Interview with Rodney D. Davies

Begin Tape 71B

Sullivan

This is continuing with Rod Davies on 30 August 1976.

Davies

Yes, David had nearly completed a receiver, which I helped him with, and we put this on a 30-foot dish that was originally on Beachy Head as a radar detecting dish across the channel.

Sullivan

During the War?

Davies

During the War, yes. Lovell had caught this and brought it to Jodrell a few years previously. And this turned out to be quite a useful instrument for the work we were doing at that stage. In fact, it was slightly bigger than the Westerbork dish that made the first hydrogen line survey.

Sullivan

Kootwijk, you mean?

Davies

Kootwijk, yes. And we were particularly interested in the distances of the radio sources and our main astronomical effort was towards getting the absorption spectra of the radio sources, Cygnus, Cassiopeia, and so on.

Sullivan

Ok, this is, of course, a very important step in the development of radio astronomy. So I’d like to pursue that further. How did this idea come up? The whole idea of absorption of optical lines, interstellar lines was around. Was it analogous to that?

Davies

Exactly analogous to that one. There’d been several meetings, at about the time I went to Jodrell, in which Baade and Minkowski had discussed the identifications of these objects, Cassiopeia and Cygnus, Cygnus X. And people were wondering what their distances were. At that stage, one didn’t have a clear idea of what the radio distances to the radio stars were. Cygnus had a redshift – an optical redshift. Now, we thought that if we could get a radio redshift of Cygnus, then this would give us the opportunity to put in an entirely radio method of getting the distance of any of the radio sources.

Sullivan

You were think of that during (?)

Davies

Yes. And the other thing was that if you could measure the absorption spectrum of an object in the Galaxy, like Cas A, then you could get a distance. This had been discussed either openly in the sessions, or amongst individuals at the meetings.

Sullivan

So this idea was around?

Davies

The idea was around, yes.

Sullivan

Did anyone actually work out the physics of column densities and so forth that would be needed to detect absorption? Or you just said, “Well, it might work, let’s try it?”

Davies

I think at that stage it was pretty obvious because we already got emission from these objects and the temperature was assigned, a spin temperature was assigned. So we thought we were going to get really quite strong signals from Cassiopeia and Cygnus in particular, because that was near the galactic plane. But it turned out that although Cas was easier to see, Cygnus wasn’t. It was down near our noise. And we know now, of course, because it’s at 4° latitude. The absorption at that sort of latitude in the spiral arms outside our own spiral arm is just very, very weak.

Sullivan

It seems like from what you just said that you did understand that given an emission profile that that same hydrogen would give you much stronger absorption signals if you had a strong signal source?

Davies

Yes.

Sullivan

You understood this even before you set out on these experiments?

Davies

Yes. I think that was from a set of notes, in fact, that Van de Hulst had given some years previously. Those notes were kicking around Harvard and a number of people there… people I guess like Bill Howard, oh, a lot of them actually had written up those notes. And we had a set of a copy of them which had a lot of this worked out explicitly for us.

Sullivan

Ok, I hadn’t remembered that it was in there.

Davies

That’s my recollection anyway. It was sort of for the theoretical bible to us at that time, because we were all pretty raw on the theoretical side, of course. We were mainly experimentalists.

Sullivan

So what happened when you got your receiver working? Were there difficulties in getting a working system?

Davies

No, it just went ahead smoothly, but slowly. There were only the two of us working on this, and we had no real expertise in the field. We just culled what information we could from people like Hanbury and a few of the firms around who were making lower noise mixers. The noise figure was the main problem as it is now, of course. We consulted with GEC and a number of other firms in an attempt to get the best mixer crystals we could get our hands on. And David was very good technically. He was able to get this thing going in quite good time. And we did quite a number of things with that receiver. The main interest was in the absorption spectrum and by 1956, we’d got a good set of absorption spectra and we’d also looked at quite a lot of other regions in the sky. Self-absorption, for example, in extended clouds of hydrogen and these gave us information on the spin temperature of the gas, cool gas, was one of the first cool clouds that were discovered, in the anti-center region.

Sullivan

Excuse me, I think we’re going to have to move.

[Interruption]

Sullivan

So, was it a matter that this absorption line came up as soon as you had a working receiver?

Davies

Yes. It was the aim of the experiment. About a year before we had the thing going, in fact. Stimulated largely by Hanbury Brown, who was working on detecting sources at that stage with the big 220-foot telescope. This was the forerunner of the Mark I telescope. We were just addressing ourselves to this question of if you could get cosmological redshifts in the radio were in a very strong position.

Sullivan

Did you know at all about the – I’m not quite sure of the timing – at NRL they accidentally discovered absorption. When did you learn about that?

Davies

We heard something I think from Hanbury who’d been over in the States. We knew that they were working on emission lines. We didn’t hear that they had gotten absorption until after we’d gotten it. That was my recollection of the situation. Although it was certainly the case that we didn’t know until after. It was of real interest a bit later on when we’d started looking for it, the real redshift and stuff (?) – 16,000 kilometers a second.

Sullivan

I’m interested in the first paper in Nature that you published. Can you tell me how you went about deriving the distance for these, especially Cas A of course, originally a point of dispute over the next (?)

Davies

That’s right. There one clearly got the absorption features at zero and minus 40 as I recall kilometers a second minus 50. And on any rotation model, the minus 50 put you about 3 Kpc away in or beyond the Perseus arm. This is where a controversy blew up because Minkowski said that the distance was like 1 Kpc (?)

Sullivan

That was based on some proper motions, radial velocities?

Davies

That’s right. And we thought it had very clear-cut distance for Cas A. And I wrote this stuff up as my thesis and offered it to my external examiner. He wasn’t having it either. When I argued this one out to the (?) it seemed to me there was enough uncertainty both in the proper motion and in the positions – there was enough position scatter that one could be completely uncertain as to whether there was a significant distance determination at all from the optical methods.

Sullivan

That’s very interesting about Oort because, of course, he is the founder of the whole galactic structure method and all. So what did he think was wrong with yours?

Davies

He felt that the absorption was OK at the minus 40 kilometers a second, he thought, but one should really look into the question as to why Minkowski was wrong rather than us. I had to justify why Minkowski was wrong.

Sullivan

Oh, I got you wrong then. What you said then was that Oort went along with you basically to believe the radio distance more than the optical. I’d thought I’d heard the other.

Davies

I think he was more concerned that I should question Minkowski and what were the reasons that I did. In my thesis I just, more or less, dismissed the optical as being really extremely uncertain. And he questioned my dismissal of that.

Sullivan

He said you can’t do it -

Davies

You can’t dismiss a man like that.

Sullivan

I see.

Davies

You ought to talk to Martin Schmidt about this one. He’s got lovely stories about Baade and Minkowski arguing this between themselves.

Sullivan

Well, I have talked to Martin Schmidt but I don’t think I talked to him about this. Of course he has been directly involved.

Davies

Well, his stories are stories.

Sullivan

Well, I’m always interested in anecdotes.

Davies

Hmmm, nice anecdotes.

Sullivan

And what was the final resolution of this distance problem?

Davies

About two years later, Minkowski redid the measurements and found, indeed, they did fit on about 3.5Kpc distance for the Cas A supernova remnant in the (?) the Perseus arm presumably. I think that’s where we still think it is.

Sullivan

You might be interested; Ed McClain has shown me a letter that Baade sent him essentially apologizing for being so vehement over the previous few years.

Davies

I see.

Sullivan

Saying that you were right. (?) Here’s an article in Monthly Notices in ’54, I guess this must have been based on your Australian work then, talking about correlation with flares of radio bursts and such. But it’s just only by you.

Davies

Yes, that was a statistical survey.

Sullivan

So you were continuing with your solar work even while you were at Jodrell?

Davies

No. This was published from the Radiophysics Division. I don’t think, apart from maybe some corrections in proof or something, but that was submitted from CSIRO, I think. I left in ’53 about September. And I think all had been accepted by that stage, I don’t recall exactly.

Sullivan

Ok, we’ve talked about Cas. Now, what was the conclusion about Cygnus based upon absorption?

Davies

Cygnus, that it was outside the Galaxy and that’s all we could say. There was absorption right in the zero velocity features and in the minus 100 kilometers a second, I guess. That was that.

Sullivan

And I notice, also, a paper here in Nature in ’55 in which you derived a distance for Sagittarius A.

Davies

That’s right.

Sullivan

Can you tell me about that?

Davies

In that one we looked at the absorption integral, the amount of hydrogen, the total absorption and compared that with the emission that was known from our own arm and all the other arms between us and the Galactic Center. And putting in a value for the kinetic temperature that was generally accepted at that stage. It looked as though one got an integral that corresponded to all the absorption occurring out into the Sagittarius arm. So we concluded the thing was in the Sagittarius arm and not in the Galactic Center.

Sullivan

In other words, -

Davies

We got it wrong.

Sullivan

It would have to be between the Sagittarius arm and the next arm towards the Center?

Davies

That’s right, yes. We assumed there were a number of such arms in between us and the Center.

Sullivan

That was an assumption really?

Davies

Well, the Kootwijk survey had shown the existence of the next arm in(?) we thought it was in front of that. You got through the Sagittarius arm and then the Norma-Scutum arm, they called it, I think at that stage, and we reckoned this source was in front of the Norma-Scutum. That wasn’t a dynamic method, that was just a line integral method. Which we now know because of the big variation in optical depth in clouds depending upon their temperatures (?)

Sullivan

Right, well, in fact, in your original Nature article (here’s a Xerox of it) you were using a combination of the two methods weren’t you? You go through what today seems a bit of a convoluted way of talking about the expected amount of absorption and the absorbed ratio, expected-to-observed, and therefore you come up with a .28 and .16 and the one is… I’m sorry, this is one arm – this is the other arm. For Cygnus, it’s clear that you were getting 6/10 of what you want so that’s close enough to all that you want. And then you get 3/10 for the second one and therefore it’s likely that’s alright. Cassiopeia -

Davies

(?)

Sullivan

That’s right. Here it is, “In order to establish the distance of the radio stars from these results it is necessary to know the total absorption to be suffered by the 21-cm radiation to completely traverse the gas in the spiral arms.” That's what I find hard to understand. Why don’t you just take the actual presence of absorption as enough indicator that it must behind? Instead you’re trying to -

Davies

This is with a very simple minded view of what spiral arms were like. In those days, we thought arms as being uniform tubes of gas and if something was behind it, you’d get absorption from everything that you saw in emission. It wasn’t… there was no clumpy model at that stage, just uniform tubes. So you thought, "We’re going to see everything in absorption we saw in emission."

Sullivan

So in order to make this a good argument, it had to agree not only in the presence of the absorption, but also the amount of absorption?

Davies

That’s right.

Sullivan

I also have a note here which I’d forgotten about that you got a much lower amount of absorption than the people at NRL did, for instance, because of your broad filter.

Davies

I guess the integral would have been the same, but the peak -

Sullivan

Well this is what you’re using, I think is the peak amount of absorption here. The peak local arm feature is 4-km per second wide, Perseus is 15 and your things were -

Davies

At 30–40 KC bandwidth there must have been 10, at least, kilometers a second.

Sullivan

I don’t see it right now -

Davies

That’s a test of memory to ask me what I wrote in a paper – whatever it was – 22 years ago. I got some of it right.

Sullivan

Now, you must have probably been at the Jodrell Bank Symposium in ’55?

Davies

That’s right.

Sullivan

That and the Paris Symposium are two rather interesting meetings to me in terms of the last of these meetings – something on radio astronomy. Do you have any impressions of the state of the art at that time that came out of that meeting?

Davies

I remember the Jodrell Bank Symposium – the great excitements at the time were the optical-radio correlations in Cygnus and Cassiopeia for the identification of Baade and Minkowski, I think. They were the center of interest as far as the radio astronomers were concerned at least. Working in the field that we were at Jodrell Bank. On the Cambridge side – I’ve just lost the timing. I’m not sure quite where they were at that stage.

Sullivan

They gave the 2C results. The 2C was published in ’55.

Davies

Fifty-five was 2C, right. That was well received as far as I remember. For instance, at that stage, there was no criticism that they had phantom sources in their lists. I think that really didn’t occur until Mills had made some progress in the South. The big controversies blew up then at the Berkeley meeting was when they set-to one another. At that stage it was all accepted (?)

Sullivan

It wasn’t so much controversy at that meeting.

Davies

I don’t remember at all. At least I don’t remember any controversy.

Sullivan

Well, while I’m going along this line, what about the Paris Symposium in ’58? How does that strike you as outstanding at that time?

Davies

I find these two almost indistinguishable in my mind now without going back to the proceedings. At that stage – let me just say something about the topics that we were involved in. One of these was the absorption in Cygnus A which maybe you wanted to talk about a bit later. But here was one of redshift absorption – we were very much involved in that at that time.

Sullivan

Did you report on that?

Davies

I’m not sure it was then or after that period.

Sullivan

At the Paris Symposium you talked about HI associated with local early type stars.

Davies

Yes. That was the Gould Belt system. That’s right, ’58 was before the Mark I was fully operational in the 21-cm, so we hadn’t done the absorption experiment at that stage.

Sullivan

Well, I’ll come back to these specific things. But what about the flavor of the Paris meeting? It seems to me like this was sort of the last time that one could define radio astronomy as a subject of a meeting.

Davies

Yes. I think – … What I remember was that you attended interest of everything that was going on. You got yourself involved what was happening in solar physics. You tried to keep up to that stage, I remember, just in case there was something relevant to your own field, or something you could observe yourself with your instruments. I think it was about that time I said, “Well, solar physics is something that could pass as far as I’m concerned although (?) (?) in Sydney but no having done very much – no solar work at Jodrell Bank unless one kept it in mind as a possible avenue of research. If something turned up that would be done with big telescopes. At that stage, one said, “Well you can’t keep up with all of radio astronomy.”

Sullivan

So you couldn’t keep up with the entire field. However, you made an interesting comment to me that even though that’s true, you still apparently don’t feel confined to one field, but rather to whatever can be done with the instruments you have at hand.

Davies

Yes, yes. But this can happen within related fields. For example, for quite a while I was involved in doing work on the planets. Because we had a good receiver that could be used for total power work on polarization work. We were interested in the polarization and the emission of the planets, particularly the search for any synchrotron emission after Jupiter had been identified as a synchrotron source. We were interested to see whether Mars and Saturn were synchrotron sources. We made a few measurements there in the planetary field just because we had the equipment to do it and physics wasn’t too far removed from what we were involved in with extragalactic sources. That really has continued much the same way up to the present day within a group at Jodrell Bank that I’m involved with. Perhaps not so much in the others, with work in galactic structure, interstellar medium, work in external galaxies, and work on magnetic fields with the Zeeman effect. With neutral hydrogen (?) mainly involving these type of measurements. [loud noise of a plane overhead] There’s one used the neutral hydrogen techniques to investigate quite a range of problems.

Sullivan

Ok, just to finish off the meetings in ’61, the IAU meeting you mentioned that Mills and Ryle were really at each other even more than the Paris Symposium. Someone else mentioned that to me this morning and I was surprised. I thought the Paris Symposium was the peak of the controversy of the log N, 2C. That had been established by that time. What was the real nature of the argument at Berkeley?

Davies

It was log-n/log-s, the slope of log-n/log-s was the real problem there. Mills was arguing that Cambridge had missed a lot of extended sources. I think they had gotten to the stage that both sides having statistics they were reasonably happy with. I’m not sure at the Berkeley meeting they had the 3C catalogue out. (?) Well, I think Cambridge felt that they had been hauled over the coals over 2C and they had been very careful with 3C and got everything right. But Mills was saying no, they’d missed a lot of extended sources. But actually Cambridge said, "Well Mills has manufactured sources from fluctuations in the background within the extended beam." This is what the basis of the controversy. Each stuck to their own statistics. They were looking at log-n/log-s. It moved, I think, into a cosmological argument whereas 2C scarcely had got that far. Much more hinged on the discussion, I think, at the Berkeley meeting.

Sullivan

Well, it’s true that 2C was not only cosmological, but I mean there was the whole technique business but the - There was a lot of cosmology mixed in. Maybe not so much Mills vs Ryle, but Hoyle vs Ryle.

Davies

That’s right, yes. As to whether the slope was different from 1.5 was an important argument.

Sullivan

I don’t quite see the publication on looking for the higher redshift absorptions in Cygnus A. Can you tell me about that? When was that done?

Davies

It had been done after the telescope was complete in -

Sullivan

Oh yes, Davies and Jennison ’59, “Non-confirmation of Lilley-McClain Cygnus A.”

Davies

That’s right, yes. They’d made this measurement and had claimed an absorption. I remember Lovell coming around and jumping up and down and saying, “Well, why haven’t you blokes observed this thing?” And we sort of gave a reason for why we thought it was highly unlikely to get an absorption. But then we had a go at this, but realizing there was another set of number you’d get out and that’s the intergalactic density. You could actually measure the trough of the redshifted gas between us and Cygnus. We were careful in getting the local absorptions as well as the end of the trough down on 16,000 kilometers a second. And we were able to use the Mark I which also gave us a lot more sensitivity than Lilley-McClain used at that time. And didn’t find a thing down there. Ableto show pretty convincingly, I think, that there was no redshift absorption in the Cygnus A itself. But, in the meantime, the interesting question of the intergalactic medium had come up and George Field had made a measurement with the Harvard dish and he’d set some limits on that. And then we realized with a stronger continuum source we could go to much lower limits than he was able to get. So we did some quite careful measurements with the galactic densities as well which was described in the paper in Monthly Notices with Jennison.

Sullivan

Was that ’61, perhaps?

Davies

It may be, around about that time. And I think soon after we’d published that Lilley-McClain admitted that they thought they’d got interference in their band. I’m not sure they ever published that.

Sullivan

It was in Scientific American, actually.

Davies

I see. They admitted it quite openly.

Sullivan

It looked quite believable, though, as published?

Davies

Yes.

Sullivan

You had not doubts as to its reliability.

Davies

And George Field made another point. I think this is still true that you don’t expect much absorption in these sources because the continuum flux is so high that it pushes the ground state hydrogen out into the upper of the two hyper-fine states, and it won’t absorb. You’ve got to go a long way outside the Galaxy before – a strong radio galaxy – before you’ll ever get absorption.

Sullivan

He made that point at that time?

Davies

I’m not sure exactly when this occurred. I can check it.

Sullivan

Now here I see something you referred to already. At the Symposium at Cambridge, Massachusetts in ’57 you talked about that the HI was cooled in some regions and I guess that was referring to your anti-center cloud?

Davies

That’s right.

Sullivan

Was that something new at that time?

Davies

Dave Heeschen had got something similar. I think it was mentioned in that meeting within a year or so and I’ve forgotten we were looking at quite independent things – he was looking at the Galactic Center and found a very narrow feature in the Galactic Center. In fact, got within a cloud about 30&° across, in fact. And he found this low temperature there – this independent one that I’d gotten, and we'd added up the story between the two made me realize that this was a cold component in the interstellar medium. Colder than the canonical 120°, I think the Dutch had come up with.

Sullivan

It’s fair to say that I think the Dutch model was really one of uniform density and temperature except for the fact of the spiral structure, and once you were within an arm it’s all sort of uniform.

Davies

That’s right. But I think (?) about that time was talking about the harmonic mean temperature – this is generally what you will see. And it was a surprise that one was coming up with these lower temperatures talked about the cooling mechanisms and so on.

Sullivan

But was anyone looking for hot clouds at that time?

Davies

They are difficult to observe, that’s the trouble. You were aware of that. I think the temperatures in the absorption spectra were only discussed in any details later in the early 1960’s outside the period you're talking about, where Verschuur and Shuter at Jodrell Bank had got all these absorption features on the strong radio sources and they went through and analyzed this data for temperatures.

Sullivan

Yes, Bill Shuter told me about that. And also Barry Clark about this same time.

Davies

Yes, they’d gotten the interferometer (?)

Sullivan

Now it’s interesting. What was the nature of this symposium in Cambridge, Massachusetts in ’57? I'm not sure i know which -

Davies

It’s published in Reviews of Modern Physics, and it was run by R. N. Thomas.

Sullivan

Oh, was that the one.

Davies

A lot of theoretical people gathered and some observers. It was very stimulating. It helped me a tremendous amount at that particular meeting because of the theoretical people were there as well discussing things in a way that we could understand. I think Thomas made sure that the theoretical and observational people were getting to know one another’s language.

Sullivan

Which doesn’t happen much, even today.

Davies

That’s right. It was a small enough meeting to be very stimulating as far as interstellar medium was concerned.

Sullivan

What did you learn?

Davies

Well, I think (Kahn?) for example, talked about his collision model for interstellar clouds, the process of cooling in clouds, the stochastic nature of the intercloud medium came up, a model which (?) were trying to get together sort of the optical end of radio absorption data so that we went away with quite a lot of stuff we then applied directly to our observations.

Sullivan

During this time, it seems like you were doing several HI projects that are related, one might say, to optical astronomy. For instance, you’re looking at dense dust clouds, you’re looking at Gould’s Belt. These are, perhaps, more optically oriented than any radio astronomy.

Davies

Yes.

Sullivan

What is the reason for this?

Davies

I think it was from a realization very early on that we were dealing with the same thing. It also – we haven’t talked about this very much yet, and that’s the Gould Belt system. This is really what signaled to me we had data that was complementary to the optical stuff. The Gould Belt region which is well known in terms of the HII region, the younger stars, the dust, is just the very same region where we get an excess of neutral hydrogen. And that was a case where I got all the optical data out and did what I could to correlate the two. And immediately, I think, thrust itself into my mind from then on that we really were very much doing work in parallel with the optical people. It was only by putting the two lots of data together that we made any sense of it.

Sullivan

What was your reasoning that Gould’s Belt should have HI also?

Davies

Well, it worked the other way around. Having found the HI one went back and saw why.

Sullivan

Oh, that’s the way it happened.

Davies

We’d found this HI well away from the galactic plane. This was using the 30-foot dish at Jodrell and I did a complete sky survey, in fact. Having finished the absorption exercise, we had a nice system going and I just surveyed the whole sky at 21-cm. The Dutch had done the Galactic Plane, but what I did was then look at intermediate latitudes. And about the same time people at DTM made a similar survey, (?) and company. And it was complementary to ours in one way or another – I didn’t have the temperature resolution or the frequency resolution and my stuff clearly showed that the Gould Belt system having the whole sky, the whole sky map. Then that was sort of obviously a part of the Gould Belt system when you looked to the stars the general asymmetry… asymmetry to the whole thing. At 20° off the plane, you see hydrogen was very, very strong in the Gould Belt region but essentially undetectable at similar latitudes elsewhere. A major symmetry. A number of other features, too, that came up in the whole sky thing. The Cepheus region is a similar thing to the Gould Belt system. Cygnus X also. Cygnus X, again, was one of the other studies that I’d done some continuum work to make it up into the whole study.

Sullivan

Were you working with any specific optical astronomers or just sort of reading books?

Davies

I was just reading books, in fact. But talking to them. About at that stage, I’d made contacts at the Harvard meeting. I remember at that time Bok was running the Harvard Radio Astronomy group, being an optical person doing radio astronomy which seemed back to front to most radio astronomy people. They didn’t expect him to get very far. In fact, he stimulated within his own group a close tie-up between optical and radio. And also, did the same thing in me, in fact. I learned quite a lot in that respect, and particular friendships have continued since then. Something that stimulated me as a result of that particular meeting (?)

Sullivan

What about the fact that major amount of HI work had, of course, been done by the Dutch by the time that you were doing your stuff? They were, of course, Oort and it all came out of an optical observatory – it was all very much associated in classical astronomy. Do you think orienting you in that way at all - ?

Davies

That, also, is another reason why one kept an eye on the optical side, because they were people who had worked in galactic structure up to then – had been the problems with optical people. So, on the hydrogen line side, I think this was a natural way to go to keep in close contact with the optical work, whereas galactic radio source work there was nothing optical to tie in with in any way by and large. So the radio astronomers there tend to be estranged from the rest of the astronomical community. The HI observers weren’t. Excepting, perhaps, the early days with Christiansen and so on, were doing their own thing, Ewen – they were essentially outside the optical fraternity. Oort and company were very much a part of that in terms of (?)

Sullivan

That’s an interesting comment. You probably knew – you probably were a correspondent to this 21-cm newsletter that we around for a couple of years. Did you get that at that time?

Davies

I got it, but I don’t know. It only existed for a short time after we got going.

Sullivan

It was only two years, ’52–’54 or something like that. But it seems to me that this is tremendous evidence of cooperation amongst the few observatories working on it. I mean, it’s impossible to imagine a radio source newsletter people giving their results before the publication.

Davies

That’s right, yes. That particular issue, though, was a question of personalities in different countries, I think. A different thing probably because of the subject. The optical people were used to talking to one another. The radio people tended to be a lot of individualists who’d come out of the War situation. They were going to do their own thing or bust almost.

Sullivan

Yes. It was a very different philosophy.

Davies

They tended to be by and large much more aggressive than their optical counterparts.

Sullivan

Now, I see a publication in ’57, in the Australian Journal of Physics, Christiansen, Warburton and Davies on the slowly varying components, and so forth. Did you go back to Australia?

Davies

No. What happened was that we had done some work together just before I left. I got out of Piddington’s domain – we were doing statistical survey or statistical analysis data. Then I got myself involved much more in observational work. At Pawsey’s instigation, really. And my own feeling that I didn’t want to get involved only in the interpretation. I wanted to get involved in learning about the equipment and getting results of my own. And I worked with Christiansen and Warburton with the 32 element interferometer, 16 elements at that stage. They were building it up to 32 about the time I left. And I had been working on the spectrohelioscope that we had observing each day the brightness of the H-alpha regions on the sun. It was while doing that I discovered the H-alpha emission was correlated with the radio bright spots that they were getting from across the dam. I was on the other side of the Potts Hill Dam with this thing (?) I was responsible for the H-alpha work. That’s where we spotted this correlation between the radio hot spots. And before anything like sunspots appeared, you get an H-alpha plage appearing. And following that up, I then measured the positions of these H-alpha plages and we correlated those with the position of the continuum emission and watched its progress across the sun and by looking at the rate of movement you can actually assess the height of the stuff. We were able to get some information on the heights and sizes of the radio emitting regions tied to the optical data. That was that ’57 paper, a result of the work that was started before I left in ’53. They did a lot more work after I left extending the whole picture through a period of time. But the essential conclusions were worked out before I left.

Sullivan

Ok, to touch upon these others. I see Davies and Jennison 1960 using the Mark I for observations of Sagittarius A, Cygnus X, and the Moon. This seems to fall in the classification as observatory which we were saying you had the dish and just measured everything in sight.

Davies

Yes. Really, the first observations with the Mark I, we put our receiver on there and -

Sullivan

Well this was ’60 -

Davies

We got going in ’59.

Sullivan

I see, so this period is early ’60.

Davies

Yes. (?)

Sullivan

Here’s one with Hanbury Brown and Hazard about the spur at about 30° longitude.

Davies

Right. That’s an interesting one. They had been working on this for ages. The distribution of continuum emission around the Galaxy and they’d been thinking more in terms of this being the radiation from the spiral arms, or one of the spiral arms near the edge, at least l = 30° being near the edge of one of the spiral arms. They had a model in which they were thinking about this. And we had a lot of discussions over coffee and tea. They tended to be very open and free discussions that we had. And I had been thinking more about supernovae for some reason or another and we got to talking about this and it seemed to me that this was a possible supernova type explanation.

Sullivan

A very nearby one?

Davies

A very nearby one. And worked on this together and developed this idea. Though it wasn’t observed in any of the observations. I came in because it was one of these topics that had been raised in the discussions and we worked out the explanation together and pushed ahead with this supernova explanation of the spur rather than geometries of the synchrotron emission.

Sullivan

What about this one in the Jodrell Bank Annals about the polarization of the Crab – was that done with the 30-foot dish, I guess?

Davies

That was done with the 30-foot dish. About the time – what I remember, Gart Westerhout had observed something around about that time.

Sullivan

Yes.

Davies

And we had a polarimeter going; we thought this was a good opportunity to see if we could check this. And we pushed right down the limits and I think they were comparable to what he got.

Sullivan

Less than 2.75 percent?

Davies

I remember something like 1.6. So we never observed any significant polarization. I’ve forgotten now just how significant that was in those days.

Sullivan

I’m not sure either. But it seems like from the abstract that you were pretty clear how that could come about, that it would be much less than the optical polarization.

Davies

That’s right. Presumably at that stage someone observed at shorter wavelengths at higher percentage polarization. Hang on, the optical had been observed, that’s right.

Sullivan

Right. But it was also just about precisely this time that – you probably didn’t know about NRL – that Mayer and company picked up at 3-cm.

Davies

Yes, that was before we knew about it, yes.

Sullivan

Well, the final thing that I see here on specific projects is the Zeeman splitting. Can you tell me about that experiment?

Davies

Well, once we’d done our observations on Cygnus, we realized that we had a dish at 21-cm that gave us good efficiency compared with smaller dishes but it had a bad side lobes. The old Mark I had something like 80-percent of its energy in the plateau around the main beam. So I knew immediately that we weren’t going to be spending any time mapping our own Galaxy because essentially you ended up with a half degree beam rather than a quarter of degree beam which is where all that energy was. And it would be incredibly difficult to interpret it. So I then looked around at significant experiments we could do with – where we could work on point sources, small angular diameter objects.

Sullivan

Was this due to just surface irregularities?

Davies

Surface irregularities in the Mark I. It was originally designed as a 1-meter dish and after the 21-cm line had been discovered from hydrogen, we thought it was worth putting the uniform sheet surface on it so that we could use it at 21-cm. No matter what its gain was. So we got quite a high gain, something like 5 flux units per degree, enough to be able to do some hydrogen work, a lot of hydrogen work. Cygnus absorption, Cas absorption, and so on. In fact, all those early programs of things you could do -

End Tape 71B