Methanol masers in NGC 253 with ALCHEMI
Methanol masers in NGC 253 with ALCHEMI
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NGC 253 中使用 ALCHEMI 的甲醇微波激射器
DOI：
10.1051/0004-6361/202243384
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发表时间：
2022
影响因子：
6.5
通讯作者：
Menten K.
中科院分区：
文献类型：
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作者：
Humire P. K.;Henkel C.;Hernandez-Gomez A.;Martin S.;Mangum J.;Harada N.;Muller S.;Sakamoto K.;Tanaka K.;Yoshimura Y.;Nakanishi K.;Muehle S.;Herrero-Illana R.;Meier D. S.;Caux E.;Aladro R.;Mauersberger R.;Viti S.;Colzi L.;Rivilla V. M.;Gorski M.;Menten K.
I类（碰撞泵浦）和II类（辐射泵浦）的甲醇微波激射器已经在我们银河系的各种天体物理环境中进行了详细的研究，例如激波和恒星形成区域，它们有助于分析致密星际介质的特性。然而，外部星系中甲醇脉泽的研究仍处于起步阶段。我们的主要目标是在附近的星暴星系NGC 253的中心分子区（CMZ；内部500pc）寻找甲醇脉泽。方法在84 - 373 GHz （λ= 3.6-0.8 mm）的高角（~ 27 pc）和光谱（~ 8-9 km s−1）分辨率范围内，利用ALCHEMI （ALMA综合高分辨率河外分子清查）探测了NGC 253 CMZ的不同区域。为了寻找候选的甲醇微波激射器，我们采用了旋转图法和一套辐射传递模型。结果我们首次在NGC 253中探测到84 GHz以上的脉泽，覆盖了J−1→（J−1）0−Eline系列（84、132、229和278 GHz）和J−1→（J−1）1−系列（95、146和198 GHz）的相当一部分。这证实了84 GHz的I类脉泽线的存在，这已经被报道过，但现在在多个位置被检测到。对于J−1→（J−1）0−Eline系列，我们观察到在中央的恒星形成盘中缺少I类脉泽候选者。结论J−1→（J−1）0−Eline系列微波激射的物理条件可能是弱激波和云-云碰撞，正如激波示踪剂（SiO和HNCO）在位于CMZ外围的双对称激波区域所提示的那样。另一方面，由于高恒星形成率而产生的光解区域的存在将需要解释在非常中心的区域缺乏I类脉泽。
ContextMethanol masers of Class I (collisionally pumped) and Class II (radiatively pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are they are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy.AimsOur main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253.MethodsCovering a frequency range between 84 and 373 GHz (λ= 3.6–0.8 mm) at high angular ( ∼ 27 pc) and spectral (∼8–9 km s−1) resolution with ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory), we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models.ResultsWe detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of theJ−1→ (J− 1)0−Eline series (at 84, 132, 229, and 278 GHz) and theJ0→ (J− 1)1−Aseries (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, which was already reported, but now being detected in more than one location. For theJ−1→ (J− 1)0−Eline series, we observe a lack of Class I maser candidates in the central star-forming disk.ConclusionsThe physical conditions for maser excitation in theJ−1→ (J− 1)0−Eline series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.