Blog #25, Worksheet #7.2, Problem #5, Red Shifts of Different Spectrums

5. You may also have 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. 

As somewhat explained in the question, on it's way to Earth, the emitted light from the quasar may hit very thick patches of gasses, notably hydrogen. As we explored earlier in the worksheet, these hydrogen molecules absorb some of the emitted energy and use it to move through energy levels. They then output the LyA wavelength as output and we use this wavelength change to measure the speed at which the the quasar, or other object, is moving away from us at. The dips in the light spectrum curve symbolize the patches of hydrogen gas that the light encounters at certain wavelengths. Instead of the steady dispersion of different wavelengths on the spectrum, certain ones are absorbed by the clouds of hydrogen and then account for a much smaller flux being received on 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 this object? 

We use the equation for red shift that we saw earlier \[z = \frac{\lambda_{observed} - \lambda_{emitted}}{\lambda_{emitted}}\]We assume that the peak in the light spectrum offers a good estimate for the observed wavelength. We also know that the emitted wavelength in a redshift is the LyA shift where \(\lambda = 1215.67 \, angstroms\). We find that the peak is around \(\lambda = 5650 \, angstroms\). Plugging into our equation and solving for z, we get a redshift of \[z = 3.65\]This shift is much more significant than the first spectrum we looked at. This larger redshift means that the quasar is moving much faster and must be significantly further away from Earth.

(c) What is the most noticeable difference between this spectrum and the spectrum of 3C 273? What conclusion might we draw regarding the incidence of gas in the early Universe as compared to the nearby Universe?

It is clear that on this spectrum, compared to the first, that there are far more peaks and valleys. Far more emission jumps and absorption lines. As we discovered in part a) these are created by light hitting patches of hydrogen along the journey from the quasar to Earth. Given that we found the second quasar to be much further than the original quasar, and the fact that there are far more absorption lines in the second spectrum, we can assume there is much more gaseous material in the earlier universe than there is in the nearby, developed universe. The larger the redshift and older the quasar is, the more absorption lines it should have.