and Cosmology

3.3 Spiral Galaxies
luminous galaxies. The specific abundance of globular
clusters in a galaxy is defined as their number, normalized
to a galaxy of absolute magnitude M V =−15.
This can be done by scaling the observed number N t
of globular clusters in a galaxy of visual luminosity
L V or absolute magnitude M V , respectively, to that of
a hypothetical galaxy with M V =−15:
S N = N t
L 15
L V
= N t 10 0.4(M V +15) . (3.13)
If the number of globular clusters were proportional to
the luminosity (and thus roughly to the stellar mass) of
a galaxy, then this would imply S N = const. However,
this is not the case: For Sa’s and Sb’s we find S N ∼ 1.2,
whereas S N ∼ 0.5 forSc’s.S N is larger for ellipticals
and largest for cD galaxies.
3.3.5 Spiral Structure
The spiral arms are the bluest regions in spirals and they
contain young stars and HII regions. For this reason,
the brightness contrast of spiral arms increases as the
wavelength of the (optical) observation decreases. In
particular, the spiral structure is very prominent in a blue
filter, as is shown impressively in Fig. 3.17.
Naturally, the question arises as to the nature of the
spiral arms. Probably the most obvious answer would be
that they are material structures of stars and gas, rotating
around the galaxy’s center together with the rest of the
disk. However, this scenario cannot explain spiral arm
structure since, owing to the differential rotation, they
would wind up much more tightly than observed within
only a few rotation periods.
Rather, it is suspected that spiral arms are a wave
structure, the velocity of which does not coincide with
the physical velocity of the stars. Spiral arms are quasistationary
density waves, regions of higher density (possibly
10–20% higher than the local disk environment).
If the gas, on its orbit around the center of the galaxy,
enters a region of higher density, it is compressed, and
this compression of molecular clouds results in an enhanced
star-formation rate. This accounts for the blue
color of spiral arms. Since low-mass (thus red) stars live
longer, the brightness contrast of spiral arms is lower in
red light, whereas massive blue stars are born in the spiral
arms and soon after explode there as SNe. Indeed,
only few blue stars are found outside spiral arms.
In order to better understand density waves we may
consider, for example, the waves on the surface of a lake.
Peaks at different times consist of different water particles,
and the velocity of the waves is by no means the
bulk velocity of the water.
3.3.6 Corona in Spirals?
Hot gas resulting from the evolution of supernova remnants
may expand out of the disk and thereby be ejected
to form a gaseous halo of a spiral galaxy. We might
103
Fig. 3.17. The galaxy NGC 1300 in the B
filter (left panel) and in the I filter (right
panel). The spiral arms are much more
prominent in the blue than in the red. Also,
the tips of the bar are more pronounced
in the blue – an indicator of an enhanced
star-formation rate