Euclid officially official

June 20, 2012

Maybe you all thought Euclid had already been selected. Well sort of. Yesterday the ESA Science Programme Committee  “adopted” Euclid, so its now officially official and all systems go. Here is the Beeb story.

I am on the Euclid consortium and must get round to doing something useful sometime. I am just realising it should be rather groovy for transients so maybe something in that line. I like big flares of the sleeping black hole variety.

Of course being on the Euclid consortium is not altogether special. There are about nine hundred of us.  I think I read somewhere that it is the biggest astronomical consortium in the history of the Outer Galaxy or something like that. Is this a good thing ?


Redshift Seven

June 29, 2011

I am very happy today to report a triumph for UKIDSS, for Dan Mortlock and Steve Warren, and for UKIRT  : the first quasar to break the redshift seven barrier. You can read the Mortlock et al Nature paper, the STFC story, the Gemini version, or the ESO version. And Telescoper has already splashed it too ! The science is best in the Gemini version, but the ESO has wonderfully gaudy animations….

When we were designing UKIDSS back in 1998-2001, SDSS was doing its thing and the redshift record climbed from 5ish to 6.3. The secret was finding i-band dropouts, as the famous forest cuts out nearly all light shortward of Lyman alpha. However this can’t work past z=6.4 as you get no visible light at all. Bring on the massive IR survey please… This was Steve Warren’s big push, along with the importance of the new Y-band, so we could tell the difference between high-z quasars and T-dwarfs. Finding these swines though is the classic needle in a haystack problem, with millions of fake candidates to weed out. Dan Mortlock has worked long hard and patiently, along with Steve Warren, Bram Venemans, Richard McMahon and others. Team UKIDSS were starting to get worried, as we were succesfully finding more 6ish quasars, but nothing past the magic 6.4… then suddenly bang – redshift 7.085.

Apart from breaking records, the new quasar is important in two ways. First, the Ly-alpha line is eaten away even redward of the peak, implying a small “near-zone” size, and so the best evidence so far for a significant neutral fraction near the quasar. Second, it has an estimated mass of two billion solar masses at just 770 million years after the Big Bang. It is generally thought that the very first stars, and so the first seed black holes, won’t be there until about z=25; then any such seed should not be able to grow by accretion to two billion solar masses until at least 900 million years…

Quasar near-zone

Artists impression of ionised bubble formed around quasar ULASJ1120+0641

Anyhoo. Although ESO and Gemini are getting lots of excellent PR, I think this is a triumph for UKIRT . Not dead yet, squire. In fact, how do we get more of these beasties, and push on to redshift 8 ? Survey the rest of the Northern Hemisphere, thats how. Another four years will do. (OK, VISTA helps too…).

Other thoughts. The Infra-red is cool. Big public surveys work. And for such massive surveys, properly processed and archived data is crucial. The whole thing would have been impossible without the selfless work of the teams at CASU and WFAU. Thank you guys.

Get your own data at the WSA ! DR9 coming your way soon.


A dim glimmer

March 3, 2011

As I write this, I am sitting in Sheffield’s fine Victorian railway station, on my way home from visiting the astronomers here. Yesterday I sang for my supper, giving a double bill seminar on the radio background and on the big blue bump. Supper duly followed, and was exceeding pleasant. The Sheffield group is a small but lively one. As normal, the postdocs were worried about job prospects and the academics were moaning about writing exam questions. To quote P.C. : “I love my job but I hate writing exam questions.” With you there, Professor C.

The Crowther mentioned my recent blog post about how the Universe is almost empty. He said he likes to set his classes ballpark estimate exercises. He tried one out on me. You can have a go too. Its quite good to start by using your instinct to make a guess, before gathering a few facts to do the quick mental calculation. That way you can get your frisson of surprise. Suppose, said Paul, you take the material of the Earth and stretch it out from here to the Sun – how wide would that rope be ? His students guess a wide range of answers, but usually around a mm. Give it a go.

Here is another one, for which I will give you the answer – how powerful is a one kg accreting black hole ? As we all know, quasars are immensely luminous because they have supermassive black holes at their hearts – those accreting black holes are the most efficient energy sources we know. We are talking big numbers. At the Eddington limit, that billion solar mass black hole can be radiating up to 10^47 erg/s. When I first came to giving an undergraduate course with some of this stuff in, I felt duty bound to do things in SI units. So I got a formula for the Eddington limit : L_Edd=6.37M. Wuh ? A one kg black hole gives me 6 Watts ? My electric fire can do better than that ! My electric fire is only a few kg,  but it gives me 2 kW.

Then comes the epiphany. You realise that accretion is not the slightest bit efficient per unit of mass of accretor ; what is impressive is the energy per unit mass of the accretee. To get 2kW out and so heat your bedsit, you need a black hole of at least 314 kg – about as big as two motorbikes. However, once you have it, the rate you have to burn fuel as it were, i.e. the rate at which you need to drop matter into the black hole, is about 2 x 10^-13 kg/s; one kg of fuel would last you roughly 140,000 years… I now leave it as an exercise for the reader how long a kg of coal would give that same 2kW. Scaling up to the most luminous quasars, you need to eat about 20 solar masses a year. Peanuts.

Then it made me think again about that electric bar fire. Its not really producing 2kW all by itself. It is plugged in and sucking that energy out of a giant power station miles away. Its really a similar story. First you need a vast power factory, and all the surrounding infrastructure. Only once you have it can you burn some fuel and give the illusion of that tiny fire producing energy. Likewise, before you can get that accretion energy goodness, you somehow have to assemble those billion solar masses. But thats another story…


Units Rant

May 10, 2010

Way Back in the 1960s (spot that quote) our physics teacher told us that in the funny old days the absorption of sound was measured in cubic yards of Standard British Cushion, but now that we lived in enlightened times we measured it in square metres of open window. That helped to teach us the necessity of converting carefully between units, but also gave us a healthy disrespect for any particular units, and reminded us that units are operationally defined. They are not philosophical magic.

Wind forward a few years and I was in the thick of my astronomy PhD, in the wonderful world of parsecs and solar masses. In X-ray astronomy we also had Uhuru Flux Units (UFU) and my good friend the milli-crab. Such weird units are of course awfully important to protect the mystique of a specialism. We don’t want any old fool thinking they can be an X-ray astronomer, now do we we ? I believe Antarctic ozone hole pundits use Dobson Units. But they are also truly natural units, in two ways. First, they give sensible human sized numbers you can think with. Would you like the distance to your star to be 3.2 parsec or 9.9 times 10^16 m ? Second, you can get on with stuff even before you have solved all your problems – quasar A is seven times as bright as quasar B whether you know the absolute flux of the Crab or not. You can sort that later.

Wind forward another few years and I was about to teach my first lectures at QMW. Time to shed those quirky units and do everything in SI. This held one or two surprises. The Eddington limit, the largest luminosity a body can have without blowing away its atmosphere, is very big for black holes. Every X-ray astronomer knows the number – 1.3 times 10^38 erg per second per solar mass. Yes, we love BIG numbers. What I do is extreme astrophysics. Black holes are the solution to quasars because they are the most efficient known power source !

Now lets have that in SI.  What I got was L=6.39M. Feeble. I checked it six times, but it was right. A one kg black hole gives you a pathetic 6 watts. My little electric fire does three hundred times better than a black hole of similar mass ! Ahh… but its not the electric fire that makes the power … its the enormous power station its connected to. In a similar way, accretion is not particularly impressive or efficient per unit mass of accretee – its efficient per unit mass of accretor. Once you have that huge black hole, you only need to drip a tiny amount of stuff onto it to generate lots of power. So whats my point here ? That its all about building physical instincts. There is no perfect global unit system. Switching between systems is like switching between physical domains, and that act in itself can teach you lessons.

You can only be “natural” within a domain.This is why the “natural units” touted by theorists, where h=c=1, are a bit irritating, just because they often come bundled with a kind of arrogant mysticism, as if this idea was revealing something deep about the structure of the universe. Au contraire, they are a convenience, just like parsecs.  They are jolly sensible if you are doing some kinds of calculations, because you don’t have to carry round awkward numbers and the equations are easier to follow. But if you are calculating real world quantities, those awkward numbers have to pop back out somewhere.

I have lots of other units rants (I bet you do too) but I had better stop there.

Oh hang on, just one more. This Galactic Guide web page is quite fun. My favourite bit is right at the end, where they recommend students answer a question by saying “The answer is 12.7 Meulens, where the Meulen is defined via this problem”.


People, galaxies, and complexity

January 19, 2009

Which is more complex, a person or a galaxy ? I have been to several seminars recently about various aspects of simulating galaxy formation and structure, on a variety of scales from large scale clustering to nuclear gas flow. Work of this kind is getting gradually more impressive, partly because better physics is going in, partly because algorithms are improving, and partly of course because folks are using bigger computers and gobbling up more CPU hours, thus achieving simulations with more resolution, more particles, or more timesteps. Obviously this must lead to more realism… but you get this strange queasy Borgesian feeling … if we simulate in enough detail to get it right, the galaxy in a box becomes indistinguishable from the real thing. What then have we learned ? Surely understanding involves some kind of encapsulation, some boiling down to a deep but simple statement ? Some kind of emergent law ?

At lunch after one such seminar, I was trying to provoke some of the locals with sceptical thoughts of this kind. Tom Abel had a good answer : the aim of massive simulations should not be to make it look right, but to see patterns and consistencies emerge. The simulations are not explanations, but experiments. Nonetheless Tom and his co-workers make very convincing looking – and stunningly beautiful – pictures. Although we love telling each other how little we know, in fact you could make a good case for the idea that we understand galaxies better than we understand people.

In popular talks I often go beyond this, claiming that people are quantifiably more complex than galaxies. (This is all about flattering the punters of course.) Now, you could go about defining  complexity in a number of ways. Its not about specifying the phase-space position of every molecule, otherwise a bucket of water is more complex than an iPod, and that ain’t right. One useful method is to ask how many instructions it takes to make something. To make a person, you take the information in the DNA molecule. The human genome has about three billion base pairs, and each of those needs 2 bits to specify one of four bases. So in total the information needed is about 750MB, or about one CD. Its instructive to compare that to printed text. A character from the alphabet needs 5 bits, but lets add other stuff and round up to one byte per character The book in front of me has 72 characters per line and 46 lines per page. So DNA is equivalent to 5.2 million lines, about 113 thousand pages, or maybe a couple of hundred moderately fat textbooks. To specify a person you don’t need an entire library, but you do need several shelves of books. By contrast, you can do a pretty good job simulating a galaxy with a few pages of C code. I believe that even the goriest codes are around the 50,000 lines mark. So there you go. A person is a thousand times more complex than a galaxy.

Well, I am sure many of you will be squirming and spotting all the cracks in that argument. It does remind me of the old story about Fred Hoyle, who claimed in a lecture that a star was a “pretty simple thing”. From the back of the room, someone called “You’d look pretty simple from ten parsecs, Fred.” (Special no-prize for anyone who can shed light on this urban myth.)

However, its pretty hard to come up with something objective. As I said, we don’t want to get into the list-every-molecule game. Perhaps this says something important about information. I can never resist the feeling that whenever somebody asks “how much information is there in X ?” the answer is “what do you want to know ?”. For this reason, I suspect that the infamous question of whether a black hole destroys information is an incoherent one. But I had better stop there or I will get stomped on by people cleverer than me. So. There you go. Galaxies simpler than people. From our point of view.


Accelerated Thinking

August 11, 2008

On Friday afternoon I had Roger Blandford and Martin Rees in my office at the same time. They have brains the size of seventeen planets each, and my office isn’t that big, so I could only just fit in at the same time. I could feel my thoughts being squeezed out through the crack under the door. I roped them back in and switched the brain into overdrive to try to keep up for a bit. Then Roger went off to a meeting, as he is wont to do, as he is a Very Important Chap (I’m not anymore ! Woohooo !) and Martin and I chatted of Scottish Physics and Quasars for bit.

Earlier that day we had both crashed lectures in the current SLAC Summer Institute, which is called Cosmic Accelerators and brings together an eclectic mix of particle physics folk, cosmic ray types, STP people, and black hole jet fans. All this non-thermal stuff is coming back into fashion, thanks to gamma ray bursts. With GLAST flying, the pulsar and blazar folks are gearing up for action too, getting their samples ready ….. The only trouble is, this stuff is so hard. We’ve done all the simple “woahh look at that” stuff, and we’re into the detailed physics and boundary conditions and so on. Every so often though a new experiment can make a big simple advance. Possibly my favourite is Meegan et al 1992 . After thirty years of utter mystery regarding gamma ray bursts, they just counted the buggers, and showed that they were isotropic on the sky, but falling off with flux faster than the 3/2 law : so they just had to be cosmological, and at large redshifts. Classic. Lets just hope GLAST does something that nice.

By the way, Martin was visiting Roger on the way to the Googleplex. For the second year in a row, Google have invited a hundred or so of the world’s top scientists for an unstructured brainstorming. Pretty special party invite. Whoever sees him next can ask if it worked …

Oh and talking of great simple ideas, happy birthday packet switching. Now that really changed the world…


Sunglasses, black holes, and unsafe partners

July 25, 2008

Oh Proud Day ! I am on the front page of the STFC web site ! This is the “polarised sunglasses see black hole disks” story. Doesn’t seem to have made the Daily Mirror, but has reasonably spread round the Internut, including Skymania, New Scientist, Universe Today, Astronomy Now and Chris Lintott in his American Manifestation . A week ago I was even more excited, as the first draft of the press release had a quote from Keef, saying how important UKIRT was to UK astronomy, but it got changed into a quote by Chris Davis. Better in some ways and a shame in others.The person who deserves nearly all the credit by the way is Makoto Kishimoto, one of the most careful but insightful astronomers I know.

If you can’t afford a subscription to Nature, you can read it on astro-ph. Here is the story. Quasar phenomenon supposedly caused by accretion onto supermassive black holes. Prediction since 1973 that accretion discs should show nu**1/3 spectrum – very blue. In optical-UV you don’t see this, but, well, there are complications. Should be a safer bet in the IR, coming from the outer bits of disc. But in the IR all you can see is the emission from the damned dust on much larger scales. Thinks. Light scattered from the disc is polarised, so maybe measuring the polarised flux shows pure uncontaminated disc ? Ahh .. but dust emission is often polarised too. Rats. But wait ! Some quasars show no polarisation in the broad emission lines.. in which case any polarisation must be caused interior to the BLR … try these.. Bingo. Nu**1/3. Or actually, about nu**0.4. Thirty year old theory finally vindicated. Phew. Sits down with glass of whisky.

So for half of today, when I wasn’t googling for references to black hole sunglasses, I was at a meeting here at SLAC today celebrating the career of ex-SLAC-Director Jonathan Dorfan. He is the man credited with pulling off PEP-II and BaBar, turning SLAC into the B-factory, focusing on CP violation and the matter-anti-matter asymmetry. But years later, he was also the man who swung the ship round to head towards a future of light sources, lasers, and biology. (But also particle astrophysics … GLAST, SNAP, LSST … thats why I’m here folks..).

One of the key talks was by Albrecht Wagner from DESY, here to remind us that actually there was still a bright future for particle physics, and indeed Linear Colliders are back in fashion. Errrr… just a shame that the US and UK seem to have changed their minds and pulled the funding plug. The only tense moment in a warm and cuddly day was when Wagner said that the US should now be considered “an unsafe partner for international projects”. Think I’ll stop there.


Follow

Get every new post delivered to your Inbox.

Join 119 other followers