SWG2: Nearby Galaxies

Key Science Case: ISM and HII regions in extreme starbursts

Additional science cases:

• Nearby low-metallicity (extreme) starburst galaxies offer an opportunity to study galaxy formation under conditions similar to those of the first galaxies. Such galaxies contain rare massive stars, are likely to host super-luminous supernovae (hypernovae), have “porous” ISM which enhances the escape of Lyα and Ly continuum radiation, and they enable studies of star formation and its feedback effects, including outflows and the enrichment of the IGM. In these respects, these galaxies are close analogues of the faint galaxies in high-z surveys, that dominate the star formation budget. Observations of massive stars, including the Wolf-Rayet populations, fundamental for the production of ionising radiation, will be fundamental to address the stochastic sampling of the IMF, a topic that has major repercussions to all fields dealing with SFR tracers based on H emission lines.
• Study of HII regions and diffuse interstellar gas properties (temperature, densities, metallicities, optical thickness) by using critical lines like [O II] λλ3726,3729 and [O III] λλ4363,4959,5007 will enable BlueMUSE to provide a new view on the interstellar medium (ISM) in general. Previous studies have revealed a pervasive turbulent nature of the ISM, but the physical mechanisms for its creation are not understood. This is fundamentally linked to gas flows in galaxies. For example, metallicity variations at galactic scales are fundamental to understand time scales for gas inflow and gas mixing. Due to BlueMUSE spectral resolution and large FoV, the determination of the supersonic gas motions and their spatial distributions within galaxies willfall for the first time within capabilities of an IFS.
• Given the faintness of the low surface brightness galaxies (LSB) it is not surprising that, while they might comprise up to 50% of local galaxies, their general properties, nature and origin are still poorly known. We still need to understand what is the shape of their luminosity function and how it combines with the general luminosity function of z=0 galaxies. LSB galaxies are sources of star formation in a low-density regime, which might be reflected in the initial mass function. The stars or emission-line gas in the LSB galaxies are likely to move almost exclusively under the gravitational influence of the dark matter, and therefore these galaxies are excellent probes of dark matter models, as well as modified gravity theories. This science case focuses on the census of the star formation and dynamical properties of LSB, as probes of galaxy evolution.

• The environment plays a major role in shaping galaxies and determining their subsequent evolution. The range of possible physical processes that operate is large, but we are missing a clear picture about which process is dominating a certain type of transformation. Moreover, while it is clear that the processes can be, broadly speaking, either linked to the gravitational perturbations or to hydrodynamical interactions (for example of the cold ISM and warm IGM), their relevance with respect to the environmental density, galaxy (stellar) mass and the interaction epoch (local or intermediate redshift universe) remains a major challenge to our understanding. BlueMUSE large FoV and high sensitivity will enable studies of individual galaxies in dense environments, or groups of galaxies and their intra-group
• Planetary nebulae are versatile probes of the distance, kinematics and stellar populations in nearby galaxies. MUSE observations demonstrated that PNe luminosity function can be used to distance of 40 Mpc, overlapping with the second rung of the distance ladder, and offering a possibility of an independent probe of the Hubble constant. Furthermore, the relative number of PNe per stellar mass of the underlying population can be an interesting probe for post-RGB evolution, especially at the high metallicities found in the cores of ellipticals. The challenge is to find PNe against the high continuum in sufficient numbers. While PNe are detectable in large numbers at larger radius in MUSE observations, galaxy cores remain challenging, which can be directly addressed using high spectral resolution of BlueMUSE.

• Detailed dynamical studies of stars in present-day galaxies provide a fossil record of their individual assembly history. In particular the high-order kinematic moments, i.e. the skewness and kurtosis of the line-of-sight velocity distribution, reveal complex stellar orbital structures that go undetected when measuring the velocity and velocity dispersion alone. High-order kinematic measurement will be crucial for revealing the dynamical properties of low-mass spirals and spheroidals (log M_∗/M_⊙ < 9.5) that break away from fundamental galaxy scaling relations. High-precision measurements of the stellar orbits in galactic disks are required to understand how minor mergers impact the formation and evolution of galaxy structure, which can be used also to place our Milky Way in a cosmological context.