Day 443

Thursday.

1306.1622
The strength of regolith and rubble pile asteroids
Sanchez, Scheeres

Hypothesis: due to van der Waals forces between regolith grains, the strength of small rubble pile asteroids is constant.  This creates a scale dependence, with relative strength increasing as size decreases.  This counters classical theory that ruble pile asteroids should behave as scale-independent cohesionless collections of rocks.  Explore a simple model for asteroid strength that is based on these weak forces, validate it through granular mechanics simulations and comparisons with properties of lunar regolith, and then show its implications and ability to explain and predict observed properties of small asteroids in the NEA and Main Belt populations.  A conclusion is that the population of rapidly rotating asteroids consists of both distributions of smaller grains (i.e., rubble piles) and of monolithic boulders whose surfaces may still retain a size distribution of finer grains, potentially of size up to centimeters.

1306.1700
Seeing patterns in noise: Gigaparsec-scale 'structures' that do not violate homogeneity
Nadathur

Clowes+ (2013) reports a Large Quasar Group (dubbed the Huge-LQG), at z~1.3 in DR7; characteristic size of ~500 Mpc and longest dimension >1 Gpc, it is claimed that this structure is incompatible with large-scale homogeneity and the cosmological principle.  If true, this would represent a serious challenge to the standard cosmological model.  However, the homogeneity scale is an average property which is not necessarily affected by the discovery of a single large structure.  Clarify this point and provide the first fractal dimension analysis of the DR7 quasar catalogue to demonstrate that it is in fact homogeneous above scales of at most 130 Mpc/h, which is much less than the upper limit for LCDM.  In addition, show that the algorithm used to identify the Huge-LQG regularly finds even larger clusters of points, extending over Gpc scales, in explicitly homogeneous simulations of a Poisson point process with the same density as the quasar catalogue.  This provides a simple null test to be applied to any cluster thus found in a real catalogue, and suggests that the interpretation of LQGs as 'structures' is misleading.  [what is a "fractal dimension analysis"?]

1306.1764
Simulating regoliths in microgravity
Murdoch et al

Despite their very low surface gravities, the surfaces of asteroids and comets are covered by granular materials - regolith - that can range from a fine dust to a gravel-like structure of varying depths.  Understanding the dynamics of granular material is vital for the interpretation of the surface geology of these small bodies and is also critical for the design and/or operations of any device planned to interact with their surfaces.  Present first measurements of transient weakening of granular material after shear reversal in microgravity, as well as a summary of experimental results recently published in other journals.  Results suggest that the force contact network within a granular material may be weaker in microgravity, although the influence of any change in the contact network is felt by the granular material over much larger distances.  This could mean that small body surfaces are even more unstable than previously imagined.  However, results also indicate that the consequences of a meteorite impact or a spacecraft landing may be very different depending on the impact angle and location, and depending on the prior history of the small body surface.

1306.1789
Externally fed star formation: a numerical study
Mohammadpour, Stahler

Study evolution of dense cores that are accreting external gas up to and beyond the point of star formation.  Model clouds are spherical, unmagnetized configurations with fixed outer boundaries, across which gas enters subsonically.  WHen starting with any near-equilibrium state, find that the cloud's internal velocity also remains subsonic for an extended period, in agreement with observations.  However, the velocity becomes supersonic shortly before the star forms.  Consequently, the accretion rate building up the protostar is much greater than the benchmark value c_s^3/G, where c_s is the sound speed in the dense core.  This accretion spike would generate a higher luminosity than those seen in even the most embedded young stars.  Moreover, find that the region of supersonic infall surrounding the protostar races out to engulf much of the cloud, again in violation of the observations, which show infall to be spatially confined.  Similar problematic results have been obtained by all other hydrodynamic simulations to date, regardless of the specific infall geometry or boundary conditions adopted.  Low-mass star formation is evidently a quasi-static process, in which cloud gas moves inward subsonically until the birth of the star itself.  Speculate that magnetic tension in the cloud's deep interior helps restrain the infall prior to this event.

1306.1804
An analytic model for redshift-space distortions
Wang, Reid, White

RSD: description of the pairwise velocities of biased tracers of matter field.  Compute the first and second moments of the pairwise velocity distribution by extending the convolution Lagrangian perturbation theory (CLPT) formalism of Carlson+ (2012).  Predictions outperform standard perturbation theory calculations in many cases when compared to statistics measured in N-body simulations.  Combine the CLPT predictions of real-space clustering and velocity statistics in the Gaussian streaming model of Reid & White (2011) to obtain predictions for the monopole and quadrupole correlation functions accurate to 1 per cent to scales of <20 Mpc/h.  Also discuss contours of the 2d correlation function and clustering "wedges".  Generalize the scheme to cross-correlation functions.

1306.1829
Confronting models of dwarf galaxy quenching with observations of the local group
Slater, Bell

Connection between SF-ing dwarf irregular galaxies with the formation of non-SF-ing dwarf spheroidal galaxies, but distinguishing between these mechanisms has been difficult.  Use the Via Lactea DM only sim to test two well-motivated simple hypothesis--transformation of irregulars into dwarf spheroidal galaxies by tidal stirring and ram pressure stripping following a close passage to the host galaxy, and transformation via mergers between dwarfs---and predict the radial distribution and inferred formation times of the resulting dSph.  Compare this to the observed distribution in the LG and show that 1) the observed dSph distribution far from the Galaxy or M31 can be matched by the VL haloes that have passed near the host galaxy at least once, though significant halo-to-halo scatter exists, 2) models that require two or more pericenter passages for dSph-formation cannot account for the dSphs beyond 500 kpc such as Cetus and Tucana, and 3) mergers predict a flat radial distribution of dSphs and cannot account for the high dSph fraction near the Galaxy, but are not ruled out at large distances.  The models also suggest that for dSphs found today beyond 500 kpc, mergers tend to occur significantly earlier than dwarf-host encounters, thus leading to a potentially observable difference in stellar populations.  Argue that tidal interactions are sufficient to reproduce the observed distribution of dSphs if and only if a single pericenter passage is sufficient to form a dSph.

1306.1836
Measures of galaxy environment - III. Difficulties in identifying proto-clusters at z~2
Shattow et al

As the title says---because of technical difficulties, hard to identify "galaxy environments" especially at high z; sensitive to both scale and metric, as well as the cluster viewing angle, evolutionary state, and the availability of either spectro or photometric data.

1306.2306
The tip of the red giant branch distances to type Ia supernova host galaixes. II. M66 and M96 in the Leo I group
Lee, Jang

Used TRGB stars in nearby galaxies with SNIa curves to determine H0; yields H0=67.6pm1.5(stat)pm3.7(sys) km/s/Mpc.  Value similar to WMAP9 and Planck results, but smaller than Cepheid calibration results (74 km/s/Mpc).

1306.2307
THe hot and energetic universe: a white paper presenting the science theme motivating the Athena+ mission
Nandra et al

(1) How does ordinary matter assemble into the large scale structures that we see today?  (2) How do black holes grow and shape the Universe?  Understand the formation and assembly of large structures: hot gas in clusters, groups and the IGM which dominate the baryonic content of the local Universe.  Requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities.  Feedback from SMBH essential ingredient in this process and in most galaxy evolution models, but it is not well understood.  X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales.  Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of SMBH through time, out to z where the first galaxies form.  X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe.  The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions to help understand the Hot and Energetic Universe.  

1306.2308
Kepler microlens planets and parallaxes
Gould, Horne

Kepler's quest for other Earths need not end just yet: it remains capable of characterizing cool Earth-mass planets by microlensing, even given its degraded pointed control.  If Kepler were pointed at the Galactic bulge, it could conduct a search for microlensing planets that would be virtually non-overlapping with ground-based surveys.  More important, by combining Kepler observations with current ground-based surveys, one could measure the mocrolens parallax" pi_E for a large fraction of the known microlensing events.  Such parallax measurements would yield mass and distance determinations for the great majority of microlensing planets, enabling much more precise study of the planet distributions as functions of planet and host mass, plane-host separation, and Galactic position (particularly bulge vs. disk).  In addition, rare systems (such as planets orbiting brown dwarfs or black holes) that are presently lost in the noise would be clearly identified.  In contrast to Kepler's current primary hunting ground of close-in planets, its microlensing planets would be in the cool outer parts of solar systems, generally beyond the snow line.  The same survey would yield a spectacular catalog of brown-dwarf binaries, probe the stellar mass function in a unique way, and still have plenty of time available for astroseismology targets.

1306.2319
THe hot and energetic universe: the evolution of galaxy groups and clusters
Pointecouteau, Reiprich, et al

[Supporting paper for Athena+ X-ray observatory]  What is the interplay of galaxy, SMBH, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters?  What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures?  How and when did the first galaxy groups in the Universe, massive enough to bind more than 1e7 K bas, form?  Focussing on the period when groups and clusters assembled (0.5<z<2.5), show that, due to the continuum and line emission of this hot intergalactic gas at X-ray wavelengths, Athena+, combining high sensitivity with excellent spectral and spatial resolution, will deliver breakthrough observations in view of the aforementioned issues.  Physical and chemical properties of the hot intra-cluster gas, and their evolution across time, are a key to understand the co-evolutionn of galaxy and SMBH within their environments.

Other titles from The Hot and Energetic Universe:
- The astrophysics of galaxy groups and clusters (1306.2322)
- AGN feedback in galaxy clusters and groups (1306.2323)
- The missing baryons and the warm-hot intergalactic medium (1306.2324)
- The formation and growth of the earliest supermassive black holes (1306.2325)
- Understanding the build-up of supermassive black holes and galaxies at the heyday of the Universe (1306.2328)  [heyday == 1<z<4]
- Astrophysics of feedback in local AGN (1306.2330)
- The close environments of supermassive black holes
- Solar system and exoplanets (1306.2332)
- Star formation and evolution (1306.2333)
- End points of stellar evolution (1306.2334)
- The astrophysics of supernova remnants and the interstellar medium (1306.2335)
- Luminous extragalactic transients (1306.2336)

1306.2424
Galaxy and Mass assembly (GAMA): linking star formation histories and stellar mass growth
Bauer et al

Present evidence for stochastic SFH in low-mass (M*<1e10 Msun) galaxies from observations within the GAMA survey.  For 73k galaxies between 0.05<z<0.32, calculate SFR and SSFR==SFR/M* from spectroscopic Ha measurements and apply dust corrections derived from Balmer decrements.  Find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for SF galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass.  Us simple parameterizations of exponentially declining SFH to investigate the dependence on stellar mass of the SF timescale and the formation redshift.  Find  that parametrizations previously fit to samples of z~1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05<z<0.32.  In particular, a large number of low-mass (M*<1e10 Msun) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05<z<0.32, even when invoking mass-dependent staged evolution.  For such a large number of galaxies to maintain low stellar masses, yet harbor such high SSFRs, requires the late onset of a weak underlying exponentially declining SFH with stochastic bursts of SF superimposed.

1306.2634
Detection massive galaxies at high redshift using the Dark Energy Survey
Davies et al

DES identification of massive (>1e12.0 Msun) galaxies at z>~4.  

1306.2678
Asteroids in the service of humanity
Crawford

[or: How We Humans Can Pollute/Raze Not Just Earth]  Need to study asteroids for (1) scientific knowledge, (2) mitigate the impact [to Earth] hazard, but also (3) utilize extraterrestrial resources.

1306.2679
The supermassive black hole mass - S\'ersic index relations for bulges and elliptical galaxies
Savorgnan et al

Study coevolution of SMBH and host galaxy: relation between M_BH and the bulge characterized by the Sersic index n may be one of the simplest and strongest such relations, requiring only uncalibrated galaxy images.  Conduct census of 54 local galaxies with M_BH values, and find a clear M_BH-n relation, despite an appreciable level of scatter due to the heterogeneity of the data.  Given the current M_BH-L_sph and the L_sph-n relations, have additionally derived the expected M_BH-n relations, which are marginally consistent at the 2 sig level with the observed relations.  Elliptical galaxies and the bulges of disc galaxies are each expected to follow two distinct bend M_BH-n relations due to Sersic/core-Sersic divide.  For the same central light concentration, predict that M_BH in the Sersic bulges of disc galaxies are an order magnitude higher than in Sersic elliptical galaxies if they follow the same M_BH-L_sph relation.

1306.2770
Solar irradiance variability and climate
Solanki et al

There is increasing evidence that variation in the brightness of sun has an influence on climate; the amplitudes of such variations depend on the wavelength and possibly on the timescale.  The main drivers of solar variability are thought to be magnetic features at the solar surface.  The climate response can be, on a global scale, largely accounted for by simple energetic considerations, but understanding the regional climate effects is more difficult.  Promising mechanisms for such a driving have been identified, including through the influence of UV irradiance on the stratosphere and dynamical coupling to the surface.  Provide an overview of the current state of knowledge, as well as main open questions.