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today's messier object of choice is messier 59 so there's a beautiful picture that came out this year from the hubble space telescope of this galaxy i've got a print out here but the online version is much better because you can pan and zoom through this image and see not only the galaxy but the truth of whenever you point the hubble space telescope at any patch of sky all the little random background spirals and other galaxies that you can see in the background it's a galaxy that happens to be part of the virgo cluster of galaxies uh near to our own at first glance it doesn't look very special like many galaxies in the virgo cluster it's an elliptical galaxy a little bit bigger
than our own galaxy the milky way and so what i want to talk about is the kind of detective work that we do as astronomers to learn about the physical nature of an object like this and its past history so the first thing that we can do with the galaxy like this is to look at what we call the surface brightness profile that just says as we look at different parts of the image how does the brightness in each pixel change and can we put that all together and make a model of what the galaxy should look like so the simplest model in this case is some sort of elliptical model something that's brighter in the center and that falls out the further away you go a more interesting step is to then take away that smooth profile we can do
this because these are all digital images so if we have a model we just subtract that bit off and see what's left in a paper using earlier observations from the hubble space telescope that's what these authors did so our galaxy messier 59 ngc 4621 is this galaxy here first of all this is zooming in right to the center of the galaxy but once we take that simple elliptical profile away you can see the light that's left underneath there's a very bright core and the light that's left is the isophotes these lines of constant brightness are quite pointed and what this is telling us is there's another component a physical component to this galaxy that's not elliptical but is more like a disc now we bring in what
we know about the physics of galaxy formation and we know that for elliptical galaxies the stars in those galaxies are orbiting fairly randomly the orbit of each star is not connected to the orbit of the other which makes this sort of overall elliptical shape in a galaxy like our own milky way which is a spiral galaxy it's quite flat and there's ordered rotation the stars are orbiting in along the same plane and so the galaxy itself we say is rotationally supported so what it looks like is happening in this galaxy is although the outskirts is a nice elliptical when you peer down into the center you've got a component of stars a population of stars that are actually rotating in a disk well at least that's our hypothesis how do we
confirm that well we need to go then from the photometry the brightness of the object to something that actually tells us about the motion of the stars and this is where we take our photons we split them up into the rainbow and we looked at the spectrum we do spectroscopy of the galaxy the reason we do this is that the light coming from the stars in each part of the galaxy will be giving off um sort of fingerprints within those the spectrum of different elements in those stars we know very precisely from laboratory experiments the wavelengths we expect those fingerprints to be at and so if we observe them at a different wavelength we know that's telling us that there's some motion involved that we can trace now the galaxies themselves the universe
itself is expanding so we have to take out that motion but what's left is to look at how those the wavelength of those spectral fingerprints varies across the galaxy and then we can learn about the internal rotation or orbital motions of the stars in the galaxy and that's a pretty amazing thing if you build a spectrograph and you look right in the central 60 parsecs you find something unusual there's a population of stars crammed into that region that are actually rotating in the opposite direction so this galaxy has what we call a counter rotating core so you've got one component of the galaxy going in one direction and you've got this little component in the middle plowing through in exactly the opposite direction so
it's elliptical it's got a disc element in the very center it's got a counter rotating core and in the center of that which we think almost all large galaxies have it has a supermassive black hole about 200 billion times the mass of our sun so when we talk about parts of the galaxy populations of stars within the galaxy that are distinct that are decoupled from the motions of all the other populations of stars in the galaxy one hypothesis that we have is that this is due to some past merging event we think that galaxies by and large are built up by hierarchical merging so little things merge together to form big things swallow up little things and so this is probably a signature that at some point in the past
history of this galaxy there's been some sort of encounter it's hard to say whether it was a major merger a minor merger of something small but essentially it's all detective work and these are all the different elements that we pull together to try to figure out the history of this galaxy as we observe it here and now closest to the sun we're going to have mercury definitely not to scale venus is over here i'm loving the oversized earth and what's really cool about the oversized earth is here you can see australia has been named new holland there definitely needs to be some work done on the moon because the moon like oh just lost two days
