A Stream of Stars (Abell 1795) - Deep Sky Videos

0:00 I have Abel 1795 which is this collection

0:04 of little faint fuzzy things in the middle

0:11 here so the center of it is that relatively big bright blob and but then

0:15 actually most of these little orange slightly fuzzy dots you can see are what

0:18 make up a 1795 it is a cluster of galaxies so each of those little

0:23 fuzzy dots is a whole galaxy and it's an enormous distance away it's about

0:28 a billion light years away amazing isn't

0:30 it just you know this little faint smattering

0:33 doesn't look like anything very much but that's

0:34 thousands of galaxies you're looking at there

0:37 all clustered around this one big bright galaxy in the middle there's got to be

0:40 stuff living in there you would certainly

0:42 think so wouldn't you that many galaxies

0:44 each with that many stars we don't live in a rich cluster we live

0:47 in a poor group so most galaxies don't

0:48 live in complete isolation few do but actually

0:51 most live in groups so maybe four five 10 20 galaxies the the rare things

0:56 are where you get thousands of galaxies all in one of these clusters the AEL

0:59 C was put together in the 1950s by George Abel there was a survey

1:04 of the sky had been put together I think called the Palomar Sky survey and he

1:07 literally went through all the photographic plates

1:09 by eye identifying objects like this I think

1:12 in his original catalog that's getting on for 3,000 of these clusters that he

1:16 found all across the sky or the northern sky because the the survey he had

1:19 actually only covered the northern sky so

1:21 they're all Galaxy clusters all clusters of galaxies

1:24 there was an extension called the Abel

1:26 Corwin uh catalog which did the southern hemisphere

1:29 as well so we now actually have these clusters all across the sky so if

1:32 you want a really long set of deep sky videos you know there's like two

1:36 or 3 thousand of these we can do and then you could do each Galaxy

1:39 within the cluster absolutely yeah all right so

1:42 here's a nice paper called mmt FH Alpha

1:45 and HST fuv Imaging of the filamentary complex in Abel 1795 this is kind

1:50 of a zoom in of the picture I showed you before so actually the orangey stuff

1:54 you can see here is exactly the same as the orangy stuff we were looking

1:57 at before but kind of zoomed in on that Big Bright galaxy in the middle

2:00 and then the pinky red stuff that's this H Al for emission and then

2:03 the blue stuff here is far ultraviolet emission

2:07 both the H for emission and the far

2:09 ultraviolet emission are the signature of young

2:12 Bright Stars so young Stars Bright Stars heat

2:16 up the gas which make it Glow to produce the H Alpha and then the Stars

2:19 themselves are actually so hot that they emit in the far fire ultraviolet so

2:23 this is actually the signature of star

2:25 formation going on within this cluster but you

2:27 can see that the star formation is actually nothing to do with the Galaxy right

2:30 it's you know I mean some of it's in the Galaxy but there's this whole

2:32 strand of star formation going on out here I mean that's on a galactic scale

2:36 that filament and Beyond right it's it's bigger than the Galaxy so how is it

2:40 that there's star formation going on quite

2:41 intense star formation going on that's nothing

2:43 at all to do with the galaxies somehow there has to be a whole L gas

2:46 that's being brought together to make stars

2:49 outside the Galaxy but let's complete this multi-wavelength

2:52 view of this cluster by looking in the X-ray part of the spectrum so

2:54 I have another paper here called a very

2:57 deep Chandra observation of a 1795 chander

3:00 is an x-ray satellite still currently doing its thing up there in space and they

3:03 pointed at it a 1795 for a very long time so here it is

3:07 in X-rays and it's a very bright x-ray

3:09 source so there masses of x-rays coming out

3:12 and actually superimposed on top of it the blue Contours here are the H Alpha

3:16 observations I was showing you before so

3:17 this is where that star formation is going

3:19 on the existence of the X-ray emission tells you there's a whole load of gas

3:23 there because the process by which x-rays

3:26 are emitted is a thing called Brimstone which

3:29 is breaking radiation which is if you have a plasma so you got if you

3:32 just think about hydrogen gas to keep life simple you got a whole load of once

3:36 you've ionized it you've got a whole load of protons and a whole load

3:38 of electrons the the electrons are no longer

3:41 bound to the protons because it's all been

3:42 ionized it's all been ripped up into this plasma but once in a while one

3:46 of these electrons will go somewhere near one of the protons and in the process

3:50 it may be decelerated so it'll scatter past it and it'll lose energy

3:54 and as with a lot of processes in physics when that when energy changes way you

4:00 kind of make sure that energy is

4:01 globally conserved is that a photon gets emitted

4:04 now these are such energetic processes

4:05 that this breaking process ends up producing x-ray photons

4:09 and the fact we see so many x-ray photons tells us there's a lot

4:12 of hot gas there so you can actually do a calculation of saying okay we got

4:16 all this x-ray emitting gas sitting there how long would we expect it to stay

4:21 there for how long does it take

4:22 to cool down and that's a reasonably straightforward

4:25 calculation to do because we can figure

4:26 out how much energy is being lost through

4:28 this brim stoling process how much energy is going away in photons and that if

4:32 you take that energy away that means the gas kind of cools down over time

4:36 and if you make the gas dense enough then the cooling time gets quite short

4:41 at least in astronomical terms um so

4:43 actually the gas that hot x-ray emitting gas

4:46 stops being hot and x-ray emitting and cools down to a point where it turns

4:50 back into normal hydrogen gas and that's what we're seeing here in this tail so

4:54 here they've kind of they they've taken

4:56 this original x-ray image and kind of remov

4:59 the smooth distribution of x-rays to see

5:01 what's left and if you remove that smooth distribution of hot gas you find there

5:06 is still x-ray emitting stuff on the scale

5:09 of this Hal Alpha filament and actually lying in kind of the same place

5:12 when you see kind of an excess of X-ray emission uh like they found when

5:16 they kind of subtracted off this smooth

5:18 stuff that's telling you that that's a place

5:19 where the the gas is particularly dense

5:22 and the fact that it's emitting more x-rays

5:25 um actually means it's going to cool down faster so this feature is cooling down

5:29 suff ly quickly that some of it's just turned into normal hydrogen gas and some

5:33 of that normal hydrogen gas is now turned into Stars this is a thing

5:36 called a cooling wake it's gone all the way from being this hot glowing diffuse

5:41 x-ray emitting gas all the way down to cooling down into clumps that then turn

5:45 into Stars it must be something to do with the fact that this galaxy is

5:48 moving through the core of the cluster so you've got this hot x-ray emitting

5:52 gas that permeates the cluster and then

5:55 you've got this galaxy moving a bit relative

5:57 to it which means that actually it's kind of dragging a wake behind it creating

6:01 this wake behind it as it's moving

6:03 of that cooling gas that physics of figuring out

6:06 the time scale on which gas cools down you can apply it to any cluster

6:11 of galaxies and what you find is that the time scale for the central parts

6:15 of clusters to cool down is relatively

6:17 short remember I said that the the cooling

6:20 time how quickly it cools down depends on the density squared so actually if you

6:24 go to the center of the cluster where there's a whole load of gas

6:27 that means that its cooling time becomes quite

6:28 short because the these emission processes become strong

6:32 and in fact they become much shorter than

6:33 the lifetime of the universe so this effect

6:36 that we see in this weird wake in AEL 1795 by a simple argument you

6:42 should see the same phenomenon in every

6:43 cluster of galaxies going on in the middle

6:45 because the cooling time is sufficiently short

6:46 that they should be turning into stars

6:49 and to some extent that's happening but not

6:51 anywhere near the extent we expected it to happen and what that's telling us

6:55 is that there's some process that's actually heating

6:58 the gas back up again stopping it from cooling down so that we end up

7:02 with loads and loads and loads of stars

7:03 forming and it's thought that the process

7:05 the main process that's heating the gas back up again is that at the center

7:09 of that massive galaxy in the middle

7:10 there's a super massive black hole that super

7:13 massive black hole once in a while becomes an active and it starts throwing out

7:17 very energetic Jets of material and so

7:19 on and that those Jets of material smashing

7:22 in to the gas that surrounds it Heats it all back up again so it's

7:26 thought that the reason why you don't see phenomena like the one one in a 1795

7:30 all over the place is that the the clusters

7:33 of galaxies themselves have their own

7:34 heating mechanism to stop it from happening too much if you look at clusters

7:37 of galaxies you actually find that as well as that sort of diffuse x-ray Mission

7:41 you actually find that there's Optical light

7:43 on large scales as well there's this thing

7:44 called intracluster light and it's thought that is

7:46 just stars that are permeating the cluster

7:49 rather than living in individual galaxies now

7:51 for the most part most of those stars are things that got ripped out of galaxies

7:55 so they probably just formed in normally

7:56 in a in a in a galaxy but that gal interacted with another galaxy

8:01 and as part of that process some of the Stars got kicked out into the cluster

8:04 space so most of that intercluster light

8:07 is probably stars that started their lives

8:09 in galaxies and then got kicked out but at least some of it would have

8:12 formed through processes like this so those are

8:14 stars that were never in a galaxy there's

8:16 something so romantic about stars not in a galaxy I think like I love

8:20 the idea of those yeah it's a it'd be a sort of it' be a sad

8:24 and lonely life wouldn't it really and look I know this probably isn't going

8:29 to win any Astro photography awards but here's

8:32 Abel 1795 photographed by Professor marfield himself

8:37 in his backyard in Nottingham with his beloved

8:40 EV scope and while I have your attention can I take this opportunity to thank

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