ALBERT

Description: We use the Galaxies-Intergalactic Medium Interaction Calculation ( gimic ) cosmological hydrodynamic simulation at z  = 0 to study the distribution and environmental dependence of neutral hydrogen (H i ) gas in the outskirts of simulated galaxies. This gas can currently be probed directly in, for example, Lyα absorption via the observation of background quasars. Radio facilities, such as the Square Kilometre Array, will provide a complementary probe of the diffuse H i in emission and will constrain the physics underpinning the complex interplay between accretion and feedback mechanisms which affect the intergalactic medium. We extract a sample of 488 galaxies from a resimulation of the average cosmic density gimic region. We estimate the neutral hydrogen content of these galaxies and the surrounding intergalactic medium within which they reside. We investigate the average H i radial profiles by stacking the individual profiles according to both mass and environment. We find high H i column densities at large impact parameters in group environments and markedly lower H i densities for non-group galaxies. We suggest that these results likely arise from the combined effects of ram pressure stripping and tidal interactions present in group environments.

Description: We present an analysis of the role of feedback in shaping the neutral hydrogen (H  i ) content of simulated disc galaxies. For our analysis, we have used two realizations of two separate Milky Way-like (~ L *) discs – one employing a conservative feedback scheme (McMaster Unbiased Galaxy Survey), the other significantly more energetic [Making Galaxies In a Cosmological Context (MaGICC)]. To quantify the impact of these schemes, we generate zeroth moment (surface density) maps of the inferred H  i distribution; construct power spectra associated with the underlying structure of the simulated cold interstellar medium, 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 The H  i Nearby Galaxy Survey (THINGS). Single power-law fits ( P    k ) to the power spectra of the stronger feedback (MaGICC) runs (over spatial scales corresponding to ~0.5 to ~20 kpc) result in slopes consistent with those seen in the THINGS sample ( ~ –2.5). The weaker feedback (MUGS) runs exhibit shallower power-law slopes ( ~ –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.  ~ –1.4 for scales in excess of ~2 kpc) and a steeper slope on scales below ~1 kpc ( ~ –5), qualitatively consistent with empirical claims, as well as our earlier work on dwarf discs. The radial H  i 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.

Description: 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  ~ 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 and 0.20 dex. We also find 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.

Description: By selecting in the Radial Velocity Experiment-fourth data release (RAVE-DR4) survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest three-component model (thin disc, thick disc and halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to [ M/H ] = -2 dex, contributing roughly 3 per cent of the entire thick disc population and having a shorter scalelength compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity ([ M/H ]) and the orbital velocity ( V ), which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find V /[ M/H ] 50 km s –1 dex –1 , which is in very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc and suggest that given the above-mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely.

Description: We aim to characterize high-velocity (HiVel) stars in the solar vicinity both chemically and kinematically using the fourth data release of the RAdial Velocity Experiment (RAVE). We used a sample of 57 HiVel stars with Galactic rest-frame velocities larger than 275 km s –1 . With 6D position and velocity information, we integrated the orbits of the HiVel stars and found that, on average, they reach out to 13 kpc from the Galactic plane and have relatively eccentric orbits consistent with the Galactic halo. Using the stellar parameters and [α/Fe] estimates from RAVE, we found the metallicity distribution of the HiVel stars peak at [ M /H] = –1.2 dex and is chemically consistent with the inner halo. There are a few notable exceptions that include a hypervelocity star candidate, an extremely HiVel bound halo star, and one star that is kinematically consistent with the halo but chemically consistent with the disc. High-resolution spectra were obtained for the metal-rich HiVel star candidate and the second highest velocity star in the sample. Using these high-resolution data, we report the discovery of a metal-rich halo star that has likely been dynamically ejected into the halo from the Galactic thick disc. This discovery could aid in explaining the assembly of the most metal-rich component of the Galactic halo.

Description: Using cosmological galaxy formation simulations from the MaGICC (Making Galaxies in a Cosmological Context) project, spanning stellar mass from ~10 7  to 3 x 10 10  M , we trace the baryonic cycle of infalling gas from the virial radius through to its eventual participation in the star formation process. An emphasis is placed upon the temporal history of chemical enrichment during its passage through the corona and circumgalactic medium. We derive the distributions of time between gas crossing the virial radius and being accreted to the star-forming region (which allows for mixing within the corona), as well as the time between gas being accreted to the star-forming region and then ultimately forming stars (which allows for 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 0.1 Z 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 simulated galaxy ( M vir ~ 2 x 10 10  M , with M * ~ 10 7  M ), by contrast, accretes primordial gas through the virial radius and on to the disc, throughout its history. Much like the case for classical analytical solutions to the so-called ‘G-dwarf problem’, overproduction of low-metallicity stars is ameliorated by the interplay between the time of accretion on to the disc and the subsequent involvement in star formation – i.e. due to the inefficiency of star formation. Finally, gas outflow/metal removal rates from star-forming regions as a function of galactic mass are presented.

Description: We report the identification of extended tidal debris potentially associated with the globular cluster NGC 3201, using the RAdial Velocity Experiment (RAVE) catalogue. We find the debris stars are located at a distance range of 1–7 kpc based on the forthcoming RAVE distance estimates. The derived space velocities and integrals of motion show interesting connections to NGC 3201, modulo uncertainties in the proper motions. Three stars, which are among the four most likely candidates for NGC 3201 tidal debris, are separated by 80° on the sky yet are well matched by the 12 Gyr, [Fe/H] = –1.5 isochrone appropriate for the cluster. This is the first time tidal debris around this cluster has been reported over such a large spatial extent, with implications for the cluster's origin and dynamical evolution.

Description: Using idealized 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 scaleheight. 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 scaleheight 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.

Description: We examine simulations of isolated galaxies to analyse the effects of localized feedback on the formation and evolution of molecular clouds. Feedback contributes to turbulence and the destruction of clouds, leading to a population of clouds that is younger, less massive, and with more retrograde rotation. We investigate the evolution of clouds as they interact with each other and the diffuse interstellar medium, and determine that the role of cloud interactions differs strongly with the presence of feedback: in models without feedback, scattering events dramatically increase the retrograde fraction, but in models with feedback, mergers between clouds may slightly increase the prograde fraction. We also produce an estimate of the viscous time-scale due to cloud–cloud collisions, which increases with increasing strength of feedback ( t  ~ 20 Gyr versus t  ~ 10 Gyr), but is still much smaller than previous estimates ( t  ~ 1000 Gyr); although collisions become more frequent with feedback, less energy is lost in each collision than in the models without feedback.

Description: The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using 72 365 red-clump stars, we examine the mean velocity components in 3D between 6 〈 R  〈 10 kpc and –2 〈 Z  〈 2 kpc, concentrating on north–south differences. Simple parametric fits to the ( R , Z ) trends for V and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V R , finding that the gradient is marked below the plane (〈 V R 〉/ R  = –8 km s –1 kpc –1 for Z  〈 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, V Z , also shows clear, large-amplitude (| V Z | = 17 km s –1 ) structure, with indications of a rarefaction–compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north–south differences in V R and V Z along the line of sight, with the V R estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.