M94 - Rings and Rotation Curves - Deep Sky Videos

all right so this is messier 94 or ngc 4736 it's a rather beautiful looking spiral galaxy about 16 million light years away and it's at the center of a group of about 20 galaxies that we call the messier 94 group it's often called the capsid galaxy but one cool thing about it is that it's got two ring structures so there's a little bit of an inner ring and then there's this huge big outer ring as well now rings are pretty rare for galaxies they're pretty rare structures you don't see that many galaxies with rings the fact that you got two rings is like what except that bubble was burst back in 2009. so there was a paper came out that sort of was like unveiling the nature of m94's outer region and they got really deep imaging on this galaxy so they exposed for 555 minutes which is a huge exposure they showed the fact that they has this huge big dispersed disc outside of what we just saw so the image we just saw was just this central bit in the middle and they're saying actually this bright white bit is what you then detect if you expose it for a really really long time and actually they've said that 25 of the galaxy's mass is actually in that disk as well so there's a huge amount of stars in there for us to sort of like ignore since messier made the list up until 2009. turns out it's also not a ring right when you look at this disc structure you actually find that it's a sort of extended spiral shape and that outer ring that we saw in the first image is actually connected to that much fainter ring on the outskirts ah no rings at all well it's got the inner ring still which are much more common to be fair than these big outer rings the kind of things that like hoax objects has but this one is definitely just spiral structure but this mass that they found in the outer regions want to keep that in mind for later right because the other thing i want to talk about is the mass of this galaxy too so another paper on this galaxy a very popular topic in astronomy this galaxy this was by iwoka and collaborators and it's all about dark matter look how excited you look okay let's talk about dark matter so what they did in this paper is they looked at the rotation curve of this galaxy this is where we look at the speed that the stars are moving at at different distances from the center of the galaxy right so this is what vera rubin did back in the 60s which rotation curve of andromeda for example and that was the very first observational piece of evidence that we had for dark matter but if we think about what this would look like say for the solar system for example right if this is you know distance from sun and we say that this is sort of like the speed that galaxies rotate at some v velocity you know if we think about mercury mercury you know orbits very quickly it takes like 88 days or something right so here's mercury and we get venus at like two two five earth at three six five mars at double that jupiter twelve years saturn 29 you know in the uranus neptune etc right the shape looks like that right ish i joined those dots right so the reason you get that shape in the solar system is because all the mass of the solar system is pretty much concentrated in the sun so that's newton's laws kepler's laws it tells us that as you get further out okay yeah those planets have got further to go in their orbits but they also just move a lot slower in terms of speed as well so if we think about when we look at a galaxy where are the brightest points in galaxies they're in the middle okay so we'd expect a similar shape they've also got a black hole in the center they have got a black hole in the center yeah but a black hole isn't 99 of the galaxy's mass it's still an appreciable amount more than you would think it's maybe something like a percent but it's not the same sort of scale as a solar system but when you do look at a galaxy's rotation curve you don't see that you don't see that shape at all so if i draw it on our solar system shape what vera rubin found way back in the 60s was that it did something like this which is just the complete opposite of this and if we think about what our understanding of gravity is telling us so our best theory of gravity einstein's theory of general relativity if we think about what that tells us of the distribution of mass in a galaxy it means the majority of the mass in a galaxy is on the outskirts not in the middle but we just said that when you look at pictures of galaxies it looks like the brightest point is in the center the brightest point is where there are more stars more stars means more mass so when that was first looked at these rotation curves as people call them that was the first evidence we had for dark matter this idea that there's matter that is invisible to us on the outskirts of galaxies that we can't see there's actually maybe even 10 times the amount of normal matter that you have that makes up stars dust gas and everything else it could just be boulders couldn't it yeah it could be anything so this is the thing so people at first thought thought it was like a combination of all the black holes all the neutron stars all the failed stars all the rogue planets all the dust all the gas but when you make that calculation it's nowhere near enough it's only something like 10 of the invisible matter in the galaxy it could even be those ducks over there it could be ducks there's a whole family of them they're science curious that's why they've come to say hello yeah was that dr becky and so well that dog is getting really just keeps sorry down and going back up again it's all this talk of dark matter they were like no no don't agree with that what are they doing i've never seen docs do this before chill out guys so the other thing that you can do along with sort of saying all right let's rule out black holes and maybe let's you know hunt for a particle that it could be it's to say maybe we've got gravity wrong maybe einstein's theory of general relativity isn't right which is sort of a bold statement to make because everyone's like oh einstein you know einstein was right and in truth like everywhere we've tested general relativity from the solar system to gravitational waves to around black holes it's been perfect but there are some people suggesting well maybe it just needs a little tweak it can stay the same on solar system scales but on the size of a galaxy there's something that comes into play that changes it slightly m94 though is very controversial and that's because its rotation curve doesn't look like all other galaxy's rotation curves in fact it has a rotation curve that suggests there's no dark matter in it at all it's not leveled off in the way that we would have expected it to if there was all this dark matter on the outskirts and obviously you've got different things going on in the middle because you've got the black hole there you've got spiral arms etcetera etcetera but essentially you don't have that leveling off so like newtonian physics could explain this thing and so that's obviously thrown a lot of spanner in the words because not only do you not need dark matter for it you don't need any of these modified gravity theories either none of those can explain this well i've just measured it wrong or missed something so this was my first thought because so this rotation curve if you look it goes out to 10 kiloparsecs kpc right a parsec is about three light years or so so a kiloparsec 3000 light years right but i was like how far is that because this came out in 2008 and that paper we looked at before showing the big ring around the outside came out in 2009 a year later this paper had the picture of the galaxy in it right and handily it does have a scale for us here you notice this this says five and then it has a little dash okay so that means it's five arc minutes across so five arc minutes is an angle one arc minute is a sixtieth of a degree so we're talking about the angular size of something on the sky say this is one arc minute for example something that's here is going to have a different absolute size to something that's here right that's going to be smaller than this one and just by basic trigonometry if we know the distance to that object and we know that angle we can work out how big the thing is right so thankfully we know the distance this from its redshift and so i worked out that at the distance of this galaxy then this line which is five arc seconds is about six and a half kilopass x this line is about three centimeters so that three centimeters six and a half kiloparsecs if we want ten kiloparsecs we need four and a half centimeters right so then i was like okay let's see where four and a half centimeters goes to and i was like would you look at that four and a half centimeters is just that little bit and this was the point that i got very excited because i was like maybe they've not even looked at this bit where there was 25 percent of the mass at all maybe they you know they didn't know about that then and i was like maybe we should be putting in telescope proposals brady we should be writing a new paper about this and then i remembered it's radius and not diameter actually they're going from the very center and they're going out to four and a half centimeters which covers that full diffuse thing and i was like oh that's disappointing like we were onto something there it would have been amazing but then i actually thought well if you look at this image you can see there's lots of gaps that are just there because obviously the spiral arms are shepherding material and so they do leave gaps in there so perhaps these sort of dips and peaks in this rotation curve they do sort of coincide if you measure it with these gaps in the data right so i was thinking well that also depends on how you take this data so the way they get at this data is with what's called a spectra right you know you take the light from the galaxy you split it into it you know prism its rainbow of light and you see the features in that galaxy of the light right usually the way that's done is with a slit so you put literally a slit along your galaxy and you take the light in that little slit and then you can very easily split it and know which part of the galaxy's radius it came from so obviously what data you get therefore depends on where you put that slit but if these sort of dips and everything in the where there isn't any stars are obviously affecting this rotation curve this much this is one of those things that it's not going to be easy to get an average across the whole thing so i'm wondering whether what you should do instead of getting a slit is do what we talked about in this channel before especially mike where you get this different type of spectral data it's called ifu integral field unit where you have this bundle of fiber optic cables that take a spectra in each one and you sort of mosaic or jigsaw piece the galaxy up and then you can get the velocity that the stars are moving all across this entire galaxy so basically you don't believe that the rotation curve is really different i am approaching this with a healthy dose of science skepticism right because every other piece of evidence we have from galaxies and from our simulations just galaxies have to have dark matter and i'm not saying you know there could be you know never say never there could be one that has somehow managed to do it but it looks very similar to all the other ones that we know do have it so yeah i want to get ifu data on this the first thing i did was like maybe somebody's already taken it i searched every ife survey that i knew that was publicly available and i couldn't find it so i still think brady we should put a telescope proposal together i reckon we get really deep really great ifu data on it and we either show yes it's got dark matter because its rotation curves flattens off or we just add more fuel to the controversy pile and be like actually no it has a rotation curve that suggests it doesn't need any dark matter in it at all i'm up for it i'm up for it too let's do it next round of proposals i'll put one in all right i'll ride it yeah okay yeah that would actually be really helpful if you could write it you're not going to have one of these and say features except the m109 is an sc galaxy so it's one of these ones with quite loose spiral arms if we look back at the image again you can see that there's lots of complex structure in there

Channel: DeepSkyVideos

Views: 11,712

Rating: 4.9825454 out of 5

Keywords: astronomy

Id: VsmTl7pn1vY

Length: 11min 25sec (685 seconds)

Published: Sun Oct 25 2020