Cosmic Structure

We use N-body/hydrodynamical simulations and observations in different bands to investigate the formation and evolution of galaxies, clusters and larger structures. We investigate the role of high-energy astrophysical processes (supernovae explosions and Active Galactic Nuclei energy emission) in the formation efficiency of stars and galaxies. We have also developed the caustic technique, a unique method to estimate the mass of galaxy clusters well beyond the region of dynamical equilibrium. The method is based on galaxy redshifts alone and is complementary to the more popular gravitational lensing techniques.

The caustic technique

In 1997, Diaferio and Geller conceived the caustic technique, a completely new and original tool for estimating the mass in the outskirts of galaxy clusters. This technique opened a new window on the mass estimation on mildly non-linear scale and has a wider application than weak lensing because it can be applied at any redshift (Diaferio 1999, Diaferio 2009 and Serra et al. 2011).

In 1999, for the first time, we measured the mass density profile of a cluster beyond its virial radius and confirmed the Navarro, Frenk and White profile on these scales (Geller et al. 1999). With extensive redshift surveys of the outer regions of clusters at low (CIRS) and intermediate redshifts (HeCS), we performed the only measurement available to date of the ratio between the masses within the infall and the virial regions; we show that the measured ratio agrees with the expected value of the ΛCDM model.
For clusters in the redshift range [0.1-0.3], the caustic masses can be compared with weak lensing measurements and we can assess the systematic errors of the two approaches (Diaferio et al. 2005, Geller et al. 2013).

A by-product of the caustic technique is the identification of cluster members to large radii: on average, within 3r200, the technique identifies 95% of the members and returns samples of galaxies contaminated by a fraction of interlopers of only 8% (Serra and Diaferio 2013). The technique was applied to both galaxies in clusters (e.g. Pranger et al. 2013, Hernandez-Fernandez et al. 2014) and stars in galaxies (Serra et al. 2010, Brown et al. 2010).

  • Figure and movie: N-body simulation of the evolution of a galaxy cluster with mass M200 = 6.28x1014Msun/h in a ΛCDM model. Top left panel: Distribution of dark matter in the x-y plane; the panel is 30 comoving Mpc/h on a side. Bottom left panel: A section of phase space; the x spatial coordinate on the abscissa ranges from -15 to +15 comoving Mpc/h; the peculiar velocity along the z spatial coordinate on the ordinate ranges from -4000 to +4000 km/s. Top right panel: Redshift diagram where the abscissa is the projected distance in the x-y plane from the cluster center at redshift z=0; the axis extends to 15 comoving Mpc/h; the ordinate is the rest-frame line-of-sight velocity and extends from -4000 to +4000 km/s. Bottom right panel: 500 particles randomly sampled from the redshift diagram shown in the top right panel. The number in the upper right corner of each panel is the redshift; the clock shows the fraction of cosmic time elapsed since the start of the simulation that is at redshift z=20. From Geller, Diaferio and Kurtz (2011).

  • Large-Scale Structure

    The Caustic group collaborates with Margaret Geller's group at the Harvard-Smithsonian Center for Astrophysics to investigate the cosmic structure on large scales. The largest structures and voids are powerful tools to constrain models of structure formation (Sheth and Diaferio 2011, Colberg et al. 2005).

    We are currently completing HectoMAP, a dense redshift survey with median redshift z=0.34 covering 50 deg2 to r = 21 (Geller, Diaferio & Kurtz 2011).

  • Figure: Logo of the HectoMAP redsfhit survey (Geller et al 2014). Left panel: the large-scale distribution of galaxies in the real Universe. Right panel: the distribution of galaxies in a mock survey by C. Park. Redshift ranges from the center outwards from 0.2 to 0.5. Credit M.J.Geller and H.S. Hwang.
  • Beyond Standard Gravity and Cosmology

    The wealth of cosmological data is currently interpreted with the ΛCDM model: Einstein's cosmological constant and cold dark matter. However, there is no obvious physical mechanism that can generate the cosmological constant or more sophisticated dark energy fluids, nor is there any convincing direct or indirect confirmation of the existence of the cold dark matter particle.
    We investigate a wide range of extended and modified gravity models, by making the best possible use of all observational constraints to test the viability of these models at various scales and epochs.


    Modified Newtonian Dynamics (MOND) is impressively successful on the scales of galaxies but fails on the scales of galaxy clusters. A model combining MOND with a dark matter component of sterile neutrinos preserves the success of MOND on small scales but does not seem to heal the problems on larger scales (Diaferio & Angus 2012, Angus, Diaferio et al. 2013).

  • Figure: Distribution of dark matter particles in a cosmological model with sterile neutrinos and MOND. The volume is 512 comoving Mpc/h on a side and 50 comoving Mpc/h deep. The densest regions are shown in yellow. From Angus and Diaferio (2011).

  • Conformal Gravity

    Conformal gravity is able to describe the rotation curves of disk galaxies but appears to fail at reproducing the X-ray properties of clusters (Diaferio & Ostorero, 2009). In addition, conformal gravity predicts an always accelerating universe. At odds with common wisdom, we show that this prediction agrees with current supernovae type Ia and GRB data (Diaferio, Ostorero & Cardone 2011).

  • Figures: Hubble diagram of the 397 SNae of the Constitution set (Hicken et al. 2009) in (1) ΛCDM and (2,3) two conformal gravity models. At odds with common wisdom, the distance modulus mu_B is NOT a directly observable quantity, but depends on the assumed cosmological model. The third panel shows that the observed quantities of this SN sample do perfectly agree with a model (KCG) where the SN distance moduli are 3 magnitudes fainter than in the standard model. Further details in Diaferio, Ostorero and Cardone (2011).

  • Quintessence

    An alternative to the cosmological constant can be found in quintessence models, in which dark energy is described by a dynamical scalar field. Several models have been proposed in which the quintessence scalar field can also be coupled to gravity (within scalar-tensor theories) and matter. Quintessence has an impact both on the growth rate of cosmological structure (De Boni et al. 2011) and on the internal properties of gravitationally bound halos (De Boni et al. 2013).

  • Figures: (1) Redshift evolution of the ratio between the value of σ8D+ for different quintessence models and the corresponding value for ΛCDM. σ8D+ is the quantity that controls structure formation and evolution, in this case normalized so that all models agree at the CMB. (2) Mass function at different redshifts for ΛCDM and the four quintessence models shown in Fig. 1. The differences in the mass function reflect the differences in σ8D+.

  • Other modified theories

    We also investigated how weak lensing surveys can constrain various extended and modified theories of gravity, including f(R), Unified Dark Matter models, and the Dvali, Gabadadze and Porrati model (Camera et al. 2011 a, b, c).

  • Figure: Weak gravitational lensing power spectrum of galaxies with median redshift z=0.91 expected from a space-based survey like Euclid in two versions of the DGP model and in the ΛCDM model. At small scales, l~1000, this survey is in principle capable of distinguishing between the two DGP models. Further details in Camera, Diaferio and Cardone (2011).