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In this paper, we use star-forming disk samples from IllustrisTNG-100 simulation to study the interconnections between a galaxy’s star formation, the circumgalactic gas motion and the large-scale environment. We divide the CGM gas into a cold phase (from 1e4 K to 2e4 K) and a hot phase (higher than 1e5 K). The motions of these phases are totally different, with cold phase displaying tangentially inwards motion (in-spiral) and hot phase exhibiting radially out-going motion. Both hot and cold phases receive angular momentum modulations from the large-scale environment and regulations from feedback, but they show different responses as is shown in the figure above: the lower the angular momentum of the cold gas, the faster it inflows into the galaxy, triggering stronger star formation activity; the more active star formation is, the faster the hot gas flows outwards, serving as a resistance to the inflowing cold CGM.

We find that the interactions (mergers and fly-bys) with the large-scale environment are often responsible for triggering new star formation episodes in the galaxy center. A crucial condition for the episodic star formation relies on a galaxy’s large-scale environment, where galaxy mergers and fly-by interactions continuously provide effective channels to send low angular momentum cold gas from large scales down to small radii inside the galaxy. This supplement of the low angular momentum cold gas will light up the star formation activity and also the feedback. Strong feedback will then prevent the further build-up of the gas reservoir. When the gas reservoir of a galaxy is consumed, the galaxy will step into a quiescent state, the weakened feedback will allow the low angular momentum cold gas to flow in -- a new star formation episode then starts. By repeating the process above, our star-forming disk samples finally present episodic star formation histories. Such episodic star formation pattern is well synced with the “breathing in and out” motion of the CGM gas which receives angular momentum modulations from large-scale environment and the regulations from the feedback. As a result, a present day star-forming galaxy may have gone through multiple star-forming with quiescent cycles in between during its evolution. We propose that this signal can be extracted from spatially resolved stellar populations for Milky Way and nearby galaxies.

We make use of the advanced magneto-hydrodynamic cosmological galaxy formation simulation, the IllustrisTNG Simulation to study the connection among the environment, the ambient CGM gas around galaxies and the star-formation/quenching activities inside galaxies. We show that the influence exerted by the large-scale galaxy environment on the CGM gas angular momentum results in either enhanced (Paper I) or suppressed (Paper II, this paper) star formation inside a galaxy. The present-day quenched early-type galaxies are seen to have higher CGM angular momentum, which is inherited from their high environmental angular momentum, resulting in less efficient gas inflow into the central star-forming gas reservoirs. Thus, they will keep the quenched status once they are quenched. As a comparison, however, the present-day star-forming disk galaxies may have experienced multiple star-forming and quenched epochs due to the "fight" between the CGM gas inflow (due to their relatively low angular momentum) and the AGN activities (Paper I).

As a hint, we show the evolution tracks on the sSFR-stellar mass plane of a present-day star-forming disk galaxy (Galaxy ID: 434303, left) and a present-day quenched elliptical galaxy (Galaxy ID: 407204, right). In each panel, the four dashed lines from top to bottom indicate the lower boundary in log sSFR for the main sequence galaxies at z=2, 1, 0.5, and 0 (indicated by colors), below which galaxies are regarded as quenched. The lower boundary is calculated as the 16th percentile of the sSFR for the galaxies with stellar masses between 3E9 to 3E10 solar masses at different redshifts.

For the example present-day star-forming disk (left), it has experienced multiple star-forming episodes, interrupted by quenched phases, most noticeably at z~1 and z~0.5, where the sSFR drops significantly below a certain star-forming threshold and the central gas fraction drops to below 5%. During these quenched phases, this system shows a deficit of cold and lower-angular momentum CGM gas. Luckily, each time a new episode of star formation manages to come back (once the feedback from the previous generation weakens) thanks to the resupply of lower-angular momentum CGM gas, as a perturbative consequence of galaxy interaction. In comparison, the present-day quenched early-type galaxy (right) was not that "lucky". It was once an active star-forming galaxy before z~2 and then became quenched and has remained quenched ever since, as indicated by a sudden drop of nearly three orders of magnitude in sSFR around z~1.

Galaxy morphologies, kinematics, and stellar populations are thought to be linked to each other. However, both simulations and observations have pointed out mismatches therein. In this work, we study the nature and origin of the present-day quenched, bulge-dominated, but dynamically cold galaxies in the IllustrisTNG-100 simulation. We compare these galaxies with a population of normal star-forming dynamically cold disc galaxies and a population of normal quenched dynamically hot elliptical galaxies within the same mass range. The populations of the present-day quenched and bulge-dominated galaxies (both being dynamically cold and hot) used to have significantly higher star formation rates and flatter morphologies at redshift of z~2. They have experienced more frequent larger mass-ratio mergers below z~0.7 in comparison to their star-forming disc counterparts, which is responsible for the formation of their bulge-dominated morphologies. The dynamically cold populations (both being star forming and quenched) have experienced more frequent prograde and tangential mergers especially below z~1, in contrast to the dynamically hot ellipticals, which have had more retrograde and radial mergers. Such different merging histories can well explain the differences on the cold and hot dynamical status among these galaxies. We point out that the real-world counterparts of these dynamically cold and hot bulge-dominated quenched populations are the fast- and slow-rotating early-type galaxies, respectively, as seen in observations and hence reveal the different evolution paths of these two distinct populations of early-type galaxies.

A key feature of a large population of low-mass, late-type disc galaxies are star-forming discs with exponential light distributions. They are typically also associated with thin and flat morphologies, blue colours, and dynamically cold stars moving along circular orbits within co-planar thin gas discs. However, the latter features do not necessarily always imply the former, in fact, a variety of different kinematic configurations do exist. In this work, we use the cosmological hydrodynamical IllustrisTNG simulation to study the nature and origin of dynamically hot, sometimes even counter-rotating, star-forming disc galaxies in the lower stellar mass range. We find that being dynamically hot arises in most cases as an induced transient state, for example due to galaxy interactions and merger activities, rather than as an age-dependent evolutionary phase of star-forming disc galaxies. The dynamically hot but still actively star-forming discs show a common feature of hosting kinematically misaligned gas and stellar discs, and centrally concentrated on-going star formation. The former is often accompanied by disturbed gas morphologies, while the latter is reflected in low gas and stellar spins in comparison to their dynamically cold, normal disc counterparts. Interestingly, observed galaxies from MaNGA with kinematic misalignment between gas and stars show remarkably similar general properties as the IllustrisTNG galaxies, and therefore are plausible real-world counterparts. In turn, this allows us to make predictions for the stellar orbits and gas properties of these misaligned galaxies.