The cosmic origin of the Hubble sequence – spin(e)

Over the past few years, our team has developed a suite of novel algorithms, based on recent progress in topology and computational geometry, to trace filaments right to the core of star forming galaxies. In parallel we have developed the corresponding theory in a series of papers. Specifically, our tools provide a formal definition of filaments, summing up the morphological properties of cold flows and the cosmic web. In particular, the “persistent skeleton” can now be implemented on noisy discrete large catalogues such as the SDSS, VIPERS, or future surveys like DESI. Combining state of the art simulations, extraction tools and observational surveys should provide us with the unique opportunity to statistically constrain the anisotropic environment of simulated and observed galaxies (geometry, connectivity and dynamic of accretion) in order to explain the Hubble sequence.
We have recently proposed a novel paradigm for the acquisition of disc angular momentum via filamentary flows. We found a closer connection between the 3D geometry and dynamics of the neighbouring cosmic web and the properties of embedded galaxies than originally suggested by the standard hierarchical formation paradigm. At these scales, in the surrounding asymmetric gravitational patch gas is streamed out from the neighbouring voids, towards their encompassing filaments where it shocks, until the cold flows are swallowed by the forming galaxy, advecting their newly acquired angular momentum. The corresponding net distribution of angular momentum hereby seems to define the morphology of that galaxy (bulge or disc). The evolution of the Hubble sequence is therefore likely to be in part driven by the geometry of the cosmic web. Conversely, the distribution of the properties of galaxies measured relative to their cosmic web should reflect this process.

The effect of the cosmic web environment on Galaxy formation has become an extremely active field of research in the last few years at the international level. The number of international conferences and publications on this topic exemplifies this. The funding of the ANR has been most useful in maintaining our lead on this topic. It is now clear to most key players in the discipline that morphology and shape must be driven in part by the environment which is best qualified by the properties of the cosmic web. Its importance is amplified by the fact that the gas follows closely the web, which represents best the anisotropy sourcing the tidal torques spinning up galaxies. This has now undisputedly been quantified in simulation, theorized, and, indirectly measured in data. Specifically, we now have a novel ab initio Lagrangian theory for galactic connectivity and spin acquisition within the cosmic web, the corresponding theory for accretion rate using excursion set theory and a novel implementation for the secular evolution of galactic discs and centers, either intrinsically or environmentally driven. We were particularly pleased to see that project-driven research remains compatible with such investigation and breakthrough.

In short we will continue our efforts in connecting theory with observations. In doing so, we will investigate the properties of our flagship simulations, and exploit our ongoing zoom simulation, New-Horizon, which resolves the disc scale height of our virtual galaxy down to redshift zero (the simulation will be completed jointly with our Korean collaborators). We are going to continue working on the variational formulation of the cosmic web, the theory of persistence, and the statistical analysis of the distribution of AM in discs. Eventually, i) we aim to connect our theoretical secular work on orbital structure evolution (carried in the context of Hamiltonian dynamics) to its anisotropic (cosmic) framework; ii) model self-consistently intrinsic alignments (consistently with colour) for lensing iii) identify dynamical signatures of cold flows iv) investigate if the cumulative effect of coherence of the large-scale inflow dominates over the stochastic effect of feedback (inflow is climate, outflow is weather?) v) resolve disc scale height numerically and observationally (IFUs), and model jointly with kinematics of flows within filaments vi) understand disc settling through the evolution of open galactic discs within known anisotropic fluctuating tides and accretion rates. vii) model secular evolution of galaxies, globular and nuclear clusters and study the Bulge-AGN-BH connection and its impact on disc stability/turbulence?

The ANR cosmicorigin.org has led to the publication of over 80 rank A papers which have been cited over 1200 times already. This topic of research has become extremely popular, and our lead has transpired in very cited papers, and the large number of prestigious postdoctoral positions former students have been offered. It defined the new framework for describing the environment in which galaxies evolve. The keyword ‘cosmic web’ collects over 50 000 citations,. Our state-of-the-art simulation horizon-simulation.org (8000 hits/month) is now cited over 1400 times. Indeed, our original work on the cosmic evolution of galactic discs and black holes, on the impact of the cosmic web on the dynamical and physical properties of galaxies have strongly contributed to a shift of paradigm towards accounting for the anisotropy of the environment. Intrinsic alignments are now a specific work package for both LSST and EUCLID missions. The main challenge for this ANR was to connect observations to theory: this goal has now been reached (Malavasi+’16, Krajic+17, Laigle+’17 etc.) over all surveys available to us. One of the highlights of this work is a complete theory for spin alignment (see Codis+’12,15, Laigle+’15) and conditional excursion (Musso et al. 17’) in the frame of filaments. Our students have been offered postdoctoral positions in the top institute worldwide, and four of our postdocs now have permanent position either at CNRS or at the University. The project now federates five international institutes (Paris, Marseille, Oxford, and more recently Yonsei KIAS and Edinburgh). The ANR has played a key role to promote such synergies, to attract strong PhD candidates and postdocs and to raise more funding (CSA, DIM-ACAV... ~ 100k€/year) e.g. to fund our dedicated post processing cluster (from 250 to 800 cores et from 1/2 PB to 3PB storage).

Observational information on the morphology of galaxies and its dependence on environment is routinely becoming available for galaxies up to redshifts two and beyond. Galaxies appear quite different at high redshifts, and quite far from the current Hubble sequence. Thanks to a well-established paradigm of cosmological structure formation, many of the boundary conditions for galaxy formation are now well defined. Modern simulations based on this paradigm have established a tight connection between the geometry and dynamics of the large-scale structure of matter on the one hand, and the evolution of the physical properties of forming galaxies on the other.
Key questions formulated decades ago are nevertheless not well answered. What are the main drivers determining the morphology of galaxies, which are responsible for establishing the Hubble sequence? When did this happen? What is the role of acquired angular momentum in shaping galaxies? Can we understand the complete "baryon cycle", where the evolution of the anisotropic IGM and galaxies are studied simultaneously? Cold flows are predicted and debated, but no smoking gun exists in the real world. This proposal aims at filling this gap: theorists and observers will work together to try and find proof of cold flows’ existence and understand the details of how they affect the structure of galaxies, in particular for the build-up of angular momentum.
Over the past few years, our team has developed a suite of novel algorithms, based on recent progress in topology and computational geometry, to trace filaments right to the core of star forming galaxies. In parallel we have developed the corresponding theory in a series of papers. The “persistent skeleton” can now be implemented on noisy discrete large catalogues such as the SDSS, VIPERS, or future surveys like e-BOSS. Combining state of the art simulations, extraction tools and observational surveys should give us the unique opportunity to statistically constrain the anisotropic environment of simulated and observed galaxies (geometry, connectivity and dynamic of accretion) in order to explain the Hubble sequence.

We have recently proposed a novel paradigm for the acquisition of disc angular momentum via filamentary flows. We found a closer connection between the 3D geometry and dynamics of the neighbouring cosmic web and the properties of embedded galaxies than originally suggested by the standard hierarchical formation paradigm. At these scales, the surrounding asymmetric gravitational patch ejects gas from the neighbouring voids towards their encompassing filaments, until the cold flows are swallowed by the galaxy, advecting their newly acquired angular momentum. The corresponding net distribution of angular momentum hereby seems to define the morphology of that galaxy. The evolution of the Hubble sequence is therefore driven by the geometry of the cosmic web. Conversely, the distribution of galaxy properties measured relative to their cosmic web should reflect this process.
Our purpose is to explore fully the impact of these propositions on the origin of the Hubble sequence. We will rely on observations and numerical expertise to automate the extraction of skeletons and morphological parameters of galaxies from mock, public and proprietary catalogues. We will interpret galaxy evolution within the complex anisotropic 3D structure of the cosmic web, and verify that filamentary flows indeed feed the disc’s angular momentum in galaxies. We will also provide a critical assessment of the respective role of merger/interaction history, secular evolution driven by instabilities, stellar feedback, wind and nuclear activity, as a function of cosmic time. We will use state-of-the-art “full-physics” cosmological simulations, theoretical priors, and observed surveys to disentangle the relative effects of all these interconnected influences, which will be crucial for a detailed understanding of the origin of the Hubble sequence.