A Stacked Search for Intermediate-Mass Black Holes in 337 Extragalactic Star Clusters

Abstract

Forbes et al. recently used the Hubble Space Telescope to localize hundreds of candidate star clusters in NGC 1023, an early-type galaxy at a distance of 11.1 Mpc. Old stars dominate the light of 92% of the clusters and intermediate-age stars dominate the light of the remaining 8%. Theory predicts that clusters with such ages can host intermediate-mass black holes (IMBHs) with masses M_BH \lesssim 10^5 M_sun. To investigate this prediction, we used 264 s of 5.5 GHz data from the Karl G. Jansky Very Large Array (VLA) to search for the radiative signatures of IMBH accretion from 337 candidate clusters in an image spanning 492 arcsec (26 kpc) with a resolution of 0.40 arcsec (22 pc). None of the individual clusters are detected, nor are weighted-mean image stacks of the 311 old clusters, the 26 intermediate-age clusters, and the 20 clusters with stellar masses M_star \gtrsim 7.5 x 10^5 M_sun. The clusters thus lack radio analogs of HLX-1, a strong IMBH candidate in a cluster in the early-type galaxy ESO 243-49. This suggests that HLX-1 is accreting gas related to its cluster's light-dominating young stars. Alternatively, the HLX-1 phenomenon could be so rare that no radio analog is expected in NGC 1023. Also, using a formalism heretofore applied to star clusters in the Milky Way, the radio-luminosity upper limit for the massive-cluster stack corresponds to a mean 3$\sigma$ IMBH mass of M_BH(massive) < 2.3 x 10^5 M_sun, suggesting mean black-hole mass fractions of M_BH(massive)/M_star < 0.05-0.29.

19 pages; 6 figures; accepted by AJ

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Cited by in corpus (10)

• The Assembly of the First Massive Black Holes
• Observational evidence for intermediate-mass black holes
• Intermediate-mass black holes in dwarf galaxies out to redshift $\sim$ 2.4 in the Chandra COSMOS Legacy Survey
• The MAVERIC Survey: Still No Evidence for Accreting Intermediate-mass Black Holes in Globular Clusters
• Intermediate Mass Black Hole Formation in compact Young Massive Star Clusters
• The Close Stellar Companions to Intermediate Mass Black Holes
• A Very Large Array Search for Intermediate-Mass Black Holes in Globular Clusters in M81
• The Lost Dwarfs of Centaurus A and the Formation of its Dark Globular Clusters
• Accessing Intermediate-Mass Black Holes in 728 Globular Star Clusters in NGC\,4472
• A Chandra Survey of Milky Way Globular Clusters. III. Searching for X-ray Signature of Intermediate-mass Black Holes