Day 1403

Friday.

1804.06930
How big is the Sun: Solar diameter changes over time
Rozelot, Kosovichev, Kilcik

The measurement of the Sun's diameter has been first tackled by the Greek astronomers from a geometric point of view.  Their estimation of ~1800", although incorrect, was not truly called into question for several centuries.  The first pioneer works for measuring the Sun's diameter with an astrometric precision were made around the year 1660 by Garbirel Mouton, then by Picard and La Hire.  A canonical value of the solar radius of 959".63 was adopted by Auwers in 1891.  Despite considerable efforts during the second half of the XXth century, involving dedicated space instruments, no consensus was reached on this issue.  However, with the advent of high sensitivity instruments on broad satellites, such as the Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SoHO) and the Helioseismic and Magnetic Imager (HMI) board NASA's Solar Dynamics Observatory (SDO), it was possible to extract with an unprecedented accuracy the surface gravity oscillation f modes, over nearly two solar cycles, from 1996 to 2017.  Their analysis in the range of angular degree l=140-300 shows that the so-called "seismic radius" exhibits a temporal variability in anti-phase with the solar activity.  Even if the link between the two radii (photospheric and seismic) can be made only through modeling, such measurements provide an interesting alternative which led to a revision of the standard solar radius by the International Astronomical Union in 2015.  This new look on such modern measurements of the Sun's global changes from 1996 to 2017 gives a new way for peering into the solar interior, mainly to better understand the subsurface fields which play an important role in the implementation of the solar cycles.

Day 1402

Thursday.

1804.06412
Circular polarization in a spherical basis
Kamionkowski

Circular polarization of the CMB arises in the standard cosmological model from Faraday conversion of the linear polarization generated at the surface of last scatter by various sources of birefringence along the line of sight.  If the sources of birefringence are generated at linear order in primordial density perturbations the principal axes of the index-of-refraction tensor are determined by gradients of the primordial density field.  Since linear polarization at the surface of last scatter is generated at linear order in density perturbations, the circular polarization thus arises at second order in primordial perturbations.  Revisit the calculation of the circular polarization using the total-angular-momentum formalism, which allows for some simplifications in the calculation of the angular power spectrum of the circular polarization --- especially for the dominant photon-photon scattering contribution --- and also provides some new intuition.


1804.06421
Tidal stripping as a possible origin of the ultra diffuse galaxy lacking dark matter
Ogiya

Recent observations revealed a mysterious UDG, NGC1052-DF2, in the group of a large elliptical galaxy, NGC1052.  Compared to expectations from abundance matching models, the DM mass contained in NGC1052-DF2 is smaller by a a factor of ~400.  Utilize controlled N-body sims of the tidal interaction between NGC1052 and a smaller satellite galaxy, that is supposedly the progenitor of NGC1052-DF2, to test if tidal stripping can explain DM deficiency at such levels.  Find that when assuming a tightly bound orbit as well as a relatively low concentration and cored density profile for the dark halo of the satellite, the simulations reproduce well both the mass profile and the effective radius inferred form the observations of NGC1052-DF2.  Orbital parameters and halo concentration are in the tail, but still consistent with measurements of their distributions from cosmological simulations.  Such strongly DM deficient galaxies, in the scenario, are thus expected to be relatively rare in groups and clusters and not present in the field.

Day 1401

Wednesday.

1804.05843
Astrophysical radio background cannot explain the EDGES signal: constraints from cooling of non-thermal electrons
Sharma

Recently the EDGES experiment has claimed the detection of an absorption feature centered at 78 MHz.  When interpreted as a signature of cosmic dawn, this feature appears at the correct wavelength (corresponding to a range of z~15-20) but is larger by at least a factor of two in amplitude compared to the standard 21-cm models.  One way to explain the excess radio absorption is by the enhancement of the diffuse radio background at nu=1.42 GHz (lambda=21cm) in the rest frame of the absorbing neutral H. Astrophysical scenarios, based on the acceleration of relativistic electrons by accretion on to SMBHs and by SN from first stars, have been proposed to produce the enhanced radio background via synchrotron emission.  In this Letter, show that either the synchrotron or the inverse-compton (IC) cooling time for such electrons is at least 3 orders of magnitude shorter than the duration of the EDGES signal centered at z~17, irrespective of the magnetic field strength.  This means that such electrons must be replenished on a timescale orders of magnitude shorter than allowed by the cosmic history of SF and growth of SMBHs.  Thus astrophysical scenarios for excess radio background proposed to explain the EDGES signal are comfortably ruled out.


1804.05865
The Aemulus Project I: Numerical simulations for precision cosmology
DeRose, Wechsler, Tinker, et al

The rapidly growing statistical precision of galaxy surveys has lead to a need for ever-more precise predictions of the observables used to constrain cosmological and galaxy formation models.  The primary avenue through which such predictions will be obtained is suites of numerical simulations.  These simulations must span the relevant model parameter spaces, be large enough to obtain the precision demanded by upcoming data, and be thoroughly validated in order to ensure accuracy.  In this paper, present one such suite of simulations, forming the basis for the AEMULUS project, a collaboration devoted to precision emulation of galaxy survey observables.  Run a set of 75 (1.05 Gpc/h)^3 simulations with mass resolution and force softening of 3.51e10 (Omega_m/0.3) Msun/h and 20 kpc/h respectively at 47 different wCDM cosmologies spanning the range of parameter space allowed by the combination of recent CMB, BAO and SNIa results.  Present convergence tests of several observables including spherical overdensity halo mass functions, galaxy projected correlation functions, galaxy clustering in z space, and matter and halo correlation functions and power spectra.  Show that these statistics are covered to 1% (2%) for halos with more than 500 (200) particles respectively and scales of r>200 kpc/h in real space or k~3Mpc/h in harmonic space for z<=1.  Find that the dominant source of uncertainty comes from varying the particle loading of the simulations.  This leads to large systematic errors for statistics using haloes with fewer than 200 particles and scales smaller than k~4Mpc/h.  Provide the halo catalogs and snapshots detailed in this work to the community at https://AemulusProject.github.io.


1804.05866
The Aemulus Project II: Emulating the halo mass funciton
McClintok, Rozo, et al

Existing models for the dependence of the halo mass function on cosmological parameters will become a limiting source of systematic uncertainty for cluster cosmology in the near future.  Present a halo mass function emulator and demonstrate improved accuracy relative to state-of-the-art analytic models.  In this work, mass is defined using an overdensity criteria of 200 relative to the mean background density.  The emulator is constructed from the AEMULUS simulations, a suite of 40 N-body simulations with snapshots from z=3 to z=0.  These simulations cover the flat wCDM parameter space allowed by recent CMB, BAO and SNIa results, varying the parameters w, Omega_m, Omega_b, sigma8, N_eff, N_s, and H_0.  Validate the emulator using 5 realizations of 7 different cosmologies, for a total of 35 test simulations. These test simulations were not used in constructing the emulator, and were run with fully independent initial conditions.  Use the test simulations to characterize the modeling uncertainty of the emulator, and introduce a novel way of marginalizing over the associated systematic uncertainty.  Confirm non-universality in the halo mass function emulator as a function of both cosmological parameters and redshift.  The emulator achieves better than 1% precision over much of the relevant parameter space, and demonstrate that the systematic uncertainty in the emulator will remain a negligible source of error for cluster abundance studies throughout at least the LSST Year 1 data set.


1804.05867
The Aemulus Project III: Emulation of the galaxy correlation function
Zhai, Tinker, et al

Using the N-body simulations of the AEMULUS project, construct an emulator for the NL clustering of galaxies in real and z space.  Construct the model of galaxy bias using the halo occupation framework, accounting for possible velocity bias. The model includes 15 parameters, including both cosmological and galaxy bias parameters.  Demonstrate that the emulator achieves ~1% precision at the scales of interest, 0.1<r<10 Mpc/h, and recovers the true cosmology when tested against independent simulations.  The primary parameters of interest are related to the growth rate of structure, f, and its degenerate combination fsigma_8.  Using this emulator, show that the constraining power on these parameters monotonically increases as smaller scales are included in the analysis, all the way down to 0.1 Mpc/h.  For a BOSS-like survey, the constraints on fsigma_8 from r<30 Mpc/h scales alone are more than a factor of two tighter than those from the fiducial BOSS analysis of redshift-space clustering using perturbation theory at larger scales.  The combination of real- and redshift-space clustering allows breaking of the degeneracy between f and sigma_8, yielding a 9% constraint on f alone for a BOSS-like analysis.  The current AEMULUS simulations limit this model to surveys of massive galaxies.  Future simulations will allow this framework to be extended to all galaxy target types, including emission-line galaxies.

Day 1400

Tuesday.

1804.03888
EDGES result versus CMB and low-redshift constraints on ionization histories
Witte, et al

Examine the results from the Experiment to Direct the Global Epoch of Reionization Signature (EDGES), which has recently claimed the detection of a strong absorption in the 21 cm hyperfine transition line of neutral hydrogen, and z demarcating the early stages of star formation.  More concretely, study the compatibility of the shape of the EDGES absorption profile, centered at a z of z~17.2, with measurements of the reionization optical depth, the Gunn-Peterson optical depth, and Lyman-alpha emission from star-forming galaxies, for a variety of possible reionization models within the standard LCDM framework.  When, conservatively, only the location of the absorption dip is attempted to have accommodated, identify a region in the parameter space of the astrophysical parameters that successfully explains all of the aforementioned observations.  However, one of the most abnormal features of the EES measurement is the absorption amplitude, which is roughly a factor of 2 larger than the maximum allowed value in the LCDM framework.  Point out that the simple considered astrophysical models that produce the largest absorption amplitudes are unable to explain the depth of the dip and of reproducing the observed shape of the absorption profile.


1804.04139
Measured and found wanting: reconciling mass-estimates of ultra-diffuse galaxies
Laporte, Agnello, Navarro

The viral masses of UDGs have been estimated using the kinematics and abundance of their globular cluster populations, leading to disparate results.  Some studies conclude that UDGs reside in massive DM haloes while other, controversially, argue for the existence of UDGs with no DM at all.  Here, show that these results arise because the uncertainties of these mass estimates have been substantially underestimated.  Applying the same procedure to the well-studied Fornax dwarf spheroidal would conclude that it has an "over massive" dark halo or, alternatively, that it lacks DM.  Corroborate the argument with self-consistent mocks of tracers in cosmological haloes, showing that masses from samples with 5<N<10 tracers (assuming no measurement errors) are uncertain by at least an order of magnitude.  Finally, estimate masses of UDGs with HST imaging in Coma and show that their recent mass measurements (with adequate uncertainties) are in agreement with that of other dwarfs, such as Fornax.  Also provide bias and scatter factors for a range of sample sizes and measurement errors, of wider applicability.

Day 1399

Monday.

1804.04722
Testing the Weak Equivalence Principle using optical and near-infrared Crab Pulses
Leung, et al

The Weak Equivalence Principle states that the geodesics of a test particle in a gravitational field are independent of the particle's constitution.  To constrain violations of the WEP, use the one-meter telescope at Table Mountain Observatory near Los Angeles to monitor the relative arrival times of pulses from the Crab Pulsar in the optical (lambda ~ 585 nm) and near-infrared (lambda ~814 nm) using an instrument which detects single photons with nanosecond-timing resolution in those two bands.  The IR pulse arrives slightly before the visible pulse.  The 3 analysis methods give delays with statistical errors of Delta t_obs=7.41±0.58, 0.4±3.6, and 7.35±4.48 microsecond (at most 1/4000 of the pulsar period).  Attribute this discrepancy to systematic error from the fact that the visible and IR pulses have slightly different shapes.  Whether this delay emerges from the pulsar, is caused by passing through wavelength-dependent media, or is caused by a violation of the equivalence principle, unless there is a fine-tuned cancellation among these, sets the first upper limit on the differential post-Newtonian parameter at these wavelengths of Delta gamma<1.07e-10 (3 sigma).  This result falls in an unexplored region of parameter space and complements existing limits on equivalence-principle violation from fast radio bursts, gamma ray bursts, as well as previous limits from the Crab.

Day 1398

Friday.

1804.04277
Wavelength dependent PSFs and their impact on weak lensing measurements
Carlsten, Strauss, Lupton, Meyers, Miyazaki

Measure and model the wavelength dependence of the PSF in the HSC SSP survey.  Find that PSF chromaticity is present in that redder stars appear smaller than bluer stars in the g, r, and i-bands at the 1-2% level and in the z and y-bands at the 0.1-0.2% level.  From the color dependence of the PSF, fit a model between the monochromatic PSF trace radius, R, and wavelength of the form R(lambda)~lambda^b.  Find values of b between -0.2 and -0.5, depending on the epoch and filter.  This is consistent with the expectations of a turbulent atmosphere with an outer scale length of ~10-100m, indicating that the atmosphere is dominating the chromaticity.  Find evidence in the best seeing data that the optical system and detector also contribute some wavelength dependence.  Meyers and Burchat (2015) showed that b must be measured to an accuracy of ~0.02 not to dominate the systematic error budget of the LSST WL survey.  Using simple image of simulations, find that b can be inferred with this accuracy in the r and i-bands for all positions in the LSST field of view, assuming a stellar density of 1 star acrmin^-2 and that the optical PSF can be accurately modeled.  Therefore, it is possible to correct for most, if not all, of the bias that the wavelength-dependent PSF will introduce into an LSST-like WL survey.

Day 1397

Wednesday.  Thursday.

1804.03667
Preparing for the cosmic shear data flood: optimal data extraction and simulation requirements for stage IV dark energy experiments
Taylor, Kitching, McEwen

It remains an open question of how to optimal extract the information and how well the matter power spectrum must be known to obtain unbiased cosmo parameter estimates.  By performing a PCA, quantify the sensitivity of 3d cosmic shear and tomography with different binning strategies to different regions of the lensing kernel and matter PS, and hence the BG geometry and growth of structure in the Universe.  Find that a large number of equally spaced tomographic bins in redshift can extract nearly all the cosmo information without the additional computational expense of 3d cosmic shear.   Meanwhile a large fraction of the information comes from small poorly understood scales in the matter PS, that can lead to biases on measurements of cosmological parameters.  In light of this, define and compute a cosmology-independent measure of the bias due to imperfect knowledge of the power spectrum.  For a Euclid-like survey, find that the PS must be known to an accuracy of less than 1% on scales with k=7 h/Mpc.  This requirement is not absolute since the bias depends on the magnitude of modeling errors, where they occur in k-z space, and the correlation between them, all of which are specific to any particular model.  Therefore compare the bias in several of the most likely modeling scenarios and introduce a general formalism and public code, RequiSim, to compute the expected bias from any non-linear model.


1804.04055
Some assembly required: assembly bias in massive dark matter halos
Chue, Dalal, White

Study halo assembly bias for cluster-sized halos.  Previous work has found little evidence for correlations between large-scale bias and halo mass assembly history for simulated cluster-sized haloes, in contrast to the significant correlation found between bias and concentration for haloes of this mass.  This difference in behavior is surprising, given that both concentration and assembly history are closely related to the same properties of the linear-density peaks that collapse to form haloes.  Using publicly available simulations, show that significant assembly bias is indeed found in the most massive haloes with M~1e15 Msun, using essentially any definition of halo age.  For lower halo masses M~1e14 Msun, no correlation is found between bias and the commonly used age indicator a_0.5, the half-mass time.  Show that this is a mere accident, and that significant assembly bias exists for other definitions of halo age, including those based on the time when the halo progenitor acquires some fraction f of the ultimate mass at z=0.  For halos with M_vir~1e14 Msun, the sense of assembly bias changes sign at f=0.5.  Explore the origin of this behavior, and argue that it arises because standard definitions of halo mass in halo finders do not correspond to the collapsed, virtualized mass that appears in the spherical collapse model used to predict large-scale clustering.  Because bias depends strongly on halo mass, these errors in mass definition can masquerade as or even obscure the assembly bias that is physically present.  More physically motivated halo dentitions using splash back should be free of this particular defect of standard halo finders.

Day 1396

Monday.  Tuesday.

1804.02406
Heating of the intergalactic medium by the cosmic microwave background during cosmic dawn
Venumadhav, Dai, Kaurov, Zaldarriaga

The IGM is expected to be at its coldest point before the formation of the first stars in the universe.  Motivated by recent results from the EDGES experiment, revisit the standard calculation of the kinetic temperature of the neutral gas through this period.  When the first UV sources turn on, photons redshift into the Lyman lines of neutral H and repeatedly scatter within the Lyman-alpha line.  They heat the gas via atomic recoils, and through the Wouthuysen-Field effect, set the spin temperature of the 21-cm hyperfine (spin-flip) line of atomic hydrogen in competition with the resonant CMB photons.  Show that the Lya photons also mediate energy transfer between the CMB photons and the thermal motions of the H atoms.  In the absence of X-ray heating, this new mechanism is the major correction to the temperature of the adiabatically cooling gas (~10% at z=17) and is several times the size of the heating rate found in previous calculations.  Also find that the effect is more dramatic in non-standard scenarios that either enhance the radio background above the CMB or invoke new physics to cool the gas in order to explain the EDGES results.  The coupling with the radio background can reduce the depth of the 21-cm absorption feature by almost a factor of 2 relative to the case with no sources of heating, and prevent the feature from developing a flattened bottom.  As an inevitable consequence of the UV BG that generates the absorption feature, this heating should be accounted for in any theoretical prediction.


1804.02667
J-PLUS: the Javalambre Photometric Local Universe Survey
Cenarro, et al

J-PLUS is an ongoing 12-band photometric optical survey, observing thousands of square degrees of the Northern Hemisphere from the dedicated JAST/T80 telescope at the Observatory Astrofisico de Javalambre.  T80Cam is a 2 sq..deg FoV camera mounted on the 83cm diameter telescope, and is equipped with a unique system of filters spanning the entire optical range.  This filter system is a combination of broad, medium and narrow-band filters, optimally designed to extract the rest-frame spectral features (the 3700-4000A Balmer break region, H_delta, Ca H+K, the G-band, the Mgb and Ca triplets) that are key to both characterize stellar types an to deliver a low-resolution photo-spectrum for each pixel of the sky observed.  With a typical depth of AB~21.25 mag per band, this filter set thus allows for an indiscriminate and accurate characterization of the stellar population in our Galaxy, it provides an unprecedented 2d photo-spectral information for all resolved galaxies in the local universe, as well las accurate photo-z estimates (Delta z ~ 0.01-0.03) for moderately bright (up to r~20 mag) extragalactic sources.  While some narrow band filters are designed for the study of particular emission features ([OII]/lambda3727, H_alpha/lambda6563) up to z<0.015, they also provide ill-defined windows for the analysis of other emission liens at higher redshifts.  As a result, J-PLUS has the potential to contribute to a wide range of fields in Astrophysics, both in the nearby universe (MW, 2D IFU-like studies, stellar populations of nearby and moderate redshift galaxies, clusters of galaxies) and at high zs (ELGs at z~0.77, 2.2 and 4.4 QSOs, etc).  With this paper, release ~36 sq.deg of J-PLUS data, containing about 1.5e5 stars and 1e5 galaxies at r<21 mag.


1804.02673
J-PLUS: Morphological star/galaxy classification by PDF analysis
López-Sanjuan, et al

The goal is to morphological classify the sources identified in the images of the J-PLUS early data release (EDR) into compact (stars) or extended (galaxies) using a suited Bayesian classifier.  J-PLUS sources exhibit two distinct populations in the r-band magnitude vs. concentration plane, corresponding to compact and extended sources.  Modeled the 2 population distribution with a skewed Gaussian for compact objects and a log-normal function for the extended ones.  The derived model and the number density prior based on J-PLUS EDR data were used to estimate the Bayesian probability of a source to be star or galaxy.  This procedure was applied pointing-by-pointing to account for varying observing conditions and sky position.  Finally, combine the morphological information from g, r, and i broad bands in order to improve the classification of ow S/N sources.  The derived probabilities are used to compute the pointing-by-pointing number counts of stars and galaxies.  The former increases as we approach the MW disk, and the latter are similar across the probed area.  The comparison with SDSS in the common regions is satisfactory up to r~21, with consistent numbers of stars and galaxies, and consistent distributions in concentration and (g-i) color spaces.  Implement a morphological star/galaxy classier based on PDF analysis, providing meaningful probabilities for J-PLUS sources to one magnitude deeper (r~21) than a classical boolean classification.  These probabilities are suited for the statistical study of 150k stars and 101k galaxies with 15<r<21 present in the 31.7 deg2 of the J-PUS EDR.  In a future version of the classifier, include J-PLUS color information from 12 photometric bands.


1804.02753
Power spectrum in the presence of large-scale overdensity and tidal fields: breaking azimuthal symmetry
Chiang, Slosar

Consider the PS of a biased tracer observed in a finite volume in the presence of a LS overdensity and tidal fields.  Expanding both the observed PS and the source fields (linear PS, scalar overdensity and tidal field tensor) in spherical harmonics, explicitly confirm that each (ell, m) source generates just the corresponding (ell, m) modes of PS in real space.  In redshift space, each (ell, m) source additionally couples only to (ell+2n,m) modes of tracer power spectra.  This generalizes the Kaiser formula for monopole, quadrupole and hexadecapole of the power spectrum to all (ell, m) modes generated to the second order in perturbation thirty.  This formalism can find applications in constraining the super-sample covariance and in the local power spectrum based bispectrum estimators.  As an example application, forecast the ability to measure these modes a survey with BOSS-like galaxy number densities.


1804.03097
The connection between galaxies and their dark matter haloes
Wechsler, TInker

In the modern understanding of galaxy formation, every galaxy forms within a DM halo.  The formation and growth of galaxies over time is connected to the growth of the haloes in which they form.  The advent of large galaxy surveys as well as high-resolution cosmological simulations has provided a new window into the statistical relationship between galaxies and halos and its evolution.  Here, define this galaxy-halo connection as the multi-variate distribution of galaxy and halo properties that can be derived from observations and simulations.  This connection provides a key test of physical galaxy formation models; it also plays an essential role in constraints of cosmological models using galaxy surveys and in elucidating the properties of DM using galaxies.  Review techniques for inferring the galaxy-halo connection and the insights that have arisen from these approaches.  Some things that were learned are that galaxy formation efficiency is a strong function of halo mass; at its peak in halos around a pivot halo mass of 1e12 Msun, less than 20% of the available baryons have turned into stars by the present day; the intrinsic scatter in galaxy stellar mass is small, less than 0.2 dex at a given halo mass above this pivot mass; below this pivot mass galaxy stellar mass is a strong function of halo mass; the majority of stars over cosmic time were formed in a narrow region around this pivot mass.  Also highlight key open questions about how galaxies and halos are connected, including understanding the correlations with secondary properties and the connection of these properties to galaxy clustering.

Day 1395

Friday.

1804.01748
Mr-Moose: an advanced SED-fitting tool for heterogeneous multi-wavelength datasets
Drouart, Falkendal

Present the public release of MrMoose, a fitting procedure that is able to perform multi-wavelength and multi-object SED fitting in a Bayesian framework.  This procedure is able to handle a large variety of cases, from an isolated source to blended multi-component sources from an heterogeneous dataset (i.e. a range of observation sensitivities and spectral/spatial resolutions).  Furthermore, MrMoose handles upper-limits during the fitting process in a continuous way allowing models to be gradually less probable as upper limits are approached.  The aim is to propose a simple-to-use, yet tightly-versatile fitting tool for handling increasing source complexity when combining multi-wavelength datasets with fully customizable filter/model databases.  The complete control of the use is one advantage, which avoids the traditional problems related to the "black box" effect, where parameter of model tuning are impossible and can lead to overfitting and/or over-interpretation of the results.  Also, while a basic knowledge of Python and statistics is required, the code aims to be sufficiently user-friendly for non-experts.  Demonstrate the procedure on 3 cases: two artificially-generated datasets and a previous result from the literature.  In particular, the most complex case (inspired by a real source, combining Herschel, ALMA and VLA data) in the context of extragalactic SED fitting makes MrMoose a particularly-attractive SED fitting tool when dealing with partially blended sources, without the need for data deconvolution.


1804.01755
Three-dimensional structure of the Magellanic System
Jacyszyn-Dobrzeniecka, The OGLE Team

Determined the 3d structure of the Magellanic Clouds and Magellanic Bridge using over 9000 classical Cepheids (CCs) and almost 23000 RR Lyrae (RRL) stars from the fourth phase of the OGLE project.  For the CCs, calculated distances based on period-luminosity relations.  CCs in the LMC are situated mainly in the bar that shows no offset from the plane of the LMC.  The northern arm is also very prominent with an additional smaller arm.  Both are located closer to us than the entire sample.  The SMC has a non-planar structure that can be described as an ellipsoid extended almost along the line of sight.  Also classified 9 of the CCs as Magellanic Bridge objects.  These Cepheids show a large spread in 3d.  For the RRL stars, calculated distances based on photometric metallicities and theoretical relations.  Both Magellanic Clouds revealed a very regular structure.  Fitted triaxial ellipsoids to the LMC and SMC samples.  In the LMC, noticed a very prominent, non-physical blend-artifact that prevented analysis of the central parts of the galaxies.  Do not see any evidence of a bridge-like connection between the Magellanic Clouds.


1804.01538
The demographics of neutron star - white dwarf mergers: rates, delay-time distributions and progenitors
Toonen, et al

Characterize the rates of NS and WD mergers.  Use binary populations synthesis models, and consider a wide rage of initial conditions and physical processes.  Consider different common-envelope evolution models and different NS natal kick distributions.  Provide detailed predictions arising from each of the models considered.  Find that the majority of NS-WD mergers are born in systems in which mass-transfer played an important role, and the WD formed before the NS.  For the majority of the mergers the WDs have a carbon-oxygen composition (60-80%) and most of the rest are with oxygen-neon WDs.  The rates of NS-WD mergers are in the rage fo 3-15% of the type Ia SNe rate.  Their delay time distribution is very similar to that of SNe Ia, but slightly biased towards earlier times.  They typically explode in young 0.1-1Gyr environments, but have attain distribution extending to long-Gyrs-timescales.  Models including significant kicks give rise to relatively wide offset distribution extending to hundreds of kpcs.  The demographic and physical properties of NS-WD mergers suggest they are likely to be peculiar type Ic-like SNe, mostly exploding in late type galaxies.  Their overall properties could be related to a class of rapidly evolving SNe recently observed, while they are less likely to be related to the class of Ca-rich SNe.

Day 1394

Thursday.

1804.01236
On the nature of the black-body stars
Serenelli, Rohrmann, Fukugita

Study the physical nature of the black-body (BB) stars recently discovered.  Use sets of white dwarf (WD) model atmospheres with pure-He, pure-H, and H/He mixtures to compute synthetic spectra and compare theoretical expectations with observations of the BB stars.  Find that the spectral properties of the BB stars in the UV and the optical are reproduced to high-accuracy by He dominated atmospheres possibly with trace amounts of H below the spectroscopic detection limit, -8 <= log (H/He) <= -6, and typical logG=9 values corresponding to WDs of 0.6 Msun.  Pure He models with LogG=8 also provide a good match from 12000 down to T_eff=8500K, but fail to simultaneously fit the UV and optical colors for the cooler stars in the sample.  However, pure-He models with logG=9 show a better match with the data, implying that BB stars could be massive WDs, around ~1.2 Msun.  Find the the emerging spectrum has a Planck distribution shifted towards the blue compared to the expected shape based on the model effective temperature.  The BB temperatures determined from colors overestimated the actual T_eff of these stars by 400K and up to 1000K, depending on temperature and log(H/He).  Finally show that precision of end-of-mission Gaia parallaxes should allow disentangling whether the BB stars have typical WD masses or represent a massive, peculiar population.


1804.01318
The standard model of cosmology: a skeptic's guide
Scott

The status of the standard cosmo model, also known as LCDM is described.  With some simple assumptions, this model fits a wide range of data, with just 6 or 7 free parameters.  One should be skeptical about this claim, since it implies that we now ave an astonishingly good picture of the statistical properties of the LS Universe.  However, the successes of the model cannot be denied, including more than 1000 sigma worth of detection of CMB anisotropy power.  The model is older than most modern astrophysicists seem to appreciate, and has not fundamentally changed for more than a quarter of a century.  Tensions and anomalies are often discussed, and while we should of course be open to the possibility of new physics, we should also be skeptical of the importance of 2-3 sigma differences between data sets until they become more significant.  Still, today's SMC is surely not the full story and we should be looking for extension or new ingredients to the model, guided through by a skeptical outlook.


1804.01427
A Gaia DR2 mock stellar catalog
Rybizki, et al

Present a mock catalog of MW stars, matching in volume and depth the content of the Gaia data release 2 (GDR2).  Generated the catalog using Galaxia, a tool to sample stars from Besancon Galactic model, together with a realistic 3D dust extinction map.  The catalog mimics the complete GDR2 data model and contains most of the entries in the Gaia source catalog: 5 parameter astrometry, 3-band photometry, radial velocities, stellar parameters, and associated scaled nominal uncertainty estimates.  In addition, supplemented the catalog with extinctions and photometry for non-Gaia bands.  This catalog can be used to prepare GDR2 queries in a realistic runtime environment, and it can serve as a Galactic model against which to compare the actual GDR2 data in the space of observables.  The catalog is hosted through the virtual observatory GAVO's Heidelberg data center service and thus can be queried using ADQL as for GDR2 data.

Day 1393

Wednesday.

1804.00659
Where are the most ancient stars in the Milky Way?
El-Badry, et al

The oldest stars in the MW bear imprints of the Galaxy's early assembly history.  Use FIRE cosmological zoom-in simulations of 3 MW-mass disk galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars (z_form >~5) in MW-like galaxies.  Predict the oldest stars to be less centrally concentrated at z=0 than stars formed at later times as a result of two processes.  First the majority of the oldest stars are not formed in situ but are accreted during hierarchical assembly.  These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighborhood, and in some simulations, even in the galactic center.  Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z>~2, similar to the process that creates DM cores and expands stellar orbits in bursty dwarf galaxies.  The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines away from the bulge and inner halo than for inward-looking sightlines.  Although the task of identifying specific stars as ancient remains challenging, anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before z=5.  Predict most of these targets to have higher metallicity (-3<[Fe/H]<-2) than the most extreme metal-poor stars.


1804.00676
CLUMP-3D: three-dimensional shape and structure of 20 CLASH galaxy clusters from combined weak and strong lensing
Chiu, Umetsu, et al

Perform a 3d triaxial analysis of 16 -ray regular and 4 high-magnification galaxy clusters selected from the CLASH survey by combining 2d WL and central SL constraints.  In a Bayesian framework, constrain the intrinsic structure and geometry of each individual cluster assuming a triaxial NFW halo with arbitrary orientations, characterized by the mass M_200c, halo concentration C_200c, and triaxial axis ratios (q_a<=q_b), and investigate scaling relations between these halo structural parameters.  From triaxial modeling of the X-ray-selected subsample, find that the halo concentration decreases with increasing cluster mass, with a mean concentration of C_200c=4.82±0.30 at the pivot mass M_200c=1e15 Msun/h.  This is consistent with the result from spherical modeling, C_200c=4.51±0.14.  Independently of the priors, the minor-to-major axis ratio q_a of the full sample exhibits a clear deviation from the spherical configuration (q_a=0.52±0.04 at 1e15Msun/h with uniform priors), with a weak dependence on the cluster mass.  Combining all 20 clusters, obtain a joint ensemble constraint on the minor-to-major axis ratio of q_a=0.652+-0.162-0.078 and a lower bound on the intermediate-to-major axis ratio of q_b>0.63 at the 2 sigma level from an analysis with uniform priors.  Assuming priors on the axis ratios derived from numerical simulations, constraint the degree of triaxiality for the full sample to be T=0.79±).03 at 1e15 Msun/h, indicating a preference for a prolate geometry of cluster halos.  Find no statistical evidence for an orientation bias (f_geo=0.93±0.07).


1804.00699
The direct imaging search for Earth 2.0: quantifying biases and planetary false positives
Guimond, Cowan

Direct imaging is likely the best way to characterize the atmospheres of Earth-Sized exoplanets in the habitable zone of Sun-like stars.  Previously, Stark+(2014,15,16) estimated the Earth twin yield of future direct imaging missions, such as LUVOIR and HabEx.  Extend this analysis to other types of planets, which will act as false positives for Earth twins.  Define an Earth twin as any exoplanet within half an e-folding of 1AU in semi-major axis and 1 R_Earth in planetary radius, orbiting a G-dwarf.  Using Monte Carlo analyses, quantify the biases and planetary false positive rates of Earth searches.  That is, given a pale dot at the correct projected separation and brightness to be a candidate Earth, what are the odds that it is, in fact, an Earth twin?  The notional telescope has a diameter of 10m, and inner working angle of 3 lambda/D, and an outer working angle of 10 lambda/D (62 was and 206 was at 1.0um).  With no precursor knowledge and one visit per star, 77% of detected candidate Earths are actually un-Earths; their mean radius is 2.3 R_Earth, a sub-Neptune.  The odds improve if every planet is imaged at its optimal orbital phase, either by relying on precursor knowledge, or by performing multi-epoch direct imaging.  In such a targeted search, 47% of detected Earth twin candidates are false positives, and they have a mean radius of 1.7 R_Earth.  The false positive rate is insensitive to stellar spectral type and the assumption of circular orbits.

Day 1392

Tuesday.

1804.00033
Exploring the production and depletion of lithium in the Milky Way stellar disk
Bensby, Lind

Determine Li abundances fo a well-studied sample of 714 F and G dwarf, turn-off, and sub giant stars in the solar neighborhood.  The analysis is based on line synthesis of the Li line at 6707A in high-resolution and high S/N ratio echelon spectra, obtained with the MIKE, FEROS, SOFIN, UVES, and FIES spectrographs.  The presented Li abundances are corrected for non-LTE effects.  Out of the sample of 714 stars, able to determine Li abundances for 420 stars and upper limits on the Li abundance for another 121 stars.  18 of the stars with well-determined Li abundances are listed as exoplanet host stars.  The main finding is that there are no signatures of Li production in the thick disk, but the Li abundance for stars of the same effective temperature is independent of metallicity for stars that can be associated with the Galactic thick disk.  Significant Li production is however seen in the thin disk, with a steady increase towards super-solar metallicities.  At the highest metallicities, however, around [Fe/H]~+0.3, tentatively confirm the recent discovery that the Li abundances level out.  Here contradict the recent finding in other studies that found that Li is also produced in the thick disk.  This is likely due to the chemically defined selection criteria those studies used to define their thick disk samples.  Age criteria that are used here produce a thick disk stellar sample that is much less contaminated by thin disk stars, and hence more reliable abundance trends.  A conclusion that can be drawn is that no significant Li production, relative to the primordial abundance, took place during the first few billion years of the Milky Way, an era coinciding with the formation and evolution of the thick disk.

Day 1391

Friday, Monday.



1803.11181

The Americal space exploration narrative from the Cold War through the Obama Administration

Holland, Burns

Document how the narrative and the policies of space exploration in the US have changed over the past 50 years.  First examine the history of the US space exploration program and also assess 3 current conditions of space exploration including: (1) the increasing role of the private sector, (2) the influence of global politics, and (3) the focus on a human mission to Mars.  Identify 5 rhetorical themes: competition, prestige, collaboration, leadership, and a new paradigm.  These themes are then used to analyze the content of 40 documents from 8 presidential administration.  The historical narrative and content analysis together suggest that space exploration has developed from a discourse about a bipolar world composed of the US and USSR into a complicated field that encompass many new players.  Make 3 observations: (1) there is a disconnect between stated US policy goals and the implementation of those goals, (2) the US communicates mixed messages regarding its intent to be both the dominant leader in space exploration and also a committed participant in international collaborations, and (3) the US cannot remain a true pioneer in space exploration if it does not embrace the realities of globalization and the changing dynamics within space exploration.  Conclude with 3 suggestions: (1) the US government and NASA should critically examine space exploration priorities and commit to implementing a program that will further realistic policy and goals, (2) the US should reexamine its intention to play a dominant leadership role in space exploration and consider emphasizing a commitment toward active participation in international collaboration in space, and (3) the US should fully embrace the new paradigm of space exploration by lowering barriers that hinder competitiveness.

1803.11198
Stellar disc strams as probes of the Galactic potential and satellite impacts
Laporte, et al

Stars aligned in thin stream-line features (feathers), with widths of delta~1-10 deg and lengths as large as Delta ell~180 deg, have been observed towards the Anticenter of our Galaxy and their properties mapped in abundances and phase-space.  Study their origin by analyzing similar features arising in an N-body simulations of a Galactic disc interacting with a Sagittarius-like dwarf spheroidal galaxy (Sgr).  By following the orbits of the particles identified as contributing to feathers backwards in time, trace their excitation to one of Sgr's previous peri centric passages.  These particles initially span a large range of phase-angles but a tight range of radii, suggesting they provide a probe of populations in distinct annuli in the outer Galactic disc.  The structures are long lived and persist after multiple passages on timescales of ~4 Gyrs.  On the sky, they exhibit oscillatory motion that can be traced with a single orbit mapped over much of their full length and with amplitudes and gradients similar to those observed.  Demonstrate how these properties of feathers may be exploited to measure the potential, its flattening, as well as infer the strength of recent potential perturbations.


1803.11526
Tensions between direct measurements of the lens power spectrum from Planck data
Motloch, Hu

Apply a recently developed method to directly measure the gravitational lensing power spectrum from CMB power spectra to the Planck satellite data.  This method allows analysis of the tension between the temperature power spectrum and lens reconstruction in a model independent way.  Even when allowing for arbitrary variations in the lensing power spectrum, the tension remains at the 2.4 sigma level.  By separating the lensing and unlensed high redshift information in the CMB power spectra, also show that under LCDM the two are in tension at a similar level whereas the unlensed information is consistent with lensing reconstruction.  These anomalies are driven by the smoother acoustic peaks relative to LCDM at ell~1250-1500.  Both tensions relax slightly when polarization data are considered.  This technique also isolates the one aspect of the lensing power spectrum that the Planck CMB power spectra currently constrain and can be straightforwardly generalized to future data when CMB power spectra constrain multiple aspects of lensing which are themselves correlated with lensing reconstruction.