5.4 Radial Light Distribution

In Figure 9 the exponential-fit parameters, scale-length alpha and shape beta are plotted for each galaxy. Pure exponential disk galaxies will have beta close to unity, while spheroidals will have beta > 1.2. And indeed the sample shows a consistent trend across Hubble sequence. Some of the galaxies have beta < 1, indicating an inverted de Vaucouleurs R1/4 fall-off in the lightłthe low surface brightness irregular, IC10, is a prime example. The angular scale length reflects both the location and physical size of the galaxy. Note that most of the ellipticals have small alpha (and large beta). The large beta, small alpha, trend may also reflect the systematically more distant spheroidals compared to disk galaxies. The more physically meaningful measure is the distance normalized scale length; e.g., for early-type spiral M31, the scale length translates to ~1.5 kpc (assuming d=770 kpc), for early-type M81 it is ~1.5 kpc (d=3.7 Mpc), and for late-type M51a it is ~3.8 kpc (d=8.7 Mpc).

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