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

Abstract

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 $\overline{M_{\mathrm{IMBH}}}<1.7\times 10^5{M}_{\odot }$ and mass fraction $\overline{M_{\mathrm{IMBH}}}/\overline{M_{\star }}<9.5\%$, 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 $L_{\mathrm{X}}<(3.3-10)\times 10^{38}$ 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.

8 pages, 4 figures, accepted 2020 July 22 by ApJ

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