Formation of galaxies in Λcold dark matter cosmologies - I. The fine structure of disc galaxies (original) (raw)
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Formation of galaxies in {\Lambda}CDM cosmologies. I. The fine structure of disc galaxies
In this work we present a detailed analysis of the global and fine structure of four middle-mass disc galaxies obtained from hydrodynamic simulations in a ΛCDM scenario. These objects have photometric disc-to-total ratios in good agreement with those observed for late-type spirals, as well as kinematic properties in agreement with the observational I-band Tully-Fisher relation. We identify the different dynamical components at zero redshift on the basis of both orbital parameters and the binding energy of stars in the galaxy. In this way, we recognize a slowly rotating centrally concentrated spheroid, and two disc components supported by rotation: a thin disc with stars in nearly circular orbits, and a thick disc with orbital parameters transitional between the thin disc and the spheroid. The spheroidal component is composed mainly by old, metal-poor and α-enhanced stars. The distribution of metals in this component shows, however, a clear bimodality with a low-metallicity peak, which could be related to a classical bulge formed from rapid collapse at early times, and a high-metallicity peak, which could be related to a pseudo-bulge formed from instabilities of the inner disc. The thin disc appears in our simulations as the youngest and most metal-rich component, with median stellar ages ranging from 3.8 to 6.7 Gyr. The radial distribution of ages and colours in this component are U-shaped: the new stars are forming in the inner regions, where the galaxy is bluer, and then migrate through secular processes reaching the outer parts. Finally, we also find, in all simulated galaxies, a thick disc containing about 16 per cent of the total stellar mass and with properties that are intermediate between those of the thin disc and the spheroid. Its low-metallicity stars are α-enhanced when compared to thin disc stars of the same metallicity. The structural parameters (e.g., the scale height) of the simulated thick discs suggest that such a component could result from the combination of different thickening mechanisms that include merger-driven processes, but also long-lived internal perturbations of the thin disc.
The Astrophysical Journal, 2003
We present a detailed analysis of the dynamical properties of a simulated disk galaxy assembled hierarchically in the ΛCDM cosmogony. At z = 0, two distinct dynamical components are easily identified solely on the basis of the orbital parameters of stars in the galaxy: a slowly rotating, centrally concentrated spheroid and a disk-like component largely supported by rotation. These components are also clearly recognized in the surface brightness profile of the galaxy, which can be very well approximated by the superposition of an R 1/4 spheroid and an exponential disk. However, neither does the dynamicallyidentified spheroid follow de Vaucouleurs' law nor is the disk purely exponential, a result which calls for caution when estimating the importance of the disk from traditional photometric decomposition techniques. The disk may be further decomposed into a thin, dynamically cold component with stars on nearly circular orbits and a hotter, thicker component with orbital parameters transitional between the thin disk and the spheroid. Supporting evidence for the presence of distinct thick and thin disk components is found, as in the Milky Way, in the double-exponential vertical structure of the disk and in abrupt changes in the vertical velocity distribution as a function of age. The dynamical origin of these components offers intriguing clues to the assembly of spheroids and disks in the Milky Way and other spirals. The spheroid is old, and has essentially no stars younger than the time elapsed since the last major accretion event; ∼ 8 Gyr ago for the system we consider here. The majority of thin disk stars, on the other hand, form after the merging activity is over, although a significant fraction (∼ 15%) of thin-disk stars are old enough to predate the last major merger event. This unexpected population of old disk stars consists mainly of the tidal debris of satellites whose orbital plane was coincident with the disk and whose orbits were circularized by dynamical friction prior to full disruption. More than half of the stars in the thick disk share this origin, part of a trend that becomes more pronounced with age: nine out of ten stars presently in the old (τ ∼ > 10 Gyr) disk component were actually brought into the disk by satellites. By contrast, only one in two stars belonging to the old spheroid are tidal debris; the rest may be traced to a major merger event that dispersed the luminous progenitor at z ∼ 1.5 and seeded the formation of the spheroid. Our results highlight the role of satellite accretion events in shaping the disk-as well as the spheroidal-component and reveal some of the clues to the assembly process of a galaxy preserved in the detailed dynamics of old stellar populations.
On the Formation of Galactic Thick Disks
The Astrophysical Journal, 2015
Recent spectroscopic observations in the Milky Way suggest that the chemically defined thick disk (stars with high [α/Fe] ratios and thus old) has a smaller scale-length than the thin disk. This is in apparent contradiction with observations of external edge-on galaxies, where the thickened components extend at least as much as the thin ones. Moreover, while observed disks do not flare (scale-height does not increase with radius), numerical simulations suggest that disk flaring is unavoidable, resulting from both environmental effects and secular evolution. Here we address these problems by studying two different suites of simulated galactic disks formed in the cosmological context. We show that the scale-heights of coeval populations always increase with radius. However, the total population can be decomposed morphologically into thin and thick disks, which do not flare. This is related to the disk inside-out formation, where younger populations have increasingly larger scale-lengths and flare at progressively larger radii. In this new picture, thick disks are composed of the imbedded flares of monoage stellar populations. Assuming that disks form inside out, we predict that morphologically defined thick disks must show a decrease in age (or [α/Fe] ratios) with radius and that coeval populations should always flare. This also explains the observed inversion in the metallicity and [α/Fe] gradients for stars away from the disk midplane in the Milky Way. The results of this work are directly linked to, and can be seen as evidence of, inside-out disk growth.
We present results concerning the internal structure and kinematics of disk galaxies formed in cosmologically motivated simulations. The calculations include dark matter, gas dynamics, radiative cooling, star formation, supernova feedback and metal enrichment. The initial model is a rigidly rotating overdense sphere with a mass of about 8 10 11 M which is perturbed by small scale uctuations according to a biased CDM power spectrum. Converging, Jeans unstable and rapidly cooling regions are allowed to form stars. Via supernovae, metal enriched gas is returned to the interstellar medium. From these initial conditions a galaxy forms which shows the main properties of spiral galaxies: a rotationally supported exponential disk which consists of young stars with about solar metallicity, a slowly rotating halo of old metal poor stars, a bulge of old metal rich stars and a slowly rotating extended halo of dark matter. Bulge, stellar and dark halo are supported by an anisotropic velocity dispersion and have a de Vaucouleurs surface density pro le. The attening of the dark and stellar halo is too large to be explained by rotation only. Whether the attening of the bulge is caused by an anisotropic velocity dispersion or by its rotation cannot be answered, because of the limited numerical resolution due to gravitational softening. The velocity dispersion and the thickness of the stellar disk increase with the age of the stars. Considering only the young stellar component, the disk is cold ( = 20 km/sec) and thin (z < 1 kpc). The dynamical formation process ends after about 4 Gyr, when the disk reaches a quasi{stationary state. During the subsequent 8 Gyr mainly gas is transformed into stars decreasing the gas fraction from 40% to 10%. The star formation rate successively decreases during the quasi{stationary state from about 5 M /yr at z = 1 to less than 0.5 M /yr at the end of the simulation (z = 0).
OBSERVATIONAL PROPERTIES OF THE METAL-POOR THICK DISK OF THE MILKY WAY AND INSIGHTS INTO ITS ORIGINS
The Astrophysical Journal, 2011
We have undertaken the study of the elemental abundances and kinematic properties of a metalpoor sample of candidate thick-disk stars selected from the RAVE spectroscopic survey of bright stars to differentiate among the present scenarios of the formation of the thick disk. In this paper, we report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 main-sequence/sub-giant branch metal-poor stars, which serves to augment our previous sample of only giant stars. We find that the thick disk [α/Fe] ratios are enhanced, and have little variation (< 0.1 dex), in agreement with our previous study. The augmented sample further allows, for the first time, investigation of the gradients in the metal-poor thick disk. For stars with [Fe/H] < −1.2, the thick disk shows very small gradients, < 0.03 ± 0.02 dex kpc −1 , in α-enhancement, while we find a +0.01 ± 0.04 dex kpc −1 radial gradient and a −0.09 ± 0.05 dex kpc −1 vertical gradient in iron abundance. In addition, we show that the peak of the distribution of orbital eccentricities for our sample agrees better with models in which the stars that comprise the thick disk were formed primarily in the Galaxy, with direct accretion of stars contributing little. Our results thus disfavor direct accretion of stars from dwarf galaxies into the thick disk as a major contributor to the thick disk population, but cannot discriminate between alternative models for the thick disk, such as those that invoke high-redshift (gas-rich) mergers, heating of a pre-existing thin stellar disk by a minor merger, or efficient radial migration of stars.
On the Significance of the Thick Disks of Disk Galaxies
arXiv (Cornell University), 2023
Thick disks are a prevalent feature observed in numerous disk galaxies including our own Milky Way. Their significance has been reported to vary widely, ranging from a few to 100% of the disk mass, depending on the galaxy and the measurement method. We use the NewHorizon simulation which has high spatial and stellar mass resolutions to investigate the issue of thick disk mass fraction. We also use the NewHorizon2 simulation that was run on the same initial conditions but additionally traced nine chemical elements. Based on a sample of 27 massive disk galaxies with M * > 10 10 M ⊙ in NewHorizon, the contribution of the thick disk was found to be 34 ± 15% in r-band luminosity or 48 ± 13% in mass to the overall galactic disk, which seems in agreement with observational data. The vertical profiles of 0, 22, and 5 galaxies are best fitted by 1, 2, or 3 sech 2 components, respectively. The NewHorizon2 data show that the selection of thick disk stars based on a single [α/Fe] cut is severely contaminated by stars of different kinematic properties while missing a bulk of kinematically thick disk stars. Vertical luminosity profile fits recover the key properties of thick disks reasonably well. The majority of stars are born near the galactic mid-plane with high circularity and get heated with time via fluctuation in the force field. Depending on the star formation and merger histories, galaxies may naturally develop thick disks with significantly different properties.
Formation of galaxies in ΛCDM cosmologies. I. The fine structure of disc galaxies
2012
We present a detailed analysis of the global and fine structure of four middle-mass disc galaxies obtained from simulations in a ΛCDM scenario. These objects have photometric D/T ratios in good agreement with those observed for late-type spirals, as well as kinematic properties in agreement with the observational Tully-Fisher relation. We identify the different dynamical components at z=0 on the basis of both orbital parameters and the binding energy of stars in the galaxy. In this way, we recognize a slowly rotating centrally concentrated spheroid, and two disc components supported by rotation: a thin disc with stars in nearly circular orbits, and a thick disc with orbital parameters transitional between the thin disc and the spheroid. The spheroidal component is composed mainly by old, metal-poor and α-enhanced stars. The distribution of metals in this component shows, however, a clear bimodality with a low-metallicity peak, which could be related to a classical bulge, and a high-...
Thickening of galactic discs through clustered star formation
Monthly Notices of the Royal Astronomical Society, 2002
The building blocks of galaxies are star clusters. These form with low-star formation efficiencies and, consequently, loose a large part of their stars that expand outwards once the residual gas is expelled by the action of the massive stars. Massive star clusters may thus add kinematically hot components to galactic field populations. This kinematical imprint on the stellar distribution function is estimated here by calculating the velocity distribution function for ensembles of star-clusters distributed as power-law or log-normal initial cluster mass functions (ICMFs). The resulting stellar velocity distribution function is non-Gaussian and may be interpreted as being composed of multiple kinematical sub-populations.
The Emergence of the Thick Disk in a CDM Universe. II. Colors and Abundance Patterns
The Astrophysical Journal, 2005
The recently emerging conviction that thick disks are prevalent in disk galaxies, and their seemingly ubiquitous old ages, means that the formation of the thick disk, perhaps more than any other component, holds the key to unravelling the evolution of the Milky Way, and indeed all disk galaxies. In Paper I, we proposed that the thick disk was formed in an epoch of gas rich mergers, at high redshift. This hypothesis was based on comparing N-body/SPH simulations to a variety of Galactic and extragalactic observations, including stellar kinematics, ages and chemical properties. Here examine our thick disk formation scenario in light of the most recent observations of extragalactic thick disks. In agreement, our simulted thick disks are old and relatively metal rich, with V-I colors that do not vary significantly with distance from the plane. Further, we show that our proposal results in an enhancement of α-elements in thick disk stars as compared with thin disk stars, consistent with observations of the relevant populations of the Milky Way. We also find that our scenario naturally leads to the formation of an old metal weak stellar halo population with high α-element abundances.
Monthly Notices of the Royal Astronomical Society
Using a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age versus metallicity or abundance ratios versus age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] versus metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary versus secondary or odd elements) and on the lifetimes of their sources (short-lived versus long-lived ones). The latter property is critical in determining the sing...