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Register a new userA Low Latitude Halo Stream around the Milky Way
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We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180 deg to l = 227 deg with turnoff magnitude $g sim 19.5$; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R = 18 +/- 2 kpc from the Galactic Center above the Galactic plane, and has R = 20 +/- 2 kpc in the region sampled below the Galactic plane. The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 degrees of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b) = (198, -27), 22 km/s at (l,b) = (225, 28), 30 km/s at (l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure rotates in the same prograde direction as the Galactic disk stars, but with a circular velocity of 110+/-25 km/s. The narrow measured velocity dispersion is inconsistent with power law spheroid or thick disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l, b) = (165, -55) deg. We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H] = -1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of $2 times 10^7$ to $5 times 10^8 rm M_odot$ which rings the Galaxy.
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Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 dataset using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. (2020). We detect LMS-1 as a $60deg$ long stream to the north of the Galactic bulge, at a distance of $sim 20$ kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from LAMOST, SDSS and APOGEE, we infer that the stream is very metal poor (${rm langle [Fe/H]rangle =-2.1}$) with a significant metallicity dispersion ($sigma_{rm [Fe/H]}=0.4$), and it possesses a large radial velocity dispersion (${rm sigma_v=20 pm 4,km,s^{-1}}$). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of $75deg$ to the Galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024 and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.
Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km/s. This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker-Planck equation, we find that present-day clusters are ~< 1 pc in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than one hundred km/s and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo, would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33.
In 1998 several papers claim the detection of an ubiquitous gaseous phase within the Galactic halo. Here we like to focus on the detections of X-ray emitting gas within the Galactic halo as well as the discovery of a pervasive neutral Galactic halo gas. We discuss critically the major differences between the recent publications as well as the limitations of the analyses.
The goal of this survey is to study the formation and evolution of the Milky Way halo to deduce its assembly history and the 3D distribution of mass in the Milky Way. The combination of multi-band photometry, Gaia proper motion and parallax data, and radial velocities and the metallicity and elemental abundances obtained from low-resolution spectra of halo giants with 4MOST, will yield an unprecedented characterisation of the Milky Way halo and its interface with the thick disc. The survey will produce a volume- and magnitude-limited complete sample of giant stars in the halo. It will cover at least 10,000 square degrees of high Galactic latitude, and measure line-of-sight velocities with a precision of 1-2 km/s as well as metallicities to within 0.2 dex.
We analyse the structure of the local stellar halo of the Milky Way using $sim$ 60000 stars with full phase space coordinates extracted from the SDSS--{it Gaia} catalogue. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended towards high radial action $J_R$ as compared to azimuthal or vertical action, $J_phi$ or $J_z$. It has a mild prograde rotation $(langle v_phi rangle approx 25$ km s$^{-1}$), is radially anisotropic and highly flattened with axis ratio $q approx 0.6 - 0.7$. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(langle v_phi rangle approx 50$ km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($qapprox 0.9$). We identify two further components of the halo in action space. There is a high energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, $omega$Centauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] $approx -3$. It has a net outward radial velocity $langle v_R rangle approx 12$ km s$^{-1}$ within the Solar circle at $|z| <3.5$ kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.
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