Blog Post 25, WS 7.2, Problem 5: More Quasar Redshift

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. 

(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?

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?

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.

SPOILER ALERT: The answer is in the question!  Gas accounts for the spikes perfectly.  Hydrogen gas in the ground state that is bombarded by high-energy radiation such as quasar emissions will absorb wavelengths of radiation corresponding to jumps in electron energy levels.  This will strip the spectrum of such wavelengths creating the jagged spectrum that we see.

(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?

Let's use the Doppler equation.

\[\frac{\lambda_{observed}-\lambda_{emitted}}{\lambda_{emitted}}=z\]
\[\frac{5600-1215.67}{1215.67}=z\]
\[\boxed{z=3.6}\]

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?

Because this object is very far away and very old, the universe was much younger when its light was first emitted, thus it passed through gas and dust from the young universe on its way to us. The spectrum picture is a little pixelated, but the dark part on the right half is actually a series of tons of absorption lines caused by the light passing through cosmic gas and dust. Since these lines are much more numerous than those in the spectra of closer objects, the gas in the earlier universe must have been much more dense in order to produce the lines. Neat!

I worked with B. Brzycki, G. Grell, and N. James on this problem.