Discovery of a molecular cloud possibly associated with the youngest Galactic SNR G1.9+0.3

TL;DR

This study identifies a molecular cloud likely associated with the youngest Galactic supernova remnant G1.9+0.3, revealing how shock interaction with dense interstellar matter causes anisotropic expansion and provides insights into the remnant's environment.

Contribution

First direct evidence linking a molecular cloud to G1.9+0.3, demonstrating shock deceleration due to dense interstellar medium interaction.

Findings

The -1 km s-1 cloud is likely associated with G1.9+0.3.

The cloud shows a cavity structure coinciding with the SNR.

Interaction with the cloud causes anisotropic expansion of the SNR.

Abstract

The youngest known Galactic supernova remnant (SNR) G1.9+0.3 has high-velocity supernova shock beyond 10000 km s-1, and it is considered to be one of the major candidates of a PeVatron. Despite these outstanding properties, the surrounding interstellar matter of this object is poorly understood. We investigated the interstellar gas toward G1.9+0.3 using the 12CO(J=3-2) data with the angular resolution of 15" obtained by the CHIMPS2 survey by the James Clerk Maxwell Telescope, and discovered three individual clouds at -1, 7, and 45 km s-1. From its morphological and velocity structures, the -1 km s-1 cloud, having the largest velocity width >20 km s-1 and located at the distance of the Galactic Center, is possibly associated with the SNR. The associated cloud shows a cavity structure both in space and velocity and coincides well with the SNR. We found that the associated cloud has higher column densities toward three bright, radio synchrotron-emitted rims where the radial expansion velocity of the supernova shock is decelerated, and the cloud is faint in the other parts of the SNR. This is the first direct evidence indicating that the highly anisotropic expansion of G1.9+0.3 observed by previous studies results from the deceleration by the interaction between the supernova shock and surrounding dense interstellar medium.