Ew(Hα) of one star forming galaxy

• 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