Simone Bianchi

Aims. We investigate the relation between molecular gas and dust extinction on kiloparsec scales. Methods. We analysed 15 253 spaxels across 51 nearby galaxies with CO(1–0) measurements and optical spectroscopy from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey. Dust extinction was traced by the Balmer decrement (H α /H β ), while molecular gas was traced by the CO(1–0) line intensity ( I _CO ). Results. Focusing on the 12 173 spaxels classified as star-forming, we found a significant and robust correlation between H α /H β and I _CO (Pearson r = 0.61, Spearman ρ = 0.59). This correlation becomes even stronger when considering deprojected quantities ( r = 0.65, ρ = 0.64), revealing a close connection between the effective V -band optical depth ( τ _V ) inferred from the Balmer decrement and the molecular gas surface density (Σ _mol ), derived assuming a constant CO-to-H ₂ conversion factor. The observed Σ _mol – τ _V relation is sub-linear, deviating from the expectation for a simple foreground-screen geometry and suggesting a more complex distribution of dust, molecular gas, and emitting regions, and/or variations in the CO-to-H ₂ conversion factor. Existing literature prescriptions fail to reproduce the observed distribution over the full extinction range. We parametrized this relation and derived a new empirical calibration linking Σ _mol and τ _V . The relation is largely independent of galaxy stellar mass, star formation rate, gas-phase metallicity, morphology, and specific star formation rate, but it shows a secondary dependence primarily on inclination, highlighting the role of geometry. The relation becomes less tight in both composite and active galactic nucleus (AGN)-dominated regions, which exhibit a substantially larger scatter than star-forming regions. Using the independent EDGE–CALIFA dataset, we show that the ALMaQUEST calibration predicts Σ _mol with a scatter of ∼0.3 dex and no significant systematic offset (−0.01 dex), demonstrating its applicability to independent kiloparsec-scale datasets. Conclusions. Our calibration thus provides a practical empirical method for estimating molecular gas surface density from optical spectroscopy alone, without requiring direct CO observations.

Context. Understanding the interstellar medium (ISM) requires high-resolution, multi-component mapping to capture its complex physical structure. Nearby spiral galaxies, with their abundant and diverse ISM, provide an ideal laboratory for such a comprehensive analysis at sub-galactic scales. Aims. We investigate dust-to-gas and dust-to-metal ratios (DGR and DMR) as functions of the gas-phase metallicity ( Z ) on spatial scales ranging from 0.6 to 2.3 kpc, in a sample of ten nearby spiral galaxies spanning more than an order of magnitude in stellar mass (9.7 ≤ log( M _* / M _⊙ )≤11.0), star formation rate (SFR, ∼0.3–3 M _⊙ yr ⁻¹ ), and metallicity ranging from 8.3 ≲ 12 + log(O/H) ≲ 8.8. We explore how the DGR– Z and DMR– Z relations are shaped by the assumptions behind the CO-to-H ₂ conversion factor ( α _CO ). Methods. We homogeneously combined maps of dust, atomic gas, molecular gas, and metallicity. Motivated by the diversity in L _{CO(1 − 0)} /SFR ratios and metallicity across our sample, we introduced a hybrid α _CO prescription to distinguish between CO-bright and CO-dark regimes. The derived DGR– Z and DMR– Z relations were compared with other global and resolved observational results, and with the predictions of dust and chemical evolution models. Results. Both DGR– Z and DMR– Z relations are dependent on the adopted α _CO prescription, and no single α _CO can reproduce the properties of the entire sample, motivating the use of a hybrid approach. The DGR increases with metallicity, spanning ∼1 dex across the sampled range, while the DMR remains approximately constant at log(DMR) = − 0.53 ± 0.13, implying that ∼30% of metals are locked into dust grains. This flat behaviour indicates an evolved dust phase where efficient ISM grain growth drives a saturation regime balancing dust formation and destruction.

Context . Observations of the rest-frame far-infrared (far-IR) emission of galaxies suggest a mild increase in dust temperature T _dust with redshift. However, constraining T _dust in high-redshift systems remains challenging due to limited sampling of the far-IR spectral energy distribution (SED). Aims . We present and discuss the redshift evolution of T _dust , predicted by a cosmological galaxy evolution simulation with dust treatment, and interpret its dependence on other galaxy physical properties. Methods . We used a semi-analytic model of galaxy formation that includes an explicit treatment of dust, post-processed with radiative transfer. Dust temperatures were derived by applying modified blackbody SED fitting to the simulated galaxies, mirroring the methodology adopted in most observational studies. Results . The dust temperature of our simulated galaxies increases with redshift, in broad agreement with observational results. A feature-importance analysis reveals that the star formation rate surface density Σ _SFR and the dust-to-gas ratio (DTG) are the main drivers of dust temperature, tracing the intensity of the interstellar radiation field and the optical depth of warm molecular clouds, respectively. Galaxies with higher star formation rate surface densities and lower DTGs – common conditions at high- z – are associated with warmer dust. We provide a simple relation to estimate DTG from Σ _SFR , T _dust , and redshift. Variations in dust grain size and chemical composition have a negligible impact on T _dust . Our results are particularly relevant to the study of dust properties in observations of high-z galaxies, where far-IR dust emission is not fully sampled.

Aims. DustPedia and Local Volume Legacy (LVL) are two samples representative of the local galaxy population, including in total ∼1000 unique objects of all morphological types, with a wide range of stellar masses and star formation activity, and a spectral coverage from the ultraviolet to the far-infrared. The purpose of this work is to show that these samples cover two complementary ranges in stellar mass and galaxy morphology, making them an ideal set for constraining the dominant processes in the evolution of the galactic dust content. Methods. Using the multi-wavelength data provided by the two surveys, we fitted the galaxies’ spectral energy distribution and estimated their physical properties, in particular the stellar mass, M _* , the specific dust mass, sM _dust = M _dust / M _* , and the specific star formation rate, sSFR = SFR/ M _* . Results. By combining DustPedia and LVL, we highlight that the trend of log ₁₀ ( sM _dust ) with log ₁₀ ( M _* ) is not monotonic. Thanks to a large number of objects across a wide range of M _* , we have been able to fit two smoothly joined linear correlations: a positive one for log ₁₀ ( M _* /M _⊙ )≲9.5 (a range populated mostly by LVL late spirals and irregulars), and a negative one for larger-mass, mainly DustPedia, spirals (with early-type galaxies being distinct and more dispersed in the same mass regime). For log ₁₀ ( M _* /M _⊙ ) > 9.5, we confirm a strong correlation between sM _dust and sSFR; dwarf galaxies, instead, lie below this trend, showing a large scatter of sM _dust for −10.5 < log ₁₀ (sSFR/yr ⁻¹ ) < − 9.0. By using chemical evolution models we find that the observed log ₁₀ ( sM _dust )–log ₁₀ ( M _* ) and log ₁₀ ( sM _dust )–log ₁₀ (sSFR) trends can be interpreted mainly by variations in the initial gas mass budget and the galaxy ages, respectively. Low-mass Sm-Irr galaxies with low sM _dust and a high sSFR can only be reproduced by the models by assuming a highly efficient photofragmentation rate of large grains, and/or low grain growth in clouds.

Supernovae (SNe) are believed to be the dominant sources of dust production at high redshift. However, the reverse shock generated by the interaction of the SN forward shock and the interstellar medium (ISM) can significantly reduce the mass of newly formed dust in SN ejecta. This study quantifies the mass, composition, and grain size distribution of surviving dust after the passage of the reverse shock using the GRASHrev model. Our analysis covers a grid of SN models with progenitor masses of 13 M _⊙ ≤ m _⋆ ≤ 120 M _⊙ and metallicity −3 ≤ [Fe/H] ≤ 0, and we explore, for the first time, the effect of stellar rotation, considering two initial velocities v = 0 and 300 km s ⁻¹ . The SN explosions are assumed to occur in a uniform ISM with densities n _ISM = 0.05, 0.5, and 5 cm ⁻³ . We find that the larger grains (≳10 nm) are more resistant to destruction by the reverse shock, with amorphous carbon dominating the surviving dust mass in most models. The surviving dust mass decreases with increasing ISM density. For nonrotating progenitors, the maximum mass of dust surviving the passage of the reverse shock is ≃0.02 M _⊙ , and it is released by SN explosions of a 120 M _⊙ progenitor with [Fe/H] = 0 in the ISM density 0.5 cm ⁻³ , corresponding to ≃4% of the initial dust mass before the passage of the reverse shock. Similarly, among the rotating progenitors, the maximum surviving mass fraction is ≃5%, with a final dust mass of ≃0.03 M _⊙ in [Fe/H] = −1 models. Although the reverse shock has a strong destructive impact, our results indicate that on very short timescales of ≲30 Myr since the onset of star formation, SNe can enrich the ISM with carbonaceous grains ranging in size from approximately 1 nm to 100 nm (up to ≃1 μm in nonrotating models). This is especially notable given the recent detection of the 2175 Å UV extinction bump in galaxies at z > 6, suggesting the early presence of such dust.

Aims. The purpose of the present study is to relate the optical attenuation inferred by the Balmer decrement, A _{V , BD} , and by the spectral energy distribution (SED) fitting, A _{V , SED} , to the dust distribution and gas surface density throughout the disc of galaxies, down to scales smaller than 0.5 kpc. Methods. We investigated five nearby Herschel -detected star-forming spiral galaxies with available far-ultraviolet to sub-millimetre observations, along with atomic and molecular gas surface density maps and optical integral-field spectroscopic data. We used the CIGALE SED-fitting code to map the dust mass surface density (Σ _dust ) and A _{V , SED} of different stellar populations. For each pixel, we independently estimated the attenuation from the Balmer decrement. Results. We find that both Σ _dust and A _{V , BD} are better at tracing the molecular and total gas mass surface density than the atomic gas. Since regions sampled in this study have high molecular fractions, atomic gas surface densities, indicative of molecular gas shielding layers, decrease as the mean dust-to-gas ratio increases from galaxy to galaxy. The fitted attenuation towards the young stellar population, A _{V , SED} ^young , is in good agreement with A _{V , BD} . It can then be used to trace the attenuation in star-forming galaxies for which integral-field observations are not available. We estimate the ratio of A _{V , BD} over the total stellar A _{V , SED} and find it slightly larger than what has been found in previous studies. Finally, we investigate which dust distribution better reproduces the estimated A _{V , BD} and A _{V , SED} . We find that the attenuation towards old stars is consistent with the expectations for a standard galactic disc, where the stellar and dust distributions are mixed, while A _{V , BD} and A _{V , SED} ^young are between the values expected for a foreground dust screen and the mixed configuration.

The Solar Tower, a characteristic feature of Arcetri hill and emblem of the Observatory’s transition to astrophysics studies, was inaugurated one hundred years ago, on 22 June 1925. We celebrate the anniversary by transcribing the account of the inauguration and the speech given by the director of the Observatory Giorgio Abetti.

Context. Dust is a fundamental component of the interstellar medium (ISM) and plays a critical role in shaping galaxy evolution. Dust grains influence the ISM by cooling the gas, altering its chemistry, absorbing stellar radiation, and re-emitting it at longer wavelengths in the FIR and submillimetre (sub-mm) regimes. The cold dust component, which constitutes the majority of the dust mass, is primarily heated by stellar radiation, with contributions from both young, massive stars and the diffuse emission from older stellar populations. It is essential to discern how dust is heated to better understand the relationship between stellar populations and their surrounding environments. Aims. This study aims to identify the dominant heating mechanisms responsible for the cold dust component in typical nearby spiral galaxies and to explore the contributions of both young and evolved stellar populations to dust heating. Methods. Using a sample of 18 large, face-on spiral galaxies from the DustPedia project, we employed two complementary approaches. In the first method, we studied the correlations between the dust temperature (T_dust), star-formation rate (SFR) surface density (Σ_SFR), and stellar mass surface density (ΣM_*). In the second method, we explored the relationship between T_dust and the dust mass surface density (Σ_dust). Results. By analyzing the median temperature radial profile, we find that T_dust peaks at ∼24 K at the galaxy centre, decreasing to ∼15 K toward the galaxy outskirts. Our analysis shows similar T_dust in galaxies with and without a central active galactic nucleus (AGN), suggesting that AGN activity does not significantly influence T_dust values and distribution on the spatial scales covered by our data, which range from 0.3 to 3 kpc. For ∼72% of the galaxies in our sample, the methods consistently identify the primary dust heating source. However, when considering the entire galaxy sample, our analysis suggests that there is no single dominant heating mechanism. We find that both young and evolved stars contribute to dust heating, with their relative contributions varying across galaxies.