ALBERT

Description: We develop a simple dynamical model for the evolution of gas in the centres of barred spiral galaxies, using the Milky Way's Central Molecular Zone (CMZ, i.e. the central few hundred pc) as a case study. We show that, in the presence of a galactic bar, gas in a disc in the central regions of a galaxy will be driven inwards by angular momentum transport induced by acoustic instabilities within the bar's inner Lindblad resonance. This transport process drives turbulence within the gas that temporarily keeps it strongly gravitationally stable and prevents the onset of rapid star formation. However, at some point the rotation curve must transition from approximately flat to approximately solid body, and the resulting reduction in shear reduces the transport rates and causes gas to build up, eventually producing a gravitationally unstable region that is subject to rapid and violent star formation. For the observed rotation curve of the Milky Way, the accumulation happens ~100 pc from the centre of the Galaxy, in good agreement with the observed location of gas clouds and young star clusters in the CMZ. The characteristic time-scale for gas accumulation and star formation is of the order of 10–20 Myr. We argue that similar phenomena should be ubiquitous in other barred spiral galaxies.

Description: We present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency (, i.e. amount of star formation happening in bound clusters) from about 26 per cent in the inner region to 8 per cent in the outer part of the galaxy. The recovered  values within different regions of M83 follow the same  versus star formation rate density relation observed for entire galaxies. We also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. We observe a significant steepening of the ICMF in the outer regions (from –1.90 ± 0.11 to –2.70 ± 0.14) and for the whole galactic cluster population (slope of –2.18 ± 0.07) of M83. We show that this change of slope reflects a more fundamental change of the ‘truncation mass’ at the high-mass end of the distribution. This can be modelled as a Schechter function of slope –2 with an exponential cutoff mass ( M c ) that decreases significantly from the inner to the outer regions (from 4.00 to 0.25  x  10 5  M ) while the galactic M c  is 1.60  x  10 5  M . The trends in  and ICMF are consistent with the observed radial decrease of the (H 2 ), hence in gas pressure. As gas pressure declines, cluster formation becomes less efficient. We conclude that the host galaxy environment appears to regulate (1) the fraction of stars locked in clusters and (2) the upper mass limit of the ICMF, consistently described by a near-universal slope –2 truncated at the high-mass end.

Description: We present an end-to-end, two-phase model for the origin of globular clusters (GCs). In the model, populations of stellar clusters form in the high-pressure discs of high-redshift ( z  〉 2) galaxies (a rapid-disruption phase due to tidal perturbations from the dense interstellar medium), after which the galaxy mergers associated with hierarchical galaxy formation redistribute the surviving, massive clusters into the galaxy haloes, where they remain until the present day (a slow-disruption phase due to tidal evaporation). The high galaxy merger rates of z  〉 2 galaxies allow these clusters to be ‘liberated’ into the galaxy haloes before they are disrupted within the high-density discs. This physically motivated toy model is the first to include the rapid-disruption phase, which is shown to be essential for simultaneously reproducing the wide variety of properties of observed GC systems, such as their universal characteristic mass-scale, the dependence of the specific frequency on metallicity and galaxy mass, the GC system mass–halo mass relation, the constant number of GCs per unit supermassive black hole mass, and the colour bimodality of GC systems. The model predicts that most of these observables were already in place at z  = 1–2, although under rare circumstances GCs may still form in present-day galaxies. In addition, the model provides important constraints on models for multiple stellar populations in GCs by putting limits on initial GC masses and the amount of pristine gas accretion. The paper is concluded with a discussion of these and several other predictions and implications, as well as the main open questions in the field.

Description: The colour–magnitude diagrams of some intermediate-age (1–2 Gyr) star clusters show unexpectedly broad main-sequence turnoffs, raising the possibility that these clusters have experienced more than one episode of star formation. Such a scenario predicts the existence of an extended main-sequence turnoff (eMSTO) only in clusters with escape velocities above a certain threshold (〉15 km s –1 ), which would allow them to retain or accrete gas that eventually would fuel a secondary extended star formation episode. This paper presents a test of this scenario based on the study of the young and massive cluster NGC 7252: W3. We use the HST photometry from Wide Field Planetary Camera 2 and Wide Field Camera 3 images obtained with UV and optical filters, as well as MagE echellette spectrograph data from the Las Campanas Clay 6.5 m telescope, in order to construct the observed UV/optical Spectral energy distribution (SED) of NGC 7252: W3. The observations are then compared with synthetic spectra based on different star formation histories consistent with those of the eMSTO clusters. We find that the SED of this cluster is best fitted by a synthetic spectrum with a single stellar population of age $570^{+70}_{-62}$  Myr and mass $1.13^{+0.14}_{-0.13}\times 10^8$ M , confirming earlier works on NGC 7252:W3. We also estimate the lower limit on the central escape velocity of 193 km s –1 . We rule out extended star formation histories, like those inferred for the eMSTO clusters in the Magellanic Clouds, at high confidence. We conclude that the escape velocity of a cluster does not dictate whether a cluster can undergo extended periods of star formation.

Description: Detailed observations of the nearest star-forming regions in the Milky Way (MW) provide the ultimate benchmark for studying star formation. The extent to which the results of these Galaxy-based studies can be extrapolated to extragalactic systems depends on the overlap of the environmental conditions probed. In this paper, we compare the properties of clouds and star-forming regions in the MW with those in nearby galaxies and in the high-redshift Universe. We find that in terms of their baryonic composition, kinematics and densities, the clouds in the solar neighbourhood are similar to those in nearby galaxies. The clouds and regions in the Central Molecular Zone (CMZ, i.e. the inner 250 pc) of the MW are indistinguishable from high-redshift clouds and galaxies. The presently low star formation rate in the CMZ therefore implies that either (1) its gas represents the initial conditions for high-redshift starbursts or (2) some yet unidentified process consistently suppresses star formation over 10 8 yr time-scales. We conclude that the MW contains large reservoirs of gas with properties directly comparable to most of the known range of star formation environments and is therefore an excellent template for studying star formation across cosmological time-scales.

Description: Galactic scaling relations between the (surface densities of) the gas mass and the star formation (SF) rate are known to develop substantial scatter or even change form when considered below a certain spatial scale. We quantify how this behaviour should be expected due to the incomplete statistical sampling of independent star-forming regions. Other included limiting factors are the incomplete sampling of SF tracers from the stellar initial mass function and the spatial drift between gas and stars. We present a simple uncertainty principle for SF, which can be used to predict and interpret the failure of galactic SF relations on small spatial scales. This uncertainty principle explains how the scatter of SF relations depends on the spatial scale and predicts a scale-dependent bias of the gas depletion time-scale when centring an aperture on gas or SF tracer peaks. We show how the scatter and bias are sensitive to the physical size and time-scales involved in the SF process (such as its duration or the molecular cloud lifetime), and illustrate how our formalism provides a powerful tool to constrain these largely unknown quantities. Thanks to its general form, the uncertainty principle can also be applied to other astrophysical systems, e.g. addressing the time evolution of star-forming cores, protoplanetary discs or galaxies and their nuclei.

Description: Whilst young massive clusters (YMCs; M 10 4  M , age 100 Myr) have been identified in significant numbers, their progenitor gas clouds have eluded detection. Recently, four extreme molecular clouds residing within 200 pc of the Galactic Centre have been identified as having the properties thought necessary to form YMCs. Here, we utilize far-IR continuum data from the Herschel infrared Galactic Plane Survey (HiGAL) and millimetre spectral line data from the Millimetre Astronomy Legacy Team 90 GHz Survey to determine their global physical and kinematic structure. We derive their masses, dust temperatures and radii and use virial analysis to conclude that they are all likely gravitationally bound – confirming that they are likely YMC progenitors. We then compare the density profiles of these clouds to those of the gas and stellar components of the Sagittarius B2 Main and North protoclusters and the stellar distribution of the Arches YMC. We find that even in these clouds – the most massive and dense quiescent clouds in the Galaxy – the gas is not compact enough to form an Arches-like ( M = 2 10 4  M , R eff = 0.4 pc) stellar distribution. Further dynamical processes would be required to condense the resultant population, indicating that the mass becomes more centrally concentrated as the (proto)cluster evolves. These results suggest that YMC formation may proceed hierarchically rather than through monolithic collapse.

Description: We recently proposed that the star-forming potential of dense molecular clouds in the Central Molecular Zone (CMZ, i.e. the central few 100 pc) of the Milky Way is intimately linked to their orbital dynamics, potentially giving rise to an absolute-time sequence of star-forming clouds. In this paper, we present an orbital model for the gas stream(s) observed in the CMZ. The model is obtained by integrating orbits in the empirically constrained gravitational potential and represents a good fit ( $\chi _{\rm red}^2=2.0$ ) to the observed position–velocity distribution of dense ( n 〉 several 10 3 cm -3 ) gas, reproducing all of its key properties. The orbit is also consistent with observational constraints not included in the fitting process, such as the 3D space velocities of Sgr B2 and the Arches and Quintuplet clusters. It differs from previous, parametric models in several respects: (1) the orbit is open rather than closed due to the extended mass distribution in the CMZ, (2) its orbital velocity (100–200 km s -1 ) is twice as high as in previous models, and (3) Sgr A* coincides with the focus of the (eccentric) orbit rather than being offset. Our orbital solution supports the recently proposed scenario in which the dust ridge between G0.253+0.016 (‘the Brick’) and Sgr B2 represents an absolute-time sequence of star-forming clouds, of which the condensation was triggered by the tidal compression during their most recent pericentre passage. We position the clouds on a common timeline and find that their pericentre passages occurred 0.30–0.74 Myr ago. Given their short free-fall times ( t ff ~ 0.34 Myr), the quiescent cloud G0.253+0.016 and the vigorously star-forming complex Sgr B2 are separated by a single free-fall time of evolution, implying that star formation proceeds rapidly once collapse has been initiated. We provide the complete orbital solution, as well as several quantitative predictions of our model (e.g. proper motions and the positions of star formation ‘hotspots’). The paper is concluded with a discussion of the assumptions and possible caveats, as well as the position of the model in the Galactic context, highlighting its relation to large-scale gas accretion, the dynamics of the bar, the x 2 orbital family, and the origin of the Arches and Quintuplet clusters.

Description: The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of ≥10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H  i –H 2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse – once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments.

Description: We recently proposed that the star-forming potential of dense molecular clouds in the Central Molecular Zone (CMZ, i.e. the central few 100 pc) of the Milky Way is intimately linked to their orbital dynamics, potentially giving rise to an absolute-time sequence of star-forming clouds. In this paper, we present an orbital model for the gas stream(s) observed in the CMZ. The model is obtained by integrating orbits in the empirically constrained gravitational potential and represents a good fit ( $\chi _{\rm red}^2=2.0$ ) to the observed position–velocity distribution of dense ( n 〉 several 10 3 cm -3 ) gas, reproducing all of its key properties. The orbit is also consistent with observational constraints not included in the fitting process, such as the 3D space velocities of Sgr B2 and the Arches and Quintuplet clusters. It differs from previous, parametric models in several respects: (1) the orbit is open rather than closed due to the extended mass distribution in the CMZ, (2) its orbital velocity (100–200 km s -1 ) is twice as high as in previous models, and (3) Sgr A* coincides with the focus of the (eccentric) orbit rather than being offset. Our orbital solution supports the recently proposed scenario in which the dust ridge between G0.253+0.016 (‘the Brick’) and Sgr B2 represents an absolute-time sequence of star-forming clouds, of which the condensation was triggered by the tidal compression during their most recent pericentre passage. We position the clouds on a common timeline and find that their pericentre passages occurred 0.30–0.74 Myr ago. Given their short free-fall times ( t ff ~ 0.34 Myr), the quiescent cloud G0.253+0.016 and the vigorously star-forming complex Sgr B2 are separated by a single free-fall time of evolution, implying that star formation proceeds rapidly once collapse has been initiated. We provide the complete orbital solution, as well as several quantitative predictions of our model (e.g. proper motions and the positions of star formation ‘hotspots’). The paper is concluded with a discussion of the assumptions and possible caveats, as well as the position of the model in the Galactic context, highlighting its relation to large-scale gas accretion, the dynamics of the bar, the x 2 orbital family, and the origin of the Arches and Quintuplet clusters.