[Abridged] The properties of observed galaxies and dark matter haloes in simulations depend on their environment. The term environment has been used to describe a wide variety of measures that may or may not correlate with each other. Popular measures of environment include the distance to the N'th nearest neighbour, the number density of objects within some distance, or the mass of the host dark matter halo. We use results from the Millennium simulation and a semi-analytic model for galaxy formation to quantify the relations between environment and halo mass. We show that the environmental parameters used in the observational literature are in effect measures of halo mass, even if they are measured for a fixed stellar mass. The strongest correlation between environment and halo mass arises when the number of objects is counted out to a distance of 1.5-2 times the virial radius of the host halo and when the galaxies/haloes are required to be relatively bright/massive. For observational studies the virial radius is not easily determined, but the number of neighbours out to 1-2 Mpc/h gives a similarly strong correlation. For the distance to the N'th nearest neighbour the correlation with halo mass is nearly as strong provided N>2. We demonstrate that this environmental parameter becomes insensitive to halo mass if it is constructed from dimensionless quantities. This can be achieved by scaling the minimum luminosity/mass of neighbours to that of the object in question and by dividing the distance to a length scale associated with either the neighbour or the galaxy under consideration. We show how such a halo mass independent environmental parameter can be defined for observational and numerical studies. The results presented here will help future studies to disentangle the effects of halo mass and external environment on the properties of galaxies and dark matter haloes.Comment: 15 pages, 9 figures, 2 tables. Accepted by MNRA
We investigate the correlation between nine different dark matter halo properties using a rank correlation analysis and a Principal Component Analysis for a sample of haloes spanning five orders of magnitude in mass. We consider mass and dimensionless measures of concentration, age, relaxedness, sphericity, triaxiality, substructure, spin, and environment, where the latter is defined in a way that makes it insensitive to mass. We find that concentration is the most fundamental property. Except for environment, all parameters are strongly correlated with concentration. Concentration, age, substructure, mass, sphericity and relaxedness can be considered a single family of parameters, albeit with substantial scatter. In contrast, spin, environment, and triaxiality are more independent, although spin does correlate strongly with substructure and both spin and triaxiality correlate substantially with concentration. Although mass sets the scale of a halo, all other properties are more sensitive to concentration.Comment: 5 pages, 2 figures. Accepted by MNRAS Letters. Minor change
We derive empirical constraints on the volume averaged 'effective' escape fraction of Lyα photons from star forming galaxies as a function of redshift, by comparing star formation functions inferred directly from observations, to observed Lyα luminosity functions. Our analysis shows that the effective escape fraction increases from f eff esc ∼ 1 − 5% at z = 0, to f eff esc ∼ 10% at z = 3 − 4, and to f eff esc = 30 − 50% at z = 6. Our constraint at z = 6 lies above predictions by models that do not include winds, and therefore hints at the importance of winds in the Lyα transfer process (even) at this redshift. We can reproduce Lyα luminosity functions with an f eff esc that does not depend on the galaxies star formation rates (ψ) over up to ∼ 2 orders of magnitude in Lyα luminosity. It is possible to reproduce the luminosity functions with an f eff esc that decreases with ψ -which appears favored by observations of drop-out galaxies -in models which include a large scatter (σ > ∼ 1.0 dex) in f eff esc , and/or in which star forming galaxies only have a non-zero f eff esc for a fraction of their life-time or a fraction of sightlines. We provide a fitting formula that summarizes our findings.
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