Stephanie Tonnesen, Renyue Cen
In a hierarchical universe, mergers may be an important mechanism not only in
increasing the mass of galaxies but also in driving the colour and
morphological evolution of galaxies. We use a large sample of ~1000 simulated
galaxies of stellar mass greater than 10^{9.6} solar masses (for ~4800
observations at multiple redshifts) from a high-resolution (0.46 kpc/h)
cosmological simulation to determine under what circumstances being a member of
a pair influences galaxy properties at z <= 0.2. We identify gravitationally
bound pairs, and find a relative fraction of blue-blue (wet), red-red (dry),
and blue-red (mixed) pairs that agrees with observations (Lin et al. 2010). All
pairs tend to avoid the extreme environments of clusters and void centres.
While pairs in groups can include galaxies that are both blue, both red, or one
of each colour, in the field it is extraordinarily rare for pair galaxies to
both be red. We find that, while physically bound pairs closer than 250 kpc/h
tend to be bluer than the galaxy population as a whole, the colour of a pair
relative to galaxies in a comparable local density environment, characterized
by the density determined by the distance to the fifth nearest neighbor, rho_5,
depends on local galaxy density. In regions of high local galaxy density pairs
are bluer, whereas the opposite is true for low local galaxy density regions.
It appears that being in a pair has an incremental, but not overwhelming,
effect on the star formation rate of the paired galaxies, compared to the more
pronounced trend where galaxies overall are reddest in clusters and bluest at
the centre of voids. This trend depends most strongly on local galaxy density
(rho_5). We find no strong evidence that pair interactions are the driver of
the colour-density relation for galaxies.
View original:
http://arxiv.org/abs/1111.0636
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