Search for: All records

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. 

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. 

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 In this study, we explore the impact of the galactic interaction/mergers on the central oxygen abundance. We analyse 234 star-forming galaxies included in the Calar Alto Legacy Integral Field Area survey with integrated molecular gas observations from the Atacama Pathfinder EXperiment millimeter telescope and the CARMA interferometer. This database has the most optical integral field spectroscopy data with CO data for yet, with integrated measurements within $$\sim 1~{R_{\rm{eff}}}$$. 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 specific star formation rate present a metallicity dilution. We suggest that both internal and external events affect the chemical composition of merging galaxies. 

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. 

Nitrogen hydrides such as NH3 and N2H+ are widely used by Galactic observers to trace the cold dense regions of the interstellar medium. In external galaxies, because of limited sensitivity, HCN has become the most common tracer of dense gas over large parts of galaxies. We provide the first systematic measurements of N2H+ (1-0) across different environments of an external spiral galaxy, NGC 6946. We find a strong correlation (r > 0.98, p < 0.01) between the HCN (1-0) and N2H+ (1-0) intensities across the inner ∼8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N2H+ (1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions (fdense). We measure an average intensity ratio of N2H+ (1-0)/HCN (1-0) = 0.15 ± 0.02 over our set of five IRAM-30m pointings. These trends are further supported by existing measurements for Galactic and extragalactic sources. This narrow distribution in the average ratio suggests that the observed systematic trends found in kiloparsec-scale extragalactic studies of fdense and the efficiency of dense gas (SFEdense) would not change if we employed N2H+ (1-0) as a more direct tracer of dense gas. At kiloparsec scales our results indicate that the HCN (1-0) emission can be used to predict the expected N2H+ (1-0) over those regions. Our results suggest that, even if HCN (1-0) and N2H+ (1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other. 

ABSTRACT ATLASGAL is an 870-µm dust survey of 420 deg2 the inner Galactic plane and has been used to identify ∼10 000 dense molecular clumps. Dedicated follow-up observations and complementary surveys are used to characterize the physical properties of these clumps, map their Galactic distribution, and investigate the evolutionary sequence for high-mass star formation. The analysis of the ATLASGAL data is ongoing: We present an up-to-date version of the catalogue. We have classified 5007 clumps into four evolutionary stages (quiescent, protostellar, young stellar objects and H ii regions) and find similar numbers of clumps in each stage, suggesting a similar lifetime. The luminosity-to-mass (Lbol/Mfwhm) ratio curve shows a smooth distribution with no significant kinks or discontinuities when compared to the mean values for evolutionary stages indicating that the star formation process is continuous and that the observational stages do not represent fundamentally different stages or changes in the physical mechanisms involved. We compare the evolutionary sample with other star formation tracers (methanol and water masers, extended green objects and molecular outflows) and find that the association rates with these increases as a function of evolutionary stage, confirming that our classification is reliable. This also reveals a high association rate between quiescent sources and molecular outflows, revealing that outflows are the earliest indication that star formation has begun and that star formation is already ongoing in many of the clumps that are dark even at 70 µm. 

Context.Recently, sensitive wide-bandwidth receivers in the millimetre regime have enabled us to combine large spatial and spectral coverage for observations of molecular clouds. The resulting capability to map the distributions of lines from many molecules simultaneously yields unbiased coverage of the various environments within star-forming regions. Aims.Our aim is to identify the dominant molecular cooling lines and characteristic emission features in the 1.3 mm window of distinct regions in the northern part of the Orion A molecular cloud. By defining and analysing template regions, we also intend to help with the interpretation of observations from more distant sources which cannot be easily spatially resolved. Methods.We analyse an imaging line survey covering the area of OMC-1 to OMC-3 from 200.2 to 281.8 GHz obtained with the PI230 receiver at the APEX telescope. Masks are used to define regions with distinct properties (e.g. column density or temperature ranges) from which we obtain averaged spectra. Lines of 29 molecular species (55 isotopologues) are fitted for each region to obtain the respective total intensity. Results.We find that strong sources like Orion KL have a clear impact on the emission on larger scales. Although not spatially extended, their line emission contributes substantially to spectra averaged over large regions. Conversely, the emission signatures of dense, cold regions like OMC-2 and OMC-3 (e.g. enhanced N₂H⁺emission and low HCN/HNC ratio) seem to be difficult to pick up on larger scales, where they are eclipsed by signatures of stronger sources. In all regions, HCO⁺appears to contribute between 3 and 6% to the total intensity, the most stable value for all bright species. N₂H⁺shows the strongest correlation with column density, but not with typical high-density tracers like HCN, HCO⁺, H₂CO, or HNC. Common line ratios associated with UV illumination, CN/HNC and CN/HCO⁺, show ambiguous results on larger scales, suggesting that the identification of UV illuminated material may be more challenging. The HCN/HNC ratio may be related to temperature over varying scales.