Simthembile Dlamini

Multiple mechanisms are proposed for the formation of giant molecular clouds (GMCs) – from gravitational free-fall caused by self-gravity, to stellar feedback-driven gas compression. Both the galactic environment and galaxy conditions could play an additional role in enhancing the formation via their gas surface density and star formation activity. In this paper, we make use of a catalog of 108 466 GMCs identified by F770W PHANGS-JWST imaging across 66 galaxies at a homogenized resolution of 30 pc. We measure the mass spectra in various galactic regions, whose power-law slopes vary from −1.2 to −2.0. We then estimate the formation time of each cloud using a model in which GMCs form by multiple feedback compression, and find that clouds with masses ≤10 ⁵ M _⊙ form in 20 Myr on average, and more massive clouds (∼10 ^6 − 7 M _⊙ ) take up to 100 Myr. We also find that cloud formation proceeds most rapidly in the central regions of galaxies, with formation timescales that are typically shorter by ∼5 − 10 Myr compared to galactic disks. This effect is most pronounced in central molecular zones with enhanced star formation. This highlights the role of intense massive star formation, high molecular gas surface densities, and strong supersonic compressions in accelerating cloud formation. However, star formation is generally inefficient as the cloud lifetime is ∼1% of the molecular depletion time. The formation time of clouds is ∼0.1 dex longer than the free-fall time. This hints that magnetic fields, stellar feedback, or other mechanisms may prolong their formation instead of immediate free-fall collapse. This indicates a longevity of massive GMCs. The GMC ages also show only limited variation with galactocentric radius in both spiral and disk galaxies, suggesting that cloud formation proceeds similarly in these galaxy types.

We investigate the relationship between star formation and X-ray emission from the hot interstellar medium (ISM) on ∼kiloparsec scales in NGC 4254 (M99) by combining spatially resolved star formation histories (SFHs) and Bayesian X-ray spectral fitting. We measure subgalactic star formation rates (SFR) by modeling spectrophotometric UV-IR data with flexible SFHs, and we produce point-source-subtracted maps of the diffuse X-ray emission using Chandra data. We extract and fit the spectra of five regions selected by their SFR density Σ _SFR , deriving hot gas luminosities and plasma temperatures. We examine the subgalactic kT –Σ _SFR and L X gas − Σ SFR scaling relations in NGC 4254 and compare to predictions from simple models of the feedback into the ISM from core collapse supernovae (CCSNe). The hot gas emission from NGC 4254 is consistent with thermalization of ≈40%–50% of the energy from CCSNe in the ISM, and mass-loading of the CCSNe ejecta, which decreases as Σ SFR − 1 / 3 . Our optimized model implies a temperature and X-ray production efficiency that scale as k T = ( 0.7 2 − 0.18 + 0.26 keV ) Σ SFR 0.34 ± 0.10 and η = ( 0.0 3 − 0.01 + 0.02 ) Σ SFR 0.34 ± 0.18 , respectively, for Σ _SFR = 0.01–0.13 M _⊙ yr ⁻¹ kpc ⁻² . We also compare the properties of the hot ISM to other ISM phases using data from the PHANGS program. The diffuse X-ray emission of a given region is on average 200 times fainter than the H α emission, and we see evidence that the hot ISM is overpressurized compared to the large-scale dynamical equilibrium pressure of the galaxy, consistent with expansion of the hot ISM into the ambient medium.

Light reprocessed by dust grains emitting in the infrared enables the study of the physics at play in dusty embedded regions, where ultraviolet and optical wavelengths are attenuated. Infrared telescopes such as JWST have made it possible to study the earliest feedback phases, when stars are shielded by cocoons of gas and dust. Comprehending this phase is crucial for unravelling the effects of feedback from young stars that leads to their emergence and the dispersal of their host molecular clouds. Here we show that the transition from the embedded to the exposed phase of star formation is short ($< 4$ Myr) and sometimes almost absent ($< 1$ Myr) across a sample of 37 nearby star-forming galaxies covering a wide range of morphologies, from massive barred spirals to irregular dwarfs. The short duration of the dust-clearing timescales suggests a predominant role of pre-supernova feedback mechanisms in revealing newborn stars, confirming previous results on smaller samples and allowing, for the first time, a statistical analysis of their dependencies. We find that the timescales associated with mid-infrared emission at 21 μm, tracing a dust-embedded feedback phase, are controlled by a complex interplay between giant molecular cloud properties (masses and velocity dispersions) and galaxy morphology. We report relatively longer durations of the embedded phase of star formation in barred spiral galaxies, while this phase is significantly reduced in low-mass irregular dwarf galaxies. We discuss tentative trends with gas-phase metallicity, which may favor faster cloud dispersal at low metallicities.

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) structures that potentially trace molecular clouds, even CO-dark regions. For this paper, we identified ISM cloud structures in PHANGS-JWST 7.7 μm PAH emission maps for 66 galaxies, smoothed to a common physical resolution of 30 pc and at native resolution. We extracted 108 466 cloud structures in the 30 pc sample and 146 040 clouds in the native resolution sample. We then calculated their molecular properties following a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these clouds and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2 − 1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. We specifically used the homogenized sample for our analysis. Upon cross-matching the PAH clouds to the ALMA CO clouds at a homogenized resolution of 90 pc in 27 galaxies, we find that 41% of the PAH clouds are associated with a CO counterpart. We also show that the converted molecular cloud properties of the PAH clouds do not differ much when compared in different galactic environments. However, outside the central environment, the highest molecular mass surface density clouds are preferentially found in spiral arms. We further apply a lognormal fit to the mass spectra to an unprecedented extragalactic completeness limit of 2 × 10 ³ M _⊙ , and find that spiral arms contain the most massive clouds compared to other galactic environments. Our findings support the idea that spiral arm gravitational potentials foster the formation of high surface density clouds, and that lower surface density clouds form in the interarm regions. The cloud Σ _mol values show a decline of a factor of ∼1.5 − 2 toward the outer 2 − 3 R _e . However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of R _gal . Factors such as large-scale processes, galaxy types, and morphologies might influence the observed trends. We note that combining homogenized molecular properties of individual galaxies leads to the loss of information about the physical processes that are driving deviations in trends of those properties across different galactic environments. We published two catalogs at the CDS, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future studies of PAH clouds, molecular clouds, and star formation.

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an upcoming radio interferometric telescope designed to constrain dark energy through the 21 cm intensity mapping of baryon acoustic oscillations. Instrumental systematics must be controlled and carefully characterized to measure the 21 cm power spectrum with fidelity and achieve high-precision constraints on the cosmological parameters. The chromaticity of the primary beam is one such complicated systematic, which can leak the power of spectrally smooth foregrounds beyond the ideal horizon limits due to the complex spatial and spectral structures of the side lobes and the main lobe. This paper investigates the chromaticity of the HIRAX Stokes I primary beam and its impact on the accurate measurement of the 21 cm power spectrum. To investigate the effect of chromaticity on the 21 cm power spectrum, we present a physically motivated beam modeling technique that utilizes a flexible basis derived from traditional optics, allowing for the accounting of higher-order radial and azimuthal structures in the primary beam. We investigate the impact of imperfect knowledge of the main lobe and side lobes' chromaticity in the power spectrum space by subtracting a simple foreground model in simulated snapshot visibilities to recover the H i power spectrum. Additionally, we find that modeling up to the octupolar azimuthal order feature (fourth-order angular variation) in the primary beam is sufficient to reduce the leakage outside the wedge with minimal bias.

Context. Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals are built up and redistributed across galaxy disks over cosmic time. Aims. We use a catalog of 22 958 H II regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced when using only the [N II ] λ 5755 electron temperature as a proxy for temperature-based, direct method metallicities. Methods. Using 534 direct detections of the temperature sensitive [N II ] λ 5755 auroral line, we measured gradients in 15 of the galaxies. Leveraging our large catalog of individual H II regions, we carried out a stacking procedure in bins of the H II region [N II ] λ 6583 luminosity and radius to recover stacked radial gradients. Results. We found a good agreement between the metallicity gradients from the stacked spectra and those gradients from individual regions and those from strong-line methods. In addition, particularly in the stacked T _e [N II ] measurements, some galaxies show very low (< 0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examined the individual high confidence (S/N > 5) outliers and identified 13 regions across nine galaxies with anomalously low metallicities, although this is not strongly reflected in the strong-line method metallicities. By stacking arm and interarm regions, we found no systematic evidence for offsets in metallicity between these environments, suggesting that enrichment within spiral arms is due to very localized processes. Conclusions. This work demonstrates the potential to systematically exploit the single [N II ] λ 5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong-line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.

Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA ¹² CO(2–1) observations at 7 pc (0 . ″ 4) spatial resolution across a 1.4 kpc × 5.6 kpc ( 1 . ′ 3 × 1 . ′ 3 ) region located in the disk of the nearby ( D = 3.5 Mpc), massive, star-forming galaxy NGC 253. We decompose this emission with a hierarchical, multiscale dendrogram algorithm to identify 2463 structures with deconvolved sizes ranging from ∼3 to 300 pc, complete to a limiting mass of 10 ⁴ M _⊙ . By comparing the virial parameter of these structures against physical properties including size, mass, surface density, velocity dispersion, and hierarchical position, we carry out a comprehensive search for a preferred scale at which gravitationally bound structures emerge. Ultimately, we do not identify evidence of an emergent scale for bound objects in our data, nor do we find a significant correlation between the virial parameter and structure sizes. These findings suggest that simple observational estimates of gravitational binding cannot be used to define molecular clouds and emphasize the need for multiscale approaches to characterize the ISM.

We characterize two candidate cool galactic outflows in two relatively low-mass, highly inclined Virgo cluster galaxies: NGC 4424 and NGC 4694. Previous analyses of observations using the Atacama Large Millimeter/submillimeter Array carbon monoxide (CO) line emission maps did not classify these sources as cool outflow hosts. Using new high-sensitivity, high-spatial-resolution, JWST mid-infrared photometry in the polycyclic aromatic hydrocarbon (PAH)–tracing F770W band, we identify extended structures present off of the stellar disk. The identified structures are bright in the MIRI F770W and F2100W bands, suggesting they include PAHs as well as other dust grains. As PAHs have been shown to be destroyed in hot, ionized gas, these structures are likely to be outflows of cool (T ≤ 10⁴ K) gas. This work represents an exciting possibility for using mid-infrared observations to identify and measure outflows in lower-mass, lower star formation galaxies.