Daisuke Kawata

Internal Alignment of the Halos of Disk Galaxies in Cosmological Hydrodynamic Simulations

Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1rvir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientations of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The lack of connection between these two regions of the halo should be taken into account when modeling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.

The dynamics of stars around spiral arms

Spiral density wave theory attempts to describe the spiral pattern in spiral galaxies in terms of a long-lived wave structure with a constant pattern speed in order to avoid the winding dilemma. The pattern is consequently a rigidly rotating, long-lived feature. We run N-body simulations of a giant disc galaxy consisting of a pure stellar disc and a static dark matter halo, and find that the spiral arms are transient features whose pattern speeds decrease with radius in such a way that the pattern speed is almost equal to the rotation curve of the galaxy. We trace particle motion around the spiral arms. We show that particles from behind and in front of the spiral arm are drawn towards and join the arm. Particles move along the arm in the radial direction and we find a clear trend that they migrate towards the outer (inner) radii on the trailing (leading) side of the arm. Our simulations demonstrate that because the spiral arm feature is corotating, the particles continue to be accelerated (decelerated) by the spiral arm for long periods, which leads to strong and efficient migration, at all radii in the disc.

Metallicity gradients in disks - Do galaxies form inside-out?

Aims. We examine radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient slope and reconcile the differences existing between extant models and observations within the canonical “inside-out” disk growth paradigm. Methods. A suite of 25 cosmological disks is used to examine the evolution of metallicity gradients; this consists of 19 galaxies selected from the RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code ramses ,i ncluding eight drawn from the “field” and six from “loose group” environments. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code gasoline. 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. Results. We first show that generically flatter gradients are observed at redshift zero when comparing older stars with those forming today, consistent with expectations of kinematically hot simulations, but counter to that observed in the Milky Way. The vertical abundance gradients at ∼1−3 disk scalelengths are comparable to those observed in the thick disk of the Milky Way, but significantly shallower than those seen in the thin disk. Most importantly, 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. Conclusions. 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.

Thin disc, thick disc and halo in a simulated galaxy

Within a cosmological hydrodynamical simulation, we form a disc galaxy with subcomponents 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 (�U,V,W = 41,31,25 kms 1 ), high [Fe/H], and low [O/Fe]. Conversely, the thick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by �21 km/s, possess higher velocity dispersion (�U,V,W = 49,44,35 kms 1 ), and have relatively low [Fe/H] and high [O/Fe]. The halo component comprises less than 4% of stars in the “solar annulus” of the simulation, has low metallicity, a velocity ellipsoid defined by (�U,V,W = 62,46,45 kms 1 ) and is formed primarily in-situ during an early merger epoch. Gas-rich mergers during this epoch play a major role in fuelling the formation of the old disc stars (the thick disc). We demonstrate that this is consistent with studies which show that cold accretion is the main source of a disc galaxy’s baryons. Our simulation initially forms a relatively short (scalelength �1.7kpc at z=1) and kinematically hot disc, primarily from gas accreted during the galaxy’s merger epoch. Far from being a competing formation scenario, we show that migration is crucial for reconciling the short, hot, discs which form at high redshift in �CDM, with the properties of the thick disc at z=0. The thick disc, as defined by its abundances maintains its relatively short scale-length at z = 0 (2.31kpc) compared with the total disc scale-length of 2.73kpc. The inside-out nature of disc growth is imprinted the evolution of abundances such that the metal poor �-young population has a larger scale-length (4.07kpc) than the more chemically evolved metal rich �-young population (2.74kpc).

Dynamics of stars around spiral arms in an N-body/SPH simulated barred-spiral galaxy

We run N-body smoothed particle hydrodynamics (SPH) simulations of a Milky Way sized galaxy. The code takes into account hydrodynamics, self-gravity, star formation, supernova and stellar wind feedback, radiative cooling and metal enrichment. The simulated galaxy is a barred-spiral galaxy consisting of a stellar and gas disc, enveloped in a static dark matter halo. Similar to what is found in our pure N-body simulation of a non-barred galaxy in Grand et. al. (2012), we find that the spiral arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is similar to the rotation of star particles. Compared to the non-barred case, we find that the spiral arm pattern speed is slightly faster than the rotation speed of star particles: the bar appears to boost the pattern speed ahead of the rotational velocity. We trace particle motion around the spiral arms at different radii, and demonstrate that there are star particles that are drawn towards and join the arm from behind (in front of) the arm and migrate toward the outer (inner) regions of the disc until the arm disappears as a result of their transient nature. We see this migration over the entire radial range analysed, which is a consequence of the spiral arm rotating at similar speeds to star particles at all radii, which is inconsistent with the prediction of classical density wave theory. The bar does not prevent this systematic radial migration, which is shown to largely preserve circular orbits. We also demonstrate that there is no significant offset of different star forming tracers across the spiral arm, which is also inconsistent with the prediction of classical density wave theory.

Disc heating: comparing the Milky Way with cosmological simulations

We present an analysis of a suite of simulations run with different particle- and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle- and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion–age relation for disc stars at z= 0 and find that four of the simulations are more consistent with observations by Holmberg, Nordstroem & Andersen in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett observations that suggest ‘saturation’ in the heating profile for young stars in the disc. None of the simulations has thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies’ evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Way’s stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger, that is, once the disc has formed. All of the simulations are significantly ‘hotter’ than the Milky Way disc; on top of the effects of mergers, we find a ‘floor’ in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical codes.

Structure, kinematics and chemical enrichment patterns after major gas-rich disc-disc mergers

We used an N-body smoothed particle hydrodynamics algorithm, with a detailed treatment of star formation, supernovae feedback and chemical enrichment, to perform eight simulations of mergers between gas-rich disc galaxies. We vary the mass ratio of the progenitors, their rotation axes and their orbital parameters and analyse the kinematic, structural and chemical properties of the remnants. Six of these simulations result in the formation of a merger remnant with a disc morphology as a result of the large gas fraction of the remnants. We show that stars formed during the merger (a sudden starburst occurs in our simulation and lasts for 0.2-0.3 Gyr) and those formed after the merger have different kinematical and chemical properties. The first ones are located in the thick disc or the halo. They are partially supported by velocity dispersion and have high [alpha/Fe] ratios even at metallicities as high as [Fe/H] = -0.5. The former ones - the young component - are located in a thin disc rotationally supported and have lower [alpha/Fe] ratios. The difference in the rotational support of both components results in the rotation of the thick disc lagging that of the thin disc by as much as a factor of 2, as recently observed. We also find counter-rotating stars in both the old and young populations. A variety of structures are formed during the merger, i.e. most simulations form a ring of young stars and two simulations formed a bar. The scalelength of the thick disc is either equal to that of the thin disc or larger by factors of up to 1.60 and in six out of the eight simulations, the thin and thick discs both have exponential luminosity profiles and are nearly coplanar. We find that, while the kinematic and structural properties of the merger remnant depend strongly upon the orbital parameters of the mergers, there is a remarkable uniformity in the chemical properties of the mergers. This suggests that general conclusions about the chemical signature of gas-rich mergers can be drawn.

Disk Evolution since z ~ 1 in a CDM Universe

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 simulated present-day 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.

Impact of radial migration on stellar and gas radial metallicity distribution

Radial migration is defined as the change in guiding centre radius of stars and gas caused by gains or losses of angular momentum that result from gravitational interaction with non-axisymmetric structure. This has been shown to have significant impact on the metallicity distribution in galactic discs, and therefore affects the interpretation of Galactic archeology. We use a simulation of a Milky Way-sized galaxy to examine the effect of radial migration on the star and gas radial metallicity distribution. We find that both the star and gas component show significant radial migration. The stellar radial metallicity gradient remains almost unchanged but the radial metallicity distribution of the stars is broadened to produce a greater dispersion at all radii. However, the metallicity dispersion of the gas remains narrow. We find that the main drivers of the gas metallicity distribution evolution are metal enrichment and mixing: more efficient metal enrichment in the inner region maintains a negative slope in the radial metallicity distribution, and the metal mixing ensures the tight relationship of the gas metallicity with the radius. The metallicity distribution function reproduces the trend in the age-metallicity relation found from observations for stars younger than 1.0 Gyr in the Milky Way.

Spiral arm pitch angle and galactic shear rate in N-body simulations of disc galaxies

Spiral galaxies are observed to exhibit a range of morphologies, in particular in the shape of spiral arms. A key diagnostic parameter is the pitch angle, which describes how tightly wound the spiral arms are. Observationally and analytically, a correlation between pitch angle and galactic shear rate has been detected. For the first time, we examine whether this effect is detected in N-body simulations by calculating and comparing pitch angles of both individual density waves and overall spiral structure in a suite of N-body simulations. We find that higher galactic shear rates produce more tightly wound spiral arms, both in individual mode patterns (density waves) and in the overall density enhancement. Although the mode pattern pitch angles by construction remain constant with time, the overall logarithmic spiral arm winds over time, which could help to explain the scatter in the relation between pitch angle versus shear seen from observations. The correlation between spiral arm pitch angle and galactic shear rate that we find in N-body simulations may also explain why late Hubble type of spiral galaxies tend to have more open arms.