Archive for galaxy formation

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Stargazing (virtually) Live

Posted in Television, The Universe and Stuff with tags , , , , , , on January 18, 2012 by telescoper

I hope you’ve all been tuning in to the BBC’s astronomy jamboree Stargazing Live. There have been two episodes so far, with one last one to follow tonight, plus a huge range of activities across the country (including Wales) giving members of the public the chance to look at the sky through telescopes. The programmes and other activities have been getting an excellent response, especially from the younger generation, which is excellent news for the future of astronomy.

Working in a School of Physics & Astronomy makes one realise just how much public interest there is in astronomy, not just among schoolkids but in the numerous amateur astronomical societies, the members of which actually know the night sky better than many professionals! Most of us astronomers and astrophysicists are regularly asked to give public lectures and Cardiff in particular runs a  host of other outreach activities related to our astronomy research. Our colleagues in mainstream physics subjects such as condensed matter physics don’t get the same level of direct public interest – I don’t think there are any amateur semiconductor physics  clubs in the UK! – but many students attracted into universities by astronomy do turn to other branches of physics when they get here, because something else catches their imagination.

But important though that role is, let’s not forget that astronomy isn’t just about outreach. It’s actually real science, making real discoveries about the way our universe works. It’s worth doing in its own right as well as being good for other branches of physics.

Anyway, being a theoretical astrophysicist I usually feel a bit left out of these stargazing actitivies because I don’t really know one end of a telescope from the other. The other day I jokingly  asked whether Stargazing Live was ever going to include a theory component…

Last night’s episode actually did, in the form of a discussion of a numerical simulation of galaxy formation between the presenters and young Dr Andrew Pontzen from Oxford University. He even made a little video about the simulation, sort of virtual reality rendition of the formation of the Milky Way, as shown on the telly:

Apparently, making this required 300,000 CPU hours on 300 processors and it is based on 16 Terabytes of raw data. Phew!

It’s a very impressive simulation, but the use of the word simulation in this context always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

It’s only the third entry that gives the intended meaning. This is worth bearing in mind if you prefer old-fashioned analytical theory!

In football, of course, you can get sent off for simulation…

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Haloes, Hosts and Quasars

Posted in The Universe and Stuff with tags , , , , , , , , on July 20, 2011 by telescoper

Not long ago I posted an item about the exciting discovery of a quasar at redshift 7.085. I thought I’d return briefly to that topic in order (a) to draw your attention to a nice guest post by Daniel Mortlock on Andrew Jaffe’s blog giving more background to the discovery, and (b) to say  something  about the theoretical interpretation of the results.

The reason for turning the second theme is to explain a little bit about what difficulties this observation might pose for the standard “Big Bang” cosmological model. Our general understanding of galaxies form is that gravity gathers cold non-baryonic matter into clumps  into which “ordinary” baryonic material subsequently falls, eventually forming a luminous galaxy forms surrounded by a “halo” of (invisible) dark matter.  Quasars are galaxies in which enough baryonic matter has collected in the centre of the halo to build a supermassive black hole, which powers a short-lived phase of extremely high luminosity.

The key idea behind this picture is that the haloes form by hierarchical clustering: the first to form are small but  merge rapidly  into objects of increasing mass as time goes on. We have a fairly well-established theory of what happens with these haloes – called the Press-Schechter formalism – which allows us to calculate the number-density N(M,z) of objects of a given mass M as a function of redshift z. As an aside, it’s interesting to remark that the paper largely responsible for establishing the efficacy of this theory was written by George Efstathiou and Martin Rees in 1988, on the topic of high redshift quasars.

Anyway, courtesy of my estimable PhD student Jo Short, this is how the mass function of haloes is predicted to evolve in the standard cosmological model (the different lines show the distribution as a function of redshift for redshifts from 0 to 9):

It might be easier to see what’s going on looking instead at this figure which shows Mn(M) instead of n(M).

You can see that the typical size of a halo increases with decreasing redshift, but it’s only at really high masses where you see a really dramatic effect.

The mass of the black hole responsible for the recently-detected high-redshift quasar is estimated to be about 2 \times 10^{9} M_{\odot}. But how does that relate to the mass of the halo within which it resides? Clearly the dark matter halo has to be more massive than the baryonic material it collects, and therefore more massive than the central black hole, but by how much?

This question is very difficult to answer, as it depends on how luminous the quasar is, how long it lives, what fraction of the baryons in the halo fall into the centre, what efficiency is involved in generating the quasar luminosity, etc.   Efstathiou and Rees argued that to power a quasar with luminosity of order 10^{13} L_{\odot} for a time order 10^{8} years requires a parent halo of mass about 2\times 10^{11} M_{\odot}.

The abundance of such haloes is down by quite a factor at redshift 7 compared to redshift 0 (the present epoch), but the fall-off is even more precipitous for haloes of larger mass than this. We really need to know how abundant such objects are before drawing definitive conclusions, and one object isn’t enough to put a reliable estimate on the general abundance, but with the discovery of this object  it’s certainly getting interesting. Haloes the size of a galaxy cluster, i.e.  10^{14} M_{\odot}, are rarer by many orders of magnitude at redshift 7 than at redshift 0 so if anyone ever finds one at this redshift that would really be a shock to many a cosmologist’s  system, as would be the discovery of quasars at  redshifts significantly higher than seven.

Another thing worth mentioning is that, although there might be a sufficient number of potential haloes to serve as hosts for a quasar, there remains the difficult issue of understanding how precisely the black hole forms and especially how long that  takes. This aspect of the process of quasar formation is much more complicated than the halo distribution, so it’s probably on detailed models of  black-hole  growth that this discovery will have the greatest impact in the short term.

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