Introduction to active galaxies
Introduction to active galaxies

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Introduction to active galaxies

7.6 Questions

Question 15

Suppose that a galaxy has emission lines in its optical spectrum. A line of wavelength 654.3 nm is broadened by 2.0 nm. Estimate the velocity dispersion of the gas giving rise to the broadened spectral line. Is it likely to be a normal galaxy?


For the gas motion use Equation 3.1, Δλ/λ = Δv/c, where Δv is the velocity dispersion. Then Δλ/λ = 2.0 nm/654.3 nm ≈ 0.0030.

Thus the overall spread of internal speeds is Δv ≈ 0.0030 × c ≈ 900 km s−1, which is too large for a normal galaxy.

Question 16

Calculate λFλ flux densities in W m−2 in the radio, the far infrared and the X-ray regions, given the Fλ and λ values listed in Table 2. Which wavelength region dominates?

Table 2

RegionλFλ/W m−2 μm−1λFλ/W m−2
Radio10 cm10−28
far-IR100 μm10−23
X-ray10−10 m10−20


In the radio wave region, λ = 105 μm so

In the far infrared region λ = 100 μm so that

In the X-ray region, λ = 10−4 μm so

The largest of these λFλ values is 10−21 W m−2, so we conclude that the far-infrared emission dominates.

Question 17

Suppose that an unusual galaxy has broadband spectral flux densities Fλ at wavelengths 500 nm, 5 μm and 50 μm, of 10−27, 10−28, and 10−28 W m−2 μm−1, respectively. By calculating λFλ, comment on whether it is likely to be a normal or an active galaxy.


The wavelengths λ are 0.5 μm, 5 μm and 50 μm, therefore the λFλ values are 5 × 10−28 W m−2, 5 × 10−28 W m−2 and 5 × 10−27 W m−2, respectively. The largest of these values is 5 × 10−27 W m−2, so the dominant flux is at 50 μm, which is in the far infrared. The object is likely to be either a starburst galaxy or an active galaxy.

Question 18

A particular galaxy has a large luminosity at X-ray wavelengths. One astronomer believes it to be a galaxy that happens to contain a large number of separate X-ray stars. Another astronomer believes that the X-rays indicate an active galaxy. How, by measuring the spectrum of the galaxy, could this question be resolved?


If the galaxy were active, one would expect to see strong emission lines in the optical and spectral excesses at non-optical wavelengths.

Skip Your course resources

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus