Interstellar anatomy of the TeV gamma-ray peak in the IC443 supernova remnant

Abstract

Context. Supernova remnants (SNRs) represent a major feedback source from stars in the interstellar medium of galaxies. During the latest stage of supernova explosions, shock waves produced by the initial blast modify the chemistry of gas and dust, inject kinetic energy into the surroundings, and may alter star formation characteristics. Simultaneously, γ-ray emission is generated by the interaction between the ambient medium and cosmic rays (CRs), including those accelerated in the early stages of the explosion. Aims. We study the stellar and interstellar contents of IC443, an evolved shell-type SNR at a distance of 1.9 kpc with an estimated age of 30 kyr. We aim to measure the mass of the gas and characterize the nature of infrared point sources within the extended G region, which corresponds to the peak of γ-ray emission detected by VERITAS and Fermi. Methods. We performed 10′ × 10′ mapped observations of 12CO, 13CO J = 1-0, J = 2-1, and J = 3-2 pure rotational lines, as well as C18O J = 1-0 and J = 2-1 obtained with the IRAM 30 m and APEX telescopes over the extent of the γ-ray peak to reveal the molecular structure of the region. We first compared our data with local thermodynamic equilibrium models. We estimated the optical depth of each line from the emission of the isotopologs 13CO and C18O. We used the population diagram and large velocity gradient assumption to measure the column density, mass, and kinetic temperature of the gas using 12CO and 13CO lines. We used complementary data (stars, gas, and dust at multiple wavelengths) and infrared point source catalogs to search for protostar candidates. Results. Our observations reveal four molecular structures: a shocked molecular clump associated with emission lines extending between -31 and 16 km s-1, a quiescent, dark cloudlet associated with a line width of ∼2 km s-1, a narrow ring-like structure associated with a line width of ∼1.5 km s-1, and a shocked knot. We measured a total mass of ∼230, ∼90, ∼210, and ∼4 M⊙, respectively, for the cloudlet, ring-like structure, shocked clump, and shocked knot. We measured a mass of ∼1100 M⊙ throughout the rest of the field of observations where an ambient cloud is detected. We found 144 protostar candidates in the region. Conclusions. Our results emphasize how the mass associated with the ring-like structure and the cloudlet cannot be overlooked when quantifying the interaction of CRs with the dense local medium. Additionally, the presence of numerous possible protostars in the region might represent a fresh source of CRs, which must also be taken into account in the interpretation of γ-ray observationsin this region.