Interview with Jan Hendrik Oort on 16 March 1972

[Begin tape 9A]

Sullivan

This is 16 March 1972. This is part 2 of interviewing Professor Oort about Dutch radio astronomy. Now, you said that Dutch radio astronomy was where you were primarily developing receivers for continuum and that the line was going to be a bonus, if anything. Now, of course, it turned out to be the other way around, that the line was the most important thing. Were there actual continuum observations done before the line observations?

Oort

No, none whatever. So, when the line was received it was evident that so much interesting work could be done observing this hydrogen line that the continuum work in comparison seemed much less interesting. We only started continuum work I don’t remember exactly now. The first survey was made by Westerhout. This is still quoted as Westerhout sources - W51, etc.

Sullivan

That was in ’56 or something?

Oort

I would have to look that up -

Sullivan

Much later anyway -

Oort

- for the exact date. It must have been around that time. And then, of course, we were very much interested in detecting polarization because that had not been demonstrated with any certainty before, and that would be the ultimate proof for the synchrotron mechanism of the radiation. This was first pushed by Charles Seeger, the polarization work. The first actual survey was made on 408 MHz frequency. And that was really the first good discovery of polarization of the radiation coming from the Milky Way.

Sullivan

About what time was this?

Oort

I’m very poor in years – I’d have to look that up.

Sullivan

So that was December of ’60 for the Seeger polarization?

Oort

That was the first observations of the polarization. These were later extended to many different frequencies. The measures were all done with the Dwingeloo telescope which was the successor of the Kootwijk 7½ meter telescope with which the original discovery of the H-line was made, and also the first survey, the first complete survey of the Galaxy. You were asking me about the particulars about the Leiden discovery of the 21cm line. We had at that time a very intensive contact with the Harvard group working on this same problem at the instigation of Purcell at Harvard, and as you know it was Ewen who first discovered this line a few weeks before it was actually observed in Holland with the receiver built by Muller. At the time of the discovery – or at the moment when we found it in Holland, Van de Hulst happened to be in the United States and was staying at that time in Cambridge, Massachusetts, and I remember that we had a long telephone conversation about this discovery and about the discovery in Holland. At that time, it so happened that we were working in a transmitting station of the PTT in Holland, and one of the directors of the PTT gave us the opportunity of having this free conversation just as long as we wished (laughter).

Sullivan

I see, so you got all the details.

Oort

It was quite an event at that time.

Sullivan

So, was the Dutch detection in the nature of an independent detection or was it a confirmation of the Harvard results? In other words, did you know about the Harvard results when you first detected?

Oort

Oh yes, we knew that they were working on it – we had very close contacts and we actually knew that they had discovered it, but it was just that the construction of the receiver was a little bit further developed there – because we were looking for it and had all the equipment ready, so to say. Perhaps it was also because Dr. Muller was a very precise man who wanted to have everything very perfect before he started observations. But it didn’t make any difference to us and we have never had that feeling of competition at all. From the beginning, I should say, there has been very intense and pleasant cooperation between the various groups that worked on H-line astronomy. We had a kind of typewritten bulletin that we distributed among the various groups – a newsletter one might say, which went very regularly during those first years. And in a most pleasant way I must say, not only the contact with Harvard, but also with the Australians. But, it so happened that in Holland we were more actively interested in the results to be obtained by this detection. In the case of Ewen and Purcell it was more an interest in the technique and in the physics part of it than in the astronomy part. While we had been starting entirely from the other direction – we saw the technical problems just as a kind of evil that you have to take into the bargain. Already right after the first observation I remember we got some data on the rotation of the Galaxy in a region which was quite inaccessible by the optical means at that time. Then the - By 1954, 3 years after the -

[some kind of break in the tape here] [interruption at the door]

Sullivan

You were talking about the – you got the very rough galactic rotation . . . one of the first things that you did.

Oort

Yes, after that, of course, we started on getting a complete rotation curve and especially also on getting the complete map of the spiral structure in the Galaxy which had from the beginning been our main hope, so to say, of getting it by the radio methods, because it was just this very large scale structure, of course which optical means were not able to see at all. The observations for this map were made in Kootwijk at the transmitting station of the PTT with the 7-1/2 meter telescope in ’52 and ’53, and were published in ’54.

Sullivan

In the BAN [Bulletin of the Astronomical Institutes of the Netherlands]?

Oort

In the BAN - yes, then very soon or about the same time we made a more complete investigation of the rotation curve. At that time the first indications were found of these long tails in the line as (?) from directions within, say, 15 degrees from the galactic center. That was actually the first indication of the expanding motions in the central region of the Galaxy, and of the very fast rotations in the innermost part. At that time, it wasn’t realized what the correct interpretation of this was, but it was fully realized that something very remarkable happened in this central region.

Sullivan

I just thought of one question – Did you think at all about looking at hydrogen absorption? I know this was discovered first at NRL, but I was just wondering if you people - ?

Oort

Well, we made absorption measurements but not so very early in the game. I would have to look up the date again, but we did find -

Sullivan

Well, I can check them later.

Oort

The absorption in the 3 kiloparsec arm in Sagittarius A at a fairly early date. This then adds the direct proof that hydrogen had very large radial motions in the central region of the Galaxy and from that observation started more complete observations of the expanding features in the central region. This was really one of the entirely new and unexpected fields that were opened by the 21cm line observations. In a way one might say that this spiral structure and rotation curve and mass distribution in the Galaxy were things that had been fully expected and it was, of course, interesting to delineate these things in the Galaxy, but they didn’t produce anything really new. But what was quite new were these expanding features in the central region. And then later the discovery of this rapidly rotating nuclear disk.

Sullivan

What about the nature of the interstellar medium? Did ideas on that change much because of the - ?

Oort

No, not so very much, in fact. Of course, one got from the 21cm line data direct values for a kind of average temperature of the cool gas in the Galaxy. But it was rather surprising at that time how well those fitted in with theoretical computations that had been made by Spitzer, for example, before the knowledge of the hydrogen line. They came to about the same kind of value of 100 degrees or so as we did.

Sullivan

Were those values based on the optical absorption lines? Were those previous computations based on the optical absorption lines primarily?

Oort

Yes, mostly . . . I would have to think a little bit about this because it is so long ago again – how this was exactly obtained. Well, it was obtained, I think, more by considering all the processes of heating and radiation that would happen in the interstellar gas and it was on the basis that Spitzer had derived these temperatures. Then - around the middle of ’57, a complete new map of the Galaxy was made. The first observations were confined to the positions quite close to the galactic circle, but in ’57 we published this whole spiral map of the Galaxy in which men like Westerhout and Schmidt collaborated to a great extent, and in a way this map, which was still based on the old Kootwijk instrument, 7½ meters, has stood up to the present as the only – more or less complete – map of the spiral structure in the Galaxy – in so far as we could see it from the northern location. That was made with a beam of about 2 degrees, but around that same time our new telescope in Dwingeloo was completed, a 25-meter telescope, with a beam of about ½ degree. That led to a very much better idea about the things that happened in the central region of the Galaxy. This whole unravelling of the expanding features and rotating disk was all done by the Dwingeloo telescope which was inaugurated in 1956. If I now think of the really new things that were found with these instruments, another new unexpected discovery was the discovery of high velocity clouds in high galactic latitudes.

Sullivan

Now, you’re getting a little later. I had a question concerning the Dwingeloo dish. Now, similar surveys of the galactic plane were carried out with the Dwingeloo dish.

Oort

Yes, but not so complete because it had this small beam and it was much more difficult to cover the whole sky with it, so we never made a very complete survey, but we did make surveys of relatively narrow zone around the galactic plane.

Sullivan

My question was; do you think that the Dwingeloo dish really added that much to the large galactic structure?

Oort

Not so very much.

Sullivan

The small dish was really more ideal?

Oort

Yes, because you get too many details with the smaller beam and it so happened that this old radar installation that was left in the sand dunes by the Germans was an ideal instrument for mapping the large scale structure of the Galaxy. But as soon as we got interested in somewhat more details like these expanding features in the center and this rotating nuclear disk, then the smaller beam of the Dwingeloo dish was a very great advantage.

Sullivan

Now, when I interrupted you, you were starting on high velocity clouds.

Oort

For the high velocity clouds – it might not have been such a great advantage to have the small beam – although, it did help to unravel the very intricate structure of these features.

Sullivan

And when were they first detected – was it around 1960 or - ?

Oort

No, considerable later. It was really at the Noordwijk Symposium on radio astronomy and the galactic system that the first somewhat complete results were presented on this.

Sullivan

That was ’66, I think?

Oort

Yes, and, well, the first indication had been discovered about a year before or so.

Sullivan

While I think of it – concerning meetings – now you mentioned that you put together this map which one sees quite often, even today, from the Kootwijk observations, and then this was published in the – was it the BAN?

Oort

Here is a combination of the fundamental articles on this – you see that’s the first one along the galactic circle, and this is the rotation curve in the inner parts and this collects all the later papers still made with the Kootwijk instrument, and the paper by Westerhout on the outer parts, and by Schmidt on the inner parts of the Galaxy.

Sullivan

Right, that’s Vol. 12 and 13 - ?

Oort

That’s where the maps are - [shuffling of paper] -

Sullivan

Now at what time did you put this map together with the Australian side of the Galaxy?

Oort

That was done about the same time because they had practically completed their map already before that time, I think. But that was very much more rudimentary and they had wide spacings between fields. So the southern map was never really completed as it might have been. But that is because in Australia there was only one man, Frank Kerr, who was really actively interested in this kind of problem -

Sullivan

And he was doing this on a -

Oort

Of course it’s the interest of the astronomer that counts -

Sullivan

This is a 60-foot antenna he was using; I think? So, -

Oort

No, no that’s the large present telescope. No, they had – yes they had about 15-meter, I think - I’m not quite sure -

Sullivan

Somewhat bigger than at Kootwijk?

Oort

Yes. . . Well, the high velocity clouds, if we may return to those, interested me particularly because it seemed to me that they gave an indication of gas still flowing into the Galaxy at the present time, and so, gave one the conviction that the galaxies are still growing, even ten billion years after their formation. And also, they indicated another interesting fact, that apparently the interaction which gave rise to these cloud formations took place at a very high elevation above the plane, and they led us to make a special study of the halo of the galactic system – the halo of cool gas of the galactic system, which was again a somewhat unexpected thing. One did expect that there would be a halo, because of cosmic rays, but we didn’t – we couldn’t really hope that there would be much neutral gas in this halo, so we were quite lucky, I think, to find there was so much neutral gas, and, at present time, this (?) should be there. Now, I don’t know how far you want to continue with this subject.

Sullivan

Well, I just wanted to ask you first about – you always had an interest in large scale galactic structure and I would – would you say cosmology also, is that why you were particularly interested in the growth of the Galaxy from the high velocity gas?

Oort

I became – I got a new interest in cosmology as a consequence of this, but actually the observations in the field of cosmology in general lay entirely outside our instrumental possibilities at that time. It was only by constructing a very much larger telescope with a very much higher resolution that we could enter that field outside our own Galaxy. And, because we wanted very much to do this, we started on a project for an array telescope which would give a very much higher resolution. You have to make a big jump there, of course, if you go from the Galaxy at 10,000 kiloparsecs to distances of 10 million more parsecs. You have to make a jump of a factor of a thousand.

Sullivan

Now, was this a fundamental shift in the philosophy of Dutch radio astronomy in your opinion – going from galactic to extragalactic - ?

Oort

It was a very natural shift – a very natural extension in a way, because we were very much interested in spiral structure – in the problem of spiral structure – and it was of evident importance to try to study these structures in other galaxies. If only because – in our own Galaxy we are in a very poor position to study this spiral structure. So, I think the main reasons for starting on this new telescope were firstly: to be able to study other galaxies – to study the phenomena that we found in our own Galaxy in other galaxies, in particular, the spiral problem. And, in the second place, to start on cosmological work in - which I, at least, was very much interested – had always been very much interested in – but these interests have been more or less sleeping because we couldn’t do any observations. But, it was certainly the intention of the new project to use the array for cosmological research and they are still hoping to do that.

Sullivan and Oort

But that’s a new subject -

Sullivan

Do you have any other scientific comments that you wanted to make about the things you’ve been involved with connected with radio astronomy in any way?

Oort

You mean before we get to Westerbork? No, I think this is enough (laughter). There are, of course, very many details and I’ve only mentioned the things in which I have personally been mainly interested. But, there was, of course, this continuum survey of Westerhout which gave quite interesting new data also – and there has been very extensive polarization work on different frequencies and discussions of the structure of the magnetic field. But, in a way one might say that these were somewhat disappointing from the point of view of the astronomer interested in large scale structure, because it turned out that so much of the magnetic field structure that you see by making radio observations pertains to spurs or loops and things that are of a very local nature and probably connected with supernova outbursts. They are very interesting in themselves, but not so interesting for the large scale structure.

Sullivan

And that was the original hope when these observations were - ?

Oort

When we started that, I had hoped that we would really be able to find out much more about the large scale structure of the magnetic fields. In particular, in the halo. At that time, we thought that much of the radiation that you see in high galactic latitudes came from a halo around the galactic system, and if that had been so, these polarization observations would have been able to unravel the structure of that halo field very well. But, it finally turned out that most of this high latitude radiation comes from these loops of nearby structures. Still, there is the possibility to get further into the structure of the galactic field by going to higher frequencies, and they are just now starting out work in Dwingeloo at the 11-cm, as you know. And, as one goes to higher frequencies, one penetrates further into the disk also, and you get more chance of getting information from the disk field. But that is still in its rudimentary stage.

Sullivan

Well, now about Westerbork. First of all, do you have time?

Oort

Yes.

Sullivan

When did you start thinking about that? – I think you were one of the moving forces in getting it going.

Oort

The first proposals to get a one-minute arc beam were made around 1959. It was especially Charles Seeger who instigated our more specific thinking about this possibility. At the time we were thinking of constructing a cross antenna telescope like a Mills Cross. We were thinking of constructing a cross for a frequency of 408 MHz. When we started this project, I had a feeling that one should try to combine, if possible, with another country in constructing such a large instrument. I have always been very much in favor of international cooperation – and this seemed to be an evident possibility in the case of such a large radio telescope. So, we took up contact with the Belgians at that time who were our closest neighbors, of similar size, one prefers to work in such a set-up with one partner that is more or less on the same order of national income (laughter) and the Belgian government was interested in the project at the time – but unfortunately, although the Dutch government -

[End Tape 9B]