Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUntangling the Galaxy I: Local Structure and Star Formation History of the Milky Way
129
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying unsupervised machine learning on DR2s 5-dimensional dataset (3d position + 2d velocity), we identify a number of clusters, associations, and co-moving groups within 1 kpc and $|b|<30^circ$ (many of which have not been previously known). We estimate their ages with the precision of $sim$0.15 dex. Many of these groups appear to be filamentary or string-like, oriented in parallel to the Galactic plane, and some span hundreds of pc in length. Most of these string lack a central cluster, indicating that their filamentary structure is primordial, rather than the result of tidal stripping or dynamical processing. The youngest strings ($<$100 Myr) are orthogonal to the Local Arm. The older ones appear to be remnants of several other arm-like structures that cannot be presently traced by dust and gas. The velocity dispersion measured from the ensemble of groups and strings increase with age, suggesting a timescale for dynamical heating of $sim$300 Myr. This timescale is also consistent with the age at which the population of strings begins to decline, while the population in more compact groups continues to increase, suggesting that dynamical processes are disrupting the weakly bound string populations, leaving only individual clusters to be identified at the oldest ages. These data shed a new light on the local galactic structure and a large scale cloud collapse.
rate research
Read More
The all-sky Milky Way Star Clusters (MWSC) survey provides uniform and precise ages and other parameters for a variety of clusters in the Solar Neighbourhood. We construct the cluster age distribution, investigate its spatial variations, and discuss constraints on cluster formation scenarios of the Galactic disk during the last 5 Gyrs. Due to the spatial extent of the MWSC, we consider spatial variations of the age distribution along galactocentric radius $R_G$, and along $Z$-axis. For the analysis of the age distribution we use 2242 clusters, which all lie within roughly 2.5 kpc of the Sun. To connect the observed age distribution to the cluster formation history we build an analytical model based on simple assumptions on the cluster initial mass function and on the cluster mass-lifetime relation, fit it to the observations, and determine the parameters of the cluster formation law. Comparison with the literature shows that earlier results strongly underestimated the number of evolved clusters with ages $tgtrsim 100$ Myr. Recent studies based on all-sky catalogues agree better with our data, but still lack the oldest clusters with ages $tgtrsim 1$ Gyr. We do not observe a strong variation in the age distribution along $R_G$, though we find an enhanced fraction of older clusters ($t>1$ Gyr) in the inner disk. In contrast, the distribution strongly varies along $Z$. The high altitude distribution practically does not contain clusters with $t<1$ Gyr. With simple assumptions on the cluster formation history, cluster initial mass function and cluster lifetime we can reproduce the observations. Cluster formation rate and cluster lifetime are strongly degenerate, which does not allow us to disentangle different formation scenarios. In all cases the cluster formation rate is strongly declining with time, and the cluster initial mass function is very shallow at the high mass end. (abridged)
We present an examination of the metallicity distribution function of the outermost stellar halo of the Galaxy based on an analysis of both local (within 4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These samples were compiled using spectroscopic metallicities from the Sloan Digital Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We detect a negative metallicity gradient in the outermost halo (r > 35 kpc from the Galactic center), and find that the frequency of very metal-poor ([Fe/H] < -2.0) stars in the outer-halo region reaches up to ~60% in our most distant sample, commensurate with previous theoretical predictions. This result provides clear evidence that the outer-halo formed hierarchically. The retrograde stars in the outermost halo exhibit a roughly constant metallicity, which may be linked to the accretion of the Sequoia progenitor. In contrast, prograde stars in the outermost halo exhibit a strong metallicity-distance dependence, indicating that they likely originated from the accretion of galaxies less massive than the Sequoia progenitor galaxy.
RR Lyrae stars being distance indicators and tracers of old population serve as excellent probes of the structure, formation, and evolution of our Galaxy. Thousands of them are being discovered in ongoing wide-field surveys. The OGLE project conducts the Galaxy Variability Survey with the aim to detect and analyze variable stars, in particular of RRab type, toward the Galactic bulge and disk, covering a total area of 3000 deg^2. Observations in these directions also allow detecting background halo variables and unique studies of their properties and distribution at distances from the Galactic Center to even 40 kpc. In this contribution, we present the first results on the spatial distribution of the observed RRab stars, their metallicity distribution, the presence of multiple populations, and relations with the old bulge. We also show the most recent results from the analysis of RR Lyrae stars of the Sgr dwarf spheroidal galaxy, including its center, the globular cluster M54.
The history of the Milky Way is encoded in the spatial distributions, kinematics, and chemical enrichment patterns of its resolved stellar populations. SEGUE-2 and APOGEE, two of the four surveys that comprise SDSS-III (the Sloan Digital Sky Survey III), will map these distributions and enrichment patterns at optical and infrared wavelengths, respectively. Using the existing SDSS spectrographs, SEGUE-2 will obtain spectra of 140,000 stars in selected high-latitude fields to a magnitude limit r ~ 19.5, more than doubling the sample of distant halo stars observed in the SDSS-II survey SEGUE (the Sloan Extension for Galactic Understanding and Exploration). With spectral resolution R ~ 2000 and typical S/N per pixel of 20-25, SEGUE and SEGUE-2 measure radial velocities with typical precision of 5-10 km/s and metallicities ([Fe/H]) with a typical external error of 0.25 dex. APOGEE (the Apache Point Observatory Galactic Evolution Experiment) will use a new, 300-fiber H-band spectrograph (1.5-1.7 micron) to obtain high-resolution (R ~ 24,000), high signal-to-noise ratio (S/N ~ 100 per pixel) spectra of 100,000 red giant stars to a magnitude limit H ~ 12.5. Infrared spectroscopy penetrates the dust that obscures the inner Galaxy from our view, allowing APOGEE to carry out the first large, homogeneous spectroscopic survey of all Galactic stellar populations. APOGEE spectra will allow radial velocity measurements with < 0.5 km/s precision and abundance determinations (with ~ 0.1 dex precision) of 15 chemical elements for each program star, which can be used to reconstruct the history of star formation that produced these elements. (abridged)
The stellar population of the Milky Way bulge is thoroughly studied, with a plethora of measurements from virtually the full suite of instruments available to astronomers. It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history. Cosmological models predict the bulge to host the Galaxys oldest stars for [Fe/H]$lesssim -1$, and this is demonstrated by RR Lyrae stars and globular cluster observations. There is consensus that bulge stars with [Fe/H]$lesssim0$ are older than $t approx10$ Gyr. However, at super-solar metallicity, there is a substantial unresolved discrepancy. Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch, the abundances of asymptotic giant branch stars, the period distribution of Mira variables, the chemistry and central-star masses of planetary nebulae, all suggest a substantial intermediate-age population ($t approx 3$ Gyr). This is in conflict with predictions from cosmologically-motivated chemical evolution models and photometric studies of the main-sequence turnoff region, which both suggest virtually no stars younger than $t approx 8$ Gyr. A possible resolution to this conflict is enhanced helium-enrichment, as this would shift nearly all of the age estimates in the direction of decreasing discrepancy. Enhanced helium-enrichment is also arguably suggested by measurements of the red giant branch bump and the R-parameter.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
