Fabio Bresolin

Accurate chemical compositions of star-forming regions provide a critical diagnostic tool for characterizing the star formation history and gas flows that regulate galaxy formation. However, the abundance discrepancy factor (ADF) between measurements derived from the “direct” optical electron temperature ( T _e ) method and those from recombination lines (RLs) introduces a ∼0.2 dex systematic uncertainty in the oxygen abundance. The degree of uncertainty for other elements is unknown. We conduct a comprehensive analysis of O ⁺⁺ and N ⁺ ion abundances using optical and far-infrared (far-IR) spectra of a star-forming region within the nearby dwarf galaxy Haro 3, which exhibits a typical ADF. Assuming homogeneous conditions, the far-IR emission indicates an oxygen abundance higher than that derived using the T _e method and consistent with the RL value, as expected from temperature fluctuations, whereas the far-IR nitrogen abundance is too large to be explained by temperature fluctuations. A two-phase analytical model reveals that differential dust obscuration associated with temperature inhomogeneity is likely required to explain all the emission-line ratios, and that the total oxygen metallicity of two phases is consistent with the RL metallicity. Our findings underscore the critical importance of resolving the cause of abundance discrepancies and understanding the biases between different metallicity methods. This work presents a promising methodology, and we identify further approaches to address the dominant sources of uncertainty.

Blue supergiant distances of nearby galaxies obtained with the flux-weighted gravity–luminosity relationship are used for a measurement of the zero-points of Tully–Fisher relationships at different photometric passbands. The Cousins I band and the infrared WISE bands W1 and W2 are investigated. The results are compared with previous work using Cepheid and tip of the red giant branch distances. No significant differences were encountered. This supports the large values of the Hubble constant greater than 73 km s ⁻¹ Mpc ⁻¹ found with the Tully–Fisher distance ladder work over the last decade. Applying blue supergiant distances on the I -band Tully–Fisher relation observations yields a Hubble constant H ₀ = 76.2 ± 6.2 km s ⁻¹ Mpc ⁻¹ . The large uncertainty is caused by the still relatively small blue supergiant galaxy sample size but will be reduced in future work.

The carbon-to-oxygen (C/O) abundance ratio is a valuable tracer of star formation history, as C and O enrichment occurs on different timescales. However, measurements based on ultraviolet (UV) collisionally excited lines and those based on optical recombination lines may be subject to biases from the abundance discrepancy factor (ADF), which is well established for oxygen but uncertain for carbon. We present precise UV-based measurements of gas-phase C ²⁺ /O ²⁺ ionic abundance in four H ii regions, which have prior optical-based measurements, combined with archival UV data for two additional H ii regions, in order to establish a reliable abundance scale and to investigate biases between the two methods. We find a clear ADF for the C ²⁺ ion, which is consistent with that of O ²⁺ , assuming a similar temperature structure in the zones of the nebula that these ions occupy. The C/O abundance derived from UV collisional lines and optical recombination lines is therefore also consistent to within 2+ /O ²⁺ for the standard T _e method. While the absolute C/H and O/H abundances are subject to large uncertainty from the ADF, our results establish that C/O abundances measured from these different methods can be reliably compared. Thus, we confirm the robustness of gas-phase C/O measurements for studying galaxy evolution and star formation timescales, including from rest-UV observations of high-redshift galaxies with JWST.

Blue supergiant distances of nearby galaxies obtained with the flux-weighted gravity-luminosity relationship are used for a measurement of the zero points of Tully-Fisher relationships at different photometric passbands. The Cousins I-band and the infrared WISE bands W1 and W2 are investigated. The results are compared with previous work using Cepheid and Tip-of-the-Red-Giant-Branch distances. No significant differences were encountered. This supports the large values of the Hubble constant greater than 73km/s/Mpc found with the Tully-Fisher distance ladder work over the last decade. Applying blue supergiant distances on the I-band Tully-Fisher relation observations yields a Hubble constant H0 = 76.2+/-6.2 km/s/Mpc. The large uncertainty is caused by the still relatively small blue supergiant galaxies sample size but will be reduced in future work.

We present a quantitative spectroscopic study of 13 blue supergiant stars in the Pinwheel Galaxy M101, based on data obtained with the Low Resolution Imaging Spectrometer available at the Keck I telescope. The average stellar metallicity decreases from ∼1.9 Z _⊙ near the center of the galaxy to ∼0.3 Z _⊙ at the optical outskirts. The galactocentric radial metallicity gradient is statistically consistent with previous studies of the gas-phase oxygen abundance from H ii regions using the direct method. The H ii region-based Cepheid metallicities used by A. G. Riess et al. in their determination of the Hubble constant H ₀ are in substantial agreement with our measurements. The direct method gas-phase metallicities of the 18 star-forming galaxies we have analyzed so far, when adjusted upward for a mean ∼0.15 dex oxygen dust depletion factor, are in good agreement with those we infer from the supergiants, over a factor of 50 in metallicity. From the same data, we derive an expression for the metal-dependent depletion of oxygen in photoionized nebulae. Utilizing the flux-weighted gravity–luminosity relationship (FGLR) of blue supergiants, we measure a distance to M101, D = 6.5 ± 0.2 Mpc (μ = 29.06 ± 0.08), which is within 1σ from determinations based on the tip of the red giant branch and Cepheids. With M101 as a nearby Type Ia supernova host and using the observed standardized B-band magnitude of the supernova, our FGLR distance yields an independent value of H ₀ = 72.5 ± 4.6 km s⁻¹ Mpc⁻¹.

We apply population synthesis techniques to analyze TYPHOON long slit spectra of the starburst barred spiral galaxy M83. The analysis covers a central square of 5′ side length. We determine the spatial distribution of dust through the analysis of reddening and extinction, together with star formation rates, ages, and metallicities of young and old stellar populations. For the first time, a spatial one-to-one comparison of metallicities derived from full spectral fitting techniques with those obtained from individual young stellar probes has been carried out. The comparison with blue supergiant stars, young massive star clusters, and super star clusters shows a high degree of concordance when wavelength coverage in the B band is available. The metallicity of the young population is supersolar and does not show a radial metallicity gradient along the investigated part of the disk, in agreement with our chemical evolution model. However, a notable decrease in metallicity is observed in a tightly confined region at the galaxy center, coinciding with circumnuclear orbits. We attribute this to matter infall either from the circumgalactic medium, a dwarf galaxy interloper, or, alternatively, to active-galactic-nucleus-interrupted chemical evolution. We confirm the presence of a dust cavity with a diameter of 260 pc close to the galaxy center. Dust absorption and molecular CO emission are spatially well correlated. We find an anticorrelation between R _V , the ratio of dust attenuation to reddening, and the emission strength of molecular species present in photodissociation regions. We confirm our results by using alternative fitting algorithms and stellar libraries.

As part of the SIGNALS (Star formation, Ionized Gas, and Nebular Abundances Legacy Survey) survey, which comprises a sample of approximately 40 nearby galaxies observed with the Fourier transform spectrometer SITELLE, we present a study of metal mixing in the spiral galaxy NGC 6946. Taking advantage of the blue sensitivity of our set-up, we measure the oxygen and nitrogen abundances of 638 H ii regions, and focus our analysis on the abundance fluctuations about the radial gradients. We detect an azimuthal variation of about 0.1 dex in these abundances across the north-east spiral arm, with the leading edge being more metal-poor than the trailing edge. This result aligns with galaxy simulations, where radial gas flows along the spiral arms lead to dilution on the leading edge and enrichment on the trailing edge, due to the presence of radial metallicity gradients. Our 2D analysis reveals that oxygen and nitrogen exhibit comparable spatial correlation scales, despite the different injection energies and distinct nucleosynthetic origins – core-collapse supernovae in the case of oxygen and primarily asymptotic giant branch stars for nitrogen. The observed similarity suggests that stellar processes drive these two elements into the interstellar medium over equivalent spatial scales.