Blog #25: WS7.2, #5

5. You may have also noticed some weak "dips" (or absorption features) in the spectrum: 

a) Suggest some plausible origins for these features. By way of inspiration, you may want to consider what might occur if the bright light from this quasar's accretion disk encounters some gaseous material on its way to Earth. That gaseous material will definitely contain hydrogen, and those hydrogen atoms will probably have electrons occupying the lowest allowed energy state. 

These dips are probably due to the absorption of photons by the gaseous material between the quasar and the Earth. The wavelengths probably correspond to the photons at appropriate energies to excite electrons from the ground state to the second, third, or higher energy levels. There is decreased flux at these wavelengths because some of those photons will be absorbed by the gas and not reach the Earth. 

b) A spectrum of a different quasar is shown below. Assuming the strongest emission line you see here is due to Lyα, what is the approximate redshift of the object? 

The observed wavelength appears to be about 5600Å. 

\(z=\frac{5600Å-1216Å}{1216Å}=3.61\) 

c) What is the most noticeable difference between this spectrum and the spectrum of 3C 273 (below)? 

What conclusion might we draw regarding the incidence of gas in the early Universe as compared to the nearby Universe? 

One noticeable feature is that the other spectrum is located at much greater wavelengths than the 3C 273 spectrum. This means that the redshift is much greater (as we calculated in part b), and therefore the object is farther away than 3C 273. We also notice all of the dips to the left of the Lyα peak. These are probably caused by the absorption of the quasar's radiation by dust and gas. This implies that gas, specifically neutral Hydrogen gas, was very prevalent in the early Universe.