ionized Gas Filaments in Early-Type Galaxies

TL;DR

This study investigates ionized gas filaments in early-type galaxies outside dense clusters, revealing diverse origins and properties, and suggesting cooling processes and hot gas reservoirs influence filament formation.

Contribution

It provides the first extensive analysis of ionized gas filaments in non-central early-type galaxies, linking their properties to galaxy evolution and hot gas interactions.

Findings

Filamentary nebulae are common in non-central ETGs, resembling those in cluster BCGs.

Warm gas in filaments cannot be explained solely by known ionization sources.

Filaments often host hot gas reservoirs, indicating cooling processes are involved.

Abstract

Multiphase filamentary nebulae are ubiquitous in the brightest cluster galaxies (BCGs) of cool-core clusters, providing insight into baryon cycling and the feeding and feedback of supermassive black holes. However, BCGs account for less than 1% of all early-type galaxies (ETGs). To broaden our understanding of how multiphase filamentary nebulae form in ETGs and connect to the greater picture of galaxy evolution, it is crucial to explore ETGs that are outside of the dense centers of galaxy clusters or groups. We present VLT-MUSE IFU observations of 126 nearby non-central ETGs, detecting warm ionized gas in 62 of them. 35/62 host rotating gas disks with the majority of them morphologically and kinematically aligned with their stellar components, suggesting stellar mass loss may dominate their warm-gas origin. The remaining 27 host filamentary nebulae, often decoupled from the stellar components, resembling those observed in BCGs. These filamentary sources display unique emission line properties that cannot be fully explained by photoionization from post-asymptotic giant branch stars, active galactic nuclei, or fast gas shocks alone. For the twelve filamentary sources that have Chandra data, their soft X-ray emission exceeds or is consistent with (within uncertainties) unresolved low-mass X-ray binary emission, indicating that filamentary systems generally host an appreciable hot gas reservoir. We suggest that cooling-related processes, e.g., self-irradiation associated with the cooling hot gas, may contribute to the powering of warm gas line emission, similar to cool-core clusters, though the detailed mechanisms and physical conditions may differ. As a case study, we investigate NGC 4374, a non-central ETG with extensive Chandra observations, and find that its warm filaments are over-pressured compared to the hot filaments - opposite to what is observed in cool-core clusters.