tokens: mass. The upper and lower limits of the SNR luminosities in Fig. 6 are found in Section 5.2 and are the same as those plotted on Fig. 4 . The solid lines of Fig. 6 show the range of luminosities that can be detected by Chandra -ACIS-S with 20 ks exposures (based on flux limits of 3 × 10 −15 and 9 × 10 −15 erg cm −2 s −1 for a 10 ks exposure). The lower horizontal axes are given in terms of the maximum resolvable distance of the SOI. This is the distance at which the (diameter of the) SOI of the SMBH is just within the core of the point-spread function of Chandra . For example, the SOI of a 10 8 M ⊙ SMBH is resolvable at any distance below ∼10 Mpc and, at 10 Mpc, the lower limit of detectable soft X-ray emission is given by the red line. At any distance less than 10 Mpc, this detection threshold drops and so fainter emission from SNRs would be detectable. Figure 6. Observable limits of the combined X-ray luminosities from young SNRs as a function of SMBH mass (upper axes) or maximum resolved distance (lower axes). The maximum resolved distance is that at which the SOI of the SMBH is within the core of the PSF of Chandra . The left-hand panel shows luminosities in the soft X-ray (0.5–2 keV), while the right-hand panel shows hard luminosities (2–8 keV). SNR luminosity limits are the dashed black lines (shown also in Fig. 5 ). Flux-limited detection thresholds from Chandra exposure times of 20 ks for galaxies at the distance limit are given as solid lines. The shaded region in the left-hand panel highlights the observable range of luminosities in the soft band. From Fig. 6 , it is evident that, even at the maximum resolved distance, the upper limit of SNR emission is detectable as it is above the 20 ks threshold in the soft band through most of the M • range (although the low end of possible X-ray luminosities is possibly not within the detection threshold of Chandra ). In the hard band, the Chandra 20 ks threshold follows very closely the high limit of predicted SNR emission. This suggests that, for galaxies at the maximum resolved distance, the emission would not be detectable in the hard band. Again, however, if a galaxy is closer than this maximum distance, the threshold marked by the blue solid line will drop; therefore, hard X-ray luminosities are still potentially detectable for more nearby galaxies. To compare this with the number of SMBHs known at these distances, in Fig. 7 , we plot the distances and masses of well-established SMBHs constructed from Tables 2 and 3 in the review of Kormendy Ho ( 2013 ) and the references therein. We show in the grey region the cases where the SOI of the SMBH is not resolved by Chandra . As for the upper axes in Fig. 6 , the limit between the grey and white area is determined by the distance at which the SOI of the SMBH equals in size the core of the point-spread function of Chandra . Figure 7. Distances and masses (with errors) of known SMBHs over the same mass range considered in this paper. Data were
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