Voids in the Local Volume galaxy sample and in high resolution simulations:
Limit on appearance of a galaxy in a Dark Matter halo

Dr. Anton Tikhonov,
St.Petersburg, Russia


Current explanation of the overabundance of dark matter subhalos in the Local Group (LG) indicates that there maybe a limit on mass of a halo, which can host a galaxy. This idea can be tested using voids in the distribution of galaxies: at some level small voids should not contain any (even dwarf) galaxies. We use observational samples complete to M_B=-12 with distances less than 8 Mpc to construct the void function (VF): the distribution of sizes of voids empty of any galaxies. There are ~30 voids with sizes ranging from 1 to 5 Mpc. We also study the distribution of dark matter halos in very high resolution simulations of the LCDM model. The theoretical VF matches the observations remarkably well only if we use halos with circular velocities larger than 30-40 km/s. This agrees with the Local Group predictions. Small voids look quite similar to their giant cousins: the density has a minimum at the center of a void and it increases as we get closer to the border. Thus, both the Local Group data and the nearby voids indicate that isolated halos below ~30 km/s must not host galaxies and that small (few Mpc) voids are truly dark. Luminosity function of Local Volume complete sample is well fitted by Schechter approximation of the average luminosity function of galaxies in the B-band in the Universe but there is an excess of very bright galaxiÂs in the LV that gives luminosity overdensity ~0.5 in K-band (that can be directly convertedto overdensity of stellar mass) with respect to the mean in the local Universe.