When Galaxies Collide... - Professor Carolin Crawford

so today's talk is about ciding galaxies and the universe is full of galaxies we estimate there are well over a hundred billion of them that we know about and they show remarkable range in both their colors and their sizes and their masses but quite a striking conformity in terms of their relative shapes you know even through all those ranges of of masses and sizes today we're going to be looking at the exceptions to the rule the about sort of one in a million galaxies that does not conform to one of these shapes it has a very peculiar very Twisted distorted shape and these are the result of the gravitational influence of one Galaxy affecting another and pulling it out of shape tugging into these these these weird morphologies we're going to be looking at lots of examples of these because they are amongst the most spectacular deep Sky objects that you can observe we'll be looking at examples we be looking at the how and the why such systems merge how we study them what their characteristics are and what they can tell us about that the physics of the interaction and also about the wider context of how galaxies change and evolve throughout the history of the universe but before we actually talk about the peculiar galaxies it might actually help just to be clear about what non- peculiar an ordinary Galaxy looks like and they tend to come in two flavors you you have a huge range in terms of the size of galaxies they can be anywhere from like a few thousand light years across to about half a million light years across they come in a huge range of Mass from just a few million times the mass of our sun to tens of trillions times the mass of our sun but despite this range from the dwarf galaxies up to the giant galaxies they tend to come in two morphologies the most common type are the elliptical galaxies you can see one down here and various other examples around these are these are about 60% of all galaxies and they tend to look the same which ever way you look at them they're sort of football or rugby ball shaped and what may vary is kind of how squish they are along one axis but very smooth very symmetrical they also have a very distinct yellow red white color indicative that they're mostly composed of older stars you know uh less massive smaller older stars stars that are you know going up to about 11 billion years old there's not a lot of very current star formation going on we know that because there are no blue colors so old stars and that's mainly because there's very little cold gas in them they've got lots of gas but it's all hot you can observe it in x-rays but there's very little coal gas that goes on to compress to form the stars and these tend to be the more massive galaxies they can be these enormous Giants and they much more massive than most of the spirro galaxies so those are the most common types of galaxies the other ones are the ones we're more familiar with the ones that tend to be in all your coffee table books and astronomy because they are the more beautiful they are not the most common they're only about like 30% or more of of all galaxies and these are the spiral systems and these really do vary according to how you look at them they're like a flat disc with a bulge in the middle in fact I think I've got a sideways view here so like a frisbee you can see it face on or you can see it Edge on and the the analogy is often like two fried eggs stuck back to back and the double egg yolk is a bulge of stars it's like a ball of stars that sits at the center and then you've got the egg white is a flat disc that extends out so in the case of our own Galaxy you know the full extent is about 100,000 light years from one side to the other and this is a Spyro Galaxy and these are rotating systems and you can see they've got very different colors hasn't really come out in this picture but they tend to have very different colors from the ellipticals they show a range of colors that bulge in the middle is yellowy white red it's old Stars the disc however is laced with very current star formation and we can see this because the stars in the disc and particularly along those spiral arms are very blue so blue stars are very massive stars massive stars don't live very long you know over the order of a few t T of millions of years I realize that may sound long but it isn't in astronomy and so if you see blue stars you know that they've been recently formed so there's Active Star formation going on in the disk so you have these knots these clusters of blue stars you also have the pink nebula here's a a zoom in on a on a Galaxy I'll be showing you later where there's a double cluster of stars and the energy from the stars is lighting up the gas cloud around it to produce this pink neb glowing hydrogen gas atoms and those are very clear indications of star formation blue star clusters pink gas clouds remember this it'll be a test later and this star formation is active in the spiral disc because there are waves compression waves that travel through the gas of the disc the disc is one large um flat pancake of cold hydrogen mainly gas and as a density wave travels through it it compresses the gas clouds and triggers them to collapse under Gravity to form Stars so those spirals are very transient features that move around through the old stars and the gas of the disc and stimulate this growth of of um of stars within the disk the other indication of course is that there is dust in the disk I talked about dust about a year ago but to remind you this is formed in the envelopes of cool you know distorted red giant stars stars at the end of their liveses and so the fact there's a lot of dust in the dis shows there have been many generations of massive star formation going on within that dis and so you've got very different star formation histories and colors and appearances between the two types of Galaxy and why you have oh well actually even between spiral galaxies you get quite a range of morphology and it's to do with the relative uh importance of the Bulge relative to the disc so you can see you've got systems the Bulge dominates and you've got quite an inter stick disc and then you move out to much looser fluffier Sparrow galaxies with perhaps a very small bulge component and you can have straightforward spiral galaxies like this and then you can also get a parallel system again from being very tightly wound to much more fluffy where the spiral arms don't go right into the Bulge but they end at each end well they come into each end of a bar shape that straddles the the Bulge so the Bulge is kind of elongated into this bar shape which can be quite small or it can be quite long and so you got different amounts of wildness of spiral galaxies you even have differences in the number of spiral arms that they contain but they still have this sort of intrinsic structure and the difference between an elliptical and a sparrow Galaxy again grossly simplifying but it comes down to rotation even the difference between the different types of spiral galaxies you find that the rate of rotation of the G galaxies varies As you move along this progression between the Bulge dis ratio from bulge dominated to the dis dominated and again how the main characteristics of how we think it happens is you have a Galaxy forms from a large hot gas Halo forms fairly symmetrically you're going to form a small bulge to begin with that's going to form relatively symmetrically but as the gas cloud collapse down any inherent rotation in it is going to get exacerbated it's going to get increased through the conservation of angular momentum so if you have a gas cloud that doesn't have much moment angular momentum so it doesn't have much net rotation to begin with stuff is going to continue to accumulate fairly randomly and you're going to end up with the elliptical shaped Galaxy and the Stars litical shaped Galaxy they they are moving but they're all kind of random orbits around the center compare that to a spiral galaxy you form the buge first but then as it collapses down it spins up rotational speed increases it flattens out and so the subsequent star formation builds in that disc around that small nucleus of the Bulge and so that's your key difference between the two so those are the characteristics of your ordinary galaxies and the main thing you have to know is they're beautifully symmetric there's an inherent symmetry about all these standard Galaxy forms the kind of ones we're looking at today are not okay they don't conform to those two forms they're not very common as I say about one in a million galaxies in the local universe but they are a scrambled mess okay they do not show spiral arms they do not show neat elliptical balls they show a whole random selection of features from both galaxies one feature you may notice as as we go through that they're very blue they've got lots of active star formation going on they've got dust cloud they've got weird tidle Tales of stars that stretch out thousands of light years into space and these began to be cataloged during the sort of middle part of the last century and the most famous catalog of these peculiar galaxies was done by guy called Halton ARP and in fact many of the objects I'm showing you today have an ARP number they come from this catalog and he systematically catalog them he's showing that they've got much Bluer colors they tend to be these very peculiar morphologies and what's very interesting is that he noticed that several of them appear to be in pairs so they are you've got these weird morphologies and two galaxies within a pair will share them and often there's some of this morphologies linking one galaxy to another so that's your first hint that is a reaction of one galaxy to another that's producing these kind of morphologies so just to be clear about this because it's a question I always get asked yes the universe is still expanding that hasn't gone away we you know galaxies are separated by hundreds of thousands millions of light years from each other space is expanding it's pulling galaxies far apart and all of that is going on that's your framework of underlying what's going on but within that expanding space there are Pockets where two galaxies can be close enough to each other that the gravitational attraction they feel for one another out you know outbalances the force that's pushing them apart it's stronger and so the galaxies will move through the space the space is still expanding everything else is moving apart but the galaxies will form Pockets when galaxies come together because they're so close and the gravity is so strong and these kind of what we call exceptions to the you know the more general what we call the Hubble flow that comes out and these are local motions so don't worry this is totally consistent with the expanding Universe they're just pockets of odd behavior that are going on and of course we see this where you've got rich clusters of galaxies hundreds of thousands of galaxies all falling together under Gravity even while different clusters may be moving apart in space space remember these are just very local motions that are going on now of course this happens around our galaxy our galaxy is not isolated in space we've got our we're in a group with about 50 other galaxies we've got two other large spirals there's the Andromeda galaxy and triangulum Galaxy and each of these Galaxies have got little dwarf galaxies that are satellites around them there at least 10 around our own Milky Way the two most prominent of which you can see from the southern night sky if you've been lucky enough to observe it these are the melanic clouds you have the large melanic cloud and the small melanic cloud and that's not a cloud that's a comet it's just a nice picture we have to have a comet in there and these are two little dwarf galaxies they've each got about hundreds of ma mass of our galaxy and they're orbiting our galaxy about once every two billion years and the large Marin alanic cloud is the closer one it's about 170,000 light years away the small melanic ones about 30,000 light years further so they're way out beyond the the Stellar disc but they're still responding to the Dark Matter around our galaxy and the mass of our galaxy even though they're Way Beyond that disc and what's happening is that they're kind of getting disintegrated if we look closely at their morphologies first of all you know they're not ball-shaped they're not elliptical they're not spiral they're completely distorted the next thing you Noti is they're very blue they've got loads of star formation going on and huge amounts of nebula around them particularly the LMC and these little galaxies are getting shredded by the gravity of our galaxy our galaxy dominates so what happens is gradually it's pulling Stars off in fact if you track the path of the melanic clouds you can see it very clearly in radio wave bands here's an all Sky picture of our galaxy that's been somewhat unwrapped you can see the melanic clouds there and there if you look in the radio you find they're trailing a long stream so imagine that looping round the Galaxy of neutral hydrogen gas that's all been stripped out poured out of these galaxies and embedded within that are stars and so what's happening is that these two galaxies as they orbit ours are slowly disintegrating materials they're leaving this huge stream of material in their weight they won't last forever they're going to get completely torn apart by the gravity of our galaxy and they're not the only ones these are the two most obvious in the sky the closest one to us was only discovered in 1994 and it was difficult to detect precisely because it is close it's called sagg or the Sagittarius dwarf elliptical galaxy it's difficult to find because it's so close so it's diffuse and it's spread out of a large part of the sky so see that a carent objects quite difficult it's also on the other side of the center of our galaxy if we're there it's right in the other side and again it's got these fantastic streams of Tails it's again being completely disintegrated and this is quite close this is like a third um of the distance out to the nearest of the melanic clouds it's only 50,000 light years out is really getting torn apart by the Dark Matter Halo of our galaxy and therefore it's a very good seeing what happens to that it's a very good Tracer of the mass distribution in our galaxy and it's leaving these wonderful tidal Tales so it's clear if you got a dwarf Galaxy in or to round a large Galaxy the dwarf Galaxy comes of worse well that's no big surprise really when you think of the relative distribution of the masses however there is an equal effect well not equal but there is an opposite effect on our galaxy the mass of these satellites does provide some interaction with our galaxy and obviously I can't show it in our galaxy but we see it in other galaxies there's a warp a distortion to our disk and you can see it's not this is slightly more exaggerated than we think it is for our galaxy but the dis is not completely flat but it tilts down at one side and up at the other and some of that Distortion is due to these satellite galaxies that orbit around it and also the perations they produce in the dark matter halo around our galaxy so the you know the small galaxies don't affect us much of course there aren't just small galaxies in our neighborhood we have our twin twin sister the andromed Galaxy it's about the same shape the same size the same mass as our own Galaxy you can even see two of its dominant um satellite galaxies in orbit around it and it's so close to us it's only 2 and a half million light years away that we feel its mass and its gravitational pole and it feels an equal and opposite one from us and it's a much more equal match than those poor satellite galaxies and we are moving towards each other speed of about a million kilometers an hour you may think you're sitting still but you are not you're moving very fast through space in the direction of andr Galaxy and it's moving towards us and so the eventual thing is what's going to happen they're going to collide and astronomers are impatient we can't we want to know what happens when you Collide two galaxies we want to model that and we reckon it's going to happen you know a few billion years in the future and the way we investigate all of this is through simulations so I'm now going to show you a neat little simulation about what happens when the Milky Way and Andromeda move together and so close that they start affecting each other quite strongly now obviously You' be glad to know this is speeded up otherwise it be a very very long lecture the the ticket for the time is down here in the bottom left hand corner and this is the Milky Way and remember as you watch this remember you were part of this Milky Way about halfway from from the center to the edge so all these stars in this spinning disc so we're going to start off looking at the Milky Way and in a minute it'll zoom out and you'll see the Andromeda galaxy down in the lower right hand corner okay and Andromeda is going to come in just about now so over time they're going to get pulled close together and you'll see that when the two galaxies meet they don't just go smack together they'll do a little dance and they'll do several little approaches to each other but watch particularly what happens in this first pass as they go close to each other they carry on going but they sploosh out long tails and arcs of stars and they don't escape each other's gravity they get pulled back towards each other on a second pass and then a third pass taking several of those little bounces before they finally come to rest and yes they are going to form some kind of merger they're going to grow a new Galaxy one that does not resemble each of the original spirals it's it's more of a very disturbed eliptical we'll see some lots of end products later but you get a much larger Galaxy from all the matter of those two galaxies combined so this is a much more equal interaction you can see by about 8 billion years in the future we've completely morphed into a different type of system and this is a very good example there a tail left behind why do you have the tail stretched out both directions why are there two tidal taals per Galaxy well that's the key indication is in the word tidal gravity drops gravit gravitational influence depends on mass but it's an inverse Square law so it depends also very sharply with distance if you go twice as far away the gravitational strength drops by a quarter and so you have a tidal fact that gravity changes a lot across the Galaxy let me explain here you've got Li SP Galaxy minding its own business pottering through space if it gets too close to another galaxy oops it feels a gravitational attraction say belong the line that joins them and that deflects it slightly you know it doesn't have to pull it round in this case onto the Galaxy just deflects it slightly the thing is the gravitational field this galaxy experiences from that one varies according with distance so for example the Stars the yellow stars and the near side are going to feel a much stronger gravitational pull than the body of the Galaxy which again feels a stronger gravitational pull than the stars on the far side shown in blue so what happens is the yellow stars get poured more sharply than the Galaxy it gets more poured sharply than the blue stars the blue stars effectively get left behind and you can imagine there effects of rotation in this as well and um other forces combining but this is the the the gist of it and so you have a tidal tail where stuff is Left Behind and a tile tail where stuff is pulled towards the other Galaxy and so the whole galaxy becomes stretched again it depends which way it's rotating compared to the directions of motion as to how far it unravels and so the first indication you see of any merger even when they haven't got that close to each other is this material in spiral arms gets distorted gets poured forward you can see how this one is beginning to unravel this one you've got this large distortion on one of the Spiral arms you start to get these these very obvious asymmetries within the structure even before the Galaxies have got very close to each other and then it just gets worse after that first encounter and then of course the end products as you saw in that simulation they are really unlike the other galaxies here's an example of one that hasn't quite finished merging and you can see it's just basically a scrambled mess you've got the dust clouds this probably was a couple of Spyro Galax this is still going to take several tens if not hundreds of millions of years to finally settle down into a coherent very smooth very organized Galaxy and so you get weird and wonderful end products from these merges so the way we study these is through simulations because you these galaxies are moving so slowly compared to human lifetimes you can't see the changes in an interaction all you have are the different snapshots but we can model what happen happens in simulations and then compare it to the the snapshots that we see around in space and from these I mean the obvious parameters that you can see that are going to affect how badly one Galaxy is affected by its interaction with another are it's due to a number of factors but the key ones to consider are obviously the types of galaxies involved spirals are very vulnerable you've got those loose fluffy very extended um discs of stars and gas that are very vulnerable to being torn off much more so than the elliptical galaxies in fact you'll see nearly every simulation I show you today involves a spiral galaxy that's because they're very good traces of what's going on because you got those visible components that get sprayed out everywhere if you merge an elliptical galaxy with elliptical galaxy you get an elliptical galaxy and nothing very exciting happens in simulation spirals you can trace what's happening to the matter much more clearly and so and and spirals as I say they're much more vulnerable you get these wonderful shapes that you you um will be seeing they're relative masses obviously you can see this from the Milky Way the satellite galaxies are not affecting us very much however Andromeda is really going to mess us up so again a very equal match of galaxies affects both galaxies equally uh a much different Mass perhaps really disrupts one and leaves the other slightly Disturbed how fast they're moving towards each other is important you can have two galaxies that get very close to each other and they're very equal Mass but they won't necessarily have much of a disturbance than the other if they're moving fast and the key thing here is the galaxies are going to move by each other slowly enough that the stars in one will recognize the gravitational influence of the other and have time to respond to it before the Galaxy moves on so you know they've got to be fairly slowly moving so for example clusters you might think are a great place for Galaxy intera actions to take place but in fact the galaxies is kind of whizzing by each other too fast and it's not a strong place for gravitational interactions so two galaxies moving relatively slowly to each other is good fast is bad it also depends how close they get remember gravity is an in inverse Square law the closer you are the stronger the gravitational disturbance and as I mentioned you know the relative directions about whether it's um moving the same it's rotating the same direction it approaches the Galaxy or in the opposite direction as to how badly it gets unraveled with the spiral arms One Direction produces spiral arms and you know Beauty other one barely shows any and all of these are done through simulations but just to show you a few examples here's one where the mass is very unevenly balanced this is the world poool Galaxy and it's got a companion that's currently lying way behind it and about 500 million years ago that companion came out from behind the screen looped up the far side of this disc did a big trajectory and then about 50 million years ago plunged through here this side and is now lying way behind the spal Galaxy and you can see how it's it's kind of pulled the the arms of the M51 the big Whirlpool Galaxy slightly out of skew but they're slightly distorted a wider view shows that there is matter still sprayed around most of it originating from this smaller Galaxy though there are other tals from the main Galaxy similarly here this is where a small galaxy has got slightly closer to a bigger one the interlop is just off off camera down here but it's plunged right through a dis of an ordinary spiral galaxy and has pulled a whole stream of matter behind it which is now condensing to form lots of stars so again you get different effects so not all of these end up looking anything like that Milky Way Andromeda Collision I showed you it depends on how fast they moving all these other things they may not even merge one can just pass by and really wreck another one as it goes and the simulations this was pioneered in the early 1970s by the brothers tum tumay who did very simplistic calculations but did the the first pass about how you generate these tial Tales how one spal Galaxy affects another now you have incredibly computer heavy end body simulations where you perhaps model each you know you can't model every individual star you have to have different point you represent a Galaxy by a collection of points where each point is several millions of stars and you step it forward in time where you kind of work at the gravitational influence affecting each point in your your maps of the two galaxies and how it's going to change their velocity their acceleration their position and you step it forward in minute sections of time and here are some visualizations of such simulations and comparing them to some of the snapshots we see in the sky now these are ones that are just showing the Stars they will of course involve the dark matter the dark matter is most of the mass of a galaxy it dominates what goes on within a gravitational Direction but we can't see it but we Trace what happens to the visible components the stars in the galaxy and that tells us whether we're getting the Dark Matter distribution around the galaxies approximately right because it would produce very different examples than the ones we're seeing and from these simulations we can experiment with you know what influences the outcome of an interaction it also tells us something about the progenitors of these merging systems that we see it tells us about their type their equal masses or not their rotational speed it tells us about that it tells about the physics of the interaction it also shows how the internal properties of each Galaxy are quite badly affected and it also helps us identify what are the end products of such um simulations and it means as I say we can then look at these snapshots and we can put them all in contact about whether it's a first or a second pass or a third pass and we can do comparisons of you know merger history in the local universe and then much further out in time so here for example a whole host of fantastic images captured by the Hubble Space Telescope and so you can see if you start simulating these there are plenty of things to go out and try and match and detail C so the stages are obviously first pass two galaxies haven't splooshed through each other yet they get they're approaching you've got this mild a symmetry where certainly this one's developing you you're leaving stars behind to form one title tail another one stretching out this is less affected so that's your F so you begin to recognize when they're getting close to each other when they've done one sploosh through each other and thrown out those tidal Tails this is known as the mice okay you nice names for these and you know simulations can match this quite exactly with one face on one Edge on spiral fairly equal Mass because they've had an equal and opposite effect on each other and you can get quite well matched what you're seeing in terms of your simulation which suggests that perhaps we're understanding what's going on in these interactions and it can tell us that in you know in a few hundred million years how perhaps that's going to merge and what the the uh the outcome would be and then as I say you can see all kinds of weird and wacky end products out there the thing about the end products again it hasn't quite come out in this picture you remember I said they were very blue the characteristic of all of these merging systems is there's lots of current star formation and when two galaxies come close to each other it's not the case that the stars collide with each other remember stars are tiny compared to the distances that separate them so if two galaxies come together you won't get a collision of stars from one against stars from another except incredibly rarely mostly the Stars pass by each other and are completely unaffected the difference is the're gas clouds remember there's an interstellar medium gas that fills the space between the stars and that can't gas clouds in one Galaxy can't avoid the gas clouds in another galaxy and they will meet and they will squeeze each other they'll compress each other and they'll get squeezed and this is the key thing if you increase the density of a gas cloud of a certain temperature you're going to make it much more likely to collapse into stars and so it's this interaction of two interstellar medium that prompt this vigorous star formation that you begin to see by the time you get maybe to that third encounter when the galaxies are combining so here's a very famous example of that this is the antenna galaxies here you got the the head with the eyes and then the long an Tenny of the insect and if I zoom in just here again with the Hubble Space Telescope telescope you will see fantastic what we call Starburst activity huge clouds of pink glass large clusters of really massive blue stars again showing where the gas clouds have been compressed and then are generating this new Starburst phenomena very transient feature within the merging of the the systems so you can see that you don't just alter the shape of a galaxy you increase its mass as you grow a new Galaxy you also generate huge amounts of star formation which affect again the internal mechanics of the Galaxy not just that remember any big spiral galaxy worth its salt contains a super massive black hole at its core probably a dormant one like our galaxy now if you model as this is doing instead not the stars but the gas within the galaxies you can see how that is redirected and sometimes you get Long Bar instabilities that develop around the nucleus of the Galaxy as they're doing here and that's a way of driving material down into the core and if there's a black hole at the center onto that black hole and as this simulation is trying to demonstrate that now becomes active and so a merger two galaxies is one way of waking up the black hole and perhaps stimulating new activity and if there's a black hole in each Galaxy both of those may get active they will as the two galaxies merge we figure the two black holes will also merge to form a bigger much more active black hole at the center of the very disturbed Galaxy and here you can see it's just like it's got so much radiation pressure it's pushing a lot of the gas away so remember this is just modeling the gas you're not seeing the galaxies in modeling the gas you have to do um you have to use different codes that takeen um things like Smooth particle hydrodynamics a slightly different technique but again very you're modeling all the physics that's going on within the gas clouds and so there's this idea that a merger of two galaxies can reawaken an even bigger black hole temporarily at the core of the new Galaxy formed and indeed when you look at many of these systems and if the birth of infrared astronomy or you know the growth and development of infrared astronomy in the 1980s found a huge population of galaxies that they called ultraluminous infrared galaxies stonkingly bright galaxies in the infrared suggest and if you looked at them in detail they all have Disturbed morphologies or they impairs they seem to be a clear association with interacting galaxies now be clear the association goes one way you look at these ultraluminous galaxies they tend to be look like they got all the characteristics of interacting galaxies not all interacting galaxies show this huge burst of Luminosity so be just be clear that you know some of the merges produce this some merges don't and it could be due to the star burst at the core it could also be due to pairing up of the active nucleus at the core the trouble is as you can see there's so much dust and obscuration really heavy obscuration we can't tell even with x-ray telescopes really which which is dominating the power output of these galaxies it's not yet determined whether it's going to be the active nucleus or the starburst but it's clear there's a lot of activity in the core and there are suggestions when you look at quazar so these are active super massive black holes and galaxies I can be telling you more about these next year and these are further back in time but again if you look at the host Galaxy there are a lot of distortions that are indicative perhaps of these features that I've been showing you characteristic of merging systems here so it's a way perhaps to reawaken the active black hole at the core and it's interesting that whenever you see a Galaxy that does not conform to your standard spiral elliptical morphology so you can reckon it's been caused by an interaction so here for example is an interesting spiral galaxy until you look at it closely compare it to a normal spiral galaxy in a normal spiral galaxy the spiral arms Trail the direction of rotation so look at this galaxy for a minute work out in your mind whether it's going clockwise or anticlockwise let going that way around then you look at this Galaxy and you can see the external arms trail that way so you'd expect it to be going around in that direction except the inside of the Galax is actually going that way it's got outer spiral arms that instead of trailing the direction rotation seem to precede it and so it's got a very disturbed kinematics where it changes halfway through you can again from the simulations that you do you can mimic such very strange kinematics that are going on within a spal Galaxy and the behavior of it in its disc from it having absorbed material from a smaller companion other examples exist this is known as the black eye Galaxy huge amounts of dust clouds that are lopsided in the Galaxy it looks like they've been input from another system and not just that if we zoom in around the center it has again got different kinematics and the center of the Galaxy is going one way compared to the outskirts of the Galaxy it's had a huge disturbance changing its rotational motion and along that Shear between the two systems you get gas clouds compressing you get lots of regions of active star formation around the center there are other things that result when you've got a head-on collision this is the cartwheel Galaxy and this was originally something like our SP our own spiral Milky Way galaxy and it's got an Invader that's just kind of plunged right through the core of the Galaxy as it does so it sets up a compressive wave that ripples out from the center and grows further I mean this is is now about 150,000 light years across and as it does so it pushes gas the shock wave pushes gas out in front of it compresses it and you've got this ring of very Brilliant Blue Star clusters recent Star formation the and you've got the remnants of The Spar Galaxy left in the core and indeed the interloper is not one of these two it's a little bit further if you Trail now in in radio there's neutral hydrogen trail that kind of identifies this one as the culprit doing the damage to this galaxy but a head on Smash is rare other examples like this you might think this is similar it's resembles the last one I just showed you and it could well be the remnant of a spiral with a Rippling ring of star formation actually what I always like about this one is if you look through the Gap here you've got another one that looks exactly the same in the background which is quite cool it could also be that this is something we're seeing PLL on which results from an interaction between a spiral and an elliptical sometimes you got small spiral going around an elliptical especially if it comes in around the poles of the elliptical galaxy you get something called a polar ring Galaxy so here for example we cannot identify the culprit that's produced this system what it seems likely is the ring is what's left of the ental oper and if we see other polar in galaxies where you have these beautiful Rings you can see how that comes in front and then goes behind this Disturbed elliptical galaxy in the same here and so maybe the other one was just a poleon version of those so these are examples now where you got a small elliptical and a spiral galaxy as I say merging elliptical with an elliptical you tend to not be able to determine really how different it looks from the original you can identify where elliptical are swallowed as spiral though because they show features you don't associate with galaxies so for example if it's got dust clouds in it you don't have dust particles within an elliptical galaxy unless like this object it swallowed a spiral in the past and you've got all these dust clouds now distributed and absorbed within the uh the the um elliptical galaxy additionally if we look just slightly further out I'm not sure this will come out you can see there are long arcs of material and shells again suggesting materials being sprayed out from that spiral galaxy and perhaps even the elliptical galaxy in the process there's another example sen nearest active galaxy to us very red spir um elliptical galaxy it's obviously absorbed some kind of spiral because you got all those dust clouds from the spiral moving around the center forming this thick band and along there where there's interaction again zooming with the Hubble Space Telescope clear star formation taking place but this still has an effect on the elliptical that's very subtle I'm willing to bet you can't see in this image you can only see if you subtract out the light of the elliptical galaxy it's a process called unsharp masking it brings out the faint um structures but you can see there are ripples concentric ripples around the elliptical galaxy these shells are again density shock waves so something comes in and it produces splashes and quite you see these round elliptical galaxies and it's you know the the wave moves out and some of these kind of it goes out one way then the other and you've got a kind of sloshing process and every time it turns around it leaves a little compression of slightly bluish Stars forming and it's kind of interleaved working out from the center and so this kind of thing suggests the elliptical has undergone an interaction you've got a case here there's another case here and even in this system where elliptical are showing these very these are very faint surface brightness features I'll just zoom in on this one just enhance it a little bit there are think it's doing the dance of the seven veils it looks very very elegant but again these are the you got to look for much more subtle features and we can reproduce these in the simulations but they're quite difficult to observe of course galaxies don't just come in pairs they come in groups of galaxies you know our own Galaxy isn't just interacting with the Andromeda but you've got the other spiral galaxy and the other satellite galaxies sometimes you've get groups of three or more G galaxies such as here where this is relatively unaffected this is the Leo triplet this is getting slightly distended but this is an edge on spiral you can see the disc has got kind of warped and thickened and again I'm not sure if it comes out but there's a long tidal tail of material already here suggesting there's been an interaction with another object already I'll just show that in a slightly different contrast there so you start looking hard you see these systems everywhere and these compact groups of galaxies are very good places to look for interaction features how about this one this is what is left of a spiral galaxy that's being tugged apart by these two giant elliptical and it's just getting completely shredded in the process very soon there won't be very much left of it and you can see how that matter will probably accumulate into this Galaxy and produce those dark dust clouds we saw in the other elliptical galaxies and then of course you get compact groups now group of galaxies like our own local group the galaxies are separated they occupy a volume of about a few million light years across compact groups is where you have smaller galaxies that are perhaps within a volume that's a few 100,000 light years so not that much bigger than our own Milky Way galaxies and this is prime place for these interactions to occur because the galaxies are very close to each other and they're moving quite slowly relative to each other and so so they are fantastic Laboratories for looking at interaction processes here for example all these uh galaxies are occupy volume that's probably about the same diameter as our Milky Way galaxy so less than 100,000 light years you got two spiral galaxies merging here another little one here perhaps the remains of one here a long chain down to the southern component or other systems such as um seaf fo sexet which is actually a bit of a cheat this is a background Galaxy that's just it's very confusing you know originally when he observed it he thought it was a sexat but this is actually way further in the background this Spyro galaxy has got a slight warp you can see this again is slightly distorted maybe it's it's an elliptical galaxy perhaps some shell structures it's absorbed dust Lane and these two galaxies obviously you know distorted this isn't a Galaxy okay the sixth member isn't even a gy Galaxy it's a big cloud of matter that's been pulled off the other galaxies in the tidal interaction so you can see you can get really big features growing from these structures out beyond the galaxies and separated from them here's another case this is stepan's quintet again slight Moma this one is about you know seven times closer than the others but these are all showing the effect of another galaxy which we think has kind of gone past and is now off screen this is relatively undisturbed we see how SP spirals got pulled out these two galaxies are combining and look at these arcs of star clusters and tidal Tales way out into space and particularly look down here you begin to see the next phase of what I'm going to talk about which is the stars that form within these tidal tals then get left behind and maybe start the growth of new galaxies so you're not just growing the giant galaxies that form from the merges of two you grow new dwarf galaxies there is a problem though with these compact groups they are such good Laboratories for interactions because they're so close but they should all interact quite quickly and we have a slight mismatch between all the compact groups we see in the nearby Universe if as these simulations suggests they all interact with each other very efficiently and very quickly we actually shouldn't get that many compact groups around us and in particular we should see lots of remnants of them having merged and there's argument about what you look for for the remnant of a compact group that's merged obviously you're looking I mean the guideline is for an isolated elliptical galaxy so an elliptical galaxy with no friends because it's basically swallowed them all and these are relatively hard to find and you need some other characteristic about them you need a giant elliptical with a deficit of neighbors and then perhaps with other properties that suggest that there was it was once in a more called a group environment there have been these are called fossil groups and there have been very few discovered so far in fact far few they need' expect given the number of compact groups around here's just one example and what you're just seeing here is you've got an optical image showing an isolated elliptical with no friends the the X-ray is shown in blue x-ray traces hot gas it traces more the kind of cluster properties and this is just a weirdo that you've got a giant elliptical surrounded by a hot x-ray Hao where the Luminosity the temperature is more indicative of a group or a cluster of galaxies than an isolated elliptical so this is a potential fossil member but there is this this big contradiction between what we observe happening in the simulations of groups compared to the lack of end products we see around in the numbers that we would expect so e you've got various get out Clauses it could be that maybe uh these compact groups are relatively recent things they only form a fairly low density area of the universe they take a while to get their act together and before they get to the stage of stage of merging it could be that these are they're continually replenished by infalling galaxies becoming part of the groups or it could be something like our simulations are wrong perhaps there's more Dark Matter around these groups you know maybe they share instead of you know having Dark Matter Halos around each Galaxy maybe they've got a more common Dark Matter Halo that slows down the interaction speed far over what we're mod in and that could account for their longevity so there's a lot of work to be done tood the to understand these particular compact groups then just to go back to Galaxy formation here's the tadpole Galaxy big Spyro Galaxy with here is the remains of the interloper that's produced this kind of vertical Distortion of the Spiral arms left a complete mess in its wake here and this tail is about 300,000 light years long these are not small structures and far away from the Galaxy you can see knots here of blue star clusters because obviously with you don't just pull out these tails of neutral gas there are compression waves that travel through them or they there are clumps that are denser than other start to collapse to form stars and if we zoom in here you can see knots of star clusters containing millions of stars these are giant star clusters and what's going to happen is most of that tail will eventually evaporate leaving behind those star clusters okay so like it true tadpole it's going to lose its tail right and it's going to leave these little knots of star clusters in its wake which are going to develop into new dwarf galaxies and we see it here in the system we saw it beginning to happen in stepan quinten that little pocket of stars just off to one side in interstellar space Here's another example biggest spiral galaxies five it's over 500,000 Li is across it's kind of being unraveled completely by this interloper lots of blue stars along here if we move to the the ultraviolet you can see there you know perhaps a new dwarf Galaxy right at the extreme of that tail where most of the gas disappeared so we're growing new dwarf satellite galaxies within this process and here's just you know one one of you know my favorite amateur telescope targets m81 and M82 big spirals one Edge on one more inclined to us you can see that m81 is slightly distorted in terms of the Spiral arms M82 off to the side clearly got a problem okay huge starb activity these big flows of hydrogen gas coming out of the center that's really getting disturbed by the proximity of m81 and there's a lot of matter in between them there even though it's not clear from the optical from the radio they're sharing streams of gas being pulled off and Knots of gas lying between them and you can begin to see some of the effects against going into slightly Bluer image there's this big loop known as ARS Loop of star clusters there's a big group of star clusters here again just zooming into an UltraViolet view that picks up the blue stars you can see a new satellite Galaxy lots of knots of stars there I'm just flipping that round on its side and identifying just one small patch you can see individual knots of blue stars masses of stars forming out in interstellar space soorry Intergalactic space even between the galaxies don't belong to either galaxies and these are going to come together and form new dwarf galaxies and in the same way over here there's an object called homog 9 which is now a satellite Galaxy of m81 but it looks like this not unlike the melanic clouds and it's close to one of those radio clumps so it looks like it's been formed in this interaction so my last point to make of course is that mergers as I've shown you can change the the whole nature of a g you can add to its mass you can trigger huge amounts of star formation you can change its morphology you can you can change galaxies immensely through this this process but merging is very rare in the nearby Universe what if it was more common in the past we think most galaxies formed within the first few hundred million years of the since the big bang and most of them formed all at once but then structure grows herar I can't say hierarchically you know that you form the galaxies but galaxies merge together to form groups and then perhaps small clusters which merge to form Rich clusters of galaxies that's our current way of thinking that um that structure builds up but perhaps mergers were incredibly common at that very early stage of the universe when you got lots of smaller galaxies and this merging process that we see so clearly in the Newby Universe can help us work out this early stages of Galaxy formation and evolution if the merger rate was a lot bigger in the past maybe this is a good way to have built up to today's galaxies that we see so one thing you do because these galaxies are so far away light takes an appreciable time to travel between us and them they're billions of years away if you look into very deep images you see very faint distant objects you're seeing them as they were as their very earliest galaxies and this is like one of the deepest pictures of galaxies in our universe that we have invisible if you go really down to the very furthest galaxies and look at their structure what you'll notice is a lot of them are Blobby okay highly technical term that but um and some sense some people even call these like Lego galaxies so the fact that you're beginning to see obviously not def faint diffuse features like tidal Tales but this multiple structure and you compare it to some of the things we're seeing you know in you know in very isolated circumstances well I mean there just aren't that many of them the nearby Universe maybe this is indicating that this process we're studying now can actually shed a lot of light in those first phases of Galaxy formation and evolution and have a much wider contextual information for how galaxies form and develop through the ede of the universe okay that's the end of colliding galaxies there are of course many more that I would have loved to have shown you and couldn't possibly pack into 55 minutes I will just put my last slide is a fly because this is my last of this year's talks but just for future notes the program will be available um printer program will be available come sort of more July time but just um to give you my dates to the next term thank [Applause] [Applause] you

Channel: Gresham College

Views: 620,727

Keywords: Astronomy, Space, Galaxies, Science, Physics, Astrophysics, Hubble telescope, Cambridge, Universe

Id: 7jPN-uFFTnc

Length: 55min 23sec (3323 seconds)

Published: Wed Apr 24 2013