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

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

http://skyserver.sdss3.org/dr10/en/tools/explore/summary.aspx?id=1237654605329661983&spec=0&apid=#imaging

• Lamost sepctrum

• 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