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Different theoretical methodologies lead to order-of-magnitude variations in predicted galaxy-galaxy merger rates. We examine how this arises and quantify the dominant uncertainties. Modeling of dark matter and galaxy inspiral/merger times contribute factor of ∼2 uncertainties. Different estimates of the halo-halo merger rate, the subhalo "destruction" rate, and the halo merger rate with some dynamical friction time delay for galaxy-galaxy mergers, agree to within this factor of ∼2, provided proper care is taken to define mergers consistently. There are some caveats: if halo/subhalo masses are not appropriately defined the major-merger rate can be dramatically suppressed, and in models with "orphan" galaxies and under-resolved subhalos the merger timescale can be severely overestimated. The dominant differences in galaxy-galaxy merger rates between models owe to the treatment of the baryonic physics. Cosmological hydrodynamic simulations without strong feedback and some older semianalytic models (SAMs), with known discrepancies in mass functions, can be biased by large factors (∼5) in predicted merger rates. However, provided that models yield a reasonable match to the total galaxy mass function, the differences in properties of central galaxies are sufficiently small to alone contribute small (factor of ∼1.5) additional systematics to merger rate predictions. But variations in the baryonic physics of satellite galaxies in models can also have a dramatic effect on merger rates. The well-known problem of satellite "over-quenching" in most current SAMs-whereby SAM satellite populations are too efficiently stripped of their gas-could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Models in which the masses of satellites are fixed by observations (or SAMs adjusted to resolve this "over-quenching") tend to predict higher merger rates, but with factor of ∼2 uncertainties stemming from the uncertainty in those observations. The choice of mass used to define "major" and "minor" mergers also matters: stellar-stellar major mergers can be more or less abundant than halo-halo major mergers by an order of magnitude. At low masses, most true major mergers (mass ratio defined in terms of their baryonic or dynamical mass) will appear to be minor mergers in their stellar mass ratio-observations and models using just stellar criteria could underestimate major-merger rates by factors of ∼3-5. We discuss the uncertainties in relating any merger rate to spheroid formation (in observations or theory): in order to achieve better than factor of ∼3 accuracy, it is necessary to account for the distribution of merger orbital parameters, gas fractions, and the full efficiency of merger-induced effects as a function of mass ratio. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

We use a suite of semi-empirical models to predict the galaxy-galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g., mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of L * bulges and the total spheroid mass density, but minor mergers contribute a non-negligible 30%. (2) This is mass dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near L *, but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with a factor 2 uncertainty in both - up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling 103 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor 2 level; uncertainties grow at the lowest masses and high redshifts. © 2010. The American Astronomical Society. All rights reserved.

Many astrophysical systems can only be accurately modelled when the behaviour of their baryonic gas components is well understood. The residual distribution (RD) family of partial differential equation (PDE) solvers produce approximate solutions to the corresponding fluid equations. We present a new implementation of the RD method. The solver efficiently calculates the evolution of the fluid, with up to second order accuracy in both time and space, across an unstructured triangulation, in both 2D and 3D. We implement a novel variable time stepping routine, which applies a drifting mechanism to greatly improve the computational efficiency of the method. We conduct extensive testing of the new implementation, demonstrating its innate ability to resolve complex fluid structures, even at very low resolution. We can resolve complex structures with as few as 3–5 resolution elements, demonstrated by Kelvin–Helmholtz and Sedov blast tests. We also note that we find cold cloud destruction time scales consistent with those predicted by a typical PPE solver, albeit the exact evolution shows small differences. The code includes three residual calculation modes, the LDA, N, and blended schemes, tailored for scenarios from smooth flows (LDA), to extreme shocks (N), and both (blended). We compare our RD solver results to state-of-the-art solvers used in other astrophysical codes, demonstrating the competitiveness of the new approach, particularly at low resolution. This is of particular interest in large scale astrophysical simulations, where important structures, such as star forming gas clouds, are often resolved by small numbers of fluid elements. [ABSTRACT FROM AUTHOR]

We present the results of a study utilizing ultradeep, rest-frame UV, spectroscopy to quantify the relationship between stellar mass and stellar metallicity for 681 star-forming galaxies at 2.5 < z < 5.0 (〈 z 〉 = 3.5 ± 0.6) drawn from the VANDELS survey. Via a comparison with high-resolution stellar population synthesis models, we determine stellar metallicities (Z ∗, here a proxy for the iron abundance) for a set of high signal-to-noise ratio composite spectra formed from subsamples selected by mass and redshift. Across the stellar mass range |$8.5\,\, \lt\,\, \mathrm{log}(\langle M_{\ast } \rangle /\rm {M}_{\odot })\,\, \lt\,\, 10.2$|⁠ , we find a strong correlation between stellar metallicity (Z ∗/Z⊙) and stellar mass, with stellar metallicity monotonically increasing from Z ∗/Z⊙ < 0.09 at |$\langle M_{\ast } \rangle = 3.2 \times 10^{8}\, \rm {M}_{\odot }$| to Z ∗/Z⊙ = 0.27 at |$\langle M_{\ast } \rangle = 1.7 \times 10^{10}\, \rm {M}_{\odot }$|⁠. In contrast, at a given stellar mass, we find no evidence for significant metallicity evolution across the redshift range of our sample. However, comparing our results to the z  = 0 stellar mass–metallicity relation for star-forming galaxies, we find that the 〈 z 〉 = 3.5 relation is consistent with being shifted to lower metallicities by ≃0.6 dex at all stellar masses. Contrasting our derived stellar metallicities with estimates of the gas-phase metallicities of galaxies at similar redshifts and stellar masses, we find evidence for enhanced |$\rm {O}/\rm {Fe}$| ratios in z  ≳ 2.5 star-forming galaxies of the order (O/Fe) ≳ 1.8 × (O/Fe)⊙. Finally, by comparing our results to the predictions of three cosmological simulations, we find that the 〈 z 〉 = 3.5 stellar mass–metallicity relation is consistent with current predictions for how outflow strength scales with galaxy stellar mass. This conclusion is supported by an analysis of one-zone analytic chemical evolution models, and suggests that the mass-loading parameter (⁠|$\eta =\dot{M}_{\mathrm{outflow}}/M_{\ast }$|⁠) scales as |$\eta \propto M_{\ast }^{\beta }$| with β ≃ −0.4. [ABSTRACT FROM AUTHOR]

The dark ages of the Universe end with the formation of the first generation of stars residing in primeval galaxies. These objects were the first to produce ultraviolet ionizing photons in a period when the cosmic gas changed from a neutral state to an ionized one, known as Epoch of Reionization (EoR). A pivotal aspect to comprehend the EoR is to probe the intertwined relationship between the fraction of ionizing photons capable to escape dark haloes, also known as the escape fraction (f esc), and the physical properties of the galaxy. This work develops a sound statistical model suitable to account for such non-linear relationships and the non-Gaussian nature of f esc. This model simultaneously estimates the probability that a given primordial galaxy starts the ionizing photon production and estimates the mean level of the f esc once it is triggered. The model was employed in the First Billion Years simulation suite, from which we show that the baryonic fraction and the rate of ionizing photons appear to have a larger impact on f esc than previously thought. A naive univariate analysis of the same problem would suggest smaller effects for these properties and a much larger impact for the specific star formation rate, which is lessened after accounting for other galaxy properties and non-linearities in the statistical model. A snippet code to reproduce the analysis of this paper is available at COIN toolbox [ABSTRACT FROM AUTHOR]

Context. Massive ultra-compact galaxies (MUGs) are common at z = 2−3, but very rare in the nearby Universe. Simulations predict that the few surviving MUGs should reside in galaxy clusters, whose large relative velocities prevent them from merging, thus maintaining their original properties (namely stellar populations, masses, sizes and dynamical state). Aims. Our goal is to obtain a complete census of the MUG population at 0.02 < z < 0.3, determining the number density, population properties and environment. Methods. We have taken advantage of the high-completeness, large-area spectroscopic GAMA survey, complementing it with deeper imaging from the KiDS and VIKING surveys. We find a set of 22 bona-fide MUGs, defined as having high stellar mass (> 8 × 1010M⊙) and compact size (Re < 2 kpc). An additional set of seven lower-mass objects (6 × 1010 < M⋆/M⊙ < 8 × 1010) are also potential candidates according to typical mass uncertainties. Results. The comoving number density of MUGs at low redshift (z < 0.3) is constrained at (1.0 ± 0.4)×10−6 Mpc−3, consistent with galaxy evolution models. However, we find a mixed distribution of old and young galaxies, with a quarter of the sample representing (old) relics. MUGs have a predominantly early or swollen disk morphology (Sérsic index 1 < n < 2.5) with high stellar surface densities (⟨Σe⟩∼1010M⊙ Kpc−2). Interestingly, a large fraction feature close companions – at least in projection – suggesting that many (but not all) reside in the central regions of groups. Halo masses show these galaxies inhabit average-mass groups. Conclusions. As MUGs are found to be almost equally distributed among environments of different masses, their relative fraction is higher in more massive overdensities, matching the expectations that some of these galaxies fell in these regions at early times. However, there must be another channel leading some of these galaxies to an abnormally low merger history because our sample shows a number of objects that do not inhabit particularly dense environments. [ABSTRACT FROM AUTHOR]

We present the results of a new study of dust attenuation at redshifts 3 <  z  < 4 based on a sample of 236 star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range 8.2 ≤ log (M⋆/M⊙) ≤ 10.6 probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at z ≃ 3.5 is similar in shape to the commonly adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of RV  = 4.18 ± 0.29. In contrast, we find that an average attenuation curve as steep as the SMC extinction law is strongly disfavoured. We show that the optical attenuation (AV) versus stellar mass (M ⋆) relation predicted using our method is consistent with recent ALMA observations of galaxies at 2 <  z  < 3 in the Hubble Ultra Deep Field (HUDF), as well as empirical AV  −  M ⋆ relations predicted by a Calzetti-like law. In fact, our results, combined with other literature data, suggest that the AV – M ⋆ relation does not evolve over the redshift range 0 <  z  < 5, at least for galaxies with log(M ⋆/M⊙) ≳ 9.5. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at lower masses log(M ⋆/M⊙) ≲ 9.0. [ABSTRACT FROM AUTHOR]

We present the results of a new study of the relationship between infrared excess (IRX ≡ L IR/ L UV), ultraviolet (UV) spectral slope (β) and stellar mass at redshifts 2 <  z  < 3, based on a deep Atacama Large Millimeter Array (ALMA) 1.3-mm continuum mosaic of the Hubble Ultra Deep Field. Excluding the most heavily obscured sources, we use a stacking analysis to show that z ≃ 2.5 star-forming galaxies in the mass range |$9.25\le \log (M_{\ast }/\rm M_{{\odot }}) \le 10.75$| are fully consistent with the IRX–β relation expected for a relatively grey attenuation curve, similar to the commonly adopted Calzetti law. Based on a large, mass-complete sample of 2 ≤ z ≤ 3 star-forming galaxies drawn from multiple surveys, we proceed to derive a new empirical relationship between β and stellar mass, making it possible to predict UV attenuation (A 1600) and IRX as a function of stellar mass, for any assumed attenuation law. Once again, we find that z ≃ 2.5 star-forming galaxies follow A 1600– M * and IRX– M * relations consistent with a relatively grey attenuation law, and find no compelling evidence that star-forming galaxies at this epoch follow a reddening law as steep as the Small Magellanic Cloud (SMC) extinction curve. In fact, we use a simple simulation to demonstrate that previous determinations of the IRX–β relation may have been biased towards low values of IRX at red values of β, mimicking the signature expected for an SMC-like dust law. We show that this provides a plausible mechanism for reconciling apparently contradictory results in the literature and that, based on typical measurement uncertainties, stellar mass provides a cleaner prediction of UV attenuation than β. Although the situation at lower stellar masses remains uncertain, we conclude that for 2 <  z  < 3 star-forming galaxies with |$\log (M_{\ast }/\rm M_{{\odot }}) \ge 9.75$|⁠, both the IRX–β and IRX– M * relations are well described by a Calzetti-like attenuation law. [ABSTRACT FROM AUTHOR]

We present the results of a study investigating the dust attenuation law at z ≃5, based on synthetic spectral energy distributions (SEDs) calculated for a sample of N = 498 galaxies drawn from the First Billion Years (FiBY) simulation project. The simulated galaxies at z ≃5, which have M1500 ⩽ -18.0 and 7.5 ⩽ log(M/M☉) ⩽ 10.2, display a mass-dependent α-enhancement, with a median value of [α/Fe]z=5 ≃4 x [α/Fe]Z☉. The median Fe/H ratio of the simulated galaxies is 0.14 ± 0.05 which produces steep intrinsic ultraviolet (UV) continuum slopes; <βi>=-2.4±0.05. Using a set of simple dust attenuationmodels, in which the wavelength-dependent attenuation is assumed to be of the form A(λ) α λn, we explore the parameter values which best reproduce the observed z=5 luminosity function (LF) and colour- magnitude relation (CMR).We find that a simple model in which the absolute UV attenuation is a linearly increasing function of log stellarmass (A1500 = 0.5 x log(M/M☉)-3.3), and the dust attenuation slope (n) is within the range -0.7 ⩽ n⩽-0.3, can successfully reproduce the LF and CMR over a wide range of stellar population synthesis model assumptions, including the effects of massive binaries. This range of attenuation curves is consistent with a power-law fit to the Calzetti attenuation law in the UV (n = -0.55). In contrast, curves as steep as the Small Magellanic Cloud extinction curve (n = -1.24) are formally ruled out. Finally, we show that our models are consistent with recent 1.3 mm Atacama Large Millimeter Array observations of the Hubble Ultra Deep Field and predict the form of the z ≃5 infrared excess (IRX)-β relation. [ABSTRACT FROM AUTHOR]

Approximately 20 per cent of early-type galaxies host small nuclear stellar discs that are tens to a few hundred parsecs in size. Such discs are expected to be easily disrupted during major galactic encounters, hence their age serve to constrain their assembly history. We use VIsible MultiObject Spectrograph integral-field spectroscopic observations for the intermediate-mass E0 galaxy NGC 4458 and age-date its nuclear disc via high-resolution fitting of various model spectra. We find that the nuclear disc is at least 6 Gyr old. A clue to gain narrow limits to the stellar age is our knowledge of the nuclear disc contribution to the central surface brightness. The presence of an old nuclear disc, or the absence of disruptive encounters since z ∼ 0.6, for a small galaxy such as NGC 4458 which belongs to the Virgo cluster, may be consistent with a hierarchical picture for galaxy formation where the smallest galaxies assembles earlier and the crowded galactic environments reduce the incidence of galaxy mergers. On the other hand, NGC 4458 displays little or no bulk rotation except for a central kpc-scale kinematically decoupled core. Slow rotation and decoupled core are usually explained in terms of mergers. The presence and age of the nuclear disc constraint these mergers to have happened at high redshift. [ABSTRACT FROM AUTHOR]

Long gamma-ray burst's (LGRB's) association with the death of massive stars suggests that they could be used to probe the cosmic star formation history (CSFH) with high accuracy, due to their high luminosities. We utilize cosmological simulations from the First Billion Years project to investigate the biases between the CSFH and the LGRB rate at z > 5, assuming various different models and constraints on the progenitors of LGRBs. We populate LGRBs using a selection based on environmental properties and demonstrate that the LGRB rate should trace the CSFH to high redshifts. The measured LGRB rate suggests that LGRBs have opening angles of 0jet = 01, although the degeneracy with the progenitor model cannot rule out an underlying bias. We demonstrate that proxies that relate the LGRB rate with global LGRB host properties do not reflect the underlying LGRB environment, and are in fact a result of the host galaxy's spatial properties, such that LGRBs can exist in galaxies of solar metallicity. However, we find a class of host galaxies that have low stellar mass and are metal rich, and that their metallicity dispersions would not allow low-metallicity environments. Detection of hosts with this set of properties would directly reflect the progenitor's environment. We predict that 10 per cent of LGRBs per year are associated with this set of galaxies that would have forbidden line emission that could be detected by instruments on the James Webb Space Telescope. Such a discovery would place strong constraints on the collapsar model and suggests other avenues to be investigated. [ABSTRACT FROM AUTHOR]

We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 × 1010 M⊙ ≲ M* ≲ 6 × 1011 M⊙. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion and higher order Gauss–Hermite moments h3 and h4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the λR-parameter. The velocity, velocity dispersion, h3 and h4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS3D survey. This includes fast (regular), slow and misaligned rotation, hot spheroids with embedded cold disc components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant (≳18 per cent) in situ formation of stars since z ≈ 2, or with additional gas-rich major mergers – resulting in a spin-up – in their formation history, form elongated (ϵ ∼ 0.45) fast rotators (λR ∼ 0.46) with a clear anticorrelation of h3 and v/σ. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants (λR ∼ 0.43). This formation path does not result in anticorrelated h3 and v/σ. The formation histories of slow rotators can include late major mergers. If the merger is gas rich, the remnant typically is a less flattened slow rotator with a central dip in the velocity dispersion. If the merger is gas poor, the remnant is very elongated (ϵ ∼ 0.43) and slowly rotating (λR ∼ 0.11). The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in situ star formation since z ∼ 2, rotate slower and have older stellar populations. We discuss general implications for the formation of fast and slowly rotating galaxies as well as the weaknesses and strengths of the underlying models. [ABSTRACT FROM AUTHOR]

According to a popular scenario supported by numerical models, the mass assembly and growth of massive galaxies, in particular the Early-Type Galaxies (ETGs), is, below a redshift of 1, mainly due to the accretion of multiple gas–poor satellites. In order to get observational evidence of the role played by minor dry mergers, we are obtaining extremely deep optical images of a complete volume limited sample of nearby ETGs. These observations, done with the CFHT as part of the ATLAS3D, NGVS and MATLAS projects, reach a stunning 28.5 – 29 mag.arcsec−2 surface brightness limit in the g' band. They allow us to detect the relics of past collisions such as faint stellar tidal tails as well as the very extended stellar halos which keep the memory of the last episodes of galactic accretion. Images and preliminary results from this on-going survey are presented, in particular a possible correlation between the fine structure index (which parametrizes the amount of tidal perturbation) of the ETGs, their stellar mass, effective radius and gas content. [ABSTRACT FROM PUBLISHER]

Recently, massive early-type galaxies have shed their red-and-dead moniker, thanks to the discovery that many host residual star formation. As part of the ATLAS-3D project, we have conducted a complete, volume-limited survey of the molecular gas in 260 local early-type galaxies with the IRAM-30m telescope and the CARMA interferometer, in an attempt to understand the fuel powering this star formation. We find that around 22% of early-type galaxies in the local volume host molecular gas reservoirs. This detection rate is independent of galaxy luminosity and environment. Here we focus on how kinematic misalignment measurements and gas-to-dust ratios can be used to put constraints on the origin of the cold ISM in these systems. The origin of the cold ISM seems to depend strongly on environment, with misaligned, dust poor gas (indicative of externally acquired material) being common in the field but completely absent in rich groups and in the Virgo cluster. Very massive galaxies also appear to be devoid of accreted gas. This suggests that in the field mergers and/or cold gas accretion dominate the gas supply, while in clusters internal secular processes become more important. This implies that environment has a strong impact on the cold gas properties of ETGs. [ABSTRACT FROM PUBLISHER]

ABSTRACT Feedback from active galactic nuclei (AGN) has become a major component in simulations of galaxy evolution, in particular for massive galaxies. AGN jets have been shown to provide a large amount of energy and are capable of quenching cooling flows. Their impact on the host galaxy, however, is still not understood. Subgrid models of AGN activity in a galaxy evolution context so far have been mostly focused on the quenching of star formation. To shed more light on the actual physics of the 'radio mode' part of AGN activity, we have performed simulations of the interaction of a powerful AGN jet with the massive gaseous disc ( [ABSTRACT FROM AUTHOR]

Context. To answer questions on the start and duration of the epoch of reionisation, periods of galaxy mergers and properties of other cosmological encounters, the cosmic star formation history, ρδ or CSFH, is of fundamental importance. Using the association of long-duration gamma-ray bursts (LGRBs) with the death of massive stars and their ultra-luminous nature (>1052 erg s-1), the CSFH can be probed to higher redshifts than current conventional methods. Unfortunately, no consensus has been reached on the manner in which the LGRB rate, ρgrb or LGRBR, traces the CSFH, leaving many of the questions mentioned mostly unexplored by this method. Aims. Observations by the gamma-ray burst near-infrared detector (GROND) over the past 4 years have, for the first time, acquired highly complete LGRB samples. Driven by these completeness levels and new evidence of LGRBs also occurring in more massive and metal rich galaxies than previously thought, the possible biases of the ρgrb- ρδ connection are investigated over a large range of galaxy properties. Methods. The CSFH is modelled using empirical fits to the galaxy mass function and galaxy star formation rates. Biasing the CSFH by means of metallicity cuts, mass range boundaries, and other unknown redshift dependencies of the form ρgrb ∞ ρδ(1 + z)δ, a ρgrb is generated and compared to the highly complete GROND LGRB sample. Results. It is found that there is no strong preference for a metallicity cut or fixed galaxy mass boundaries and that there are no unknown redshift effects (δ = 0), in contrast to previous work which suggest values of Z/Zω ~ 0.1-0.3. From the best-fit models obtained, we predict that ~1.2% of the LGRB burst sample exists above z = 6. Conclusions. The linear relationship between ρgrb and ρδ suggested by our results implies that redshift biases present in previous LGRB samples significantly affect the inferred dependencies of LGRBs on their host galaxy properties. Such biases can lead to, for example, an interpretation of metallicity limitations and evolving LGRB luminosity functions. [ABSTRACT FROM AUTHOR]

We report the discovery of a powerful molecular wind from the nucleus of the non-interacting nearby S0 field galaxy NGC 1266. The single-dish CO profile exhibits emission to ±400 km s-1 and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally concentrated molecular component with a mass of 1.1 × 109 M ☉ and a molecular outflow with a molecular mass of ≈2.4 × 107 M ☉. The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1 × 108 M ☉ within a radius of ≈60 pc. This compact molecular nucleus has a surface density of ≈2.7 × 104 M ☉ pc-2, more than two orders of magnitude larger than that of giant molecular clouds in the disk of the Milky Way, and it appears to sit on the Kennicutt-Schmidt relation despite its extreme kinematics and energetic activity. We interpret this nucleus as a disk that confines the outflowing wind. A mass outflow rate of ≈13 M ☉ yr-1 leads to a depletion timescale of lsim85 Myr. The star formation in NGC 1266 is insufficient to drive the outflow, and thus it is likely driven by the active galactic nucleus. The concentration of the majority of the molecular gas in the central 100 pc requires an extraordinary loss of angular momentum, but no obvious companion or interacting galaxy is present to enable the transfer. NGC 1266 is the first known outflowing molecular system that does not show any evidence of a recent interaction. [ABSTRACT FROM AUTHOR]

Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm-length asymmetries of jets and the properties of extended emission-line regions in high-redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium (ISM) of lognormal density distribution and analyse the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm-length asymmetries: disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission-line nebulae are generally found to be brighter on the side of the shorter lobe in high-redshift radio galaxies. This interaction of jets with the cold gas phase suggests that star formation in evolving high-redshift galaxies may be affected considerably by jet activity. [ABSTRACT FROM AUTHOR]

The mass assembly of galaxies leaves various imprints on their surroundings, such as shells, streams and tidal tails. The frequency and properties of these fine structures depend on the mechanism driving the mass assembly: e.g. a monolithic collapse, rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry/wet mergers. Therefore, by studying the outskirts of galaxies, one can learn about their main formation mechanism. I present here our on-going work to characterize the outskirts of Early-Type Galaxies (ETGs), which are powerful probes at low redshift of the hierarchical mass assembly of galaxies. This work relies on ultra–deep optical images obtained at CFHT with the wide-field of view MegaCam camera of field and cluster ETGs obtained as part of the ATLAS3D and NGVS projects. State of the art numerical simulations are used to interpret the data. The images reveal a wealth of unknown faint structures at levels as faint as 29 mag arcsec−2 in the g-band. Initial results for two galaxies are presented here. [ABSTRACT FROM PUBLISHER]

The molecular gas content of local early-type galaxies is constrained and discussed in relation to their evolution. First, as part of the ATLAS3D survey, we present the first complete, large (260 objects), volume-limited single-dish survey of CO in normal local early-type galaxies. We find a surprisingly high detection rate of 22%, independent of luminosity and at best weakly dependent on environment. Second, the extent of the molecular gas is constrained with CO synthesis imaging, and a variety of morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in over a third of the systems, although this behaviour is drastically diffferent between field and cluster environments. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information on the molecular gas, current star formation, and young stars. Last, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation, following the standard Schmidt-Kennicutt law but not the far infrared-radio correlation. This may suggest a greater diversity in star formation processes than observed in disk galaxies. Using multiple molecular tracers, we are thus starting to probe the physical conditions of the cold gas in early-types. [ABSTRACT FROM PUBLISHER]

Nuclear stellar discs (NSDs), of a few tens to hundreds of parsec across, are a common and yet poorly studied feature of early-type galaxies. Still, such small discs represent a powerful tool to constrain the assembling history of galaxies, since they can be used to trace to the epoch when galaxies experienced their last major merger event. By studying the fraction and stellar age of NSDs, it is thus possible to test the predictions for the assembly history of early-type galaxies according to the current hierarchical paradigm for galaxy formation. In this paper we have produced the most comprehensive census of NSDs in nearby early-type galaxies by searching for such discs in objects within 100 Mpc and by using archival images from the Hubble Space Telescope. We found that NSDs are present in approximately 20 per cent of early-type galaxies, and that the fraction of galaxies with NSDs depends neither on their Hubble type nor on their galactic environment, whereas the incidence of NSDs appears to decline in the most massive systems. Furthermore, we have separated the light contribution of 12 such discs from that of their surrounding stellar bulge in order to extract their physical properties. This doubles the number of decomposed NSDs and although the derived values for their central surface brightness and scalelength are consistent with previous studies, they also give a hint of possible different characteristics due to different formation scenario between nuclear discs and other kinds of large galactic discs. [ABSTRACT FROM AUTHOR]

We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly rotating elliptical galaxies often requires a resolution that is beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200 pc and 105 particles per component (stars, gas and dark matter), i.e. a gas mass resolution of , typical of some recently published major merger simulations, to up to 32 pc and in simulations using collisionless particles and gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low-residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component (0–10 per cent in mass). The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. Moreover, the effect of gas in a galaxy merger is not limited to helping the survival/rebuilding of rotating disc components: at high resolution, gas actively participates in the relaxation process and the formation of slowly rotating stellar systems. [ABSTRACT FROM AUTHOR]

The star formation history of nearby early-type galaxies is investigated via numerical modelling. Idealized hydrodynamical N-body simulations with a star formation prescription are used to study the minor merger process between a giant galaxy (host) and a less massive spiral galaxy (satellite) with reasonable assumptions for the ages and metallicities of the merger progenitors. We find that the evolution of the star formation rate is extended over several dynamical times and shows peaks which correspond to pericentre passages of the satellite. The newly formed stars are mainly located in the central part of the satellite remnant while the older stars of the initial disc are deposited at larger radii in shell-like structures. After the final plunge of the satellite, star formation in the central part of the remnant can continue for several Gyr depending on the star formation efficiency. Although the mass fraction in new stars is small, we find that the half-mass radius differs from the half-light radius in the V and H bands. Moreover synthetic 2D images in J, H, NUV, Hβ and V bands, using the characteristic filters of the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, reveal that residual star formation induced by gas-rich minor mergers can be clearly observed during and after the final plunge, especially in the near-ultraviolet band, for interacting systems at ( ) over moderate numbers of orbits (approximately two orbits correspond to typical exposure times of ∼3600 s). This suggests that WFC3 has the potential to resolve these substructures, characterize plausible past merger episodes and give clues to the formation of early-type galaxies. [ABSTRACT FROM AUTHOR]

We investigate the role that dry mergers play in the build-up of massive galaxies within the cold dark matter paradigm. Implementing an empirical shut-off mass scale for star formation, we find a nearly constant dry merger rate of at and a steep decline at larger z. Less than half of these mergers are between two galaxies that are morphologically classified as early-types, and the other half is mostly between an early- and late-type galaxy. Latter are prime candidates for the origin of tidal features around red elliptical galaxies. The introduction of a transition mass scale for star formation has a strong impact on the evolution of galaxies, allowing them to grow above a characteristic mass scale of by mergers only. As a consequence of this transition, we find that around , the fraction of 1:1 mergers is enhanced with respect to unequal mass major mergers. This suggests that it is possible to detect the existence of a transition mass scale by measuring the relative contribution of equal mass mergers to unequal mass mergers as a function of galaxy mass. The evolution of the high-mass end of the luminosity function is mainly driven by dry mergers at low z. We however find that only 10–20 per cent of galaxies more massive than experience dry major mergers within their last Gyr at any given redshift . [ABSTRACT FROM AUTHOR]

We combine deep optical and NIR ( UBVRIzJK) photometry from the Multiwavelength Survey by Yale–Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 < z < 1) early-type galaxies, drawn from the Extended Chandra Deep Field-South (E-CDFS). Galaxy morphologies are determined through visual inspection of Hubble Space Telescope ( HST) images taken from the GEMS survey. We harness the sensitivity of the UV to young (<1-Gyr old) stars to quantify the recent star formation history of early-type galaxies across a range of luminosities [−23.5 < M( V) < −18]. Comparisons to simple stellar populations forming at high redshift indicate that ∼1.1 per cent of early-types in this sample are consistent with purely passive ageing since z= 2 – this value drops to ∼0.24 per cent and ∼0.15 per cent for z= 3 and 5, respectively. Parametrizing the recent star formation (RSF) in terms of the mass fraction of stars less than a Gyr old, we find that the early-type population as a whole shows a typical RSF between 5 and 13 per cent in the redshift range 0.5 < z < 1. Early-types on the broad UV ‘red sequence’ show RSF values less than 5 per cent, while the reddest early-types (which are also the most luminous) are virtually quiescent with RSF values of ∼1 per cent. In contrast to their low-redshift ( z < 0.1) counterparts, the high-redshift early-types in this sample show a pronounced bimodality in the rest-frame UV–optical colour, with a minor but significant peak centred on the blue cloud. Furthermore, star formation in the most active early-types is a factor of 2 greater at z∼ 0.7 than in the local universe. Given that evolved sources of UV flux (e.g. horizontal branch stars) should be absent at z > 0.5, implying that the UV is dominated by young stars, we find compelling evidence that early-types of all luminosities form stars over the lifetime of the Universe, although the bulk of their star formation is already complete at high redshift. This ‘tail-end’ of star formation is measurable and not negligible, with luminous [ ] early-types potentially forming 10–15 per cent of their mass since z= 1, with less luminous early-types [ ] potentially forming 30–60 per cent of their mass after z= 1. This, in turn, implies that intermediate-age stellar populations should be abundant in local early-type galaxies, as expected in hierarchical cosmology. [ABSTRACT FROM AUTHOR]

We combine deep UBV RIzJK photometry from the MUSYC survey with redshifts from the COMBO-17 survey to study the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 < z < 1) early-type galaxies, drawn from the Extended Chandra Deep Field South (E-CDFS). Galaxy morphologies are determined through visual inspection of Hubble Space Telescope (HST) images taken from the GEMS survey. We harness the sensitivity of the UV to young (<1 Gyrs old) stars to quantify the recent star formation history of the early-type population. We find compelling evidence that early-types of all luminosities form stars over the lifetime of the Universe, although the bulk of their star formation is already complete at high redshift. Luminous (−23 < M(V) < −20.5) early-types form 10-15 percent of their mass after z = 1, while their less luminous (M(V) > −20.5) counterparts form 30-60 percent of their mass in the same redshift range. [ABSTRACT FROM PUBLISHER]

We investigate the stellar composition of bulges and elliptical galaxies as predicted by the cold dark matter paradigm using semi-analytical modelling. We argue that spheroid stars are built up of two main components, merger and quiescent, according to the origin of the stars. The merger component is formed during major mergers by gas driven to the centre, while the quiescent component is formed in gaseous discs and added later to the spheroid during major mergers. Galaxies more massive than have on average only a 15 per cent merger component in their spheroids, while smaller galaxies can have up to 30 per cent. The merger component increases with redshift due to mergers involving more gas. However, we do not find mergers with gas fraction above ∼40 per cent of the remnants mass. Generally, the gas fraction is a decreasing function of the redshift at which the merger occurs and the mass of the remnant, with more massive remnants having smaller gas fraction and hence smaller merger components. This trend is independent of the environment of the galaxy with the only impact of the environment being that galaxies less massive than MC have slightly larger merger components in dense environments. The fraction of stars in bulges for galaxies more massive than MC is larger than 50 per cent. We find that the majority of stars in galaxies more massive than MC reside within bulges and ellipticals independent of redshift and that the fraction increases with redshift. The most massive galaxies at each redshift are elliptical galaxies. [ABSTRACT FROM AUTHOR]

Using semi-analytical models of galaxy formation, the origin of boxy and discy elliptical galaxies is investigated. We find that the simple scenario, motivated byN-body simulations, in which the isophotal shape is only dependent on the mass ratio of the last major merger, is not able to reproduce the observation that the fraction of boxy and discy ellipticals depends on galaxy luminosity. The observations can, however, be reproduced with the following reasonable assumptions: (i) equal-mass mergers lead to boxy ellipticals and unequal-mass mergers produce discy ellipticals (as motivated byN-body simulations); (ii) major mergers between bulge-dominated galaxies result always in boxy ellipticals, independent of the mass ratio; (iii) merger remnants that subsequently accrete gas leading to a secondary stellar disc with more than 20 per cent of the total stellar fraction are always discy. This scenario indicates that the isophotal shapes of merger remnants are sensitive to the morphology of their progenitors and subsequent gas infall. Boxy and discy ellipticals can be divided into two subclasses, depending on their formation history. Boxy ellipticals are formed either by equal-mass mergers of disc galaxies or by major mergers of early-type galaxies. We find that discy ellipticals are indicators of unequal-mass mergers or late gas infall. Discy ellipticals with high luminosities are preferentially one-component systems that result from unequal-mass mergers, whereas low-luminosity discy ellipticals are more likely to harbour secondary disc components. In addition, the fraction of discy ellipticals with secular disc components should increase in regions with higher galaxy densities. Taking into account the conversion of cuspy cores into flat low-density cores by black hole merging, we find that discy ellipticals should contain central density cusps whereas boxy ellipticals should in general be characterized by flat cores. Only rare low-luminosity boxy ellipticals, resulting from equal-mass mergers of disc galaxies, could have power-law cores. [ABSTRACT FROM AUTHOR]

In around ≈25% of early-type galaxies (ETGs) UV emission from young stellar populations is present. Molecular gas reservoirs have been detected in these systems (e.g. Young et al. (2011), providing the fuel for this residual star-formation. The environment in which this molecular gas is found is quite different than that in spiral galaxies however, with harsher radiation fields, deeper potentials and high metallicity and alpha-element abundances. Here we report on one element of our multi-faceted programme to understand the similarities and differences between the gas reservoirs in spirals and ETGs. We use spatially resolved observations from the CARMA mm-wave interferometer to investigate the size of the molecular reservoirs in the the CO-rich ATLAS3D ETGs (survey described in Alatalo et al. 2012, submitted). We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scale-lengths (Fig 1, left). Amongst ETGs, we find that the extent of the molecular gas is independent of the kinematic misalignment, despite the many reasons why misaligned gas might have a smaller extent. The extent of the molecular gas does depend on environment, with Virgo cluster ETGs having less extended molecular gas reservoirs (Fig 1, right). Whatever the cause, this further emphases that cluster ETGs follow different evolutionary pathways from those in the field. Full details of this work will be presented in Davis et al. (2012), submitted. [ABSTRACT FROM PUBLISHER]

NGC 1266 is a nearby field galaxy observed as part of the ATLAS3D survey (Cappellari et al. 2011). NGC 1266 has been shown to host a compact (< 200 pc) molecular disk and a mass-loaded molecular outflow driven by the AGN (Alatalo et al. 2011). Very Long Basline Array (VLBA) observations at 1.65 GHz revealed a compact (diameter < 1.2 pc), high brightness temperature continuum source most consistent with a low-level AGN origin. The VLBA continuum source is positioned at the center of the molecular disk and may be responsible for the expulsion of molecular gas in NGC 1266. Thus, the candidate AGN-driven molecular outflow in NGC 1266 supports the picture in which AGNs do play a significant role in the quenching of star formation and ultimately the evolution of the red sequence of galaxies. [ABSTRACT FROM PUBLISHER]

Excluding those unsettled systems undergoing mergers, bright galaxies come in two flavours: with and without discs. In this work we look for photometric evidence for presence of discs and compare it with kinematic results of the ATLAS3D survey (Cappellari et al. 2011). We fit a Sérsic (1968) function to azimuthally averaged light profiles of ATLAS3D galaxies to derive single component fits and, subsequently, we fit a combination of the Sérsic function (free index n) and an exponential function (n=1) with the purpose of decomposing the light profiles into “bulge” and “disc” components (B+D model) of all non-barred sample galaxies. We compare the residuals of the B+D models with those of the single Sérsic fits and select the B+D model as preferred only when the improvement is substantial and there are no correlations within residuals. We find that the high angular momentum objects (fast rotators) are disc dominated systems with bulges of typically low n (when their light profiles can be decomposed) or are best represented with a single Sérsic function with a low Sérsic index (n<3). Single component systems with large Sérsic indices are characteristic of low angular momentum objects (slow rotators). [ABSTRACT FROM PUBLISHER]

We detail the rich molecular story of NGC 1266, its serendipitous discovery within the ATLAS3D survey (Cappellari et al. 2011) and how it plays host to an AGN-driven molecular outflow, potentially quenching all of its star formation (SF) within the next 100 Myr. While major mergers appear to play a role in instigating outflows in other systems, deep imaging of NGC 1266 as well as stellar kinematic observations from SAURON, have failed to provide evidence that NGC 1266 has recently been involved in a major interaction. The molecular gas and the instantaneous SF tracers indicate that the current sites of star formation are located in a hypercompact disk within 200 pc of the nucleus (Fig. 1; SF rate ≈ 2 M⊙ yr−1). On the other hand, tracers of recent star formation, such as the Hβ absorption map from SAURON and stellar population analysis show that the young stars are distributed throughout a larger area of the galaxy than current star formation. As the AGN at the center of NGC 1266 continues to drive cold gas out of the galaxy, we expect star formation rates to decline as the star formation is ultimately quenched. Thus, NGC 1266 is in the midst of a key portion of its evolution and continued studies of this unique galaxy may help improve our understanding of how galaxies transition from the blue to the red sequence (Alatalo et al. 2011). [ABSTRACT FROM PUBLISHER]

Early-type galaxies (ETGs) satisfy a now classic scaling relation Re ∝ σ1.2eI−0.8e, the Fundamental Plane (FP; Djorgovski & Davis 1987; Dressler et al. 1987), between their size, stellar velocity dispersion and mean surface brightness. A significant effort has been devoted in the past twenty years to try to understand why the coefficients of the relation are not the ones predicted by the virial theorem Re ∝ σ2eI−1e. [ABSTRACT FROM PUBLISHER]

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination, no consensus has emerged on whether it is universal among different types of galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS(3D) sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy's IMF depends intimately on the galaxy's formation history.

Detailed high-resolution observations of the innermost regions of nearby galaxies have revealed the presence of supermassive black holes. These black holes may interact with their host galaxies by means of 'feedback' in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to observed relations between the black hole and the host. Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies.