We present our broadband study of GW170817 from radio to hard X-rays, including NuSTAR and Chandra observations up to 165 days after the merger, and a multi-messenger analysis including LIGO constraints. The data are compared with predictions from a wide range of models, providing the first detailed comparison between non-trivial cocoon and jet models. Homogeneous and power-law shaped jets, as well as simple cocoon models are ruled out by the data, while both a Gaussian shaped jet and a cocoon with energy injection can describe the current dataset for a reasonable range of physical parameters, consistent with the typical values derived from short GRB afterglows. We propose that these models can be unambiguously discriminated by future observations measuring the post-peak behaviour, with slope -1.0 for the cocoon and -2.5 for the jet model.

E. Troja L. Piro G. Ryan H. van Eerten R. Ricci M. H. Wieringa S. Lotti T. Sakamoto S. B. Cenko

Broadening the variety of two-dimensional (2D) materials and improving the synthesis of ultrathin films are crucial to the development of the semiconductor industry. As a state-of-the-art 2D material, Ga2Se2 has attractive optoelectronic properties when it reaches the atomically-thin regime. However, its van der Waals epitaxial growth, especially for the atomically-thin films, has seldom been studied. In this paper, we used molecular beam epitaxy to synthesize Ga2Se2 single-crystal films with a surface roughness down to 1.82 nm on c-plane sapphire substrates by optimizing substrate temperature, Se:Ga flux ratio, and growth rate. Then we used a 3-step mode to grow Ga2Se2 films with a thickness as low as 3 tetralayers and a surface roughness as low as 0.61 nm, far exceeding the performance of direct growth. Finally, we found that the surface morphology strongly depends on the Se:Ga flux ratio, and higher growth rates widened the suitable flux ratio window for growing Ga2Se2. Overall, this work advances the understanding of the vdW epitaxy growth mechanism for post-transition metal monochalcogenides on sapphire substrates.

Mingyu Yu Lottie Murray Matthew Doty Stephanie Law

Separation logic was conceived in order to make the verification of pointer programs scalable to large systems and it has proven extremely effective. The key idea is that programs typically access only small parts of memory, allowing for local reasoning. This idea is implemented in separation logic by extending first-order logic with separating connectives, which inspect local regions of memory. It turns that this approach not only applies to pointer programs, but also to programs involving other resource structures. Various theories have been put forward to extract and apply the ideas of separation logic more broadly. This resulted in algebraic abstractions of memory and many variants of separation logic for, e.g., concurrent programs and stochastic processes. However, none of the existing approaches formulate the combination of first-order logic with separating connectives in a theory that could immediately yield program logics for different resources. In this paper, we propose a framework based on the idea that separation logic can obtained by making first-order logic resource-aware. First-order logic can be understood in terms of categorical logic, specifically fibrations. Our contribution is to make these resource-aware by developing categorical logic internally in categories of sheaves, which is what we call sheafeology. The role of sheaves is to model views on resources, through which resources can be localised and combined, which enables the scalability promised by separation logic. We contribute constructions of an internal fibration in sheaf categories that models predicates on resources, and that admits first-order and separating connectives. Thereby, we attain a general framework of separation logic for generic resources, a claim we substantiate by instantiating our framework to various memory models and random variables.

Berend van Starkenburg Henning Basold Chase Ford

We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites.

Else Starkenburg Amina Helmi Heather L. Morrison Paul Harding Hugo van Woerden Mario Mateo Edward W. Olszewski Thirupathi Sivarani John E. Norris Kenneth C. Freeman Stephen A. Shectman R. C. Dohm-Palmer Lucy Frey Dan Oravetz

In the $\Lambda$CDM model of structure formation, a stellar spheroid grows by the assembly of smaller galaxies, the so-called building blocks. Combining the Munich-Groningen semi-analytical model of galaxy formation with the high resolution Aquarius simulations of dark matter haloes, we study the assembly history of the stellar spheroids of six Milky Way-mass galaxies, focussing on building block properties such as mass, age and metallicity. These properties are compared to those of the surviving satellites in the same models. We find that the building blocks have higher star formation rates on average, and this is especially the case for the more massive objects. At high redshift these dominate in star formation over the satellites, whose star formation timescales are longer on average. These differences ought to result in a larger $\alpha$-element enhancement from Type II supernovae in the building blocks (compared to the satellites) by the time Type Ia supernovae would start to enrich them in iron, explaining the observational trends. Interestingly, there are some variations in the star formation timescales of the building blocks amongst the simulated haloes, indicating that [$\alpha$/Fe] abundances in spheroids of other galaxies might differ from those in our own Milky Way.

Pim van Oirschot Else Starkenburg Amina Helmi Gijs Nelemans

Neutron star mergers (NSM) are likely to be the main production sites for the rapid (r-) neutron capture process elements. We study the r-process enrichment of the stellar halo of the Milky Way through NSM, by tracing the typical r-process element Eu in the Munich-Groningen semi-analytic galaxy formation model, applied to three high resolution Aquarius dark matter simulations. In particular, we investigate the effect of the kick velocities that neutron star binaries receive upon their formation, in the building block galaxies (BBs) that partly formed the stellar halo by merging with our Galaxy. When this kick is large enough to overcome the escape velocity of the BB, the NSM takes place outside the BB with the consequence that there is no r-process enrichment. We find that a standard distribution of NS kick velocities decreases [Eu/Mg] abundances of halo stars by $\sim 0.5$~dex compared to models where NS do not receive a kick. With low NS kick velocities, our simulations match observed [Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities [Mg/H]$\geq -1.5$. Only in Aquarius halo B-2 also the lower metallicity stars have [Eu/Mg] values similar to observations. We conclude that our assumption of instantaneous mixing is most likely inaccurate for modelling the r-process enrichment of the Galactic halo, or an additional production site for r-process elements is necessary to explain the presence of low-metallicity halo stars with high Eu abundances.

Pim van Oirschot Gijs Nelemans Onno Pols Else Starkenburg

We present deep near-infrared (NIR) Js, H, and Ks-band ISAAC imaging of the WFPC2 field of the HDF-S. The 2.5'x 2.5' high Galactic latitude field was observed with the VLT under the best seeing conditions with integration times amounting to 33.6 hours in Js, 32.3 hours in H, and 35.6 hours in Ks. We reach total AB magnitudes for point sources of 26.8, 26.2, and 26.2 respectively (3 sigma), which make it the deepest ground-based NIR observations to date, and the deepest Ks-band data in any field. The effective seeing of the coadded images is ~0.45" in Js, ~0.48" in H, and ~0.46" in Ks. Using published WFPC2 optical data, we constructed a Ks-limited multicolor catalog containing 833 sources down to Ks,tot ~< 26 (AB), of which 624 have seven-band optical-to-NIR photometry. These data allow us to select normal galaxies from their rest-frame optical properties to high redshift (z ~< 4). The observations, data reduction and properties of the final images are discussed, and we address the detection and photometry procedures that were used in making the catalog. In addition, we present deep number counts, color distributions and photometric redshifts of the HDF-S galaxies. We find that our faint Ks-band number counts are flatter than published counts in other deep fields, which might reflect cosmic variations or different analysis techniques. Compared to the HDF-N, we find many galaxies with very red V-H colors at photometric redshifts 1.95 < z < 3.5. These galaxies are bright in Ks with infrared colors redder than Js-Ks > 2.3 (in Johnson magnitudes). Because they are extremely faint in the observed optical, they would be missed by ultraviolet-optical selection techniques, such as the U-dropout method.

I. Labbe M. Franx G. Rudnick N. M. Forster Schreiber P. van der Werf H. Rottgering L. van Starkenburg A. van de Wel K. Kuijken H. -W. Rix A. Moorwood E. Daddi P. G. van Dokkum

Using deep near-infrared imaging of the Hubble Deep Field South with ISAAC on the Very Large Telescope we find 6 large disk-like galaxies at redshifts z = 1.4-3.0. The galaxies, selected in K_s (2.2 micron), are regular and surprisingly large in the near-infrared (rest-frame optical), with face-on effective radii r_e = 0.65"-0.9" or 5.0-7.5 h_70^-1 kpc in a Lambda-CDM cosmology, comparable to the Milky Way. The surface brightness profiles are consistent with an exponential law over 2-3 effective radii. The WFPC2 morphologies in Hubble Space Telescope imaging (rest-frame UV) are irregular and show complex aggregates of star-forming regions ~2" (~15 h_70^-1 kpc) across, symmetrically distributed around the K_s-band centers. The spectral energy distributions show clear breaks in the rest-frame optical. The breaks are strongest in the central regions of the galaxies, and can be identified as the age-sensitive Balmer/4000 Angstrom break. The most straightforward interpretation is that these galaxies are large disk galaxies; deep NIR data are indispensable for this classification. The candidate disks constitute 50% of galaxies with L_V > 6 x 10^10 h_70^-2 L_sun at z = 1.4-3.0. This discovery was not expected on the basis of previously studied samples. In particular, the Hubble Deep Field North is deficient in large galaxies with the morphologies and profiles we report here.

I. Labbe G. Rudnick M. Franx E. Daddi P. G. van Dokkum N. M. Forster Schreiber K. Kuijken A. Moorwood H. -W. Rix H. Rottgering L. van Starkenburg Ignacio Trujillo A. van der Wel P. van der Werf

We investigate the nature of the substantial population of high-z galaxies with Js-Ks>2.3 discovered as part of our FIRES survey. This colour cut efficiently isolates z>2 galaxies with red rest-frame optical colors ("Distant Red Galaxies" or DRGs). We select objects in the 2.5'x2.5' HDF-South (HDF-S) and 5'x5' field around the MS1054-03 cluster; the surface densities at Ks<21 are 1.6+-0.6 and 1.0+-0.2 arcmin^-2. We discuss the 34 DRGs at 2<z<3.5: 11 at Ks<22.5 in HDF-S and 23 at Ks<21.7 in the MS1054-03 field. We analyze the SEDs constructed from our deep near-infrared (NIR) and optical imaging from the ESO VLT and HST. We develop diagnostics involving I-Js, Js-H, and H-Ks to argue that the red NIR colors of DRGs cannot be attributed solely to extinction and require for many an evolved stellar population with prominent Balmer/4000A break. In the rest-frame, the optical colours of DRGs fall within the envelope of normal nearby galaxies and the UV colours suggest a wide range in star formation activity and/or extinction. This contrasts with the much bluer and more uniform SEDs of Lyman break galaxies (LBGs). From evolutionary synthesis models with constant star formation, solar metallicity, Salpeter IMF, and Calzetti et al. extinction law, we derive for the HDF-S (MS1054-03 field) DRGs median ages of 1.7(2.0) Gyr, A_V = 2.7(2.4) mag, stellar masses 0.8(1.6)x10^11 Msun, M/L_V = 1.2(2.3) Msun/LVsun, and SFR = 120(170) Msun/yr. Models assuming declining SFRs with e-folding timescales of 10Myr-1Gyr generally imply younger ages, lower A_V's and SFRs, but similar stellar masses within a factor of two. Compared to LBGs at similar redshifts and rest-frame L_V's, DRGs are older, more massive, and more obscured for any given star formation history. [ABRIDGED]

N. M. Forster Schreiber P. G. van Dokkum M. Franx I. Labbe G. Rudnick E. Daddi G. D. Illingworth M. Kriek A. F. M. Moorwood H. -W. Rix H. Rottgering I. Trujillo P. van der Werf L. van Starkenburg S. Wuyts

The galactic halo likely grew over time in part by assembling smaller galaxies, the so-called building blocks. We investigate if the properties of these building blocks are reflected in the halo white dwarf (WD) population in the Solar neighborhood. Furthermore, we compute the halo WD luminosity functions (WDLFs) for four major building blocks of five cosmologically motivated stellar haloes. We couple the SeBa binary population synthesis model to the Munich-Groningen semi-analytic galaxy formation model, applied to the high-resolution Aquarius dark matter simulations. Although the semi-analytic model assumes an instantaneous recycling approximation, we model the evolution of zero-age main sequence stars to WDs, taking age and metallicity variations of the population into account. Although the majority of halo stars is old and metal-poor and therefore the WDs in the different building blocks have similar properties (including present-day luminosity), we find in our models that the WDs originating from building blocks that have young and/or metal-rich stars can be distinguished from WDs that were born in other building blocks. In practice however, it will be hard to prove that these WDs really originate from different building blocks, as the variations in the halo WD population due to binary WD mergers result in similar effects. The five joined stellar halo WD populations that we modelled result in WDLFs that are very similar to each other. We find that simple models with a Kroupa or Salpeter initial mass function (IMF) fit the observed luminosity function slightly better, since the Chabrier IMF is more top-heavy, although this result is dependent on our choice of the stellar halo mass density in the Solar neighborhood.

Pim van Oirschot Gijs Nelemans Else Starkenburg Silvia Toonen Amina Helmi Simon Portegies Zwart