Day 513

Monday.

1309.4771
Constraining the primordial power spectrum from SNIa lensing dispersion
Ben-Dayan, Kalaydzhyan

The (absence of detecting) lensing dispersion of SNe Ia can be used as a novel and extremely efficient probe of cosmology.  In this preliminary example, analyze its consequences for the primordial PS.  The main setback is the [lack of?] knowledge of the PSF in the NL regime, 1 Mpc^-1 <k< 1e2-3 Mpc^-1 at z~1.  By using the lensing dispersion and conservative estimates in this regime of wavenumbers, show how the current upper bound sigma_mu(z=1)<0.12 [what is sigma_mu?] on existing data gives strong indirect constraints on the primordial PS.  The probe extends handle on the spectrum to a total of 12-15 inflation e-folds. These constraints are so strong that they are already ruling out a large portion of the parameter space allowed by PLANCK for running alpha = d n_s/d ln k and running of running beta = d^2 n_s / d ln k^2.  The bounds follow a linear relation to a very good accuracy.  A conservative bound disfavors any enhancement above the line beta(k0)=0.032-0.41 alpha(k0) and a realistic estimate disfavors any enhancement above the line beta(k0)=0.019-0.45 alpha(k0).

1309.4774
The FMOS-COSMOS survey of star-forming galaxies at z~1.5 I. H\alpha -based star formation rates and dust extinction
Kashino et al

First results from a NIR spectroscopic survey of COSMOS field, with FMOS on Subaru, designed to characterize the SF galaxy population at 1.4<z<1.7. The high-resolution mode is implemented to detect Ha in emission.  Specifically focus on 271 sBzK-selected galaxies that yield Ha detection, providing a redshift and emission-line luminosity to establish the relation between SFR and stellar mass.  With futher J-band spectroscopy for 89 of these, the level of dust extinction is assessed by measuring the Balmer decrement using co-added spectra.  Find that the extinction rises with stellar mass and is elevated at high masses compared to low-redshift galaxies.  Using this subset of the spectroscopic sample, further find that the differential extinction between stellar adn nebular emission E_star(B_V)/E_neb(B_V) is 0.7-0.8, dissimilar to that typically seen at low redshift.  After correcting for extinction, derive an Ha-based MS having a slope (0.81 pm 004) and normalization similar to previous studies at these redshifts.

1309.4780
Prospects for detecting gamma rays from annihilating dark matter in dwarf galaxies in the Era of DES and LSST
He, Bechtol, Hearin, Hooper

Among the most stringent constraints on the DM annihilation cross section are those derived from observations of dwarf galaxies by Fermi.  As current (DES) and future (LSST) optical imaging surveys discover more of the MW's ultra-faint satellite galaxies, they may increase Fermi's sensitivity to DM annihilations.  In this study, use SAM of the MW's satellite population to predict the characteristics of the dwarfs likely to be discovered by DES and LSST, and project how these discoveries will impact Fermi's sensitivity to DM.  Find that modest improvements are likely, but the dwarf galaxies discovered by DES and LSST are unlikely to increase Fermi's sensitivity by more than a factor of ~2.

1309.4809
On the nature of the brightest globular cluster in M81
Mayya et al

Analysis of the photometric, chemical, SFH and structural properties of the brightest GC in M81 ("GC1"), with the intention of establishing its nature and origin.  It is a metal-rich [Fe/H]=-0.6pm0.1, alpha-enhanced [alpha/Fe]=0.20pm0.05, core-collapased (core radius r_c=1.2 pc, tidal radius r_t = 76r_c, old (>13 Gyr) cluster.  It has UV excess equivalent of ~2500 blue horizontal branch stars.  Detected in X-rays, indicative of the presence of low-mass binaries.  With a mass of 1e7 Msun, the cluster is comparable in mass to M31-G1 and is 4x more massive than Omega Cen.  The values of r_c, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.

1309.4838
Mass extinction and the structure of the Milky Way
Filipović et al

Use the most up-to-date MW model and solar orbit data in order to test the hypothesis that the Sun's galactic spiral arm crossings cause mass extinction events on Earth.  Create a new model of spiral arms from data from several surveys; combine this model with a recently derived solution for the solar orbit to determine the timing of the Sun's historical passages through the Galaxy's spiral arms.  New model is designed with a symmetrical appearance, with the major alteration being the addition of a spur at the far side of the Galaxy.  A correlation found between the times at which the Sun crosses the spiral arms and six known mass extinction events.  Identify 5 additional historical mass extinction events that might be explained by the motion of the Sun around the Galaxy: significant drops in marine genera that include significant reductions in diversity at 415, 322, 300, 145 and 33 Myr ago.  Simulations in indicate that the Sun has spent ~60% of its time passing through the Galaxy's various spiral arms.  Briefly discuss and combine previous work on the Galactic Habitable Zone with the new Milky Way model.

1309.4839
Mars encounters cause fresh surfaces on some near-Earth asteroids
DeMeo et al

All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1My) timescales.  The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying that they have been recently been exposed.  Previous work found that Earth encounders were the dominant freshing mechanism and could be responsible for all NEO Q-types.  In this work, increase the known NEO Q-type sample by 3x.  Present the orbital distributions of 64 Q-type NE asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces.  Sample reveals two important results: i) the relatively steady fraction of Q-types with increasing semi-major axis and ii) the existence of Q-type NE asteroids with minimum orbit intersection distances (MOID) that do not have orbit solutions that cross Earth.  Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened.  The high Earth-MOID asteroids represent 10% of the Q-type population and are all in Amor orbits [?].  While surface refreshing could also be cause by Main Belt collisions or mass shedding from YORP spinup [?], all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.

1309.4866
Linear structures in the core of the Coma cluster of galaxies
Sanders et al

Chandra show presence of quasi-linear high-density arms spanning 150 kpc, consisting of low-entropy material likely stripped from merging subclusters.  Two appear to be connected with a subgroup of galaxies at 650 kpc radius that is merging into the cluster, implying coherence over several hundred Myr [merging timescale?].  Such long lifetime implies that strong isotropic turbulence and conduction are suppressed in the core, despite the unrelaxed state of the cluster.  Magnetic fields are presumably responsible.  The structures seen in coma present insight into the past Gyr of subcluster merger activity.

1309.4924
Remarks on mechanical approach to observable Universe
Eingorn, Zhuk

Consider the Universe deep inside the cell of uniformity.  At these scales, the Universe is filled with inhomogeneously distributed discrete structures (galaxies, groups and clusters of galaxies), which perturb the background Friedmann model.  The mechanical approach is appropriate to describe the dynamics of the inhomogeneities which is defined, by gravitational potentials of inhomogeneities and also by the cosmological expansion of the Universe.  Here, present additional arguments in favor of this approach.  First, estimate the size of the cell of uniformity (order of 190 Mpc from stat mech; close to observations [?]); then, show that the non-relativistic approximation (wrt the peculiar velocities) is valid for z<~10, approximately for 13 billion years from present.  Consider scalar perturbations and, within the LCDM model, justify the main equations.  Demonstrate that radiation can be naturally incorporated into the scheme, emphasizing the viability of the approach.  Gives a possibility to analyze different cosmological models and compare them with the observable Universe.  Indicate some problematic aspects of the spatially flat models.  Such models require a rather specific distribution of the inhomogeneities to get a finite potential at any points outside gravitating masses.  Also criticize the application of the Schwarzschild-deSitter solution to the description of the motion of test bodies on the cosmological background.

1309.5087
Sparse sampling, galaxy bais, and voids
Sutter, ... Hamaus, Weinberg, et al

Study the impact of sparsity and galaxy bias on void statistics: use a single large-volume, high-res N-body sim to compare voids in multiple levels of subsampled DM, halo populations, and mock galaxies from an HOD model tuned to different galaxy survey densities.  Focus comparison on 3 key observational statistics: number functions, ellipticity distributions, and radial density profiles.  Use the hierarchical tree structure of voids to interpret the impacts of sampling density and galaxy bias, and theoretical and empirical functions to describe the statistics in all the sample populations.  While theoretical calculations do not make immediate contact with the results, still able to make simple adjustments to offer prescriptions for translating from analytics to the void properties measured in realistic observations.  Find that sampling density has a much larger effect on void sizes than galaxy bias.  At lower tracer density, small voids disappear and the remaining voids are larger, more spherical, and have slightly steeper profiles.  Radial density profiles follow a self-similar trend as a function of sampling density and void size.  When a proper lower mass threshold is chosen, voids in halo distributions largely mimic those found in galaxy populations, except for ellipticities, where galayx bias leads to higher values.  Added the mock void catalogs used in this work to the Public Cosmic Void Catalog.

1309.5244
Why do Earth satellites stay up?
Tremaine, Yavetz

Satellites in low Earth orbits must accurately conserve their orbital eccentricity, since a decrease in perigee of only 5-10% would cause them to crash.  However, these satellites are subject to gravitational perturbations from the Earth's multipole moments, the Moon, and the Sun that are not spherically symmetric and hence do not conserve angular momentum, especially over the tens of thousands of orbits made by a typical satellite.  Why do then satellites do not crash?  Describe a vector-based analysis of the long-term behavior of satellite orbits and apply this to several toy systems containing a single non-Keplerian perturbing potential.  If only the quadrupole potential from the Earth's equatorial bulge is present, all near-circular orbits are stable.  If only the octupole potential is present, all such orbits are unstable.  If only the lunar or solar potential is present, all near-circular orbits with inclinations to the ecliptic exceeding 39 degrees are unstable.  Describe the behavior of satellites in the simultaneous presence of all these perturbations and show that almost all low Earth orbits are stable because of an accidental property of the dominant quadrupole potential.  Also relate these results to the phenomenon of Lidov-Kozai oscillations.