Toward Astrometric Constraints on a Supermassive Black Hole Binary in the Early-Type Galaxy NGC\,4472

Abstract

The merger of two galaxies, each hosting a supermassive black hole (SMBH) of mass $10^6$\,M${}_{\odot }$ or more, could yield a bound SMBH binary. For the early-type galaxy NGC\,4472, we study how astrometry with a next-generation Very Large Array (ngVLA) could be used to monitor the reflex motion of the primary SMBH of mass $M_{\mathrm{pri}}$, as it is tugged on by the secondary SMBH of mass $M_{\sec }$. Casting the orbit of the putative SMBH binary in terms of its period $P$, semimajor axis $a_{\mathrm{bin}}$, and mass ratio $q=M_{\sec }/M_{\mathrm{pri}}\le 1$, we find the following: (1) Orbits with fiducial periods of $P=4$\,yr and 40\,yr could be spatially resolved and monitored. (2) For a 95\% accuracy of $2\mu$as per monitoring epoch, sub-parsec values of $a_{\mathrm{bin}}$ could be accessed over a range of mass ratios notionally encompassing major ($q>\frac{1}{4}$) and minor ($q<\frac{1}{4}$) galaxy mergers. (3) If no reflex motion is detected for $M_{\mathrm{pri}}$ after 1(10)\,yr of monitoring, a SMBH binary with period $P=4(40)$\,yr and mass ratio $q>0.01(0.003)$ could be excluded. This would suggest no present-day evidence for a past major merger like that recently simulated, where scouring by a $q\sim 1$ SMBH binary formed a stellar core with kinematic traits like those of NGC\,4472. (4) Astrometric monitoring could independently check the upper limits on $q$ from searches for continuous gravitational waves from NGC\,4472.

arXiv comment7 pages, 2 figures, accepted by ApJ; builds upon our ngVLA Memo. #90 (arXiv:2108.07341)

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