Let us know take a look at the structural
or photometric properties of spiral
galaxies.
First, about spiral galaxies in general,
they do have a lot of diversity in their
structure and components.
First of all, there is the degree of, in
which their spiral arms are prominent,
which was, of course, the basis of Hubble
classification, as well as the ratio bulge
to disk.
And because bulge and disk have different
stellar populations, bulge being an old
one, disk being younger, there are all
these differences in star-formation rates,
and colors, and so on.
The disks, of course, have interstellar
material called gas and dust, from which
stars are forming.
Also, spirals tend to avoid dense regions
of the large scale structure, because this
is where galaxy mergers can happen, or do
happen.
And those spirals have been turning into
ellipticals.
Recall the trends that we discussed about
Hubble sequence.
Spirals, of course, participate in all of
them.
Most of the trends are, is along the
spiral branch, and they are listed here.
But basically, as we go from early Hubble
types towards the late ones, there is an
increasing amount of star formation,
increasing amount of gas and increase in
the disk to bulge ratio.
All of this manifest itself in colors,
they get bluer and other measured for the
metric properties.
So, let's look at individual subcomponents
of spiral galaxies.
First, there are the disks and they're
characterized by exponential distribution.
We'll show that in a moment.
And there may be more than one kind of
disks.
There are thin disks within which star
formation occurs.
That's where the interstellar material is.
But they may be embedded into thicker
disks, composed mainly of intermediate to
old aged stars.
The bulges are essentially ellipticals in
middle of spirals.
Not all spirals have bulges, some simply
have disks that go all the way to the
middle, maybe rounded off, but no distinct
elliptical-like component.
Now, unlike Baade's original idea of
Population II stars that's supposed to be
metal-poor, stars in halo are metal poor,
those in galactic bulge as well as in
ellipticals are actually metal-rich.
They did result from a substantial amount
of chemical evolution, but very early on.
And again, there is the important
difference in dynamics.
The disks are rotationally supported
against self-gravity, and bulges are
pressure supported, random motions.
About half of all spirals contain bar-like
feature, and you've seen pictures,
sometimes spiral arms begin at the ends of
the bar.
Bars are similar to bulges in their
composition, but dynamically, they're
distinct.
In their very centers, some people argue
that spirals contain an additional
component concentric with the bulge
itself, which can be very dense,
sometimes, may contain supermassive black
hole, which may or may not be active and
there is usually some star formation.
If you remember, numerical experiments and
collisions of spiral galaxies, the gas
tends to sink to the center, because it
loses energy.
And if you accumulate a lot of gas with
high density, it'll tend to make stars.
In contrast to that, the most extended
stellar component is the stellar halo.
And that is composed out of metal-poor
stars, the kind of stars that make
[unknown] galaxies.
So, we believe that all of the stellar
content of the galactic stellar halo, is
from disrupted, merged [unknown] galaxies
that have been torn apart by tidal, tidal
forces.
And there are simply, they are, they're
populated the halo.
And there is lot of good experimental
evidence for that.
And of course, there is the dark halo.
Dark halos are more extended than visible
parts.
There are some hints as to whether radial
distribution would be, as well as their
shape.
They're probably triaxial ellipsoids.
So, the way we can quantify distribution
of visible material is through surface
odometry meaning.
But before we look at the distribution of
stars, we first need to correct for the
inclination effects, because phase on
spirals are more or less circular, so
their apparent distance in the sky tells
you about the inclination is.
Then, we have to correct for the
interstellar extinction, both in the
galaxies themselves, but also in the Milky
Way and the direction that we look at
stellars.
Usually, these radial surface brightness
profiles are obtained by averaging in
circular or elliptical annuli.
And so, that tends to average over the
spiral arms.
As it turns out, typical surface
brightness profiles of spiral disks are
exponential.
The projected surface brightness declines
exponentially from the center out.
And the enfolding lengths or disk scale
lengths served the order few kiloparsecs.
So, if you measure a surface brightness
profile of a galaxy, then you'll see
something like this.
If plotted on semi log plot, log surface
brightness versus linear radius, an
exponential looks like a straight line,
and indeed that's what you see in the
outer regions.
There is extra light in the middle, and
that's due to the bulge.
And so, one can then fit an exponential
disk plus the bulge component, using one
of the elliptical galaxy-fitting formulae
that we will look at in the next chapter,
and does decompose the light in, into
bulge and to disk.
So, this is now well-established procedure
and many galaxies have been studied in
this way.
Here are examples of a few.
Now, here are some contour maps of the
surface brightness distribution of spiral
galaxies in the sky.
And these are lines of equal brightness.
And to anticipate something I'll show in a
moment, they do seem to stop at some
point.
So, what about perpendicular structure?
I mean, along the radius from center out,
their exponential.
But it turns out, the distribution of
density in spiral galaxies perpendicular
to the plane of the disk, is also
exponential.
And the typical scale height is of the
order of hundreds of parsecs.
Much shorter than the disk scale length of
several kiloparsecs.
So, one interesting thing about spiral
galaxies is that, their disks have
cutoffs, they do have an edge.
Past certain radius, there are no more
stars.
However, hydrogen, from which stars are
made usually extends further and the dark
halo, further yet.
So, that suggests that disks may be
forming from the inside out, in terms of
star formation.
So, if you measure the profile, you can
take the integral under it.
And that gives you the total luminosity of
the galaxy, or its two components, the
bulge and the disk.
Inclination correction tends to be the
trickiest of them all.
But, now we know how to do that.
And the extinction correction depends on
estimating the amount of dust along the
line of sight.
Because interstellar extinction causes
reddening, absorbing more blue light than
red light, using colors can give us
indication how much reddening there is.
And from that, how much of total
absorption there is.
Next time, we will talk about interstellar
medium, gas in spiral galaxies.