Letters of Intent received in 2016

LoI 2018-1908
Focus Meeting (GA): Galactic Angular Momentum

Topics

+ The cosmological origin of angular momentum (AM)
+ Recent precision measurements of galactic AM using integral field spectroscopy and interferometry
+ Numerical simulations of acquisition, transport, growth and ejection of galactic AM
+ The role of AM in cold gas physics and star formation
+ Connection between AM and galactic sub-structure: Hubble type, clumps, velocity/dispersion support
+ Alignment of galactic spin and cosmic large scale structure

Rationale

Since the time of Kant Angular Momentum (AM) has been recognised as the cause of the rotating 'nebulae', now known as galaxies. This fundamental role of AM was cemented in the original works on the formation of galaxies in cold dark matter (CDM) haloes in the 1970s and 80s. However, quantitative progress was hampered by observational and theoretical obstacles. The numerical modelling of galaxies with realistic AM turned out to be a severe difficulty, which we only began to overcome in the last five to ten years. Simultaneously, the fast rise of Integral Field Spectroscopy (IFS) and millimetre/radio interferometry have opened the door for systematic AM measurements out to large galactic radii, across representative samples and cosmic volumes. This Focus Meeting at the IAU GA 2018 will commemorate the first decade of precision AM astronomy, bridge between observations and simulations of AM, and debate new emerging views on galaxy evolution. The framework is detailed as follows.

According to the current theory of galaxy evolution, CDM haloes grow from primordial density fluctuations while acquiring AM through tidal torques to nearby haloes. Two processes dominate the subsequent formation of baryonic galactic disks at the halo centres: dissipation of energy and conservation of AM. More precisely, the cooling baryons naturally exchange AM with their haloes, but the mass-size relation of local star-forming galaxies implies that, on average, the specific AM of the baryons must remain approximately conserved during galaxy formation. Explaining this conservation has been a long-standing problem for theory: until recently (early 2010), hydro-gravitational simulations (using both particle-based and grid-based techniques) systematically failed at reproducing disks as large and thin as normal late-type galaxies, such as the Milky Way. The simulated galaxies were systematically deficient in AM, making them too small and too bulgy - a problem so severe that it became known as the 'AM catastrophe'. Semi-analytic models (SAMs), which constitute a faster and more global approach to galactic modeling, did not suffer from this catastrophe, though not by solving it. SAMs bypass the AM problem by imposing conservation of specific AM during disk formation (or slight variations of this assumption). Solving the catastrophe has been one of the major recent success stories of hydro-gravitational galaxy simulations. The solution required much higher numerical resolution and the inclusion of strong supernova winds. A lesson learnt from the AM catastrophe is that AM is one of the most critical quantities for simulating galaxies with realistic morphologies. If AM is erroneous, realistic disks fail to form.

This insight motivates a deeper exploration of the link between galaxy morphologies and AM. The significant development of IFS has enabled simultaneous measurements of the composition and Doppler velocity at every position in a 2D galaxy image, hence enabling a pixel-by-pixel integration of the AM. Such measurements of AM in early-type galaxies (ATLAS-3D survey, 2011) led to the surprising discovery that most of these seemingly featureless objects exhibit a rotational structure akin to that of normal spiral galaxies, hence containing more AM than previously suspected. The fewer actual 'slow-rotator' host up to an order of magnitude less AM at a fixed mass (modulo de-projection uncertainties). AM thus offers a more fundamental, albeit harder to measure, classification of galaxy types than the classical Hubble sequence. This conclusion also follows from more recent AM measurements in disks, for instance from high-precision measurements based on the THINGS survey that account for the AM in stars and cold gas out to ten effective radii. These data reveal a tight relationship between the relative mass in the central stellar over-density (bulge) and the location of the galaxy in the baryonic mass-AM plane, again suggesting that the Hubble morphology sequence might be substituted for a more fundamental and physically motivated classification by AM. The precise form of this new AM-based classification scheme remains nonetheless a source of much argument. Many recent hydro-gravitational simulations (e.g. Illustris, EAGLE, Horizon, Magneticum, NIHAO) contribute to this discussion, as do most major kinematic observing efforts. Prominent examples include optical IFS/IFS-like surveys (e.g. ATLAS-3D, CALIFA, MaNGA, SLUGGS, PN.S, KROSS, SAMI Survey), interferometric radio surveys (e.g. THINGS on the VLA) and many other kinematic observations on new and future instruments (e.g. KMOS, MUSE, SINFONI, HECTOR, ALMA, NOEMA, SKA and precursors). Bridging between these surveys and theory is a key aim of this Focus Meeting.

The strong correlations between morphology and AM of local galaxies raises the question as to whether the cosmic evolution of morphologies is paralleled, or even driven, by the evolution of AM. Observationally, the Hubble Space Telescope's (HST) exquisite spatial resolution showed that star-forming galaxies at redshift z>1 had very different structures to local grand-design spirals: The rapidly star-forming early galaxies showed a predominance of 'clumpy' and 'irregular' morphologies caused by super-giant (300 to 1000 pc) star-forming complexes. The physical origin of these clumpy morphologies and the processes that drive the large star formation rates are currently heavily debated. High-z IFS observations surprised with the finding that most of the clumpy star-forming galaxies have a regular, rotating disk structure. Interestingly, the emission line velocity dispersions appear to be about five times larger than in mass-matched local disks, which presents a major puzzle, because high velocity dispersions are predicted to stabilise the disks, preventing them from fragmenting into star-forming clumps. While high gas fractions could explain instabilities in spite of high dispersion, deep IFS studies (on Keck-OSIRIS, Gemini-GMOS) in rare nearby clumpy disks suggest that low AM is the more dominant driver of instabilities. This motivates the arguable conjecture that the cosmic evolution of AM plays indeed a major role in the morphological transformation of the star-forming population - a hypothesis to be debated at this Focus Meeting.

Answers to key questions regarding the cosmic evolution of AM are about to emerge from new high-z IFS observations on 8m-class telescopes (e.g. KMOS and MUSE on the VLT), as well as from an array of cosmological hydro-gravitational simulations (see above). Meanwhile multi-wavelength surveys are about to pile up evidence for strong correlations between AM and various baryonic processes (e.g. star formation rates, the transition from atomic to molecular gas). Moreover, ongoing and near-future surveys (see above) are about to expand AM science to smaller and larger scales: for the first time, enough spatially resolved velocity maps are available to systematically study the spatial distribution of the baryon AM in galaxies, which offers an extremely nuanced test of different galaxy evolution models. On large scales, the number and spatial completeness of galaxies mapped using IFS are about to become sufficient to test the weak correlations between AM and cosmic large scale structure predicted by simulations.

In summary, observational and computational studies of AM have induced major progress in galaxy evolution theory over the last decade. The rich and fast evolving diversity of AM-related aspects, as well as the need for bringing observers and theoreticians together, makes this topic an optimal theme for an IAU Focus Meeting. SOC members will include highly qualified lead investigators of kinematic surveys and cosmological galaxy simulations.