# Galaxy evolution

## Halo interactions

**B. L'Huillier**, C. Park, & J. Kim,
MNRAS 451, 527,
arXiv:1505.00788.

Mergers have long been thought to have a strong influence on galaxy evolution, shaping their morphology and triggering star formation. Recently, interest on distant interaction has been growing too. Together with Prof. Changbom Park and Juhan Kim, we have used our large Horizon Run 4 N-body simulation to study the interaction rate of dark matter haloes as a function of the environment and the redshift, in order to understand how interactions shape galaxies.

We define target haloes of mass $M_\mathrm{T}$ to be interacting if they are located within the virial radius of a neighbour more massive than $0.4M_\mathrm{T}$.

Figure 1 shows the two-dimensional distribution of target haloes in the HR4 simulation (top left) and interactions (top right). The bottom-left panel shows the interaction fraction, i.e., the fraction of targets undergoing an interaction as defined in the previous paragraph. We fit the following formula: $$ \Gamma(M|\delta,z) = A_0\mathrm{erfc}\left(b\log_{10}\frac M {M_*}\right) $$ at fixed large-scale density $\delta$ and redshift $z$.

We also fit the interaction rate as a function of the scale factor $a$ for different bins of mass and density, as shown in Figure 2, using the following formula: $$\Gamma(a) = B\mathrm{exp}\left(\left(-\frac{1-a}{A}\right)^\gamma\right)$$

## Galaxy mass assembly

**B. L'Huillier**, F. Combes & B. Semelin 2012,
A&A 544, 68

With Francoise Combes and Benoit Semelin, we have used multi-zoom simulations to investigate the way galaxies assemble their baryonic mass. We ran a series of N-body only and N=body + hydrodynamics simulations, zooming on a dense region (15 times the mean density of the Universe at z=0.46). I studied the mass assembly of galaxies, namely, how they assemble their baryonic mass, discriminating between the mass asseembled by mergers and by smooth accretion. To do this, I detected the dark matter and baryonic structures using a modified version of AdaptaHOP (Aubert et al 2004, Tweed et al. 2009), and buit merger trees. I found a mean accretion fraction of 77%, with low mass galaxies assembling their mass mostly from accretion. I also found some evidence for downsizing in these simulations: massive galaxies have have an old stellar populations, whereas low-mass galaxies form stars at every epoch.

I am currently comparing the statistical (mass function) and internal (concentration, spin) properties of haloes between the DM only and the hydrodynamic simulations, to understand how the baryonic physics shapes the haloes.