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Observations indicate that nearly all galaxies contain supermassive blackholes (SMBHs) at their centers. When galaxies merge, their component blackholes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves(GWs) that can be detected by pulsar timing arrays (PTAs). We have searched therecently-released North American Nanohertz Observatory for Gravitational Waves(NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits.As we did not find strong evidence for GWs in our data, we placed 95\% upperlimits on the strength of GWs from such sources as a function of GW frequencyand sky location. We placed a sky-averaged upper limit on the GW strain of $h_0< 7.3(3) \times 10^{-15}$ at $f_\mathrm{gw}= 8$ nHz. We also developed atechnique to determine the significance of a particular signal in each pulsarusing ``dropout' parameters as a way of identifying spurious signals inmeasurements from individual pulsars. We used our upper limits on the GW strainto place lower limits on the distances to individual SMBHBs. At themost-sensitive sky location, we ruled out SMBHBs emitting GWs with$f_\mathrm{gw}= 8$ nHz within 120 Mpc for $\mathcal{M} = 10^9 \, M_\odot$, andwithin 5.5 Gpc for $\mathcal{M} = 10^{10} \, M_\odot$. We also determined thatthere are no SMBHBs with $\mathcal{M} > 1.6 \times 10^9 \, M_\odot$ emittingGWs in the Virgo Cluster. Finally, we estimated the number of potentiallydetectable sources given our current strain upper limits based on galaxies inTwo Micron All-Sky Survey (2MASS) and merger rates from the Illustriscosmological simulation project. Only 34 out of 75,000 realizations of thelocal Universe contained a detectable source, from which we concluded it wasunsurprising that we did not detect any individual sources given our currentsensitivity to GWs.

The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries