ALMA CO Observations of Gamma-Ray Supernova Remnant N132D in the Large Magellanic Cloud: Possible Evidence for Shocked Molecular Clouds Illuminated by Cosmic-Ray Protons

Abstract

N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out 12CO(J = 1–0, 3–2) observations toward the SNR using the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of 5″, corresponding to a spatial resolution of ∼1 pc at the distance of the LMC. Typical cloud sizes and masses are ∼2.0 pc and ∼100 M ⊙, respectively. High intensity ratios of CO J = 3–2/1–0 > 1.5 are seen toward the molecular clouds, indicating that shock heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud but also in front of it. Considering the absence of a thermal component associated with the forward shock toward one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of the remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be ∼0.5–3.8 × 1049 erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.

LMC N132D: a mature supernova remnant with a youthful gamma-ray spectrum

“…N132D is located near a molecular cloud complex, and it has recently been shown that the SNR is interacting with some of the molecular gas [19]. For the hadronic model, preferred by our analysis, the local gas density has to be high n p 10 cm −3 in order to explain the gamma-ray luminosity of N132D.…”

“…For the hadronic model, preferred by our analysis, the local gas density has to be high n p 10 cm −3 in order to explain the gamma-ray luminosity of N132D. The analysis of observations of HI and CO line emission by [19] shows that the density requirements are easily met. The HI gas cloud around the SNR has a typical density of n H ≈ 30 cm −3 , implying W p ≈ 1 × 10 50 erg.…”

“…1), but a few smaller, shocked clouds are present at the southern edge N132D and at the center of the remnant, with estimated densities ranging from 100 cm −3 to 900 cm −3 [19]. Sano et al [19] argue that the shocked cloudlets toward the center of N132D are also physically located within the SNR shell, and have survived shock passage due to their high density.…”

“…In [18] it was argued that the SNR has been evolving in a wind-blown cavity, and that the shock has relatively recently started interacting with the surrounding wind shell. The SNR, or its progenitor star, has carved out a hole on the northeastern edge of an extended HI cloud, in which several dense molecular cloud are present southwest of the SNR [19]. The presence of enhanced CO J = 3 → 2 over J = 1 → 0 emission at southern edge of the SNR as well as toward the center of the SNR shows that the SNR shock is interacting with molecular gas in some locations of the SNR [19].…”

The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma $-rays

“…Collaboration (2018) showed that within statistical errors, there is no significant difference between the TeV γ -rays and the X-ray synchrotron Fig. 15 CO results toward the five Magellanic SNRs N63A (Sano et al 2019b), N49 (Yamane et al 2018), RX J0046.5−7308 (Sano et al 2019c), N132D (Sano et al 2020a), N103B (Sano et al 2018), and the Magellanic superbubble 30 Doradus C (Yamane et al 2021). The CO observations of the LMC SNRs N63A, N49, N132D, N103B, and the LMC superbubble 30 Doradus C were obtained using ALMA, while the SMC SNR RX J0046.5−7308 was observed using the Atacama Submillimeter Telescope Experiment (ASTE).…”