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Register a new userTowards a consistent model of the Galaxy: I. kinematic properties, star counts and microlensing observations
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D. Mera
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We examine the most recent observational constraints arising from i) small-scale and large-scale Galactic dynamical properties, ii) star counts at faint magnitude and iii) microlensing experiments. From these constraints, we determine the halo and disk stellar mass functions and stellar content down to the bottom of the main sequence, which yields the normalization of the halo/disk total stellar population, and we infer the contributions of sub-stellar objects to the mass budget of the various Galactic regions. The consistent analysis of star counts and of the overall microlensing observations in the Bulge are compatible with a small contribution of brown dwarfs to the Galactic mass budget $rho_{BD}/rho_* leq 0.2 $. However the separate bulge/disk analysis based on the bulge clump giants is compatible with a substantial population of disk brown dwarfs, $Sigma_{BD}/Sigma_*leq 1 $. More statistics of microlensing events towards the Galactic center and a better determination of the velocity dispersions in the bulge should break this degeneracy of solutions. For the halo, we show that a steep mass-function in the dark halo is excluded and that low-mass stars and brown dwarfs represent a negligible fraction of the halo dark matter, and thus of the observed events towards the LMC. The nature of these events remains a puzzle and halo white dwarfs remain the least unlikely candidates.
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We use the calculations derived in a previous paper (Mera, Chabrier and Schaeffer, 1997), based on observational constraints arising from star counts, microlensing experiments and kinematic properties, to determine the amount of dark matter under the form of stellar and sub-stellar objects in the different parts of the Galaxy. This yields the derivation of different mass-models for the Galaxy. In the light of all the afore-mentioned constraints, we discuss two models that correspond to different conclusions about the nature and the location of the Galactic dark matter. In the first model there is a small amount of dark matter in the disk, and a large fraction of the dark matter in the halo is still undetected and likely to be non-baryonic. The second, less conventional model is consistent with entirely, or at least predominantly baryonic dark matter, under the form of brown dwarfs in the disk and white dwarfs in the dark halo. We derive observational predictions for these two models which should be verifiable by near future infrared and microlensing observations.
Aims. We aim to perform consistent comparisons between observations and simulations on the mass dependence of the galaxy major merger fraction at low redshift over an unprecedentedly wide range of stellar masses (10^9 to 10^12 solar masses). Methods. We first carry out forward modelling of ideal synthetic images of major mergers and non-mergers selected from the Next Generation Illustris Simulations (IllustrisTNG) to include major observational effects. We then train deep convolutional neural networks (CNNs) using realistic mock observations of galaxy samples from the simulations. Subsequently, we apply the trained CNNs to real the Kilo-Degree Survey (KiDS) images of galaxies selected from the Galaxy And Mass Assembly (GAMA) survey. Based on the major merger samples, which are detected in a consistent manner in the observations and simulations, we determine the dependence of major merger fraction on stellar mass at z around 0.15 and make comparisons between the two. Results. The detected major merger fraction in the GAMA/KiDS observations has a fairly mild decreasing trend with increasing stellar mass over the mass range 10^9 < M_sun < M_star < 10^11.5 M_sun. There is good agreement in the mass dependence of the major merger fraction in the GAMA/KiDS observations and the IllustrisTNG simulations over 10^9.5 M_sun < M_star < 10^10.5 M_sun. However, the observations and the simulations show some differences at M_star > 10^10.5M_sun, possibly due to the supermassive blackhole feedback in its low-accretion state in the simulations which causes a sharp transition in the quenched fractions at this mass scale. The discrepancy could also be due to the relatively small volume of the simulations and/or differences in how stellar masses are measured in simulations and observations.
Measuring the proper motions and geometric distances of galaxies within the Local Group is very important for our understanding of the history, present state and future of the Local Group. Currently, proper motion measurements using optical methods are limited only to the closest companions of the Milky Way. However, Very Long Baseline Interferometry (VLBI) provides the best angular resolution in astronomy and phase-referencing techniques yield astrometric accuracies of ~ 10 micro-arcseconds. This makes a measurement of proper motions and angular rotation rates of galaxies out to a distance of ~ 1 Mpc feasible. This article presents results of VLBI observations of regions of H2O maser activity in the Local Group galaxies M33 and IC10. These measurements promise a new handle on dynamical models for the Local Group and the mass and dark matter halo of Andromeda and the Milky Way. (Abridged)
We used FORS2 in MXU mode to mimic a coarse IFU in order to measure the 3D large-scale kinematics around the central Hydra I cluster galaxy NGC 3311. Our data show that the velocity dispersion field varies as a function of radius and azimuthal angle and violates point symmetry. Also, the velocity field shows similar dependence, hence the stellar halo of NGC 3311 is a dynamically young structure. The kinematic irregularities coincide in position with a displaced diffuse halo North-East of NGC 3311 and with tidal features of a group of disrupting dwarf galaxies. This suggests that the superposition of different velocity components is responsible for the kinematic substructure in the Hydra I cluster core.
Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from white dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L-star) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated green valley region of the galaxy colour-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.
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