Origin of the high v_los feature in the Galactic bar

We analyse a controlled N-body + smoothed particle hydrodynamics simulation of a growing disc galaxy within a non-growing, live dark halo. The disc is continuously fed with gas and star particles on near-circular orbits and develops a bar comparable in size to the one of the Milky Way (MW). We extract line of sight velocity v_los distributions from the model and compare it to data recently obtained from the APOGEE survey which show distinct high velocity features around v_los ~ 200 km/s. With an APOGEE like selection function, but without any scaling nor adjustment, we find v_los distributions very similar to those in APOGEE. The stars that make up the high v_los features at positive longitudes l are preferentially young bar stars (age <~ 2-3 Gyr) which move away from us along the rear side of the bar. At negative l, we find the corresponding low v_los feature from stars moving towards us. At l>10 degrees the highest v_los stars are a mixture of bar and background disc stars which complicates the interpretation of observations. The main peak in v_los is dominated by fore- and background stars. At a given time, ~40-50 per cent of high v_los stars occupy x_1 like orbits, but a significant fraction are on more complex orbits. The observed feature is likely due to a population of dynamically cool, young stars formed from gas just outside the bar and subsequently captured by the growing bar. The high v_los features disappear at high latitudes |b|>~2 degrees which explains the non-detection of such features in other surveys.Comment: 21 pages, 17 figures, accepted for publication in MNRA

Assessing distances and consistency of kinematics in Gaia/TGAS

We apply the statistical methods by Schoenrich, Binney & Asplund to assess the quality of distances and kinematics in the RAVE-TGAS and LAMOST-TGAS samples of Solar neighbourhood stars. These methods yield a nominal distance accuracy of 1-2%. Other than common tests on parallax accuracy, they directly test distance estimations including the effects of distance priors. We show how to construct these priors including the survey selection functions (SSFs) directly from the data. We demonstrate that neglecting the SSFs causes severe distance biases. Due to the decline of the SSFs in distance, the simple 1/parallax estimate only mildly underestimates distances. We test the accuracy of measured line-of-sight velocities (v_los) by binning the samples in the nominal v_los uncertainties. We find: a) the LAMOST v_los have a ~ -5 km/s offset; b) the average LAMOST measurement error for v_los is ~7 km/s, significantly smaller than, and nearly uncorrelated with the nominal LAMOST estimates. The RAVE sample shows either a moderate distance underestimate, or an unaccounted source of v_los dispersion (e_v) from measurement errors and binary stars. For a subsample of suspected binary stars in RAVE, our methods indicate significant distance underestimates. Separating a sample in metallicity or kinematics to select thick-disc/halo stars, discriminates between distance bias and e_v. For LAMOST, this separation yields consistency with pure v_los measurement errors. We find an anomaly near longitude l~(300+/-60)deg and distance s~(0.32+/-0.03)kpc on both sides of the galactic plane, which could be explained by either a localised distance error or a breathing mode.Comment: 21 pages, 14 figures accepted by MNRAS; now also includes comparison to Astraatmadja & Bailer-Jones distance

The structural evolution of galaxies with both thin and thick discs

We perform controlled N-body simulations of disc galaxies growing within live dark matter (DM) haloes to present-day galaxies that contain both thin and thick discs. We consider two types of models: a) thick-disc initial conditions to which stars on near-circular orbits are continuously added over ~10 Gyr and b) models in which the birth velocity dispersion of stars decreases continuously over the same time-scale. We show that both schemes produce double-exponential vertical profiles similar to that of the Milky Way (MW). We indicate how the spatial age structure of galaxies can be used to discriminate between scenarios. We show that the presence of a thick disc significantly alters and delays bar formation and thus makes possible models with a realistic bar and a high baryon-to-DM mass ratio in the central regions, as required by microlensing constraints. We examine how the radial mass distribution in stars and DM is affected by disc growth and non-axisymmetries. We discuss how bar buckling shapes the vertical age distribution of thin- and thick-disc stars in the bar region. The extent to which the combination of observationally motivated inside-out growth histories and cosmologically motivated dark halo properties leads to the spontaneous formation of non-axisymmetries that steer the models towards present-day MW-like galaxies is noteworthy.Comment: Accepted for publication in MNRAS, 22 pages, 13 figures, 2 table

Migration and kinematics in growing disc galaxies with thin and thick discs

We analyse disc heating and radial migration in N-body models of growing disc galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies with appropriate non-axisymmetric structures reproduce observational constraints on radial disc heating in and migration to the Solar Neighbourhood (Snhd). The presence of thick discs can suppress non-axisymmetries and thus higher baryonic-to-dark matter fractions are required than in models that only have a thin disc. Models that are baryon-dominated to roughly the Solar radius R_0 are favoured, in agreement with data for the Milky Way. For inside-out growing discs, today's thick-disc stars at R_0 are dominated by outwards migrators. Whether outwards migrators are vertically hotter than non-migrators depends on the radial gradient of the thick disc vertical velocity dispersion. There is an effective upper boundary in angular momentum that thick disc stars born in the centre of a galaxy can reach by migration, which explains the fading of the high-alpha sequence outside R_0. Our models compare well to Snhd kinematics from RAVE-TGAS. For such comparisons it is important to take into account the azimuthal variation of kinematics at R ~ R_0 and biases from survey selection functions. The vertical heating of thin disc stars by giant molecular clouds is only mildly affected by the presence of thick discs. Our models predict higher vertical velocity dispersions for the oldest stars than found in the Snhd age-velocity dispersion relation, possibly because of measurement uncertainties or an underestimation of the number of old cold stars in our models.Comment: accepted for publication in MNRAS, 22 pages, 11 figures, 1 Table, Appendi

Simulating the formation and evolution of disc galaxies in a LambdaCDM universe

The majority of stars in the universe has formed in disc galaxies with masses similar to that of the Milky Way. Ab-initio cosmological hydrodynamical simulations of the formation and evolution of galaxies in a Lambda Cold Dark Matter universe have long suffered from serious problems in correctly modelling the star-formation history and structure of disc galaxies. We first use idealized semi-cosmological simulations to gain a better understanding of processes leading to problems in disc formation simulations. We add rotating spheres of hot gas to cosmological dark-matter-only simulations of individual haloes and follow the formation and evolution of galaxy discs from the cooling gas. The initial orientation of the baryonic angular momentum with respect to the halo has a major effect on disc formation. Despite the coherently rotating initial conditions, the orientations of the disc and the outer gas and the relative angle between the components can all change by more than 90 degrees over several billion years. Dominant discs with realistic structural and kinematical properties form preferentially if slow cooling times shift disc formation to later times, if the initial angular momentum is aligned with the halo minor axis and if there is little reorientation of the disc. We then present a new set of fully cosmological simulations with an updated multiphase smoothed particle hydrodynamics galaxy formation code. The update includes improved treatment of metal-line cooling, metal production, turbulent diffusion of metals, kinetic and thermal supernova feedback and radiation pressure from massive young stars. We compare the models to a variety of observations at high and low redshifts and find good agreement for morphologies, stellar-to-dark-matter mass ratios, star formation rates, gas fractions and heavy element abundances. Agreement is better at redshift z=1 than at present day as discrepancies in star formation histories for the lowest and highest simulated galaxy masses become apparent at late times. 18 out of 19 of our model galaxies at z=0 contain stellar discs with kinematic disc fractions up to 65 %, higher than in any previous simulations. We finally compare our model galaxies in detail with recent observations of the structural evolution of stellar galactic discs and the structure of z=0 gas discs. Stellar surface density profiles agree well with observations at z>1, but reveal too little central growth afterwards. This is likely connected to a lack of bars in our simulations resulting from overly strong feedback. Discs at z=0 are too extended by a factor \sim 2. The discs have diverse formation histories ranging from pure inside-out growth in systems with quiescent merger histories to continuous mass growth at all radii. Central mass growth in our models is driven by mergers and misaligned infall events, which leave signatures in the present day distributions of radii and element abundances as functions of stellar age. Gas discs agree well with observations in terms of sizes and profile shapes, but on average have overly high gas-to-stellar mass ratios. Our models agree well with the observed neutral hydrogen mass-size relation. Despite significant progress, our models continue to suffer from various problems illustrating that we are still far away from capturing all relevant physical processes accurately

Age velocity dispersion relations and heating histories in disc galaxies

We analyse the heating of stellar discs by non axisymmetric structures and giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies. The analysis resolves long-standing discrepancies between models and data by demonstrating the importance of distinguishing between measured age-velocity dispersion relations (AVRs) and the heating histories of the stars that make up the AVR. We fit both AVRs and heating histories with formulae proportional to t^beta and determine the exponents beta_R and beta_z derived from in-plane and vertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_z are in almost all simulations larger than values of ~beta_z, whereas values of beta_R are similar to or mildly larger than values of ~beta_R. Moreover, values of beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). The dominant cause of these relations is the decline over the life of the disc in importance of GMCs as heating agents relative to spiral structure and the bar. We examine how age errors and biases in solar neighbourhood surveys influence the measured AVR: they tend to decrease beta values by smearing out ages and thus measured dispersions. We compare AVRs and velocity ellipsoid shapes sigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude that for the expected disc mass and dark halo structure, combined GMC and spiral/bar heating can explain the AVR of the Galactic thin disc. Strong departures of the disc mass or the dark halo structure from expectation spoil fits to the data.Comment: Accepted for publication in MNRAS, 19 pages, 8 figures, 1 tabl

Kinematic Detection of the Galactic Nuclear Disc

We report the detection of the Galactic nuclear disc in line-of-sight kinematics of stars, measured with infrared spectroscopy from APOGEE. This stellar component of the nuclear disc has an extent and rotation velocity V ~ 120kms comparable to the gas disc in the central molecular zone. The current data suggest that this disc is kinematically cool and has a small vertical extent of order 50pc. The stellar kinematics suggest a truncation radius/steep decline of the stellar disc at a galactocentric radius R ~ 150pc, and provide tentative evidence for an overdensity at the position of the ring found in the molecular gas disc.Comment: 6 pages, 4 figures, accepted by ApJ Letter

Towards a more realistic population of bright spiral galaxies in cosmological simulations

We present an update to the multiphase SPH galaxy formation code by Scannapieco et al. We include a more elaborate treatment of the production of metals, cooling rates based on individual element abundances, and a scheme for the turbulent diffusion of metals. Our SN feedback model now transfers energy to the ISM in kinetic and thermal form, and we include a prescription for the effects of radiation pressure from massive young stars on the ISM. We calibrate our new code on the well studied Aquarius haloes and then use it to simulate a sample of 16 galaxies with halo masses between 1x10^11 and 3x10^12 M_sun. In general, the stellar masses of the sample agree well with the stellar mass to halo mass relation inferred from abundance matching techniques for redshifts z=0-4. There is however a tendency to overproduce stars at z>4 and to underproduce them at z<0.5 in the least massive haloes. Overly high SFRs at z<1 for the most massive haloes are likely connected to the lack of AGN feedback in our model. The simulated sample also shows reasonable agreement with observed star formation rates, sizes, gas fractions and gas-phase metallicities at z=0-3. Remaining discrepancies can be connected to deviations from predictions for star formation histories from abundance matching. At z=0, the model galaxies show realistic morphologies, stellar surface density profiles, circular velocity curves and stellar metallicities, but overly flat metallicity gradients. 15 out of 16 of our galaxies contain disk components with kinematic disk fraction ranging between 15 and 65 %. The disk fraction depends on the time of the last destructive merger or misaligned infall event. Considering the remaining shortcomings of our simulations we conclude that even higher kinematic disk fractions may be possible for LambdaCDM haloes with quiet merger histories, such as the Aquarius haloes.Comment: 26 pages, 20 figures, accepted for publication in MNRA

Why stellar feedback promotes disc formation in simulated galaxies

We study how feedback influences baryon infall onto galaxies using cosmological, zoom-in simulations of haloes with present mass $M_{vir}=6.9\times10^{11} M_{\odot}$ to $1.7\times10^{12} M_{\odot}$. Starting at z=4 from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because 50% of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z=0 dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because 25-30% of the total accreted gas is ejected entirely before z~1, removing primarily low angular momentum material which enriches the nearby inter-galactic medium. Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion. These processes lower galaxy formation efficiency in addition to promoting disc formation.Comment: 23 pages, 29 figures, accepted for publication in MNRA

Pet attitudes predicting preferences for pets over people

The preference for animal companionship over human companionship may be predicted by attitudes about pet ownership. We hypothesized that pet attitudes could predict preferences for relationships with pets over humans. We sampled 182 people who named a person and a pet they love and care about. Participants rated their feelings of love, time spent, enjoyment, and equity in both their human and pet relationships. We also presented seven hypothetical negative event scenarios that involve both the pet and human and asked participants to predict their feelings and reactions based on these events. The Pet Attitudes Scale (Templer et al., 1981) was used to assess attitudes towards pets. People had similar positive feelings about their pet‑human and human‑human relationships. However, people were more likely to react negatively towards a human compared to a pet. Positive pet a;itudes predicted more positive and less negative reactions to pets. Positive pet a;itudes can predict preference for pet relationships over human relationships and may help researchers identify what relationships work best depending on a personʹs pet attitudes