We point out a natural mechanism for quenching of star formation in early-type galaxies (ETGs). It automatically links the color of a galaxy with its morphology and does not require gas consumption, ...removal or termination of gas supply. Given that star formation takes place in gravitationally unstable gas disks, it can be quenched when a disk becomes stable against fragmentation to bound clumps. This can result from the growth of a stellar spheroid, for instance by mergers. We present the concept of morphological quenching (MQ) using standard disk instability analysis, and demonstrate its natural occurrence in a cosmological simulation using an efficient zoom-in technique. We show that the transition from a stellar disk to a spheroid can be sufficient to stabilize the gas disk, quench star formation, and turn an ETG red and dead while gas accretion continues. The turbulence necessary for disk stability can be stirred up by sheared perturbations within the disk in the absence of bound star-forming clumps. While other quenching mechanisms, such as gas stripping, active galactic nucleus feedback, virial shock heating, and gravitational heating are limited to massive halos, MQ can explain the appearance of red ETGs also in halos less massive than ~1012 M. The dense gas disks observed in some of today's red ellipticals may be the relics of this mechanism, whereas red galaxies with quenched gas disks could be more frequent at high redshift.
We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multicomponent model for stellar feedback and reaching numerical convergence in the global ...star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericentre passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated with the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultracompact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic centre. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.
ABSTRACT
High-redshift star-forming galaxies often have irregular morphologies with giant clumps containing up to 108−109 solar masses of gas and stars. The origin and evolution of giant clumps are ...debated both theoretically and observationally. In most cosmological simulations, high-redshift galaxies have regular spiral structures or short-lived clumps, in contradiction with many idealized high-redshift disc models. Here, we test whether this discrepancy can be explained by the low gas fractions of galaxies in cosmological simulations. We present a series of simulations with varying gas fractions, from 25 per cent, typical of galaxies in most cosmological simulations, to 50 per cent, typical of observed galaxies at 1.5 < z < 3. We find that gas-poor models have short-lived clumps, that are unbound and mostly destroyed by galactic shear, even with weak stellar feedback. In contrast, gas-rich models form long-lived clumps even with boosted stellar feedback. This shows that the gas mass fraction is the primary physical parameter driving violent disc instabilities and the evolution of giant clumps on ∼108 yr time-scales, with lower impact from the calibration of the stellar feedback. Many cosmological simulations of galaxy formation have relatively gas-poor galactic discs, which could explain why giant clumps are absent or short-lived in such models. Similar baryonic and dark matter mass distribution could produce clumpy galaxies with long-lived clumps at z ∼ 2 if the gas fraction was in better agreement with observations.
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 scattering. 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.
We study the evolution of galactic bars and the link with disk and spheroid formation in a sample of zoom-in cosmological simulations. Our simulation sample focuses on galaxies with present-day ...stellar masses in the 10 super(10)-10 super(11) Mmiddot in circle range, in field and loose group environments, with a broad variety of mass growth histories. Our models hence predict that the decrease in the bar fraction with increasing redshift should continue with a fraction of observable bars not larger than 10%-15% in disk galaxies at z > 1. We find that the characteristic epoch of bar formation, namely redshift z approximately 0.8-1 in the studied mass range, corresponds to the epoch at which today's spirals acquire their disk-dominated morphology. Early bars at z > 1 are often short-lived, while most of the bars formed at z < or =, slant 1 persist down to z = 0, late cosmological gas infall being necessary to maintain some of them.
We utilize zoom-in cosmological simulations to study the nature of violent disc instability in clumpy galaxies at high redshift, z = 1–5. Our simulated galaxies are not in the ideal state assumed in ...Toomre instability, of linear fluctuations in an isolated, uniform, rotating disc. There, instability is characterized by a Q parameter below unity, and lower when the disc is thick. Instead, the high-redshift discs are highly perturbed. Over long periods they consist of non-linear perturbations, compact massive clumps and extended structures, with new clumps forming in interclump regions. This is while the galaxy is subject to frequent external perturbances. We compute the local, two-component Q parameter for gas and stars, smoothed on a ∼1 kpc scale to capture clumps of 108–9 M⊙. The Q < 1 regions are confined to collapsed clumps due to the high surface density there, while the interclump regions show Q significantly higher than unity. Tracing the clumps back to their relatively smooth Lagrangian patches, we find that Q prior to clump formation typically ranges from unity to a few. This is unlike the expectations from standard Toomre instability. We discuss possible mechanisms for high-Q clump formation, e.g. rapid turbulence decay leading to small clumps that grow by mergers, non-axisymmetric instability, or clump formation induced by non-linear perturbations in the disc. Alternatively, the high-Q non-linear VDI may be stimulated by the external perturbations such as mergers and counter-rotating streams. The high Q may represent excessive compressive modes of turbulence, possibly induced by tidal interactions.
We present hydrodynamic simulations of a major merger of disk galaxies, and study the interstellar medium (ISM) dynamics and star formation (SF) properties. High spatial and mass resolutions of 12 pc ...and 4 x 10{sup 4} M {sub sun} allow us to resolve cold and turbulent gas clouds embedded in a warmer diffuse phase. We compare lower-resolution models, where the multiphase ISM is not resolved and is modeled as a relatively homogeneous and stable medium. While merger-driven bursts of SF are generally attributed to large-scale gas inflows toward the nuclear regions, we show that once a realistic ISM is resolved, the dominant process is actually gas fragmentation into massive and dense clouds and rapid SF therein. As a consequence, SF is more efficient by a factor of up to {approx}10 and is also somewhat more extended, while the gas density probability distribution function rapidly evolves toward very high densities. We thus propose that the actual mechanism of starburst triggering in galaxy collisions can only be captured at high spatial resolution and when the cooling of gas is modeled down to less than 10{sup 3} K. Not only does our model reproduce the properties of the Antennae system, but it also explains the 'starburst mode' recently revealed in high-redshift mergers compared to quiescent disks.
Context. Feedback processes and the galactic shear regulate star formation. Aims. We investigate the effects of differential galactic rotation and stellar feedback on the interstellar medium (ISM) ...and on the star formation rate (SFR). Methods. A numerical shearing box is used to perform three-dimensional simulations of a 1 kpc stratified cubic box of turbulent and self-gravitating interstellar medium (in a rotating frame) with supernovae and H II feedback. We vary the value of the velocity gradient induced by the shear and the initial value of the galactic magnetic field. Finally, the different star formation rates and the properties of the structures associated with this set of simulations are computed. Results. We first confirm that the feedback has a strong limiting effect on star formation. The galactic shear has also a great influence: the higher the shear, the lower the SFR. Taking the value of the velocity gradient in the solar neighbourhood, the SFR is too high compared to the observed Kennicutt law, by a factor approximately three to six. This discrepancy can be solved by arguing that the relevant value of the shear is not the one in the solar neighbourhood, and that in reality the star formation efficiency within clusters is not 100%. Taking into account the fact that star-forming clouds generally lie in spiral arms where the shear can be substantially higher (as probed by galaxy-scale simulations), the SFR is now close to the observed one. Different numerical recipes have been tested for the sink particles, giving a numerical incertitude of a factor of about two on the SFR. Finally, we have also estimated the velocity dispersions in our dense clouds and found that they lie below the observed Larson law by a factor of about two. Conclusions. In our simulations, magnetic field, shear, H II regions, and supernovae all contribute significantly to reduce the SFR. In this numerical setup with feedback from supernovae and H II regions and a relevant value of galactic shear, the SFRs are compatible with those observed, with a numerical incertitude factor of about two.
We study outflows driven by active galactic nuclei (AGNs) using high-resolution simulations of idealized z ∼ 2 isolated disc galaxies. Episodic accretion events lead to outflows with velocities >1000 ...km s−1 and mass outflow rates of about the star formation rate (several tens of M⊙ yr−1). Outflowing winds escape perpendicular to the disc with wide opening angles, and are typically asymmetric (i.e. unipolar) because dense gas above or below the AGN in the resolved disc inhibits outflow. Owing to rapid variability in the accretion rates, outflowing gas may be detectable even when the AGN is effectively ‘off’. The highest velocity outflows are sometimes, but not always, concentrated within 2–3 kpc of the galactic centre during the peak accretion. With our purely thermal AGN feedback model – standard in previous literature – the outflowing material is mostly hot (≳106 K) and diffuse (n
H ≲ 10− 2 cm−3), but includes a cold component entrained in the hot wind. Despite the powerful bursts and high outflow rates, AGN feedback has little effect on the dense gas in the galaxy disc. Thus AGN-driven outflows in our simulations do not cause rapid quenching of star formation, although they may remove significant amounts of gas over long (≳Gyr) time-scales.
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 X108 or 109 M . 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.
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