Scientific Rationale

In the standard model of galaxy formation, cold dark matter (CDM) haloes grow from primordial density fluctuations while acquiring AM through tidal torques. Galactic disks then condense at the halo centres by radiative dissipation of energy. 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. 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 deficient in AM, making them too small and too bulgy – a problem so severe that it became known as the ‘AM catastrophe’. Overcoming this catastrophe via much increased computational power and refined feedback physics has been one of the major recent success stories of galaxy simulations. However, there is still debate about exactly how this is achieved: is outflowing gas torqued so that re-accreted gas has higher AM, is low AM material preferentially removed from galaxies, or do the winds prevent loss of AM by making inflows smoother?

A certain result from the AM catastrophe is that AM is one of the most critical quantities for explaining galaxy morpholgies, opening a new bridge between theory and observation. The recent fast rise 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 (Atlas3D 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, thus containing more AM than previously suspected. The fewer actual ‘slow-rotators’ host up to ten times less AM at a fixed mass. AM thus offers a more fundamental, albeit harder to measure, classification of galaxy types than the classical Hubble sequence. This conclusion was cemented by recent AM measurements in spiral galaxies, for instance 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 physical 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, MAGICC, CLUES, NIHAO) contribute to this discussion, as do most major kinematic observing programs. Prominent examples include optical IFS/IFS-like surveys (e.g. Atlas3D, CALIFA, MaNGA, SLUGGS, PN.S, KROSS, SAMI Survey), interferometric radio surveys (e.g. THINGS on the VLA) and many other kinematic observations on modern and future instruments (e.g. KMOS, MUSE, SINFONI, HECTOR, ALMA, NOEMA, JWST, SKA and precursors).

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 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 FM.

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, and the growth of black holes). 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.