Day 1390

Thursday.

1803.10237
A galaxy lacking dark matter
van Dokkum, et al

Studies of galaxy surveys in the context of the CDM paradigm have shown that the mass of the DM halo and the total stellar mass are coupled through a function that varies smoothly with mass.  Their average ratio M_halo/M_stars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5e10 Msun) and increases both towards lower masses and towards higher masses.  The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies.  Here, report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2e8 Msun.  Infer that its velocity dispersion is less than 10.5 km/s with 90 % confidence, and determine from this that its total mass within a radius of 7.6 kpc is less than 3.4e8 Msun.  This implies that the ratio M_halo/M_stars is of order unity (and consistent with zero), a factor of at least 400 lower than expected.  NGC1052-DF2 demonstrates that DM is not always coupled with baryonic matter on galactic scales.


1803.10240
An enigmatic population of luminous globular clusters in a galaxy lacking dark matter
van Dokkum, et al

Recently found an ultra diffuse galaxy (UDG) with half-light radius of R_e=2.2 kpc and little or no DM.  The total mass of NGC1052-DF2 was measured from the radial velocities of bright compact objects that are associated with the galaxy.  Here, analyze these objects using a combination of HST imaging and Keck spectroscopy.  Their average size is <r_h>=6.2±0.5 pc and their average ellipticity is <epsilon>=0.18±0.02.  From a stacked Keck spectrum, derive an age >9 Gyr and a metallicity of [Fe/H]=-1.35±0.12.  Their properties are similar to omega Centauri, the brightest and largest globular cluster in the MW, and the results demonstrate that the luminosity function of metal-poor globular clusters is not universal.  The fraction of the total stellar mass that is in the globular cluster system is similar to that in other UDGs, and consistent with "failed galaxy" scenarios where star formation terminated shortly after the clusters were formed.  However, the galaxy is a factor of ~1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies, including other UDGs.  Infer that a DM halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high redshift galaxies.


1803.10741
The existence of the cosmic neutrino background is predicted from the hot big bang model.  These neutrinos were a dominant component of the energy density in the early universe and, therefore, played an important role in the evolution of cosmological perturbations.  In particular, fluctuations in the neutrino density produced a distinct shift in the temporal phase of sound waves in the primordial plasma, which has recently been detected in the CMB.  In this paper, report the first measurement of this neutrino-induced phase shift in the spectrum of BAO of BOSS DR12 data.  Constraining the acoustic scale using Planck data, and marginalizing over the effects of neutrinos in the CMB, find evidence for a non-zero phase shift at greater than 95% CL.  Also demonstrate the robustness of this result in simulations and forecasts.  Besides being a new measurement of the cosmic neutrino background, this work is the first application of the BAO signal to early universe physics and a non-trivial confirmation of the standard cosmological history.

Day 1389

Tuesday+Wednesday.

1803.09410
Projection effects of large-scale structures on weak-lensing peak abundances
Yuan, et al

High peaks in WL maps originate dominantly from the lensing effects of single massive haloes.  Their abundance is therefore closely related to the halo mass function and thus a powerful cosmological probe.  On the other hand, however, besides individual massive halos, LSS along lines of sight also contribute to the peak signals.  In this paper, with ray tracing simulations, investigate the LSS projection effects.  Show that for current surveys with a large shape noise, the stochastic LSS effects are subdominant.  For future WL surveys with source galaxies having a median redshift z_med~1 or higher, however, they are significant.  For the cosmological constraints derived from observed WL high peak counts, severe biases can occur if the LSS effects are not taken into account properly.  Extend the model of Fan2010 by incorporating the LSS projection effects into the theoretical considerations.  By comparing with simulation results, demonstrate the good performance of the improved model and its applicability in cosmological studies.


1803.09463
Gaia: 3-dimensional census of the Milky Way Galaxy
Gilmore

Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable, and temperature and gravity invariant operating environment delivers precision, accuracy and sample volume several orders of magnitude greater than ground-based results.  Even more importantly, absolute astrometry is possible.  The ESA Cornerstone mission Gaia is delivering that promise.  Gaia provides 5-D phase space measurements - 3 spatial coordinates and two space motions in the plane of the sky - for a representative sample of the MW's stellar populations, including over two billion stars, being about 1% of the stars over about 50% of the radius.  Full 6-D phase space data is delivered from Gaia's line-of-sight (radial) velocities for the 300 million brightest stars.  These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to Cosmology.  A knowledge revolution is underway.


1803.09746
Lenstronomy: multi-purpose gravitational lens modeling software package
Birrer, Amara

Present Lenstronomy, a multi-purpose open-source gravitational lens modeling python package.  Lenstronomy is able to reconstruct the lens mass and surface brightness distributions of strong lensing systems using forward modeling.  Lenstronomy supports wide range of analytic lens and light models in arbitrary combination.  The software is also able to reconstruct complex extended sources (Birrer+2015) as well as being able to model point sources.  Lenstronomy was designed to be stable, flexible and numerically accurate, with a clear user interface that could be deployed across different platforms.  Throughout its development, have actively used Lenstronomy to make several measurements including deriving constraints on DM properties in strong lenses, measuring the expansion history of the universe with time-delay cosmography, measuring cosmic shear with Einstein rings and decomposing quasar and host galaxy light.  The SW is distributed under the MIT license.  The documentation, starter guild, example notebooks, source code and installation guidelines can be found at https://lenstronomy.readthdocs.io.


1803.09795
DES Y1 results: validating cosmological parameter estimation using simulated Dark Energy Surveys
MacCrann, et al

Use mock galaxy survey simulations designed to resemble the DES Y1 data to validate and inform cosmological parameter estimation.  When similar analysis tools are applied to both simulations and real survey data, they provide powerful validation tests of the DES Y1 cosmo analyses presented in companion papers.  Use two suites of galaxy simulations produced using different methods, which therefore provide independent tests of the cosmological parameter inference.  The cosmological analysis that is aimed to validate is presented in DES Collaboration+(2017) and uses angular two-point correlation functions of galaxy number counts and WL shear, as well as their cross-correlation, in multiple redshift bins.  While the constraints depend on the specific set of simulated realizations available, for both suites of simulations find that the input cosmology is consistent with the combined constraints from multiple simulated DES Y1 realizations in the Omega_m-sigma_8 plane.  For one of the suites, able to show with high confidence that any biases in the inferred S8=sigma_8(Omega_m/0.3)^0.5 and Omega_m are smaller than the DES Y1 1sigma uncertainties.  For the other suite, for which there are fewer realizations, was unable to be this conclusive; infer a roughly 70% probability that systematic biases in the recovered Omega_m and S8 are sub-dominant to the DES Y1 uncertainty.  As cosmological analyses of this kind become increasingly more precise, validation of parameter inference using survey simulations will be essential to demonstrate robustness.

Day 1388

Monday.

1803.08564
The WFIRST Exoplanet microlensing survey
Bennett, et al

WFIRST was the top ranked large space mission in the 2010 New Worlds, New Horizons decadal survey, and it was formed by merging the science programs of 3 different mission concepts, including the Microlensing Planet Finder (MPF) concept (Bennett+2010).  The WFIRST science program (Spergel+2015) consists of a general observer program, a wavefront controlled technology program, and two targeted science programs: a program to study dark energy, and a statistical census of exoplanets with a microlensing survey, which uses nearly one quarter of WFIRST's observing time in the current design reference mission.  The New Worlds, New Horizons (decadal survey) midterm assessment summarizes the science case for the WFIRST exoplanet microlensing survey with this statement: "WFIRST's microlensing census of planets beyond 1AU will perfectly complement Kepler's census of combat systems, and WFIRST will also be able to detect free-floating planets unbound from their parent stars."


1803.08717
Weak lensing by voids n weak lensing maps
Davies, et al

Cosmic voids are an important probe of LSS that allows us to constrain cosmological parameters and test cosmo models.  Present a new paradigm for void studies: void detection in WL convergence maps.  This approach identifies objects that relate directly to the theoretical understanding of voids as under densities in the total matter field and presents several advantages compared to the customary method of finding voids in the galaxy distribution.  Exemplify this approach by identifying voids using the WL peaks as tracers of the LSS.  Find self-similarity in the void abundance across a range of peak S/N selection thresholds.  The voids obtained via this approach give a tangential shear signal up to ~50 times larger than voids identified in the galaxy distribution.


1803.08682
A comprehensive understanding of planet formation is required for assessing planetary habitability and for the search for life
Apai, et al

Dozens of habitable zone, approximately earth-sized exoplanets are known today.  An emerging frontier of exoplanet studies is identifying which of these habitable zone, small planets are actually habitable (have all necessary conditions for life) and, of those, which are earth-like.  Many parameters and processes influence habitability, ranging from the orbit through detailed composition including volatiles and organics, to the presence of biological activity and plate tectonics.  While some properties will soon be directly observable, others cannot be probed by remote sensing for the foreseeable future.  Thus, statistical understanding of planetary systems' formation and evolution is a key supplement to the direct measurements of planet properties.  Probabilistically assessing parameters we cannot directly measure is essential to reliably assess habitability, to prioritizing habitable-zone planets for follow-up, and for interpreting possible bio signatures.

Day 1387

Friday.

1803.08185
The Zeldovich approximation and wide-angle redshift-space distortions
Castorina, White

The contribution of the LoS peculiar velocities to the observed redshift of objects breaks the translational symmetry of the underlying theory, modifying the predicted 2pt functions.  These 'wide angle effects' have mostly been studied using linear perturbation theory in the context of the multipoles of the correlation function and power spectrum.  In this work, present the first calculation of wide angle terms in the Zeldovich approximation, which is known to be more accurate than linear theory on scales probed by the next generation of galaxy surveys.  Present the exact result for dark matter and perturbatively biased tracers as well as the small angle expansion of the configuraiton- and Fourier-space 2pt functions and the connection to the multi-frequency angular power spectrum.  Compare different definitions of the LoS direction and discuss how to translate between them.  Show that wide angle terms can reach tens of percent of the total signal in a measurement at low redshift in some approximations, and that a generic feature of wide angle effects is to slightly shift the BAO scale.


1803.08461
Weak lensing peak statistics in the era of large scale cosmological surveys
Fluri, Kacprzak, Sgier, Refregier, Amara

WL peak counts are a powerful statistical tool for constraining cosmological parameters.  So far, this method has been applied only to surveys with relatively small areas, up to several hundred square degrees.  As future surveys will provide WL datasets with size of thousands of square degrees, the demand on the theoretical prediction of the peak statistics will become heightened.  In particular, large simulations of increased cosmological volume are required.  In this work, investigate the possibility of using simulations generated with the fast Comoving-Lagrangian acceleration (COLA) method, coupled to the convergence map generator Ufalcon, for predicting the peak counts.  Examine the systematics introduced by the COLA method by comparing it with a full TreePM code.  Find that for a 2000 deg^2 survey, the systematic error is much smaller than the statistical error.  This suggests that the COLA method is able to generate promising theoretical predictions for WL peaks.  Also examine the constraining power of various configurations of data vectors, exploring the influence of splitting the sample into tomographic bins and combining different smoothing scales.  Find the combination of smoothing scales to have the most constraining power, improving the constraints on the S8 amplitude parameter by at least 40% compared to a single shooting scale, with tomography bringing only limited increase in measurement precision.

Day 1386

Thursday.

1803.07569
Prediction of supernova rates in known galaxy-galaxy strong-lens systems
Shu, et al

Propose a strategy of finding strongly-lensed SNe by monitoring known galaxy-scale SL systems.  Although being powerful tools in cosmology, galaxy evolution, and stellar physics, strongly-lensed SNe are extremely rare.  Built upon the prior knowledge of the lens nature, the strategy will significantly boost the detection efficiency compared with a blind search.  In particular, build a compilation of 128 gg SL systems from the Sloan Lens ACS survey (SLACS), the SLACS for the Masses Survey, and the BOSS Emission-Line Lens Survey.  Within this compilation, estimate the event rates of strongly-lensed Type Ia SN (SNIa) and core-collapse SN (CCSN) to be 1.23±0.12 and 10.4±1.1 per year, respectively.  The lensed SN images are expected to be widely separated with a median separation of 2 arcsec.  Forecast that a time-domain survey with a single-visit depth of 24.7 mag (5 sigma for point source, r band), which is the same as that of the LSST, can detect in this compilation 0.58 SL SNIa event and 2.4 SL CCSN events every year, when a conservative, fiducial lensing magnification factor of 5 is assumed for the most-magnified SN image.  The proposed strategy requires no extra time or any scanning strategy change to ongoing and scheduled high-cadence, all-sky surveys.  It also allows telescopes with much smaller field of views and limited time to deliver a comparable yield of strongly-lensed SNe with a pencil-beam scanning strategy.


1803.07570
Optimal target stars in the search for life
Lingam, Loeb

The selection of optimal targets in the search for life represents a highly important strategic issue.  In this paper, evaluate the relative benefits of searching for life around a potentially habitable planet orbiting a star of arbitrary mass relative to a Sun-like star.  If recent physical arguments implying that the habitability of planets orbiting low-mass stars is selectively suppressed are correct, find that planets around solar-type stars may represent the optimal targets.


1803.07601
Gravitationally lensed quasars in Gaia: II. Discovery of 24 lensed quasars
Lemon, Auger, McMahon, Ostrovski

Report the discovery, spectroscopic confirmation and preliminary characterization of 24 gravitationally lensed quasars identified using Gaia observations.  Candidates were selected in the Pan-STARRS footprint with quasar-like WISE colors or as photometric quasars from SDSS, requiring either multiple detections in Gaia or a single Gaia detection near a morphological galaxy.  The Pan-STARRS grizY images were modeled for the most promising candidates and 60 candidate systems were followed up with Herschel.  13 of the lenses were discovered as Gaia multiples and 10 as single Gaia detections near galaxies.  Also discover 1 lens identified through a quasar emission line in an SDSS galaxy spectrum.  The lenses have median image separation 2..13 arcs and the source redshifts range from 1.06 to 3.36.  4 systems are quadruply-imaged and 20 are doubly-imaged.  Deep CFHT data reveal an Einstein ring in one double system.  Also report 12 quasar pairs, 10 of which have components at the same redshift and require further follow-up to rule out the lensing hypothesis. Compare the properties of these lenses and other known lenses recovered by this search method to a complete sample of simulated lenses to show the lenses lenses that are missing are mainly those with small separations and higher source redshifts.  The initial Gaia data release only catalogues all images of ~30% of known bright lensed quasars, however the improved completeness of Gaia data release 2 will help fill all bright lensed quasars on the sky.


1803.07851
Strong-lensing of gravitational waves by galaxy clusters
Smith, et al

Discovery of strongly-lensed GW sources will unveil binary compact objects at higher redshifts and lower intrinsic luminosities than is possible without lensing. Such systems will yield unprecedented constraints on the mass distribution in galaxy clusters, measurements of the polarization of GWs, tests of GR, and constraints on the Hubble parameter.  Excited by these prospects, and intrigued by the presence of so-called "heavy BHs" in the early detections by LIGO-Virgo, commence a search for strongly-lensed GWs and possible EM counterparts in the latter stages of the second LIGO observing run (O2).  Summarise the calculation of the detection rate of SL GWs, describe the review of BBH detections from O1, outline the observing strategy in O2, summarize the follow-up observations of GW170814, and discuss the future prospects of detection.

Day 1385

Tuesday.  Wednesday.

1803.07082
A new strong-lensing galaxy at z=0.066: another elliptical galaxy with a lightweight IMF
Collier, Smith, Lucey

Report the discovery of a new low-redshift galaxy-scale gravitational lens, identified from a systematic search of publicly available MUSE observations.  The lens galaxy, 2MASXJ04035024-0239275, is a giant elliptical at z=0.06604 with a velocity dispersion of sigma=325 km/s.  The lensed source has a redshift of 0.19165 and formed a pair of bright images either side of the lens center.  The Einstein radius is 1.5 arcsec, projecting to 1.8 kpc, which is just one quarter of the galaxy effective radius.  After correcting for an estimated 19% DM contribution, find that the stellar mass-to-light ratio from lensing is consistent with that expected for a MW IMF.  Combining the new system with three previously-studied low-redshift lenses of similar sigma, the derived mean mass excess factor (relative to a Kroupa IMF) is <alpha>=1.09±0.08.  With all four systems, the intrinsic scatter in alpha for massive elliptical galaxies can be limited to <0.32, at 90% CL.


1803.07083
Consequences of giant impacts on early Uranus for rotation, internal structure, debris, and atmospheric erosion
Kegerreis, et al

Perform a suite of smoothed particle hydrodynamics sims to investigate in detail the results of a giant impact on the young Uranus.  Study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit.  Most of the material from the impactor's rocky core falls in to the core of the target.  However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer.  Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ~3 earth radii.  This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field.  Verify the results from the single previous study of lower resolution insulations that an impactor with a mass of at least 2 Earth masses can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta.  At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 Earth mass impactor.  This atmospheric erosion peaks for intermediate impactor angular moment (~3e36 km/m^2/s).  Rock is more efficiently placed into orbit and made available for satellite formation by 2 Earth mass impactors than 3 Earth mass ones, because it requires tidal disruption that is suppressed by the more massive impactors.


1803.07533
Void lensing as a test of gravity
Beker, Clampitt, Jain, Trodden

Investigate the potential of WL by voids to test for deviations from GR.  Calculate the expected lensing signal of a scalar field with derivative couplings, finding that it has the potential to boost the tangential shear both within and outside the void radius.  Use voids traced by LRGs in SDSS to demonstrate the methodology of testing these predictions.  Find that the void central density parameter, as inferred from the lensing signal, can shift from its GR value by up to 20% in some galleon gravity models.  Since this parameter can be estimated independently using the galaxy tracer profiles of voids, the method provides a consistency check of the gravity theory.  Although galleon gravity is now disfavoured as a source of cosmic acceleration by other datasets, the methods demonstrated here can be used to test for more general fifth source effects with upcoming void lensing data.

Day 1384

Monday.

1803.05917
The Telltale Heartbeat: detection and characterization of eccentric orbiting planets via tides on their host star
Penoyre, Stone

Present an analytic description of tides raised on a star by a small orbiting body.  In particular, highlight the disproportionate effect of eccentricity and thus the scope for using these tides to detect and characterize the orbits of exoplanets and brown dwarfs.  The tidal distortions of the star produced by an eccentric orbit are, in comparison to a circular orbit, much richer in detail, and potentially visible from any viewing angle.  The magnitude of these variations is much larger than that in a circular orbit of the same semi-major axis.  These variations are visible in both photometric and spectroscopic data, and dominate other regular sources of phase variability (e.g. reflection and Doppler beaming) over a particularly interesting portion of parameter space.  These tidal signatures will be a useful tool for planet detection on their own, and used in concert with other methods provide powerful constraints on planetary and stellar properties.


1803.05952
Observational constraints on the sub-galactic matter-power spectrum from galaxy-galaxy strong gravitational lensing
Bayer, et al

Constraining the sub-galactic matter-power spectrum in 1-10 kpc scales would make it possible to distinguish between the concordance LCDM model and various alternative DM models due to the significantly different levels of predicted mass structure.  Here, demonstrate a novel approach to observationally constrain the population of overall low-mass density fluctuations in the inner regions of massive elliptical lens galaxies, based on the power spectrum of the associated surface-brightness perturbations observable in highly magnified galaxy-scale Einstein rings and gravitational arcs.  The application of the method to the SLACS lens system SDSS J0252+0039 results in the following limits (at the 99% CL) on the dimensionless convergence-power spectrum (and the associated standard deviation in aperture mass): Delta^2_{delta kappa}< 1 (sigma_AM<0.8e8 Msun) on 0.5 kpc scale, Delta^2_{delta kappa}<0.1 (sigma_AM < 1e8 Msun) on 1kpc scale and Delta^2_{delta kappa}<0.01 (sigma_AM<3e8 Msun) on 3 kpc scale.  The estimated effect of CDM sub-haloes lies considerably below these first observational upper-limit constraints on the level of inhomogeneities in the projected total mass distribution of galactic haloes.  Future analysis for a larger sample of gg SL systems will narrow down these constraints and rule out all cosmological models predicting a significantly larger level of clumsiness on these critical sub-galactic scales.

Day 1383

Friday.

1803.05436
Evidence for a new component of high-energy solar gamma-ray production
Linden, et al

The observed multi-GeV gamma-ray emission from the solar disk --- sourced by hadronic cosmic rays interacting with gas, and affected by complex magnetic fields --- is not understood.  Utilizing and improve analysis of the Fermi-LAT data that includes the first resolved imaging of the disk, find strong evidence that this emission is produced by two separate mechanisms.  Between 2010-2017 (the rise to and foo from solar maximum), the gamma-ray emission is dominated by a polar components.  Between 2008-2009 (solar minimum) this component remains present, but the total emission is instead dominated by a new equatorial component with a brighter flux and harder spectrum.  Most strikingly, although 6 gamma rays above 100 GeV are observed during the 1.4 years of solar minimum, none are observed during the next 7.8 years.  These features, along with a 30-50 GeV spectral dip which will be discussed in a companion paper, were not anticipated by theory.  To understand the underlying physics, Fermi and HAWC observations of the imminent Cycle25 solar minimum are crucial.

Day 1382

Thursday.

1803.05076
Galaxy and mass assembly (GAMA): impact of the group environment on galaxy star formation
Barsanti, et al

Explore how the group environment may affect the evolution of star-forming galaxies.  Select 1197 GAMA groups at 0.05<=z<=0.2 and analyze the projected phase space (PPS) diagram, i.e. the galaxy velocity as a function of projected group-centric radius, as a local environmental metric in the low-mass halo regime 1e12 <=(M200/Msun) < 1e14.  Study the properties of star-forming group galaxies, exploring the correlation of SFR with radial distance and stellar mass.  Find that the fraction of star-forming group members is higher in the PPS regions dominated by recently accreted galaxies, whereas passive galaxies dominate the virtualized regions.  Observe a small decline in specific SFR of star-forming galaxies towards the group center by a factor ~1.2 with respect to field galaxies.  Similar to cluster studies, conclude for low-mass halos that star-forming group galaxies represent an infalling population from the field to the halo and show suppressed star formation.

Day 1381

Wednesday.

1803.04470
Conservative cosmology: combining data with allowance for unknown systematics
Bernal, Peacock

When combining data sets to perform parameter inference, the results will be unreliable if there are unknown systematics in data or models.  Here, introduce a flexible methodology, BACCUS: BAyesian Conservative Constraints and Unknown Systematics, which deals in a conservative way with the problem of data combination, for any degree of tension between experiments.  Introduce hyper parameters that describe a bias in each model parameter for each class of experiments.  A conservative posterior for the model parameters is then obtained by marginalization both over these unknown shifts and over the width of their prior.  Contrast this approach with an existing hyper parameter method in which each individual likelihood is scaled, comparing the performance of each approach and their combination in application to some idealized models.  Using only these rescaling hyper parameters is not a suitable approach for the current observational situation, in which internal null tests of the errors are passed, and yet different experiments prefer models that are in poor agreement.  The possible existence of large shift systematics cannot be constrained with a smaller number of data sets, leading to extended tails on the conservative posterior distributions.  Illustrate the method with the case of the H0 tension between results from the cosmic distance ladder and physical measurements that rely on the standard cosmological model.


1803.04642
The value of astrometry for exoplanet science
Bendek, et al

Exoplanets mass measurements will be a critical next step to assess the habitability of Earth-like planets a key aspect of the 2020 vision in the previous decadal survey and also central to NASA's strategic priorities.  Precision astrometry delivers measurement of exoplanet masses, allowing discrimination of rocky planets from water worlds and enabling much better modeling of their atmosphere improving species retrieval from spectroscopy.  The scientific potential of astrometry will be enormous.  The intrinsic astrophysical noise floor set by star spots and stellar surface activity is about a factor of 10 more benign for astrometry than for the more established technique of Radial Velocity, widening the discovery region and pushing detection thresholds to lower masses than previously possible.  On the instrumental side, precision astrometry is limited by optical field distortion and detector calibration issues.  Both technical challenges are now being addressed successfully in the laboratory.  However, identified the need to continue these technology development efforts to achieve sub-microarcsecond astrometry precision necessary for detection and characterization of Earth-like planets around nearby FGK stars.  The international community has realized the importance of astrometry, and various astrometry missions have been proposed and under development, with a few high profile missions now operational.  Believe that it is vital for the U.S. scientific community to participate in the development of these new technologies and scientific discoveries.  Recommend exploring alternatives to incorporate astrometric capabilities into future exoplanet flagship missions such as HABEX and LUVOIR, substantially increasing the scientific return associated with the expected yield of earth-like planets to be recovered.


1803.04869
Rapidly evolving transients in the Dark Energy Survey
Pursiainen, Childress, et al

Present the results of a search for rapidly evolving transients in the DES SN Programme. These events are characterized by fast light curve evolution (rise to peak in <~10d and exponential decline in <~30d after peak).  Discovered 72 events, including 37 transients with a spectroscopic redshift from host galaxy spectral features.  The 37 events increase the total number of rapid optical transients by more than factor of 2.  They are found at a wide range of redshifts (0.05<z<1.56) and peak brightnesses (-15.75>M_g>-22.25).  The multi band photometry is well fit by a blackbody up to few weeks after peak.  The events appear to be hot (T~10000-30000K) and large (R~1e14-2e15 cm) at peak, and generally expand and cool in time, though some events show evidence for a receding photosphere with roughly constant temperature.  Spectra taken around peak are dominated by a blue featureless continuum consistent with hot, optical thick ejecta.  Compare the events with a previously suggested physical scenario involving shock breakout in an optically thick wind surrounding a core-collapse supernova (CCSNe), conclude that current models for such a scenario might need an additional power source to describe the exponential decline.  Find these transients tend to favor star-forming host galaxies, which could be consistent with a core-collapse origin.  However, more detailed modeling of the light curves is necessary to determine their physical origin.

Day 1380

Monday.  Tuesday.


1803.04319
Recognizing the value of the Solar gravitational lens for direct multipixel imaging and spectroscopy of an Exoplanet
Turyshev, et al

The Solar Gravitational Lens (SGL) allows for major brightness amplification (~1e11 at wavelength of 1 um) and extreme angular resolution (~1e-10 arcsec) within a narrow field of view.  A meter-class telescope, with a modest coronagraph to back solar light with 1e-6 suppression placed in the focal area of the SGL, can image an exoplanet at a distance of 30 parsec with few kilometer-scale resolution on its surface.  Notably, spectroscopic broadband SNR is ~1e-6 in two weeks of integration time, providing this instrument with incredible remote sensing capabilities.  A mission capable of exploiting the remarkable optical properties of the SGL allows for direct high-resolution imaging/spectroscopy of a potentially habitable exoplanet.  Such missions could allow exploration of exoplanets relying on the SGL capabilities decades, if not centuries, earlier than possible with other extant technologies.

Day 1379

Friday.

1803.02383
Constraining Density Fluctuations with Big Bang Nucleosynthesis in the era of precision cosmology
Barrow, Scherrer

Reexamine big bang nucleosynthesis with large-scale baryon density inhomogeneities when the length scale of the density fluctuations exceeds the neutron diffusion length (~1e7-8 cm at BBN), and the amplitude of the fluctuations is sufficiently small to prevent gravitational collapse.  In this limit, the final light element abundances can be determined by simply mixing the abundances from regions with different baryon/photon ratios without interactions.  Examine gaussian, lognormal, and gamma distributions for the baryon/photon ratio, eta.  Find that the deuterium and lithium-7 abundances increase with the RMS fluctuation in eta, while the effect on He-4 is much smaller.  Observational upper limits on the primordial D abundance constrain the RMS fluctuation in eta to be less than 10-20% of the mean value of eta.  Larger RMS fluctuations can be obtained for less physically realistic distributions for eta.  This provides with a new limit on the graininess of the early universe.

Day 1378

Tuesday.  Wednesday.  Thursday.

1803.02771
Testing the equivalence principle on cosmological scales
Bonvin, Fleury

The Equivalence Principle, that is one of the main pillars of GR, is very well tested in the Solar system; however, its validity is more uncertain on cosmological scales, or when DM is concerned.  This article shows that relativistic effects in the large-scale structure can be used to directly test whether DM satisfies Euler's equation, i.e. whether its free fall is characterized by geodesic motion, just like baryons and light.  After having proposed a general parameterization for deviations from Euler's equation, perform Fisher-matrix forecasts for future surveys like DESI and the SKA, and show that such deviations can be constrained with a precision of order 10%.  Deviations from Euler's equation cannot be tested directly with standard methods like z-space distortions and gravitational lensing, since these observables are not sensitive to the time component of the metric.  The analysis shows therefore that relativistic effects bring new and complementary constraints to alternative theories of gravity.

Day 1377

Monday.

1803.00689
Measuring the microlensing parallax from various space observatories
Bachelet, Hinse, Street

A few observational methods allow the measurement of the mass and distance of the lens-star for a micro lensing event.  A first estimate can be obtained by measuring the microlensing parallax effect produced by either the motion of the Earth (annual parallax) or the contemporaneous observation of the lensing event from two (or more) observatories (space or terrestrial parallax) sufficiently separated from each other.  Further developing ideas originally outlined by Gould (2013) and Mogavero & Beaulieu (2016), review the possibility of measuring systematically the microlensing parallax using a telescope based on the Moon surface and other space-based observing platforms including the upcoming WFIRST space-telescope.  First generalize the Fisher matrix formulation and present results demonstrating the advantage for each observing scenario.  Conclude by outlining the limitation of the Fisher matrix analysis when submitted to a practical data modeling process.  By considering a lunar-based parallax observation, find that parameter correlations introduce a significant loss in detection efficiency of the probed lunar parallax effect.


1803.00728
Accurate determination of halo velocity bias in simulations and its cosmological implications
Chen, Zhang, Zheng, Yu, Jing

A long standing issue in peculiar velocity cosmology is whether the halo/galaxy velocity bias b_v=1 at large scale.  The resolution of this important issue must resort to high precision cosmological simulations.  However, this is hampered by another long standing "sampling artifact" problem in volume weighted velocity measurement.  Circumvent this problem with a hybrid approach.  First measure statistics free of sampling artifact, then link them to volume weighted statistics in theory, and finally solve for the velocity bias.  b_v determined by this method is not only free of sampling artifact, but also free of cosmic variance.  Apply this method to a LCDM N-body sim of 3072^3 simulation particles and 1200 Mpc/h box size.  For the first time, determine the halo velocity bias of various mass and redshift to 0.1%-1% accuracy.  Major findings are as follows.  (1) b_v != 1 at k>0.1 h/Mpc.  The deviation from unity increases with k.  Depending on the halo mass and redshift, it may reach O(0.01) at k=0.2 h/Mpc and O(0.05) at k~0.3 h/Mpc.  The discovered b_v != 1 has statistically significant impact on structure growth rate measurement by spectroscopic redshift surveys, including DESI, Euclid and SKA.  (2) Both the sign and the amplitude of b_v-1 depend on mass and redshift.  These results disagree with the peak model prediction on porto-halos in that b_v has much weaker deviation from unity, varies with redshift, and can be bigger than unity.  (3) Most of the mass and redshift dependencies can be compressed into a single dependence on the halo density bias.  Biased on this finding, provide an approximate two-parameter fitting formula.