2024A&A...689A.121V

Context. The expanding molecular ring (EMR) manifests itself as a parallelogram in the position-velocity diagram of spectral line emission from the Central Molecular Zone (CMZ) surrounding the Galactic centre (GC). It is a high velocity (| V_LSR | > 100 km s^–1) extended molecular gas structure. The formation of the EMR is believed to be associated with the bar driven inflow onto the nuclear region of the Galaxy. The physical and chemical properties, as well as the evolution of the EMR and its connection to other GC clouds and the CMZ as a whole, are not yet fully comprehended.
Aims. Using multiwavelength data, we investigate the gas kinematics, star formation activity, and the presence of shocked gas in a 200 pc long high velocity gas stream (V_LSR ~+150 km s^–1) with a double helix morphology named the helix stream, that is located 15-55 pc above the CMZ (l ∼ 0°-1.5°; b - 0.05°-0.4°) and is kinematically associated with the EMR/parallelogram.
Methods. To study the kinematics of the helix stream, we used ¹³CO (J = 2-1) data from the SEDIGISM survey and ¹²CO (J = 1-0) archival data from the Nobeyama telescope. Additional multiwavelength archival data from infrared to radio wavelengths were used to investigate the star formation activity. We carried out molecular line observations using the IRAM 30m, Yebes 40m, and APEX 12m telescopes. The detection of four rotational transitions of the SiO molecule (J = 1-0, 2-1, 5-4, 7-6) indicate the presence of shocks. We derived the SiO column densities and abundances in different regions of the helix stream using the rotational diagram method. We also performed non-local thermodynamic equilibrium (non-LTE) modelling of the SiO emission to analyse the excitation conditions of the shocked gas.
Results. The presence of clumps with submillimetre continuum emission from dust and a candidate H II region signify the ongoing star formation activity within the helix stream. The cloud is massive (2.5 × 10⁶M_⊙) and highly turbulent (ΔV_mean = 18 km s^–1). We find evidence of cloud-cloud collisions towards the eastern edge (l ∼ 1.3°), suggesting a dynamic interaction with the CMZ. An expanding shell is detected within the cloud with radius of 6.7 pc and an expansion velocity of 35 km s^–1. The shell might be powered by several supernovae or a single hypernova. The relative abundance of SiO within the helix stream with respect to H₂ implies extensive shock processes occurring on large scales (X(SiO) ∼10^–9). The helical or cork-screw velocity structure observed within the individual strands of the helix stream indicates twisting and turning motions occurring within the cloud.
Conclusions. We propose that the helix stream is the continuation of the near side bar lane, that is overshooting after “brushing” the CMZ and interacting with it at the location of the G1.3 cloud. This interpretation finds support both from numerical simulations and prior observational studies of the CMZ. Our findings carry profound implications for understanding star formation in extreme conditions and they elucidate the intricate properties of gas and dust associated with nuclear inflows in barred spiral galaxies.