The Rise of Ionized Gas Filaments in Early-Type Galaxies

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.

Figures (13)

• Figure 1: Distribution of spectral coverage and median physical spatial resolution of our MUSE sample (red circle) compared to the ATLAS$^{\mathrm{3D}}$, CALIFA, SAMI, and MaNGA IFU surveys (black squares). The sub-kpc resolution and broad optical wavelength coverage of our data allow for a detailed view of the warm ionized phase of the ISMs of our sample galaxies.
• Figure 2: Top: Frequency histogram showing the full stellar mass distributions of early (orange dashed) and late (purple dotted)-type galaxies within the 50MGC catalog. The solid black histogram shows the stellar mass distribution of our subsample described in Sec. \ref{['sec:sample']}. Bottom: Total count histogram for the sources within our sample with H$\alpha$ emission detected in their MUSE spectra. The red solid (blue dotted) histogram depicts sources with detected H$\alpha$ in the form of filaments (rotating disks). The total number of galaxies belonging to each group is provided in the legend of both panels.
• Figure 3: Three example ETGs with rotating H$\alpha$ disks. Each row corresponds to one of the three general subclasses of rotating disks we find in our sample: symmetric, volume filling disks (top), smaller nuclear disk surrounded by an outer ring (middle), and asymmetric disks with features likely indicating interaction and tidal effects (bottom). The name (stellar mass) of the galaxy is given along the side (top) of the leftmost panel. The first, second, third, and fourth columns correspond to the optical continuum, H$\alpha$ flux, warm gas LOS velocity (km s$^{-1}$), and warm gas velocity dispersion (km s$^{-1}$), respectively. The physical scale is given in the bottom right of each panel. The star denotes the catalog's published RA and Dec values.
• Figure 4: All 27 ETGs with filamentary H$\alpha$ morphologies. Each row of four panels corresponds to an individual source. The name (stellar mass) of the galaxy is given along the side (top) of the leftmost panel. The first, second, third, and fourth columns correspond to the optical continuum, H$\alpha$ flux, warm gas LOS velocity (km s$^{-1}$), and warm gas velocity dispersion (km s$^{-1}$), respectively. The physical scale is given in the bottom right of each panel.
• Figure 5: 0.5-2.0 keV luminosity normalized by the stellar mass (vertical axis) vs. stellar mass (horizontal axis). Sources with filamentary H$\alpha$ nebulae are shown as red X's while rotating H$\alpha$ disks are blue circles. Sources without H$\alpha$ emission are depicted as gray diamonds. For very faint sources, Chandra upper-limits are shown as filled inverted triangles. Open inverted triangles represent upper-limits on $L_X$ provided in eRASS1 for filamentary sources not observed with Chandra. The black square represents NGC 4477 - the smallest known ETG with X-ray cavities and cooling signatures li2018x. The solid blue line is a rough expectation of the emission from unresolved point sources likely influencing the eROSITA upper-limits. The dashed blue line denotes the estimated 0.5-2.0 keV emission from unresolved LMXBs as given in hou2021x. We expect the ETGs sitting above this line to host diffuse, hot gas, and are thus systems where cooling is a feasible origin to their warm gas.