Toward Astrometric Constraints on a Supermassive Black Hole Binary in
  the Early-Type Galaxy NGC\,4472
  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 $M_{\rm pri}$, as it is tugged on by the secondary SMBH of
mass $M_{\rm sec}$. Casting the orbit of the putative SMBH binary in terms of
its period $P$, semimajor axis $a_{\rm bin}$, and mass ratio $q = M_{\rm sec} /
M_{\rm pri} \le 1$, 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 $a_{\rm
bin}$ 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 $M_{\rm pri}$ 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.
