Accurate Determination of Chemical Abundances near a Supermassive Black Hole

Abstract

The metal abundances in galactic nuclei carry key information on the history of star formation and mass transfer in central regions of galaxies. X-ray fluorescence analysis is a unique tool to reliably measure the abundances of various elements via simple physics. Here we present a new observation of the active nucleus in the Circinus Galaxy with the XRISM satellite at unprecedented X-ray energy resolution. The fluorescent iron-K$α$ line profile modified by Compton scattering indicates that the material responsible for its emission is cold, metal-rich, and is located $\gtrsim$0.024 parsecs (pc) from the supermassive black hole, consistent with the dusty torus region. The abundance pattern derived from comparing fluorescent line intensities of different metals shows sub-solar ratios of argon- and calcium-to-iron, and a super-solar ratio of nickel-to-iron. This abundance pattern is best produced by a combination in number fraction of $92^{+2}_{-4}$\% core-collapse supernovae from progenitor stars less massive than $20^{+3}_{-2} M_\odot$ and $8^{+4}_{-2}$\% type-Ia SNe. This suggests that gas feeding the super-massive black hole was enriched by recent core-collapse supernovae. Our findings imply that in metal-rich environments stars more massive than about 20 $M_\odot$ directly collapse into black holes or make faint SNe without ejecting heavy metals into the space.

Figures (15)

• Figure 1: XRISM/Resolve spectrum of the Circinus galaxy in the 2.8--10 keV band. It is folded by the instrumental response in units of $E F_E$, where $F_E$ is the energy flux at energy $E$ (i.e., corrected for the effective area). The spectrum is binned according to the "optimal binning" techniqueKaastra16, which is used for the spectral analysis throughout the paper. The non X-ray background component is subtracted. Major fluorescence lines from cold matter are labelled. The error bars denote the 1$\sigma$ confidence limits in photon statistics.
• Figure 1: Contribution of the non X-ray background (NXB) in the Circinus spectrum. The red line denotes the modelled NXB spectrum. The black data points are the observed spectrum of Circinus (including the NXB) with error bars denoting the 1$\sigma$ confidence limits in photon statistics.
• Figure 1: The XMM-Newton/MOS1 image in the 0.2--12 keV band around the Circinus centre. The small cyan circle and three magenta circles are the source and background extraction regions for the "XMM-CGX2" spectrum. The large green circle corresponds to the source region for the "XMM-all" (pn) spectrum.
• Figure 2: Enlarged plots of the XRISM/Resolve spectrum of the Circinus galaxy around the major fluorescence lines from neutral matter, overlaid with the best-fit model. (a) Ar K$\alpha$. (b) Ca K$\alpha$. (c) Cr K$\alpha$. (d) Mn K$\alpha$. (e) Fe K$\alpha$. (f) Fe K$\beta$. (g) Ni K$\alpha$. They are folded by the instrumental response in units of $E F_E$, where $F_E$ is the energy flux at energy $E$. The observed data are plotted in black with error bars denoting the 1$\sigma$ confidence limits in photon statistics. The blue, cyan, green, and red lines represent the emission lines from cold matter, the reflection continuum from cold matter, emission from ionized matter together with the Thomson-scattered component, and the total, respectively, based on our best-fitting model (Methods Section \ref{['sec:specmodel']}). For visualization, the bin size is set at either 2 eV (Ar K$\alpha$, Cr K$\alpha$, and Mn K$\alpha$), 1 eV (Ca K$\alpha$, Fe K$\beta$, and Ni K$\alpha$), or 0.5 eV (Fe K$\alpha$) in these plots.
• Figure 2: Comparison of the XRISM/Resolve spectrum and Chandra/HETG first order spectrum around the iron K$\alpha$ line of Circinus. The former (this work) is binned with 0.5 eV. The latter is adopted from Uematsu21 and is binned with 9 eV. The error bars denote the 1$\sigma$ confidence limits in photon statistics.