The sample of the Local Volume galaxies (distance smaller than 10 Mpc) gives a unique opportunity to study the properties of galaxies including the weakest objects: the absolute magnitude M_B≈-10 and the virial mass M_vir≈10⁹M☼. We found out that the standard ΛCDM model precisely describes the distribution function of galactic circular velocities for massive objects (V_circ>~70 km s^-1 and M_vir>~5x10¹⁰M☼) but five times overestimates the number of dwarf galaxies with circular velocities of V_circ~30-40 km s^-1. Such a considerable overabundance of predicted big dwarf field galaxies with masses M_vir≈10¹⁰M☼ causes a complicated problem: to prove they are lost, these galaxies should be of extremely low surface brightness, with no star formation processes and no neutral hydrogen. Up to now, there has been no such galaxy detected.
    The abundance of galaxies as a function of their circular velocities dN/dV is fundamental statistics that is very sensitive for theoretical prediction checking. These studies became possible only recently, as they are complicated to obtain. The Local Volume (D<10 Mpc) allows us to study the circular velocity function for the galaxies of all morphological types up to the smallest ones. We have found that the observed velocity function shows a small inclination α≈-1 for small velocities and relatively steep inclination at great velocities.
    The velocity function of the Local Volume comply with the same function in the significantly fuller sample ALFAFA based on the HI observations corrected for 10-20% fraction of early-type galaxies non-detectable in neutral hydrogen. Thus, we can now construct the velocity function with ~10% accuracy for galaxies of all types in the velocity range of 10 to 200 km s^-1.
    We compared the galaxies of the Local Volume with theoretical predictions and found that for comparatively massive galaxies with V>~70 km s^-1 and masses M_vir>~5x10¹⁰M☼, the ΛCDM standard model is in agreement with the observed abundance of galaxies. However, the small inclination and the normalization of velocity function for dwarf galaxies with V<40 km s^-1 contradict the standard model which predicts the inclination α≈-3 for the dark matter halo from the cosmological N-body modeling. Moreover, the models with warm dark matter also can not explain the observed data irrespective of particle masses.
    The overabundance of the field galaxies differs in a number of aspects from the similar problem of the overabundance of satellites in the Local Group. As distinct from the Local Group, where the problem arises with gas-poor dwarf spheroidal galaxies with V~10 km s^-1 and at small distances (≤1 kpc), in the field, the problem arises with gas-rich galaxies with star formation process with circular velocities of about 30-40 km s^-1 and at great distances (≥2 kpc) from massive objects.
A.Klypin, I.Karachentsev, D.Makarov, O.Nasonova
Published:
A.Klypin, I.Karachentsev, D.Makarov, and O.Nasonova "Abundance of field galaxies", 2015, MNRAS, 454, 1798