On the Tidal Dependence of Galaxy Properties
  Using volume-limited samples drawn from The Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we measure the tidal environment of galaxies, which we characterize by the ellipticity e of the potential field calculated from the smoothed spatial number density 1 + {\delta} of galaxies. We analyze if galaxy properties, including color, Dn4000, concentration and size correlate with e, in addition to depending on 1 + {\delta}. We find that there exists a transition smoothing scale at which correlations/anti-correlations with e reverse. This transition scale is well represented by the distance to the 3rd- nearest-neighbor of a galaxy in a volume limited sample with Mr < -20 which has a distribution peaked at ~ 2 h-1Mpc. We further demonstrate that this scale corresponds to that where the correlation between the color of galaxies and environmental density 1+{\delta} is the strongest. For this optimal smoothing R0 no additional correlations with e are observed. The apparent dependence on tidal ellipticity e at other smoothing scales Rs can be viewed as a geometric effect, arising from the cross correlation be- tween (1+{\delta}o) and e(Rs). We perform the same analysis on numerical simulations with semi-analytical modeling (SAM) of galaxy formation. The e dependence of the galaxy properties shows similar behavior to that in the SDSS, although the color-density cor- relation is significantly stronger in the SAM. The 'optimal adaptive smoothing scale' in the SAM is also closely related to the distance to the 3rd-nearest-neighbor of a galaxy, and its characteristic value is consistent with, albeit slightly smaller than for SDSS. 
Heling Yan, Zuhui Fan, Simon D.M. White
astro-ph.GA