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Register a new userUncovering fossils of the distant Milky Way with UNIONS: NGC 5466 and its stellar stream
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We examine the spatial clustering of blue horizontal branch (BHB) stars from the $textit{u}$-band of the Canada-France Imaging Survey (CFIS, a component of the Ultraviolet Near-Infrared Optical Northern Survey, or UNIONS). All major groupings of stars are associated with previously known satellites, and among these is NGC 5466, a distant (16 kpc) globular cluster. NGC 5466 reportedly possesses a long stellar stream, although no individual members of the stream have previously been identified. Using both BHBs and more numerous red giant branch stars cross-matched to $textit{Gaia}$ Data Release 2, we identify extended tidal tails from NGC 5466 that are both spatially and kinematically coherent. Interestingly, we find that this stream does not follow the same path as the previous detection at large distances from the cluster. We trace the stream across 31$^{circ}$ of sky and show that it exhibits a very strong distance gradient ranging from 10 $<$ R$_{helio}$ $<$ 30 kpc. We compare our observations to simple dynamical models of the stream and find that they are able to broadly reproduce the overall path and kinematics. The fact that NGC 5466 is so distant, traces a wide range of Galactic distances, has an identified progenitor, and appears to have recently had an interaction with the Galaxys disk, makes it a unique test-case for dynamical modelling of the Milky Way.
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We investigate the metallicity distribution function (MDF) in the Galactic halo and the relative fraction of Carbon-normal and Carbon-rich stars. To this aim, we use an improved version of the semi-analytical code GAlaxy MErger Tree and Evolution (GAMETE), that reconstructs the hierarchical merger tree of the MW, following the star formation history and the metal and dust evolution in individual progenitors. The predicted scaling relations between the dust, metal and gas masses for MW progenitors show a good agreement with observational data of local galaxies and of Gamma Ray Burst (GRB) host galaxies at 0.1 < z < 6.3. We find that in order to reproduce the observed tail of the MDF at [Fe/H] < -4, faint SN explosions have to dominate the metal yields produced by Pop III stars, disfavoring a Pop III IMF that extends to stellar masses > 140 M_{sun}, into the Pair-Instability SN progenitor mass range. The relative contribution of C-normal and C-enhanced stars to the MDF and its dependence on [Fe/H] points to a scenario where the Pop III/II transition is driven by dust-cooling and the first low-mass stars form when the dust-to-gas ratio in their parent clouds exceeds a critical value of D_crit = 4.4 x 10^{-9}.
We study the globular clusters (GCs) in the spiral galaxy NGC~5907 well-known for its spectacular stellar stream -- to better understand its origin. Using wide-field Subaru/Suprime-Cam $gri$ images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disk. We estimate the total number of globular clusters in NGC 5907 to be $154pm44$, with a specific frequency of $0.73pm0.21$. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts of the stellar disk. Using the properties of the stream GCs, we estimate that the disrupted galaxy has a stellar mass similar to the Sagittarius dwarf galaxy accreted by the Milky Way, i.e. $sim10^8~M_odot$.
We explore the local volume of the Milky Way via chemical and kinematical measurements from high quality astrometric and spectroscopic data recently released by the Gaia, APOGEE and GALAH programs. We chemically select $1137$ stars up to $2.5$~kpc of the Sun and $rm{[Fe/H]} le -1.0$~dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies $163$ objects into eight kinematical groups, whose origin is further investigated with high resolution N-body numerical simulations of single merging events. The two retrograde groups appear associated with Gaia-Sausage-Enceladus, while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date ($langle Z_{rm max} rangle lesssim 0.5$~kpc, $langle V+V_{rm{LSR}}rangle simeq 231~rm{km~s^{-1}}$). Its peculiar chemical ($langle rm{[Fe/H]}rangle simeq -1.45$, $langle rm{[Mg/Fe]}rangle simeq -0.02$) and dynamical (mean eccentricity $simeq 0.11$) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of $sim 10^9 M_sun$ on an initial prograde low-inclination orbit, $sim 10^circ$. The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination.
The narrow GD-1 stream of stars, spanning 60 deg on the sky at a distance of ~10 kpc from the Sun and ~15 kpc from the Galactic center, is presumed to be debris from a tidally disrupted star cluster that traces out a test-particle orbit in the Milky Way halo. We combine SDSS photometry, USNO-B astrometry, and SDSS and Calar Alto spectroscopy to construct a complete, empirical 6-dimensional phase-space map of the stream. We find that an eccentric orbit in a flattened isothermal potential describes this phase-space map well. Even after marginalizing over the stream orbital parameters and the distance from the Sun to the Galactic center, the orbital fit to GD-1 places strong constraints on the circular velocity at the Suns radius V_c=224 pm 13 km/s and total potential flattening q_Phi=0.87^{+0.07}_{-0.04}. When we drop any informative priors on V_c the GD-1 constraint becomes V_c=221 pm 18 km/s. Our 6-D map of GD-1 therefore yields the best current constraint on V_c and the only strong constraint on q_Phi at Galactocentric radii near R~15 kpc. Much, if not all, of the total potential flattening may be attributed to the mass in the stellar disk, so the GD-1 constraints on the flattening of the halo itself are weak: q_{Phi,halo}>0.89 at 90% confidence. The greatest uncertainty in the 6-D map and the orbital analysis stems from the photometric distances, which will be obviated by Gaia.
We present NGC 4565 and NGC 5746 as structural analogs of our Milky Way. All three are giant, SBb - SBbc galaxies with two pseudobulges, i. e., a compact, disky, star-forming pseudobulge embedded in a vertically thick, red and dead, boxy pseudobulge that really is a bar seen almost end-on. The stars in the boxy bulge of our Milky Way are old and enhanced in alpha elements, indicating that star formation finished within ~ 1 Gyr of when it started. Here, we present Hobby-Eberly Telescope spectroscopy of the boxy pseudobulges of NGC 4565 and NGC 5746 and show that they also are made of old and alpha-element-enhanced stars. Evidently it is not rare that the formation of stars that now live in bars finished quickly and early, even in galaxies of intermediate Hubble types whose disks still form stars now. Comparison of structural component parameters leads us to suggest that NGC 4565 and NGC 5746 are suitable analogs of the Milky Way, because they show signatures of similar evolution processes.
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