Dark supernova remnant buried in the Galactic Center “Brick” G0.253+0.016 revealed by an expanding CO-line bubble

We performed a $^{12}$CO- and $^{13}$CO-line study of the “Brick” (G0.253+0.016) in the Galactic Center (GC) by analyzing archival data obtained with the Nobeyama 45 m telescope. We present kinematics and molecular gas distributions in the longitude–velocity diagram, and suggest that the Brick is located along the GC Arm I in the central molecular zone (CMZ), which yields a distance from the Sun of 8 kpc and a Galactocentric distance of 0.2 kpc. The major- and minor-axis diameters of the Brick are $D_x\times D_y=8.4 \times 4.1\, {\rm pc}$ at position angles of 40 and $130^\circ$, respectively, and the scale radius is $r_{\rm Bri}=\sqrt{D_x D_y}=2.96\, {\rm pc}$. The molecular mass inferred from the $^{12}$CO-line integrated intensity is $M_{\mathrm{Bri};X_\mathrm{CO}}\sim 5.1\times 10^4\, M_\odot$ for a conversion factor $X_{\rm CO;GC}=1.0\times 10^{20}$ H$_2$ cm $^{-2}$ [K km s$^{-1}]^{-1}$. On the other hand, the dynamical (virial) mass for the measured velocity dispersion of $\sigma _v=10.0\, {\rm km\ s^{-1}}$ is calculated to be $M_{\rm Bri;vir}\sim 6.8 \times 10^4\,M_\odot$, which yields a new conversion factor of $X_{\rm CO;Bri}=1.3\times 10^{20}$ H$_2$ cm $^{-2}$ [K km s$^{-1}]^{-1}$. The Brick’s center has a cavity surrounded by a spherical molecular bubble of radius $r_{\rm bub}=1.85$ pc and mass $\sim 1.7\times 10^4\,M_\odot$ expanding at $v_{\rm exp}\simeq 10 \ {\rm km\ s^{-1}}$ with a kinetic energy of $E_0\sim 1.7\times 10^{49}$ erg. If the bubble is approximated by an adiabatic spherical shock wave, the age is estimated to be $t\sim 2/5 r_{\rm bub}/v_{\rm exp}\sim 7.2\times 10^4$ yr. Neither non-thermal radio structures nor thermal radio emission indicative of the H ii region are found in the archival data from MeerKAT. We suggest that the molecular bubble is a dark supernova remnant buried in the Brick, which has therefore experienced past (${\sim}0.1$ Myr ago) massive star formation with a supernova explosion.