退休星系与星暴后星系的红外光度：恒星形成率测量的警示故事
The infrared luminosity of retired and post-starburst galaxies: A cautionary tale for star formation rate measurements
AI总结：
本研究结合SDSS光谱与GAMA多波段测光，发现退休及星暴后星系的L$_{TIR}$/L$_{Hα}$显著升高，红外与Hα仅在特定W$_{Hα}$阈值内可可靠估计SFR。
摘要：
在具有显著持续恒星形成的星系中，当Hα光度（L$_{Hα}$）经过恒星吸收和尘埃衰减的适当校正后，总红外光度（L$_{TIR}$）与Hα光度之间存在极其紧密的相关关系。这一长期存在的结果使人们相信，尽管这两种测量的来源不同，但都能准确估计星系的恒星形成率（SFR）。为了检验这一关系在具有较低比恒星形成率（sSFR=SFR/M$_{gal}$）的星系中适用到何种程度，我们将SDSS的光学光谱与GAMA的多波段（FUV至FIR）测光观测相结合。我们发现，L$_{TIR}$/L$_{Hα}$随着Hα等值宽度（W$_{Hα}$，sSFR的代理指标）的减小而稳步增加，这表明在低于标准恒星形成序列的星系中，这两种光度不能提供有效的SFR测量。对于“退休星系”和“星暴后星系”，L$_{TIR}$/L$_{Hα}$最高可比恒星形成星系大30倍。L$_{TIR}$/L$_{Hα}$的平滑变化与恒星形成历史、电离或加热源、尘埃温度或其他性质无关，这表明L$_{TIR}$/L$_{Hα}$的值由恒星形成区与对L$_{TIR}$和Hα均有贡献的周围星际介质之间的平衡决定，而不是这些光度所探测的恒星形成时间尺度不同所致。虽然L$_{Hα}$只能用于估计W$_{Hα}$ > 3Å（sSFR $\gtrsim 10^{-11.5}$/年）的星系的SFR，但我们认为中红外和远红外只能用于估计位于恒星形成序列上、W$_{Hα}$ >10Å（sSFR $\gtrsim 10^{-10.5}$/年）的星系的SFR。我们没有发现本地星暴后星系中存在被尘埃遮蔽的恒星形成的证据。
英文摘要：
In galaxies with significant ongoing star formation there is an impressively tight correlation between total infrared luminosity (L$_{TIR}$) and H$α$ luminosity (L$_{Hα}$), when H$α$ is properly corrected for stellar absorption and dust attenuation. This long-standing result gives confidence that both measurements provide accurate estimates of a galaxy's star formation rate (SFR), despite their differing origins. To test the extent to which this holds in galaxies with lower specific SFR (sSFR=SFR/M$_{gal}$), we combine optical spectroscopy from the SDSS with multi-wavelength (FUV to FIR) photometric observations from GAMA. We find that L$_{TIR}$/L$_{Hα}$ increases steadily with decreasing H$α$ equivalent width (W$_{Hα}$, a proxy for sSFR), indicating that both luminosities cannot provide a valid measurement of SFR in galaxies below the canonical star-forming sequence. For both `retired galaxies' and `post-starburst galaxies', L$_{TIR}$/L$_{Hα}$ can be up to a factor of 30 larger than for star-forming galaxies. The smooth change in L$_{TIR}$/L$_{Hα}$, irrespective of star formation history, ionisation or heating source, dust temperature or other properties, suggests that the value of L$_{TIR}$/L$_{Hα}$ is determined by the balance between star-forming regions and ambient interstellar medium contributing to both L$_{TIR}$ and H$α$. It is not a result of the differing timescales of star formation that these luminosities probe. While L$_{Hα}$ can only be used to estimate the SFR for galaxies with $W_{Hα}$ > 3A (sSFR $\gtrsim 10^{-11.5}$/yr), we argue that the mid- and far-infrared can only be used to estimate the SFR of galaxies on the star-forming sequence with W$_{Hα}$ >10A (sSFR $\gtrsim 10^{-10.5}$/yr). We find no evidence for dust obscured star-formation in local post-starburst galaxies.
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