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

we are making a video about Abel 1 since we finished the messia catalog I thought I would follow the lead of my lustrious colleague Professor Maryfield who's branched out into looking at other cataloges the obvious catalog for me to talk about is the one that I've probably used the most in my career which is the Abel catalog of Galaxy clusters and I thought I'd go and figure out what Abel one was because it wasn't one I was familiar with so the a catalog isn't a catalog of individual objects it's clusters yeah so the cluster is the object but it's a concentration of objects a concentration of galaxies and just like with the Messier catalog if we have a list of these things we can go off and do exciting things with them there's a lot of history in this video um and it was really interesting for me to go away and read about this because it all starts from the Palomar Sky survey so this is a big survey that took place at the Palmer observatory in Southern California in the 1950s mostly and it was at a time where a we had photographic plates so we were able to take records of large patches of sky and B there was a specific kind of telescope called a Schmid telescope the design of which was optimized for covering wide areas of the sky at once so the idea here was to put the this technology together and do a really big survey of as much of the sky as was possible that would then act as a repository for astronomers to do work on and to understand you know what was out there basically rather than targeting specific objects to look at but no one telescope on the surface of the Earth can see the whole sky this was a survey that did start in the north and they did most of the northern sky down to a declination of minus 30° they got quite a long way into the South it was finished later um by using another Schmid telescope in the South and then it covered the entire celestial sphere so now let's introduce Abel George Abel he was a student at Caltech the California Institute of Technology and he was employed to work on this survey he was one of The Observers his job was literally to look at plate after plate and note down anything interesting he literally had a little card file of interesting objects uh that came along so actually there are I should qualify this video there are two objects called Abel one because the first thing he did was make a list of planetary nebula so he made a list of 86 of them most of which were later confirmed but we're not going to really concentrate on that Abel one right now if you go and look at it is kind of boring it's a little disc kind of reddish in color really featureless just marks the end stage of a star's life but as he was going through this um this project of looking at all these plates he noticed that there were a lot of clusters of galaxies so the galaxies weren't uniformly distributed there were very obvious regions where they were sort of clumped together and he thought this was interesting now here this is where I had a lot of fun researching this video is that we can actually go to Abel's words himself to hear what he thought about this and how he came up with this idea because the American Institute of physics has for a long time conducted really indepth oral histories interviews with prominent physicists which are now available the transcripts are available and they're really long they're really detailed they go into everything from their early childhood and education to the really nitty-gritty of what they were doing but also you know why they were doing it what it was like the time who else they were working with what sort of feuds and personalities were involved it's like all the gossip so I had a look back at Abel's interview which was really interesting reading first of all the interviewer made a point that is the sort of point you might make Brady which says it sounds like it must have been a very tedious job but Abel said oh no it's very exciting to look at the plate I was the first person to look at these plates and I think that's kind of neat the idea that you are the first person to see these objects that are out there in the universe so that must have that sort of motivation must have sustained him for a while in terms of the idea of the Clusters what he said was during that time I was impressed with how many clusters of galaxies kept showing up I thought this was an interesting thing and I was keeping lists of them anyway so it occurred to me why not make a complete survey of the distribution of these rich clusters so I organized that thing probably around the end of the first year and submitted a thesis proposal which got a approved and

I did it and so that was his PhD work that was his thesis to compile this catalog of clusters and to do something like this you don't just write down everything you see for a catalog to be useful it has to have really clear criteria so he set out these rules of what a cluster had to be in order to make it to this list and that allows other astronomers to use that list with confidence that they are getting a clean sample that they're getting a well-described sample and that any statistical analysis they do will be meaningful and unbiased I'm just going to say this that sounds like an easy PhD to me am I wrong um well um I don't know what the standards of astronomy were like at that time I mean nobody had ever done this was new information he made some conclusions derived from that catalog which Advanced the field somewhat but you know the pace of astronomy was really different back then and the big work was being done at the telescope actually taking the data we're so far removed from that now even in the time scale of my training in my career we've become quite divorced as professional astronomers from that staying up all night doing the work at the telescope and we download cataloges and we play with things and we write computer code but they couldn't do that we're a really data Rich field right now but this was the front line if we were going to do astronomy somebody had to be up the telescope collecting those photons they were all precious I always feel like one of the great insults you can make to a scientist or a field of science is to call it stamp collecting and this feels a lot like stamp collecting don't get me wrong it's cool it tickles my brain I no one loves lists more than me as evidenced by my work but it does feel very stamp collecty is this thing useful do you still use it I still do use it I mean you have to think he started from a position of looking at the universe in a way that nobody had been able to look at it before because we just hadn't have this Widefield view of so much of the sky and he made an observation that the Galaxy seemed to Cluster and then beyond that he made an observation that he thought there was second order clust clustering that the cluster is clustered and now we're getting into questions of cosmology what kind of universe do we live in how does structure form in the universe and remember this was the 1950s the model of cosmology was Far different their understanding of what the universe was Far different than the detailed understanding we have today so no I don't think it's just stamp collecting I think it's stamp collecting as a first step because that how you impose order onto knowledge and then it's trying to find patterns and trying to find understanding from your stamp collection that's cool so we've talked about Abel's work as a graduate student doing this PhD but he was a very eminent American astronomer in the 20th century he was chair of the University of California Los Angeles astronomy Department by selecting among various Optical Arrangements the telescope can be used in a variety of ways he played a big role in both education and public understanding of science which I feel is kind of meta here because we are having an interview about the business of doing science using an oral history where someone sat down with an interviewer talking about the business of doing science the photographs taken for this poar Sky survey revealed the images of hundreds of millions of individual galaxies he also had a connection with the UK because he worked up in Edinburgh quite a bit and he made films with the BBC he was an early advocate of doing popularization of science because he thought it was valuable for the public to understand this work so he made a series of programs with the open University and the BBC and I learned that the observatory at the open University here in the UK is named after him it's the George Abel Observatory which I thought was kind of nice we neia Abel one um well honestly I don't have much to say about Abel one Brady because I looked it up and there is nothing special about it is a little cluster of galaxies and I did the usual thing where there are databases where you can say here are all of the papers that have referenced this object and sometimes it can be hundreds and hundreds of entries long there were 16 most of them it was just it's an entry in this catalog here's a catalog of clusters here's a compilation of something else no one to my knowledge has ever looked at it as an individual object and studied it in that

perspective which made me feel kind of sad for it to be honest well if only you knew an astronomer oh hang on why don't you do it so what I did was I actually so this original Sky survey that we're talking about it was in photographic plates and this is something that big observatories used to have a collection like there be a room you could go and look at the you get the plate out that you wanted to this was before my time but you You' go to the plate collection get the plate out take a Polaroid picture of it and then take that with you to the telescope as sort of your map of the sky but in the 1980s this survey and there was a successor survey that was done again with better technology an all Sky survey in the 1980s the sort of Mark 2 these two surveys all of the photographic plates were digitized so they were scanned and they were scanned in such a way that the digital copies could be used for science they weren't just sort of you know take a picture and stick it on the wall so we can now access these so I thought I would download and have a look at the actual data there were two plates taken of each piece of Sky one in a blue filter one in the red filter he did all his work on the red filter so here is the digitized scan of the plate that Abel would have looked at when he was doing his inspection that's Abel one and I'm going to change this a little bit I'm going to invert the color map because this is what Abel would have seen so in this case now dark means something that's bright and it's just a little bit easier for our eyes to see contrast because it's digital I can change the color scale and I can bring that contrast out so you can see a little bit more fuzzy detail Abel wouldn't have had that capability and so here here's a Galaxy the thing next to it with a little cross indicates that that's a star so that's in our galaxy and that one's much further away if we zoom out you can sort of see the whole plate you can see how big it is and yeah perhaps you can guess that there is some sort of concentration of galaxies here now to get into Abel's catalog the criteria he used where there had to be at least 50 members they had to be within a certain radius that depended on the distance to the cluster but it was quite a general one so it's quite out here um and they had to be 50 between the brightness of the third brightest Galaxy um and two magnitudes fainter so magnitudes are a funny unit that astronomers use to measure brightness but essentially sort of a factor of six in flux in brightness and so yep so there's 50 of those galaxies in that region 49 you don't get in because otherwise you know well to be honest I think some of the criteria were a little bit uh a little bit generously applied um but generally speaking you have these rules for a reason um some get in some don't um and then you have your catalog so there you go that's just Abel one so we don't know if there's spirals or ellipticals or B there's no greater zooms that have since been done with hubbles and things no there's no Hubble data the SLO digital Sky survey didn't cover it nobody's gone G through and classified those galaxies at least not that I could find in the literature that's it was pretty much Abel and now us as far as I can tell oh we've got to get a closer look can you get us some telescope time well to get telescope time I'd have to have a reason why and so I'd have to build a case as to for why it was worth spending expensive time to look at this object and a better look isn't quite going to do it um we could do like a Kickstarter or something and buy the time can you buy telescope time um not on any of the telescopes that I am familiar with um I want to okay now I need to pause here i' I've been a bit unfair um because of course I've been talking about professional astronomers the kind of telescopes that you know we put on top of mountain tops and the results that get published in journals that is not the only kind of astronomy that gets done here so I want to take back that what statement I just said because I know that there are people we might call amateurs but are very skilled who have observed AEL one who have beautiful photos of it and have probably looked at it and thought carefully and deeply about it so I don't want to diss the contributions of any of those very important people but speaking from the sort of professional sphere yeah that that's what we got but I thought this would be interesting so the reason I

started this video is I said I have used the Abel catalog many times and that's because I study clusters of galaxies and the Abel catalog encompasses all of the nearby clusters of galaxies okay the ones that we've been able to see you know for quite a long time and for some of those are as familiar to me as my own address so Abel 2218 Abel 2219 Abel 1689 if you showed me a picture of them I would recognize them instantly and so I thought it was quite interesting the anonymity of Abel one in contrast and I know you can see over my shoulder right now that poster which we've made a video about before this is a patch of sky that has two Abel clusters in it so Abel would have gone through this patch of sky as well and put two different objects in his catalog which two Abels are in it well do you want to come over and see yeah so this is of course not the picture that Abel would have had okay and so when he went through this he thought there was one cluster of galaxies up in this region Abel 901 and one cluster down 902 now this field got the luck of the draw unlike Abel 1 because myself and some of my colleagues decided to focus on this and we spent 10 15 years working on this patch of sky and we've looked at it with lots of different telescopes including the ones that's that have made this image with Hubble we've seen all sorts of different wavelengths and we've learned so much about the structure and all of the individual galaxies here but we also learned that Abel didn't see the whole story because where he saw one collection here we now know that there are two very distinct different clusters of galaxies and the whole thing makes up some sort of super cluster which was exactly the sort of thing he was pointing out he thought existed so I thought i' I'd drag out the images from the digital Sky survey and have a look at that as well just to see what Abel would have seen when he made that determination and here I love this because you can see it's actually the corner of the plate and you can even see the label up there it says National Geographic Society because the survey was funded by a grant from the National Geographic society and it says Palmer Observatory Sky survey and so right there in the middle you can see the same region we've just seen up there and this is essentially the region he thought was one cluster and the other is down off the bottom of the screen look how ropey it is you can see all these artifacts looks like a bit of I don't know dust or something a hair or something like that you know this is the real deal actual stuff that he would have been looking at and it's not that he got it wrong it's just that to the level of information that he had he made one determination and when we looked a little bit more closely we could see a lot more richer detail maybe that would be the case with Abel one as well I'm sure there are lots of interesting things lurking there we just don't know what they are how did he decide it was number one did he start at the North Pole like we do with cordwell or so they are ordered they are in a list which is very pleasing and the order is in right Ascension so this is the astronomical coordinate that is analogous to longitude on the Earth but up on the celestial spere it's the one with the lowest right Ascension the one closest to zero that's it that's why it's in there at number one it's not grenwich that we use uh for our zero point there it's the uh position of the sun at the Spring Equinox very arbitrary but this is another thing the coordinates he would have been using are not the same as the coordinates we use now because the orientation of the earth is moving relative to the fixed Stars There's A procession so every few decades we have to sort of update what we mean by all of these coordinates so that's in the coordinates of the epoch that he was working in back in the 1950s which might not be the same as the ones we use now we had a conversation about whether this was stamp collecting or whether he was gleaning some understanding of the universe and we talked about the super clustering of galaxies which we've seen Illustrated over here and again that wasn't uh an a widely accepted idea before he introduced it and in fact uh one of the um big astronomers in California at the time Fritz wikii who we know as being one of the first people to postulate that dark matter existed we also know him as someone you might politely refer to as a strong personality or a character this is what I love about the oral history is because it makes all of this human ins side of science come alive so Abel

worked you know in the same sphere as zi and Zi was adamant that clusters were uniformly distributed so this idea that there could be super clusters of galaxies that clusters could themselves be clustered he was dead against right from the beginning um and it's really interesting to read how that sort of um argument developed and to get a sense of the person ities involved um so there's lots of stories about zi but I really liked what Abel said about him he said I got to know zwicki very well I always liked zwicki he was a lot of fun but he would be an impossible person to take a course with he was an ego maniac to be sure but he made a lot of contributions a bright guy and this phrase is brilliant if you could buy zi for what we think he's worth and sell him for what he thinks he's worth you'd be a millionaire but nevertheless that doesn't attract back from the fact that he was also a great man there you go you know these are the these are people doing science they're not robots um and it was clearly a very exciting time to be part of all of this endeavor here's another list we really like here at Deep sky videos these are most of our patreon supporters people who make contributions each month no matter how small to help keep our Channel going if you'd like to join them go to patreon /de skyvideos it means a lot to us but no matter what thank you very much just for being here and watching our videos do share them with your friends give us a like make sure you subscribe it all helps keep the channel going 50 by 100 like years so that that's still fairly Hefty as well it's a stellar Nursery so it's got all the raw materials to form young Stars the reason it glows is that it's full of ionized hydrogen so there's so much radiation from the hot young stars that are being born