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Register a new userThe Milky Ways bar and bulge revealed by APOGEE DR16 and Gaia DR3
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Anna B\\'arbara de Andrade Queiroz
Publication date
2020
fields
Physics
and research's language is
English
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We investigate the inner regions of the Milky Way with a sample of unprecedented size and coverage thanks to APOGEE DR16 and {it Gaia} DR3 data. Our inner Galactic sample has more than 26,000 stars within $|X_{rm Gal}| <5$ kpc, $|Y_{rm Gal}| <3.5$ kpc, $|Z_{rm Gal}| <1$ kpc, and we also make the analysis for a foreground-cleaned sub-sample of 8,000 stars more representative of the bulge-bar populations. The inner Galaxy shows a clear chemical discontinuity in key abundance ratios [$alpha$/Fe], [C/N], and [Mn/O], probing different enrichment timescales, which suggests a star formation gap (quenching) between the high- and low-$alpha$ populations. For the first time, we are able to fully characterize the different populations co-existing in the innermost regions of the Galaxy via joint analysis of the distributions of rotational velocities, metallicities, orbital parameters and chemical abundances. The chemo-kinematic analysis reveals the presence of the bar; of an inner thin disk; of a thick disk, and of a broad metallicity population, with a large velocity dispersion, indicative of a pressure supported component. We find and characterize chemically and kinematically a group of counter-rotating stars, which could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disk, which migrated into the bulge. Finally, based on the 6D information we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpcs. Even stars with a high probability of belonging to the bar show the chemical bimodality in the [$alpha$/Fe] vs. [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant distinct chemical abundance ratio signature.
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We construct a large set of dynamical models of the galactic bulge, bar and inner disk using the Made-to-Measure method. Our models are constrained to match the red clump giant density from a combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of $39.0 pm 3.5 ,rm{km,s^{-1},kpc^{-1}}$, placing the bar corotation radius at $6.1 pm 0.5 rm{kpc}$ and making the Milky Way bar a typical fast rotator. We evaluate the stellar mass of the long bar and bulge structure to be $M_{rm{bar/bulge}} = 1.88 pm 0.12 times 10^{10} , rm{M}_{odot}$, larger than the mass of disk in the bar region, $M_{rm{inner disk}} = 1.29pm0.12 times 10^{10} , rm{M}_{odot}$. The total dynamical mass in the bulge volume is $1.85pm0.05times 10^{10} , rm{M}_{odot}$. Thanks to more extended kinematic data sets and recent measurement of the bulge IMF our models have a low dark matter fraction in the bulge of $17%pm2%$. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of $sim0.2times10^{10} ,rm{M}_{odot}$, probably in a nuclear disk or disky pseudobulge.
The upcoming LISA mission offers the unique opportunity to study the Milky Way through gravitational wave radiation from Galactic binaries. Among the variety of Galactic gravitational wave sources, LISA is expected to individually resolve signals from $sim 10^5$ ultra-compact double white dwarf (DWD) binaries. DWDs detected by LISA will be distributed across the Galaxy, including regions that are hardly accessible to electromagnetic observations such as the inner part of the Galactic disc, the bulge and beyond. We quantitatively show that the large number of DWD detections will allow us to use these systems as tracers of the Milky Way potential. We demonstrate that density profiles of DWDs detected by LISA may provide constraints on the scale length parameters of the baryonic components that are both accurate and precise, with statistical errors of a few percent to $10$ percent level. Furthermore, the LISA sample is found to be sufficient to disentangle between different (commonly used) disc profiles, by well covering the disc out to sufficiently large radii. Finally, up to $sim 80$ DWDs can be detected through both electromagnetic and gravitational wave radiation. This enables multi-messenger astronomy with DWD binaries and allows one to extract their physical properties using both probes. We show that fitting the Galactic rotation curve constructed using distances inferred from gravitational waves {it and} proper motions from optical observations yield a unique and competitive estimate of the bulge mass. Instead robust results for the stellar disc mass are contingent upon knowledge of the Dark Matter content.
We compare distance resolved, absolute proper motions in the Milky Way bar/bulge region to a grid of made-to-measure dynamical models with well defined pattern speeds. The data are obtained by combining the relative VVV Infrared Astrometric Catalog v1 proper motions with the Gaia DR2 absolute reference frame. We undertake a comprehensive analysis of the various errors in our comparison, from both the data and the models, and allow for additional, unknown, contributions by using an outlier-tolerant likelihood function to evaluate the best fitting model. We quantify systematic effects such as the region of data included in the comparison, with or without possible overlap from spiral arms, and the choice of synthetic luminosity function and bar angle used to predict the data from the models. Resulting variations in the best-fit parameters are included in the final error budget. We measure the bar pattern speed to be Omega_b=35.4+-0.9 km/s/kpc and the azimuthal solar velocity to be V_phi_sun= 251.4+-1.7 km/s. These values, when combined with recent measurements of the Galactic rotation curve, yield the distance of corotation, 6.3 < R_(CR) [kpc] < 6.8, the outer Lindblad resonance (OLR), 10.5 < R_(OLR) [kpc] < 11.5, and the higher order, m=4, OLR, 8.5 < R_(OLR_4) [kpc] < 9.0. The measured low pattern speed provides strong evidence for the long-slow bar scenario.
Numerous studies of integrated starlight, stellar counts, and kinematics have confirmed that the Milky Way is a barred galaxy. However, far fewer studies have investigated the bars stellar population properties, which carry valuable independent information regarding the bars formation history. Here we conduct a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in the on-bar and off-bar regions to study the azimuthal variation of star formation history (SFH) in the inner Galaxy. We find that the on-bar and off-bar stars at Galactocentric radii 3 $< r_{rm GC}<$ 5 kpc have remarkably consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]--[Fe/H] relation, suggesting a common SFH shared by the long bar and the disc. In contrast, the bar and disc at smaller radii (2 $< r_{rm GC} <$ 3 kpc) show noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars but fewer solar abundance stars in the bar. Given the three-phase star formation history proposed for the inner Galaxy in Lian et al. (2020b), these differences could be explained by the off-bar disc having experienced either a faster early quenching process or recent metal-poor gas accretion. Vertical variations of the abundance distributions at small $r_{rm GC}$ suggest a wider vertical distribution of low-$alpha$ stars in the bar, which may serve as chemical evidence for vertical heating through the bar buckling process. The lack of such vertical variations outside the bulge may then suggest a lack of vertical heating in the long bar.
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