南方星暴星系的次秒差距尺度射电特性。II。NGC 4945星暴中的超新星遗迹、超新星速率和电离介质
The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies. II. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC 4945 Starburst
摘要：
用澳大利亚LBA对附近的星暴星系NGC 4945进行了宽视场VLBI观测，在两个时期内产生了2.3 GHz的图像，最大空间分辨率为0.3 pc。检测到15个源，其中13个对应于在较高频率（3 cm和12 mm）ATCA图像中识别的源。其中四个来源被分解成直径在1.1到2.1 pc之间的壳状结构。根据这些数据，我们模拟了NGC 4945中13个致密射电源的光谱，其中9个符合自由-自由吸收幂律，4个符合简单幂律谱。自由-自由不透明度最高的是向核，但在整个核区域变化显着，这意味着电离介质的整体结构是cluberous。在13个源中，有10个源的本征谱较陡，可能与超新星遗迹的同步辐射有关，其余源的本征谱较平坦，可能与热射电辐射有关。在距活动星系核假定位置约1 "处探测到一个具有喷流状形态的非热源。在95%置信水平下，基于1.9年观测中没有发现新的超新星源和超新星遗迹演化的简单模型，确定了银河系内部250 pc区域的II型超新星（SN）速率上限为15.3/年。根据超新星遗迹源的数量、大小和膨胀率的估计，II型超新星的SN速率> 0.1（v/1e4）/yr，其中v是超新星遗迹的径向膨胀速度，单位为km/s。直接从SN速率极限估算出的SFR为2.4（v/1e4）<SFR（M>= 5Msun）<370 Msun/yr，与从FIR和射电光度积分确定的速率具有相同的数量级。
英文摘要：
Wide-field VLBI observations of the nearby starburst galaxy NGC 4945, obtained with the Australian LBA, have produced 2.3 GHz images over two epochs with a maximum spatial resolution of 0.3 pc. 15 sources were detected, 13 of which correspond to sources identified in higher frequency (3 cm and 12 mm) ATCA images. Four of the sources are resolved into shell-like structures ranging between 1.1 to 2.1 pc in diameter. From these data the spectra of 13 compact radio sources in NGC 4945 were modelled; nine were found to be consistent with free-free absorbed power laws and four with a simple power law spectrum. The free-free opacity is highest toward the nucleus but varies significantly throughout the nuclear region, implying that the overall structure of the ionised medium is clumpy. Of the 13 sources, 10 have steep intrinsic spectra associated with synchrotron emission from supernova remnants, the remaining sources have flat intrinsic spectra which may be associated with thermal radio emission. A non-thermal source with a jet-like morphology is detected ~1" from the assumed location of the AGN. A type II supernova (SN) rate upper limit of 15.3/yr is determined for the inner 250 pc region of the galaxy at the 95% confidence level, based on the lack of detection of new sources in observations spanning 1.9 years and a simple model for the evolution of supernova remnants. A type II SN rate of >0.1(v/1e4)/yr is implied from estimates of supernova remnant source counts, sizes and expansion rates, where v is the radial expansion velocity of the supernova remnant in km/s. A SFR of 2.4(v/1e4)<SFR(M>=5Msun)<370 Msun/yr has been estimated directly from the SN rate limits and is of the same order of magnitude as rates determined from integrated FIR and radio luminosities.
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