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We have explored the outskirts of dark matter haloes out to 2.5 times the virial radius using a large sample of halos drawn from Illustris, along with a set of zoom simulations (MUGS). Using these, we make a systematic exploration of the shape profile beyond R$_{vir}$. In the mean sphericity profile of Illustris halos we identify a dip close to the virial radius, which is robust across a broad range of masses and infall rates. The inner edge of this feature may be related to the virial radius and the outer edge with the splashback radius. Due to the high halo-to-halo variation this result is visible only on average. However, in four individual halos in the MUGS sample, a decrease in the sphericity and a subsequent recovery is evident close to the splashback radius. We find that this feature persists for several Gyr, growing with the halo. This feature appears at the interface between the spherical halo density distribution and the filamentary structure in the environment. The shape feature is strongest when there is a high rate of infall, implying that the effect is due to the mixing of accreting and virializing material. The filamentary velocity field becomes rapidly mixed in the halo region inside the virial radius, with the area between this and the splashback radius serving as the transition region. We also identify a long-lasting and smoothly evolving splashback region in the radial density gradient in many of the MUGS halos., Comment: 22 pages, 15 figures, 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

Detailed studies of galaxy formation require clear definitions of the structural components of galaxies. Precisely defined components also enable better comparisons between observations and simulations. We use a subsample of eighteen cosmological zoom-in simulations from the NIHAO project to derive a robust method for defining stellar kinematic discs in galaxies. Our method uses Gaussian Mixture Models in a 3D space of dynamical variables. The NIHAO galaxies have the right stellar mass for their halo mass, and their angular momenta and S\'ersic indices match observations. While the photometric disc-to-total ratios are close to 1 for all the simulated galaxies, the kinematic ratios are around ~0.5. Thus, exponential structure does not imply a cold kinematic disc. Above log(M*)~9.5, the decomposition leads to thin discs and spheroids that have clearly different properties, in terms of angular momentum, rotational support, ellipticity, [Fe/H] and [O/Fe]. At log(M*)<9.5, the decomposition selects discs and spheroids with less distinct properties. At these low masses, both the discs and spheroids have exponential profiles with high minor-to-major axes ratios, i.e. thickened discs., Comment: Accepted for publication in MNRAS

We explore the chemical distribution of stars in a simulated galaxy. Using simulations of the same initial conditions but with two different feedback schemes (MUGS and MaGICC), we examine the features of the age-metallicity relation (AMR), and the three-dimensional age-metallicity-[O/Fe] distribution, both for the galaxy as a whole and decomposed into disc, bulge, halo, and satellites. The MUGS simulation, which uses traditional supernova feedback, is replete with chemical substructure. This sub- structure is absent from the MaGICC simulation, which includes early feedback from stellar winds, a modified IMF and more efficient feedback. The reduced amount of substructure is due to the almost complete lack of satellites in MaGICC. We identify a significant separation between the bulge and disc AMRs, where the bulge is considerably more metal-rich with a smaller spread in metallicity at any given time than the disc. Our results suggest, however, that identifying the substructure in observations will require exquisite age resolution, on the order of 0.25 Gyr. Certain satellites show exotic features in the AMR, even forming a 'sawtooth' shape of increasing metallicity followed by sharp declines which correspond to pericentric passages. This fact, along with the large spread in stellar age at a given metallicity, compromises the use of metallicity as an age indicator, although alpha abundance provides a more robust clock at early times. This may also impact algorithms that are used to reconstruct star formation histories from resolved stellar populations, which frequently assume a monotonically-increasing AMR., Comment: 25 pages, 14 Figures, 2 Tables. Accepted for publication in MNRAS

We show that the cool gas masses of galactic discs reach a steady state that lasts many Gyr after their last major merger in cosmological hydrodynamic simulations. The mass of disc gas, M$_{\rm gas}$, depends upon a galaxy halo's spin and virial mass, but not upon stellar feedback. Halos with low spin have high star formation efficiency and lower disc gas mass. Similarly, lower stellar feedback leads to more star formation so the gas mass ends up nearly the same irregardless of stellar feedback strength. Even considering spin, the M$_{\rm gas}$ relation with halo mass, M$_{200}$ only shows a factor of 3 scatter. The M$_{\rm gas}$--M$_{200}$ relation show a break at M$_{200}$=$2\times10^{11}$ M$_\odot$ that corresponds to an observed break in the M$_{\rm gas}$--M$_\star$ relation. The constant disc mass stems from a shared halo gas density profile in all the simulated galaxies. In their outer regions, the profiles are isothermal. Where the profile rises above $n=10^{-3}$ cm$^{-3}$, the gas readily cools and the profile steepens. Inside the disc, rotation supports gas with a flatter density profile except where supernova explosions disrupt the disc. Energy injection from stellar feedback also provides pressure support to the halo gas to prevent runaway cooling flows. The resulting constant gas mass makes simpler models for galaxy formation possible, either using a "bathtub" model for star formation rates or when modeling chemical evolution., Comment: To be submitted to MNRAS. Comments very welcome in the meantime

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 present a first study of the effect of local photoionising radiation on gas cooling in smoothed particle hydrodynamics simulations of galaxy formation. We explore the combined effect of ionising radiation from young and old stellar populations. The method computes the effect of multiple radiative sources using the same tree algorithm used for gravity, so it is computationally efficient and well resolved. The method foregoes calculating absorption and scattering in favour of a constant escape fraction for young stars to keep the calculation efficient enough to simulate the entire evolution of a galaxy in a cosmological context to the present day. This allows us to quantify the effect of the local photoionisation feedback through the whole history of a galaxy`s formation. The simulation of a Milky Way like galaxy using the local photoionisation model forms ~ 40 % less stars than a simulation that only includes a standard uniform background UV field. The local photoionisation model decreases star formation by increasing the cooling time of the gas in the halo and increasing the equilibrium temperature of dense gas in the disc. Coupling the local radiation field to gas cooling from the halo provides a preventive feedback mechanism which keeps the central disc light and produces slowly rising rotation curves without resorting to extreme feedback mechanisms. These preliminary results indicate that the effect of local photoionising sources is significant and should not be ignored in models of galaxy formation., Comment: Accepted for Publication in MNRAS, 13 pages, 13 figures

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

The high-resolution, SPH galaxies of the McMaster Unbiased Galaxy Survey (MUGS) are used to examine the satellite systems of sixteen model host galaxies. Each galaxy has a different mass, angular momentum and merger history that yield a rich set of satellite luminosity functions. With new observations of distant satellite systems, we can compare these luminosity functions to satellite systems beyond the Local Group. We find that the luminosity functions of our simulations compare well to observations when the luminosity functions are scaled according to host mass. We use the recently-found relationship between dwarf satellites and host mass in distant satellite systems (Trentham & Tully 2009) to normalize a theoretical, complete luminosity function for the Milky Way (Koposov et al. 2008). The luminosity function of satellites, expressed as a function of the host mass, is given by dN/dM_V= 3.5 x M_host^(0.91) x 10^(0.1 x M_V-10.2), where host mass is given in M_sun. The mass of a host galaxy can be used to predict the number of dwarf satellites and even when considering spiral and elliptical hosts separately this relation holds., Comment: 7 pages, 5 figures, 1 table, accepted by MNRAS

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

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

We compare the results of thirteen cosmological gasdynamical codes used to simulate the formation of a galaxy in the LCDM structure formation paradigm. The various runs differ in their hydrodynamical treatment (SPH, moving-mesh and AMR) but share the same initial conditions and adopt their latest published model of cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z=0, due mainly to the different implementations of feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas-rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disk is discernible in most simulations, though its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observation. Models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disk component. Our conclusions hold at two different numerical resolutions. Some differences can also be traced to the numerical techniques: more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high-angular momentum stellar disks remains a challenge., Comment: 28 pages, 16 figures, MNRAS accepted. High resolution figures and additional images can be found at http://www.aip.de/People/cscannapieco/aquila/

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

We present a study of satellites in orbit around a high-resolution, smoothed particle hydrodynamics (SPH) galaxy simulated in a cosmological context. The simulated galaxy is approximately the same mass as the Milky Way. The cumulative number of luminous satellites at z = 0 is similar to the observed system of satellites orbiting the Milky Way although an analysis of the satellite mass function reveals an order of magnitude more dark satellites than luminous. Some of the dark subhalos are more massive than some of the luminous subhalos at z = 0. What separates luminous and dark subhalos is not their mass at z = 0, but the maximum mass the subhalos ever achieve. We study the effect of four mass loss mechanisms on the subhalos: ultraviolet (UV) ionising radiation, ram pressure stripping, tidal stripping, and stellar feedback, and compare the impact of each of these four mechanisms on the satellites. In the lowest mass subhalos, UV is responsible for the majority of the baryonic mass loss. Ram pressure stripping removes whatever mass remains from the low mass satellites. More massive subhalos have deeper potential wells and retain more mass during reionisation. However, as satellites pass near the centre of the main halo, tidal forces cause significant mass loss from satellites of all masses. Satellites that are tidally stripped from the outside can account for the luminous satellites that are lower mass than some of the dark satellites. Stellar feedback has the greatest impact on medium mass satellites that had formed stars, but lost all their gas by z = 0. Our results demonstrate that the missing satellite problem is not an intractable issue with the cold dark matter cosmology, but is rather a manifestation of baryonic processes., Comment: 15 pages, 18 figures, accepted at MNRAS

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

The next generation mass probes will investigate DE nature by measuring non-linear power spectra at various z, and comparing them with high precision simulations. Producing a complete set of them, taking into account baryon physics and for any DE state equation w(z), would really be numerically expensive. Regularities reducing such duty are essential. This paper presents further n-body tests of a relation we found, linking models with DE state parameter w(z) to const.-w models, and also tests the relation in hydro simulations., Comment: PASCOS 2010, the 16th International Symposium on Particles, Strings and Cosmology, Valencia (Spain), July 19th - 23rd, 2010

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.

We trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr) galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in extended gas disks. Our simulations thus show that star forming galaxies observed at z~2 are likely the main progenitors of central galaxies in galaxy groups at z=0. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. In one of the three simulated groups the short central cooling time leads to a dramatic rejuvenation of the central group galaxy at z<1, affecting its morphology, kinematics and colors. This episode is eventually terminated by a group-group merger. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.[Abridged], Comment: 28 pages, 23 figures, 9 tables, accepted to APJ (revised to match accepted version)

We simulate the collapse of isolated dwarf galaxies using SPH + N-Body simulations including a physically motivated description of the effects of supernova feedback. As the gas collapses and stars form, the supernova feedback disrupts enough gas to temporarily quench star formation. The gas flows outward into a hot halo, where it cools until star formation can continue once more and the cycle repeats. The star formation histories of isolated Local Group dwarf galaxies exhibit similar episodic bursts of star formation. We examine the mass dependence of the stellar velocity dispersions and find that they are no less than half the velocity of the halos measured at the virial radius., Comment: 5 pages, 3 figures, accepted ApJ. Full resolution figures and movies available at http://hpcc.astro.washington.edu/feedback

We examine the origin and evolution of the mass-metallicity relationship (MZR, M-Z) for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of supernovae feedback and subsequent metal enrichment. We discriminate between two sources that may contribute to the origin of the MZR: 1) metal and baryon loss due to gas outflow, or 2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies reproduce the observed MZR in shape and normalization both at z=0 and z=2. We find that baryon loss occurs due to UV heating before star formation turns on in galaxies with M_baryon < 10^8 M_sun, but that some gas loss due to supernovae induced winds is required to subsequently reproduce the low effective chemical yield observed in low mass galaxies. Despite this, we show that low star formation efficiencies, regulated by supernovae feedback, are primarily responsible for the lower metallicities of low mass galaxies and the overall M-Z trend. We find that the shape of the MZR is relatively constant with redshift, but that its normalization increases with time. Simulations with no energy feedback from supernovae overproduce metals at low galaxy masses by rapidly transforming a large fraction of their gas into stars. Despite the fact that our low mass galaxies have lost a majority of their baryons, they are still the most gas rich objects in our simulations due to their low star formation efficiencies., Comment: 3 figures. Accepted for ApJL, in press

We used fully cosmological, high resolution N-body + SPH simulations to follow the formation of disk galaxies with rotational velocities between 135 and 270 km/sec in a Lambda CDM universe. The simulations include gas cooling, star formation, the effects of a uniform UV background and a physically motivated description of feedback from supernovae. The host dark matter halos have a spin and last major merger redshift typical of galaxy sized halos as measured in recent large scale N--Body simulations. The simulated galaxies form rotationally supported disks with realistic exponential scale lengths and fall on both the I-band and baryonic Tully Fisher relations. An extended stellar disk forms inside the Milky Way sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a Milky Way sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy's stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies., Comment: Revised version after the referee's comments. Conclusions unchanged. 2 new plots. MNRAS in press. 20 plots. 21 pages