اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدBeta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter for Stellar Halos
85
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The velocity anisotropy parameter, beta, is a measure of the kinematic state of orbits in the stellar halo which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for beta as a function of radius from three suites of simulations, including accretion only and cosmological hydrodynamic simulations. We find that both types of simulations are consistent and predict strong radial anisotropy (<beta>~0.7) for Galactocentric radii greater than 10 kpc. Previous observations of beta for the MWs stellar halo claim a detection of an isotropic or tangential dip at r~20 kpc. Using the N-body+SPH simulations, we investigate the temporal persistence, population origin, and severity of dips in beta. We find dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z~1 can result in a present day low (isotropic to tangential) value of beta over a wide range of radii and angular expanse. While all of these mechanisms are plausible drivers for the beta dip observed in the MW, in the simulations, each mechanism has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of beta(r) is required for measuring the mass of the MW halo, we note significant transient dips in beta could cause an overestimate of the halos mass when using spherical Jeans equation modeling.
قيم البحث
اقرأ أيضاً
The hierarchical theory of galaxy formation rests on the idea that smaller galactic structures merge to form the galaxies that we see today. The past decade has provided remarkable observational support for this scenario, driven in part by advances i
n spectroscopic instrumentation. Multi-object spectroscopy enabled the discovery of kinematically cold substructures around the Milky Way and M31 that are likely the debris of disrupting satellites. Improvements in high-resolution spectroscopy have produced key evidence that the abundance patterns of the Milky Way halo and its dwarf satellites can be explained by Galactic chemical evolution models based on hierarchical assembly. These breakthroughs have depended almost entirely on observations of nearby stars in the Milky Way and luminous red giant stars in M31 and Local Group dwarf satellites. In the next decade, extremely large telescopes will allow observations far down the luminosity function in the known dwarf galaxies, and they will enable observations of individual stars far out in the Galactic halo. The chemical abundance census now available for the Milky Way will become possible for our nearest neighbor, M31. Velocity dispersion measurements now available in M31 will become possible for systems beyond the Local Group such as Sculptor and M81 Group galaxies. Detailed studies of a greater number of individual stars in a greater number of spiral galaxies and their satellites will test hierarchical assembly in new ways because dynamical and chemical evolution models predict different outcomes for halos of different masses in different environments.
Stellar clusters are important for astrophysics in many ways, for instance as optimal tracers of the Galactic populations to which they belong or as one of the best test bench for stellar evolutionary models. Gaia DR1, with TGAS, is just skimming the
wealth of exquisite information we are expecting from the more advanced catalogues, but already offers good opportunities and indicates the vast potentialities. Gaia results can be efficiently complemented by ground-based data, in particular by large spectroscopic and photometric surveys. Examples of some scientific results of the Gaia-ESO survey are presented, as a teaser for what will be possible once advanced Gaia releases and ground-based data will be combined.
The expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar population of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images
and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically: case A) at the center of the disk of a giant spiral in the Centaurus Group, (mu B~21, distance of 4.6 Mpc); and, case B) at half of the effective radius of a giant elliptical in the Virgo Cluster (mu~19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental set up, i.e. a 42 m Telescope operating close to the diffraction limit. The effect of crowding is discussed in detail showing how stars are measured preferentially brighter than they are as the confusion limit is approached. We find that (i) accurate photometry (sigma~0.1, completeness >90%) can be obtained for case B) down to I~28.5, J~27.5 allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ~0.1 dex; (ii) the same photometric accuracy holds for the science case A) down to J~28.0, K~27.0, enabling to reconstruct of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For this latter case we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their Horizontal Branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological type up to the distance of the Virgo cluster.
The ellipsoid of stellar random motions is a fundamental ingredient of galaxy dynamics. Yet it has long been difficult to constrain this component in disks others than the Milky Way. This article presents the modeling of the azimuthal-to-radial axis
ratio of the velocity ellipsoid of galactic disks from stellar dispersion maps using integral field spectroscopy data of the CALIFA survey. The measured azimuthal anisotropy is shown to be not strongly dependent on the assumed vertical-to-radial dispersion ratio of the ellipsoid. The anisotropy distribution shows a large diversity in the orbital structure of disk galaxies from tangential to radial stellar orbits. Globally, the orbits are isotropic in inner disk regions and become more radial as a function of radius, although this picture tends to depend on galaxy morphology and luminosity. The Milky Way orbital anisotropy profile measured from the Second Gaia Data Release is consistent with those of CALIFA galaxies. A new correlation is evidenced, linking the absolute magnitude or stellar mass of the disks to the azimuthal anisotropy. More luminous disks have more radial orbits and less luminous disks have isotropic and somewhat tangential orbits. This correlation is consistent with the picture in galaxy evolution in which orbits become more radial as the mass grows and is redistributed as a function of time. With the help of circular velocity curves, it is also shown that the epicycle theory fails to reproduce the diversity of the azimuthal anisotropy of stellar random motions, as it predicts only nearly radial orbits in the presence of flat curves. The origin of this conflict is yet to be identified. It also questions the validity of the vertical-to-radial axis ratio of the velocity ellipsoid derived by many studies in the framework of the epicyclic approximation.
Orbital properties of stars, computed from their six-dimensional phase space measurements and an assumed Galactic potential, are used to understand the structure and evolution of the Galaxy. Stellar actions, computed from orbits, have the attractive
quality of being invariant under certain assumptions and are therefore used as quantitative labels of a stars orbit. We report a subtle but important systematic error that is induced in the actions as a consequence of local midplane variations expected for the Milky Way. This error is difficult to model because it is non-Gaussian and bimodal, with neither mode peaking on the null value. An offset in the vertical position of the Galactic midplane of $sim15,text{pc}$ for a thin disk-like orbit or $sim 120,text{pc}$ for a thick disk-like orbit induces a $25%$ systematic error in the vertical action $J_z$. In FIRE simulations of Milky Way-mass galaxies, these variations are on the order of $sim100,text{pc}$ at the solar circle. From observations of the mean vertical velocity variation of $sim5text{--}10,text{km},text{s}^{-1}$ with radius, we estimate that the Milky Way midplane variations are $sim60text{--}170,text{pc}$, consistent with three-dimensional dust maps. Action calculations and orbit integrations, which assume the global and local midplanes are identical, are likely to include this induced error, depending on the volume considered. Variation in the local standard of rest or distance to the Galactic center causes similar issues. The variation of the midplane must be taken into account when performing dynamical analysis across the large regions of the disk accessible to Gaia and future missions.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
