and Cosmology

3.3 Spiral Galaxies
99
Fig. 3.14. Types of spiral galaxies. Top left: M94, an Sab
galaxy. Top middle: M51, an Sbc galaxy. Top right: M101, an
Here, μ e is the surface brightness at the effective radius
R e which is defined such that half of the luminosity
is emitted within R e (see (2.40)). The central surface
brightness and the scale-length of the disk are denoted
by μ 0 and h r , respectively. It has to be noted that μ 0
is not directly measurable since μ 0 is not the central
surface brightness of the galaxy, only that of its disk
component. To determine μ 0 , the exponential law (3.11)
is extrapolated from large R inwards to R = 0.
When Ken Freeman analyzed a sample of spiral galaxies,
he found the remarkable result that the central
surface brightness μ 0 of disks has a very low spread,
i.e., it is very similar for different galaxies (Freeman’s
law, 1970). For Sa’s to Sc’s, a value of μ 0 = 21.52 ±
0.39 B-mag/arcsec 2 is observed, and for Sd spirals and
later types, μ 0 = 22.61 ± 0.47 B-mag/arcsec 2 . This result
was critically discussed, for example with regard to
Sc galaxy. Lower left: M83, an SBa galaxy. Lower middle:
NGC 1365, an SBb galaxy. Lower right: M58, an SBc galaxy
its possible dependence on selection effects. Their importance
is not implausible since the determination of
precise photometry of galaxies is definitely a lot easier
for objects with a high surface brightness. After
accounting for such selection effects in the statistical
analysis of galaxy samples, Freeman’s law was confirmed
for “normal” spiral galaxies. However, galaxies
exist which have a significantly lower surface brightness,
the low surface brightness galaxies (LSBs). They
seem to form a separate class of galaxies whose study
is substantially more difficult compared to normal spirals
because of their low surface brightness. In fact,
the central surface brightness of LSBs is much lower
than the brightness of the night sky, so that searching
for these LSBs is problematic and requires very
accurate data reduction and subtraction of the sky
background.