VLA Limits on Intermediate-Mass Black Holes in 19 Massive Globular Clusters

The NSF's Karl G.\ Jansky Very Large Array (VLA) was used at 3~cm to search for accretion signatures from intermediate-mass black holes (IMBHs) in 19 globular star clusters (GCs) in NGC\,3115, an early-type galaxy at a distance of 9.4 Mpc. The 19 have stellar masses $M_{\star} \sim (1.1 - 2.7) \times 10^6~M_\odot$, with a mean $\overline{M_{\star}} \sim 1.8 \times 10^6~M_\odot$. None were detected. An IMBH accretion model was applied to the individual GCs and their radio stack. The radio-stacked GCs have an IMBH mass $\overline{M_{\rm IMBH}} < 1.7 \times 10^5~M_\odot$ and mass fraction $\overline{M_{\rm IMBH}} / \overline{M_{\star}} < 9.5\%$, with each limit being uncertain by a factor of about 2.5. The latter limit contrasts with the extremes of some stripped nuclei, suggesting that the set of stacked GCs in NGC\,3115 is not a set of such nuclei. The radio luminosities of the individual GCs correspond to X-ray luminosities $L_{\rm X} < (3.3 - 10) \times 10^{38}$ erg~s$^{-1}$, with a factor of about 2.5 uncertainty. These limits predicted for putative IMBHs in the GCs are consistent with extant {\em Chandra} observations. Finally, a simulated observation with a next-generation VLA (ngVLA) demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs.

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