The role of disk instabilities, such as bars and spiral arms, and the associated resonances, in growing bulges in the inner regions of disk galaxies have long been studied in the low-redshift nearby Universe. There it has long been probed observation
ally, in particular through peanut-shaped bulges. This secular growth of bulges in modern disk galaxies is driven by weak, non-axisymmetric instabilities: it mostly produces pseudo-bulges at slow rates and with long star-formation timescales. Disk instabilities at high redshift (z>1) in moderate-mass to massive galaxies (10^10 to a few 10^11 Msun of stars) are very different from those found in modern spiral galaxies. High-redshift disks are globally unstable and fragment into giant clumps containing 10^8-10^9 Msun of gas and stars each, which results in highly irregular galaxy morphologies. The clumps and other features associated to the violent instability drive disk evolution and bulge growth through various mechanisms, on short timescales. The giant clumps can migrate inward and coalesce into the bulge in a few 10^8 yr. The instability in the very turbulent media drives intense gas inflows toward the bulge and nuclear region. Thick disks and supermassive black holes can grow concurrently as a result of the violent instability. This chapter reviews the properties of high-redshift disk instabilities, the evolution of giant clumps and other features associated to the instability, and the resulting growth of bulges and associated sub-galactic components.
We provide evidence for a correlation between the presence of giant clumps and the occurrence of active galactic nuclei (AGN) in disk galaxies. Giant clumps of 10^8-9 Msun arise from violent gravitational instability in gas-rich galaxies, and it has
been proposed that this instability could feed supermassive black holes (BH). We use emission line diagnostics to compare a sample of 14 clumpy (unstable) disks and a sample of 13 smoother (stable) disks at redshift z~0.7. The majority of clumpy disks in our sample have a high probability of containing AGN. Their [OIII] emission line is strongly excited, inconsistent with low-metallicity star formation alone. [NeIII] excitation is also higher. Stable disks rarely have such properties. Stacking ultra sensitive Chandra observations (4 Ms) reveals an X-ray excess in clumpy galaxies, which cannot be solely due to star formation and confirms the presence of AGN. The clumpy galaxies in our intermediate-redshift sample have properties typical of gas-rich disk galaxies rather than mergers, being in particular on the Main Sequence of star formation. This suggests that our findings apply to the physically-similar and numerous gas-rich unstable disks at z>1. Using the observed [OIII] and X-ray luminosities, we conservatively estimate that AGN hosted by clumpy disks have typical bolometric luminosities of the order of a few 10^43 erg/s, BH growth rates ~10^-2 Msun/yr, and that these AGN are substantially obscured in X-rays. This moderate-luminosity mode could be sufficient to provide a large fraction of todays BH mass over a couple of Gyr given that our observations suggest a high duty cycle (>10%), accretion bursts with higher luminosities being possible over shorter phases. The observed evolution of disk instabilities with mass and redshift could explain the simultaneous downsizing of star formation and of BH growth.
Disk galaxies at high redshift have been predicted to maintain high gas surface densities due to continuous feeding by intense cold streams leading to violent gravitational instability, transient features and giant clumps. Gravitational torques betwe
en the perturbations drive angular momentum out and mass in, and the inflow provides the energy for keeping strong turbulence. We use analytic estimates of the inflow for a self-regulated unstable disk at a Toomre stability parameter Q~1, and isolated galaxy simulations capable of resolving the nuclear inflow down to the central parsec. We predict an average inflow rate ~10 Msun/yr through the disk of a 10^11 Msun galaxy, with conditions representative of z~2 stream-fed disks. The inflow rate scales with disk mass and (1+z)^{3/2}. It includes clump migration and inflow of the smoother component, valid even if clumps disrupt. This inflow grows the bulge, while only a fraction ~ 10^-3 of it needs to accrete onto a central black hole (BH), in order to obey the observed BH-bulge relation. A galaxy of 10^11 Msun at z~2 is expected to host a BH of ~10^8 Msun, accreting on average with moderate sub-Eddington luminosity L_X ~ 10^42-43 erg/s, accompanied by brighter episodes when dense clumps coalesce. We note that in rare massive galaxies at z~6, the same process may feed 10^9 Msun BH at the Eddington rate. High central gas column densities can severely obscure AGN in high-redshift disks, possibly hindering their detection in deep X-ray surveys.
This lecture reviews the fundamental physical processes involved in star formation in galaxy interactions and mergers. Interactions and mergers often drive intense starbursts, but the link between interstellar gas physics, large scale interactions, a
nd active star formation is complex and not fully understood yet. Two processes can drive starbursts: radial inflows of gas can fuel nuclear starbursts, triggered gas turbulence and fragmentation can drive more extended starbursts in massive star clusters with high fractions of dense gas. Both modes are certainly required to account for the observed properties of starbursting mergers. A particular consequence is that star formation scaling laws are not universal, but vary from quiescent disks to starbursting mergers. High-resolution hydrodynamic simulations are used to illustrate the lectures.
Major progress has been made over the last few years in understanding hydrodynamical processes on cosmological scales, in particular how galaxies get their baryons. There is increasing recognition that a large part of the baryons accrete smoothly ont
o galaxies, and that internal evolution processes play a major role in shaping galaxies - mergers are not necessarily the dominant process. However, predictions from the various assembly mechanisms are still in large disagreement with the observed properties of galaxies in the nearby Universe. Small-scale processes have a major impact on the global evolution of galaxies over a Hubble time and the usual sub-grid models account for them in a far too uncertain way. Understanding when, where and at which rate galaxies formed their stars becomes crucial to understand the formation of galaxy populations. I discuss recent improvements and current limitations in resolved modelling of star formation, aiming at explicitely capturing star-forming instabilities, in cosmological and galaxy-sized simulations. Such models need to develop three-dimensional turbulence in the ISM, which requires parsec-scale resolution at redshift zero.
Disk galaxies at high redshift (z~2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger
simulations, however, model the ISM as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion-dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early-type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.
The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scatter
ing. The bulge and thick disks of modern galaxies may form this way. Simulations of minor mergers make thick disks too, but there is an important difference. Thick disks made by internal processes have a constant scale height with galactocentric radius, but thick disks made by mergers flare. The difference arises because in the first case, perpendicular forcing and disk-gravity resistance are both proportional to the disk column density, so the resulting scale height is independent of this density. In the case of mergers, perpendicular forcing is independent of the column density and the low density regions get thicker; the resulting flaring is inconsistent with observations. Late-stage gas accretion and thin disk growth are shown to preserve the constant scale heights of thick disks formed by internal evolution. These results reinforce the idea that disk galaxies accrete most of their mass smoothly and acquire their structure by internal processes, in particular through turbulent and clumpy phases at high redshift.
A number of theoretical and simulation results on star and structure formation in galaxy interactions and mergers is reviewed, and recent hydrodynamic simulations are presented. The role of gravity torques and ISM turbulence in galaxy interactions, i
n addition to the tidal field, is highlighted. Interactions can drive gas inflows towards the central kpc and trigger a central starburst, the intensity and statistical properties of which are discussed. A kinematically decoupled core and a supermassive central black hole can be fueled. Outside of the central kpc, many structures can form inside tidal tails, collisional ring, bridges, including super star clusters and tidal dwarf galaxies. The formation of super star clusters in galaxy mergers can now be directly resolved in hydrodynamic simulations. Their formation mechanisms and long-term evolution are reviewed, and the connection with present-day early-type galaxies is discussed.
Tidal dwarf galaxies form during the interaction, collision or merger of massive spiral galaxies. They can resemble normal dwarf galaxies in terms of mass, size, and become dwarf satellites orbiting around their massive progenitor. They nevertheless
keep some signatures from their origin, making them interesting targets for cosmological studies. In particular, they should be free from dark matter from a spheroidal halo. Flat rotation curves and high dynamical masses may then indicate the presence of an unseen component, and constrain the properties of the missing baryons, known to exist but not directly observed. The number of dwarf galaxies in the Universe is another cosmological problem that can be significantly impacted if tidal dwarf galaxies formed frequently at high redshift, when the merger rate was high, and many of them survived until today.
Galaxies above redshift 1 can be very clumpy, with irregular morphologies dominated by star complexes as large as 2 kpc and as massive as a few 10^8 or 10^9 Mo. Their co-moving densities and rapid evolution suggest that most present-day spirals could
have formed through a clumpy phase. The clumps may form by gravitational instabilities in gas-rich turbulent disks; they do not appear to be separate galaxies merging together. We show here that the formation of the observed clumps requires initial disks of gas and stars with almost no stabilizing bulge or stellar halo. This cannot be achieved in models where disk galaxies grow by mergers. Mergers tend to make stellar spheroids even when the gas fraction is high, and then the disk is too stable to make giant clumps. The morphology of high-redshift galaxies thus suggests that inner disks assemble mostly by smooth gas accretion, either from cosmological flows or from the outer disk during a grazing interaction.
