Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userExploring the stellar age distribution of the Milky Way Bulge using APOGEE
95
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present stellar age distributions of the Milky Way (MW) bulge region using ages for $sim$6,000 high-luminosity ($log(g) < 2.0$), metal-rich ($rm [Fe/H] ge -0.5$) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Ages are derived using {it The Cannon} label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars ($>$8 Gyr). We find evidence that the planar region of the bulge ($|Z_{rm GC}| le 0.25$ kpc) enriched in metallicity, $Z$, at a faster rate ($dZ/dt sim$ 0.0034 ${rm Gyr^{-1}}$) than regions farther from the plane ($dZ/dt sim$ 0.0013 ${rm Gyr^{-1}}$ at $|Z_{rm GC}| > 1.00$ kpc). We identify a non-negligible fraction of younger stars (age $sim$ 2--5 Gyr) at metallicities of $rm +0.2 < [Fe/H] < +0.4$. These stars are preferentially found in the plane ($|Z_{rm GC}| le 0.25$ kpc) and between $R_{rm cy} approx 2-3$ kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found in and outside of the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at super-solar metallicities in a thin disk at 2 kpc $lesssim R_{rm cy} lesssim$ 3 kpc until $sim$2 Gyr ago.
rate research
Read More
The Galactic bulge of the Milky Way is made up of stars with a broad range of metallicity, -3.0 < [Fe/H] < 1 dex. The mean of the Metallicity Distribution Function (MDF) decreases as a function of height z from the plane and, more weakly, with galactic radius. The most metal rich stars in the inner Galaxy are concentrated to the plane and the more metal poor stars are found predominantly further from the plane, with an overall vertical gradient in the mean of the MDF of about -0.45 dex/kpc. This vertical gradient is believed to reflect the changing contribution with height of different populations in the inner-most region of the Galaxy. The more metal rich stars of the bulge are part of the boxy/peanut structure and comprise stars in orbits which trace out the underlying X-shape. There is still a lack of consensus on the origin of the metal poor stars ([Fe/H] < -0.5) in the region of the bulge. Some studies attribute the more metal poor stars of the bulge to the thick disk and stellar halo that are present in the inner region, and other studies propose that the metal poor stars are a distinct old spheroid bulge population. Understanding the origin of the populations that make up the MDF of the bulge, and identifying if there is a unique bulge population which has formed separately from the disk and halo, has important consequences for identifying the relevant processes in the the formation and evolution of the Milky Way.
We use the extensive $Gaia$ Data Release 2 set of Long Period Variables to select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disk and bulge as a function of period. We find the morphology of both components evolves as a function of stellar age/chemistry with the stellar disk being stubby at old ages, becoming progressively thinner and more radially extended at younger stellar ages, consistent with the picture of inside-out and upside-down formation of the Milky Ways disk. We see evidence of a perturbed disk, with large-scale stellar over-densities visible both in and away from the stellar plane. We find the bulge is well modelled by a triaxial boxy distribution with an axis ratio of $sim [1:0.4:0.3]$. The oldest of the Miras ($sim$ 9-10 Gyr) show little bar-like morphology, whilst the younger stars appear inclined at a viewing angle of $sim 21^{circ}$ to the Sun-Galactic Centre line. This suggests that bar formation and buckling took place 8-9 Gyr ago, with the older Miras being hot enough to avoid being trapped by the growing bar. We find the youngest Miras to exhibit a strong peanut morphology, bearing the characteristic X-shape of an inclined bar structure.
Recent observational programmes are providing a global view of the Milky Way bulge that serves as template for detailed comparison with models and extragalactic bulges. A number of surveys (i.e. VVV, GIBS, GES, ARGOS, BRAVA, APOGEE) are producing comprehensive and detailed extinction, metallicity, kinematics and stellar density maps of the Galactic bulge with unprecedented accuracy. However, the still missing key ingredient is the distribution of stellar ages across the bulge. To overcome this limitation, we aim to age-date the stellar population in several bulge fields with the ultimate goal of deriving an age map of the Bulge. This paper presents the methodology and the first results obtained for a field along the Bulge minor axis, at $b=-6^circ$. We use a new PSF-fitting photometry of the VISTA Variables in the V{i}a L{a}ctea (VVV) survey data to construct deep color-magnitude diagrams of the bulge stellar population down to $sim$ 2 mag below the Main Sequence turnoff. We find the bulk of the bulge stellar population in the observed field along the minor axis to be at least older than $sim$ 7.5 Gyr. In particular, when the metallicity distribution function spectroscopically derived by GIBS is used, the best fit to the data is obtained with a combination of synthetic populations with ages in between $sim$ 7.5 Gyr and 11 Gyr. However, the fraction of stars younger than $sim$ 10 Gyr strongly depends upon the number of Blue Straggler Stars present in the bulge. Simulations show that the observed color-magnitude diagram of the bulge in the field along the minor axis is incompatible with the presence of a conspicuous population of intermediate-age/young (i.e. $lesssim 5$ Gyr) stars.
We use data of $sim$13,000 stars from the SDSS/APOGEE survey to study the shape of the bulge MDF within the region $|ell|leq11^circ$ and $|b|leq13^circ$, and spatially constrained to ${rm R_{GC}leq3.5}$ kpc. We apply Gaussian Mixture Modeling and Non-negative Matrix Factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find the shape and spatial variations of the MDF (at ${rm [Fe/H]geq-1}$ dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H]=+$0.32$, $-0.17$ and $-0.66$ dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disk into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help to link a present-day structure with those observed in formation in the center of high redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.
The detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction particularly toward the Galactic plane and center, where the severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, suggesting the need for higher resolutions. These undercorrections affect any stellar study sensitive to color (e.g. star formation history analysis via color-magnitude diagram fitting), either making them inaccurate or limiting them to small low/stable extinction windows where this value is better constrained. We aim at providing a high-resolution (2 arcmin to $sim$ 10 arcsec) color excess map for the VVV bulge area, in $mathrm{J}-mathrm{K}_s$ color. We use the MW-BULGE-PSFPHOT catalogs sampling $sim$ 300 deg$^2$ across the Galactic bulge ($|l| < 10^circ$ and $-10^circ < b < 5^circ$) to isolate a sample of red clump and red giant branch stars, for which we calculate average $mathrm{J}-mathrm{K}_s$ color in a fine spatial grid in $(l, b)$ space. We obtain a E$(mathrm{J}-mathrm{K}_s)$ map spanning the VVV bulge area of roughly 300 deg$^2$, with the equivalent to a resolution between $sim$ 1 arcmin for bulge outskirts ($l < -6^circ$) to below 20 arcsec within the central $|l| < 1^circ$, and below 10 arcsec for the innermost area ($|l| < 1^circ$ and $|b| < 3^circ$). The result is publicly available at http://basti-iac.oa-teramo.inaf.it/vvvexmap/
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
