Open Questions on Dust, Cold Gas, Star Formation, and Feedback in Nearby Galaxies

Determine the formation mechanisms and properties of interstellar dust and cold gas phases in nearby galaxies; ascertain the processes that lead to star formation activity in those cold phases and quantify the local fraction of gas mass converted into stars and the time scale for such conversion; and characterize the feedback mechanisms that regulate star and black hole formation and drive galaxy growth on intermediate and large spatial scales.

Background

The report emphasizes nearby galaxies as prime laboratories for studying the baryon cycle because they allow high spatial resolution and sensitivity. Within this context, the authors explicitly list several open questions concerning the formation and properties of interstellar dust and cold gas, the processes that control star formation efficiency and timescales, and the feedback mechanisms that influence both star and black hole formation and thus galaxy growth.

Addressing these questions requires observations across cm-to-FIR wavelengths, including polarimetry and spectroscopy, and motivates enhanced capabilities in facilities such as ALMA, ngVLA, and potential FIR space missions.

References

Some of the important open questions include the formation and properties of interstellar dust and the cold gas phases, the processes that lead to star formation activity in those cold phases and determine locally the fraction of gas mass that is converted into stars and the time scale for such conversion, and the feedback mechanisms that regulate star and black hole formation and consequently galaxy growth on intermediate and large spatial scales.

— Report of the Kavli-IAU Workshop on Global Coordination, "Probing the Universe from far-infrared to millimeter wavelengths: future facilities and their synergies"  (2409.07570 - Committee et al., 2024) in Section 5 (Nearby Galaxies), p. 25

Some of the most pressing open questions are: Metal mixing and the ISM complexity. How do metals mix and distribute in the ISM? On what spacial and time scales? What are the chemical properties of ISM clouds? Are there cloud-to-cloud differences? How do stars enrich the surrounding ISM? What does the ISM complexity tell us about the chemical evolution of galaxies? The low-metallicity Universe. What are the chemical properties of the neutral ISM in low-metalllicity galaxies? What are the physical conditions of the cold ISM, specially at low metallicity? How do detailed studies of the local low-metallicity Universe enable a more meaningful understanding of the distant Universe?

— The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission  (2609.09329 - Cia et al., 8 Sep 2026) in Section 5, subsection “Metals in the Interstellar Medium”

Taken together, these comparisons point to a broader open problem: the absolute rate of recent local star formation remains uncertain, and different tracers give different answers. This is important since the adopted rate sets the amount of massive star feedback available to shape the present-day ISM.

— The Nearby Star Formation and Supernova Histories Reconstructed from Young Star Clusters  (2608.20307 - Swiggum et al., 20 Aug 2026) in Section 6.4, “The absolute star formation and ccSN rates in the Solar Neighborhood”

Whether low-mass galaxies are capable of retaining their dust and metals in the presence of active star formation is uncertain, with stellar feedback potentially clearing new star forming regions over time.

— The Physical Conditions of Low-Mass Galaxies at z=3.5-7.0 from JWST Spectroscopy: The Behaviour of Spectral Line Ratios with Burstiness  (2609.01205 - Cooper et al., 1 Sep 2026) in Section “Main Line Ratios versus Stellar Mass”

The exact functional form of the metallicity dependence of $\alpha_{\rm CO}$ remains uncertain, and a variety of empirical calibrations have been proposed in the literature \citep[e.g.,][]{Genzel_2012,Schruba_2012,Sandstrom_2013,Hunt_2015,Amorin_2016,Accurso_2017, Bisbas_2025}.

— Resolved Dust-Gas-Metallicity relations in nearby spiral galaxies  (2609.04320 - Tailor et al., 3 Sep 2026) in Section 3.2.1, “The CO-to-H₂ conversion factor”

As such, without knowing exactly how long their present period of star formation will be sustained, it is difficult to directly connect these galaxies to their quenched descendants.

— SQuIGG$\vec{L}$E: Spatially resolved NIRSpec Pa$α$ reveals that the most CO-luminous, $z\sim0.7$ post-starburst galaxies exhibit falling---but still active---star formation behind optically thick dust  (2609.31307 - Setton et al., 25 Sep 2026) in Section 4.1, “Can the rapid depletion of molecular gas reservoirs be explained by buried star formation alone?”