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The problem of the charged-particle motion in an axisymmetric magnetic geometry is used to assess the validity of higher-order Hamiltonian guiding-center theory, which includes higher-order corrections associated with gyrogauge invariance as well as guiding-center polarization induced by magnetic-field non-uniformity. Two axisymmetric magnetic geometries are considered: a magnetic mirror geometry and a simple tokamak geometry. When a magnetically-confined charged-particle orbit is regular (i.e., its guiding-center magnetic moment is adiabatically invariant), the guiding-center approximation, which conserves both energy and azimuthal canonical angular momentum, is shown to be faithful to the particle orbit when higher-order corrections are taken into account., Comment: 13 pages, 10 figures

Numerical simulations within a cold dark matter (DM) cosmology form halos whose density profiles have a steep inner slope (`cusp'), yet observations of galaxies often point towards a flat central `core'. We develop a convolutional mixture density neural network model to derive a probability density function (PDF) of the inner density slopes of DM halos. We train the network on simulated dwarf galaxies from the NIHAO and AURIGA projects, which include both DM cusps and cores: line-of-sight velocities and 2D spatial distributions of their stars are used as inputs to obtain a PDF representing the probability of predicting a specific inner slope. The model recovers accurately the expected DM profiles: $\sim$82$\%$ of the galaxies have a derived inner slope within $\pm$0.1 of their true value, while $\sim$98$\%$ within $\pm$0.3. We apply our model to four Local Group dwarf spheroidal galaxies and find results consistent with those obtained with the Jeans modelling based code GravSphere: the Fornax dSph has a strong indication of possessing a central DM core, Carina and Sextans have cusps (although the latter with large uncertainties), while Sculptor shows a double peaked PDF indicating that a cusp is preferred, but a core can not be ruled out. Our results show that simulation-based inference with neural networks provide a innovative and complementary method for the determination of the inner matter density profiles in galaxies, which in turn can help constrain the properties of the elusive DM., Comment: 14 pages, 11 figures, submitted to MNRAS. comments welcome

We use Gaia DR2 systemic proper motions of 45 satellite galaxies to constrain the mass of the Milky Way using the scale free mass estimator of Watkins et al. (2010). We first determine the anisotropy parameter $\beta$, and the tracer satellites' radial density index $\gamma$ to be $\beta$=$-0.67^{+0.45}_{-0.62}$ and $\gamma=2.11\pm0.23$. When we exclude possible former satellites of the Large Magellanic Cloud, the anisotropy changes to $\beta$=$-0.21^{+0.37}_{-0.51}$. We find that the index of the Milky Way's gravitational potential $\alpha$, which is dependent on the mass itself, is the parameter with the largest impact on the mass determination. Via comparison with cosmological simulations of Milky Way-like galaxies, we carried out a detailed analysis of the estimation of the observational uncertainties and their impact on the mass estimator. We found that the mass estimator is biased when applied naively to the satellites of simulated Milky Way halos. Correcting for this bias, we obtain for our Galaxy a mass of $0.58^{+0.15}_{-0.14}\times10^{12}$M$_\odot$ within 64 kpc, as computed from the inner half of our observational sample, and $1.43^{+0.35}_{-0.32}\times10^{12}$M$_\odot$ within 273 kpc, from the full sample; this latter value extrapolates to a virial mass of $M_\mathrm{vir\,\Delta=97}$=$1.51^{+0.45}_{-0.40} \times 10^{12}M_{\odot}$ corresponding to a virial radius of R$_\mathrm{vir}$=$308\pm29$ kpc. This value of the Milky Way mass lies in-between other mass estimates reported in the literature, from various different methods., Comment: 17 pages, 10 figures, 2 tables. Accepted for publication in MNRAS

A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological context requires knowledge of their 3D velocities and orbits. However, proper motion (PM) measurements have generally been of limited accuracy and available only for more massive dwarfs. We therefore present a new study of the kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that have been spectroscopically observed in the literature. We derive systemic PMs for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good consistency for the subset with measurements available from other studies. We derive the implied Galactocentric velocities, and calculate orbits in canonical MW halo potentials of "low" ($0.8 \times 10^{12} M_\odot$) and "high" mass ($1.6 \times 10^{12} M_\odot$). Comparison of the distributions of orbital apocenters and 3D velocities to the halo virial radius and escape velocity, respectively, suggests that the satellite kinematics are best explained in the high-mass halo. Tuc III, Crater II, and additional candidates have orbital pericenters small enough to imply significant tidal influences. Relevant to the missing satellite problem, the fact that fewer galaxies are observed to be near apocenter than near pericenter implies that there must be a population of distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital poles that do not align with the MW plane of satellites (given reasonable assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to establish whether they align; and 17 satellites align, of which 11 are co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are counter-orbiting. Group infall might have contributed to this, but no definitive association is found for the members of the Crater-Leo group., Comment: 18 pages, 7 figures, 4 tables, accepted for publication in A&A

Understanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via spectroscopic analysis is a challenging task requiring very deep observations and exquisite data reduction. In this work we perform one of the most complete characterisations of the stellar component of UDGs to date using deep optical spectroscopic data from OSIRIS at GTC. We measure radial and rotation velocities, star formation histories (SFH) and mean population parameters, such as ages and metallicities, for a sample of five UDG candidates in the Coma cluster. From the radial velocities, we confirm the Coma membership of these galaxies. We find that their rotation properties, if detected at all, are compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old (~ 7 Gyr), metal-poor ([M/H] ~ -1.1) and alpha-enhanced ([Mg/Fe] ~ 0.4) populations followed by a smooth or episodic decline which halted ~ 2 Gyr ago, possibly a sign of cluster-induced quenching. We find no obvious correlation between individual SFH shapes and any UDG morphological properties. The recovered stellar properties for UDGs are similar to those found for DDO44, a local UDG analogue resolved into stars. We conclude that the UDGs in our sample are extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster., Comment: Accepted for publication in MNRAS

Recent determinations of the radial distributions of mono-metallicity populations (MMPs, i.e., stars in narrow bins in [Fe/H] within wider [$\alpha$/Fe] ranges) by the SDSS-III/APOGEE DR12 survey cast doubts on the classical thin - thick disk dichotomy. The analysis of these observations lead to the non-$[\alpha$/Fe] enhanced populations splitting into MMPs with different surface densities according to their [Fe/H]. By contrast, $[\alpha$/Fe] enhanced (i.e., old) populations show an homogeneous behaviour. We analyze these results in the wider context of disk formation within non-isolated halos embedded in the Cosmic Web, resulting in a two-phase mass assembly. By performing hydrodynamical simulations in the context of the $\rm \Lambda CDM$ model, we have found that the two phases of halo mass assembly (an early, fast phase, followed by a slow one, with low mass assembly rates) are very relevant to determine the radial structure of MMP distributions, while radial mixing has only a secondary role, depending on the coeval dynamical and/or destabilizing events. Indeed, while the frequent dynamical violent events occuring at high redshift remove metallicity gradients, and imply efficient stellar mixing, the relatively quiescent dynamics after the transition keeps [Fe/H] gaseous gradients and prevents newly formed stars to suffer from strong radial mixing. By linking the two-component disk concept with the two-phase halo mass assembly scenario, our results set halo virialization (the event marking the transition from the fast to the slow phases) as the separating event marking periods characterized by different physical conditions under which thick and thin disk stars were born., Comment: accepted for publication in ApJ

We have developed a novel Markov Chain Mote Carlo (MCMC) chemical "painting" technique to explore possible radial and vertical metallicity gradients for the thick disc progenitor. In our analysis we match an N-body simulation to the data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We assume that the thick disc has a constant scale-height and has completed its formation at an early epoch, after which time radial mixing of its stars has taken place. Under these assumptions, we find that the initial radial metallicity gradient of the thick disc progenitor should not be negative, but either flat or even positive, to explain the current negative vertical metallicity gradient of the thick disc. Our study suggests that the thick disc was built-up in an inside-out and upside-down fashion, and older, smaller and thicker populations are more metal poor. In this case, star forming discs at different epochs of the thick disc formation are allowed to have different radial metallicity gradients, including a negative one, which helps to explain a variety of slopes observed in high redshift disc galaxies. This scenario helps to explain the positive slope of the metallicity-rotation velocity relation observed for the Galactic thick disc. On the other hand, radial mixing flattens the slope of an existing gradient., Comment: 12 pages, 9 figures, 3 tables, accepted for publication in MNRAS

Using idealised N-body simulations of a Milky Way-sized disc galaxy, we qualitatively study how the metallicity distributions of the thin disc star particles are modified by the formation of the bar and spiral arm structures. The thin disc in our numerical experiments initially has a tight negative radial metallicity gradient and a constant vertical scale-height. We show that the radial mixing of stars drives a positive vertical metallicity gradient in the thin disc. On the other hand, if the initial thin disc is flared, with vertical scale-height increasing with galactocentric radius, the metal poor stars originally in the outer disc become dominant in regions above the disc plane at every radii. This process can drive a negative vertical metallicity gradient, which is consistent with the current observed trend. This model mimics a scenario where the star-forming thin disc was flared in the outer region at earlier epochs. Our numerical experiment with an initial flared disc predicts that the negative vertical metallicity gradient of the mono-age relatively young thin disc population should be steeper in the inner disc, and the radial metallicity gradient of the mono-age population should be shallower at greater heights above the disc plane. We also predict that the metallicity distribution function of mono-age young thin disc populations above the disc plane would be more positively skewed in the inner disc compared to the outer disc., Comment: 12 pages, 12 figures, 2 tables, accepted for publication in MNRAS

We compare the Baryonic Tully-Fisher relation (BTFR) of simulations and observations of galaxies ranging from dwarfs to spirals, using various measures of rotational velocity Vrot. We explore the BTFR when measuring Vrot at the flat part of the rotation curve, Vflat, at the extent of HI gas, Vlast, and using 20% (W20) and 50% (W50) of the width of HI line profiles. We also compare with the maximum circular velocity of the parent halo, Vmax, within dark matter only simulations. The different BTFRs increasingly diverge as galaxy mass decreases. Using Vlast one obtains a power law over four orders of magnitude in baryonic mass, with slope similar to the observed BTFR. Measuring Vflat gives similar results as Vlast when galaxies with rising rotation curves are excluded. However, higher rotation velocities would be found for low mass galaxies if the cold gas extended far enough for Vrot to reach a maximum. W20 gives a similar slope as Vlast but with slightly lower values of Vrot for low mass galaxies, although this may depend on the extent of the gas in your galaxy sample. W50 bends away from these other relations toward low velocities at low masses. By contrast, Vmax bends toward high velocities for low mass galaxies, as cold gas does not extend out to the radius at which halos reach Vmax. Our study highlights the need for careful comparisons between observations and models: one needs to be consistent about the particular method of measuring Vrot, and precise about the radius at which velocities are measured., Comment: Accepted for publication MNRAS

The Square Kilometre Array (SKA) is called to revolutionise essentially all areas of Astrophysics. With a collecting area of about a square kilometre, the SKA will be a transformational instrument, and its scientific potential will go beyond the interests of astronomers. Its technological challenges and huge cost requires a multinational effort, and Europe has recognised this by putting the SKA on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI). The Spanish SKA White Book is the result of the coordinated effort of 120 astronomers from 40 different research centers. The book shows the enormous scientific interest of the Spanish astronomical community in the SKA and warrants an optimum scientific exploitation of the SKA by Spanish researchers, if Spain enters the SKA project., Comment: Editors: Miguel A. Perez-Torres (Editor-in-chief), L. Verdes-Montenegro, J. C. Guirado, A. Alberdi, J. Martin-Pintado, R. Bachiller, D. Herranz, J. M. Girart, J. Gorgas, C. Hernandez-Monteagudo, S. Migliari and J. M. Rodriguez Espinosa; 289 pages, published in June 2015 by Sociedad Espa\~nola de Astronom\'ia

Using cosmological galaxy simulations from the MaGICC project, we study the evolution of the stellar masses, star formation rates and gas phase abundances of star forming galaxies. We derive the stellar masses and star formation rates using observational relations based on spectral energy distributions by applying the new radiative transfer code GRASIL-3D to our simulated galaxies. The simulations match well the evolution of the stellar mass-halo mass relation, have a star forming main sequence that maintains a constant slope out to redshift z $\sim$ 2, and populate projections of the stellar mass - star formation - metallicity plane, similar to observed star forming disc galaxies. We discuss small differences between these projections in observational data and in simulations, and the possible causes for the discrepancies. The light-weighted stellar masses are in good agreement with the simulation values, the differences between the two varying between 0.06 dex and 0.20 dex. We also find a good agreement between the star formation rate tracer and the true (time-averaged) simulation star formation rates. Regardless if we use mass- or light-weighted quantities, our simulations indicate that bursty star formation cycles can account for the scatter in the star forming main sequence., Comment: 12 pages, 8 figures, submitted to MNRAS

We present an analysis of the role of feedback in shaping the neutral hydrogen (HI) content of simulated disc galaxies. For our analysis, we have used two realisations of two separate Milky Way-like (~L*) discs - one employing a conservative feedback scheme (MUGS), the other significantly more energetic (MaGICC). To quantify the impact of these schemes, we generate zeroth moment (surface density) maps of the inferred HI distribution; construct power spectra associated with the underlying structure of the simulated cold ISM, in addition to their radial surface density and velocity dispersion profiles. Our results are compared with a parallel, self-consistent, analysis of empirical data from THINGS (The HI Nearby Galaxy Survey). Single power-law fits (P~k^gamma) to the power spectra of the stronger-feedback (MaGICC) runs (over spatial scales corresponding to 0.5 kpc to 20 kpc) result in slopes consistent with those seen in the THINGS sample (gamma = -2.5). The weaker-feedback (MUGS) runs exhibit shallower power law slopes (gamma = -1.2). The power spectra of the MaGICC simulations are more consistent though with a two-component fit, with a flatter distribution of power on larger scales (i.e., gamma = -1.4 for scales in excess of 2 kpc) and a steeper slope on scales below 1 kpc (gamma = -5), qualitatively consistent with empirical claims, as well as our earlier work on dwarf discs. The radial HI surface density profiles of the MaGICC discs show a clear exponential behaviour, while those of the MUGS suite are essentially flat; both behaviours are encountered in nature, although the THINGS sample is more consistent with our stronger (MaGICC) feedback runs., Comment: Accepted for publication in MNRAS; 9 pages; 7 figures

We introduce a new model for the spectral energy distribution of galaxies, GRASIL-3D, which includes a careful modelling of the dust component of the interstellar medium. GRASIL-3D is an entirely new model based on the formalism of an existing and widely applied spectrophotometric model, GRASIL, but specifically designed to be interfaced with galaxies with any arbitrarily given geometry, such as galaxies calculated by theoretical hydrodynamical galaxy formation codes. GRASIL-3D is designed to separately treat radiative transfer in molecular clouds and in the diffuse cirrus component. The code has a general applicability to the outputs of simulated galaxies, either from Lagrangian or Eulerian hydrodynamic codes. As an application, the new model has been interfaced to the P-DEVA and GASOLINE smoothed-particle hydrodynamic codes, and has been used to calculate the spectral energy distribution for a variety of simulated galaxies from UV to sub-millimeter wavelengths, whose comparison with observational data gives encouraging results. In addition, GRASIL-3D allows 2D images of such galaxies to be obtained, at several angles and in different bands., Comment: 24 pages, 13 Figures and 6 Tables. Accepted in MNRAS

We contend that a single power law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated local group realisations, which we determine using constrained local universe simulations (CLUES). For the stellar mass range 10$^7$

Using cosmological galaxy formation simulations from the MaGICC project, spanning more than three magnitudes in stellar mass (~10^7-3x10^{10} Msun), we trace the baryonic cycle of infalling gas from the virial radius through to its participation in the star formation process. An emphasis is placed upon the temporal history of chemical enrichment during its passage through the corona and CGM. We derive the distributions of time between gas crossing the virial radius and being accreted to the star forming region (which allows mixing within the corona), as well as the time between gas being accreted to the star forming region and then forming stars (which allows mixing within the disc). Significant numbers of stars are formed from gas that cycles back through the hot halo after first accreting to the star forming region. Gas entering high mass galaxies is pre-enriched in low mass proto-galaxies prior to entering the virial radius of the central progenitor, with only small amounts of primordial gas accreted, even at high redshift (z~5). After entering the virial radius, significant further enrichment occurs prior to the accretion of the gas to the star forming region, with gas that is feeding the star forming region surpassing 0.1Z by z=0. Mixing with halo gas, itself enriched via galactic fountains, is thus crucial in determining the metallicity at which gas is accreted to the disc. The lowest mass simulation (Mvir~2x10^{10}Msun, with M*~10^7Msun), by contrast, accretes primordial gas through the virial radius and onto the disc at all times. Much like the classical analytical solutions to the `G-dwarf problem', overproduction of low-metallicity stars is ameliorated by the inefficiency of star formation. Finally, gas outflow/metal removal rates from star forming regions as a function of galactic mass are presented., Comment: published in MNRAS

We examine the role of energy feedback in shaping the distribution of metals within cosmological hydrodynamical simulations of L* disc galaxies. While negative abundance gradients today provide a boundary condition for galaxy evolution models, in support of inside-out disc growth, empirical evidence as to whether abundance gradients steepen or flatten with time remains highly contradictory. We made use of a suite of L* discs, realised with and without `enhanced' feedback. All the simulations were produced using the smoothed particle hydrodynamics code Gasoline, and their in situ gas-phase metallicity gradients traced from redshift z~2 to the present-day. Present-day age-metallicity relations and metallicity distribution functions were derived for each system. The `enhanced' feedback models, which have been shown to be in agreement with a broad range of empirical scaling relations, distribute energy and re-cycled ISM material over large scales and predict the existence of relatively `flat' and temporally invariant abundance gradients. Enhanced feedback schemes reduce significantly the scatter in the local stellar age-metallicity relation and, especially, the [O/Fe]-[Fe/H] relation. The local [O/Fe] distribution functions for our L* discs show clear bimodality, with peaks at [O/Fe]=-0.05 and +0.05 (for stars with [Fe/H]>-1), consistent with our earlier work on dwarf discs. Our results with `enhanced' feedback are inconsistent with our earlier generation of simulations realised with `conservative' feedback. We conclude that spatially-resolved metallicity distributions, particularly at high-redshift, offer a unique and under-utilised constraint on the uncertain nature of stellar feedback processes., Comment: Accepted for publication in A&A; 5 pages; 3 figures; inset/legend to figure fixed (misaligned symbol fixed!)

We analyse the structure and chemical enrichment of a Milky Way-like galaxy with a stellar mass of 2 10^{10} M_sun, formed in a cosmological hydrodynamical simulation. It is disk-dominated with a flat rotation curve, and has a disk scale length similar to the Milky Way's, but a velocity dispersion that is ~50% higher. Examining stars in narrow [Fe/H] and [\alpha/Fe] abundance ranges, we find remarkable qualitative agreement between this simulation and observations: a) The old stars lie in a thickened distribution with a short scale length, while the young stars form a thinner disk, with scale lengths decreasing, as [Fe/H] increases. b) Consequently, there is a distinct outward metallicity gradient. c) Mono-abundance populations exist with a continuous distribution of scale heights (from thin to thick). However, the simulated galaxy has a distinct and substantive very thick disk (h_z~1.5 kpc), not seen in the Milky Way. The broad agreement between simulations and observations allows us to test the validity of observational proxies used in the literature: we find in the simulation that mono-abundance populations are good proxies for single age populations (<1 Gyr) for most abundances., Comment: 11 page, 10 figures, submitted. Comments welcome

We analyze and compare the bulges of a sample of L* spiral galaxies in hydrodynamical simulations in a cosmological context, using two different codes, P-DEVA and GASOLINE. The codes regulate star formation in very different ways, with P-DEVA simulations inputing low star formation efficiency under the assumption that feedback occurs on subgrid scales, while the GASOLINE simulations have feedback which drives large scale outflows. In all cases, the marked knee-shape in mass aggregation tracks, corresponding to the transition from an early phase of rapid mass assembly to a later slower one, separates the properties of two populations within the simulated bulges. The bulges analyzed show an important early starburst resulting from the collapse-like fast phase of mass assembly, followed by a second phase with lower star formation, driven by a variety of processes such as disk instabilities and/or mergers. Classifying bulge stellar particles identified at z=0 into old and young according to these two phases, we found bulge stellar sub-populations with distinct kinematics, shapes, stellar ages and metal contents. The young components are more oblate, generally smaller, more rotationally supported, with higher metallicity and less alpha-element enhanced than the old ones. These results are consistent with the current observational status of bulges, and provide an explanation for some apparently paradoxical observations, such as bulge rejuvenation and metal-content gradients observed. Our results suggest that bulges of L* galaxies will generically have two bulge populations which can be likened to classical and pseudo-bulges, with differences being in the relative proportions of the two, which may vary due to galaxy mass and specific mass accretion and merger histories., Comment: 24 pages, 10 figures, accepted for publication in ApJ

We test the validity of comparing simulated field disk galaxies with the empirical properties of systems situated within environments more comparable to loose groups, including the Milky Way's Local Group. Cosmological simulations of Milky Way-mass galaxies have been realised in two different environment samples: in the field and in environments with similar properties to the Local Group. Apart from the environments of the galaxies, the samples are kept as homogeneous as possible with equivalent ranges in last major merger time, halo mass and halo spin. Comparison of these two samples allow for systematic differences in the simulations to be identified. Metallicity gradients, disk scale lengths, colours, magnitudes and age-velocity dispersion relations are studied for each galaxy in the suite and the strength of the link between these and environment of the galaxies is studied. The bulge-to-disk ratio of the galaxies show that these galaxies are less spheroid dominated than many other simulated galaxies in literature with the majority of both samples being disk dominated. We find that secular evolution and mergers dominate the spread of morphologies and metallicity gradients with no visible differences between the two environment samples. In contrast with this consistency in the two samples there is tentative evidence for a systematic difference in the velocity dispersion-age relations of galaxies in the different environments. Loose group galaxies appear to have more discrete steps in their velocity dispersion-age relations. We conclude that at the current resolution of cosmological galaxy simulations field environment galaxies are sufficiently similar to those in loose groups to be acceptable proxies for comparison with the Milky Way provided that a similar assembly history is considered., Comment: 16 pages, 11 figures, abstract abridged for arXiv. Accepted for publication in Astronomy & Astrophysics

We introduce the Making Galaxies in a Cosmological Context (MaGICC) program of smoothed particle hydrodynamics (SPH) simulations. We describe a parameter study of galaxy formation simulations of an L* galaxy that uses early stellar feedback combined with supernova feedback to match the stellar mass--halo mass relationship. While supernova feedback alone can reduce star formation enough to match the stellar mass--halo mass relationship, the galaxy forms too many stars before z=2 to match the evolution seen using abundance matching. Our early stellar feedback is purely thermal and thus operates like a UV ionization source as well as providing some additional pressure from the radiation of massive, young stars. The early feedback heats gas to >10^6 K before cooling to 10^4 K. The pressure from this hot gas creates a more extended disk and prevents more star formation prior to z=1 than supernovae feedback alone. The resulting disk galaxy has a flat rotation curve, an exponential surface brightness profile, and matches a wide range of disk scaling relationships. The disk forms from the inside-out with an increasing exponential scale length as the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies that match observational results at low and high redshifts., Comment: 13 pages, 14 figures, accepted MNRAS, movies at http://www.mpia.de/~stinson/magicc

We examine the properties and evolution of a simulated polar disc galaxy. This galaxy is comprised of two orthogonal discs, one of which contains old stars (old stellar disc), and the other, containing both younger stars and the cold gas (polar disc) of the galaxy. By exploring the shape of the inner region of the dark matter halo, we are able to confirm that the halo shape is a oblate ellipsoid flattened in the direction of the polar disc. We also note that there is a twist in the shape profile, where the innermost 3 kpc of the halo flattens in the direction perpendicular to the old disc, and then aligns with the polar disc out until the virial radius. This result is then compared to the halo shape inferred from the circular velocities of the two discs. We also use the temporal information of the simulation to track the system's evolution, and identify the processes which give rise to this unusual galaxy type. We confirm the proposal that the polar disc galaxy is the result of the last major merger, where the angular moment of the interaction is orthogonal to the angle of the infalling gas. This merger is followed by the resumption of coherent gas infall. We emphasise that the disc is rapidly restored after the major merger and that after this event the galaxy begins to tilt. A significant proportion of the infalling gas comes from filaments. This infalling gas from the filament gives the gas its angular momentum, and, in the case of the polar disc galaxy, the direction of the gas filament does not change before or after the last major merger., Comment: Accepted for publication in MNRAS; 14 pages; 14 figures

Within a cosmological hydrodynamical simulation, we form a disc galaxy with sub- components which can be assigned to a thin stellar disc, thick disk, and a low mass stellar halo via a chemical decomposition. The thin and thick disc populations so selected are distinct in their ages, kinematics, and metallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity dispersion ({\sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The thick disc stars are old (6.6

We examine the chemical properties of 5 cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown to be consistent with the morphological characteristics and bulk scaling relations expected of late-type spirals. These simulations are part of the Making Galaxies In a Cosmological Context (MaGICC) Project, in which stellar feedback is tuned to match the stellar mass -- halo mass relationship. Each realisation employed identical initial conditions and assembly histories, but differed from one another in their underlying baryonic physics prescriptions, including (a) the efficiency with which each supernova energy couples to the ISM, (b) the impact of feedback associated with massive star radiation pressure, (c) the role of the minimum shut-off time for radiative cooling of Type II SNe remnants, (d) the treatment of metal diffusion, and (e) varying the IMF. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) `solar neighbourhood' and central `bulge' region. We compare the simulated MDFs' skewness, kurtosis, and dispersion (inter-quartile, inter-decile, inter-centile, and inter-tenth-percentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarfs. We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally. We can trace this difference to the simulations' tight and correlated age-metallicity relations (compared with that of the Milky Way), suggesting that these relations within `dwarf' discs might be steeper than in L* discs and/or the degree of stellar orbital re-distribution and migration inferred locally has not been captured in their entirety, at the resolution of our simulations. The important role of metal diffusion in ameliorating the over-production of extremely metal-poor stars is highlighted., Comment: Accepted for publication in MNRAS; 12 pages; 7 figures

By means of high-resolution cosmological hydrodynamical simulations of Milky Way-like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born "in situ" relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness., Comment: 20 pages, 14 figures, MNRAS, accepted

We examine radial and vertical metallicity gradients using a suite of disk galaxy simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient and reconcile differences between extant models and observations within the `inside-out' disk growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+) and Kobayashi & Nakasato (GRAPE-SPH). Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. We find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical `inside-out' growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z=1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required., Comment: Accepted for publication in A&A; 12 pages; 5 figures (several typos corrected and author affiliations updated)

We use the same physical model to simulate four galaxies that match the relation between stellar and total mass, over a mass range that includes the vast majority of disc galaxies. The resultant galaxies, part of the Making Galaxies in a Cosmological Context (MaGICC) program, also match observed relations between luminosity, rotation velocity, size, colour, star formation rate, HI mass, baryonic mass, and metallicity. Radiation from massive stars and supernova energy regulate star formation and drive outflows, balancing the complex interplay between cooling gas, star formation, large scale outflows, and recycling of gas in a manner which correctly scales with the mass of the galaxy. Outflows also play a key role in simulating galaxies with exponential surface brightness profiles, flat rotation curves and dark matter cores. Our study implies that large scale outflows are the primary driver of the dependence of disc galaxy properties on mass. We show that the amount of outflows invoked in our model is required to meet the constraints provided by observations of OVI absorption lines in the circum-galactic-media of local galaxies., Comment: 9 pages. accepted version MNRAS. Comments welcome: cbabrook@gmail.com

Encoded within the morphological structure of galaxies are clues related to their formation and evolutionary history. Recent advances pertaining to the statistics of galaxy morphology include sophisticated measures of concentration (C), asymmetry (A), and clumpiness (S). In this study, these three parameters (CAS) have been applied to a suite of simulated galaxies and compared with observational results inferred from a sample of nearby galaxies. The simulations span a range of late-type systems, with masses between ~1e10 Msun and ~1e12 Msun, and employ star formation density thresholds between 0.1 cm^-3 and 100 cm^-3. We have found that the simulated galaxies possess comparable concentrations to their real counterparts. However, the results of the CAS analysis revealed that the simulated galaxies are generally more asymmetric, and that the range of clumpiness values extends beyond the range of those observed. Strong correlations were obtained between the three CAS parameters and colour (B-V), consistent with observed galaxies. Furthermore, the simulated galaxies possess strong links between their CAS parameters and Hubble type, mostly in-line with their real counterparts., Comment: Accepted for publication in MNRAS; 11 pages; 12 figures; replacement includes minor amendments to match the published version

Using a suite of simulations (Governato et al. 2010) which successfully produce bulgeless (dwarf) disk galaxies, we provide an analysis of their associated cold interstellar media (ISM) and stellar chemical abundance patterns. A preliminary comparison with observations is undertaken, in order to assess whether the properties of the cold gas and chemistry of the stellar components are recovered successfully. To this end, we have extracted the radial and vertical gas density profiles, neutral hydrogen velocity dispersion, and the power spectrum of structure within the ISM. We complement this analysis of the cold gas with a brief examination of the simulations' metallicity distribution functions and the distribution of alpha-elements-to-iron., Comment: To appear in the proceedings of the JENAM 2010 Symposium "Dwarf Galaxies: Keys to Galaxy Formation and Evolution" (Lisbon, 9-10 September 2010), P. Papaderos, S. Recchi, G. Hensler (eds.), Springer Verlag (2011), in press

We present an analysis of the neutral hydrogen (HI) properties of a fully cosmological hydrodynamical dwarf galaxy, run with varying simulation parameters. As reported by Governato et al. (2010), the high resolution, high star formation density threshold version of this galaxy is the first simulation to result in the successful reproduction of a (dwarf) spiral galaxy without any associated stellar bulge. We have set out to compare in detail the HI distribution and kinematics of this simulated bulgeless disk with what is observed in a sample of nearby dwarfs. To do so, we extracted the radial gas density profiles, velocity dispersion (e.g., velocity ellipsoid, turbulence), and the power spectrum of structure within the cold interstellar medium from the simulations. The highest resolution dwarf, when using a high density star formation threshold comparable to densities of giant molecular clouds, possesses bulk characteristics consistent with those observed in nature, though the cold gas is not as radially extended as that observed in nearby dwarfs, resulting in somewhat excessive surface densities. The lines-of-sight velocity dispersion radial profiles have values that are in good agreement with observed dwarf galaxies, but due to the fact that only the streaming velocities of particles are tracked, a correction to include the thermal velocities can lead to profiles that are quite flat. The ISM power spectra of the simulations appear to possess more power on smaller spatial scales than that of the SMC. We conclude that unavoidable limitations remain due to the unresolved physics of star formation and feedback within pc-scale molecular clouds., Comment: Accepted for publication in MNRAS; 9 pages; 5 figures; Fig.4 de-graded to meet arXiv restrictions

Within a fully cosmological hydrodynamical simulation, we form a galaxy which rotates at 140 km/s, and is characterised by two loose spiral arms and a bar, indicative of a Hubble Type SBc/d galaxy. We show that our simulated galaxy has no classical bulge, with a pure disc profile at z=1, well after the major merging activity has ended. A long-lived bar subsequently forms, resulting in the formation of a secularly-formed "pseudo" bulge, with the final bulge-to-total light ratio B/T=0.21. We show that the majority of gas which loses angular momentum and falls to the central region of the galaxy during the merging epoch is blown back into the hot halo, with much of it returning later to form stars in the disc. We propose that this mechanism of redistribution of angular momentum via a galactic fountain, when coupled with the results from our previous study which showed why gas outflows are biased to have low angular momentum, can solve the angular momentum/bulgeless disc problem of the cold dark matter paradigm., Comment: 9 Pages, 10 Figures, accepted MNRAS version. Comments welcome

Using high resolution, fully cosmological smoothed particle hydro-dynamical simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show how baryons attain a final angular momentum distribution which allows pure disc galaxies to form. Blowing out substantial amounts of gas through supernovae and stellar winds, which is well supported observationally, is a key ingredient in forming bulgeless discs. We outline why galactic outflows preferentially remove low angular momentum material, and show that this is a natural result when structure forms in a cold dark matter cosmology. The driving factors are a) the mean angular momentum of accreted material increases with time, b) lower potentials at early times, c) the existence of an extended reservoir of high angular momentum gas which is not within star forming regions, meaning that only gas from the inner region (low angular momentum gas) is expelled and d) the tendency for outflows to follow the path of least resistance which is perpendicular to the disc. We also show that outflows are enhanced during mergers, thus expelling much of the gas which has lost its angular momentum during these events, and preventing the formation of "classical", merger driven bulges in low mass systems. Stars formed prior to such mergers form a diffuse, extended stellar halo component., Comment: accepted version: MNRAS in press

We present the McMaster Unbiased Galaxy Simulations (MUGS), the first 9 galaxies of an unbiased selection ranging in total mass from 5$\times10^{11}$ M$_\odot$ to 2$\times10^{12}$ M$_\odot$ simulated using n-body smoothed particle hydrodynamics (SPH) at high resolution. The simulations include a treatment of low temperature metal cooling, UV background radiation, star formation, and physically motivated stellar feedback. Mock images of the simulations show that the simulations lie within the observed range of relations such as that between color and magnitude and that between brightness and circular velocity (Tully-Fisher). The greatest discrepancy between the simulated galaxies and observed galaxies is the high concentration of material at the center of the galaxies as represented by the centrally peaked rotation curves and the high bulge-to-total ratios of the simulations determined both kinematically and photometrically. This central concentration represents the excess of low angular momentum material that long has plagued morphological studies of simulated galaxies and suggests that higher resolutions and a more accurate description of feedback will be required to simulate more realistic galaxies. Even with the excess central mass concentrations, the simulations suggest the important role merger history and halo spin play in the formation of disks., Comment: 16 pages, 16 figures, submitted to MNRAS, movies available at http://mugs.mcmaster.ca . Comments welcome.

Using high resolution SPH simulations in a fully cosmological Lambda CDM context we study the formation of a bright disk dominated galaxy that originates from a "wet" major merger at z=0.8. The progenitors of the disk galaxy are themselves disk galaxies that formed from early major mergers between galaxies with blue colors. A substantial thin stellar disk grows rapidly following the last major merger and the present day properties of the final remnant are typical of early type spiral galaxies, with an i band B/D ~0.65, a disk scale length of 7.2 kpc, g-r = 0.5 mag, an HI line width (W_{20}/2) of 238 km/sec and total magnitude i = -22.4. The key ingredients for the formation of a dominant stellar disk component after a major merger are: i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, ii) supernova feedback that is able to partially suppress star formation during mergers and iii) relative fading of the spheroidal component. The gas fraction of the progenitors' disks does not exceed 25% at z<3, emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of disks and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disk galaxies by the high likelihood of interactions and mergers for galaxy sized halos at relatively low z., Comment: MNRAS in press. Minor additions to text. Movie at: http://www.astro.washington.edu/fabio/movies/Merger.mpg

We use high resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas accretion, particularly along filaments, modifies the standard picture of gas accretion and cooling onto galaxy disks. In the standard picture, all gas is initially heated to the virial temperature of the galaxy as it enters the virial radius. Low mass galaxies are instead dominated by accretion of gas that stays well below the virial temperature, and even when a hot halo is able to develop in more massive galaxies there exist dense filaments that penetrate inside of the virial radius and deliver cold gas to the central galaxy. For galaxies up to ~L*, this cold accretion gas is responsible for the star formation in the disk at all times to the present. Even for galaxies at higher masses, cold flows dominate the growth of the disk at early times. Within this modified picture, galaxies are able to accrete a large mass of cold gas, with lower initial gas temperatures leading to shorter cooling times to reach the disk. Although star formation in the disk is mitigated by supernovae feedback, the short cooling times allow for the growth of stellar disks at higher redshifts than predicted by the standard model., Comment: accepted to ApJ

We present a new set of multi-million particle SPH simulations of the formation of disk dominated galaxies in a cosmological context. Some of these galaxies are higher resolution versions of the models already described in Governato et al (2007). To correctly compare simulations with observations we create artificial images of our simulations and from them measure photometric Bulge to Disk (B/D) ratios and disk scale lengths. We show how feedback and high force and mass resolution are necessary ingredients to form galaxies that have flatter rotation curves, larger I band disk scale lengths and smaller B/D ratios. A new simulated disk galaxy has an I-band disk scale length of 9.2 kpc and a B/D flux ratio of 0.64 (face on, dust reddened)., Comment: To appear in proceedings of "Formation and Evolution of Galaxy Disks", Rome, October 2007, Eds. J.G. Funes, S.J. and E.M. Corsini. Bigger figures than in printed version

The difference in density profiles of the contributions from different density peaks to dark matter halos results in certain expectations about the Milky Way's stellar halo. We cut our simulated halo stars into two populations: those forming before/during the last major merger, and those accreted after the last major merger. The former population are more centrally located at z=0, while stars forming in low mass late forming proto-galaxies are spread through the halo. A difference in observed binding energy distinguishes these two populations. We look at possible chemical abundance signatures of the two populations. We also show that galaxies forming in isolated low sigma peaks will form from primordial material. Thus, even though the oldest stars are centrally concentrated as they originated in the early collapsing, densest regions, primordial stars would be found distributed throughout the halo. Thus, the lack of observed metal free stars can be taken as directly constraining the Population III IMF, and the lowest metallicity observed stars can be interpreted as holding clues to the chemical yields of Pop III stars., Comment: Conference proceedings linked in ADS, 6 short pages

Increasingly large populations of disk galaxies are now being observed at increasingly high redshifts, providing new constraints on our knowledge of how such galaxies evolve. Are these observations consistent with a cosmology in which structures form hierarchically? To probe this question, we employ SPH/N-body galaxy scale simulations of late-type galaxies. We examine the evolution of these simulated disk galaxies from redshift 1 to 0, looking at the mass-size and luminosity-size relations, and the thickness parameter, defined as the ratio of scale-height to scale-length. The structural parameters of our simulated disks settle down quickly, and after redshift z=1 the galaxies evolve to become only slightly flatter. Our present day simulated galaxies are larger, more massive, less bright, and redder than at z=1. The inside-out nature of the growth of our simulated galaxies reduces, and perhaps eliminates, expectations of evolution in the size-mass relation., Comment: accepted version, to appear in ApJ 01 March 2006, v639

Despite having entered the era of "precision cosmology," the formation of galaxies within the favored CDM cosmological paradigm remains problematic. By relating our N-body/smooth particle hydro-dynamical simulation to an extensive range of Galactic and extragalactic observations, we shed light on the formation and evolution of the Milky Way and other late type galaxies. In light of recent observations of the stellar populations of extragalactic thick disks, we examine the proposal that the thick disk formed in a high redshift period characterized by gas rich merging. We show such a scenario to be consistent with color observations. We then follow the evolution of structural parameters of the subsequently formed disk galaxy, from redshift ~1 to the present. Consistent with observation, little evolution in the ratio of scale-height to scale-length is found in our simulated galaxy, despite its somewhat chaotic origins. The simulated galaxy in this report forms part of a larger study of a suite of galaxies, with which these issues are to be studied in detail., Comment: Poster Presentation at "The XXII Texas Symposium on Relativistic Astrophysics", at Stanford University, Dec. 2004, (TSRA04-1402) 5 pages, LaTeX, 8 ps figures

Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked-up in white dwarf stellar remnants. We first construct a 10^9 M_sun stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low- and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (10^12 M_sun) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z ~ 3) would have resulted in halos more than 100 times more luminous than those expected from conventional initial mass functions alone, and (2) any putative halo white dwarf progenitor dominated initial mass function component, regardless of its dynamical importance, would be virtually impossible to detect at the present-day, due to its extremely faint surface brightness., Comment: 4 pages, Refereed contribution to the 5th Galactic Chemodynamics conference held in Swinburne, July 2003. Accepted for publication in PASA

The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation - hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation, supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of ``rapid collapse'', we suggest that many such stars may have come from recently accreted satellites., Comment: 4 pages, 2 figures, to appear in the proceedings of Eurokiel2002 "The Evolution of Galaxies III. From Simple Approaches to Self-Consistent Models"

Halo initial mass functions (IMFs) heavily biased toward white dwarf (WD) precursors ~1-8 Msun have been suggested as a suitable mechanism for explaining MACHO events. However, by simple chemical evolution argument, Gibson & Mould (1997; GM97) pointed out that such WD-heavy IMFs (wdIMF) cause the overproduction of carbon and nitrogen. We re-examine this problem using numerical simulations., Comment: 2 pages, 1 table, to appear in the proceedings of IAU Symp. 208, "Astrophysical Supercomputing Using Particles", eds. J. Makino & P. Hut