Ew(Hα) of one star forming galaxy
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Equivalent Width : EW(Hα)
Recall EW(Hα) measures the relative strength of Hα to the continuum under the line It therefore acts like a long baseline color index UV(Hα)<--> λ6550 A
Although it cannot be converted to a current SFR, it has another important use : It measures the ratio of the current SFR (from Hα) to the integrated past SF (from the continuum) Using synthesis models, this relation can be quantified, to give : EW(Hα)--> (current SFR) / (mean past SFR) ; written SFR/<SFR> or "b" http://www.astro.virginia.edu/class/whittle/astr553/Topic11/Lecture_11.html
Exercise: measure the Ew(Hα) of one SFing galaxy in the Lamost survey
- RA=169.02567, DEC=4.79585, z=0.02729 (Lamost)
- SDSS image
- Lamost sepctrum
文件:Spec-55977-F5597703 sp10-191.ps
- Calculation of the Ew(Hα)
- ignore the stellar absorption (usually ~2A) and the dust distinction
- central wavelength of this galaxy: 6563*(1+z)=6742 A
- continuum (blue dashed line): linear fit of the wavelength range [6650,6850] (masked the emission line region [6710,6780])
- Ha emission (thick red solid line)
- Ew(Hα)=area (between thick red line and blue dashed line)/strength of continuum = 67.95A