Day 849

Wednesday, Thursday, Friday, Monday.


1503.02665
Zooming in on accretion - I. The structure of halo gas
Nelson, Genel, ..., Vogelsberger, Springel, Hernquist

Study the properties of gas in and around 1e12 Msun at z=2 using a suite of high-res cosmo hydro 'zoom' simulations.  Quantify the thermal and dynamical structure of these gaseous reservoirs in terms of their mean radial distributions and angular variability along different sightlines.  With each halo simulated at 3 levels of increasing resolution, the highest reaching a baryon mass resolution of 10k Msun, study the interaction of filamentary inflow and the quasi-static hot halo atmosphere.  Highlight the discrepancy between the spatial resolution available in the halo gas as opposed to within the galaxy itself, and find that stream morphologies become increasingly complex at higher resolution, with large coherent flows revealing density and temperature structure at progressively smaller scales.  Moreover, multiple gas components co-exist at the same radius within the halo, making radially averaged analyses misleading.  This is particularly true where the hot, quasi-static, high entropy halo atmosphere interacts with cold, rapidly inflowing, low entropy accretion.  Investigate the process of gas virialization and identify different regimes for the heating of gas as it accretes from the intergalactic medium.  Haloes at this mass have a well-defined viral shock, associated with a sharp jump in temperature and entropy at ~1.25 r_vir.  The presence, radius, and radial width of this boundary feature, however, vary not only from halo to halo, but also as a function of angular direction, covering roughly ~85% of the 4pi sphere.  Findings are relevant for the proper interpretation of observations pertinent to the circumgalactic medium, including evidence for large amounts of cold gas surrounding massive haloes at intermediate redshifts.


1503.02942
The extent of gravitationally bound structure in a {\Lambda}CDM universe
Pearson

A new analytical model for constraining the extent of gravitationally bound structure in the Universe is presented.  This model is based on a simple modification of the spherical collapse model (SCM), and its performance in predicting the limits of bound structure in N-body simulations is compared to that of two previous models with the aid new software named COLDGaS- compute unified device architecture (CUDA) object location determination in GADGET2 snapshots.  All of these models can be distilled down to a single unique parameter \xi, the critical parameter, which was found to have values of 3 and 1.18 from the previous studies and a value of 1.89 from the modified SCM.  While still on the conservative side, this new model tends to better identify what structure is gravitationally bound in simulations.  All three analytical models are applied to the Corona Borealis supercluster, with the modified SCM and /xi=1.18 model making predictions that are in agreement with recent work showing that A2056, 61, 65, 67 and 89 comprise a gravitationally bound supercluster.  As an additional test, the modified SCM is used to estimate the mass within the turnaround radius of the Virgo cluster, providing results in good agreement with studies relating the viral mass of clusters to the total mass within turnaround.


1503.03070
A fundamental plane of spiral structure in disk galaxies
Davis, et al

Many versions of the density wave theory demand that the pitch angle is uniquely determined by the distribution of mass in the bulge and the disk of the galaxy.  Present evidence that the tangent of the pitch angle of logarithmic spiral arms in disk galaxies correlates strongly with the density of neutral atomic H in the disk and with the central stellar bulge mass of the galaxy.  These three quantities, when plotted against each other, form a planar relationship which should be fundamental to the understanding of spiral structure in disk galaxies.   Further argue that any successful theory of spiral structure must be able to explain this relationship.


1503.03078
A cosmological context for compact massive galaxies
Stringer, et al

In the Bolshoi sim, find that over 80% of objects that are "compact" are substructures of more massive groups of clusters, and that the probability of a given massive substructure being this compact increases significantly with the mass of the host structure; rising to ~30% for the most massive clusters in the sim.  Tracking the main progenitors of this subsample back to z=2, find them all to be distinct structures with scale radii and densities representative of the population as a whole at this epoch.  These substructures are characterized as: (1) they mostly become substructures, and (2) they have almost experienced below-average mass accretion since z=2; 1/3 of them barely retaining, or even losing mass during the intervening 10 Gyr.


1503.03475
Water formation in the early universe
Bialy, Sternberg, Loeb

Demonstrate that high abundances of water vapor could have existed in extremely low metallicity (1e-3 solar) partially shielded gas, during the epoch of first metal enrichment of the ISM of galaxies at high redshifts.


1503.03487
The angle-averaged squeezed limit of nonlinear matter N-point functions
Wagner, Schimdt, Chiang, Komatsu

Show that in a certain, angle-averaged squeezed limit, the N-pt function of matter is related to the response of the matter PS to a long-wavelength density perturbation, P^-1 d^n P(k|delta_L) / d delta^n_L @ (delta_L=0), with n = N-2.  By performing N-bondy sims with a homogeneous overdensity superimposed on a flat FRLW universe using the separate universe approach, obtain measurements of the NL matter PS response up to n=3., which is equivalent to measuring the fully NL matter 3- to 5-pt function in this squeezed limit.  The sub-percent to few percent accuracy of those measurements is unprecedented.  Then test the hypothesis that NL N-pt functions at a given time are a function of the linear PS at that time, which is credited by SPT and its variants that are based on the ideal pressures fluid equations.  Specifically, compare the responses compared from the separate universe simulations and sims with a rescaled initial (linear) PS amplitude.  Find discrepancies of 10% at k~0.2-0.5 h/Mpc for 5- to 3-pt functions at z=0.  The discrepancy occurs at higher wavenumber at z=2.  Thus, SPT and its variants, carried out to arbitrarily high order, are guaranteed to fail to describe matter N-pt functions (N>2) around that scale.


1503.03876
Planet formation around binary stars: Tatooine made easy
Bromley, Kenyon

Examine characteristics of circumbinary orbits in the context of current planet formation scenarios.  Analytical perturbation theory predicts the existence of nested circumbinary orbits that are generalizations of circular orbits in Keplerian potential.  They contain forced epicyclic motion aligned with the binary as well as higher frequency oscillations, yet they do not cross, even in the presence of massive disks and perturbations from large planets.  For this reason, dissipative gas and planetesimals can settle onto these "most circular" orbits, facilitating the growth of protoplanets.  Outside a region close to the binary where orbits are generally unstable, circumbinary planets form in much the same way as their cousins around a single star.  Review the theory and confirm its predictions with a suite of representative simulations.  Then consider the circumbinary planets discovered with NASA's Kepler satellite.  These Neptune- and Jupiter-size planets, or their planetesimal precursors, may have migrated inward to reach their observed orbits, since their current positions are outside of unstable zones caused by overlapping resonances.  In situ formation without migration seems less likely, only because the surface density of the protoplanetary disks must be implausibly high.  Otherwise, the circumbinary environment is friendly to planet formation, and expect that many Earth-like "Tatooines" will join the growing census of circumbinary planets.