Spatial and Dymanical Properties of Voids in a LambdaCDM Universe

NELSON PADILLA; LAURA CECCARELLI; DIEGO G LAMBAS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2005 vol. 367 p. 977 - 990

0035-8711

We study statistical properties of voids in the distribution of mass, dark-matter haloes and galaxies ($B_J<-16$) in a $Lambda$CDM numerical simulation populated with galaxies using a semi-analytic galaxy formation model(GALFORM, Cole et al. 2000). We find that the properties of voids selected from GALFORM galaxies are compatible with those of voids identified from a population of haloes with mass $M>10^{11.5}$h$^{-1}$M$_{odot}$, similar to the median halo mass, $M_{ m med}=10^{11.3}$h$^{-1}$M$_{odot}$.  We also find that the number density of galaxy- and halo-defined voids can be up to two orders of magnitude higher than mass-defined voids for large void radii, however, we observe that this difference is reduced to about half an order of magnitude when the positions are considered in  redshift-space.  As expected, there are outflow velocities which show their maximum at larger void-centric distances for larger voids. We find a linear relation for the maximum outflow velocity, $v_{ m max}=v_0 r_{ m void}$.  The void-centric distance where this maximum occurs,follows a suitable power law fit of the form, $log(d_{v m max})=(r_{ m void}/A)^B$. At sufficiently large distances, we find mild infall motions onto the sub-dense regions. The galaxy velocity field around galaxy-defined voids is consistent with the results of haloes with masses above the median, showing milder outflows than the mass around mass-defined voids. We find that a similar analysis in redshiftspace would make both outflows and infalls to appear with a lower amplitude. We also find that the velocity dispersion of galaxies and haloes is larger in the direction parallel to the void walls by$simeq10-20%$. Given that voids are by definition sub-dense regions,the cross-correlation function between galaxy-defined voids and galaxies are close to $xi=-1$ out to separations comparableto the void size, and at larger separations the correlation function level increases approaching the values of the auto-correlation function of galaxies.  The cross-correlation amplitude of mass-defined voids vs. mass has a more gentle behaviour remaining negative at larger distances. The cross- to auto-correlation function ratio as a function ofthe distance normalised to the void radius, shows a small scatter around a relation that depends only on the object used to define the voids (galaxies or haloes for instance). The distortion pattern observed in$xi(sigma,pi)$ is that of an elongation along the line of sight which extends out to large separations.  Positive $xi$ contours evidence finger-of-god motions at the void walls.  Elongations along the line of sight are roughly comparable between galaxy-, halo- and mass-defined voids.