Dry minor mergers and size evolution of high-z compact massive early-type galaxies

Recent observations show evidence that high-z (z\sim 2 - 3) early-type galaxies (ETGs) are more compact than those with comparable mass at z\sim 0. Such a size evolution is most likely explained by the `Dry Merger Sceanario'. However, previous studies based on this scenario are not able to consistantly explain both the properties of the high-z compact massive ETGs and the local ETGs. We investigate the effect of multiple sequential dry minor mergers on the size evolution of the compact massive ETGs. From an analysis of the Millennium Simulation Database, we show that such minor (stellar mass ratio $M_{2}/M_{1} < 1/4$) mergers are extremely common during hierarchical structure formation. We perform N-body simulations of sequential minor mergers with parabolic and head-on orbits, including a dark matter component and a stellar component. Typical mass ratios of the minor mergers are $1/20 < M_{2}/M_{1} < 1/10$. We show that sequential minor mergers of compact satellite galaxies are the most efficient at promoting size growth and decreasing the velocity dispersion of the compact massive ETGs in our simulations. The change of stellar size and density of the merger remnants is consistent with recent observations. Furthermore, we construct the merger histories of candidates for the high-z compact massive ETGs using the Millennium Simulation Database, and estimate the size growth of the galaxies by the dry minor merger scenario. We can reproduce the mean size growth factor between $z=2$ and $z=0$, assuming the most efficient size growth obtained during sequential minor mergers in our simulations. However, we note that our numerical result is only valid for merger histories with typical mass ratios between 1/20 and 1/10 with parabolic and head-on orbits, and that our most efficient size growth efficiency is likely to an upper limit.Comment: 23 pages, 15 figures, accepted for MNRAS, Fig. 5 is fixe

The Effect of Self-gravity of Gas on Gas Fueling in Barred Galaxies with a Supermassive Black Hole

In our previous paper, we have shown that a gas disk in the nuclear region of a barred galaxy which contains a central supermassive black hole (SMBH) rapidly evolves into a nuclear gas ring by the effect of an additional inner Lindblad resonance caused by the SMBH. In this paper, we investigate the fate of the gas ring, involving self-gravity of gas, using two-dimensional hydrodynamical simulations. We find that the gas ring becomes gravitationally unstable for a gas surface density of gas above a critical value, and fragments into several gas clumps. Some denser clumps increase their mass via the accretion of the surrounding gas and collisions with other clumps, and finally a very massive gas clump (10^7 M_sun) is formed. Due to the torque from the most massive clump, a part of the gas in the ring loses its angular momentum and falls into the galactic center. As a result, a nuclear gas disk (50 pc) is formed around the SMBH. The accretion rate for $R<50$ pc attains about 1 M_sun/yr for 3.5*10^7 yr. At the final phase of the bar-driven fueling, self-gravity is crucial for the angular momentum transfer of the gas. This is a new mechanism for gas fueling to the vicinity of the SMBH.Comment: 14 pages, 7 figures, AASTeX, submitted to Ap

Cosmic evolution of bars in simulations of galaxy formation

We investigate the evolution of two bars formed in fully self-consistent hydrodynamic simulations of the formation of Milky Way-mass galaxies. One galaxy shows higher central mass concentration and has a longer and stronger bar than the other at $z = 0$. The stronger bar evolves by transferring its angular momentum mainly to the dark halo. Consequently the rotation speed of the bar decreases with time, while the amplitude of the bar increases with time. These features qualitatively agree with the results obtained by idealized simulations. The pattern speed of the stronger bar largely goes up and down within a half revolution in its early evolutionary stage. These oscillations occur when the bar is misaligned with the $m = 4$ mode Fourier component. These oscillations correlate with the oscillations in the triaxilality of the dark matter halo, but differently from the way identified by idealized simulations. The amplitude of the weaker bar does not increase despite the fact that its rotation slows down with time.This result contradicts what is expected from idealized simulations and is caused by the decline of the central density associated with the mass loss and feedback from the stellar populations. The amplitude of the weaker bar is further weakens by the angular momentum injection by the interactions with stellar clumps in the disk. In the both galaxies, the bars are terminated around the 4:1 resonance.Comment: 17 pages, 18 figures, accepted for publication in PAS

Does feedback help or hinder star formation? The effect of photoionisation on star formation in Giant Molecular Clouds

We investigated the effect of photoionising feedback inside turbulent star-forming clouds, comparing the resultant star formation in both idealised profiles and more realistic cloud structures drawn from a global galaxy simulation. We performed a series of numerical simulations which compared the effect of star formation alone, photoionisation and photoionisation plus supernovae feedback. In the idealised cloud, photoionisation suppresses gas fragmentation at early times, resulting in the formation of more massive stars and an increase in the star formation efficiency. At later times, the dispersal of the dense gas causes the radiative feedback effect to switch from positive to negative as the star formation efficiency drops. In the cloud extracted from the global simulation, the initial cloud is heavily fragmented prior to the stellar feedback beginning and is largely structurally unaffected by the late injection of radiation energy. The result is a suppression of the star formation. We conclude that the efficiency of feedback is heavily dependent on the gas structure, with negative feedback dominating when the density is high.Comment: Accepted to MNRA

Environmental dependence of star formation induced by cloud collisions in a barred galaxy

Cloud collision have been proposed as a way to link the small-scale star formation process with the observed global relation between the surface star formation rate and gas surface density. We suggest that this model can be improved further by allowing the productivity of such collisions to depend on the relative velocity of the two clouds. Our adjustment implements a simple step function that results in the most successful collisions being at the observed velocities for triggered star formation. By applying this to a high resolution simulation of a barred galaxy, we successfully reproduce the observational result that the star formation efficiency (SFE) in the bar is lower than that in the spiral arms. This is not possible when we use an efficiency dependent on the internal turbulence properties of the clouds. Our results suggest that high velocity collisions driven by the gravitational pull of the clouds are responsible for the low bar SFE.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS Letter

Effects of Minor Mergers on Coalescence of a Supermassive Black Hole Binary

We study the possibility that minor mergers resolve the loss cone depletion problem, which is the difficulty occured in the coalescence process of the supermassive black hole (SMBH) binary, by performing numerical simulations with a highly accurate $N$-body code. We show that the minor merger of a dwarf galaxy disturbs stellar orbits in the galactic central region of the host galaxy where the loss cone depletion is already caused by the SMBH binary. The disturbed stars are supplied into the loss cone. Stars of the dwarf galaxy are also supplied into the loss cone. The gravitational interactions between the SMBH binary and these stars become very effective. The gravitational interaction decreases the binding energy of the SMBH binary effectively. As a result, the shrink of the separation of the SMBH binary is accelerated. Our numerical results strongly suggest that the minor mergers are one of the important processes to reduce the coalescence time of the SMBH binary much less than the Hubble time.Comment: 6 pages, 16 figures. accepted for publication in PAS

Bar-driven Fueling to a Galactic Central Reagin in a Massive Gas Disk

We have found an effective fueling process to a central region of galaxies with weak bar-like distortion by two dimensional hydrodynamical simulations. Gravitational instability of an elongated gas ring at the inner Lindblad resonance (ILR), which has been reported as an effective fueling mechanism, are not needed for this fueling process. A massive gaseous disk in a central region of galaxies sensitively responds to the weakly distorted potential, and a large amount of gas can be fed into within $1/20$ of a core radius of the potential in several $10^7$ yr. The ILRs, the dissipative nature of the gas, and self-gravity of the gas are essential for triggering this effective fueling. The accumulation process has not been ever known: the gas accumulates to form a dense `linear' structure inclined at about 45 degree with respect to the bar potential in a {\it leading sense}. We also found that a counter rotating gaseous core can be formed as a result of the fueling. The sense of the rotation of the core depends on a fraction of the gas mass to the background mass. Physical mechanism of the fueling process can be understood using a linear theory of gaseous orbits in a weak barred potential.Comment: 8 pages, uuencoded gziped Postscript file without figures. Figures will be sent upon request ([email protected]). Accepted by MNRA