An amazing composite image of M31 in Andromeda using both infra-red and X-rays was recently obtained using Herschel and XMM space observatories. It featured in the BBC Stargazing Live programme earlier this week and I’m told that, typically for astronomy, the inspiration behind it was … beer.
via The Herschel Space Observatory
I’m told that there was a partial eclipse of the Sun visible from the UK this morning, although it was so cloudy here in Cardiff that I wouldn’t have seen anything even if I had bothered to get up in time to observe it. For more details of the event and pictures from people who managed to see it, see here. There’s also a nice article on the BBC website. The BBC are coordinating three days of programmes alongside a host of other events called Stargazing Live presumably timed to coincide with this morning’s eclipse. It’s taking a chance to do live broadcasts about astronomy given the British weather, but I hope they are successful in generating interest especially among the young.
As a spectacle a partial solar eclipse is pretty exciting – as long as it’s not cloudy – but even a full view of one can’t really be compared with the awesome event that is a total eclipse. I’m lucky enough to have observed one and I can tell you it was truly awe-inspiring.
If you think about it, though, it’s a very strange thing that such a thing is possible at all. In a total eclipse, the Moon passes between the Earth and the Sun in such a way that it exactly covers the Solar disk. In order for this to happen the apparent angular size of the Moon (as seen from Earth) has to be almost exactly the same as that of the Sun (as seen from Earth). This involves a strange coincidence: the Moon is small (about 1740 km in radius) but very close to the Earth in astronomical terms (about 400,000 km away). The Sun, on the other hand, is both enormously large (radius 700,000 km) and enormously distant (approx. 150,000,000 km). The ratio of radius to distance from Earth of these objects is almost identical at the point of a a total eclipse, so the apparent disk of the Moon almost exactly fits over that of the Sun. Why is this so?
The simple answer is that it is just a coincidence. There seems no particular physical reason why the geometry of the Earth-Moon-Sun system should have turned out this way. Moreover, the system is not static. The tides raised by the Moon on the Earth lead to frictional heating and a loss of orbital energy. The Moon’s orbit is therefore moving slowly outwards from the Earth. I’m not going to tell you exactly how quickly this happens, as it is one of the questions I set my students in the module Astrophysical Concepts I’ll be starting in a few weeks, but eventually the Earth-Moon distance will be too large for total eclipses of the Sun by the Moon to be possible on Earth, although partial and annular eclipses may still be possible.
It seems therefore that we just happen to be living at the right place at the right time to see total eclipses. Perhaps there are other inhabited moonless planets whose inhabitants will never see one. Future inhabitants of Earth will have to content themselves with watching eclipse clips on Youtube.
Things may be more complicated than this though. I’ve heard it argued that the existence of a moon reasonably close to the Earth may have helped the evolution of terrestrial life. The argument – as far as I understand it – is that life presumably began in the oceans, then amphibious forms evolved in tidal margins of some sort wherein conditions favoured both aquatic and land-dwelling creatures. Only then did life fully emerge from the seas and begin to live on land. If it is the case that the existence of significant tides is necessary for life to complete the transition from oceans to solid ground, then maybe the Moon played a key role in the evolution of dinosaurs, mammals, and even ourselves.
I’m not sure I’m convinced of this argument because, although the Moon is the dominant source of the Earth’s tides, it is not overwhelmingly so. The effect of the Sun is also considerable, only a factor of three smaller than the Moon. So maybe the Sun could have done the job on its own. I don’t know.
That’s not really the point of this post, however. What I wanted to comment on is that astronomers basically don’t question the interpretation of the occurence of total eclipses as simply a coincidence. Eclipses just are. There are no doubt many other planets where they aren’t. We’re special in that we live somewhere where something apparently unlikely happens. But this isn’t important because eclipses aren’t really all that significant in cosmic terms, other than that the law of physics allow them.
On the other hand astronomers (and many other people) do make a big deal of the fact that life exists in the Universe. Given what we know about fundamental physics and biology – which admittedly isn’t very much – this also seems unlikely. Perhaps there are many other worlds without life, so the Earth is special once again. Others argue that the existence of life is so unlikely that special provision must have been made to make it possible.
Before I find myself falling into the black hole marked “Anthropic Principle” let me just say that I don’t see the existence of life (including human life) as being of any greater significance than that of a total eclipse. Both phenomena are (subjectively) interesting to humans, both are contingent on particular circumstances, and both will no doubt cease to occur at some point in perhaps not-too-distant the future. Neither tells us much about the true nature of the Universe.
Let’s face it. We’re just not significant.
I hope the blogosphere hasn’t got too bad a hangover this morning. I don’t, although I did have a nice lie in until about 11am when the lure of the Guardian prize crossword drew me out of bed and down to the newsagents. Luckily, I remembered to get dressed first. The crossword turned out to be quite a nice one to start the year with, by the perennial Araucaria, but it didn’t take all that long to do so I’ve got time to do a bit of shopping and a go on my exercise bike. Yes, that’s my New Year’s resolution. More shopping.
I know 2010 was a tough year for many people for many different reasons. I wouldn’t say it’s exactly been brilliant for me either, but I am looking forward to 2011 whatever it might bring. The first results from Planck will be released very soon (on 11th January, in fact), which will give me something exciting to blog about. More generally, the recent financial settlement for STFC was not as poor as many of us expected so the future doesn’t look quite as grim for UK astronomy as we feared.
There are exciting developments in store for the School of Physics & Astronomy at Cardiff University, where I work, with (hopefully) a number of new staff members joining us soon. Later on in the year we’ll be rolling out a completely redsigned set of physics courses which we’ve been working on for over a year. In addition we’ll be starting to work more closely with Swansea University in order to provide a broader range of advanced options for physics students at both institutions.
Of course behind all this there’s still considerable uncertainty about the funding situation for universities which are facing big cuts in government grants and having to increase tuition fees charged to students. Whether and to what extent this will deter students from going to university remains to be seen. The financial pressure will certainly lead to mergers and possibly to closures across the UK over the next few years, although only time will tell how many.
On the cultural side there’s a large number of concerts at St David’s Hall and a full season of Opera at WNO to look forward to, including a performance of Cosi fan Tutte on my birthday. Cardiff plays host to the First Test match between England and Sri Lanka at the end of May, and a one-day international against India in September. I might even get myself a membership of Glamorgan Cricket Club, something I’ve toyed with doing for a couple of years now. There’s also a good chance that Cardiff City F.C. might get themselves promoted to the Premiership, something that would be great for the city of Cardiff. It wouldn’t be beyond them to fall at the last fence, as they have a habit of doing..
May 2011 will also see the Welsh Assembly elections, and there will be a referendum on further law-making powers for the WAG on 3rd March.
On the wider political scene the question is whether the governing coalition’s cuts will force the economy back into recession or not. I don’t know the answer to that, but I do know that many ordinary working people are going to lose their jobs and many less advantaged members of society will have their benefits cut. Meanwhile the people who took us to the brink of economic ruin will no doubt carry on getting their bonuses.
In spite of all that, let me end by wishing you peace and prosperity for the New Year and beyond. And if that’s not possible, just remember Nil Illegitimi Carborundum.
Excuse the very quick and sketchy post on such an important topic, but I’ve got a lot of things to do before the dreaded Christmas lunch.
This morning the allocations of funding for the research councils were announced. The statement accompanying the ensuing Delivery Plan for the Science and Technology Facilities Council can be found here, while the plan itself is here. You’ll probably also want to read Paul Crowther’s analysis here.
Other research councils have also published their plans; you can find the one for EPSRC here.
The headline announcement reads:
After transferring responsibility for space science to the UK Space Agency, STFC’s overall baseline allocation for 2011-12 for resource funding (previously termed “near-cash”) is £377.5m rising to an allocation of £381.14m in 2014-15. This excludes administration which will be separately allocated. Our capital baseline allocation for 2011-12 is £91m, with an indicative allocation for the remainder of the spending review period reducing to £68m in 2014-15.
So not at all bad news for resource funding, but the implications of the capital cut are unclear (at least to me).
I haven’t had time to read the entire document, but did have a quick look at the crucial Appendix D which shows how each discipline is expected to fare:
Within an approximately flat-cash settlement, therefore, Astronomy is a clear loser (although much of the cuts in expenditure relate to decisions already made, such as withdrawal from the Gemini Telescopes). Confusingly, much of the increase in Particle Physics expenditure relates to an increase in the CERN subscription, which I thought was supposed to be falling …
As far as I understand it, the plan also maintains grant funding at the current level (although it will move into the new consolidated grant system as quickly as this can be achieved).
Anyway, that’s all I’ve got time for right now, and comments/reactions/corrections/clarifications are very welcome through the box below.
by Sarah Williams (1837-1868)
Donning my community service hat, I’ll just pass on some important news from the Science and Technology Facilities Council (STFC) concerning Astronomy research grants. The message is contained in an email that has been circulated concerning the new grant system and you can also find it at Paul Crowther’s website here. I urge all astronomers to read the text in full. I believe separate instructions are going out to particle physics and nuclear physics groups concerning their grants.
The main points are that:
A lot of questions remain to be answered, such as how on Earth people are going to be able to write a big proposal in the short time available when there are as yet no proper instructions, how groups with several existing grants will go about consolidating them when they all have different start and end dates, how the consolidated grants will be assessed, etc.
Also, it is now clear that results of the existing grant round (for grants due to start in April 2011) will not be forthcoming until January at the earliest, so that Swindon Office will be trying to sort out the new system at the same time as trying to complete the last round of the old one.
The combinations of delays to this round with the hasty implementation of a drastically different scheme for the next round is bound to cause a lot of problems both for STFC staff and researchers wanting to apply for grants, not to mention the Astronomy Grants Panel (of which I am a member).
The main purpose of this change is to save administrative costs at STFC, but it seems to me the main effect will be transfer an increased burden to universities, at least in the short term. Once again everything’s being done by the seat of the pants, with a complete lack of joined-up thinking.
Please don’t shoot the messenger, or anyone else on the AGP!
Not for the first time in my life I find myself a bit of a laughing stock, after blowing my top during a seminar at Cardiff yesterday by retired Professor Mike Disney. In fact I got so angry that, much to the amusement of my colleagues, I stormed out. I don’t often lose my temper, and am not proud of having done so, but I reached a point when the red mist descended. What caused it was bad science and, in particular, bad statistics. It was all a big pity because what could have been an interesting discussion of an interesting result was ruined by too many unjustified assertions and too little attention to the underlying basis of the science. I still believe that no matter how interesting the results are, it’s the method that really matters.
The interesting result that Mike Disney talked about emerges from a Principal Components Analysis (PCA) of the data relating to a sample of about 200 galaxies; it was actually published in Nature a couple of years ago; the arXiv version is here. It was the misleading way this was discussed in the seminar that got me so agitated so I’ll give my take on it now that I’ve calmed down to explain what I think is going on.
In fact, Principal Component Analysis is a very simple technique and shouldn’t really be controversial at all. It is a way of simplifying the representation of multivariate data by looking for the correlations present within it. To illustrate how it works, consider the following two-dimensional (i.e. bivariate) example I took from a nice tutorial on the method.
In this example the measured variables are Pressure and Temperature. When you plot them against each other you find they are correlated, i.e. the pressure tends to increase with temperature (or vice-versa). When you do a PCA of this type of dataset you first construct the covariance matrix (or, more precisely, its normalized form the correlation matrix). Such matrices are always symmetric and square (i.e. N×N, where N is the number of measurements involved at each point; in this case N=2) . What the PCA does is to determine the eigenvalues and eigenvectors of the correlation matrix.
The eigenvectors for the example above are shown in the diagram – they are basically the major and minor axes of an ellipse drawn to fit the scatter plot; these two eigenvectors (and their associated eigenvalues) define the principal components as linear combinations of the original variables. Notice that along one principal direction (v1) there is much more variation than the other (v2). This means that most of the variance in the data set is along the direction indicated by the vector v1, and relatively little in the orthogonal direction v2; the eigenvalue for the first vector is consequently larger than that for the second.
The upshot of this is that the description of this (very simple) dataset can be compressed by using the first principal component rather than the original variables, i.e. by switching from the original two variables (pressure and temperature) to one variable (v1) we have compressed our description without losing much information (only the little bit that is involved in the scatter in the v2 direction.
In the more general case of N observables there will be N principal components, corresponding to vectors in an N-dimensional space, but nothing changes qualitatively. What the PCA does is to rank the eigenvectors according to their eigenvalue (i.e. the variance associated with the direction of the eigenvector). The first principal component is the one with the largest variance, and so on down the ordered list.
Where PCA is useful with large data sets is when the variance associated with the first (or first few) principal components is very much larger than the rest. In that case one can dispense with the N variables and just use one or two.
In the cases discussed by Professor Disney yesterday the data involved six measurable parameters of each galaxy: (1) a dynamical mass estimate; (2) the mass inferred from HI emission (21cm); (3) the total luminosity; (4) radius; (5) a measure of the central concentration of the galaxy; and (6) a measure of its colour. The PCA analysis of these data reveals that about 80% of the variance in the data set is associated with the first principal component, so there is clearly a significant correlation present in the data although, to be honest, I have seen many PCA analyses with much stronger concentrations of variance in the first eigenvector so it doesn’t strike me as being particularly strong.
However, thinking as a physicist rather than a statistician there is clearly something very interesting going on. From a theoretical point of view one would imagine that the properties of an individual galaxy might be controlled by as many as six independent parameters including mass, angular momentum, baryon fraction, age and size, as well as by the accidents of its recent haphazard merger history.
Disney et al. argue that for gaseous galaxies to appear as a one-parameter set, as observed here, the theory of galaxy formation and evolution must supply at least five independent constraint equations in order to collapse everything into a single parameter.
This is all vaguely reminiscent of the Hertzsprung-Russell diagram, or at least the main sequence thereof:
You can see here that there’s a correlation between temperature and luminosity which constrains this particular bivariate data set to lie along a (nearly) one-dimensional track in the diagram. In fact these properties correlate with each other because there is a single parameter model relating all properties of main sequence stars to their mass. In other words, once you fix the mass of a main sequence star, it has a fixed luminosity, temperature, and radius (apart from variations caused by age, metallicity, etc). Of course the problem is that masses of stars are difficult to determine so this parameter is largely hidden from the observer. What is really happening is that luminosity and temperature correlate with each other, because they both depend on the hidden parameter mass.
I don’t think that the PCA result disproves the current theory of hierarchical galaxy formation (which is what Disney claims) but it will definitely be a challenge for theorists to provide a satisfactory explanation of the result! My own guess for the physical parameter that accounts for most of the variation in this data set is the mass of the dark halo within which the galaxy is embedded. In other words, it might really be just like the Hertzsprung-Russell diagram…
But back to my argument with Mike Disney. I asked what is the first principal component of the galaxy data, i.e. what does the principal eigenvector look like? He refused to answer, saying that it was impossible to tell. Of course it isn’t, as the PCA method actually requires it to be determined. Further questioning seemed to reveal a basic misunderstanding of the whole idea of PCA which made the assertion that all of modern cosmology would need to be revised somewhat difficult to swallow. At that point of deadlock, I got very angry and stormed out.
I realise that behind the confusion was a reasonable point. The first principal component is well-defined, i.e. v1 is completely well defined in the first figure. However, along the line defined by that vector, P and T are proportional to each other so in a sense only one of them is needed to specify a position along this line. But you can’t say on the basis of this analysis alone that the fundamental variable is either pressure or temperature; they might be correlated through a third quantity you don’t know about.
Anyway, as a postscript I’ll say I did go and apologize to Mike Disney afterwards for losing my rag. He was very forgiving, although I probably now have a reputation for being a grumpy old bastard. Which I suppose I am. He also said one other thing, that he didn’t mind me getting angry because it showed I cared about the truth. Which I suppose I do.
More sad news. Allan Sandage, one of the founding fathers of observational cosmology, passed away on 13th November, aged 84, of pancreatic cancer.
You can read a fuller appreciation of Allan Sandage’s contributions to astronomy and cosmology by Julianne Dalcanton over at Cosmic Variance.
I found this intruiging and impressive image over at Cosmic Variance (there’s also a press release at the Hubble Space Telescope website with higher resolution images). It shows the giant cluster of galaxies Abell 1689 with, superimposed on it, a map of the matter distribution as reconstructed from the pattern of distortions of background galaxy images caused by gravitational lensing.
This picture confirms the existence of large amounts of dark matter in the cluster – the mass distribution causing lensing quite different from what you can see in the luminous matter – but it also poses a problem, in that the matter is much more concentrated in the centre of the cluster than current theoretical ideas seem to suggest it should be…
You can find the full paper here.
It’s nice to have the chance to blog for once about some exciting astrophysics rather than doom and gloom about budget cuts. Tomorrow (5th November) sees the publication of a long-awaited article (by Negrello et al.) in the journal Science (abstract here) that presents evidence of discovery of a number of new gravitational lens systems using the Herschel Space Observatory.
There is a press release accompanying this paper on the Cardiff University website, and a longer article on the Herschel Outreach website, from which I nicked the following nice graphic (click on it for a bigger version).
This shows rather nicely how a gravitational lens works: it’s basically a concentration of matter (in this case a galaxy) along the line of sight from the observer to a background source (in this case another galaxy). Light from the background object gets bent by the foreground object, forming multiple images which are usually both magnified and distorted. Gravitational lensing itself is not a new discovery but what is especially interesting about the new results are that they suggest a much more efficient way of finding lensed systems than we have previously had.
In the past they have usually been found by laboriously scouring optical (or sometimes radio) images of very faint galaxies. A candidate lens (perhaps a close-set group of images with similar colours), then this candidate is followed up with detailed spectroscopy to establish whether the images are actually all at the same redshift, which they should be if they are part of a lens system. Unfortunately, only about one-in-ten of candidate lens systems found this way turn out to be actual lenses, so this isn’t a very efficient way of finding them. Even multiple needles are hard to find in a haystack.
The new results have emerged from a large survey, called H-ATLAS, of galaxies detected in the far-infrared/submillimetre part of the spectrum. Even the preliminary stages of this survey covered a sufficiently large part of the sky – and sufficiently many galaxies within the region studied – to suggest the presence of a significant population of galaxies that bear all the hallmarks of being lensed.
The new Science article discusses five surprisingly bright objects found early on during the course of the H-ATLAS survey. The galaxies found with optical telescopes in the directions of these sources would not normally be expected to be bright at the far-infrared wavelengths observed by Herschel. This suggested that the galaxies seen in visible light might be gravitational lenses magnifying much more distant background galaxies seen by Herschel. With the relatively poor resolution that comes from working at long wavelengths, Herschel can’t resolve the individual images produced by the lens, but does collect more photons from a lensed galaxy than an unlensed one, so it appears much brighter in the detectors.
Detailed spectroscopic follow-up using ground-based radio and sub-millimetre telescopes confirmed these ideas : the galaxies seen by the optical telescopes are much closer, each ideally positioned to create gravitational lenses.
These results demonstrate that gravitational lensing is probably at work in all the distant and bright galaxies seen by Herschel. This in turn, suggests that in the full H-ATLAS survey might provide huge numbers of gravitational lens systems, enough to perform a number of powerful statistical tests of theories of galaxy formation and evolution. It’s a bit of a cliché to say so, but it looks like Herschel will indeed open up a new window on the distant Universe.
P.S. For the record, although I’m technically a member of the H-ATLAS consortium, I was not directly involved in this work and am not among the authors.
P.P.S. This announcement also gives me the opportunity to pass on the information that all the data arising from the H-ATLAS science demonstration phase is now available online for you to play with!
In the Dark 