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.