Abstract
We present a novel interpretation of the previously puzzling different behaviours of stellar populations of the Milky Way’s bulge. We first show, by means of pure N-body simulations, that ...initially co-spatial stellar populations with different in-plane random motions separate when a bar forms. The radially cooler populations form a strong bar, and are vertically thin and peanut-shaped, while the hotter populations form a weaker bar and become a vertically thicker box. We demonstrate that it is the radial, not the vertical, velocity dispersion that dominates this evolution. Assuming that early stellar discs heat rapidly as they form, then both the in-plane and vertical random motions correlate with stellar age and chemistry, leading to different density distributions for metal-rich and metal-poor stars. We then use a high-resolution simulation, in which all stars form out of gas, to demonstrate that this is what happens. When we apply these results to the Milky Way we show that a very broad range of observed trends for ages, densities, kinematics
and chemistries, that have been presented as evidence for contradictory paths to the formation of the bulge, are in fact consistent with a bulge which formed from a continuum of disc stellar populations which were kinematically separated by the bar. For the first time, we are able to account for the bulge’s main trends via a model in which the bulge formed largely in situ. Since the model is generic, we also predict the general appearance of stellar population maps of external edge-on galaxies.
Abstract
The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive ...satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the
α
/Fe–Fe/H abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the
α
-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.
Detailed elemental-abundance patterns of giant stars in the Galactic halo measured by the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have revealed the existence of a unique and ...significant stellar subpopulation of silicon-enhanced (Si/Fe ≳ +0.5) metal-poor stars, spanning a wide range of metallicities (−1.5 ≲ Fe/H ≲ −0.8). Stars with over-abundances in Si/Fe are of great interest because these have very strong silicon (
28
Si) spectral features for stars of their metallicity and evolutionary stage, offering clues about rare nucleosynthetic pathways in globular clusters (GCs). Si-rich field stars have been conjectured to have been evaporated from GCs, however, the origin of their abundances remains unclear, and several scenarios have been offered to explain the anomalous abundance ratios. These include the hypothesis that some of them were born from a cloud of gas previously polluted by a progenitor that underwent a specific and peculiar nucleosynthesis event or, alternatively, that they were due to mass transfer from a previous evolved companion. However, those scenarios do not simultaneously explain the wide gamut of chemical species that are found in Si-rich stars. Instead, we show that the present inventory of such unusual stars, as well as their relation to known halo substructures (including the in situ halo,
Gaia
-Enceladus, the Helmi Stream(s), and Sequoia, among others), is still incomplete. We report the chemical abundances of the iron-peak (Fe), the light- (C and N), the
α
- (O and Mg), the odd-
Z
(Na and Al), and the
s
-process (Ce and Nd) elements of 55 newly identified Si-rich field stars (among more than ∼600 000 APOGEE-2 targets), which exhibit over-abundances of Si/Fe as extreme as those observed in some Galactic GCs, and they are relatively well distinguished from other stars in the Si/Fe−Fe/H plane. This new census confirms the presence of a statistically significant and chemically-anomalous structure in the inner halo:
Jurassic
. The chemo-dynamical properties of the
Jurassic
structure is consistent with it being the tidally disrupted remains of GCs, which are easily distinguished by an over-abundance of Si/Fe among Milky Way populations or satellites.
ABSTRACT Red clump stars are commonly used to map the reddening and morphology of the inner regions of the Milky Way. We use the new photometric catalogues of the VISTA Variables in the Vía Láctea ...survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes $\rm -10^{\circ }\lt l\lt 10^{\circ }$ and latitudes $\rm -1.5^{\circ }\lt b\lt 1.5^{\circ }$. We use these de-reddened catalogues to construct the $\rm K_{s}$ luminosity function around the red clump in the Galactic plane. We show that the secondary peak (fainter than the red clump) detected in these regions does not correspond to the bulge red-giant branch bump alone, as previously interpreted. Instead, this fainter clump corresponds largely to the over-density of red clump stars tracing the spiral arm structure behind the Galactic bar. This result suggests that studies aiming to characterize the bulge red-giant branch bump should avoid low galactic latitudes ($\rm |b|\lt 2^{\circ }$), where the background red clump population contributes significant contamination. It furthermore highlights the need to include this structural component in future modelling of the Galactic bar.
We investigate aluminum abundance variations in the stellar populations of globular clusters using both literature measurements of sodium and aluminum and APOGEE measurements of nitrogen and aluminum ...abundances. For the latter, we show that the Payne is the most suitable of the five available abundance pipelines for our purposes. Our combined sample of 42 globular clusters spans approximately 2 dex in Fe/H and 1.5 dex in . We find no fewer than five globular clusters with significant internal variations in nitrogen and/or sodium with little to no corresponding variation in aluminum, and that the minimum present-day cluster mass for aluminum enrichment in metal-rich systems is . We demonstrate that the slopes of the Al/Fe versus Na/Fe and Al/Fe versus N/Fe relations for stars without field-like abundances are approximately log-linearly dependent on both the metallicity and the stellar mass of the globular clusters. In contrast, the relationship between Na/Fe and N/Fe shows no evidence of such dependencies. This suggests that there were (at least) two classes of non-supernova chemical polluters that were common in the early universe, and that their relative contributions within globular clusters somehow scaled with the metallicity and mass of globular clusters. The first of these classes is predominantly responsible for the CNO and NeNa abundance variations, and likewise the second for the MgAl abundance variations. Particularly striking examples of this dichotomy include 47 Tuc and M4. As an auxiliary finding, we argue that abundance variations among Terzan 5 stars are consistent with it being a normal globular cluster.
We completed the search for microlensing events in the zero latitude area of the Galactic bulge using the VVV Survey near-infrared (near-IR) data obtained between 2010 and 2015. We have now a total ...sample of N = 630 events Using the near-IR color-magnitude diagram we selected the red clump (RC) sources to analyze the longitude dependence of microlensing across the central region of the Galactic plane. The events show a homogeneous distribution, smoothly increasing in numbers toward the Galactic center, as predicted by different models. We find a slight asymmetry, with a larger number of events toward negative longitudes than positive longitudes. This asymmetry is seen both in the complete sample and the subsample of RC giant sources, and it is possibly related with the inclination of the bar along the line of sight. The timescale distribution is fairly symmetric with a peak in 17.4 1.0 days for the complete sample (N = 630 events), and 20.7 1.0 days for the RC stars (N = 291 events), in agreement with previous results.
The stellar velocity distribution function in the solar vicinity is reexamined using data from the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey's ...DR16 and Gaia DR2. By exploiting APOGEE's ability to chemically discriminate with great reliability the thin-disk, thick-disk, and (accreted) halo populations, we can, for the first time, derive the three-dimensional velocity distribution functions (DFs) for these chemically separated populations. We employ this smaller but more data-rich APOGEE+Gaia sample to build a data-driven model of the local stellar population velocity DFs and use these as basis vectors for assessing the relative density proportions of these populations over the 5 < R < 12 kpc and −1.5 < z < 2.5 kpc range as derived from the larger, more complete (i.e., all-sky, magnitude-limited) Gaia database. We find that 81.9% 3.1% of the objects in the selected Gaia data set are thin-disk stars, 16.6% 3.2% are thick-disk stars, and 1.5% 0.1% belong to the Milky Way stellar halo. We also find the local thick-to-thin-disk density normalization to be T(R )/ t(R ) = 2.1% 0.2%, a result consistent with, but determined in a completely different way from, typical star-count/density analyses. Using the same methodology, the local halo-to-disk-density normalization is found to be H(R )/( T(R ) + t(R )) = 1.2% 0.6%, a value that may be inflated due to the chemical overlap of halo and metal-weak thick-disk stars.
We investigate interstellar extinction curve variations towards ∼4 deg2 of the inner Milky Way in VIJKs
photometry from the OGLE-III (third phase of the Optical Gravitational Lensing Experiment) and ...VVV (VISTA Variables in the Via Lactea) surveys, with supporting evidence from diffuse interstellar bands and F435W, F625W photometry. We obtain independent measurements towards ∼2000 sightlines of AI, E(V − I), E(I − J) and E(J − Ks
), with median precision and accuracy of 2 per cent. We find that the variations in the extinction ratios AI
/E(V − I), E(I − J)/E(V − I) and E(J − Ks
)/E(V − I) are large (exceeding 20 per cent), significant and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how RV
is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve have at least two degrees of freedom, and not one (e.g. RV
), which we confirm with a principal component analysis. We derive a median value of 〈AV
/AKs
〉 = 13.44, which is ∼60 per cent higher than the ‘standard’ value. We show that the Wesenheit magnitude WI
= I − 1.61(I − J) is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that RV
variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of Type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ∼100 per cent and lack sensitivity to extinction curve variations.
Aims. The windows of low extinction in the Milky Way (MW) plane are rare but important because they enable us to place structural constraints on the opposite side of the Galaxy, which has hither to ...been done rarely. Methods. We use the near-infrared (near-IR) images of the VISTA Variables in the Vía Láctea (VVV) Survey to build extinction maps and to identify low extinction windows towards the Southern Galactic plane. Here we report the discovery of VVV WIN 1713−3939, a very interesting window with relatively uniform and low extinction conveniently placed very close to the Galactic plane. Results. The new window of roughly 30 arcmin diameter is located at Galactic coordinates (l, b) = (347.4, −0.4) deg. We analyse the VVV near-IR colour-magnitude diagrams in this window. The mean total near-IR extinction and reddening values measured for this window are AKs = 0.46 and E(J − Ks) = 0.95. The red clump giants within the window show a bimodal magnitude distribution in the Ks band, with peaks at Ks = 14.1 and 14.8 mag, corresponding to mean distances of D = 11.0 ± 2.4 and 14.8 ± 3.6 kpc, respectively. We discuss the origin of these red clump overdensities within the context of the MW disk structure.
ABSTRACT
We report the identification of possible extended star debris candidates beyond the cluster tidal radius of NGC 6362 based on the second Gaia data release (Gaia DR2). We found 259 objects ...possibly associated with the cluster lying in the vicinity of the giant branch and 1–2 magnitudes fainter/brighter than the main-sequence turn-off in the cluster colour–magnitude diagram and which cover an area on the sky of ∼4.1 deg2 centred on the cluster. We traced back the orbit of NGC 6362 in a realistic Milky Way potential, using the gravpot16 package, for 3 Gyr. The orbit shows that the cluster shares similar orbital properties as the inner disc, having peri-/apogalactic distances, and maximum vertical excursion from the Galactic plane inside the corotation radius (CR), moving inwards from CR radius to visit the inner regions of the Milky Way. The dynamical history of the cluster reveals that it has crossed the Galactic disc several times in its lifetime and has recently undergone a gravitational shock, ∼15.9 Myr ago, suggesting that less than 0.1 per cent of its mass has been lost during the current disc-shocking event. Based on the cluster’s orbit and position in the Galaxy, we conclude that the possible extended star debris candidates are a combined effect of the shocks from the Galactic disc and evaporation from the cluster. Lastly, the evolution of the vertical component of the angular momentum shows that the cluster is strongly affected dynamically by the Galactic bar potential.
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