Keywords

This paper presents an alternative scenario to explain the observed properties of the Milky Way dwarf Spheroidals (MW dSphs). We show that instead of resulting from large amounts of dark matter (DM), the large velocity dispersions observed along their lines of sight (σ_los) can be entirely accounted for by dynamical heating of DM-free systems resulting from MW tidal shocks. Such a regime is expected if the progenitors of the MW dwarfs are infalling gas-dominated galaxies. In this case, gas lost through ram-pressure leads to a strong decrease of self-gravity, a phase during which stars can radially expand, while leaving a gas-free dSph in which tidal shocks can easily develop. The DM content of dSphs is widely derived from the measurement of the dSphs self-gravity acceleration projected along the line of sight. We show that the latter strongly anti-correlates with the dSph distance from the MW, and that it is matched in amplitude by the acceleration caused by MW tidal shocks on DM-free dSphs. If correct, this implies that the MW dSphs would have negligible DM content, putting in question, e.g., their use as targets for DM direct searches, or our understanding of the Local Group mass assembly history. Most of the progenitors of the MW dSphs are likely extremely tiny dIrrs, and deeper observations and more accurate modeling are necessary to infer their properties as well as to derive star formation histories of the faintest dSphs.

Much data on the Galactic polarized radio emission has been gathered in the last five decades. All-sky surveys have been made, but only in narrow, widely spaced frequency bands—and the data are inadequate for the characterization of Faraday rotation, the main determinant of the appearance of the polarized radio sky at decimeter wavelengths. We describe a survey of polarized radio emission from the southern sky, aiming to characterize the magneto-ionic medium, particularly the strength and configuration of the magnetic field. This work is part of the Global Magneto-Ionic Medium Survey (GMIMS). We have designed and built a feed and receiver covering the band 300–900 MHz for the CSIRO Parkes 64 m Telescope. We have surveyed the entire sky between decl. −90° and +20°. We present data covering 300–480 MHz with angular resolution 81'–45'. The survey intensity scale is absolutely calibrated, based on measurements of resistors at known temperatures and on an assumed flux density and spectral index for Taurus A. Data are presented as brightness temperatures. We have applied Rotation Measure Synthesis to the data to obtain a Faraday depth cube of resolution 5.9 rad m⁻², sensitivity of 60 mK of polarized intensity, and angular resolution 135. The data presented in this paper are available at the Canadian Astronomy Data Centre.

Observations reveal a universal stellar mass–stellar metallicity relation (MZR) existing in Local Group dwarfs of different types, ${Z}_{* }\propto {M}_{* }^{\alpha }$ with α = 0.30 ± 0.02. In this work, we investigate the "universality" of the MZRs for both satellites and central galaxies in a large number of different host dark matter halos covering a large mass range of 10⁹–10¹⁵ h⁻¹$\,{M}_{\odot }$, by using a semianalytical galaxy formation and evolution model. We obtain the following results: (1) The exponents (α) for the MZRs of the satellites in halos with the same mass as the Milky Way halo, but different individual assembly histories, are mostly ∼0.2–0.4, i.e., having a scatter of ∼0.2; and the scatter of α increases with decreasing halo masses. (2) The MZR relations are changed little by the variation of halo masses and the classification between central galaxies and satellites, if many halos with the same mass are stacked together. (3) A double power law exists in the MZR relations for both central galaxies and stacked satellites, with α ∼ 0.2–0.4 at 10³$\,{M}_{\odot }$ ≲ M_* ≲ 10⁸$\,{M}_{\odot }$ and a relatively higher α ∼ 0.5 at 10⁸$\,{M}_{\odot }$ ≲ M_* ≲ 10¹¹$\,{M}_{\odot }$. (4) The high-mass satellites (M_* ≳ 10⁸$\,{M}_{\odot }$) existing mostly in high-mass halos can lead to an apparent increase of α (from ∼0.2 to ∼0.4) with increasing host halo masses shown in the single-power-law fitting results of stacked satellites. The universality of the MZR suggests the common physical processes in stellar formation and chemical evolution of galaxies can be unified over a large range of galaxy masses and halo masses.

We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg² field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320–1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg² region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r ∼ 27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.

Although core helium-burning red clump (RC) stars are faint at ultraviolet wavelengths, their ultraviolet (UV)–optical color is a unique and accessible probe of their physical properties. Using data from the Galaxy Evolution Explorer All Sky Imaging Survey, Gaia Data Release 2, and the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR14 survey, we find that spectroscopic metallicity is strongly correlated with the location of an RC star in the UV–optical color–magnitude diagram. The RC has a wide spread in (NUV–G)₀ color of over 4 mag compared to a 0.7 mag range in (G_BP–G_RP)₀. We propose a photometric, dust-corrected, UV–optical (NUV–G)₀ color–metallicity [Fe/H] relation using a sample of 5,175 RC stars from APOGEE. We show that this relation has a scatter of 0.16 dex and is easier to obtain for large, wide-field samples than for spectroscopic metallicities. Importantly, the effect may be comparable to the spread in RC color attributed to extinction in other studies.

In this paper, we investigate how the chemical and kinematic properties of stars vary as a function of age. Using data from a variety of photometric, astrometric, and spectroscopic surveys, we calculate the ages, phase space information, and orbits for ∼125,000 stars covering a wide range of stellar parameters. We find indications that the inner regions of the disk reached high levels of enrichment early, while the outer regions were more substantially enriched in intermediate and recent epochs. We consider these enrichment histories through comparison of the ages of stars, their metallicities, and kinematic properties, such as their angular momentum in the solar neighborhood (which is a proxy for orbital radius). We calculate rates at which the velocity dispersions evolve, investigate the Oort constants for populations of different ages (finding a slightly negative ∂V_C/∂R and ∂V_R/∂R for all ages, which is most negative for the oldest stars), as well as examine the behavior of the deviation angle of the velocity vertex as a function of age (which we find to fall from ∼15° for the 2 Gyr old population to ∼6° at around 6.5 Gyr of age, after which it remains unchanged). We find evidence for stellar churning, and find that the churned stars have a slightly younger age distribution than the rest of the data.

Most of the known RR Lyraes are type ab RR Lyraes (RRLab), and they are an excellent tool to map the Milky Way and its substructures. We find that 1148 RRLab stars determined by Drake et al. have been observed by spectroscopic surveys of SDSS and LAMOST. We derived the radial velocity dispersion, circular velocity, and mass profile from 860 halo tracers in Paper I. Here we present the stellar densities and radial velocity distributions of the thick disk and halo of the Milky Way. The 288 RRLab stars located in the thick disk have the mean metallicity of [Fe/H] = −1.02. Three thick disk tracers have a radial velocity lower than 215 km s⁻¹. With 860 halo tracers, which have a mean metallicity of [Fe/H] = −1.33, we find a double power law of n(r) ∝ r^−2.8 and n(r) ∝ r^−4.8 with a break distance of 21 kpc to express the halo stellar density profile. The radial velocity dispersion at 50 kpc is around 78 km s⁻¹.

Relativistic electrons accelerated by both the first-order and the second-order Fermi accelerations in some synchrotron sources have a hybrid shape of thermal and nonthermal energy distribution. This particle acceleration result is supported by some recent numerical simulations. We calculate the synchrotron polarization by applying this electron energy distribution. The polarization degrees in the cases of active galactic nucleus jets and gamma-ray bursts are given as examples. The possible application for the polarization study of Sgr A* is also mentioned. We finally suggest high-energy polarization measurements for these synchrotron sources to test our results.