The Hubble Frontier Fields, together with other cluster lensing surveys with Hubble, have revealed hundreds of high-redshift (z≳6) dropout galaxy candidates. We summarize recent efforts taken to spectroscopically follow up magnified high-redshift galaxies behind those clusters and in the field, including our detection of Lyα emission from a redshift z=8.68 object.

We have conducted a LABOCA 870 μm follow-up of ten massive lensing clusters of the Herschel Lensing Survey (HLS, Egami et al. 2010) aiming at unveiling the yet hidden part of dusty star formation in the distant Universe. Among these clusters, A2744 and AS1063 are part of the Frontier Fields HST program. We also obtained 2 mm bolometer observations of A2744 and A370 with the GISMO array at the IRAM 30 m. We detected sources that are undetected with Herschel (PACS and SPIRE) implying either a very high redshift (z > 4) or a very low dust temperature (T<25 K). Their flux also imply a low intrinsic luminosity, LFIR < 1012L⊙. Some of them are extended and could correspond to multiple sources or to multiple images of a lensed source. Substructures in the Sunyaev-Zeldovich effect could also contribute this very red submm emission. An ALMA program is ongoing to unveil the nature of these sources.

We present MUSE observations of a typical (M⋆ = 6 × 109M⊙) young lensed galaxy at z=3.5, for which we obtain 2D resolved spatial information of Ly α and, for the first time, of C iii] emission. We also derive important physical properties from several UV emission and absorption lines, rarely seen at these redshifts. Stellar and gas-phase metallicities point towards a low metallicity object. We model the Ly α line and surface brightness profile using a radiative transfer code in an expanding gas shell, finding that this model provides a reasonable description of both observables.

In 1964, Sjur Refsdal first considered the possibility that the light from a background supernova could traverse multiple paths around a strong gravitational lens towards us. He showed that the arrival times of the supernova's light would depend on the cosmic expansion rate, as well as the distribution of matter in the lens. I discussed the discovery of the first such multiply imaged supernova, which exploded behind the MACS J1149.6+2223 galaxy cluster. We have obtained Hubble Space Telescope grism and ground-based spectra of the four images of the supernova, which form an Einstein Cross configuration around an elliptical cluster member. These spectra as well as rest-frame optical light curves have allowed us to learn about the properties of the peculiar core-collapse supernova explosion, which occurred 4.3 Gyr after the Bang Bang, and contain information about the lenses matter distribution as well as their stellar populations. A delayed image of the supernova is expected close to the galaxy cluster center as early as this Fall, and will serve as an unprecedented probe of the potential of a massive galaxy cluster.

Supernovae are important probes of the properties of stars at high redshifts because they can be detected at early epochs and their masses can be inferred from their light curves. Direct detection of the first cosmic explosions in the universe will only be possible with JWST, WFIRST and the next generation of extremely large telescopes. But strong gravitational lensing by massive clusters, like those in the Frontier Fields, could reveal supernovae at slightly lower redshifts now by magnifying their flux by factors of 10 or more. We find that Frontier Fields will likely discover dozens of core-collapse supernovae at 5 < z < 12. Future surveys of cluster lenses similar in scope to Frontier Fields by JWST might find hundreds of these events out to z ~ 15 - 17. Besides revealing the masses of early stars, these ancient supernovae could also constrain cosmic star formation rates in the era of first galaxy formation.

In the Astronomy in Focus series, volume XXIXB, Focus Meeting 4 was omitted from table 2 of the preface (Benvenuti 2016) on page xxi. The editor apologizes for this error.

In the Astronomy in Focus series, volume XXIXB, a number of contributions from Focus Meeting 4 and Focus Meeting 17 were omitted. The editor apologizes for this error.

We report on the most recent progresses on the modelling of the asymptotic giant branch (AGB) phase. We show that dust formation in the wind of AGBs is sensitive to the mass of the precursor, low–mass AGB with mass below ~ 3 M⊙ producing mainly carbonaceous dust, whereas their counterparts of higher mass produce silicates and alumina dust. We discuss the uncertainty of the results, due to the poor knowledge of physical mechanisms highly important for AGB evolution, primarily convection and mass loss. We show how the chemical composition of planetary nebulae can be used to draw information on the main features of AGB evolution.

The diversity of collimated outflows in post-asymptotic-giant-branch stars and their planetary nebula progeny are often explained by a combination of close binary interactions and accretion. The viability of such scenarios can be tested by comparing kinematic outflow data to determine minimum accretion rates necessary to power observed outflows. While many binary channels have been ruled out with this technique, common envelope interactions can accommodate the current observational constraints, are potentially common, lead to a diverse array of planetary-nebula shapes, and naturally produce period gaps for companions to white dwarfs.

Extra-galactic planetary nebulae (PNe) permit the study of dust and molecules in metallicity environments other than the Galaxy. Their known distances lower the number of free parameters in the observations vs. models comparison, providing strong constraints on the gas-phase and solid-state astrochemistry models. Observations of PNe in the Galaxy and other Local Group galaxies such as the Magellanic Clouds (MC) provide evidence that metallicity affects the production of dust as well as the formation of complex organic molecules and inorganic solid-state compounds in their circumstellar envelopes. In particular, the lower metallicity MC environments seem to be less favorable to dust production and the frequency of carbonaceous dust features and complex fullerene molecules is generally higher with decreasing metallicity. Here, I present an observational review of the dust and molecular content in extra-galactic PNe as compared to their higher metallicity Galactic counterparts. A special attention is given to the level of dust processing and the formation of complex organic molecules (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene precursors) depending on metallicity.

The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) allow us to study late stellar evolution in environments that are respectively about a half and a quarter the metallicity of the Milky Way. With a known distance and low reddening, the LMC is an excellent environment to study PNe and conduct multiple studies. Over the past twelve months we have used the UKST Hα survey to complete our search for faint PNe in the outer most LMC beyond the 64 deg2 area previously covered. Follow-up spectroscopy using AAOmega on the AAT and the 2.3-m telescope at Siding Spring Observatory have yielded a further 22 new LMC PNe while confirming the 8 previously known in the outer LMC. Medium- and high-resolution spectra have been used to measure fluxes and derive densities, mass and central star temperatures. A strong correlation is found between PNe and stellar density. This is visually displayed and given an empirical value of α = 1 PN / 2.5 × 106 L⊙. The current [O iii]-based PNLF, apart from providing an excellent standard candle, contains information about the parent population. The new PNLF, which extends down nine magnitudes, permits investigation of the faint end, the overall effects of internal extinction and provides clues to explain the insensitivity of the PNLF cutoff. When compared to the ionised density and mass of LMC PNe, the PNLF reveals it’s bimodal characteristics. Two separate evolutionary paths are evident for young, evolving PNe.

Recent studies of planetary nebulae and deep color-magnitude diagrams in M 31 have unveiled some unusual star-formation events beyond its spiral arms into its outer regions extending to 100 kpc in projected distance. In this paper I summarize these observations. I shall also highlight some special interpretive surprises suggested by observations of PNe related to their O/H abundances, ages, and the luminosities on PNe well beyond the inner disk.

The framework of the present review is as follows. The Local Group (LG) is populated by dwarf galaxies belonging to different morphological types. Apparently these galaxies are very different. The questions naturally raised from this framework are many. Is there an evolutionary sequence among them? Do they share common progenitors? Is the environment at the origin of their differences? The method we propose to answer to these questions is by investigating the chemical evolution of dwarf galaxies and their mass-metallicity relation (MZR). To this aim we use metallicities derived from planetary nebulae, since this stellar population is present in the star-forming (dwarf irregular, dIrr) as well as in the quiescent (dwarf spheroidal, dSph) galaxies. The results, actually, show that both dIrr and dSph galaxies of the LG follow the same MZR, at variance with the differences claimed in the past. These results are in good agreement with the recently derived MZR, based on stellar instead of the nebular metallicities of the LG dwarf galaxies. Moreover, our MZR is also consistent with the global MZR of SLOAN star-forming galaxies, which spans a wider stellar mass range (~ 106 - 1011 M⊙) than the LG dwarfs.

While the chemical abundances observed in bright planetary nebulae in local spiral galaxies are less varied than their counterparts in dwarfs, they provide new insight. Their helium abundances are typically enriched by less than 50% compared to the primordial abundance. Nitrogen abundances always show some level of secondary enrichment, but the absolute enrichment is not extreme. In particular, type I PNe are rare among the bright PNe in local spirals. The oxygen and neon abundances are very well correlated and follow the relation between these abundances observed in star-forming galaxies, implying that either the progenitor stars of these PNe modify neither abundance substantially or that they modify both to maintain the ratio (not predicted by theory). According to theory, these results imply that the progenitor stars of bright PNe in local spirals have masses of about 2 M⊙ or less. If so, the progenitors of these PNe have substantial lifetimes that allow us to use them to study the recent history of their host galaxies, including gravitational interactions with their neighbours. Areas that require further study include the systematic differences observed between spectroscopy obtained through slits and fibres, the uncertainties assigned to chemical abundances, including effects due to ionization correction factors, and the physics that gives rise to the PN luminosity function. Indeed, so long as we lack an understanding of how the last arises, our ability to use bright PNe as probes to understand the evolution of their host galaxies will remain limited.

The stellar phase of Thermally-Pulsating Asymptotic giant branch is the last major evolutionary stage of intermediate-mass stars which afterwards evolve into planetary nebulae. The TP-AGB phase is affected by mass-loss and instabilities which notoriously make its theoretical modelling uncertain. This review focuses on the effects such modelling has on stellar population models for galaxies, with particular focus on the high-z Universe where galaxies are young and contain a large number of short-living TP-AGB stars. I shall present the models, discuss how different prescriptions for the treatment of the TP-AGB affect the theoretical integrated spectral energy distribution and how these compare to galaxy data, and discuss implications for the PN nebulae luminosity function stemming from the various assumptions. Finally I shall discuss the inclusion of hot evolved stars on stellar population models and how they compare to data for old galaxies at our present time.

The bright-end cutoff of the [O III] λ5007 planetary nebula luminosity function (PNLF) is insensitive to population age and metallicity, making it an excellent extragalactic standard candle. We review our knowledge of the function and discuss the challenges related to modeling it. We show that, while there has been a great deal of progress in understanding the nuances of its shape, there is still no solid theory which explains the luminosity of the PNLF cutoff in old stellar populations. This is an extremely serious problem, as it affects a myriad of astrophysical issues, from the determination of the stellar masses of galaxies to our understanding of alternative channels of stellar evolution.

The kinematics and dynamical properties of galaxy stellar halos are difficult to measure, given the faint surface brightness that characterizes these regions. Gas-rich systems such as spiral galaxies can be probed using the radio Hi emission. Early-type galaxies contain less gas, therefore alternative kinematic tracers need to be used. Planetary Nebulae (PNe) can be easily detected far out in the halo thanks to their bright [Oiii] emission at 5007 Å. It is therefore possible to map the halo kinematics also in early-type galaxies, typically out to 5 effective radii or beyond. Thanks to the recent spectroscopic surveys targeting extra-galactic PNe, we can now rely on few tens of galaxies where the kinematics of the stellar halos are measured. I will discuss the most important results: (a) the relation of the stellar surface brightness and the PNe number density; (b) the velocity and velocity dispersion two-dimensional fields; (c) the radial profiles of angular momentum; and (d) the relation between the derived kinematics physical properties of the host galaxies.

Since the first detection of intracluster planetary nebulae in 1996, imaging and spectroscopic surveys identified such stars to trace the radial extent and the kinematics of diffuse light in clusters. This topic of research is tightly linked with the studies of galaxy formation and evolution in dense environment, as the spatial distribution and kinematics of planetary nebulae in the outermost regions of galaxies and in the cluster cores is relevant for setting constraints on cosmological simulations. In this sense, extragalactic planetary nebulae play a very important role in the near-field cosmology, in order to measure the integrated mass as function of radius and the orbital distribution of stars in structures placed in the densest regions of the nearby universe.

We report our recent analyses of the RR Lyrae Optical Gravitational Lensing Experiment (OGLE) photometry, which resulted in several new discoveries and important results. We present the in-depth analysis of the Blazhko effect discovered in several double-mode RR Lyr (RRd) stars for the first time. Typically, these stars show multiple modulation periods and somewhat atypical first overtone to fundamental mode period ratio. Our analysis of the Galactic bulge stars allowed a significant (~factor of 10) increase of the members of the recently detected group of double-mode radial-non-radial RR Lyr stars, pulsating in the first overtone and in an yet unidentified shorter period mode, with characteristic period ratio of the two modes around 0.61. With more than 200 new members of the group first statistical analysis of the group properties is possible. Finally, we report the discovery of an even more intriguing group of double-periodic RR Lyr stars. The dominant pulsation mode in these stars corresponds to the radial first overtone. The additional period is longer, the period ratios (first overtone to a period of an yet unidentified origin) tightly cluster around 0.686. Hence the additional period is longer than the unseen fundamental mode and cannot correspond to a purely acoustic pulsation mode.

We present a grid of nonlinear convective pulsation models of type-II Cepheids: BL Her stars, W Vir stars and RV Tau stars. The models have 0.6M⊙ and cover a wide range of luminosities, effective temperatures and chemical compositions. The most important result is the detection of two domains in the HR diagram, in which period doubling effect is present. First domain, with pulsation periods in between 2 d and 5.5 d, was also detected in earlier radiative computations. The first star showing the period doubling effect with period in the predicted range (≈ 2.4 d) was discovered recently. The second domain extends for periods longer than 9.5 d but so far no period-doubled type-II Cepheid that could be associated with this domain was discovered. We also detect very small domains in which modulation of pulsation is possible. The origin of these phenomena may be assocaited with half-integer resonances between radial pulsation modes. In the most luminous models a dynamical instability is detected, which indicates that pulsation driven mass loss is important process occurring in type-II Cepheids.