Brightest cluster galaxies (BCGs) are the very luminous galaxies that inhabit the cores of rich galaxy clusters ( Research Highlight November 2003 , Research Highlight March 2007 ). These galaxies are among the most luminous and most massive systems in the Universe at the present epoch, and their formation involves complex physical processes like "cooling flows" and "cannibalism". The former mechanism is related to the fact that the hot gas (typically 2-14 keV) in the cores of galaxy clusters can lose energy via X-ray radiation. As the gas cools, the pressure decreases so that the intra-cluster medium gradually contracts. The second mechanism is due to the loss of momentum and kinetic energy of moving bodies through gravitational interaction with the surrounding matter. As a consequence, galaxies orbiting in a galaxy cluster, gradually sink towards the centre, eventually merging with the central galaxy.

Scientists at MPA have studied the formation and evolution of BCGs taking advantage of the largest dark matter cosmological simulation carried out so far - the Millennium Simulation ( Research Highlight August 2004). In order to model the observable properties of the galaxies (like their mass, luminosity, colour, etc.), Gabriella De Lucia & Jeremy Blaizot have coupled the high-resolution N-body simulation to semi-analytic models of galaxy formation ( Research Highlight May 2004). In these models, the evolution of the baryonic components (gas and stars) is "painted" on top of the history of dark matter haloes using simple relations that are supported by observational data and by theoretical arguments.

Fig.1 shows the "history" of a BCG. In the astronomical jargon, this diagram is referred to as "galaxy merger tree", and it can be considered as the equivalent of a family tree. The BCG itself is plotted at the top, and all its progenitors (and also their histories) are plotted backward in time recursively. Galaxies with stellar masses larger than ~10^10 Msun are shown as symbols, while lower mass galaxies are shown as lines. The size of the symbols scales with stellar mass, while the colour scales with the colour of the galaxy itself, which can be considered as a proxy for the age of the stellar population. The left-most branch in Fig.1 represents what is usually called "main branch". This is obtained by connecting the galaxy at each time-step to the progenitor with the largest stellar mass (the main progenitor) at the previous time-step. Fig.1 shows that this particular branch does not contain all the information needed to explain the properties of the final galaxy. When viewed at earlier times, the BCG is no longer a single object, rather it is the "ensemble" of its progenitors at that time. At late times, the main branch contains most of the final mass, but this is not true at earlier times when the main progenitor is only marginally more massive than other branches of the tree.

Given the complexity of the merger history shown in Fig.1, it is helpful to define some "times" that mark important phases in the evolution of a BCG. In particular, we call "assembly time" the time when the main progenitor contains half the final stellar mass of the BCG, and "formation time" as the time when the total mass of stars formed reaches half of the final mass. By total mass, we mean, at each cosmic time, the sum of the stellar masses of all the progenitors present at a given time, i.e. the projection of Fig. 1 onto the vertical time axis.

Fig. 2 shows the evolution of the mass in the main branch (blue) and of the total mass of stars formed (green) as a function of cosmic time. The solid lines and the dashed regions in Fig.2 show the median and the scatter obtained considering an ensemble of 125 BCGs. For essentially all the objects in the sample, half of the stars that end up in the final galaxy are already formed more than 10 billion years ago. Only at much later times (typically after about 7 billion years) these stars merge toghether to form a single massive system. Since, however, most of the galaxies accreted onto BCGs have little gas and are already dominated by old stars, this late accretion does not change the apparent age of BCGs at present time.

Model BCGs are therefore old (i.e. their stars formed very early), but their mass assembled quite late. The increase in mass in the last ~8 billion years found by these models, appears to be in qualitative agreement with observational data. Future detailed comparisons with observations will provide strong constraints on the physical processes regulating the formation and evolution of this special class of objects.

Gabriella De Lucia and Jeremy Blaizot

Publication

Gabriella De Lucia, Jeremy Blaizot
"The hierarchical formation of the brightest cluster galaxies",
Mon. Not. R. Astron. Soc., 375, 2 (2007)