Accretion from Winds of Red Giant Branch Stars May Reveal the Supermassive Black Hole in Leo I

TL;DR

This paper suggests that winds from evolved stars near the SMBH in Leo I could produce detectable accretion signals, offering a new way to confirm and study the black hole in a galaxy with minimal gas and star formation.

Contribution

It introduces a model where stellar winds within the SMBH's Bondi radius drive accretion rates consistent with ADAF, predicting observable microwave and radio emissions for the first time in Leo I.

Findings

Predicted microwave spectrum peaks at 0.1-1 THz.

Estimated radio flux at 6 GHz is about 0.1 mJy.

Accretion rates are consistent with ADAF mode.

Abstract

A supermassive black hole (SMBH) of $\sim 3\times 10^6 \, \rm M_\odot$ was recently detected via dynamical measurements at the center of the dwarf galaxy Leo I. Standing $\sim 2$ orders of magnitude above standard scaling relations, this SMBH is hosted by a galaxy devoid of gas and with no significant star formation in the last $\sim 1$ Gyr. This detection can profoundly impact the formation models for black holes and their hosts. We propose that winds from a population of $\sim 100$ evolved stars within the Bondi radius of the SMBH produce a sizable accretion rate, with Eddington ratios between $9\times10^{-8}$ and $9\times10^{-7}$, depending on the value of the stellar mass loss. These rates are typical of SMBHs accreting in advection-dominated accretion flow (ADAF) mode. The predicted spectrum peaks in the microwaves at $\sim 0.1-1$ THz ($300-3000 \, \mathrm{\mu m}$) and exhibits significant variations at higher energies depending on the accretion rate. We predict a radio flux of $\sim 0.1$ mJy at $6$ GHz, mildly dependent on the accretion properties. Deep imaging with Chandra, VLA, and ALMA can confirm the presence of this SMBH and constrain its accretion flow.