from the SDSS-DR6 spectroscopic main galaxy sample. The first and second column indicate the sample name and number of galaxies in them, the third column indicates the number density, and the fourth column the host-halo mass.[]{data-label="table"}

We study the alignment signal in the observational data for different subsamples defined by galaxy colour and luminosity in order to detect trends that can be associated to the results from the numerical simulation. The subsamples we study are sample O1, consisting of all the edge-on galaxies with axial ratio $b/a<0.7$ in the SDSS-DR6; O2, red galaxies with $g-r>0.7$; O3, blue galaxies with $g-r<0.7$; O4, faint galaxies with $M_r>-19.5$; and O5 composed by bright galaxies with $M_r<-19.5$. The selection of subsamples is done so as to ensure a number of target galaxies to allow good statistics ($> 1000$); the number of galaxies in each sample is shown in Table \[table\], where we also show the galaxy number density calculated out to the redshift of completeness of each sample.

In order to characterise in a quantitative way the subsamples selected from the SDSS-DR6, we calculate the median mass of $\Lambda$CDM haloes predicted to show the same 2-halo regime clustering amplitude as the galaxies in each 