Day 577

Tuesday.

1401.2458
Jumping the gap: the formation conditions and mass function of Pebble-Pile Planetesimals
Hopkins

In a turbulent proto-planetary disk, dust grains undergo large density fluctuations and under the right circumstances, these grain over densities can overcome shear, turbulence, and gas pressure support to collapse under self-gravity (forming a 'pebble pile' planetesimal).  Using insights from simulations and a new analytic model for the fluctuations, calculate the rate-of-formation and mass function of self-gravitating, collapsing planetesimal-mass bodies formed by this mechanism.  Statistics depend on size/stopping time of largest grains, disk surface density, and turbulent Mach numbers.  When collapse occurs, predict that the resulting planetesimal mass function is broad and quasi-universal, with a slope dN/dM~1/M, spanning a size/mass range ~10-1e4 km (1e-9-5.0 M_Earth).  Collapse to planetesimal through super-Earth masses is possible.  The key condition is that grain density fluctuations reach large amplitudes on large scales, where gravitational instability proceeds most easily (collapse of small grains is strongly suppressed by turbulent vorticity).  Show this leads to a new criterion for 'pebble-pile' formation in terms of the dimensionless particle stopping time (tau_stop>f(Q,Z,alpha)).  In a MMSN [?], this requires grains larger than a=(50,1,0.1)cm at r=(1,30,100) au.  At large radii, this can easily occur and seed core accretion.  At small radii, it would depend on the existence of large boulders.  However, because density fluctuations depend super-exponentially on tau_stop (inversely proportional to disk surface density), lower-density disks are more unstable.  In fact, predict that cm-sized grains at ~1au will form pebble piles in a disk with ~10% of the MMSN density, so planet formation at ~au may generically occur late, as disks are evaporating.

1401.2460
Environmental dependence of bulge-dominated galaxy sizes in hierarchical models of galaxy formation.  Comparison with the local Universe
Shankar, Mei, … Sheth, et al

Compare SAMs as well as SHAMs with a large sample of early-type galaxies from SDSS at z<0.3.  Focus on dependence of median sizes of central galaxies on host halo mass.  The data do not show any difference in the structural properties of early-type galaxies with environment, at fixed stellar mass.  All hierarchical models considered in this work instead tend to predict a moderate to strong environmental dependence, with the median size increasing by a factor of about 1.5-3 when moving from low to high mass host haloes.  At face value the discrepancy with the data is highly significant, especially at the cluster scale, for haloes above M_halo>1e14.  The convolution with (correlated) observational errors reduces some of the tension.  Despite the observational uncertainties, the data tend to disfavor hierarchical models characterized by a relevant contribution of disc instabilities to the formation of spheroids, strong gas dissipation in (major) mergers, short dynamical friction timescales, and very short quenching timescales in infalling satellites.  Also discuss a variety of additional related issues, such as the slope and scatter in the local size-stellar mass relation, the fraction of gas in local early-type galaxies, and the general predictions on satellite galaxies.

1401.2463
Microlens masses from astrometry and parallax in space-based surveys: from planets to black holes
Gould, Yee

Show that space-based microlensing experiments can recover lens masses and distances for a large fraction of all events (those with individual photometric errors <~0.01 mag) using a combination of 1d micro lens parallaxes and astrometric microlensing.  This will provide a powerful probe of the mass distributions of planets, BHs, and NS, the distribution of planets as a function of Galactic environment, and the velocity distributions of BHs and NSs.  While systematics are in principle a significant concern, show that it is possible to vet against all systematics (known and unknown) using single-epoch precursor observations with the HST roughly 10 years before the space mission.

1401.2593
HEIDI: an automated process for the identification hand extraction of photometric light curves from astronomical images
Todd

As the title says.  Seems thoroughly tested (on a single-night exposure following astroid 939 Isberga, and on various Linux platforms).

1401.2636
Resampling images in Fourier domain
Bernstein, Gruen

For sky image simulation, using pixellated image + some interpolation kernel, and produce a new sampled image representing this galaxy as it would appear with a different PSF, rotation, shearing, magnification, and/or different pixel scale.  These operations are sometimes only possible, or most efficiently executed, as resampling of the Fourier transform F(u) of the image onto a u-space grid that differs from the one produced by a discrete Fourier transform (DFT) of the samples.  In some applications, it is essential that the resampled image be accurate to better than 1e-3, so in this paper, use standard Fourier techniques to show that Fourier-domain interpolation with a wrapped since function yields the exact value of F(u) in terms of the input samples and kernel.  This operations scales with image dimension as N^4 and can be prohibitively slow, so investigate the error accrued from approximating the since function with a compact kernel.  Show that these approximations produce a multiplicative error plus a pair of ghost images (in each dimension) in the simulated image.  Standard Lanczos or cubic interpolators, when applied in Fourier domain, produce unacceptable artifacts.  Find that errors <1e-3 can be obtained by (1) 4-fold zero-padding of the original image before executing the z=>u DFT, followed by (2) resampling to the desired u grid using a 6-pt, piecewise-quintic interplant that is designed expressly to minimize the ghosts, then (3) executing the DFT back to x-domain.