# A What is the theory behind large velocity/frequency widths due to cosmic expansion

1. Feb 8, 2017

I was going through http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2001ASPC..249..639B&defaultprint=YES&filetype=.pdf [Broken] where the velocity of the maser is on the order of 80 000 km/s due to cosmic expansion (z = 0.265). But as you may see in Figure 1, the velocity width of the signal is about 2200 km s${^-1}$ . When plugging those values for this OH transition into here, I get frequency widths on the order 10 MHz.

Are not these frequency widths quite large for megamasers? I was under the impression these masers resulted in narrow lines.

Also, I was led to believe there is velocity coherence in masers, but in this case, how can a maser have a velocity width of 2200 km s$^{-1}$ ? Would not the molecules moving at different velocities result in collisions and dephasing throughout the masing region resulting in thermal equilibrium eventually? Even when looking at Figure 1, why does there appear to be two peaks as opposed to a single pulse as normally observed?

And finally, can the velocity widths theoretically be even higher than 2200 km s$^{-1}$ ? I suppose a better initial question is what is causing these widths of 2200 km s$^{-1}$ in the first place?

Last edited by a moderator: May 8, 2017
2. Feb 16, 2017