The Story of Galaxy Clusters (and Abell 1) - Deep Sky Videos

0:00 we are making a video about Abel 1 since

0:03 we finished the messia catalog I thought

0:05 I would follow the lead of my lustrious

0:08 colleague Professor Maryfield who's

0:10 branched out into looking at other

0:13 cataloges the obvious catalog for me to

0:16 talk about is the one that I've probably

0:18 used the most in my career which is the

0:21 Abel catalog of Galaxy clusters and I

0:24 thought I'd go and figure out what Abel

0:26 one was because it wasn't one I was

0:28 familiar with so the a catalog isn't a

0:31 catalog of individual objects it's

0:33 clusters yeah so the cluster is the

0:36 object but it's a concentration of

0:39 objects a concentration of galaxies and

0:41 just like with the Messier catalog if we

0:44 have a list of these things we can go

0:46 off and do exciting things with them

0:48 there's a lot of history in this video

0:52 um and it was really really interesting

0:54 for me to go away and read about this

0:56 because it all starts from the Palomar

1:00 Sky survey so this is a big survey that

1:03 took place at the Palmer observatory in

1:06 Southern California in the 1950s mostly

1:10 and it was at a time where a we had

1:13 photographic plates so we were able to

1:16 take records of large patches of sky and

1:19 B there was a specific kind of telescope

1:21 called a Schmid telescope the design of

1:24 which was optimized for covering wide

1:26 areas of the sky at once so the idea

1:29 here was to put the this technology

1:31 together and do a really big survey of

1:35 as much of the sky as was possible that

1:37 would then act as a repository for

1:39 astronomers to do work on and to

1:43 understand you know what was out there

1:44 basically rather than targeting specific

1:47 objects to look at but no one telescope

1:50 on the surface of the Earth can see the

1:52 whole sky this was a survey that did

1:54 start in the north and they they did

1:55 most of the northern sky down to a

1:58 declination of minus 30° they got quite

2:01 a long way into the South it was

2:02 finished later um by using another

2:05 Schmid telescope in the South and then

2:07 it covered the the entire celestial

2:10 sphere so now let's introduce Abel

2:13 George Abel he was a student at Caltech

2:16 the California Institute of Technology

2:18 and he was employed to work on this

2:19 survey he was one of The Observers his

2:22 job was literally to look at plate after

2:24 plate after plate and note down anything

2:28 interesting he literally had a little

2:29 card file of interesting objects uh that

2:32 came along so actually there are I

2:35 should qualify this video there are two

2:37 objects called Abel one because the

2:40 first thing he did was make a list of

2:42 planetary nebula so he made a list of 86

2:45 of them most of which were later

2:47 confirmed but we're not going to really

2:49 concentrate on that Abel one right now

2:52 if you go and look at it it is kind of

2:54 boring it's a little disc kind of

2:57 reddish in color really featureless just

3:00 marks the end stage of a star's life but

3:04 as he was going through this um this

3:08 project of looking at all these plates

3:10 he noticed that there were a lot of

3:14 clusters of galaxies so the galaxies

3:16 weren't uniformly distributed there were

3:18 very obvious regions where they were

3:20 sort of clumped together and he thought

3:23 this was interesting now here this is

3:26 this is where I had a lot of fun

3:27 researching this video is that we can

3:29 actually go to Abel's words himself to

3:31 hear what he thought about this and how

3:33 he came up with this idea because the

3:36 American Institute of physics has for a

3:39 long time conducted really indepth oral

3:42 histories interviews with prominent

3:44 physicists which are now available the

3:47 transcripts are available and they're

3:48 really long they're really detailed they

3:50 go into everything from their early

3:52 childhood and education to the really

3:55 nitty-gritty of what they were doing but

3:57 also you know why they were doing it

3:59 what it was like the time who else they

4:01 were working with what sort of feuds and

4:03 personalities were involved it's like

4:05 all the gossip so I had a look back at

4:09 Abel's interview which was really

4:11 interesting reading first of all the

4:13 interviewer made a point that is the

4:14 sort of point you might make Brady which

4:16 says it sounds like it must have been a

4:18 very tedious job but Abel said oh no

4:21 it's very exciting to look at the plate

4:23 I was the first person to look at these

4:24 plates and I think that's that's kind of

4:27 neat the idea that you are the first

4:29 person person to see these objects that

4:31 are out there in the universe so that

4:33 must have that sort of motivation must

4:35 have sustained him for a while in terms

4:37 of the idea of the Clusters what he said

4:40 was during that time I was impressed

4:42 with how many clusters of galaxies kept

4:44 showing up I thought this was an

4:45 interesting thing and I was keeping

4:47 lists of them anyway so it occurred to

4:49 me why not make a complete survey of the

4:51 distribution of these rich clusters so I

4:54 organized that thing probably around the

4:56 end of the first year and submitted a

4:58 thesis proposal which got a approved and

5:00 I did it and so that was his his PhD

5:02 work that was his thesis to compile this

5:05 catalog of clusters and to do something like this

5:11 you don't just write down everything you

5:13 see for a catalog to be useful it has to

5:17 have really clear criteria so he set out

5:20 these rules of what a cluster had to be

5:24 in order to make it to this list and

5:27 that allows other astronomers to use

5:30 that list with confidence that they are

5:33 getting a clean sample that they're

5:35 getting a well-described sample and that

5:37 any statistical analysis they do will be

5:40 meaningful and unbiased I'm just going

5:43 to say this that sounds like an easy PhD

5:46 to me am I wrong um well um I don't know

5:51 what the standards of astronomy were

5:54 like at that time I mean nobody had ever

5:56 done this this was new information he

6:00 made some conclusions derived from that

6:02 catalog which Advanced the field

6:04 somewhat but you know the pace of

6:06 astronomy was really really different

6:09 back then and the big work was being

6:11 done at the telescope actually taking

6:14 the data we're so far removed from that

6:16 now even in the the time scale of my

6:18 training in my career we've become quite

6:21 divorced as professional astronomers

6:23 from that staying up all night doing the

6:26 work at the telescope and we download

6:28 cataloges and we play with things and we

6:30 write computer code but they couldn't do

6:32 that we're a really data Rich field

6:35 right now but this was the front line if

6:38 we were going to do astronomy somebody

6:40 had to be up the telescope collecting

6:42 those photons they were they were all

6:44 precious I always feel like one of the

6:46 great insults you can make to a

6:48 scientist or a field of science is to

6:50 call it stamp collecting and this feels

6:52 a lot like stamp collecting don't get me

6:54 wrong it's cool it tickles my brain I no

6:57 one loves lists more than me as

6:58 evidenced by my work

7:00 but it does feel very stamp collecty is

7:02 this thing useful do you still use it I

7:04 still do use it I mean you have to think

7:07 he he started from a position of looking

7:10 at the universe in a way that nobody had

7:13 been able to look at it before because

7:15 we just hadn't have this Widefield view

7:17 of so much of the sky and he made an

7:19 observation he made an observation that

7:22 the the Galaxy seemed to Cluster and

7:24 then beyond that he made an observation

7:27 that he thought there was second order

7:29 clust clustering that the cluster is

7:31 clustered and now we're getting into

7:33 questions of cosmology what kind of

7:35 universe do we live in how does

7:38 structure form in the universe and

7:40 remember this was the 1950s the model of

7:43 cosmology was Far different their

7:46 understanding of what the universe was

7:48 was Far Far different than than the

7:51 detailed understanding we have today so

7:54 no I don't think it's just stamp

7:55 collecting I think it's stamp collecting

7:57 as a first step because that how you

8:00 impose order onto knowledge and then

8:03 it's trying to find patterns and trying

8:05 to find understanding from your stamp

8:07 collection that's cool so we've talked

8:09 about Abel's work as a graduate student

8:11 doing this PhD but he was a very eminent

8:13 American astronomer in the 20th century

8:16 he was chair of the University of

8:18 California Los Angeles astronomy

8:20 Department by selecting among various

8:23 Optical Arrangements the telescope can

8:25 be used in a variety of ways he played a

8:28 big role in both education and public

8:31 understanding of science which I feel is

8:33 kind of meta here because we are having

8:35 an interview about the business of doing

8:37 science using an oral history where

8:39 someone sat down with an interviewer

8:41 talking about the business of doing

8:42 science the photographs taken for this

8:45 poar Sky survey revealed the images of

8:48 hundreds of millions of individual

8:50 galaxies he also had a connection with

8:53 the UK because he worked up in Edinburgh

8:56 quite a bit and he made films with the

8:58 BBC he was an early advocate of doing

9:02 popularization of science because he

9:04 thought it was valuable for the public

9:08 to understand this work so he made a

9:10 series of programs with the open

9:12 University and the BBC and I learned

9:14 that the observatory at the open

9:16 University here in the UK is named after

9:19 him it's the George Abel Observatory

9:21 which I thought was kind of nice we neia

9:23 Abel one um well honestly I don't have

9:26 much to say about Abel one Brady because

9:28 I looked it up

9:30 and there is nothing special about it it

9:32 is a little cluster of galaxies and I

9:34 did the usual thing where there are

9:36 databases where you can say here are all

9:38 of the papers that have referenced this

9:41 object and sometimes it can be hundreds

9:43 and hundreds of entries long there were

9:45 16 most of them it was just it's an

9:49 entry in this catalog here's a catalog

9:51 of clusters here's a compilation of

9:52 something else no one to my knowledge

9:56 has ever looked at it as an individual

9:59 object and studied it in that in that

10:01 perspective which which made me feel

10:03 kind of sad for it to be honest well if

10:05 only you knew an astronomer oh hang

10:08 on why don't you do it so what I did was

10:11 I actually so this this original Sky

10:14 survey that we're talking about it was

10:16 in photographic plates and this is

10:18 something that that big observatories

10:20 used to have a collection like there be

10:22 a room you could go and look at the you

10:25 get the plate out that you wanted to

10:28 this was before my time but you You' go

10:29 to the plate collection get the plate

10:32 out take a Polaroid picture of it and

10:35 then take that with you to the telescope

10:37 as sort of your map of the sky but in

10:39 the 1980s this survey and and there was

10:42 there was a successor survey that was

10:44 that was done again with with better

10:47 technology an all Sky survey in the

10:48 1980s the sort of Mark 2 these two

10:51 surveys all of the photographic plates

10:53 were digitized so they were scanned and

10:56 they were scanned in such a way that the

10:57 digital copies could be used for science

11:00 they weren't just sort of you know take

11:01 a picture and stick it on the wall so we

11:04 can now access these so I thought I

11:05 would download and have a look at the

11:08 actual data there were two plates taken

11:10 of each piece of Sky one in in a blue

11:13 filter one in the red filter he did all

11:15 his work on the red filter so here is

11:17 the digitized scan of the plate that

11:20 Abel would have looked at when he was

11:22 doing his inspection that's Abel one and

11:26 I'm going to change this a little bit

11:28 I'm going to to invert the color map

11:31 because this is what Abel would have

11:32 seen so in this case now dark means

11:35 something that's bright and it's just a

11:37 little bit easier for our eyes to see

11:39 contrast because it's digital I can

11:41 change the color scale and I can bring

11:44 that contrast out so you can see a

11:46 little bit more fuzzy detail Abel

11:47 wouldn't have had that capability and so

11:50 here here's a a Galaxy the thing next to

11:53 it with a little cross indicates that

11:56 that's a star so that's in our galaxy

11:58 and that one's much much further away if

12:00 we zoom out you can sort of see the

12:04 whole plate you can see how big it is

12:07 and yeah perhaps you can guess that

12:10 there is some sort of concentration of

12:12 galaxies here now to get into Abel's

12:15 catalog the criteria he used where there

12:18 had to be at least 50 members they had

12:22 to be within a certain radius that

12:25 depended on the distance to the to the

12:27 cluster but it was quite a general one

12:29 so it's quite out here um and they had

12:32 to be 50 between the brightness of the

12:36 third brightest Galaxy um and two

12:40 magnitudes fainter so magnitudes are a

12:43 funny unit that that astronomers use to

12:45 to measure brightness but essentially

12:47 sort of a factor of six in in flux in

12:51 brightness and so yep so there's there's

12:54 50 of those galaxies in that region 49

12:57 you don't get in you don't get in

13:00 because otherwise you know well to be

13:02 honest I think some of the criteria were

13:04 a little bit uh a little bit generously

13:06 applied um but but generally speaking

13:08 you have these rules for a reason um

13:10 some get in some don't um and then you

13:13 have your your catalog so there you go

13:16 that's just Abel one so we don't know if

13:18 there's spirals or ellipticals or B

13:21 there's no greater zooms that have since

13:23 been done with hubbles and things no

13:25 there's no Hubble data the SLO digital

13:27 Sky survey didn't cover it nobody's gone

13:29 G through and classified those galaxies

13:31 at least not that I could find in the

13:33 literature that's it it was pretty much

13:36 Abel and now us as far as I can tell oh

13:40 we've got to get a closer look can you

13:42 get us some telescope time well to get

13:44 telescope time I'd have to have a reason

13:46 why and so I'd have to build a case as

13:48 to for why it was worth spending

13:51 expensive time to look at this object

13:54 and a better look isn't isn't quite

13:57 going to do it um we could do like a

13:59 Kickstarter or something and and buy the

14:01 time can you buy telescope time um not

14:05 on any of the telescopes that I am

14:07 familiar with um I want to okay now I

14:11 need to pause here i' I've been I've

14:13 been a bit unfair um because of course I've been

14:18 talking about professional astronomers

14:20 I've been talking about the kind of

14:21 telescopes that you know we put on top

14:23 of mountain tops and the results that

14:25 get published in journals that is not

14:27 the only kind of astronomy that that

14:29 gets done here so I want to I want to

14:31 take back that what statement I just

14:34 said because I know I know that there

14:37 are people we might call amateurs but

14:39 are very very skilled who have observed

14:42 AEL one who have beautiful photos of it

14:45 and have probably looked at it and

14:47 thought carefully and deeply about it so

14:49 I don't want to diss the contributions

14:50 of any of those very important people

14:52 but speaking from the the sort of

14:55 professional sphere yeah that that

14:57 that's what we got but I thought this

14:59 would be interesting so the reason I

15:01 started this video is I said I have used

15:03 the Abel catalog many times and that's

15:06 because I study clusters of galaxies and

15:08 the Abel catalog encompasses all of the

15:11 nearby clusters of galaxies okay the

15:14 ones that we've been able to see you

15:16 know for for quite a long time and for

15:18 some of those those are as familiar to

15:20 me as my own address so Abel 2218 Abel

15:23 2219 Abel 1689 if you showed me a

15:26 picture of them I would recognize them

15:28 instantly and so I thought it was quite

15:30 interesting the anonymity of Abel one in

15:34 contrast and I know you can see over my

15:36 shoulder right now that poster which

15:39 we've made a video about before this is

15:41 a patch of sky that has two Abel

15:43 clusters in it so Abel would have gone

15:45 through this patch of sky as well and

15:48 put two different objects in his catalog

15:52 which two Abels are in it well do you

15:53 want to come over and see yeah so this

15:55 is of course not the picture that Abel

15:58 would have had okay and so when he went through this he

16:02 thought there was one cluster of

16:03 galaxies up in this region Abel 901 and

16:07 one cluster down in this region Abel 902

16:11 now this field got the luck of the draw

16:13 unlike Abel 1 because myself and and

16:16 some of my colleagues decided to focus

16:18 on this and we spent 10 15 years working

16:21 on this patch of sky and we've looked at

16:24 it with lots of different telescopes

16:26 including the ones that's that have made

16:27 this image with Hubble we've seen all

16:29 sorts of different wavelengths and we've

16:31 learned so much about the structure and

16:33 all of the individual galaxies here but

16:35 we also learned that Abel didn't see the

16:37 whole story because where he saw one

16:40 collection here we now know that there

16:42 are two very distinct different clusters

16:45 of galaxies and the whole thing makes up

16:48 some sort of super cluster which was

16:50 exactly the sort of thing he was

16:52 pointing out he thought existed so I

16:55 thought i' i' I'd drag out the images

16:58 from the digital Sky survey and and have

17:00 a look at that as well just to see what

17:02 Abel would have seen when he made that

17:05 determination and here I love this

17:08 because you can see it's actually the

17:09 corner of the plate and you can even see

17:11 the label up there it says National

17:13 Geographic Society because the survey

17:16 was funded by a grant from the National

17:18 Geographic society and it says Palmer

17:21 Observatory Sky survey and so right

17:23 there in the middle you can see the same

17:25 region we've just seen up there and this

17:29 is essentially the region he thought was

17:30 one cluster and the region he thought

17:33 was the other is is down off the bottom

17:34 of the screen look how ropey it is you

17:37 can see all these artifacts looks like a

17:39 bit of I don't know dust or or something

17:42 a hair or something a hair or something

17:44 like that you know this is the real deal

17:46 you know this is the actual stuff that

17:48 he would have been looking at and it's

17:49 not that he got it wrong it's just that

17:52 to the level of information that he had

17:56 he made one determination and when we

17:58 looked a little bit more closely we

18:00 could see a lot more richer detail maybe

18:03 that would be the case with Abel one as

18:05 well I'm sure there are lots of

18:06 interesting things lurking there we just

18:08 don't know what they are how did he

18:10 decide it was number one did he start at

18:12 the North Pole like we do with cordwell

18:14 or so they are ordered they are in a

18:17 list which is very pleasing and the

18:19 order is in right Ascension so this is

18:22 the astronomical coordinate that is

18:25 analogous to longitude on the Earth but

18:27 up on the celestial spere it's the one

18:29 with the lowest right Ascension the one

18:31 closest to zero that's it that's why

18:34 it's in there at number one it's not

18:35 grenwich that that we use uh for our

18:39 zero point there it's the uh position of

18:42 the sun at the Spring

18:46 Equinox very arbitrary but this is another thing the

18:50 coordinates he would have been using are

18:52 not the same as the coordinates we use

18:54 now because the orientation of the earth

18:57 is moving relative to the fixed Stars

18:59 There's A procession so every few

19:01 decades we have to sort of update what

19:03 we mean by all of these coordinates so

19:07 that's in the coordinates of the epoch

19:10 that he was working in back in the 1950s

19:13 which might not be the same as the ones

19:14 we use now we we had a conversation

19:17 about whether this was stamp collecting

19:19 or whether he was gleaning some

19:21 understanding of the universe and we

19:22 talked about the super clustering of

19:25 galaxies which we've seen Illustrated

19:27 over here and again again that wasn't uh

19:30 an a widely accepted idea before he

19:33 introduced it and in fact uh one of the

19:38 um big astronomers in in California at

19:40 the time Fritz wikii who we we know as

19:43 as being one of the first people to

19:45 postulate that dark matter existed we

19:47 also know him as someone you you might

19:49 politely refer to as a strong

19:52 personality or a character this is what

19:54 I love about the oral history is because

19:56 it makes all of this human ins side of

19:59 science come alive so Abel

20:03 worked you know in the same sphere as zi

20:07 and Zi was adamant that that clusters

20:09 were uniformly distributed so this idea

20:12 that that there could be super clusters

20:14 of galaxies that clusters could

20:15 themselves be clustered he was dead

20:17 against right from the beginning um and

20:20 it's really interesting to to read how

20:22 that sort of um argument developed and

20:26 and to get a sense of the person ities

20:29 involved um so there's lots of stories

20:32 about zi but I really liked what Abel

20:35 said about him he said I got to know

20:36 zwicki very well I always liked zwicki

20:39 he was a lot of fun but he would be an

20:41 impossible person to take a course with

20:43 he was an ego maniac to be sure but he

20:46 made a lot of contributions a bright guy

20:48 and this phrase is brilliant if you

20:49 could buy zi for what we think he's

20:51 worth and sell him for what he thinks

20:54 he's worth you'd be a

20:56 millionaire but nevertheless that

20:58 doesn't attract back from the fact that

20:59 he was also a great man there you go you

21:02 know these are the these are these are

21:04 people doing science they're not robots

21:07 um and it was clearly a very exciting

21:09 time to be to be part of all of this

21:13 endeavor here's another list we really

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21:42 going 50 by 100 like years so that

21:45 that's still fairly Hefty as well it's a

21:47 stellar Nursery so it's got all the raw

21:50 materials to form young Stars the reason

21:53 it glows is that it's full of ionized

21:56 hydrogen so there's so much radiation

21:59 from the hot young stars that are being

22:01 born