Eric Emsellem

Galactic bars are thought to play a critical role in the secular evolution of their hosts by, for example, reorganising the interstellar medium (ISM). We used a sample of 57 star-forming disc galaxies observed with JWST at 3 and 7.7 µm to probe how the spatial distribution of polycyclic aromatic hydrocarbon (PAH) emission as a structural marker of the cold ISM might depend on galaxy stellar mass and the presence of a bar. We find evidence for a ’watershed’ at a stellar mass of 10 ¹⁰ M _⊙ , marking a fundamental transition in the bar-driven distribution of PAH emission. This confirms trends previously predicted by numerical simulations and observed via ionised gas or UV light. While lower-mass galaxies exhibit a disordered and clumpy distribution of PAH emission regardless of bar presence, higher-mass barred hosts display well-structured dynamical features traced by PAH emission with significant gas reservoirs (e.g., discs and rings) within the central 15% of the bar radius (R _b ). Furthermore, we observed a systematic depletion of PAH emission within the [0.2–0.8] R _b range in barred systems with stellar masses above 10 ¹⁰ M _⊙ . Such central discs, rings, and associated radial dips (‘bar deserts’) appear to be a mass-dependent phenomenon as they are ubiquitous in massive galaxies but mostly absent in their lower-mass counterparts. In contrast to the structured features in massive hosts, the disorganised ISM in lower-mass galaxies masks the commonly observed bar-driven signatures. This suggests that tracer selection and dust obscuration may significantly bias observed bar fractions. Our study underlines the existence of two regimes of secular evolution, with different impacts and observability of bar-driven processes, and it reaffirms the role of bars as the primary drivers of rapid secular evolution in galaxies above 10 ¹⁰ M _⊙ , while the impact of bars is significantly reduced or delayed below this threshold. Our results further underscore the need to critically account for these processes when modelling galaxy evolution in cosmological simulations.

We present the largest study to date of star cluster mass and age distributions in nearby galaxies. The analysis is based on a uniformly selected catalog of over 15,000 (36,000) human- (machine-) classified compact clusters across 38 spiral galaxies in the PHANGS-Hubble Space Telescope survey, with improved photometric SED age dating that mitigates key degeneracies with reddening and metallicity. The galaxies span a factor of ∼100 in star formation rate (SFR) and in their surface density of star formation and molecular gas (Σ _SFR , Σ H 2 ), and cover a broad range of morphologies. We find that cluster mass functions are well described by a power law, dN / dM ∝ M ^β , with β = −1.9 ± 0.2, and that this shape is remarkably similar across all galaxies in our sample and eight composite groups of galaxies with similar SFRs, with no significant dependence on SFR, Σ _SFR , or Σ H 2 . Cluster age distributions decline steeply and can be described by dN / dτ ∝ τ ^γ with γ = −0.83 ± 0.24, again with little dependence on host-galaxy properties or cluster mass. The independence of the mass and age distributions implies a separable joint distribution, g ( M , τ ) ∝ M ^β τ ^γ , and suggests that cluster populations in spiral disks are dominated by strong, mass-independent dissolution over their first approximately gigayear. These results establish a benchmark for interpreting cluster demographics across diverse galactic environments and provide context for studies of young cluster populations at high redshift.

The Next Generation Virgo Cluster Survey (NGVS) is a deep, high-resolution imaging campaign that used the 1 deg ² MegaCam instrument on the Canada–France–Hawaii Telescope to carry out a comprehensive optical survey of the Virgo cluster, from its core to its virial radius. The NGVS covers a contiguous area of 104 deg ² (8.63 Mpc ² at the 16.5 Mpc distance of Virgo) in the u *, g , i , and z bands, with additional limited coverage in r . In this paper, we present the final catalog of Virgo galaxies across the entire NGVS area. The catalog includes 3680 galaxies considered to be bona fide members of the cluster, spanning a factor of 2.5 million in luminosity, from g = 8.42 mag to g = 24.41 mag ( M _g = −22.67 mag to M _g = −6.68 mag). With 2100 previously uncataloged galaxies, the NGVS catalog augments the number of known Virgo members by a factor of 2.3. The catalog is complete down to g = 18.6 mag ( M _g = −12.5 mag, corresponding to a stellar mass M _⋆ ∼ 1.6 × 10 ⁷ M _⊙ for an old stellar population) and 50% complete at g = 22.0 mag ( M _g = −9.1 mag, M _⋆ ∼ 6.2 × 10 ⁵ M _⊙ ), 3 mag deeper than the venerable Virgo Cluster Catalog (VCC), which for over 40 yr has served as the reference standard for Virgo. Photometric and structural parameters are derived for all NGVS galaxies and presented in a series of tables, alongside nuclear and morphological classification, as well as stellar masses and, when available, radial velocities.

We present 50–100 pc-resolution JWST/MIRI and NIRCam measurements of mid-infrared (mid-IR) color variations in the diffuse interstellar medium (ISM) of 71 nearby star-forming galaxies from the PHANGS-JWST survey. Mid-IR emission traces the dust column density, intensity ( U ) and hardness of the interstellar radiation field, and the physical state (charge, size) and abundance of polycyclic aromatic hydrocarbons (PAHs). Mid-IR colors that trace PAH band ratios remain fairly constant in the diffuse ISM of star-forming disks. However, they show stark variations in extreme environments: highly star-forming central molecular zones (CMZs) and star formation deserts/quiescent bulges. In CMZs, PAH-to-continuum (3.3/21, 7.7/21, and 11.3/21 μ m) and the 10/21 μ m continuum colors are 0.2–0.4 dex lower than in normal disks. We attribute this to higher U based on the far-infrared dust colors and the high 21 μ m/Σ _Mol , which we suggest to be a good tracer of U outside star-forming regions. Meanwhile, star formation deserts show low 7.7 μ m PAH emission, resulting in low 7.7/21 μ m and 7.7/11.3 μ m, while all other mid-IR colors remain typical. This suggests the presence of more neutral PAHs in star formation deserts, where low 7.7 μ m likely reflects ISM conditions similar to early-type and elliptical galaxies. All environments form part of a continuous trend in 7.7/11.3 μ m versus specific star formation rate.

We present early science results from the Multiphase Astrophysics to Unveil the Virgo Environment (MAUVE) program, which targets 40 Virgo Cluster galaxies to investigate the effect of environment on the interstellar medium (ISM) at ∼100 pc scales. From 12 galaxies in the MAUVE–Multi Unit Spectroscopic Explorer (MUSE) early sample, we find systematically elevated line ratios compared to Physics at High Angular resolution in Nearby GalaxieS (PHANGS)–MUSE field disks, with higher medians of [N ii ]/H α (0.75 versus 0.50), [S ii ]/H α (0.57 versus 0.49), and [O iii ]/H β (1.04 versus 0.68). Spatially resolved Baldwin–Phillips–Terlevich diagrams show 74% of MAUVE spaxels ionized by sources other than H ii regions versus 61% in the field, and we find these ionization differences to be closely coupled to broadened kinematics. In all, 44% of MAUVE spaxels exceed H α σ _LOS = 40 km s ⁻¹ (versus 26% in the field), driven mainly by non-star-forming gas with σ _LOS between 40 and 80 km s ⁻¹ , consistent with enhanced contribution of diffuse ionized gas (DIG). A subdominant tail of 5% of spaxels at σ _LOS > 100 km s ⁻¹ , largely absent in PHANGS–MUSE (1%), points to shocks or turbulent mixing layers from intracluster interactions. Our results show that environmental quenching primarily suppresses star formation, unveiling DIG as the dominant ionized component in cluster disks. The elevated line ratios and broadened kinematics observed in the MAUVE sample reflect the physical state of the ISM in the absence of vigorous star formation, rather than widespread direct environmental excitation. The observed shock-like emission provides an additional, secondary contribution likely driven by active interactions with the intracluster medium.

We present stellar kinematics of the MUSE Most Massive Galaxies (M3G) Survey, comprising 25 galaxies brighter than −25.7 mag in the K _s band and stellar masses above ≈6 × 10 ¹¹ M _⊙ . The galaxies are divided into brightest cluster galaxies (BCGs) and lower-ranked (in brightness) galaxies (non-BCGs) in three rich galaxy clusters within the core of the Shapley Supercluster. We find several velocity maps with rich kinematic structures, including multiple spin reversals within the region encompassing two central effective radii, typically associated with BCGs. Additionally, the majority of the BCGs show rotation around the major axis of the galaxy at least in one of the visible velocity components. These kinematic structures are possible only if galaxies have non-axisymmetric shapes and contain several orbital families with both prograde and retrograde rotations. We argue that such velocity maps provide further evidence that the evolution of very massive galaxies is influenced by, possibly repeated, gas-poor major merging. There are six fast rotators in the M3G sample, and they are all among non-BCGs and typically ranked below the third brightest galaxy in their clusters. Based on the properties of the h ₃ Gauss-Hermite moment, fast rotation can be linked to the dominance of prograde rotating short-axis tubes in the orbital distribution. Slow rotators are BCGs or the second or sometimes third brightest galaxies, indicating that the galaxy mass (brightness) is not the only driver of low spin and that the location within the local environment also plays a role. Slow rotators, as evidenced from their multi-spin velocity maps, require more complex orbital structures. Furthermore, some BCGs show kinematic evidence for a secondary component at larger radii, which is likely not in equilibrium with the main galaxy and possibly made of stars accreted from other cluster galaxies. Multi-spin velocity maps, low angular momentum, and additional kinematic components highlight the differences in the evolutionary histories of BCGs (including second-ranked galaxies) and non-BCGs.

Context. Nearby galaxies exhibit a variety of structures, including so-called central or (circum-)nuclear rings that are similar to the Milky Way (MW) Central Molecular Zone (CMZ). These rings are common in barred galaxies and can be gas-rich and highly star-forming. Aims. We aim to study the molecular gas content and star formation rate of central rings within nearby galaxies and link them to global galaxy properties, especially the bar morphology. Methods. We utilized 1″(≲100 pc) resolution CO(2–1) observations from the PHANGS-ALMA survey, visually identifying 20 central rings and determine their properties. For 14 of these rings, MUSE observations tracing star formation rate (SFR) surface density were available. We derived the rings’ geometry, integrated molecular gas masses, SFRs, depletion times, and compared them to host galaxy and bar properties from the literature. Results. Molecular gas is an effective tracer for central rings. Previous studies have used ionized gas and dust tracers to identify central rings in galaxies of similar morphological types as the PHANGS galaxies (numerical Hubble type T ∼ −3 to T ∼ 9). In comparison, molecular gas yields similar fractions of galaxies hosting central rings and similar radii distributions. The gaseous central rings have typical radii of ∼ 400 ⁺²⁵⁰ ₋₁₅₀ pc, molecular gas masses of log( M / M _⊙ ) ∼ 8.1 ^+0.17 _−0.23 , and SFRs of ∼ 0.21 ^+0.15 _−0.16 M _⊙ /yr. As a result, they contribute 5.6 ^+4.5 _−2.1 % and 13 ⁺¹⁰ ₋₅ % to their host galaxies’ molecular gas mass and SFR, respectively. While the MW CMZ sits at the lower end of the radius, molecular gas mass, and SFR distribution, it matches well in terms of ring molecular gas mass and SFR fraction, and depletion time. Longer bars contain more massive molecular central rings, but there is no correlation between the classical bar strength parameters ( Q _b , ε _bar , A ₂ ^max ) and the ring’s molecular gas content. Conclusions. Although absolute central ring properties (ring radius, molecular gas mass, SFR) likely depend on host galaxy properties, the similarities between the MW CMZ and PHANGS central rings in relative parameters (molecular gas and SFR fraction, depletion time) suggest that the processes of gas inflow and star formation are similar for central rings across nearby galaxies.

We present high-resolution Very Large Array continuum observations at the S band (3 GHz, 560 pc scale) and X band (10 GHz, 200 pc scale) of the ram-pressure-stripped Virgo galaxy NGC 4522 to investigate the characteristics of its radio continuum, spectral index, and magnetic field under the influence of the intracluster medium (ICM). The total radio continuum shows an asymmetry that extends northwest, mirroring the H i gas distribution but showing distinct features in the extraplanar regions. The spectral index steepens systematically from α ∼ −0.6 in the main disk to α ∼ −1.1 in the outer disk. We find that the spectral index behavior of the outer disk is mainly due to an ICM shock that can reaccelerate electrons and a significant reduction of thermal emission. Intriguingly, extraplanar clouds exhibit exceptionally flat spectral indices ( α ∼ −0.2 to 0), resulting from a combination of significantly enhanced thermal emission and pronounced spectral aging of the nonthermal component. Although some of these regions correlate with H α , others do not. We propose that the mixing between the ICM and interstellar medium (ISM) is an alternative mechanism that enhances thermal emission independently of star formation. Polarized continuum emissions are highly asymmetric, preferentially distributed along the ICM wind side, and the polarization fraction increases radially outward from the galactic midplane, indicating that the polarized emission is strongly influenced by the ICM wind. Our results show how and where the ICM substantially affects the ISM and also demonstrate that high-frequency observations are crucial for analyzing the radio continuum of ram pressure stripping galaxies.

We use the full-disk Very Large Telescope/MUSE mosaic of NGC 253 to identify 2492 H ii regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic H ii regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved core with radius < 10 pc surrounded by a more extended halo of emission with radius = 20–30 pc. Approximately 80% of the emission of a region originates from the halo component. As a result of this compact structure, the luminosity–radius relations for core and effective radii of H ii regions depend sensitively on the adopted methodology. Only the isophotal radius yields a robust relationship in NGC 253, but this measurement has an ambiguous physical meaning. We invert the measured emission profiles to infer density profiles and find central densities of n _e ≈ 10–100 cm ⁻³ . In the brightest regions, these agree well with densities inferred from the [S ii ] λλ 6716, 6730 doublet. The central density of H ii regions correlates well with the surface brightness within the effective radius. We show that this same scaling relation applies to the recent MUSE + Hubble Space Telescope catalog for 19 nearby galaxies. We also discuss potential limitations, including completeness, impacts of background subtraction and spatial resolution, and the generality of our results when applied to other galaxies.