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Molecular clouds (MCs) are active sites of star formation in galaxies, and their formation and evolution are largely affected by stellar feedback. This includes outflows and winds from newly formed stars, radiation from young clusters, and supernova explosions. High-resolution molecular line observations allow for the identification of individual star-forming regions and the study of their integrated properties. Moreover, state-of-the-art simulations are now capable of accurately replicating the evolution of MCs, including all key stellar feedback processes. We present¹³CO(2–1) synthetic observations of the STARFORGE simulations produced using the radiative transfer code RADMC-3D, matching the observational setup of the SEDIGISM survey. From these synthetic observations, we identified the population of MCs using hierarchical clustering and analysed them to provide insights into the interpretation of observed MCs as they evolve. The flux distributions of the post-processed synthetic observations and the properties of the MCs, namely, radius, mass, velocity dispersion, virial parameter, and surface density, are consistent with those of SEDIGISM. Both samples of MCs occupy the same regions in the scaling relation plots; however, the average distributions of MCs at different evolutionary stages do not overlap on the plots. This highlights the reliability of our approach in modelling SEDIGISM and suggests that MCs at different evolutionary stages contribute to the scatter in observed scaling relations. We study the trends in MC properties, morphologies, and fragmentation over time to analyse their physical structure as they form, evolve, and are destroyed. MCs appear as small diffuse cloudlets in early stages, and this is followed by their evolution to filamentary structures before being shaped by stellar feedback into 3D bubbles and getting dispersed. These trends in the observable properties of MCs are consistent with other realisations of simulations and provide strong evidence that clouds exhibit distinct morphologies over the course of their evolution. 

ABSTRACT In this study, we investigate how the merging process influences the radial variations of the specific star formation rate, star formation efficiency, and molecular gas fraction ($$\mathrm{f_{mol} }$$) in galaxies. We analyse 33 isolated galaxies and 34 galaxies in four different merger stages from pairs, merging galaxies, post-mergers, and merger remnants. Our sample is included in the EDGE-CALIFA survey, which provides spatially resolved optical integral-field unit and CO spectroscopy data. We show that, in comparison with the isolated sample, the mergers increase the molecular gas fraction non-uniformly across different galactocentric distances. Also, we find that the main driver (efficiency or molecular gas) of both enhanced and suppressed star formation changes independently of galactocentric radius and merger stage. However, efficiency appears to be the primary driver of variations in star formation (except during the merging stage), where we find an enhancement in star formation driven by the available fuel. Our results suggest that in interacting and merging galaxies, the efficiency plays a crucial role in the star formation variations throughout the galaxy, regardless of the available molecular gas content. 

Galaxy quenching, the intricate process through which galaxies transition from active star-forming states to retired ones, remains a complex phenomenon that requires further investigation. This study investigates the role of active galactic nuclei (AGNs) in regulating star formation by analyzing a sample of 643 nearby galaxies with redshifts between 0.005 and 0.03 from the Calar Alto Legacy Integral Field Area (CALIFA) survey. Galaxies were classified according to the Quenching Stages and Nuclear Activity (QueStNA) scheme, which categorizes them based on their quenching stage and the presence of nuclear activity. We further utilized the integrated Extragalactic Database for Galaxy Evolution (iEDGE), which combined homogenized optical integral field unit and CO observations. This allowed us to examine how AGNs influence the molecular gas reservoirs of active galaxies compared to their non-active counterparts at similar evolutionary stages. Our Kolmogorov–Smirnov andχ²tests indicate that the star formation property distributions and scaling relations of AGN hosts are largely consistent with those of non-active galaxies. However, AGN hosts exhibit systematically higher molecular gas masses across all quenching stages except for the quiescent nuclear ring stage. We find that AGN hosts follow the expected trends of non-active quenching galaxies, characterized by a lower star formation efficiency and molecular gas fraction compared to star-forming galaxies. Our results suggest that signatures of instantaneous AGN feedback are not prominent in the global molecular gas and star formation properties of galaxies. 

Abstract The quest for the mechanisms that halt star formation in galaxies is essential to understand their evolution. Here, we use the APEX-CALIFA survey, which includes 560 galaxies (0.005 <z< 0.08), so far the largest sample of galaxies in the nearby universe with both Integral Field Spectroscopic, Calar Alto Legacy Integral Field Area (CALIFA) and single-aperture millimeter observations, as well as the extended CALIFA sample (823 targets). Using these observations we derive (i) the deficit or excess of star formation for a given stellar mass with respect to the star formation main sequence (ΔSFMS), (ii) the gas fraction, and (iii) the star formation efficiency (SFE) for two apertures (central and global apertures using the APEX-CALIFA and CALIFA samples, respectively). We confirm the so-called “inside-out” quenching, that is, for quiescent galaxies the central values of ΔSFMS are usually smaller than those values derived from global measurements. However, for a given ΔSFMS we find that for retired galaxies the central gas fraction is larger in comparison to global measurements. Furthermore, the central SFE is significantly smaller in comparison to global counterparts. In general, in comparison to the global measurements, the deficit of star formation at the center of retired galaxies is primarily caused by the inefficiency to form new stars rather than the lack of molecular gas. We suggest that even though at the center of retired galaxies the gas fraction is larger, morphological structures could prevent that the molecular gas is transformed into new stars. Even more so in the outskirts of some retired galaxies with small gas fractions, star formation activity is still occurring. 

Studying galaxy evolution requires knowledge not only of the stellar properties, but also of the interstellar medium (in particular the molecular phase) out of which stars form, using a statistically significant and unbiased sample of galaxies. To this end, we introduce here the integrated Extragalactic Database for Galaxy Evolution (iEDGE), a collection of integrated stellar and nebular emission lines, and molecular gas properties from 643 galaxies in the local Universe. These galaxies are drawn from the CALIFA datasets, and are followed up in CO lines by the APEX, CARMA, and ACA telescopes. As this database is assembled from data coming from a heterogeneous set of telescopes (including IFU optical data and single-dish and interferometric CO data), we adopted a series of techniques (tapering, spatial and spectral smoothing, and aperture correction) to homogenise the data. Due to the application of these techniques, the database contains measurements from the inner regions of the galaxies and for the full galaxy extent. We used the database to study the fundamental star formation relationships between star formation rate (SFR), stellar mass (M_*), and molecular gas mass (M_mol) across galaxies with different morphologies. We observed that the diagrams defined by these quantities are bi-modal, with early-type passive objects well separated from spiral star-forming galaxies. Additionally, while the molecular gas fraction (f_mol=M_mol/M_*) decreases homogeneously across these two types of galaxies, the star formation efficiency (SFE=SFR/M_mol) in the inner regions of passive galaxies is almost two orders of magnitude lower compared to the global values. This indicates that inside-out quenching requires not only lowf_mol, but also strongly reduced SFE in the galactic centres. 

Galaxy evolution is largely driven by star formation activity or by the cessation of it, also called star formation quenching. In this paper, we present fundamental star formation scaling relations for groups of galaxies at different evolutionary stages. To do so, we used the integrated Extragalactic Database for Galaxy Evolution (iEDGE), which collects homogenised CO, optical continuum, and emission line information for 643 galaxies drawn from the CALIFA IFU dataset. By considering the patterns described by star-forming and retired regions across the galactic disc, we grouped the galaxies into different quenching stages using the emission line classification scheme,QueStNA. We observed that the molecular gas mass (M_mol) decreases from star-forming to retired systems and so does the molecular-to-stellar mass ratio (f_mol). In contrast, star formation efficiency (SFE) is largely constant in the quenching stages dominated by star formation and rapidly declines afterwards. Additionally, we observed that this rapid decline is more pronounced in the centre of the galaxies compared to the rest of the discs, reflecting the inside-out quenching often displayed by nearby galaxies. We also noticed that the relations betweenM_moland the stellar mass (M_*) become increasingly shallow with the quenching stages; however, the relations between the star formation rate andM_molsteepen when moving from star-forming to retired systems. We also observed that a three-dimensional relation between star formation rate,M_*, andM_molexists only for purely star-forming galaxies, while data points from other quenching groups are scattered across the parameter space. Taken together, these pieces of evidence indicate that the quenching of the galaxies cannot be explained solely by a depletion of the molecular gas and that a significant decrease in the SFE is necessary to retire the centre of the galaxies beyond the star formation green valley. 

Abstract In this study, we explore the impact of the galactic interaction/mergers on the central oxygen abundance. We analyze 234 star-forming galaxies included in the CALIFA survey with integrated molecular gas observations from the APEX millimeter telescope and the CARMA interferometer. This database has the most optical Integral Field Spectroscopy (IFS) data with CO data for yet, with integrated measurements within $$\sim 1~\mbox{$$R_{\mathrm{eff}}$$}$$. Our sample includes 125 isolated galaxies (control sample) and 109 galaxies in different merging stages. We find that despite whether the merging galaxies show an increase or decrease in their molecular gas fraction, the oxygen abundance does not vary significantly, in comparison to our control sample. Therefore the enhancement and suppression of oxygen abundance are similar in both isolated galaxies and interacting/merging galaxies. On the contrary, regardless of the merger stage (including isolated sample), galaxies that present an increase in their sSFR present a metallicity dilution. We suggest that both internal and external events affect the chemical composition of merging galaxies. 

Stars form in dense cores within molecular clouds, and newly formed stars influence their natal environments. How stellar feedback impacts core properties and evolution has been the subject of extensive investigation. We performed a hierarchical clustering (dendrogram) analysis of a STARFORGE (STAR FORmation in Gaseous Environments) simulation, modelling a giant molecular cloud to identify gas overdensities (cores) and study changes in their radius, mass, velocity dispersion, and virial parameter with respect to stellar feedback. We binned these cores on the basis of the fraction of gas affected by protostellar outflows, stellar winds, and supernovae and analysed the property distributions for each feedback bin. We find that cores that experience more feedback influence are smaller. Feedback notably enhances the velocity dispersion and virial parameter of the cores, more so than it reduces their radius. This is also evident in the linewidth–size relation, according to which cores in higher-feedback bins exhibit higher velocities than their similarly sized pristine counterparts. We conclude that stellar feedback mechanisms, which impart momentum to the molecular cloud, simultaneously compress and disperse the dense molecular gas. 

ABSTRACT The evolutionary sequence for high-mass star formation starts with massive starless clumps that go on to form protostellar, young stellar objects and then compact H ii regions. While there are many examples of the three later stages, the very early stages have proved to be elusive. We follow-up a sample of 110 mid-infrared dark clumps selected from the ATLASGAL catalogue with the IRAM telescope in an effort to identify a robust sample of massive starless clumps. We have used the HCO+ and HNC (1-0) transitions to identify clumps associated with infall motion and the SiO (2-1) transition to identity outflow candidates. We have found blue asymmetric line profile in 65 per cent of the sample, and have measured the infall velocities and mass infall rates (0.6–36 × 10−3 M⊙ yr−1) for 33 of these clumps. We find a trend for the mass infall rate decreasing with an increase of bolometric luminosity to clump mass, i.e. star formation within the clumps evolves. Using the SiO 2-1 line, we have identified good outflow candidates. Combining the infall and outflow tracers reveals that 67 per cent of quiescent clumps are already undergoing gravitational collapse or are associated with star formation; these clumps provide us with our best opportunity to determine the initial conditions and study the earliest stages of massive star formation. Finally, we provide an overview of a systematic high-resolution ALMA study of quiescent clumps selected that allows us to develop a detailed understanding of earliest stages and their subsequent evolution. 

ABSTRACT The low-J rotational transitions of 12CO are commonly used to trace the distribution of molecular gas in galaxies. Their ratios are sensitive to excitation and physical conditions in the molecular gas. Spatially resolved studies of CO ratios are still sparse and affected by flux calibration uncertainties, especially since most do not have high angular resolution or do not have short-spacing information and hence miss any diffuse emission. We compare the low-J CO ratios across the disc of two massive, star-forming spiral galaxies NGC 2903 and NGC 3627 to investigate whether and how local environments drive excitation variations at GMC scales. We use Atacama Large Millimeter Array (ALMA) observations of the three lowest-J CO transitions at a common angular resolution of 4 arcsec (190 pc). We measure median line ratios of $$R_{21}=0.67^{+0.13}_{-0.11}$$, $$R_{32}=0.33^{+0.09}_{-0.08}$$, and $$R_{31}=0.24^{+0.10}_{-0.09}$$ across the full disc of NGC 3627. We see clear CO line ratio variation across the galaxy consistent with changes in temperature and density of the molecular gas. In particular, towards the centre, R21, R32, and R31 increase by 35  per cent, 50  per cent, and 66  per cent, respectively, compared to their average disc values. The overall line ratio trends suggest that CO(3–2) is more sensitive to changes in the excitation conditions than the two lower J transitions. Furthermore, we find a similar radial R32 trend in NGC 2903, albeit a larger disc-wide average of $$\langle R_{32}\rangle =0.47^{+0.14}_{-0.08}$$. We conclude that the CO low-J line ratios vary across environments in such a way that they can trace changes in the molecular gas conditions, with the main driver being changes in temperature.