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We examine the impact of dark matter particle resolution on the formation of a baryonic core in high resolution adaptive mesh refinement simulations. We test the effect that both particle smoothing and particle splitting have on the hydrodynamic properties of a collapsing halo at high redshift (z > 20). Furthermore, we vary the background field intensity, with energy below the Lyman limit (< 13.6 eV), as may be relevant for the case of metal-free star formation and super-massive black hole seed formation. We find that using particle splitting methods greatly increases our particle resolution without introducing any numerical noise and allows us to achieve converged results over a wide range of external background fields. Additionally, we find that for lower values of the background field a lower dark matter particle mass is required. We define the radius of the core as the point at which the enclosed baryonic mass dominates over the enclosed dark matter mass. For our simulations this results in $\rm{R_{core} \sim 5\ pc}$. We find that in order to produce converged results which are not affected by dark matter particles requires that the relationship ${M_{\rm{core}} / M_{\rm{DM}}} > 100.0$ be satisfied, where ${M_{\rm{core}}}$ is the enclosed baryon mass within the core and $M_{\rm{DM}}$ is the minimum dark matter particle mass. This ratio should provide a very useful starting point for conducting convergence tests before any production run simulations. We find that dark matter particle smoothing is a useful adjunct to already highly resolved simulations., Comment: 16 pages. Updated to match MNRAS accepted version

We study the evolution of the gaseous components in massive simulated galaxies and show that their early formation is fuelled by cold, low entropy gas streams. At lower redshifts of z<3 the simulated galaxies are massive enough to support stable virial shocks resulting in a transition from cold to hot gas accretion. The gas accretion history of early-type galaxies is directly linked to the formation of their stellar component in the two phased formation scenario, in which the central parts of the galaxy assemble rapidly through in situ star formation and the later assembly is dominated primarily by minor stellar mergers., Comment: 4 pages, 3 figures. To appear in the Proceedings of the IAU Symposium No. 308: "The Zeldovich Universe: Genesis and Growth of the Cosmic Web", R. van de Weygaert, S. Shandarin, E. Saar and J. Einasto, eds

We present a catalogue and images of visually detected features, such as asymmetries, extensions, warps, shells, tidal tails, polar rings, and obvious signs of mergers or interactions, in the faint outer regions (at and outside of R_25) of nearby galaxies. This catalogue can be used in future quantitative studies that examine galaxy evolution due to internal and external factors. We are able to reliably detect outer region features down to a brightness level of 0.03 MJy/sr per pixel at 3.6 microns in the Spitzer Survey of Stellar Structure in Galaxies (S4G). We also tabulate companion galaxies. We find asymmetries in the outer isophotes in 22+/-1 per cent of the sample. The asymmetry fraction does not correlate with galaxy classification as an interacting galaxy or merger remnant, or with the presence of companions. We also compare the detected features to similar features in galaxies taken from cosmological zoom re-simulations. The simulated images have a higher fraction (33 per cent) of outer disc asymmetries, which may be due to selection effects and an uncertain star formation threshold in the models. The asymmetries may have either an internal (e.g., lopsidedness due to dark halo asymmetry) or external origin., Comment: 26 pages, 16 embedded + 100 online-only figures, accepted for publication in MNRAS

The direct collapse model of supermassive black hole seed formation provides an attractive solution to the origin of the quasars now routinely observed at $z \gtrsim 6$. We use the adaptive mesh refinement code Enzo to simulate the collapse of gas at high redshift, including a nine species chemical model of H, He, and H$_2$. The direct collapse model requires that the gas cools predominantly via atomic hydrogen. To this end we simulate the effect of an anisotropic radiation source on the collapse of a halo at high redshift. The radiation source is placed at a distance of 3 kpc (physical) from the collapsing object. The source is set to emit monochromatically in the center of the Lyman-Werner (LW) band only at $12.8 \ \rm{eV}$. The LW radiation emitted from the high redshift source is followed self-consistently using ray tracing techniques. We find that, due to self-shielding, a small amount of H$_2$ is able to form at the very center of the collapsing halo even under very strong LW radiation. Furthermore, we find that a radiation source, emitting $> 10^{54}\ (\sim10^3\ \rm{J_{21}})$ photons per second is required to cause the collapse of a clump of $\rm{M \sim 10^5}$ M$_{\odot}$. The resulting accretion rate onto the collapsing object is $\sim 0.25$ M$_{\odot}$ $\rm{yr^{-1}}$. Our results display significant differences, compared to the isotropic radiation field case, in terms of H$_2$ fraction at an equivalent radius. These differences will significantly effect the dynamics of the collapse. With the inclusion of a strong anisotropic radiation source, the final mass of the collapsing object is found to be $\rm{M \sim 10^5}$ M$_{\odot}$. This is consistent with predictions for the formation of a supermassive star or quasi-star leading to a supermassive black hole., Comment: 18 pages. ApJ accepted, with minor changes to submitted version. High resolution movies available at http://www.helsinki.fi/~regan/visualisations.html

We use a large suite of hydrodynamical simulations of binary galaxy mergers to construct and calibrate a physical prescription for computing the effective radii and velocity dispersions of spheroids. We implement this prescription within a semi-analytic model embedded in merger trees extracted from the Bolshoi Lambda-CDM N-body simulation, accounting for spheroid growth via major and minor mergers as well as disk instabilities. We find that without disk instabilities, our model does not predict sufficient numbers of intermediate mass early-type galaxies in the local universe. Spheroids also form earlier in models with spheroid growth via disk instabilities. Our model correctly predicts the normalization, slope, and scatter of the low-redshift size-mass and Fundamental Plane relations for early type galaxies. It predicts a degree of curvature in the Faber-Jackson relation that is not seen in local observations, but this could be alleviated if higher mass spheroids have more bottom-heavy initial mass functions. The model also correctly predicts the observed strong evolution of the size-mass relation for spheroids out to higher redshifts, as well as the slower evolution in the normalization of the Faber-Jackson relation. We emphasize that these are genuine predictions of the model since it was tuned to match hydrodynamical simulations and not these observations., Comment: Submitted to MNRAS

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A and B, observed between first passage and final coalescence. The spectra provide stellar and gas kinematics, ionised gas properties and stellar population diagnostics, over the full optical extent of both galaxies. The Mice provide a perfect case study highlighting the importance of IFS data for improving our understanding of local galaxies. The impact of first passage on the kinematics of the stars and gas has been significant, with strong bars likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong twist in both its stellar and ionised gas disk. On the other hand, the impact of the merger on the stellar populations has been minimal thus far: star formation induced by the recent close passage has not contributed significantly to the global star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc above the disk plane. The measured ram pressure and mass outflow rate (~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that provided by the mechanical energy of the starburst appears to be required to drive the outflow. We compare the observations to mock kinematic and stellar population maps from a merger simulation. The models show little enhancement in star formation during and following first passage, in agreement with the observations. We highlight areas where IFS data could help further constrain the models., Comment: 23 pages, 13 figures, accepted to A&A. A version with a complete set of high resolution figures is available here: http://www-star.st-and.ac.uk/~vw8/resources/mice_v8_astroph.pdf

We have performed high-resolution numerical simulations with the hydrodynamical AMR code Enzo to investigate the formation of massive seed black holes in a sample of six dark matter haloes above the atomic cooling threshold. The aim of this study is to illustrate the effects of varying the maximum refinement level on the final object formed. The virial temperatures of the simulated haloes range from $\rm{T} \sim 10000\ \rm{K} - 16000\ \rm{K}$ and they have virial masses in the range $\rm{M} \sim 2 \times 10^7 \rm{M_{\odot}}$ to $\rm{M} \sim 7 \times 10^7 \rm{M_{\odot}}$ at $z \sim 15$. The outcome of our six fiducial simulations is both generic and robust. A rotationally supported, marginally gravitationally stable, disk forms with an exponential profile. The mass and scale length of this disk depends strongly on the maximum refinement level used. Varying the maximum refinement level by factors between 1 / 64 to 256 times the fiducial level illustrates the care that must be taken in interpreting the results. The lower resolution simulations show tentative evidence that the gas may become rotationally supported out to 20 pc while the highest resolution simulations show only weak evidence of rotational support due to the shorter dynamical times for which the simulation runs. The higher resolution simulations do, however, point to fragmentation at small scales of the order of $\sim 100$ AU. In the highest resolution simulations a central object of a few times $10^2\ \rm{M_{\odot}}$ forms with multiple strongly bound, Jeans unstable, clumps of $\sim 10\ \rm{M_{\odot}}$ and radii of 10 - 20 AU suggesting the formation of dense star clusters in these haloes., Comment: Accepted for publication in MNRAS. 18 pages. High Resolutions visualisations and movies available at http://www.helsinki.fi/~regan/visualisations.html

The stellar velocity dispersion as a function of the galactocentric radius of an early-type galaxy can generally be well approximated by a power law $\sigma \propto r^{\beta}$. However, some observed dispersion profiles show a deviation from this fit at intermediate radii, usually between one and three $R_{\mathrm{eff}}$, where the velocity dispersion remains constant with radius, showing a bump-like behavior, which we term the "$\sigma$-"bump. To understand the origin of this $\sigma$-bump, we study a set of simulated early-type galaxies formed in major mergers. We find the $\sigma$-bump in all of our simulated early-type galaxies, with the size and position of the bump slightly varying from galaxy to galaxy, leading to the assumption that the bump is a characteristic of the major merger formation scenario. The feature can be seen both in the intrinsic and projected stellar velocity dispersions. In contrast to shells that form during the merger event but evolve with time and finally disappear, the $\sigma$-bump stays nearly constant with radius and is a permanent feature that is preserved until the end of the simulation. The $\sigma$-bump is not seen in the dark matter and gas components and we therefore conclude that it is a purely stellar feature of merger remnants., Comment: 7 pages, 4 figures, submitted to APJ Letters

We study the effect of AGN mechanical and radiation feedback on the formation of bulge dominated galaxies via mergers of disc galaxies. The merging galaxies have mass-ratios of 1:1 to 6:1 and include pre-existing hot gaseous halos to properly account for the global impact of AGN feedback. Using smoothed particle hydrodynamics simulation code (GADGET-3) we compare three models with different AGN feedback models: (1) no black hole and no AGN feedback; (2) thermal AGN feedback; and (3) mechanical and radiative AGN feedback. The last model is motivated by observations of broad line quasars which show winds with initial velocities of $v_w \ge$ 10,000 km/s and also heating associated with the central AGN X-ray radiation. The primary changes in gas properties due to mechanical AGN feedback are lower thermal X-ray luminosity from the final galaxy - in better agreement with observations - and galactic outflows with higher velocity $\sim 1000$ km/s similar to recent direct observations of nearby merger remnants. The kinetic energy of the outflowing gas is a factor of $\sim$ 20 higher than in the thermal feedback case. All merger remnants with momentum-based AGN feedback with $v_w \sim 10,000$ km/s and $\epsilon_w=2 \times 10^{-3}$, independent of their progenitor mass-ratios, reproduce the observed relations between stellar velocity dispersion and black hole mass ($M_{\rm bh} - \sigma$) as well as X-ray luminosity ($L_X - \sigma$) with $10^{37.5}$ erg/s $\lesssim L_X (0.3-8~{\rm keV}) \lesssim 10^{39.2}$ erg/s for velocity dispersions in the range of 120 km/s $\lesssim \sigma \lesssim$ 190 km/s. In addition, the mechanical feedback produces a much greater AGN variability. We also show that gas is more rapidly and impulsively stripped from the galactic centres driving a moderate increase in galaxy size and decrease in central density with the mechanical AGN feedback model., Comment: 16 pages, 10 figures, resubmitted to MNRAS

Dynamical modeling and strong lensing data indicate that the total density profiles of early-type galaxies are close to isothermal, i.e., rho_tot ~ r^gamma with gamma approx -2. To understand the origin of this universal slope we study a set of simulated spheroids formed in isolated binary mergers as well as the formation within the cosmological framework. The total stellar plus dark matter density profiles can always be described by a power law with an index of gamma approx -2.1 with a tendency toward steeper slopes for more compact, lower-mass ellipticals. In the binary mergers the amount of gas involved in the merger determines the precise steepness of the slope. This agrees with results from the cosmological simulations where ellipticals with steeper slopes have a higher fraction of stars formed in situ. Each gas-poor merger event evolves the slope toward gamma ~ -2, once this slope is reached further merger events do not change it anymore. All our ellipticals have flat intrinsic combined stellar and dark matter velocity dispersion profiles. We conclude that flat velocity dispersion profiles and total density distributions with a slope of gamma ~ -2 for the combined system of stars and dark matter act as a natural attractor. The variety of complex formation histories as present in cosmological simulations, including major as well as minor merger events, is essential to generate the full range of observed density slopes seen for present-day elliptical galaxies., Comment: Accepted by the Astrophysical Journal, 17 pages, 12 figures

We demonstrate that massive simulated galaxies assemble in two phases, with the initial growth dominated by compact in situ star formation, whereas the late growth is dominated by accretion of old stars formed in subunits outside the main galaxy. We also show that 1) gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy colour bimodality that 2) the observed galaxy downsizing can be explained naturally in the two-phased model and finally that 3) the details of the assembly histories of massive galaxies are directly connected to their observed kinematic properties., Comment: 4 pages, 3 figures. To appear in the Proceedings of IAU Symposium No. 295: "The intriguing life of massive galaxies", D. Thomas, A. Pasquali and I. Ferreras, eds

We study the growth of black holes (BHs) in galaxies using three-dimensional smoothed particle hydrodynamic simulations with new implementations of the momentum mechanical feedback, and restriction of accreted elements to those that are gravitationally bound to the BH. We also include the feedback from the X-ray radiation emitted by the BH, which heats the surrounding gas in the host galaxies, and adds radial momentum to the fluid. We perform simulations of isolated galaxies and merging galaxies and test various feedback models with the new treatment of the Bondi radius criterion. We find that overall the BH growth is similar to what has been obtained by earlier workers using the Springel, Di Matteo, & Hernquist algorithms. However, the outflowing wind velocities and mechanical energy emitted by winds are considerably higher (v_w ~ 1000-3000 km/s) compared to the standard thermal feedback model (v_w ~ 50-100 km/s). While the thermal feedback model emits only 0.1 % of BH released energy in winds, the momentum feedback model emits more than 30 % of the total energy released by the BH in winds. In the momentum feedback model, the degree of fluctuation in both radiant and wind output is considerably larger than in the standard treatments. We check that the new model of the BH mass accretion agrees with analytic results for the standard Bondi problem., Comment: 15 pages, 9 figures, ApJ in press

We present a sample of nine high resolution cosmological simulations in the mass range of M_vir=7x10^11-4x10^12 M_sun starting from LambdaCDM initial conditions. Our simulations include primordial radiative cooling, photoionization, star formation, supernova II feedback, but exclude supernova driven winds and AGN feedback. The simulated galaxies assemble in two phases, with the initial growth dominated by compact (rr_eff) resulting in an assembly redshift of z_{f,acc}~0.5-1.5 with much larger scatter. We find a positive correlation between the fraction of accreted stars and the final mass of our galaxies. We show that gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy color bimodality. The accretion of stellar material is also responsible for the observed size growth of early-type galaxies. In addition, we find that the dark matter fractions within the stellar half-mass radii continuously increase towards lower redshift from about f_DM~0.05 at z~3 to f_DM~0.1-0.3 at z=0. Furthermore, the logarithmic slope of the total density profile is nearly isothermal at the present-day (gamma'~1.9-2.2). Finally, the input of gravitational heating lowers the central dark matter densities in the galaxies, with the effect being smaller compared to simulations without supernova feedback., Comment: 23 pages, 16 figures, accepted for publication in ApJ

We analyze 40 cosmological re-simulations of individual massive galaxies with present-day stellar masses of $M_{*} > 6.3 \times 10^{10} M_{\odot}$ in order to investigate the physical origin of the observed strong increase in galaxy sizes and the decrease of the stellar velocity dispersions since redshift $z \approx 2$. At present 25 out of 40 galaxies are quiescent with structural parameters (sizes and velocity dispersions) in agreement with local early type galaxies. At z=2 all simulated galaxies with $M_* \gtrsim 10^{11}M_{\odot}$ (11 out of 40) at z=2 are compact with projected half-mass radii of $\approx$ 0.77 ($\pm$0.24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of $\approx$ 262 ($\pm$28) kms$^{-1}$ (3 out of 11 are already quiescent). Similar to observed compact early-type galaxies at high redshift the simulated galaxies are clearly offset from the local mass-size and mass-velocity dispersion relations. Towards redshift zero the sizes increase by a factor of $\sim 5-6$, following $R_{1/2} \propto (1+z)^{\alpha}$ with $\alpha = -1.44$ for quiescent galaxies ($\alpha = -1.12$ for all galaxies). The velocity dispersions drop by about one-third since $z \approx 2$, following $\sigma_{1/2} \propto (1+z)^{\beta}$ with $\beta = 0.44$ for the quiescent galaxies ($\beta = 0.37$ for all galaxies). The simulated size and dispersion evolution is in good agreement with observations and results from the subsequent accretion and merging of stellar systems at $z\lesssim 2$ which is a natural consequence of the hierarchical structure formation. A significant number of the simulated massive galaxies (7 out of 40) experience no merger more massive than 1:4 (usually considered as major mergers). On average, the dominant accretion mode is stellar minor mergers with a mass-weighted mass-ratio of 1:5. (abridged), Comment: 9 pages, 6 figures, accepted for publication in ApJ

Hydrodynamic simulations of galaxies with active galactic nuclei (AGN) have typically employed feedback that is purely local: i.e., an injection of energy to the immediate neighborhood of the black hole. We perform GADGET-2 simulations of massive elliptical galaxies with an additional feedback component: an observationally calibrated X-ray radiation field which emanates from the black hole and heats gas out to large radii from the galaxy center. We find that including the heating and radiation pressure associated with this X-ray flux in our simulations enhances the effects which are commonly reported from AGN feedback. This new feedback model is twice as effective as traditional feedback at suppressing star formation, produces 3 times less star formation in the last 6 Gyr, and modestly lowers the final BH mass (30%). It is also significantly more effective than an X-ray background in reducing the number of satellite galaxies., Comment: 9 emulateapj pages, 8 figures; accepted to ApJ

We derive expressions for the electromagnetic Green's function for a layered system using a transfer matrix technique. The expressions we arrive at makes it possible to study symmetry properties of the Green's function, such as reciprocity symmetry, and the long-range properties of the Green's function which involves plasmon waves as well as boundary waves, also known as Norton waves. We apply the method by calculating the light scattering cross section off a chain of nanoholes in a thin Au film. The results highlight the importance of nanohole interactions mediated by surface plasmon propagating along the chain of holes., Comment: 14 pages, 9 figures

Many observed massive star-forming z\approx2 galaxies are large disks that exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We present the largest sample to date of high-resolution cosmological SPH simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong stellar feedback parameterized as momentum-driven galactic winds. This model reproduces many characteristic features of this observed class of galaxies, such as their clumpy morphologies, smooth and monotonic velocity gradients, high gas fractions (f_g\sim50%) and high specific star-formation rates (\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps (Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities. However, the galactic winds are critical for their subsequent evolution. The giant clumps we obtain are short-lived and are disrupted by wind-driven mass loss. They do not virialise or migrate to the galaxy centers as suggested in recent work neglecting strong winds. By phenomenologically implementing the winds that are observed from high-redshift galaxies and in particular from individual clumps, our simulations reproduce well new observational constraints on clump kinematics and clump ages. In particular, the observation that older clumps appear closer to their galaxy centers is reproduced in our simulations, as a result of inside-out formation of the disks rather than inward clump migration., Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the Astrophysical Journal

Cosmological simulations of galaxy formation appear to show a two-phase character with a rapid early phase at z>2 during which in-situ stars are formed within the galaxy from infalling cold gas followed by an extended phase since z<3 during which ex-situ stars are primarily accreted. In the latter phase massive systems grow considerably in mass and radius by accretion of smaller satellite stellar systems formed at quite early times (z>3) outside of the virial radius of the forming central galaxy. These tentative conclusions are obtained from high resolution re-simulations of 39 individual galaxies in a full cosmological context with present-day virial halo masses ranging from 7e11 M_sun h^-1 < M_vir < 2.7e13 M_sun h^-1 and central galaxy masses between 4.5e10 M_sun h^-1 < M_* < 3.6e11 M_sun h^-1. The simulations include the effects of a uniform UV background, radiative cooling, star formation and energetic feedback from SNII. The importance of stellar accretion increases with galaxy mass and towards lower redshift. In our simulations lower mass galaxies ($M_* < 9e10 M_sun h^-1) accrete about 60 per cent of their present-day stellar mass. High mass galaxy ($M_* > 1.7e11 M_sun h^-1) assembly is dominated by accretion and merging with about 80 per cent of the stars added by the present-day. In general the simulated galaxies approximately double their mass since z=1. For massive systems this mass growth is not accompanied by significant star formation. The majority of the in-situ created stars is formed at z>2, primarily out of cold gas flows. We recover the observational result of archaeological downsizing, where the most massive galaxies harbor the oldest stars. We find that this is not in contradiction with hierarchical structure formation. Most stars in the massive galaxies are formed early on in smaller structures, the galaxies themselves are assembled late., Comment: 13 pages, 13 figures, accepted for publication in ApJ

We use a series of idealized, numerical SPH simulations to study the formation and evolution of galactic, gas-rich disks forming from gas infall within dark matter halos. The temperature and density structure of the gas is varied in order to differentiate between (i) simultaneous gas infall at a large range of radii and (ii) the inside-out build-up of a disk. In all cases, the disks go through phases of ring formation, gravitational instability and break-up into massive clumps. Ring formation can be enhanced by a focal point effect. The position of the ring is determined by the angular momentum distribution of the material it forms from. We study the ring and clump morphologies, the characteristic properties of the resulting velocity dispersion field and the effect of star formation. In the early phases, gas accretion leads to a high vertical velocity dispersion. We find that the disk fragmentation by gravitational instability and the subsequent clump-clump interactions drive high velocity dispersions mainly in the plane of the disk while at the same time the vertical velocity dispersion dissipates. The result is a strong variation of the line-of-sight velocity dispersion with inclination angle. For a face-on view, clumps appear as minima in the (vertical) dispersion, whereas for a more edge-on view, they tend to correspond to maxima. There exists observational evidence of a systematic variation of the velocity dispersion with inclination angle in high-redshift disks, which could be partly explained by our simulation results. Additional energetic sources to drive velocity dispersion that are not included in our models are also expected to contribute to the observational results., Comment: 16 pages, 12 figures, accepted for publication in ApJ

We present a new high-resolution N-body/SPH simulation of an encounter of two gas-rich disk galaxies which closely matches the morphology and kinematics of the interacting Antennae galaxies (NGC 4038/39). The simulation includes radiative cooling, star formation and feedback from SNII. The large-scale morphology and kinematics are determined by the internal structure and the orbit of the progenitor disks. The properties of the central region, in particular the starburst in the overlap region, only match the observations for a very short time interval of ~20 Myr after the second encounter. This indicates that the Antennae galaxies are in a special phase only about 40 Myr after the second encounter and 50 Myr before their final collision. This is the only phase in the simulation when a gas-rich overlap region between the nuclei is forming accompanied by enhanced star formation. The star formation rate as well as the recent star formation history in the central region agree well with observational estimates. For the first time this new model explains the distributed extra-nuclear star formation in the Antennae galaxies as a consequence of the recent second encounter. The proposed model predicts that the Antennae are in a later merger stage than the Mice (NGC 4676) and would therefore lose their first place in the classical Toomre sequence., Comment: 6 pages, 4 figures, 1 table. Accepted for publication in ApJL

We study the evolution of black holes (BHs) on the M_BH-sigma and M_BH-M_bulge planes as a function of time in disk galaxies undergoing mergers. We begin the simulations with the progenitor black hole masses being initially below (Delta log M_BH=-2), on (Delta log M_BH=0) and above (Delta log M_BH=0.5) the observed local relations. The final relations are rapidly established after the final coalescense of the galaxies and their BHs. Progenitors with low initial gas fractions (f_gas=0.2) starting below the relations evolve onto the relations (Delta log M_BH=-0.18), progenitors on the relations stay there (Delta log M_BH=0) and finally progenitors above the relations evolve towards the relations, but still remaining above them (Delta log M_BH=0.35). Mergers in which the progenitors have high initial gas fractions (f_gas=0.8) evolve above the relations in all cases (Delta log M_BH=0.5). We find that the initial gas fraction is the prime source of scatter in the observed relations, dominating over the scatter arising from the evolutionary stage of the merger remnants. The fact that BHs starting above the relations do not evolve onto the relations, indicates that our simulations rule out the scenario in which overmassive BHs evolve onto the relations through gas-rich mergers. By implication our simulations thus disfavor the picture in which supermassive BHs develop significantly before their parent bulges., Comment: 6 pages, 4 figures, accepted to ApJL (minor revisions to match accepted version)

Understanding the details of how the red sequence is built is a key question in galaxy evolution. What are the relative roles of gas-rich vs. dry mergers, major vs. minor mergers or galaxy mergers vs. gas accretion? In Wild et al. 2009 we compare hydrodynamic simulations with observations to show how gas-rich major mergers result in galaxies with strong post-starburst spectral features, a population of galaxies easily identified in the real Universe using optical spectra. Using spectra from the VVDS deep survey with z~0.7, and a principal component analysis technique to provide indices with high enough SNR, we find that 40% of the mass flux onto the red-sequence could enter through a strong post-starburst phase, and thus through gas-rich major mergers. The deeper samples provided by next generation galaxy redshift surveys will allow us to observe the primary physical processes responsible for the shut-down in starformation and build-up of the red sequence., Comment: 4 pages, 7 figures, proceedings of IAU symposium 262 "Stellar populations, planning for the next decade"

We find that the amount and nature of the assumed ionizing background can strongly affect galaxy formation and evolution. Galaxy evolution simulations typically incorporate an ultraviolet background which falls off rapidly above z=3; e.g., that of Haardt & Madau (1996). However, this decline may be too steep to fit the WMAP constraints on electron scattering optical depth or observations of intermediate redshift (z ~ 2-4) Ly-alpha forest transmission. As an alternative, we present simulations of the cosmological formation of individual galaxies with UV backgrounds that decline more slowly at high redshift: both a simple intensity rescaling and the background recently derived by Faucher-Giguere (2009), which softens the spectrum at higher redshifts. We also test an approximation of the X-ray background with a similar z-dependence. We find for the test galaxies that an increase in either the intensity or hardness of ionizing radiation generically pushes star formation towards lower redshifts: although overall star formation in the simulation boxes is reduced by 10-25%, the galaxies show a factor of ~2 increase in the fraction of stars within a 30 kpc radius that are formed after z=1. Other effects include late gas inflows enhanced up to 30 times, stellar half-mass radii decreased by up to 30%, central velocity dispersions increased up to 40%, and a strong reduction in substructure. The magnitude of the effects depends on the environmental/accretion properties of the particular galaxy., Comment: 10 emulateapj pages, 10 figures; accepted by ApJ

We perform numerical simulations of clouds in the Galactic Centre (GC) engulfing the nuclear super-massive black hole and show that this mechanism leads to the formation of gaseous accretion discs with properties that are similar to the expected gaseous progenitor discs that fragmented into the observed stellar disc in the GC. As soon as the cloud hits the black hole, gas with opposite angular momentum relative to the black hole collides downstream. This process leads to redistribution of angular momentum and dissipation of kinetic energy, resulting in a compact gaseous accretion disc. A parameter study using thirteen high resolution simulations of homogeneous clouds falling onto the black hole and engulfing it in parts demonstrates that this mechanism is able to produce gaseous accretion discs that could potentially be the progenitor of the observed stellar disc in the GC. A comparison of simulations with different equations of state (adiabatic, isothermal and full cooling) demonstrates the importance of including a detailed thermodynamical description. However the simple isothermal approach already yields good results on the radial mass transfer and accretion rates, as well as disc eccentricities and sizes. We find that the cloud impact parameter strongly influences the accretion rate whereas the impact velocity has a small affect on the accretion rate., Comment: 21 pages, 18 figures, Accepted for publication in MNRAS

We study the effect of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. We produce a suite of minor merger simulations for Milky Way-like galaxies. This suite consists of collisionless simulations as well as hydrodynamical runs including a gaseous component in the galactic disc. We find that in dissipationless simulations minor mergers cause the scale height of the disc to increase by up to a factor of ~2. When the presence of gas in the disc is taken into account this thickening is reduced by 25% (50%) for an initial disc gas fraction of 20% (40%), leading to a final scale height z0 between 0.6 and 0.7 kpc, regardless of the initial scale height. We argue that the presence of gas reduces disc heating via two mechanisms: absorption of kinetic impact energy by the gas and/or formation of a new thin stellar disc that can cause heated stars to recontract towards the disc plane. We show that in our simulations most of the gas is consumed during the merger and thus the regrowth of a new thin disc has a negligible impact on the z0 of the post merger galaxy. Final disc scale heights found in our simulations are in good agreement with studies of the vertical structure of spiral galaxies where the majority of the systems are found to have scale heights of 0.4 kpc < z0 < 0.8 kpc. We also found no tension between recent measurements of the scale height of the Milky Way thin disc and results coming from our hydrodynamical simulations. We conclude that the existence of a thin disc in the Milky Way and in external galaxies is not in obvious conflict with the predictions of the CDM model., Comment: 12 pages, 13 figures, two additional simulations, discussion enlarged, two figures added, updated references, accepted for publication in MNRAS