Category Archives: EAGLE project

The dark nemesis of galaxy formation: why hot haloes trigger black hole growth and bring star formation to an end

I’ve just submitted a new paper to MNRAS, the professional journal of astronomy… its all about black holes and how they bring star formation in galaxies to an end, creating two distinct galaxy types and making the Universe look the way it is. Far from being exotic predictions of Einsteins General Relativity, it seems that black holes play a fundamental role in shaping the observable universe.

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Here’s the paper’s abstract, click to get a link to the paper… Continue reading The dark nemesis of galaxy formation: why hot haloes trigger black hole growth and bring star formation to an end

EAGLE takes off

The first EAGLE paper is now available on the astro-ph archive. It will tell you everything you wanted to know about the project (and more probably more!).  Most importantly, it sets out the project philosophy and shows just how much the simulated universe looks like the real one.

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This plot counts the numbers of galaxies of different masses. It compares the main Eagle simulation (the dark blue line) to previous work (the other lines) and observational data (the fray points)… you’re meant to see that EAGLE fits the data really well – it captures both the flat part (similar numbers of galaxies with masses to our Milky Way galaxy and with mass 10 times smaller), and the exponential break at higher masses (galaxies much larger than our own are incredibly rare). EAGLE is a huge step forward compared to previous work. There are many more comparisons like this in the paper.

Here’s the paper’s abstract…. Continue reading EAGLE takes off

Cosmic Own Goal

gas imageOne of the very exciting things about the Eagle project is that it is possible to pick out interesting objects, and then re-simulate the volume around them at higher resolution giving a much better view of what’s going on and letting us find even the smallest galaxies.

So what are the smallest galaxies? Surely galaxies can be as small as you want? Well – no. There’s a limit to the smallest size of galaxies, and Till Sawala has just written an exciting paper demonstrating why.

The science paper is available here…  http://arxiv.org/abs/1406.6362

but we’ve also issued a press realease with some very cool pictures. The full pictures are here… http://star-www.dur.ac.uk/~till/

Here’s the story ….

Continue reading Cosmic Own Goal

EAGLE in the News

Images from our EAGLE simulations appeared on the BBC’s Newsnight program last night. Journalist Rebecca Morelle used them to explain the concept of Dark Matter: the exotic and invisible matter that makes up most of the mass of the Universe. The images contrast the universe as we see it (from looking at starlight) to the way it would appear if it were possible to see the Dark Matter.

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Follow the on-line story at the BBC’s news website.

If you would like to explore more of the Universe, download our app “Cosmic Universe” from the apple store.

Angular momentum, black holes and the characteristic mass of galaxies

One of my PhD students, Yetli Rosas-Guevara, has just submitted a very exciting paper. In it, we discuss the important of angular momentum in black hole accretion.  In effect, our new model that takes account of the whirlpool that forms as matter tries to fall into the black hole!

This can have a major impact on the accretion rates of black hole and strongly affects how black holes affect the galaxies that host them. The graph below shows an example of how one black hole grows during the simulation. The colouring of the points indicates the importance of angular momentum (redder colours mean the angular momentum is important). Focus on the upper panel which shows the rate at which matter falls in. Notice how the accretion onto the black hole is concentrated into a few events, these are triggered by galaxy mergers. As the system grows in mass, however, the gas halo around the galaxy takes longer and longer to recover from the outburst. The effect of concentrating the accretion into a few events is to create a characteristic size for galaxies. We live in a galaxy of this “characteristic size” — the Milky Way. In the near future, we’ll publish a detailed comparison between the simulations and observations as part of the EAGLE project.

 

plots_historyHere’s the abstract of the paper:

“Feedback from energy liberated by gas accretion onto black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function (SMF). Semi-analytic models of galaxy formation in which this form of feedback is assumed to suppress cooling in haloes where the gas cooling time is large compared to the dynamical time do indeed achieve a good match to the observed SMF. Furthermore, hydrodynamic simulations of individual halos in which gas is assumed to accrete onto the central BH at the Bondi rate have shown that a self-regulating regime is established in which the BH grows just enough to liberate an amount of energy comparable to the thermal energy of the halo. However, this process is efficient at suppressing the growth not only of massive galaxies but also of galaxies like the Milky Way, leading to disagreement with the observed SMF. The Bondi accretion rate, however, is inappropriate when the accreting material has angular momentum. We present an improved accretion model that takes into account the circularisation and subsequent viscous transport of infalling material and include it as a “subgrid” model in hydrodynamic simulations of the evolution of halos with a wide range of masses. The resulting accretion rates are generally low in low mass ($\lsim 10^{11.5} \msun$) halos, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion occurs continuously, typically at  10 % of the Eddington rate, which is conducive to the formation of radio jets. The resulting dependence of the accretion behaviour on halo mass induces a break in the relation between galaxy stellar mass and halo mass in these simulations that matches observations.”

You can down load the paper from astro-ph … http://arxiv.org/abs/1312.0598… where you will find the full caption to the figure at the top!