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We revisit the origin of Larson's scaling laws describing the structure and kinematics of molecular clouds. Our analysis is based on recent observational measurements and data from a suite of six simulations of the interstellar medium, including effects of self-gravity, turbulence, magnetic field and multiphase thermodynamics. Simulations of isothermal supersonic turbulence reproduce observed slopes in linewidth-size and mass-size relations. Whether or not self-gravity is included, the linewidth-size relation remains the same. The mass-size relation, instead, substantially flattens below the sonic scale, as prestellar cores start to form. Our multiphase models with magnetic field and domain size 200 pc reproduce both scaling and normalization of the first Larson law. The simulations support a turbulent interpretation of Larson's relations. This interpretation implies that: (i) the slopes of linewidth-size and mass-size correlations are determined by the inertial cascade; (ii) none of the three Larson laws is fundamental; (iii) instead, if one is known, the other two follow from scale invariance of the kinetic energy transfer rate. It does not imply that gravity is dynamically unimportant. The self-similarity of structure established by the turbulence breaks in star-forming clouds due to the development of gravitational instability in the vicinity of the sonic scale. The instability leads to the formation of prestellar cores with the characteristic mass set by the sonic scale. The high-end slope of the core mass function predicted by the scaling relations is consistent with the Salpeter power-law index. Larson (1981) established that for many MCs their internal velocity dispersion, σu, is well correlated with the cloud size, L, and mass, m. Since the power-law form of the correlation, σu ∝ L 0.38 , and the power index, 0.38 ∼ 1/3, were similar to those of the Kolmogorov (1941a,b, hereafter K41) turbulence, he suggested that observed non-thermal linewidths may originate from a 'common hierarchy of interstellar turbulent motions'. The clouds would also appear mostly gravitationally bound and in approximate virial equilibrium, as there was a close positive correlation between their velocity dispersion and mass, σu ∝ m 0.20 . However, Larson suggested that these structures 'cannot have formed by simple gravitational collapse' and should be at least partly created by supersonic turbulence. This seminal paper preconceived many important ideas in the field and strongly influenced its development for the past 30 years.Myers (1983) studied 43 smaller dark clouds and confirmed the existence of significant linewidth-size and density-size correlations found earlier by Larson for larger MCs. He acknowledged that two distinct interpretations of the data are possible: (i) the linewidth-size relation also known as Larson's first law (σu ∝ R 0.5 ) arises from a Kolmogorov-like cascade of turbulent energy; (ii) the same c 2013 RAS

We study how properties of discrete dark matter halos depend on halo environment, characterized by the mass density around the halos on scales from 0.5 to 16 h −1 Mpc. We find that low mass halos (those less massive than the characteristic mass M C of halos collapsing at a given epoch) in high-density environments have lower accretion rates, lower spins, higher concentrations, and rounder shapes than halos in median density environments. Halos in median and low-density environments have similar accretion rates and concentrations, but halos in low density environments have lower spins and are more elongated. Halos of a given mass in high-density regions accrete material earlier than halos of the same mass in lower-density regions. All but the most massive halos in high-density regions are losing mass (i.e., being stripped) at low redshifts, which causes artificially lowered NFW scale radii and increased concentrations. Tidal effects are also responsible for the decreasing spins of low mass halos in high density regions at low redshifts z < 1, by preferentially removing higher angular momentum material from halos. Halos in low-density regions have lower than average spins because they lack nearby halos whose tidal fields can spin them up. We also show that the simulation density distribution is well fit by an Extreme Value Distribution, and that the density distribution becomes broader with cosmic time.

SubHalo Abundance Matching (SHAM) assumes that one (sub)halo property, such as mass M vir or peak circular velocity V peak , determines properties of the galaxy hosted in each (sub)halo such as its luminosity or stellar mass. This assumption implies that the dependence of Galaxy Luminosity Functions (GLFs) and the Galaxy Stellar Mass Function (GSMF) on environmental density is determined by the corresponding halo density dependence. In this paper, we test this by determining from an SDSS sample the observed dependence with environmental density of the ugriz GLFs and GSMF for all galaxies, and for central and satellite galaxies separately. We then show that the SHAM predictions are in remarkable agreement with these observations, even when the galaxy population is divided between central and satellite galaxies. However, we show that SHAM fails to reproduce the correct dependence between environmental density and g − r color for all galaxies and central galaxies, although it better reproduces the color dependence on environmental density of satellite galaxies.

We study the effects of the local environmental density and the cosmic web environment (filaments, walls, and voids) on key properties of dark matter haloes using the Bolshoi-Planck cold dark matter cosmological simulation. The z = 0simulationisanalysedintofilaments, walls, and voids using the SpineWeb method and also the VIDE package of tools, both of which use the watershed transform. The key halo properties that we study are the specific mass accretion rate, spin parameter, concentration, prolateness, scale factor of the last major merger, and scale factor when the halo had half of its z = 0m a s s .F o ra l lt h e s ep r o p e r t i e s , we find that there is no discernible difference between the halo properties in filaments, walls, or voids when compared at the same environmental density. As a result, we conclude that environmental density is the core attribute that affects these properties. This conclusion is in line with recent findings that properties of galaxies in redshift surveys are independent of their cosmic web environment at the same environmental density at z ∼ 0. We also find that the local web environment around galaxies of Milky Way's and Andromeda's masses that are near the centre of a cosmic wall does not appear to have any effect on the properties of those galaxies' dark matter haloes except on their orientation, although we find that it is rather rare to have such massive haloes near the centre of a relatively small cosmic wall.