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We study of the role of galaxy–galaxy interactions and disk instabilities in producing starburst activity in galaxies out to z = 4. For this, we use a sample of 387 galaxies with robust total star formation rate measurements from Herschel, gas masses from the Atacama Large Millimeter/submillimeter Array, stellar masses and redshifts from multiband photometry, and JWST/NIRCam rest-frame optical imaging. Using mass-controlled samples, we find an increased fraction of interacting galaxies in the starburst regime at all redshifts out to z = 4. This increase correlates with star formation efficiency (SFE) but not with gas fraction. However, the correlation is weak (and only significant out to z = 2), which could be explained by the short duration of SFE increase during interaction. In addition, we find that isolated disk galaxies make up a significant fraction of the starburst population. The fraction of such galaxies with star-forming clumps (“clumpy disks”) is significantly increased compared to the main-sequence disk population. Furthermore, this fraction directly correlates with SFE. This is direct observational evidence for a long-term increase of SFE maintained due to disk instabilities, contributing to the majority of starburst galaxies in our sample and hence to substantial mass growth in these systems. This result could also be of importance for explaining the growth of the most massive galaxies at z > 6.

A quasar on the edge Astrophysicists may be forgiven a bout of déjà vu following the discovery of a quasar that is not at the centre of a massive host galaxy. The claim that ‘naked quasars’ had been observed caused a flurry of excitement in the mid-1990s. Quasars are among the most powerful energy sources in the Universe, and their energy is thought to derive from the infall of matter into a black hole at the centre of a massive galaxy. Quasars with no galaxy to power them seemed an anachronism and on inspection ‘naked quasars’ were nothing of the sort: the host galaxies were just hidden by the extreme luminosity of the quasars. The ‘new’ quasar is not naked, but it is not wearing much. HE0450–2958 is at the edge of a large gas cloud; if it has a host galaxy it is too small to drive the quasar, which might be feeding on mass from a nearby ultraluminous infrared galaxy with which it may be interacting. This discovery could change our ideas about how these objects form and suggests that isolated supermassive black holes may exist in the Universe. A quasar is thought to be powered by the infall of matter onto a supermassive black hole at the centre of a massive galaxy 1 , 2 . Because the optical luminosity of quasars exceeds that of their host galaxy, disentangling the two components can be difficult. This led in the 1990s to the controversial claim of the discovery of ‘naked’ quasars 3 , 4 , 5 , 6 , 7 . Since then, the connection between quasars and galaxies has been well established 8 . Here we report the discovery of a quasar lying at the edge of a gas cloud, whose size is comparable to that of a small galaxy, but whose spectrum shows no evidence for stars. The gas in the cloud is excited by the quasar itself. If a host galaxy is present, it is at least six times fainter than would normally be expected 8 , 9 for such a bright quasar. The quasar is interacting dynamically with a neighbouring galaxy, whose gas might be feeding the black hole.

A next main step in understanding star formation is to link the sharp but narrow view of Galactic molecular cloud studies to the wider context accessed by less detail by extragalactic work. In this proceeding, we discuss how new technology and large programs at millimeter wavelengths are improving our ability to access physical conditions in the interstellar medium (ISM) of other galaxies. We highlight results from the multi-line survey of Usero et al. (2015), which measured density sensitive lines across nearby galaxy disks, and two new mapping studies of M51: the high resolution Plateau de Bure Arcsecond Whirlpool Survey (PAWS) and the EMPIRE multi-line mapping survey. These results argue for a context-dependent role for gas density in star formation; that is, gas at a particular density does not appear to form stars in a universal way. They also demonstrate the influence of cloud-scale conditions, especially surface density and the velocity dispersion, in setting the small-scale density distribution and highlight gravitational boundedness as a main driver of the ability of gas to form stars. Beyond these specific results, we argue that ability to gauge detailed physical conditions in the star-forming gas of other galaxies promises major advances that will help unify the fields of Galactic and extragalactic star formation in the next few years.

This review reports on recent results obtained from HST/WFPC2 high resolution observations of field stars and star clusters in the bulge and the halo of the nearby galaxy M31.[PUBLICATION ABSTRACT]

It is now well established that chemistry in external galaxies is rich and complex. In this review I will explore whether one can use molecular emissions to determine their physical conditions. There are several considerations to bear in mind when using molecular emission, and in particular molecular ratios, to determine the densities, temperatures and energetics of a galaxy, which I will briefly summarise here. I will then present an example of a study that uses multiple chemical and radiative transfer analyses in order to tackle the too often neglected ‘degeneracies’ implicit in the interpretation of molecular ratios and show that only via such analyses combined with multi-species and multi-lines high spatial resolution data one can truly make molecules into powerful diagnostics of the evolution and distribution of molecular gas.

I review some steps in the conversion of molecular cloud gas into stars and planets, with an emphasis in this presentation on the early stage molecular cloud fragmentation that leads to elongated filaments/ribbons. Magnetic fields can play a crucial role in all stages and need to be invoked particularly for early stage fragmentation as well as in late core collapse where it may control disk formation. I also review some elements of hydrodynamic modeling of disk evolution.

The Central Molecular Zone (CMZ; inner ~100 pc) hosts some of the most dense and massive molecular clouds of the Milky Way. These clouds might serve as local templates for dense clouds seen in nearby starburst galaxies or in the early universe. The clouds have a striking feature: they form stars at a very slow pace, considering their mass and high average density. Here we use interferometer data from ALMA and the SMA to show that this slow star formation is a consequence of the cloud density structure: CMZ clouds have a very flat density structure. They might, for example, exceed the average density of the Orion A molecular cloud by an order of magnitude on spatial scales ~5 pc, but CMZ “cores” of ~0.1 pc radius have masses and densities lower than what is found in the Orion KL region. This absence of highest–density gas probably explains the suppression of star formation. The clouds are relatively turbulent, and ALMA observations of H2CO and SiO indicate that the turbulence is induced by high–velocity shocks. We speculate that these shocks might prevent the formation of high–mass cores. It has been argued that the state of CMZ clouds depends on their position along the orbit around Sgr A*. Our incomplete data indicate no evolution in the density structure, and only a modest evolution in star formation activity per unit mass.

Dense cores are the simplest star-forming sites. They represent the end stage of the fragmentation hierarchy that characterizes molecular clouds, and they likely control the efficiency of star formation via their relatively low numbers. Recent dust continuum observations of entire molecular clouds show that dense cores often lie along large-scale filamentary structures, suggesting that the cores form by some type of fragmentation process in an approximately cylindrical geometry. To understand the formation mechanism of cores, additional kinematic information is needed, and this requires observations in molecular-line tracers of both the dense cores and their surrounding cloud material. Here I present some recent efforts to clarify the kinematic structure of core-forming regions in the nearby Taurus molecular cloud. These new observations show that the filamentary structures seen in clouds are often more complex than suggested by the maps of continuum emission, and that they consist of multiple fiber-like components that have different velocities and sonic internal motions. These components likely arise from turbulent fragmentation of the large-scale flows that generate the filamentary structures. While not all these fiber-like components further fragment to form dense cores, a small group of them does so, likely by gravitational instability. This fragmentation produces characteristic chain-like groups of dense cores that further evolve to form stars.

The cosmic star formation rate density first increases with time towards a pronounced peak 10 Gyrs ago (or z=1-2) and then slows down, dropping by more than a factor 10 since z=1. The processes at the origin of the star formation quenching are not yet well identified: either the gas is expelled by supernovae and AGN feedback, or prevented to inflow. Morphological transformation or environment effects are also invoked. Recent IRAM/NOEMA and ALMA results are reviewed about the molecular content of galaxies and its dynamics, as a function of redshift. Along the main sequence of massive star forming galaxies, the gas fraction was higher in the past (up to 80%), and galaxy disks were more unstable and more turbulent. The star formation efficiency increases with redshift, or equivalently the depletion time decreases, whatever the position of galaxies, either on the main sequence or above. Attempts have been made to determine the cosmic evolution of the H2 density, but deeper ALMA observations are needed to effectively compare with models.

In the current paradigm of turbulence-regulated interstellar medium (ISM), star formation rates of entire galaxies are intricately linked to the density structure of the individual molecular clouds. This density structure is essentially encapsulated in the probability distribution function of volume densities (ρ-PDF), which directly affects the star formation rates predicted by analytic models. Contrasting its fundamental role, the ρ-PDF function has remained virtually unconstrained by observations. I describe in this contribution the recent progress in attaining observational constraints for the column density PDFs (N-PDFs) of molecular clouds that function as a proxy of the ρ-PDFs. Specifically, observational works point towards a universal correlation between the shape of the N-PDFs and star formation activity in molecular clouds. The correlation is in place from the scales of a parsec up to the scales of entire galaxies, making it a fundamental, global link between the ISM structure and star formation.