Resolving the sub-parsec circumnuclear density profiles of quiescent galaxies: Evidence for Bondi accretion flows in tidal disruption event hosts

Abstract

The sub-parsec circumnuclear density profiles of galaxies represent a key element in our understanding of the accretion history and fuel availability of supermassive black holes (SMBHs). Observations that directly resolve sub-parsec scales in galaxies require extremely high resolution and generally hot (bright) environments, making this impossible in all but the nearest active galaxies. Transient accretion events onto previously quiescent SMBHs, such as a tidal disruption event (TDE), offer a new avenue to understand SMBHs and their environments. Radio-bright outflows from TDEs directly probe the ambient density at $10^{-3}-1$ pc scales, allowing direct constraints on the circumnuclear density of TDE host galaxies (i.e., quiescent galaxies). Here we present, using radio observations of a sample of 11 TDE hosts, a new methodology for fitting observed TDE radio emission to constrain their sub-parsec circumnuclear density profiles. Our findings reveal that TDE host galaxies exhibit circumnuclear density profiles remarkably consistent with the expectations of a simple Bondi accretion flow ($n_e\propto R^{-3/2}$). Under the assumption of a Bondi profile, we present a new method to jointly fit the outflow mass and ambient densities, in order to constrain the Bondi accretion rate and temperature. For the TDE host galaxies in our sample, we constrain a sample average Bondi accretion rate Eddington fraction of $\log_{10}f_{\rm{Edd}} = -3.96^{+0.30}_{-0.38}$ (as well as individual fits to each host). This work provides a methodology by which radio observations of TDEs can provide powerful constraints on the sub-parsec density distribution of quiescent SMBHs -- well inside the Bondi sphere. This opens up a new observational avenue to constrain sub-parsec gas distributions in a broad range of galaxies.

Figures (11)

• Figure 1: The dependence of the electron number densities of Bondi-accretion profiles on the three parameters of interest (i) the background Bondi accretion rate in units of Eddington (top), (ii) the black hole mass in the TDE (middle) and (iii) the temperature of the gas far from the black hole (bottom). When not varied, the parameters take the values $M_\bullet=10^{6.5}M_\odot$, $f_{\rm Edd} = 10^{-5}$ and $T_\infty = 10^6$ K.
• Figure 2: The mass swept up by spherical winds launched into Bondi profiles, as a function of the three parameters of interest (i) the background Bondi accretion rate in units of the Eddington (top), (ii) the black hole mass in the TDE (middle) and (iii) the temperature of the gas far from the black hole (bottom). When not varied, the parameters take the values $M_\bullet=10^{6.5}M_\odot$, $f_{\rm Edd} = 10^{-5}$ and $T_\infty = 10^6$ K.
• Figure 3: Top: The ambient density with radius. Bottom: The mass swept up by the outflow with radius. Both profiles assume $\epsilon_e = 0.1$ and a spherical geometry, assumptions which will be improved upon later in this work. These profiles are shown only to highlight the broad scale of the parameters. In both panels the observed TDE population is plotted (white points with black error bars) over the theoretical density profile for Bondi accretion with $T_\infty = 10^6$ K, $\log_{10}M_\bullet/M_\odot=6.5$. The colour shading indicates different $f_{\rm{Edd}}$, the fraction of the Eddington accretion rate of the Bondi accretion rate.
• Figure 4: The different fits for $n_e(R)$ under different geometry and equipartition fraction assumptions. We compare conical geometry (blue), spherical geometry (pink), and reduced equipartition fraction of $\epsilon_e=5\times10^{-4}$ (purple). Evidently, changes in geometry assumptions do not affect the measured density profile, but changes in the relative equipartition fraction may. This motivates developing a framework where $\epsilon_e$ is constrained in parallel with the density profile.
• Figure 5: Top row: The individual joint mass and density TDE fits for free $k$, $A$, and $\epsilon_e$ plotted against black hole mass, where $\log_{10} n_e = k \log_{10}R/R_0 + A$ and $R_0=10^{16}$ cm. Bottom row: The individual TDE Bondi profile fits for $f_{\rm{Edd}}$, $T_{\inf}$, and $\epsilon_e$ plotted against black hole mass. We find no apparent correlation between black hole mass and either of the fit parameters, although there are large uncertainties for many of the individual TDE fits due to low numbers of epochs of observations.