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 GCs have stellar masses M ⋆  ∼ (1.1–2.7) × 10 6 M ⊙ , with a mean . 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 and mass fraction , 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 X  < (3.3–10) × 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 Chandra observations. Finally, a simulated observation with a next-generation VLA demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet they elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs.