The Milky Way's Shell Structure Reveals the Time of a Radial Collision

Introduction hello my name is tom donlon from rensselaer polytechnic institute and i'm excited to share with you today our work about a dwarf galaxy collision with the milky way three billion years ago other people who worked with me on this were dr heidi jo newberg at rensselaer polytechnic institute dr robin sanderson at the university of pennsylvania and dr lawrence woodrow at queen's university What is a Radial Collision so today i'm going to be talking about our work that came out in a recently published paper this year and it talks about what we call the virgo radial merger which is a head-on collision between the milky way our galaxy and a smaller dwarf galaxy and this collision happened about three billion years ago so what happened was this smaller galaxy fell straight into the center of the milky way and it collided with it and the smaller galaxy was completely torn apart by the strong gravity in the center of the milky way so all the stars kind of exploded outwards from what used to be this dwarf galaxy and then eventually they were pulled back by the milky way's gravity and then they turned around to fall right back in and as they turned around they're going to form what we call shell structure which look like these big spherical umbrellas in the galaxy and then as they fall back in they're going to pass through the galaxy again and then they're going to pop out on the other side and make another shell structure and then that process will come will continue over and over again until eventually you have so many shells that you can no longer tell one shell from another and by tracing these shells backwards in time we're able to actually compare with what we see in the milky way and find what time this type of collision happened and we find that it happened three billion years ago and so i'm going to talk to you more about shell structures what about the virgo radio merger and what it is and how do we know that this actually happened three billion years ago Star Formation in the Milky Way so most of the stars that are in the milky way today didn't actually form in the milky way or at least stars that are in the outer portions of the milky way a lot of stars were formed in smaller galaxies that were way out on the outskirts of the milky way called dwarf galaxies and these dwarf galaxies were then eventually pulled by the milky way strong gravity and they fell into the milky way and a lot of the times they formed what we call stellar streams so here i have a video of a simulation of a dwarf galaxy in pink falling into the milky way in white and as the dwarf galaxy falls into the milky way stronger gravity it gets stretched out because the stars in that are closer to the milky way are pulled more by the milky way and the stars that are farther away are pulled less and that's actually the same process that creates tides on earth and there are many tidal streams in the milky way lots of them are fairly small but the process that generates these structures is pretty well understood and they're very good uh tools that we can use in order to determine what the shape of the milky way is or where things like dark matter is distributed in the milky way so title streams are are a nice tool for understanding the milky way but we have another type of collision event that we are talking The Virgo Over Density about here today in order to do that we have to go into what we call the virgo over density and the vertical over density is just a region of the milky way that has more stars than expected and we've known about the virgo density for a couple of decades now and what they kept seeing was that there were stars that were moving away from the milky way and there was also stars that are moving towards the milky way so what you see in the plot here is on the vertical axis you have the number of stars that are moving with some velocity and on the bottom you have velocity on the left that is negative or moving towards us and velocity on the right that is positive or moving away from us and this is all stars within the virgo over density in a simulation that we made that are within some distance from the sun and so what you see is that there's two big peaks here at about there's one at minus 150 kilometers per second and one at plus 150 kilometers per second so you see that this virgo over density has two main clusters of stars that are moving in two different directions typically when you see this you think that there must be multiple structures involved there must be two title streams here because when you have stars that are all in a tidal stream they all move together you wouldn't expect to see stars moving in different directions in one tidal stream and so they thought oh this must be two streams in the virgo density but actually just last year uh we were able to show that the virgo over density could be formed from a single dwarf galaxy that collided with the milky way and what happens is that as that single dwarf galaxy falls in it falls in directly into the center of the milky way and so it turns it goes through and then it will go some distance turn around and come back and instead of getting this stream that's extended on the sky you just get a cluster of stars that are within a very small area but some are moving out in order to start to turn around again and some are moving back in because they've already gone and turned around and uh collision events like that will actually create what are called shell structures so you Shell Structures can go talk about those a little bit when dwarf galaxies pass very close to the center of larger galaxies they're going to make these shells which are these big either umbrella-like things that you see there in the galaxy on the right or just big like spherical distributions of stars and we see these in other galaxies in the universe right typically they're very prominent shell structures that we think oh we don't see those at all in the milky way so we wouldn't have expected to get a radio merger like this before however now that we have this evidence that says that the virgo radial merger is in the milky way and that it passed very close to the center of the milky way then maybe it did in fact make shells and maybe we can find them and so we ended up looking and we only considered stars that were at their turnaround points so the stars that would actually be on shells as they went down from the center of the galaxy and they turned around you can isolate the stars that are turning around and then you can pull out out of all the stars in milky way only the stars that should be in these structures and doing that we were able to find four shells in the milky way two were in that virgo density that we talked about and there were two on the other side of the galaxy in a region called the hercules aquila cloud and so if you look on the on the picture here on the right you can see that there's usually big strong shells that are on either side of the galaxy and that's the same in the milky way there's a shell structure in the virgo over density on one side and the hercules aquila cloud on the other Pendulums so shell structure acts very similarly physically to a group of pendulums with different lengths and in this video here i'm going to show you what happens if you if you start out with some pendulums and so the pendulums are going to be our stars here and we're going to start them all out at the same position on the same side of the galaxy right they're going to start on the left and then they're going to pass through the galaxy in the center and go the other side on the right and all these pendulums are going to be moving very closely together initially and as time goes on they're going to be moving different uh they're going to be moving farther and farther apart and becoming more mixed over time and so initially you're going to be able to see that there are collections of these pendulums on either side of well the galaxy but as time goes on you're going to not be able to really follow where the collections of pendulums are and that's going to be this mixing that we talk about but so as you let it let the pendulums fall through the galaxy the first time you see that there are collections of pendulums that are on their turnaround point and there's also pendulums that are coming out to start to turn around and there's pendulums that are already falling back in towards the middle and coming out to the other side so the pendulums that are on the outside and are turning around are analogous to the shells that we see in the milky way whereas the stars that are falling inwards are more like the pendulums that are in the middle and right now you only see one shell on that right side but as you let the video go it's going to turn into two shells as it goes again and then it's going to become three and then it's going to continue to mix until suddenly there's going to be so many shells that now you can't really tell if a pendulum is in a shell or not and that's what we think happens in the milky way as well so we use computer modeling on the shells that we saw in the milky way and we kind of ran them backwards in time like we could do with these pendulums here and we found that they all lined up at one nice time 2.7 billion years ago and we say that is probably when the dwarf galaxy fell into the milky way and so we said the virgo radial merger or the collision of that event happened about 2.7 billion years ago Simulation so what do shells look like in the milky way actually because we saw that they looked very nice in um in a big spherical galaxy that we could see outside of the milky way but the milky way has this big disk of stars which is where the sun is and the disc is going to change the way that the shells look so what i have here is a simulation of stars in the milky way that i made and it's going to start out with a nice dwarf galaxy and you can see that in those bright white stars on the right is this dwarf galaxy that's falling into the milky way which is uh the galaxy that's in the center of the image and it's going to pass through very quickly and it's going to go on the other side and it's going to turn around and it's going to oscillate back and forth kind of like a pendulum what you're going to see is that things get mixed very quickly and it'll pause at one point and show you about the stars that are at their turnaround points which we would say are in shells and it's not going to look like what you would expect from the nice spherical galaxy you were looking at earlier in ngc 474. right so now we can see that there are in fact shells in the milky way and the dwarf galaxy that fell in however they're a lot messier they're not quite as nice and spherical they don't look quite as umbrella like and so that's probably the reason that no one found shelves for so long is because they don't look as nice they're not as prominent as we would have thought and this is about two and a half billion years into the simulation which is about the time that we expect the milky way to look like right now and if we keep running the simulation we're going to see that these shells are going to mix over time very rapidly and then you're not going to be able to see them anymore after some point and we find that that point happens about five billion years into simulation and so the the virgo radio merger can't really be any older than like five billion years because the shelves would be too mixed and we see clearly not very mixed shells in the milky way so those two things don't match up and the virgo radio merger just can't be that old and so now you see in the simulation that the stars are very well mixed and you can't really pull out one shelf from another there so before i talk a little bit about the effects of the virgo radio merger there is a another major collision in the milky way called the gaia sausage and the gaia sausage was a collision between the milky way and another major galaxy called the sausage galaxy and it's thought to have happened about 8 to 11 billion years ago and people have been saying that this is the last major merger of the milky way or this is the really only time that the milky way has collided with another large galaxy like this and last year we showed that the virgo radial merger and the gaia sausage actually produced the same signature structure of stars around the sun and that you could actually produce the effects that we're seeing in the data from either the virgo radio merger or the gaia sausage and so it's possible that either the gaia sausage and the virgo radio merger are in fact the same thing in which case this gaia sausage can't be actually 8 to 11 billion years old because we just showed that from the shell structure in the milky way you can only be about 5 billion years old at max and we actually found a time of 2.7 billion years ago in our simulations also either the other option in that problem is that the virgo radio merger and the guy sausage are actually not the same thing and that there is the old gaia sausage that did impact the milky way eight to eleven billion years ago and then the virgo radio merger happened much much later than that much more recent in the history of the milky way and that the two are actually unrelated and different uh collision events in the milky way's history but so uh going back to the effects of the virgo radial merger think about if you're standing uh at a pond and you toss a rock in then the rock's going to create a little bit of a splash in the surface of the pond and then that splash will create ripples that go out on the surface of the water and the same thing's actually going to happen in the milky way so if you toss or toss i guess a dwarf galaxy into another larger galaxy then you're going to get a splash it's going to it's going to toss stars out of the center of the large galaxy and uh that is actually something that other scientists have found they called it the biggest splash and it is a collection of stars that could possibly have been thrown out of the milky way center a long time ago and that could possibly be because of the virgo radio merger also scientists have found that there are in fact ripples in the milky way's disk so in the image on the right that's what you see is that the milky way isn't flat the spiral arms are uh also have ripples that go up and down as you move outwards in the galaxy and these might be in part due to the virgo radio merger and the collision between the dwarf galaxy and the larger galaxy and finally uh you would expect that the dwarf galaxy for the virgo radio merger would have some other gas in it just like our milky way has gas that is around and forms stars and as you have a dwarf galaxy that collides with the milky way the gas is all going to collide and then the gas in the milky way will be compressed in order to form new stars and there's actually evidence of lots of new star formation at the time that we suspect that this dwarf galaxy collided with the milky way which is more evidence that supports that in fact this merger event did happen three billion years ago and so to sum up there was a pre there Summary we have identified a previously unknown head-on collision between a dwarf galaxy and the milky way and we call this the virgo radial merger and this this head-on collision is going to create shell structure which we've identified in the milky way for the first time no one else has found this before and this shell structure is going to tell us that the collision happened three billion years ago partially because we were able to rewind the shells like how we could see how pendulums move over time and also because over five billion years ago you wouldn't be able to see shells or if the collision happened over five billion years ago you wouldn't be able to see shells and milky way anymore however we do see shells and so we know that this collision has to be more recent than that and the virgo radio merger is also potentially responsible for other interesting things in milky way such as new star formation or ripples in the disk for more information please see the paper which is domino 2020 published in the astrophysical journal or you can see rensselaer's press release for october 20th 2020 and just to get into the funding information for this work this work uses data from the esa's gaia data release 2 as well as the sloan digital sky survey and the lemos project it's also supported by a couple different nsf grants and plenty of individual contributions and contributions from the 2015 crowdfunding campaign from milky way research so um thank you for all the people who help support this work and thank you for listening to the talk today and have a good day