用NGC6946中的33 GHz自由-自由辐射校准无消光恒星形成率诊断
Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946
摘要：
摘要：使用在Ka波段（26- 40 GHz）测量的附近星系NGC 6946中10个恒星形成区域的自由发射，包括其恒星爆发核，我们比较了一些通常被认为不受星际消光影响的SFR诊断：即，非热无线电（即，1.4GHz）、全红外（IR; 8- 1000 um）和暖尘（即，24 um）发射，以及Hα+24um和UV+ IR的混合（遮蔽+未遮蔽）指示器。假定33 GHz自由-自由发射提供当前SFR的最准确测量。在双核恒星形成区中，24 μ m、Hα +24 μ m和UV+IR的SFR刻度与33 GHz的自由-自由SFR符合得很好。然而，每个SFR校准依赖于某种形式的尘埃排放高估了核SFR的一个因素~2。这更有可能是由于非电离恒星的积累而导致的过度尘埃加热，而不是尘埃对电离光子的竞争。使用非热射电连续谱的SFR校准产生的值仅与核的自由-自由SFR一致，并且低估了来自核内恒星形成区域的SFR约2倍。这一结果可能是由于CR电子在星暴区域内衰变，与年轻的双核恒星形成区域相比，其逃逸可以忽略不计。最后，我们发现，估计使用的总33 GHz的发射的SFR同意以及与自由的SFR由于在这些频率存在的大的热分数，即使当本地扩散背景不被删除。因此，静止坐标系33 GHz观测可以作为一种可靠的方法来测量星系的SFR在越来越高的红移，而不需要辅助的无线电数据来解释的非热发射。
英文摘要：
Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10 star-forming regions in the nearby galaxy NGC6946, including its starbursting nucleus, we compare a number of SFR diagnostics that are typically considered to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e., 1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission, along with the hybrid (obscured + unobscured) indicators of Hα+24um and UV+IR. The 33GHz free-free emission is assumed to provide the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24um, Hα+24um and UV+IR SFR calibrations are in good agreement with the 33GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ~2. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by a factor of ~2. This result likely arises from the CR electrons decaying within the starburst region with negligible escape compared to the young extranuclear star-forming regions. Finally, we find that the SFRs estimated using the total 33GHz emission agree well with the free-free SFRs due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.
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