Archive for UKIDSS

Bright and Early

Posted in The Universe and Stuff with tags , , , , , , on June 29, 2011 by telescoper

Some interesting astronomy news emerged this evening relating to a paper published in 30th June issue of the journal Nature. The press release from the European Southern Observatory (ESO) is quite detailed, so I’ll refer you there for the minutiae, but in a nutshell:

A team of European astronomers has used ESO’s Very Large Telescope and a host of other telescopes to discover and study the most distant quasar found to date. This brilliant beacon, powered by a black hole with a mass two billion times that of the Sun, is by far the brightest object yet discovered in the early Universe.

and the interesting numbers are given here (with links from the press release):

The quasar that has just been found, named ULAS J1120+0641 [2], is seen as it was only 770 million years after the Big Bang (redshift 7.1, [3]). It took 12.9 billion years for its light to reach us.

Although more distant objects have been confirmed (such as a gamma-ray burst at redshift 8.2, eso0917, and a galaxy at redshift 8.6, eso1041), the newly discovered quasar is hundreds of times brighter than these. Amongst objects bright enough to be studied in detail, this is the most distant by a large margin.

When I was a lad, or at least a postdoc, the most distant objects known were quasars, although in those days the record holders had redshifts just over half that of the newly discovered one. Nowadays technology has improved so much that astronomers can detect “normal” galaxies at even higher redshifts but quasars remain interesting because of their extraordinary luminosity. The standard model for how a quasar can generate so much power involves a central black hole onto which matter falls, liberating vast amounts of gravitational energy.

You can understand how efficient this is by imagining a mass m falling onto a black hole of Mass M from a large distance to the horizon of the black hole, which is at the Schwarzschild radius R=2GM/c^2. Since the gravitational potential energy at a radius R is -GMm/R the energy involved in bringing a mass m from infinity to the horizon is a staggering \frac{1}{2} mc^2, i.e. half the rest mass energy of the infalling material. This is an overestimate  for various reasons but it gives you an idea of how much energy is available if you can get gravity to do the work; doing the calculation properly still gives an answer much higher than the amount of energy that can be released by, e.g., nuclear reactions.

The point is, though, that black holes aren’t built in a day, so if you see one so far away that its light has taken most of the age of the Universe to reach us then it tells us that its  black hole must have grown very quickly. This one seems to be a particularly massive one, which means it must have grown very quickly indeed. Through observations like this  we learn something potentially very interesting about the relationship between galaxies and their central black holes, and how they both form and evolve.

On the lighter side, ESO have also produced the following animation which I suppose is quite illustrative, but what are the sound effects all about?

What Counts as Productivity?

Posted in Bad Statistics, Science Politics, The Universe and Stuff with tags , , , , on March 18, 2011 by telescoper

Apparently last year the United Kingdom Infra-Red Telescope (UKIRT) beat its own personal best for scientific productivity. In fact here’s a  graphic showing the number of publications resulting from UKIRT to make the point:

The plot also demonstrates that a large part of recent burst of productivity has been associated with UKIDSS (the UKIRT Infrared Deep Sky Survey) which a number of my colleagues are involved in. Excellent chaps. Great project. Lots of hard work done very well.  Take a bow, the UKIDSS team!

Now I hope I’ve made it clear that  I don’t in any way want to pour cold water on the achievements of UKIRT, and particularly not UKIDSS, but this does provide an example of how difficult it is to use bibliometric information in a meaningful way.

Take the UKIDSS papers used in the plot above. There are 226 of these listed by Steve Warren at Imperial College. But what is a “UKIDSS paper”? Steve states the criteria he adopted:

A paper is listed as a UKIDSS paper if it is already published in a journal (with one exception) and satisfies one of the following criteria:

1. It is one of the core papers describing the survey (e.g. calibration, archive, data releases). The DR2 paper is included, and is the only paper listed not published in a journal.
2. It includes science results that are derived in whole or in part from UKIDSS data directly accessed from the archive (analysis of data published in another paper does not count).
3. It contains science results from primary follow-up observations in a programme that is identifiable as a UKIDSS programme (e.g. The physical properties of four ~600K T dwarfs, presenting Spitzer spectra of cool brown dwarfs discovered with UKIDSS).
4. It includes a feasibility study of science that could be achieved using UKIDSS data (e.g. The possiblity of detection of ultracool dwarfs with the UKIRT Infrared Deep Sky Survey by Deacon and Hambly).

Papers are identified by a full-text search for the string ‘UKIDSS’, and then compared against the above criteria.

That all seems to me to by quite reasonable, and it’s certainly one way of defining what a UKIDSS paper is. According to that measure, UKIDSS scores 226.

The Warren measure does, however, include a number of papers that don’t directly use UKIDSS data, and many written by people who aren’t members of the UKIDSS consortium. Being picky you might say that such papers aren’t really original UKIDSS papers, but are more like second-generation spin-offs. So how could you count UKIDSS papers differently?

I just tried one alternative way, which is to use ADS to identify all refereed papers with “UKIDSS” in the title, assuming – possibly incorrectly – that all papers written by the UKIDSS consortium would have UKIDSS in the title. The number returned by this search was 38.

Now I’m not saying that this is more reasonable than the Warren measure. It’s just different, that’s all.  According to my criterion however UKIDSS measures 38 rather than 226. It sounds less impressive (if only because 38 is a smaller number than 226),  but what does it mean about UKIDSS productivity in absolute terms?

Not very much, I think is the answer.

Yet another way you might try to judge UKIDSS using bibliometric means is to look at its citation impact. After all, any fool can churn out dozens of papers that no-one ever reads. I know that for a fact. I am that fool.

But citation data also provide another way of doing what Steve Warren was trying to measure. Presumably the authors of any paper that uses UKIDSS data in any significant way would cite the main UKIDSS survey paper led by Andy Lawrence (Lawrence et al. 2007). According to ADS, the number of times this has been cited since publication is 359. That’s higher than the Warren measure (226), and much higher than the UKIDSS-in-the-title measure (38).

So there we are, three different measures, all in my opinion perfectly reasonable measures of, er,  something or other, but each giving a very different numerical value. I am not saying any  is misleading or that any is necessarily better than the others. My point is simply that it’s not easy to assign a numerical value to something that’s intrinsically difficult to define.

Unfortunately, it’s a point few people in government seem to be prepared to acknowledge.

Andy Lawrence is 57.


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