Galaxy formation and evolution

Abstract:

Dust is a fundamental component of the interstellar medium within galaxies, as dust grains are highly efficient absorbers of ultraviolet (UV) and optical photons. Accurately quantifying this obscuration is crucial for interpreting galaxy spectral energy distributions (SEDs). The extinction curves in the Milky Way (MW) and Large Magellanic Cloud exhibit a strong feature known as the 2175 Å UV bump, most often attributed to small carbonaceous dust grains. This feature was recently detected in faint galaxies out to , suggesting rapid formation channels. Here, we report the detection of a strong UV bump in a luminous Lyman-break galaxy at , GNWY-7379420231, through observations taken as part of the NIRSpec Wide GTO survey. We fit a dust attenuation curve that is consistent with the MW extinction curve within , in a galaxy just Myr after the big bang. From the integrated spectrum, we infer a young mass-weighted age ( Myr) for this galaxy, however spatially resolved SED fitting unveils the presence of an older stellar population ( Myr). Furthermore, morphological analysis provides evidence for a potential merger. The underlying older stellar population suggests the merging system could be pre-enriched, with the dust illuminated by a merger-induced starburst. Moreover, turbulence driven by stellar feedback in this bursty region may be driving polycyclic aromatic hydrocarbon formation through top-down shattering. The presence of a UV bump in GNWY-7379420231 solidifies growing evidence for the rapid evolution of dust properties within the first billion years of cosmic time.

Abstract:

Recent JWST mid-infrared (mid-IR) images, tracing polycyclic aromatic hydrocarbons (PAHs) and dust continuum emission, provide detailed views of the interstellar medium (ISM) in nearby galaxies. Leveraging PHANGS-JWST Cycle 1 and PHANGS-MUSE data, we measure the PAH and dust continuum emission lifetimes of gas clouds across 17 nearby star-forming galaxies by analyzing the relative spatial distributions of mid-IR (7.7–11.3 μm) and Hα emission at various scales. We find that the mid-IR emitting timescale of gas clouds in galactic disks (excluding centers) ranges from 10–30 Myr. After star formation is detected in Hα, mid-IR emission persists for 3–7 Myr during the stellar feedback phase, covering 70%–80% of the Hα emission. This significant overlap is due to intense radiation from star-forming regions, illuminating the surrounding PAHs and dust grains. In most galaxies, the mid-IR time-scale closely matches the molecular cloud lifetime measured with CO. Although mid-IR emission is complex, as influenced by ISM distribution, radiation, and abundances of dust and PAHs, the similarity between the two timescales suggests that once gas clouds form with compact mid-IR emission, they quickly provide sufficient shielding for stable CO formation. This is likely due to our focus on molecular gas-rich regions of galaxies with near-solar metallicity. Finally, we find that the mid-IR emitting timescale is longer in galaxies with well-defined H ii regions and less structured backgrounds, allowing photons to more efficiently heat the ambient ISM surrounding the H ii regions, rather than contributing to diffuse emission. This suggests that the shape of the ISM also influences mid-IR emission.

Abstract:

We present high-resolution ( or pc) Atacama Large Millimeter/sub-millimeter Array CO(J = 2–1) observations of the spiral galaxy NGC 5064. Our study identifies 478 molecular clouds, of which 387 are resolved both spatially and spectrally. These clouds exhibit similarities to those of the Milky Way in terms of their sizes, molecular gas masses, velocity dispersions, velocity gradients, and Larson relations. However, the NGC 5064 clouds stand out with slightly higher gas mass surface densities, lower virial parameters ( assuming a standard conversion factor cm (K km s; for a lower conversion factor of cm (K km s), and an unusually high fraction of retrograde rotation (). Retrograde clouds are 18 per cent larger, 58 per cent more massive, 15 per cent more turbulent and have 17 per cent larger gas mass surface densities than prograde clouds. The velocity gradients in the clouds seem to arise from turbulence rather than cloud’s intrinsic rotation or large-scale galaxy rotation. Cloud–cloud collisions provide the most plausible explanation for the elevated retrograde fraction, though further investigation is needed to confirm this scenario. Projection effects due to the galaxy’s high inclination () may further enhance the apparent retrograde fraction. Confirmation using less inclined systems is essential to determine whether the observed dominance of retrograde rotation reflects a genuine physical phenomenon or is significantly shaped by projection effects.

Abstract:

The young neutron star X-ray binary, Cir X-1, resides within its natal supernova remnant and experiences ongoing outbursts every 16.5 d, likely due to periastron passage in an eccentric orbit. We present the deepest ever radio image of the field, which reveals relativistic jet-punched bubbles that are aligned with the mean axis of the smaller scale jets observed close to the X-ray binary core. We are able to measure the minimum energy for the bubble, which is around = erg. The nature and morphological structure of the source were investigated through spectral index mapping and numerical simulations. The spectral index map reveals a large fraction of the nebula’s radio continuum has a steep slope, associated with optically thin synchrotron emission, although there are distinct regions with flatter spectra. Our data are not sensitive enough to measure the spectral index of the protruding bubbles. We used the pluto code to run relativistic hydrodynamic simulations to try and qualitatively reproduce the observations with a combined supernova-plus-jet system. We are able to do so using a simplified model in which the asymmetrical bubbles are best represented by supernova explosion which is closely followed (within 100 yr) by a phase of very powerful jets lasting less than 1000 yr. These are the first observations revealing the initial breakout of neutron star jets from their natal supernova remnant, and further support the scenario in which Cir X-1 is a younger relation of the archetypal jet source SS433.

Abstract:

CO(1–0) and CO(2–1) are commonly used as bulk molecular gas tracers. The CO line ratios (especially CO(2–1)/CO(1–0)–R21) vary within and among galaxies, yet previous studies on R21 and alike often rely on measurements constructed by combining data from facilities with substantial relative calibration uncertainties that have the same order as physical line ratio variations. Hence, robustly determining systematic R21 variations is challenging. Here, we compare CO(1–0) and CO(2–1) mapping data from ALMA for 14 nearby galaxies, at a common physical resolution of 1.7 kpc. Our data set includes new ALMA (7 m+TP) CO(1–0) maps of 12 galaxies. We investigate R21 variation to understand its dependence on global galaxy properties, kiloparsec-scale environmental factors, and its correlation with star formation rate (SFR) surface density and metallicity. We find that the galaxy-to-galaxy scatter is 0.05 dex. This is lower than previous studies, which reported over 0.1 dex variation, likely reflecting significant flux calibration uncertainties in single-dish surveys. Within individual galaxies, R21 has a typical mean value of ∼0.64 and 0.1 dex variation, with an increase to ∼0.75 toward galactic centers. We find strong correlations between R21 and various galactic parameters, particularly SFR surface density, which shows a power-law slope of 0.10–0.11 depending on the adopted binning/fitting methods. Our findings suggest that, for studies covering main-sequence galaxy samples, assuming a fixed R21 = 0.64 does not significantly bias kiloparsec-scale molecular gas mass estimates from CO(2–1). Instead, systematic uncertainties from flux calibration and the CO-to-H2 conversion factor account for more systematic scatter of CO-derived molecular gas properties.