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Register a new userVoronoi volume function: A new probe of cosmology and galaxy evolution
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Aseem Paranjape
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We study the Voronoi volume function (VVF) -- the distribution of cell volumes (or inverse local number density) in the Voronoi tessellation of any set of cosmological tracers (galaxies/haloes). We show that the shape of the VVF of biased tracers responds sensitively to physical properties such as halo mass, large-scale environment, substructure and redshift-space effects, making this a hitherto unexplored probe of both primordial cosmology and galaxy evolution. Using convenient summary statistics -- the width, median and a low percentile of the VVF as functions of average tracer number density -- we explore these effects for tracer populations in a suite of N-body simulations of a range of dark matter models. Our summary statistics sensitively probe primordial features such as small-scale oscillations in the initial matter power spectrum (as arise in models involving collisional effects in the dark sector), while being largely insensitive to a truncation of initial power (as in warm dark matter models). For vanilla cold dark matter (CDM) cosmologies, the summary statistics display strong evolution and redshift-space effects, and are also sensitive to cosmological parameter values for realistic tracer samples. Comparing the VVF of galaxies in the GAMA survey with that of abundance matched CDM (sub)haloes tentatively reveals environmental effects in GAMA beyond halo mass (modulo unmodelled satellite properties). Our exploratory analysis thus paves the way for using the VVF as a new probe of galaxy evolution physics as well as the nature of dark matter and dark energy.
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We present high signal-to-noise galaxy-galaxy lensing measurements of the BOSS CMASS sample using 250 square degrees of weak lensing data from CFHTLenS and CS82. We compare this signal with predictions from mock catalogs trained to match observables including the stellar mass function and the projected and two dimensional clustering of CMASS. We show that the clustering of CMASS, together with standard models of the galaxy-halo connection, robustly predicts a lensing signal that is 20-40% larger than observed. Detailed tests show that our results are robust to a variety of systematic effects. Lowering the value of $S_{rm 8}=sigma_{rm 8} sqrt{Omega_{rm m}/0.3}$ compared to Planck2015 reconciles the lensing with clustering. However, given the scale of our measurement ($r<10$ $h^{-1}$ Mpc), other effects may also be at play and need to be taken into consideration. We explore the impact of baryon physics, assembly bias, massive neutrinos, and modifications to general relativity on $DeltaSigma$ and show that several of these effects may be non-negligible given the precision of our measurement. Disentangling cosmological effects from the details of the galaxy-halo connection, the effects of baryons, and massive neutrinos, is the next challenge facing joint lensing and clustering analyses. This is especially true in the context of large galaxy samples from Baryon Acoustic Oscillation surveys with precise measurements but complex selection functions.
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We propose the existence of ultracompact minihalos as a new type of massive compact halo object (MACHO) and suggest an observational test to discover them. These new MACHOs are a powerful probe into the nature of dark matter and physics in the high energy Universe. Non-Gaussian energy-density fluctuations produced at phase transitions (e.g., QCD) or by features in the inflaton potential can trigger primordial black hole (PBH) formation if their amplitudes are delta > 30%. We show that a PBH accumulates over time a sufficiently massive and compact minihalo to be able to modify or dominate its microlensing magnification light curve. Perturbations of amplitude 0.03% < delta < 30% are too small to form PBHs, but can nonetheless seed the growth of ultracompact minihalos. Thus, the likelihood of ultracompact minihalos as MACHOs is greater than that of PBHs. In addition, depending on their mass, they may be sites of formation of the first PopIII stars. Ultracompact minihalos and PBHs produce a microlensing light curve that can be distinguished from that of a point-like object if high-quality photometric data are taken for a sufficiently long time after the peak of the magnification event. This enables them to be detected below the stellar-lensing background toward both the Magellanic Clouds and the Galactic bulge.
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