Wind-fed Supermassive Black Hole Accretion in the Ultracompact Dwarf Galaxy M60-UCD1

TL;DR

This study tests whether stellar winds from an old population can feed a central SMBH in a gas-poor ultracompact dwarf galaxy. Using Schwarzschild dynamical modeling to define the SMBH potential and 3D hydrodynamic PLUTO simulations of wind injection from ~$1500$ TP-AGB stars, the authors reproduce a three-component gas structure (cold disk, warm halo, inner hot corona) that yields an accretion rate of $\dot{M}_{acc}\approx1.6\times10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$ and an X-ray luminosity of $L_X\approx7\times10^{37}$ erg s$^{-1}$, consistent with M60-UCD1 observations. Including ram-pressure from the Virgo ICM or M60's ISM reduces accretion and $L_X$ by about a factor of two, while the disk mass stabilizes near $10^3\,M_\odot$ (varying with environment) and warps under strong ISM pressure. The results support the wind-fed SMBH scenario as the origin of the X-ray counterpart in M60-UCD1 and offer broader implications for SMBH feeding and X-ray emission in other UCDs, though distinguishing this from LMXB contributions will require deeper multiwavelength data. Overall, the work demonstrates that wind-fed accretion can sustain a weakly accreting SMBH in UCDs, providing a clean laboratory to study feeding mechanisms in compact stellar systems.

Abstract

Ultracompact dwarf galaxies (UCDs) are thought to be remnants of stripped galactic nuclei, among which a handful are known to host a central supermassive black hole (SMBH). As in stripped nuclear star clusters, the SMBHs in UCDs may be fed by stellar winds from old stellar populations, in the absence of substantial gas reservoirs and galactic inflows. In this work, we investigate such a wind-fed accretion scenario for M60-UCD1, which harbors a confirmed $2\times10^7~M_\odot$ SMBH and exhibits X-ray emission suggestive of SMBH accretion signature. Using three-dimensional hydrodynamical simulations, we simulate the SMBH accreting stellar winds from approximately 1500 asymptotic giant branch stars, and explore the role of ram pressure from the ambient interstellar or intracluster medium. After 5 Myr, the majority of the stellar winds form a cold gas disk ($\sim1000~M_\odot$) within $\sim10~\rm pc$ as well as the SMBH's gravitational sphere of influence. Within the inner $10^4~r_{\rm g}$, this disk transitions into a hot ($\sim10^7-10^9~\rm K$), geometrically thick corona that dominates the X-ray emission. The SMBH achieves an accretion rate of $\sim10^{-5}~M_\odot~\rm yr^{-1}$, yielding an X-ray luminosity of $\sim7\times10^{37}~\rm erg~s^{-1}$, well consistent with observations. Including ram pressure stripping reduces both the accretion rate and luminosity by about a factor of two. Our results suggest that the X-ray counterpart of M60-UCD1 originates from a weakly accreting SMBH fed by stellar winds, with broader insights into the feeding mechanisms of central massive black holes and the origins of X-ray sources in other UCDs.

Figures (10)

• Figure 1: Enclosed mass profile of the dynamical model of M60-UCD1. The black solid line denotes the total mass while the red dashed line denotes the stellar component. The grey dashed line indicates the sphere of influence of the SMBH with $r_{\rm infl}=14.1~\rm pc$.
• Figure 2: Radius percentiles of the sampled AGB stars versus time in the simulation. For comparison, the resolutions of different runs (Section \ref{['sec:grid']}) are indicated by the dashed horizontal lines, with "Level1" representing the coarsest run and "Level4" representing the finest run. The discontinuity at 3.9 Myr is due to the random sampling of AGB stars.
• Figure 3: Projected gas density and temperature of the Fiducial simulation. The top and bottom two rows represent the face-on and edge-on view, respectively. From left to right are for snapshots at 0.5, 1.0, 2.5, and 5.0 Myr. The dashed black circles mark the gravitational sphere of influence of the SMBH. Each panel has a length of 50 pc. The black solid lines indicate a spatial scale of 24 pc and an angular scale of 0.3 arcsec at the distance of M60-UCD1 2009ApJ...694..556B, corresponding to the observed half-light radius of M60-UCD1 2013ApJ...775L...6S. The gas disk has a clockwise rotation in the face-on view.
• Figure 4: A zoom-in view of the Fiducial simulation at 5.3 Myr, the last snapshot of the finest run. The left half shows the azimuthally-averaged gas density with superposed streamlines indicating the fluid velocity, and the right half shows the azimuthally-averaged gas temperature.
• Figure 5: Temperature versus density distribution of the Fiducial simulation at 5.3 Myr. The color bar represents the mass occupation of the gas. Three dominant components (hot corona, cold disk, and warm halo) are indicated by the arrows.