Calvin College - Physics and Astronomy

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Astr212 project : NGC891 edge-on galaxy

Contents: NGC891 is a spiral galaxy approximately 24,000,000 light years from Earth. Although it occupies a tiny portion of our sky, the galaxy is nearly 120,000 light years across. We can only view this galaxy from the side (edge-on), but this galaxy is thought to be very similar to our own galaxy - the Milky Way. The majority of the light emitted from the galaxy originates in the central bulge, but dust orbiting the bulge blocks most of the light. This process, called dust extinction, is most visible near the central plane of the galaxy, where most of the dust would be found. The light of the galaxy is emitted by stars, which become increasingly dense towards the center of the galaxy. The galaxy was first observed by Caroline and William Herschel in 1783, and remains an excellent example of dust extinction.

Observations and Processing:

NGC 891

The exposures were taken by Catherine Boersma on October 1, 2002. She took 144 18-second exposures using the clear filter. Each image was then dark subtracted and flat field corrected. The final image is a combination of about 30 of the best exposures.

To process the data, we plotted the data using DS9 and examined compared counts along cross-sections of the galaxy. We compared measured counts to known magnitudes in The Sky to convert counts to magnitudes. To calculate the amount of dust required to cause the extinction observed, we assumed that, along a given cross-section, the galaxy should be at least as bright along the dust lane as slightly above or below it. This is confirmed by observations of similar galaxies at a variety of viewing angles.

We know that along one cross-section of NGC891, it has a brightness of 15.79 at a bright part of the core and 16.48 along the dust lane. Then the number of magnitudes of extinction is 0.69. It has a radius of 60,000 ly. If we assume the extinction coefficient, Q_λ = 1.5 and the dust grains are spherical with radii (a) of 0.2 micrometers, then we can calculate the density of the material in the dust lanes.

The optical depth, τ_λ is (Carroll and Ostlie )
τ_λ = a_λ / 1.086 = 0.69 / 1.086 = .63

Now, the scattering cross-section, σ_λ is
σ_λ = πa²Q_λ = π(0.2 μm)²(1.5) = 1.88 x 10^-13 m²

The column density of the dust along the line of sight is
N_d = τ_λ / σ_λ = (.63) / ( 1.88 x 10^-13 m²) = 2.73 x 10¹² m^-2

Then the column density (N_d) is also related to the number density (n) of material in NGC891
N_d = ∫ n(s)ds from 0 to s = n*60,000 ly = n(60,000 ly)(9.46 x 10¹⁵ m/ly)

Finally,
n = 2.73 x 10¹² m^-2/(60,000 ly*9.46 x 10¹⁵ m/ly) = 5.89 x 10^-9 m^-3

So, the average density of material between NGC891 and us is 5.89 x 10^-9 m^-3.

Orientation and scale:
  Image Dimensions:
      53.5 x 35 arcsecs
  Coordinates:
      RA: 02h 22m 43.5s
      Dec: +42d 21m 30s
  Constellation: Andromeda

Links:
    National Optical Astronomy Observatory:
        NGC891 Dust Features:     NGC891 Color Optical
    Robert Gendler:
        NGC891 Color Optical
    Infrared Space Observatory:
        NGC891 Dark Matter

Group:
    Catherine Boersma
    Dave Meyer
    Eliot Eshelman

Content updated 12/10/02