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

Publications of the Astronomical Society of Japan, 2024, 76(4), 773–786 https://doi.org/10.1093/pasj/psae047 Advance access publication date: 2024 June 13 Dark supernova remnant buried in the Galactic Center “Brick” G0.253+0.016 revealed by an expanding CO-line bubble Yoshiaki SOFUE ∗ Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan ∗ E-mail: 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 Dx × Dy = 8.4 × 4.1 pc at position angles of 40 and 130 , respectively, and the scale radius is rBri = Dx Dy = 2.96 pc. The molecular mass inferred from the 12 CO-line integrated intensity is MBri;XCO ∼ 5.1 × 104 M for a conversion factor XCO;GC = 1.0 × 1020 H2 cm −2 [K km s−1 ]−1 . On the other hand, the dynamical (virial) mass for the measured velocity dispersion of σv = 10.0 km s−1 is calculated to be MBri;vir ∼ 6.8 × 104 M , which yields a new conversion factor of XCO;Bri = 1.3 × 1020 H2 cm −2 [K km s−1 ]−1 . The Brick’s center has a cavity surrounded by a spherical molecular bubble of radius rbub = 1.85 pc and mass ∼ 1.7 × 104 M expanding at vexp  10 km s−1 with a kinetic energy of E0 ∼ 1.7 × 1049 erg. If the bubble is approximated by an adiabatic spherical shock wave, the age is estimated to be t ∼ 2/5rbub /vexp ∼ 7.2 × 104 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 (∼0.1 Myr ago) massive star formation with a supernova explosion. Keywords: Galaxy: Center — ISM: bubbles — ISM: clouds — ISM: molecules — ISM: supernova remnant 1 Introduction The “Brick” (G0.253+0.016, M0.25+0.01) near the Galactic Center (GC) is a dense dust cloud detected in the submillimeter emission (Guesten & Henkel 1983; Lis et al. 1994; Johnston et al. 2014). Submillimeter photometry showed a molecular gas column density of ∼ 1023 H2 cm−2 and a mass of ∼ 1.5 × 105 M (Longmore et al. 2012). It exhibits severe extinction at infrared wavelengths, silhouetted against the central stellar disk and bulge (Henshaw et al. 2019; Ginsburg et al. 2023). The cloud is detected by a molecular line with radial velocity vLSR ∼ 30 km s−1 (Lis & Carlstrom 1994; Johnston et al. 2014). Interferometric observations with ALMA (Atacama Large Millimeter/Submillimeter Array) revealed a bubble/cavity structure near the center of the Brick, consisting of a semi-loop with numerous arcs concentric with the center of the bubble (Higuchi et al. 2014; Henshaw et al. 2022). Filament and turbulent structures associated with magnetic fields are also highlighted (Federrath et al. 2016). There have been two models to explain the bubble/arc structure: One idea is that it is a cavity produced by a collision of a compact molecular cloud at high speed from the highlatitude direction (Higuchi et al. 2014). The other attributes the bubble to stellar feedback such as wind from young stars (Johnston et al. 2014; Henshaw et al. 2019, 2022). While the signature of star formation has been found by detection of maser lines and outflow from young stars (Lis et al. 1994; Longmore et al. 2012; Walker et al. 2021), no clear evidence has yet been reported of H ii regions indicative of massive star formation (Anderson et al. 2014; Wenger et al. 2023). Infrared photometry of the bulge stars and extinction study indicated a distance of ∼7 kpc (Longmore et al. 2012; Zoccali et al. 2021), locating the cloud about 1 kpc in front of the GC. On the other hand, a color–magnitude study of bulge stars (Nogueras-Lara et al. 2021) and an extinction study of GC stars with known proper motions (Martínez-Arranz et al. 2022) suggested that the Brick is inside the central molecular zone (CMZ). In this paper we study the kinematics and energetics of the Brick by analyzing the CO-line data from the GC survey with the Nobeyama 45 m millimeter-wave telescope (Tokuyama et al. 2019). We argue that the Brick is more likely to be associated with the CMZ located at a distance of ∼8 kpc. We derive fundamental physical parameters such as the size, mass, and kinetic and gravitational energies of the Brick based on the Nobeyama 12 CO- and 13 CO-line data, which, because of the single-dish aperture, do not suffer from the missing-mass problem in interferometric measurments. So, the present study will be complementary to the current interferometer works in the sense that the present analysis provides more information about physics that has not been explored in the recent studies, while it does not add much information about the detailed morphology in the cloud. We then focus on the expanding molecular bubble centered on (l, b) = (0.◦ 253, +0.◦ 016) (G0.253+0.016), and present a new model attributing it to a dark supernova remnant (SNR) Received: 2024 January 11; Accepted: 2024 May 5 © The Author(s) 2024. Published by Oxford University Press on behalf of the Astronomical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract 774 Publications of the Astronomical Society of Japan (2024), Vol. 76, No. 4 buried in the dense molecular Brick (Shull 1980; Wheeler et al. 1980; Lucas et al. 2020; Sofue 2020, 2021) in order to explain the kinetic energy an order of magnitude greater than that estimated from the interferometer observations. 2 The Brick in the CO line 2.1 CO-line data 2.2 Intensity distribution: moment-0 map Figure 1 shows integrated intensity (moment-0) maps in the 13 CO-line emission between vLSR = 0 and 60 km s−1 for the ◦ 0. 5 × 0.◦ 4 (top) and 0.◦ 16 × 0.◦ 16 (middle) regions around the Brick. The middle panel shows contours overlaid on the Spitzer (GLIMPSE) 8 μm intensity map in grayscale (Churchwell et al. 2009). The map shows the close correlation of the CO cloud in emission with the dust Brick in silhouette against the stellar background of the central bulge and GC stellar disk. The bottom panel shows perpendicular crosssections of the Brick along the dashed line in the middle panel, showing plateaued intensity profiles both in the infrared and 13 CO indicative of a density cavity inside the Brick, as will be discussed later based on channel maps and position–velocity diagrams. Figure 2 shows integrated intensi (...truncated)