Statistical study of C$\mathsf{^{18}}$O dense cloud cores and star formation

The realization that most stars form in clusters, raises the question of whether star/planet formation are influenced by the cluster environment. The stellar density in the most prevalent clusters is the key factor here. Whether dominant modes of clustered star formation exist is a fundamental question. Using near-neighbour searches in young clusters Bressert et al. (2010) claim this not to be the case and conclude that star formation is continuous from isolated to densely clustered. We investigate under which conditions near-neighbour searches can distinguish between different modes of clustered star formation. Near-neighbour searches are performed for model star clusters investigating the influence of the combination of different cluster modes, observational biases, and types of diagnostic and find that the cluster density profile, the relative sample sizes, limitations in observations and the choice of diagnostic method decides whether modelled modes of clustered star formation are detected. For centrally concentrated density distributions spanning a wide density range (King profiles) separate cluster modes are only detectable if the mean density of the individual clusters differs by at least a factor of ~65. Introducing a central cut-off can lead to underestimating the mean density by more than a factor of ten. The environmental effect on star and planet formation is underestimated for half of the population in dense systems. A analysis of a sample of cluster environments involves effects of superposition that suppress characteristic features and promotes erroneous conclusions. While multiple peaks in the distribution of the local surface density imply the existence of different modes, the reverse conclusion is not possible. Equally, a smooth distribution is not a proof of continuous star formation, because such a shape can easily hide modes of clustered star formation (abridged)

We combine SAURON integral field data of a representative sample of local early-type, red sequence galaxies with Spitzer/Infrared Array Camera imaging in order to investigate the presence of trace star formation in these systems. With the Spitzer data, we identify galaxies hosting low-level star formation, as traced by polycyclic aromatic hydrocarbon emission, with measured star formation rates that compare well to those estimated from other tracers. This star formation proceeds according to established scaling relations with molecular gas content, in surface density regimes characteristic of disc galaxies and circumnuclear starbursts. We find that star formation in early-type galaxies happens exclusively in fast-rotating systems and occurs in two distinct modes. In the first, star formation is a diffuse process, corresponding to widespread young stellar populations and high molecular gas content. The equal presence of co- and counter-rotating components in these systems strongly implies an external origin for the star-forming gas, and we argue that these star formation events may be the final stages of (mostly minor) mergers that build up the bulges of red sequence lenticulars. In the second mode of star formation, the process is concentrated into well-defined disc or ring morphologies, outside of which the host galaxies exhibit uniformly evolved stellar populations. This implies that these star formation events represent rejuvenations within previously quiescent stellar systems. Evidence for earlier star formation events similar to these in all fast-rotating early-type galaxies suggests that this mode of star formation may be common to all such galaxies, with a duty cycle of roughly 1/10, and likely contributes to the embedded, corotating inner stellar discs ubiquitous in this population.

The stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) are studied to understand the correlation between star formation and cluster formation rates. We used the stellar data base of the OGLE II LMC survey and the star cluster catalogues. The observed distributions of stellar density in the colour−magnitude diagrams (CMDs) were compared with synthetic ones generated from stellar evolutionary models. By minimising the reduced χ 2 values, the star formation history of the regions were obtained in terms of star formation rates (SFR). All the regions were found to show large SFRs between the ages 500−2 Gyr with lower values for younger and older ages. A correlated peak in the cluster and SFRs is found for ages ∼1 Gyr, and for ages less than 100 Myr. Five of the six regions show significant cluster formation in the age range of 100−300 Myr, when the SFRs were found to be very low. This indicates anti-correlation between star and cluster formation rates for the 100−300 Myr age range. A possible reason may be that the stars are predominantly formed in clusters, whether bound or unbound, as a result of star formation during the above age range. The enhanced cluster formation rate in the 100−300 Myr age range could be correlated with the encounter of the LMC with the Small Magellanic Cloud, while the enhanced star and cluster formation at ∼1 Gyr does not correspond to any interaction. This could indicate that the star formation induced by interactions is biased towards group or cluster formation of stars.

Context. It is now well known that certain massive galaxies undergo enormous enhancements in their star formation rate (SFR) when they undergo major mergers. These enhancements can be as high as 100 times the SFR of unperturbed galaxies of the same stellar mass. Previous works have found that the size of this boost in star formation (SF) is related to the morphology of and the proximity to the companion. The same trend has also been observed for the fraction of active galactic nuclei (AGN), where galaxies that are closer together tend to have higher AGN fractions. Aims. We aim to analyse the SF enhancement and AGN fraction evolution during the merger process by using a more timeline-like merger sequence. Additionally, we aim to determine the relation between the SF enhancement in mergers and the morphology of the galaxies involved. Methods. Taking advantage of the stellar masses (M*) and SFRs of the ∼600 nearby isolated mergers obtained in our previous study, we calculated the distance of each of our galaxies from the star-forming main sequence (MS; specific SFR (sSFR)/sSFRMS), which werefer to as the SF mode. We then analysed how the SF mode varies during the merger process as a function of morphology and M*. Additionally, we analysed the AGN content of our mergers, using multiple diagnostics based on emission line ratios and WISE colours. Results. We observed that, overall, merging galaxies show an SF mode that is governed by their morphology. Spirals typically show high SF mode values, while highly disturbed (HD) galaxies are generally even more enhanced (median values of +0.8 dex and +1.08 dex above the MS, respectively). In contrast, elliptical and lenticular galaxies show the lowest SF modes, as expected. However, even they show SF enhancement compared to their unperturbed counterparts. For example, their median SF mode is just within the 1-sigma scatter of the MS, and this can occur even before the galaxies have coalesced. We observed a trend for the SF mode to gradually increase with increasing merger stage. We did not find a clear dependency of the observed AGN fraction on the merger stage for the majority of our classification methods. Conclusions. We find mergers can significantly enhance SF in galaxies of all morphologies. For early-type galaxies, this could suggest that some gas was present prior to the merger, which may be triggered to form stars by the tidal interaction. As the SF enhancement continues throughout the merger process, this suggests that the enhancement may be a long-lived event, contrary to the short starbursts seen in some models.

The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCH framework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCH by implementing the N-body code PETAR, thereby enabling TORCH to handle massive clusters forming from 106 M⊙ clouds with ≥105 individual stars. We present results from TORCH simulations of star clusters forming from 104, 105, and 106 M⊙ turbulent spherical gas clouds (named M4, M5, M6) of radius R = 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 101,2,3 M⊙ pc−2, after a free-fall time of tff = 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 are ϵ⋆ = M⋆/Mcloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies of ϵinst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 102 M⊙ pc−2) and massive (Mcloud ≥ 105 M⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars.

We analyse extinction maps of nearby Giant Molecular Clouds to forge a link between driving processes of turbulence and modes of star formation. Our investigation focuses on cloud structure in the column density range above the self shielding threshold of 1mag Av and below the star formation threshold -- the regime in which turbulence is expected to dominate. We identify clouds with shallow mass distributions as cluster forming. Clouds that form stars in a less clustered or isolated mode show a steeper mass distribution. Structure functions prove inadequate to distinguish between clouds of different star formation mode. They may, however, suggest that the turbulence in the average cloud is governed by solenoidal forcing. The same is found using the Delta-variance analysis which also indicates that clouds with a clustered mode of star formation show an enhanced component of compressive driving in the turbulent field. Thus, while star formation occurs in each cloud, independent of the turbulent driving mechanism, compressive forcing appears to be associated with the formation of stellar clusters.

The efficiency of star formation, defined as the ratio of the stellar to total (gas and stellar) mass, is observed to vary from a few percent in regions of dispersed star formation to about a third in cluster-forming cores. This difference may reflect the relative importance of magnetic fields and turbulence in controlling star formation. We investigate the interplay between supersonic turbulence and magnetic fields using numerical simulations, in a sheet-like geometry. We demonstrate that star formation with an efficiency of a few percent can occur over several gravitational collapse times in moderately magnetically subcritical clouds that are supersonically turbulent. The turbulence accelerates star formation by reducing the time for dense core formation. The dense cores produced are predominantly quiescent, with subsonic internal motions. These cores tend to be moderately supercritical. They have lifetimes long compared with their local gravitational collapse time. Some of the cores collapse to form stars, while others disperse away without star formation. In turbulent clouds that are marginally magnetically supercritical, the star formation efficiency is higher, but can still be consistent with the values inferred for nearby embedded clusters. If not regulated by magnetic fields at all, star formation in a multi-Jeans mass cloud endowed with a strong initial turbulence proceeds rapidly, with the majority of cloud mass converted into stars in a gravitational collapse time. The efficiency is formally higher than the values inferred for nearby cluster-forming cores, indicating that magnetic fields are dynamically important even for cluster formation.

We investigate the hypothesis that stars form in aggregates of binary systems and that the dynamical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field. We assume that the initial distribution of periods is flat in logP, where P is the orbital period in days, and 3<logP<7.5. We distribute 200 binaries in aggregates with half mass radii R corresponding to the range from tightly clustered to isolated star formation, and follow the subsequent evolution of the stellar systems by direct N-body integration. Hardening and softening of binary systems do not significantly increase the number of orbits with logP<3 and logP>7.5. After the cluster with R=0.8pc disintegrates we obtain a population which consists of about 60 per cent binary systems with a period distribution for logP>4 as is observed and in which the G-dwarf binaries have a mass ratio distribution which agrees with the observed distribution. This result indicates that the majority of Galactic field stars may originate from a clustered star formation mode. We invert the orbit depletion function and obtain an approximation to the initial binary star period distribution for star formation in the dominant mode cluster. Comparison with the measured distribution of orbits for pre-main sequence stars suggests that the initial distribution may not depend on the star formation environment. Inverse dynamical population synthesis suggests that the Galactic field stellar mass function may be related to the stellar density at birth in the most common, or dominant, mode of star formation.

Massive stars in our Galaxy are born predominantly within the dense cores of giant molecular clouds. They affect their environment very soon after a stellar core has formed through their large rate of ionizing photons and their strong stellar winds. Massive stars are formed in clusters with stellar densities n* ~ 104 stars/pc3 and sizes 0.2 — 0.4 pc, and seem to preferentially form near the central region of the cores. Disks have been found in several sources (see Cesaroni, this volume). Associated with massive stars also are bipolar molecular outflows, which have masses, mass loss rates, and energies that are factors of ~ 100 larger than those of low mass stars (see review of Churchwell 1999).An understanding of the physical processes that dominate during the early stages of formation of massive stars and their influence back on the molecular gas from which they formed requires a detailed knowledge of the physical conditions of the environment prior to and after the formation of the star. Here we present an abstract of an extensive review on this subject by Garay &amp; Lizano (1999). For compactness, most of the references have been omitted.

Cold gas is the raw material for star formation (SF) in galaxies. Owing to gravitational force, cold gas collapses into dense molecular cloud cores and will eventually continue to collapse and transform into stars. Moreover, SF feedback drives the formation and evolution of galaxies. A detailed understanding of the properties of molecular gas, the description of the physical relationship between SF and galaxies, and SF feedback in galaxies are extremely important studies today. Herein, we only summarize the physical laws of SF in extragalactic galaxies and discuss the relationship between SF and cold gas. In particular, with further physical understanding of star formation, we will summarize our series of related studies on the relationship between SF and dense molecular gas. From the overall global study of entire galaxies, distant external galaxies, and the spatially resolved decomposition of nearby galaxies down to the Galactic dense cloud cores, the linear relationship is valid with a span of 10 orders of magnitude. At even higher densities traced by higher- J rotation ladders of dense gas tracers, this 10-orders-of-magnitude correlation is again tightly maintained from entire galaxies to the spatial decomposition of the denser gas in nearby galaxies, spanning further with the Galactic dense cloud cores. We further discuss future development prospects of this research direction in the Atacama Large Millimeter/submillimeter Array (ALMA) era. The James Clerk Maxwell Telescope large project MALATANG provides an understanding of the large-scale distribution of extremely dense and warm molecular gas in nearby galaxies and probes the relationships among the various gas phases, the dense molecular gas, and SF in the centers of the galaxies and most of the inner spiral disks, ultimately elucidating the physical connection in the relationship between SF and dense molecular gas on different physical scales. With the advancement of ALMA/NOrthern Extended Millimeter Array observations and the investment in next-generation telescopes, we expect to finally obtain the physical properties and laws of dense molecular gas and SF on different physical scales, from the molecular cloud scale to different regions in galaxies (distant and/or nearby galaxies). Eventually, a breakthrough understanding of contemporary issues, such as multispectral diagnosis and local SF efficiency of galaxies, will be in sight.

Dense cores of molecular clouds are the basic units of isolated low-mass star formation. They have been observed extensively in various molecule lines and dust continuum with the aim of revealing their chemical and dynamical state. In a previous paper, we formulated a coupled dynamical and chemical model for data interpretation and carried out an initial investigation focusing on the effects of a magnetic field on the core dynamics and chemistry. Here, we update our chemical network and the treatment of magnetic field-matter coupling and explore the effects of changing various parameters, including the initial gas-phase metal abundances, adsorption energies, cosmic-ray ionization rate, sticking probability onto dust grains, cloud mass, as well as magnetic field strength. The model results are compared with the velocity field and column density distributions of CO, CS, CCS, NH3, N2H+, and HCO+ inferred observationally for the well-studied starless core L1544. We find that, in agreement with previous work, models with the so-called high metal abundances produce excessive CS and CCS by more than 2 orders of magnitude. Models of magnetized clouds with "low metal" and "mixed metal" (with a strong initial depletion of sulphur) abundances can fit the available data on L1544 reasonably well, with the low-metal model fitting somewhat better the chemical data (except for CS) and the mixed-metal model the velocity field. Taking into account of a newly recalculated rate for the neutral-neutral reaction S + CCH → CCS + H increases the abundance of CCS substantially, leading to a better agreement with observation for the mixed-metal model. We considered two sets of adsorption energies, compiled respectively by Aikawa et al. and Hasegawa & Herbst. Our results favor the former over the latter. For our standard models, we adopted a cosmic ionization rate of 1.3 × 10-17 s-1 and a sticking probability of 0.3. Increasing their values does not improve the model fits. Somewhat surprisingly, removing the magnetic support of the cloud leads to relatively modest changes in the peak column densities of the species except for CS. However, the spatial distributions of CS and CCS become more centrally concentrated than observed in L1544, and the infall speed is too large to be acceptable. This illustrates the need for both chemical and dynamical data to provide the tightest possible model constraints. A generic feature of our coupled dynamical and chemical model is that NH3 and, to a lesser extent, N2H+ are concentrated in the slowly contracting, central plateau region of the growing core, whereas CS and CCS are most abundant in the lower density envelope surrounding the plateau, which has a faster infall motion. The chemical differentiation offers an exciting possibility of directly probing the velocity field of core evolution leading to star formation.

We study the specific star formation rate (SSFR) and its evolution at z ≳ 4, in models of galaxy formation, where the star formation is driven by cold accretion flows. We show that constant star formation and feedback efficiencies cannot reproduce the observed trend of SSFR with stellar mass and its observed lack of evolution at z &amp;gt; 4. Model galaxies with log (M*) ≲ 9.5 M⊙ show systematically lower SSFRs by orders of magnitudes, while massive galaxies with M* ≳ 5 × 1010 M⊙ have up to an order of magnitude larger SSFRs, compared to recent observations by Stark et al. To recover these observations we apply an empirical star formation efficiency in galaxies that scales with the host halo velocity dispersion as ∝ 1/σ3 during galaxy mergers. We find that this modification needs to be of stochastic nature to reproduce the observations, i.e. only applied during mergers and not during accretion driven star formation phases. Our choice of star formation efficiency during mergers allows us to capture both, the boost in star formation at low masses and the quenching at high masses, and at the same time produce a constant SSFR–stellar mass relation at z ≳ 4 under the assumption that most of the observed galaxies are in a merger-triggered star formation phase. Our results suggest that observed high-z low-mass galaxies with high SSFRs are likely to be frequently interacting systems, which experienced bursts in their star formation rate and efficiency (mode 1), in contrast to low redshift z ≲ 3 galaxies which are cold accretion-regulated star forming systems with lower star formation efficiencies (mode 2).

We present new griffin project hydrodynamical simulations that model the formation of galactic star cluster populations in low-metallicity (Z = 0.00021) dwarf galaxies, including radiation, supernova, and stellar wind feedback of individual massive stars. In the simulations, stars are sampled from the stellar initial mass function (IMF) down to the hydrogen-burning limit of 0.08 M⊙. Mass conservation is enforced within a radius of 1 pc for the formation of massive stars. We find that massive stars are preferentially found in star clusters and follow a correlation set at birth between the highest initial stellar mass and the star cluster mass that differs from pure stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in the galactic tidal field according to mass-loss rates observed in nearby galaxies. Of the released stellar feedback, 60 per cent of the supernova material and up to 35 per cent of the wind material reside either in the hot interstellar medium (ISM) or in gaseous, metal-enriched outflows. While stellar winds (instantaneously) and supernovae (delayed) start enriching the ISM right after the first massive stars form, the formation of supernova-enriched stars and star clusters is significantly delayed (by &amp;gt;50 Myr) compared to the formation of stars and star clusters enriched by stellar winds. Overall, supernova ejecta dominate the enrichment by mass, while the number of enriched stars is determined by continuous stellar winds. These results present a concept for the formation of chemically distinct populations of stars in bound star clusters, reminiscent of multiple populations in globular clusters.

The star formation history of a galaxy is modulated by a plethora of internal processes and environmental conditions. The details of how these evolve and couple together are not fully understood yet. In this work, we study the effects that galaxy mergers and morphological transformations have on setting different modes of star formation at galactic scales and across cosmic time. We monitor the global properties of vintergatan, a 20 pc resolution cosmological zoom-in simulation of a Milky Way-type galaxy. Between redshifts 1 and 5, we find that major mergers trigger multiple starburst episodes, corresponding to a tenfold drop of the gas depletion time down to 100 Myr. Bursty star formation is enabled by the emergence of a galactic disc, when the rotational velocity of gas starts to dominate over its velocity dispersion. Coherent motions of gas then outweigh disordered ones, such that the galaxy responds to merger-induced forcings by redistributing large amounts of gas towards high densities. As a result, the overall star formation rate (SFR) is enhanced with an associated decrease in the depletion time. Before redshift 5, mergers were expected to be even more frequent. However, a more turbulent interstellar medium is incapable of reacting in such a collective manner so as to spark rapid star formation. Thus, a constant long depletion time of 1 Gyr is kept, along with a low, but gradually increasing SFR. After the last major merger at redshift 1, vintergatan spends the next 8 Gyr evolving secularly. It has a settled and adiabatically growing disc, and a constant SFR with gas depletion times of 1–2 Gyr. Our results are compatible with the observed rapid transition between different modes of star formation when galaxies leave the main sequence.

JWST has revealed a population of ``dormant'' galaxies at z&gt;5 that have recently halted their star formation and are characterized by weak emission lines and significant Balmer breaks. Until now, only four such galaxies have been reported at z&gt;5, three with low stellar masses, M_*&lt;10^9M_⊙ (so-called mini-quenched galaxies), and one massive quiescent galaxy with M_*=10^ 10.2 M_⊙; no such galaxy had been reported at intermediate masses. Here, we present a systematic search for dormant galaxies at 5&lt;z&lt;7.4 that halted star formation at least 10 Myr before the time of observation. To do this, we made use of all the publicly available NIRSpec prism data in the DAWN JWST Archive (DJA) and select galaxies with low Hα equivalent widths (EW_ &lt;50Å) and strong Balmer breaks (F_ ν,4200 /F_ ν,3500 &gt;1.4). We find 14 dormant galaxies with stellar masses ranging from 10^ 7.6 -10^ 10.5 , revealing an intermediate-mass population. By construction, these 14 sources are located about 1 dex below the star-forming main sequence. Their star formation histories suggest that they halted star formation between 10 and 25 Myr before the time of observation which, according to models, is comparable with the timescales of internally regulated bursts driving a ``breathing'' mode of star formation. Our results show that sim1% of the galaxies in the DJA are in a dormant phase of their star formation histories, and they span a wide stellar mass range. These galaxies can be empirically selected using only their spectral features in NIRSpec prism data.