Ivan Lacerna, Nelson Padilla, Federico Stasyszyn
We analyse catalogues of galaxies from the Sloan Digital Sky Survey (SDSS)
and find a weak but significant assembly-type bias, where old galaxies have a
higher clustering amplitude at scales r > 1 Mpc than young galaxies of equal
magnitude. When using the definition of age based on the luminosity-weighted
stellar age, the difference increases from -19 < M_r < -18 to -22 < M_r < -21
SDSS galaxies, from a 25 percent to a factor of two at r ~ 2.5 Mpc. In
contrast, when using the definition based on the Dn4000 index, the clustering
amplitude at large scales shows a weaker signal-to-noise for the assembly bias
detection. In semi-analytic galaxies the difference in clustering using the
stellar age is similar at distances beyond 3 Mpc, but they overpredict the
assembly bias found in SDSS galaxies at smaller scales. We then adapt the model
presented by Lacerna & Padilla (Paper I) to redefine the overdensity peak
height, that traces the assembly bias such that galaxies in equal density peaks
show the same clustering regardless of their age, but this time using
observational features. The proxy of peak height for observational data
consists in the luminosity inside cylinders of different radii centred in each
galaxy. This radius is parameterized as a function of stellar age and magnitude
M_r. The best-fitting set of parameters that make the assembly bias practically
absent for both simulated and real galaxies are similar (a=0.75, b=-0.36 and
a=1, b=-0.36, respectively). Even though in this case we consider neighbour
galaxies in cylinders in contrast to using a smooth density field, e.g. the
dark matter particles distribution, our simulations show that the new method is
correlated to the definition of Paper I as both measure the crowding around
objects that traces the assembly bias effect.
View original:
http://arxiv.org/abs/1110.6174
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