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Register a new userThe effects of a hot gaseous halo in galaxy major mergers
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Cosmological hydrodynamical simulations as well as observations indicate that spiral galaxies are comprised of five different components: dark matter halo, stellar disc, stellar bulge, gaseous disc and gaseous halo. While the first four components have been extensively considered in numerical simulations of binary galaxy mergers, the effect of a hot gaseous halo has usually been neglected even though it can contain up to 80% of the total gas within the galaxy virial radius. We present a series of hydrodynamic simulations of major mergers of disc galaxies, that for the first time include a diffuse, rotating, hot gaseous halo. Through cooling and accretion, the hot halo can dissipate and refuel the cold gas disc before and after a merger. This cold gas can subsequently form stars, thus impacting the morphology and kinematics of the remnant. Simulations of isolated systems with total mass M~10^12Msun show a nearly constant star formation rate of ~5Msun/yr if the hot gaseous halo is included, while the star formation rate declines exponentially if it is neglected. We conduct a detailed study of the star formation efficiency during mergers and find that the presence of a hot gaseous halo reduces the starburst efficiency (e=0.5) compared to simulations without a hot halo (e=0.68). Moreover we find cases where the stellar mass of the merger remnant is lower than the sum of the stellar mass of the two progenitor galaxies when evolved in isolation. This suggests a revision to semi-analytic galaxy formation models which assume that a merger always leads to enhanced star formation. We show that adding the hot gas component has a significant effect on the kinematics and internal structure of the merger remnants, like an increased abundance of fast rotators and an r^(1/4) surface brightness profile at small scales.
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Currently-proposed galaxy quenching mechanisms predict very different behaviours during major halo mergers, ranging from significant quenching enhancement (e.g., clump-induced gravitational heating models) to significant star formation enhancement (e.g., gas starvation models). To test real galaxies behaviour, we present an observational galaxy pair method for selecting galaxies whose host haloes are preferentially undergoing major mergers. Applying the method to central L* (10^10 Msun < M_* < 10^10.5 Msun) galaxies in the Sloan Digital Sky Survey (SDSS) at z<0.06, we find that major halo mergers can at most modestly reduce the star-forming fraction, from 59% to 47%. Consistent with past research, however, mergers accompany enhanced specific star formation rates for star-forming L* centrals: ~10% when a paired galaxy is within 200 kpc (approximately the host halos virial radius), climbing to ~70% when a paired galaxy is within 30 kpc. No evidence is seen for even extremely close pairs (<30 kpc separation) rejuvenating star formation in quenched galaxies. For galaxy formation models, our results suggest: (1) quenching in L* galaxies likely begins due to decoupling of the galaxy from existing hot and cold gas reservoirs, rather than a lack of available gas or gravitational heating from infalling clumps, (2) state-of-the-art semi-analytic models currently over-predict the effect of major halo mergers on quenching, and (3) major halo mergers can trigger enhanced star formation in non-quenched central galaxies.
The circumgalactic region of the Milky Way contains a large amount of gaseous mass in the warm-hot phase. The presence of this warm-hot halo observed through $z=0$ X-ray absorption lines is generally agreed upon, but its density, path-length, and mass is a matter of debate. Here I discuss in detail why different investigations led to different results. The presence of an extended (over 100 kpc) and massive (over ten billion solar masses) warm-hot gaseous halo is supported by observations of other galaxies as well. I briefly discuss the assumption of constant density and end with outlining future prospects.
We report on the merger-induced generation of a shock-heated gas wind and formation of a remnant gas halo in simulations of colliding disk galaxies. The simulations use cosmologically motivated initial conditions and include the effects of radiative cooling, star formation, stellar feedback and the non-adiabatic heating of gas. The non-adiabatic heating, i.e. shocks, generated in the final merger forces gas out of the central region of the merger remnant and into the dark-matter halo. We demonstrate that the amount of heating depends on the size of the progenitor disk galaxy as well as the initial orbit the galaxies are placed on. Based upon these dependencies, we motivate a possible recipe for including this effect in semi-analytic models of galaxy formation.
Fine-grained estimation of galaxy merger stages from observations is a key problem useful for validation of our current theoretical understanding of galaxy formation. To this end, we demonstrate a CNN-based regression model that is able to predict, for the first time, using a single image, the merger stage relative to the first perigee passage with a median error of 38.3 million years (Myrs) over a period of 400 Myrs. This model uses no specific dynamical modeling and learns only from simulated merger events. We show that our model provides reasonable estimates on real observations, approximately matching prior estimates provided by detailed dynamical modeling. We provide a preliminary interpretability analysis of our models, and demonstrate first steps toward calibrated uncertainty estimation.
We use stellar mass functions to study the properties and the significance of quenching through major galaxy mergers. In addition to SDSS DR7 and Galaxy Zoo 1 data, we use samples of visually selected major galaxy mergers and post merger galaxies. We determine the stellar mass functions of the stages that we would expect major merger quenched galaxies to pass through on their way from the blue cloud to the red sequence: 1: major merger, 2: post merger, 3: blue early type, 4: green early type and 5: red early type. Based on the similar mass function shapes we conclude that major mergers are likely to form an evolutionary sequence from star formation to quiescence via quenching. Relative to all blue galaxies, the major merger fraction increases as a function of stellar mass. Major merger quenching is inconsistent with the mass and environment quenching model. At z~0 major merger quenched galaxies are unlikely to constitute the majority of galaxies that transition the green valley. Furthermore, between z~0-0.5 major merger quenched galaxies account for 1-5% of all quenched galaxies at a given stellar mass. Major galaxy mergers are therefore not a significant quenching pathway, neither at z~0 nor within the last 5 Gyr. The majority of red galaxies must have been quenched through an alternative quenching mechanism which causes a slow blue to red evolution.
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