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Massive galaxy overdensities at the peak epoch of cosmic star formationprovide ideal testbeds for the formation theories of galaxies and large-scalestructure. We report the confirmation of two massive galaxy overdensities at$z=2.24$, BOSS1244 and BOSS1542, selected from the MAMMOTH project usingLy$\alpha$ absorption from the intergalactic medium over the scales of 15$-$30$h^{-1}$ Mpc imprinted on the quasar spectra. We use H$\alpha$ emitters (HAEs)as the density tracer and identify them using deep narrowband $H_2S1$ andbroadband $K_{\rm s}$ imaging data obtained with CFHT/WIRCam. In total, 244 and223 line emitters are detected in these two fields, and $196\pm 2$ and $175\pm2$ are expected to be HAEs with an H$\alpha$ flux of $> 2.5\times 10^{-17}$ ergs$^{-1}$ cm$^{-2}$ (corresponding to an SFR of $>$5 M$_\odot$ yr$^{-1}$). Thedetection rate of HAE candidates suggests an overdensity factor of $\delta_{\rmgal}=5.6\pm0.3$ and $4.9\pm0.3$ over the volume of $54\times32\times32$cMpc$^3$. The overdensity factor increases $2-3$ times when focusing on thehigh-density regions of scales $10-15$ cMpc. Interestingly, the HAE densitymaps reveal that BOSS1244 contains a dominant structure, while BOSS1542manifests as a giant filamentary structure. We measure the H$\alpha$ luminosityfunctions (HLF), finding that BOSS1244's HLF is nearly identical to that of thegeneral field at the same epoch, while BOSS1542 shows an excess of HAEs withhigh H$\alpha$ luminosity, indicating the presence of enhanced star formationor AGN activity. We conclude that the two massive MAMMOTH overdensities areundergoing a rapid galaxy mass assembly.

MAMMOTH: Confirmation of Two Massive Galaxy Overdensities at $z=2.24$ with H$\alpha$ Emitters