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Register a new userThe Hercules stream as seen by APOGEE-2 South
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The Hercules stream is a group of co-moving stars in the Solar neighbourhood, which can potentially be explained as a signature of either the outer Lindblad resonance (OLR) of a fast Galactic bar or the corotation resonance of a slower bar. In either case, the feature should be present over a large area of the disc. With the recent commissioning of the APOGEE-2 Southern spectrograph we can search for the Hercules stream at $(l,b)=(270^circ,0)$, a direction in which the Hercules stream, if caused by the bars OLR, would be strong enough to be detected using only the line-of-sight velocities. We clearly detect a narrow, Hercules-like feature in the data that can be traced from the solar neighbourhood to a distance of about 4 kpc. The detected feature matches well the line-of-sight velocity distribution from the fast-bar (OLR) model. Confronting the data with a model where the Hercules stream is caused by the corotation resonance of a slower bar leads to a poorer match, as the corotation model does not predict clearly separated modes, possibly because the slow-bar model is too hot.
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Using 3D positions and kinematics of stars relative to the Sagittarius (Sgr) orbital plane and angular momentum, we identify 166 Sgr stream members observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) that also have Gaia DR2 astrometry. This sample of 63/103 stars in the Sgr trailing/leading arm are combined with an APOGEE sample of 710 members of the Sgr dwarf spheroidal core (385 of them newly presented here) to establish differences of 0.6 dex in median metallicity and 0.1 dex in [$alpha$/Fe] between our Sgr core and dynamically older stream samples. Mild chemical gradients are found internally along each arm, but these steepen when anchored by core stars. With a model of Sgr tidal disruption providing estimated dynamical ages (i.e., stripping times) for each stream star, we find a mean metallicity gradient of 0.12 +/- 0.03 dex/Gyr for stars stripped from Sgr over time. For the first time, an [$alpha$/Fe] gradient is also measured within the stream, at 0.02 +/- 0.01 dex/Gyr using magnesium abundances and 0.04 +/- 0.01 dex/Gyr using silicon, which imply that the Sgr progenitor had significant radial abundance gradients. We discuss the magnitude of those inferred gradients and their implication for the nature of the Sgr progenitor within the context of the current family of Milky Way satellite galaxies, and suggest that more sophisticated Sgr models are needed to properly interpret the growing chemodynamical detail we have on the Sgr system.
The nature of the Triangulum-Andromeda (TriAnd) system has been debated since the discovery of this distant, low-latitude Milky Way (MW) overdensity more than a decade ago. Explanations for its origin are either as a halo substructure from the disruption of a dwarf galaxy or a distant extension of the Galactic disk. We test these hypotheses using chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Surveys 14th Data Release of Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare to APOGEE abundances of stars with similar metallicity from both the Sagittarius (Sgr) dSph, and the outer MW disk. We find that TriAnd stars are chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca, Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on the element. Instead, the TriAnd stars, with a median metallicity of about -0.8, exhibit chemical abundance ratios similar to those of the lowest metallicity ([Fe/H] ~ -0.7) stars in the outer Galactic disk, and are consistent with expectations of extrapolated chemical gradients in the outer disk of the MW. These results suggest that TriAnd is associated with the MW disk, and, therefore, that the disk extends to this overdensity --- i.e., past a Galactocentric radius of 24 kpc --- albeit vertically perturbed about 7 kpc below the nominal disk midplane in this region of the Galaxy.
We present the kinematic and chemical profiles of red giant stars observed by the APOGEE-2 survey in the direction of the Jhelum stellar stream, a Milky Way substructure located in the inner halo of the Milky Way at a distance from the Sun of $approx$ 13 kpc. From the six APOGEE-2 Jhelum pointings, we isolate stars with log($g$) $<$ 3.5, leaving a sample of 289 red giant stars. From this sample of APOGEE giants, we identified seven stars that are consistent with the astrometric signal from $Gaia$ DR2 for this stream. Of these seven, one falls onto the RGB along the same sequence as the Jhelum stars presented by cite{ji20}. This new Jhelum member has [Fe/H]=-2.2 and is at the tip of the red giant branch. By selecting high orbital eccentricity, metal-rich stars, we identify red giants in our APOGEE sample that are likely associated with the $Gaia$-Enceladus-Sausage (GES) merger. We compare the abundance profiles of the Jhelum stars and GES stars and find similar trends in $alpha$-elements, as expected for low-metallicity populations. However, we find that the orbits for GES and Jhelum stars are not generally consistent with a shared origin. The chemical abundances for the APOGEE Jhelum star and other confirmed members of the stream are similar to stars in known stellar streams and thus are consistent with an accreted dwarf galaxy origin for the progenitor of the stream, although we cannot rule out a globular cluster origin.
It has been suggested that a resonance between a rotating bar and stars in the solar neighbourhood can produce the so called Hercules stream. Recently, a second bar may have been identified in the Galactic centre, the so called long bar, which is longer and much flatter than the traditional Galactic bar, and has a similar mass. We looked at the dynamical effects of both bars, separately and together, on orbits of stars integrated backwards from local position and velocities, and a model of the Galactic potential which includes the bars directly. Both bars can produce Hercules like features, and allow us to measure the rotation rate of the bar(s). We measure a pattern speed, for both bars, of 1.87 +/- 0.02 times the local circular frequency. This is on par with previous measurements for the Galactic bar, although we do adopt a slightly different Solar motion. Finally, we identify a new kinematic feature in local velocity space, caused by the long bar, which is tempting to identify with the high velocity Arcturus stream.
Based on the [Fe/H] versus [Mg/Fe] diagram and distances from APOGEE data release 14, we compare the spatial distributions, the l-Vlos diagram and the abundance gradients between high-[Mg/Fe] and low-[Mg/Fe] sequences. The two sequences are clearly shown at 5<|Z|<10 kpc in the metallicity range of -1.6 <[Fe/H] <-0.7, where the halo at |Z| > 10 kpc consists of low-[Mg/Fe] stars only. In the intermediate-metallicity range of -1.1 <[Fe/H]<-0.7, a [Mg/Fe] gradient is detected for stars at |Z|=10-30 kpc and it flattens out at |Z|>30 kpc. The l-Vlos diagram is adopted to separate halo stars from the disk by defining the transition metallicity, which is of [Fe/H]~ -1.1 dex for the high-[Mg/Fe] sequence and of [Fe/H]~-0.7 dex for the low-[Mg/Fe] sequence. The R and |Z| distributions for the high-[Mg/Fe] sequence, the thick disk at -1.1<[Fe/H]<-0.7 and the in situ halo at -1.6<[Fe/H]<-1.1, have a cutoff at R~15 kpc and |Z|~10 kpc, beyond which low-[Mg/Fe] halo stars are the main contributions. In the metallicity range of -1.6<[Fe/H]<-0.7, there is a negative metallicity gradient for the high-[Mg/Fe] halo at |Z|<8-10 kpc, while only a marginal or no slope in the [Fe/H] versus |Z| diagram for the low-[Mg/Fe] halo at |Z|<8-10 kpc, beyond which both the high-[Mg/Fe] halo and low-[Mg/Fe] halo flatten out toward |Z| > 20 kpc. These results indicate a complicated formation history of the Galaxy and we may see a hint of a three-section halo, i.e. the inner in situ halo within $|Z|~8-10$ kpc, the intermediately outer dual-mode halo at |Z|~10-30 kpc, and the extremely outer accreted halo with |Z|>30 kpc.
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