We present a systematic numerical relativity study of the dynamical ejecta, winds, and nucleosynthesis in neutron star (NS) merger remnants. Binaries with the chirp mass compatible with GW170817, different mass ratios, and five microphysical equations of state (EOSs) are simulated with an approximate neutrino transport and a subgrid model for magnetohydrodynamic turbulence up to 100 ms postmerger. Spiral density waves propagating from the NS remnant to the disk trigger a wind with mass flux ∼0.1–0.5 M ⊙ s−1, which persists for the entire simulation as long as the remnant does not collapse to a black hole. This wind has average electron fraction ≳0.3 and average velocity ∼0.1–0.17 c and thus is a site for the production of weak r-process elements (mass number A < 195). Disks around long-lived remnants have masses ∼0.1–0.2 M ⊙, temperatures peaking at ≲10 MeV near the inner edge, and a characteristic double-peak distribution in entropy resulting from shocks propagating through the disk. The dynamical and spiral-wave ejecta computed in our targeted simulations are not compatible with those inferred from AT2017gfo using two-components kilonova models. Rather, they indicate that multicomponent kilonova models including disk winds are necessary to interpret AT2017gfo. The nucleosynthesis in the combined dynamical ejecta and spiral-wave wind in the long-lived mergers of comparable mass robustly accounts for all the r-process peaks, from mass number ∼75 to actinides in terms of solar abundances. Total abundances are weakly dependent on the EOS, while the mass ratio affects the production of first-peak elements.

We compare the predictions of three independently developed semi-analytic galaxy formation models (SAMs) that are being used to aid in the interpretation of results from the CANDELS survey. These models are each applied to the same set of halo merger trees extracted from the "Bolshoi" high-resolution cosmological N-body simulation and are carefully tuned to match the local galaxy stellar mass function using the powerful method of Bayesian Inference coupled with Markov Chain Monte Carlo or by hand. The comparisons reveal that in spite of the significantly different parameterizations for star formation and feedback processes, the three models yield qualitatively similar predictions for the assembly histories of galaxy stellar mass and star formation over cosmic time. Comparing SAM predictions with existing estimates of the stellar mass function from z = 0-8, we show that the SAMs generally require strong outflows to suppress star formation in low-mass halos to match the present-day stellar mass function, as is the present common wisdom. However, all of the models considered produce predictions for the star formation rates (SFRs) and metallicities of low-mass galaxies that are inconsistent with existing data. The predictions for metallicity-stellar mass relations and their evolution clearly diverge between the models. We suggest that large differences in the metallicity relations and small differences in the stellar mass assembly histories of model galaxies stem from different assumptions for the outflow mass-loading factor produced by feedback. Importantly, while more accurate observational measurements for stellar mass, SFR and metallicity of galaxies at 1 < z < 5 will discriminate between models, the discrepancies between the constrained models and existing data of these observables have already revealed challenging problems in understanding star formation and its feedback in galaxy formation. The three sets of models are being used to construct catalogs of mock galaxies on light cones that have the same geometry as the CANDELS survey, which should be particularly useful for quantifying the biases and uncertainties on measurements and inferences from the real observations.

The density profiles of dwarf galaxies are a highly varied set. If the dark matter is an Ultra-light particle such as axions, then simulations predict a distinctive and unique profile. If the axion mass is large enough to fit the ultra-faint dwarf (UFD) satellites(m 10 −21 eV), then the models do not fit the density profile of Fornax and Sculptor and are ruled out by more than 3 − σ confidence. If the axion mass is in the mass range that can fit mass profiles of Fornax and Sculptor dwarf spheroidals, then its extended profile implies enormous masses (≈ 10 11 − 10 12 M ) for the UFDs. These large masses for the UFDS are ruled out by more than 3 − σ confidence by dynamical friction arguments. The tension would increase further considering star formation histories and stellar masses of the UFDs. Moreover, light mass axions are inconsistent with the sub-halo mass function in the Milky Way.

The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass range 2.2 M largely unpopulated. Objects found in this so-called lower mass gap likely originate from a distinct astrophysical process. Such an object, with mass 2.6 M was recently detected in the binary merger GW190814 through gravitational waves by LIGO/Virgo. Here we show that black holes in the mass gap are naturally assembled through mergers and accretion in active galactic nucleus (AGN) disks, and can subsequently participate in additional mergers. We compute the properties of AGN-assisted mergers involving neutron stars and black holes, accounting for accretion. We find that mergers in which one of the objects is in the lower mass gap represent up to 4% of AGN-assisted mergers detectable by LIGO/Virgo. The lighter object of GW190814, with mass 2.6 M , could have grown in an AGN disk through accretion. We find that the unexpectedly high total mass of 3.4 M observed in the neutron star merger GW190425 may also be due to accretion in an AGN disk.

The recent EDGES collaboration detection of an absorption signal at a central frequency of ν = 78 ± 1 MHz points to the presence of a significant Lyman-α background by a redshift of z = 18. The timing of this signal constrains the dark matter particle mass (m χ ) in the warm dark matter (WDM) cosmological model. WDM delays the formation of small-scale structures, and therefore a stringent lower limit can be placed on m χ , based on the presence of a sufficiently strong Ly-α background due to star formation at z = 18, Our results show that the coupling the spin temperature to the gas through Ly-α pumping requires a minimum mass of m χ > 3 keV if atomic cooling halos dominate the star formation rate at z = 18, and m χ > 2 keV if H 2 cooling halos also form stars efficiently at this redshift. These limits match or exceed the most stringent limits cited to date in the literature, even in the face of the many uncertainties regarding star-formation at high redshift.

To explain the high observed abundances of r-process elements in local ultrafaint dwarf (UFD) galaxies, we perform cosmological zoom simulations that include r-process production from neutron star mergers (NSMs). We model star-formation stochastically and simulate two different halos with total masses ≈ 10 8 M at z = 6. We find that the final distribution of [Eu/H] vs. [Fe/H] is relatively insensitive to the energy by which the r-process material is ejected into the interstellar medium, but strongly sensitive to the environment in which the NSM event occurs. In one halo the NSM event takes place at the center of the stellar distribution, leading to high-levels of r-process enrichment such as seen in a local UFD, Reticulum II (Ret II). In a second halo, the NSM event takes place outside of the densest part of the galaxy, leading to a more extended r-process distribution. The subsequent star formation occurs in an interstellar medium with shallow levels of r-process enrichment which results in stars with low levels of [Eu/H] compared to Ret II stars even when the maximum possible r-process mass is assumed to be ejected. This suggests that the natal kicks of neutron stars may also play an important role in determining the r-process abundances in UFD galaxies, a topic that warrants further theoretical investigation.

The recent aLIGO/aVirgo discovery of gravitational waves from the neutron star merger (NSM) GW170817 and the follow up kilonova observations have shown that NSMs produce copious amount of r -process material. However, it is difficult to reconcile the large natal kicks and long average merging times of Double Neutron Stars (DNSs), with the levels of r -process enrichment seen in ultrafaint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming that such dwarf systems have lost a significant fraction of their stellar mass through tidal stripping, we conclude that contrary to most current models, it is the DNSs with rather large natal kicks but very short merging timescales that can enrich UFD-type galaxies. These binaries are either on highly eccentric orbits or form with very short separations due to an additional mass-transfer between the first-born neutron star and a naked helium star, progenitor of the second-born neutron star. These DNSs are born with a frequency that agrees with the statistics of the r -process UFDs, and merge well within the virial radius of their host halos, therefore contributing significantly to their r -process enrichment. arXiv:1810.04176v2 [astro-ph.HE]

We study the relationship between the UV continuum slope and infrared excess ( º L L IRX IR FUV ) predicted by performing dust radiative transfer on a suite of hydrodynamical simulations of galaxies. Our suite includes both isolated disk galaxies and mergers intended to be representative of galaxies at bothz 0 and~-z 2 3. Our lowredshift systems often populate a region around the locally calibrated Meurer et al. relation but move above the relation during merger-induced starbursts. Our high-redshift systems are blue and IR luminous and therefore lie above the Meurer et al. relation. The value of β strongly depends on the dust type used in the RT simulation: Milky-Way-type dust leads to significantly more negative (bluer) slopes compared with Small-Magellanic-Cloudtype dust. The effect on β due to variations in the dust composition with galaxy properties or redshift is the dominant model uncertainty. The dispersion in β is anticorrelated with specific star formation rate (sSFR) and tends to be higher for the~-z 2 3 simulations. In the actively star-forming~-z 2 3 simulated galaxies, dust attenuation dominates the dispersion in β, whereas in thez 0 simulations, the contributions of star formation history (SFH) variations and dust are similar. For low-sSFR systems at both redshifts, SFH variations dominate the dispersion. Finally, the simulated~-z 2 3 isolated disks and mergers both occupy a region in the b -IRX plane consistent with observed~-z 2 3 dusty star-forming galaxies (DSFGs). Thus, contrary to some claims in the literature, the blue colors of high-z DSFGs do not imply that they are short-lived starbursts.