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Evolution stellar populations in galaxies
is one of the key ingredients of our

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understanding of galaxy evolution in
general.

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If we look at colors of galaxies nearby we
can immediately see that there is a

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bimodel distribution.
There is a fairly narrow red, peak, which

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is due to the elliptical galaxies and
bulges.

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And there is a fairly broad blue
distribution, that corresponds to the

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discs of spirals.
In more modern renderings, you can plot

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colors as a function of intrinsic
luminosity.

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And you still see those same two blobs.
But they're a little tilted.

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In the case of elliptical galaxies that
ridge line.

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[inaudible] is really mass[INAUDIBLE].
The more luminous the.

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The ellipticals retain more of their,
chemical evolution products.

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Their more metal rich metals absorb more
like in the ultraviolet galaxy look

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redder.
So this is largely mass metallicity

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sequence.
For spirals there isn't a very simple

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relation like that.
Their colors are combination of mixture of

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stellar ages.
Star formation rates as well as extinction

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by dust.
So we can produce predicted theoretical

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spectra of evolving galaxies in the
following fashion.

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We have to make an assumption of what the
star formation history is.

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This is a purely pre-parametering the
model, but we can make reasonable guesses.

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We need to know what is the initial mass
function, the distribution of stars by

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mass when they're formed, because stars of
different masses can be involved in very

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different[UNKNOWN].
Then for each stellar mass and, and, and

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age we need a spectrum.
So we need libraries of stellar spectra

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that can be associated with all components
of the stellar population at any given

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age.
This is actually not an easy thing to do,

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because we can observe a lot near us.
But for example, we have no spectra of

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very metal poor very massive stars,
because those have burnt out long time

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ago.
And, again from theory, tested by

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observations of star clusters and so on,
we need stellar population evolution

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tracks.
How does distribution of stars and color

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magnitudes space the HR Diagram changes
the function of time for a given amount

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of.
Chemical enrichment and so on.

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You take a quantity of gas and turn it all
into stars instantaneously.

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A delta function star formation rate
distributed according to your initial mass

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function.
And then follow it's evolution in time.

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This is what's known as a simple stellar
population.

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There is no such thing in reality,
although globular clusters come close.

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And then it can represent net total star
formation history of a galaxy is a

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collection of these, and follow the map.
One popular way of expressing star

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formation histories is a exponentially
declining rate.

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Which is probalby not a bad overall
assumption Average or many galaxies.

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In this case, for early type galaxies
there is a lot of star formation early on.

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The exponential very steep.
For this galaxy very shallow.

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For irregular galaxies may be essentially
flat or even rising.

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Stellar evolution is one of the best
understood Segments of astrophysics.

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We really do know how stars work and
evolve.

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And that's been confirmed again and again
through many decades of observations.

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There are certainly details that still
need to be ironed out.

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But at the level that we care about here,
we really do understand that.

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An important thing to remember is that
more massive stars They are much faster,

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they are more luminous, they are also
hotter, so they are[UNKNOWN] more

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important but not for long.
Thus, spectrum of an evolving galaxy will

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change more rapidly in the blue parts of
the spectrum, thanks to the short life

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times of these massive stars.
And will be changing relatively slowly in

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the red parts of the spectrum where the
most of the light may be coming from well

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of all slowly evolving population of red
giants.

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There are many stellar populations indices
methods out there.

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And they sometimes disagree on some
details.

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But by and large the agreement's pretty
good.

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One popular set is called Giselle.
Which is a library of The galaxy evolution

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spectra by[UNKNOWN] and[UNKNOWN].
We know how stars evolve.

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We even have spectra of lots of different
kinds of stars.

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And we have some idea of the initial mass
function.

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At least we measure it locally in the
milky way.

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And then we have to make assumptions what
it's saying in all galaxies at all times.

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We can test some of those assumptions.
But then we have to assume star formation

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rate which again is a completely free
parameter in this exercise.

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This is what stellar evolution tracks look
like.

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They're computed from stellar evolution
models.

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And a star evolves it moves in the color
magnitude space.

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How and where depends very much or almost
entirely on its mass with minor dependence

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on its metallicity.
So we get these stellar evolutionary

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tracks from theory, our understanding of
stellar structure and evolution it's

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pretty solid.
Then, we need to have their spectra.

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And for kinds of stars that we can observe
near us, that's an easy thing to acquire.

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For kinds of stars that are no longer with
us, say, very young, metal poor, very high

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mass stars.
This will require some theoretical

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modeling, and stripulation.
We make an assumption about initial mass

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function, and again we understand that in
the local Milky Way condition, but it

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would be very different in the early
universe where, say, stars were made out

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of, Of hydrogen and helium, and hardly
anything else.

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In fact, we believe that was the case,
that the initial mass function of

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primordial stars was very different.
And then again, we have to assume star

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formation history.
In reality, we assume some types of star

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formation histories.
Compute the consequences, compared it to

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iterations and iterate until we have a
model that seems to fit observations and

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that's telling us what the likely star
formation history of these galaxies was.

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So stars evolve, the blue ones keep
disappearing faster.

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Galaxies will be changing their colors and
here in color, color space you can follow

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the, theoretical behavior of evolving
stellar populations.

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You can also see where the galaxies are,
and indeed they form something of a.

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Sequence that corresponds to the younger,
hotter stars on late Hubble types, older,

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redder stars for the early Hubble types.
Here is a comparison of predictions of

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three different types of stellar
population and synthesis models by

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different groups.
And it shows predicted colors and mass to

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light ratios for model galaxies.
By and large they agree very well.

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The minor disagreements usually are very
young ages and there is some debate

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actually what is the correct thing to
assume but.

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Qualitatively at least, we understand very
well how things are working.

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And so this is what evolving spectra of
stellar populations look like.

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This is for a simple stellar population.
Remember this is a scoop of stars made all

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at once, and let go.
Note, these are logarithmic plots of log

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flux versus log wavelength.
So.

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They really hide the strong contrast.
And if you stare at curves which are

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labeled by their age, those near the top
tends to be youngest because younger stars

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are more luminous.
You find out that in the ultraviolet, blue

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part of the spectrum, the flux will
plummet very quickly.

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Whereas it would be changing in the red
part of the spectrum, but much slower

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which is exactly what we expect.
Here is a comparison of predicted model

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spectral of different ages assuming
different libraries of stellar population

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evolution tracs and spectra.
And again you see that.

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Different models seem to be in an
excellent mutual agreement.

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Which is giving us some confidence that we
actually do understand how this works.

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Another kind of models that they are now
more popular are so called,

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semi-analytical, or hybrid models, where
one can use telepopulation synthesis

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models.
Associate them with galaxies that they're

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being assembled through hierarchical
structure formation.

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Either from a numerical simulation, or
from some statistical description thereof.

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And make predictions of how will they ch,
colors change and so on.

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There are unfortunately way too many two
level parameters in these models.

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And so they have some modest success, but
they illustrate of all different things

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that have to be taken into account, and a
lot of assumptions that have to be made.

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Next time we will turn to the actual
observations of galaxy evolution.