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?

## Answer

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

Region | λ | Fλ /W m−2 μm−1 | λFλ /W m−2
--- | --- | --- | ---
Radio | 10 cm | 10−28 |
far-IR | 100 μm | 10−23 |
X-ray | 10−10 m | 10−20 |

## Answer

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.

## Answer

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?

## Answer

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

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