The splashback radius as a physical halo boundary and the growth of halo mass

The boundaries of cold dark matter halos are commonly defined to enclose a density contrast $\Delta$ relative to a reference (mean or critical) density. We argue that a more physical boundary of halos is the radius at which accreted matter reaches its first orbital apocenter after turnaround. This splashback radius, $R_{sp}$, manifests itself as a sharp density drop in the halo outskirts, at a location that depends upon the mass accretion rate. We present calibrations of $R_{sp}$ and the enclosed mass, $M_{sp}$, as a function of the accretion rate and alternatively peak height. We find that $R_{sp}$ varies between $\approx0.8-1R_{200m}$ for rapidly accreting halos and $\approx1.5R_{200m}$ for slowly accreting halos. The extent of a halo and its associated environmental effects can thus extend well beyond the conventionally defined "virial" radius. We show that $M_{sp}$ and $R_{sp}$ evolve relatively strongly compared to other commonly used definitions. In particular, $M_{sp}$ evolves significantly even for the smallest dwarf-sized halos at $z=0$. We also contrast $M_{sp}$ with the mass enclosed within four scale radii of the halo density profile, $M_{<4rs}$, which characterizes the inner halo. During the early stages of halo assembly, $M_{sp}$ and $M_{<4rs}$ evolve similarly, but in the late stages $M_{<4rs}$ stops increasing while $M_{sp}$ continues to grow significantly. This illustrates that halos at low $z$ can have "quiet" interiors while continuing to accrete mass in their outskirts. We discuss potential observational estimates of the splashback radius and show that it may already have been detected in galaxy clusters.Comment: 15 pages, 12 figures, comments most welcom

Log-normal Star Formation Histories in Simulated and Observed Galaxies

Gladders et al. have recently suggested that the star formation histories (SFHs) of individual galaxies are characterized by a log-normal function in time, implying a slow decline rather than rapid quenching. We test their conjecture on theoretical SFHs from the cosmological simulation Illustris and on observationally inferred SFHs. While the log-normal form necessarily ignores short-lived features such as starbursts, it fits the overall shape of the majority of SFHs very well. In particular, 85% of the cumulative SFHs are fitted to within a maximum error of 5% of the total stellar mass formed, and 99% to within 10%. The log-normal performs systematically better than the commonly used delayed-τ model, and is superseded only by functions with more than three free parameters. Poor fits are mostly found in galaxies that were rapidly quenched after becoming satellites. We explore the log-normal parameter space of normalization, peak time, and full width at half maximum, and find that the simulated and observed samples occupy similar regions, though Illustris predicts wider, later-forming SFHs on average. The ensemble of log-normal fits correctly reproduces complex metrics such as the evolution of Illustris galaxies across the star formation main sequence, but overpredicts their quenching timescales. SFHs in Illustris are a diverse population not determined by any one physical property of galaxies, but follow a tight relation, where width ∝ (peak time)^(3/2). We show that such a relation can be explained qualitatively (though not quantitatively) by a close connection between the growth of dark matter halos and their galaxies