Investigating Velocity & Frequency Widths of Megamasers

[TheCanadian]

I was going through http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2001ASPC..249..639B&defaultprint=YES&filetype=.pdf 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?

 

[gtoguy97]

The frequency widths of megamaser lines are indeed quite large compared to thermal masers, which typically have frequency widths of just a few MHz or less. This is because the large recessional velocities of megamasers result in a much larger Doppler shift of the maser line relative to the rest frequency, resulting in a broad range of frequencies being emitted from the maser.The velocity widths of megamasers are caused by the fact that the maser emission is generated over an extended region, with different portions of the masing region having slightly different velocities due to the cosmic expansion. This results in a broadening of the maser line, with the width of the line increasing with increasing distance from the source. The two peaks in Figure 1 are likely due to multiple regions of maser emission that have slightly different velocities, resulting in two distinct peaks in the spectrum.Theoretically, the velocity width of a maser line could be higher than 2200 km s$^{-1}$ if the maser emission is generated over a larger region, or if the recessional velocity of the source is higher. However, there is a limit to how broad a maser line can be, as the line will eventually become too broad for the maser to remain coherent. As such, masers with very broad line widths are not likely to be observed.