We now turn our attention to galaxies.
First, we'll talk about morphology and
classification.
You can think of galaxies as the basic
constituents of the universe obviously
distinct units when you look at the sky.
And the reason for that is that they are
over dense by a factor of 1,000 relative
to the extrapolation of their large scale
structure.
And, as we covered earlier, this is due to
the additional collapse due to the cooling
of variants.
One thing about galaxies that, that have a
remarkably large range of properties, and
yet there are some irregularities among
them.
Both the wide range and irregularities are
indicative of different processes of
galaxy formation and galaxy evolution.
In some sense, they represent the fossil
evidence of galaxy formation.
To the first order, these differences in
formative processes leads to the
observable differences in galaxy
morphology today.
And, in addition to measuring cosmological
parameters, understanding of galaxy
formation, structure formation in general,
is one of the key goals of modern
cosmology.
A couple of basic facts are that there are
about 100 billion galaxies within the
observable universe now, and their masses
range from hundreds of million to maybe
trillion solar masses, containing up to
couple hundred billion stars.
The first thing that any empirical science
would do is to catalog what's out there
and catalogs of galaxies have been.
A basic data for astronomy for a long
time.
The very first one was due to Charles
Messier in the 18th century.
Messier cataloged a little over a hundred
objects.
We still call them Messier in their
number, or m then the number.
And about half of them are galaxies, the
rest of them are different kinds of
nebulae or start clusters.
Following him, Herschel finally has
produced a more extensive catalogs.
And an astronomer, by the way, of Dreyer
has recompiled those and published as the
new general catalog of nebulae.
Remember, people didn't know that galaxies
were galaxies back then, and the acronym
of this, NGC, is now commonly seen.
You see a lot of objects, they're not just
galaxies but star clusters, gaseous
nebulae and so on.
This was supplemented by a so-called index
catalog that extended things further to
the southern hemisphere, and sometimes you
see that designation IC and the number.
In the 20th century, people started
compiling more systematic collections of
data on galaxies.
Shapley and Ames were among the first,
Tamman and Sandage followed.
With the advent of first systematic sky
surveys, originally the photographic ones
from Palomar Observatory, it became
possible to do this in a systematic
fashion and go much fainter.
And the most notable of those is the
Uppsala General Catalogue which has the
designation EUGC and that catalogue all
galaxies on the northern sky down to the
apparent angular diameter for about 1
arcmin.
Notice, this was the angular diameter
selection and not magnitude selection
which is more common.
Ugc was extended to the southern sky using
the southern sky photographic survey to
ESO Uppsala Catalogue.
In parallel, Vorontsov-Vel'yaminov, in the
Soviet Union, produced what he called
Morphological Catalog of Galaxies.
[unknown] also had a catalog of galaxies
as well as clusters.
Gerard deVaucoleur and his collaborators
compiled not simple catalogs that would
list positions, magnitudes, maybe
diameters, but everything that was known
about Bright Galaxies at the time, and
those were called Reference Catalogs.
Ending up with the third one of those, and
those are sort of the last postmodern
compilations of galaxy and data.
Nowadays, it's all down in fully automated
objective fashion from digital sky surveys
and modern catalogs of galaxies contains
tens to hundreds of millions of objects.
So, after you catalog what's out there,
the next step is the superficial
morphology.
This is sort of like biology was in the
early days.
You do not yet understand what is going
on, but you see that there are
differences.
And indeed Edwin Hubble was the first one
to address this in a, in a somewhat
quantitative fashion, and a lot of his
early work is still relevant today.
He proposed what's still used Hubble
classification or Hubble types in a
popular book, The Realm of Nebulae.
And that led to famous tuning fork diagram
which I will show you in a moment.
Hubble's classification subsequently
refined or extended by deVaucouleurs, van
den Bergh and others.
But the basic things haven't really
changed.
These days, we're actually classify
galaxies in a more quantitative fashion
using correlations and distributions of
their physical properties rather than
their superficial appearance on pictures.
Although, it turns out that, that
superficial appearance of pictures of
morphology does correlate with the number
of real physical or astrophysical
properties of galaxies.
And we'll talk about that in more detail.
Another thing you noticed about galaxies
is that they all seem to have a limited
subset of subsystems building blocks.
And, for example, bulges, which also look
like elliptical galaxies and disks spiral
arms.
It turns out that these subsystems in
galaxies correspond to differences in
stellar content and internal kinematics,
age, star formation, history and so on.
And so, maybe that would be a better way
of classifying galaxies.
So, here is Hubble's famous tuning fork
diagram.
Hubble noted that there is one obvious
variable which is the relative prominence
of the bulge component, the central
reddish component.
And the disk with spiral arms, which said
to be fairly blue.
So, he sorted galaxies from redder, the
smoothest, to the bluer, which are also
having more prominent disks and more
prominent spiral arms.
He split the spiral galaxies into two.
Those that showed also bar-like structure
in the middle from which spiral arms
originated and those that didn't.
This turns out to be some of superficial,
but it does have some physical meaning.
So, Hubble talked that this was a
evolutionary sequence, that goes from left
to right, from elliptical galaxies into
spirals.
And now, think that it is the exactly
opposite of that really happens, Hubble
had simply no other way of telling.
And so, because of that, the elliptical
galaxies which are actually older are
called early types, according to Hubble.
And spiral galaxies which on average are
younger are called late types.
It's a little confusing but it's just the
way the usage is.
Within the elliptical family, not knowing
anything else about them, Hubble simply
classified them by the apparent
elipstisity in the sky.
It turns out that, that actually doesn't
correlate with anything whatsoever in
terms of physical properties of
ellipticals, but that's what he had, he
classified spirals by the relative
prominence of spiral arms.
Sa's being the least prominent, Sc's being
the most prominent.
Spiral arms, that also correlates with
relative importance of bulge versus disk.
And then, there are galaxies that simply
looked like scrambled eggs, and he
couldn't classify them, he just called
them irregular.
Turns out that's also very heterogeneous
group of things, some of which are mergers
of large galaxies, some of which are
genuine dwarf galaxies with spotty star
formation.
But that wasn't known at that time.
So, elliptical galaxies account for maybe
up to 20% of all galaxies uh,delays and
most of them are found in clusters or
maybe dense groups relatively modest
number in general field.
Hubble classify them according to the
apparent ellipticity projected on sky,
having no other physical properties to
use.
And subsequently, there were subcategories
of ellipticals based on their luminosity.
This time, giant ellipticals, central
dominant galaxies and clusters, dwarf
ellipticals and so-called dwarf
spheroidals, which actually a completely
different family of galaxies, but more
about that later.
One of the Hubble criteria for calling
something an elliptical galaxy is that
they have the smooth appearance, meaning
no spiral arms.
They have no dust lanes obscuring them.
They have no star formation.
Turns out, actually, elliptical galaxies
have all of that.
They have a lot of gas except that gas
tends to be extra gas.
They sometimes have dust lanes from spiral
galaxies they gobbled up.
And sometimes have a little bit of star
formations, too.
So Hubble sub-typed according to the
apparent ellipticity, E0's being the round
ones, E7's being the most elegant that he
could see.
Now, we think that E7's actually contain
disks but without star formation.
And here, couple of examples of elliptical
galaxies from our immediate intergalactic
neighborhood, the Virgo cluster M87, the
central elliptical galaxy in Virgo on the
left.
A lot of little dots that you see around
it, around it are not stars.
They're actually globular star clusters
that belong to M87.
M87 has about 10,000 globular clusters
around it.
To put this in perspective, Milky Way has
about 150, and M84, M86.
Remember, M is for Messier Catalog our two
other big ellipticals in Virgo.