# How to convert between spectra given in terms of velocity (km/s) to wavelength spectra?

I have H alpha spectra of H II regions in our galaxy. In the paper, the x-axis is in units of km/s and the corresponding line width as well. From this, how can I get equivalent width of the H alpha line?

• $v = c\Delta \lambda/\lambda_0$ – ProfRob Nov 15 '20 at 21:58
• It would be much better if you mentioned and linked to the paper and gave the actual value of the line width in km/s. The comment above works to first order, but if it's really wide the answer could turn out to be more interesting! – uhoh Nov 17 '20 at 3:39
• @uhoh- Thank You for replying. The link for the paper I am looking at is link. I needed to know the H alpha equivalent width of the H2 regions in units of wavelength. Am I missing something here? – user1796 Nov 17 '20 at 16:20