- Dette emne har 4 svar og 1 stemme, og blev senest opdateret for 5 år, 10 måneder siden af Bjarne. This post has been viewed 624 times
-
Indlæg
-
Bjarne- Super Nova
GAIA Data Release 2 er frigivet. Der blev på førstedagen efter frigivelsen lagt 18 artikler på arXiv.org. Flere artikler følger uden tvivl i de kommende dage, uger og måneder. Jeg kan ikke på nogen måde gennemgå alle artikler. Her følger en nyhedstekst taget fra Science efterfulgt af link til de 18 artikler. Af umiddelbar interesse er “Observational Hertzsprung-Russell diagrams”, som fortæller meget om, hvad man kan vente sig.
European satellite reveals motions of more than 1 billion stars and shape of the Milky Way
By Daniel Clery |
“It’s like waiting for Christmas,” said Vasily Belokurov, an astronomer at the University of Cambridge in the United Kingdom last week. Today, the gifts arrived: the exact positions, motions, brightnesses, and colors of 1.3 billion stars in and around the Milky Way, as tracked by the European Space Agency’s (ESA’s) €750 million Gaia satellite, which after launch in 2013 began measuring the positions of stars and, over time, how they move. On 25 April, ESA made Gaia’s second data set—based on 22 months of observations—publicly available, which should enable a precise 3D map of large portions of the galaxy and the way it moves. “Nothing comes close to what Gaia will release,” Belokurov says.
One might think that the galaxy is completely mapped. But large parts of it are obscured by gas and dust, and it is hard to discern structure from the vantage of the solar system. Gaia is not only expected to clarify the spiral structures of the galaxy today, but because the satellite traces how stars move, astronomers can wind the clock backward and see how the galaxy evolved over the past 13 billion years—a field known as galactic archaeology. With Gaia’s color and brightness information, astronomers can classify the stars by composition and identify the stellar nurseries where different types were born, to understand how chemical elements were forged and distributed.
Gaia isn’t only about the Milky Way. For solar system scientists, the new data set will contain data on 14,000 asteroids. That’s a small fraction of the roughly 750,000 known minor bodies, but Gaia provides orbit information 100 times more accurate than before, says University of Cambridge astronomer Gerry Gilmore, who heads the U.K. branch of Gaia’s data processing consortium. That should help astronomers identify families of asteroids and trace how they relate to each other, shedding light on the solar system’s past and how planets formed from smaller bodies.
For cosmologists, the data set will improve distance measures to stars of known brightness such as Cepheid variables, crucial stepping stones that allow a “distance ladder” to be built out to other galaxies—so that the expansion rate of the universe, also known as the Hubble constant, can be calculated. And exoplanet hunters expect that Gaia will eventually see thousands of stars shifting from side to side because of the gravitational tugs of Jupiter-size planets in distant orbits, but these won’t emerge until the satellite’s precision improves in later data releases. “No one in the world knows what we’ll find,” says David Hogg of New York University in New York City.
The Gaia team released an initial catalog in 2016 and, although it contained more than a billion stars, it only provided motions for 2 million of them. It was a “sampler to get people used to handling Gaia-type data,” Gilmore says. The 2016 release showed that the Milky Way was larger in size than previously thought. The first paper exploiting the data appeared on the arXiv preprint server on the same day. Ever since, Gilmore says, there’s been an average of one paper per day.
This time, astronomers are even more geared up with algorithms that can crunch the tabular data. Belokurov says he and his group have about 50 ideas to pursue, including an assessment of the distribution of mass across the Milky Way and the Large Magellanic Cloud (LMC), a nearby satellite galaxy. Astronomers have long estimated the LMC’s mass at about a billion times that of the sun, but recently studies have suggested it may be heftier. With Gaia data, they may be able to see Milky Way objects that are perturbed by the LMC, which would be a sign of its more massive gravitational influence. “There’s going to be a complete science explosion,” Belokurov says. “I’m planning on not sleeping for a week or two.”
Hogg is also ready for some heavy-duty Gaia hacking. For the release, he invited colleagues from around the world to gather in New York City to work on analyzing the data. He plans to start by drawing up plots that were not possible previously, to look for new trends. Graphing color versus brightness for white dwarf stars, for example, could illuminate how these stellar remnants change as they cool off and eventually become black stellar cinders. Following Gaia’s first release, “almost every plot led to a paper,” he says.
The 450-strong Gaia consortium is already at work on a third data release, planned for 2020. “There are very clear areas we can improve,” says ESA’s Timo Prusti, Gaia project scientist, at the European Space Research and Technology Centre in Noordwijk, the Netherlands. For example, the team wants to return to the very brightest stars, which saturate the detector, in a special short-exposure observing mode. The team also wants to improve on ways to deal with stray light getting onto the detector, a problem which only emerged after launch.
Gaia is also unusual because the scientists who work on the mission are not given a period of exclusive access to the data, a common practice in astronomy. Although Gaia consortium members get to intimately know the way the data are collected and processed, they cannot use the data to do science until after the release, just like everyone else. “It’s brave and very admirable,” Belokurov says.
Gilmore says his team members have been laying bets on how many papers will hit the preprint servers on Day One. Belokurov says: “It’s like going to a festival—the festival of Gaia.”
Gaia Data Release 2. Summary of the contents and survey properties
Gaia Data Release 2: The astrometric solution
Gaia Data Release 2: processing of the photometric data
Gaia Data Release 2: Photometric content and validation
Gaia Radial Velocity Spectrometer
Gaia Data Release 2: The catalogue of radial velocity standard stars
Gaia Data Release 2: Processing the spectroscopic data
Gaia Data Release 2: Properties and validation of the radial velocities
Gaia Data Release 2: Summary of the variability processing & analysis results
Gaia Data Release 2: first stellar parameters from Apsis
Gaia Data Release 2: Catalogue validation
Gaia Data Release 2: using Gaia parallaxes
Gaia Data Release 2: The Celestial reference frame (Gaia-CRF2)
Gaia Data Release 2: Observational Hertzsprung-Russell diagrams
Gaia Data Release 2: Observations of solar system objects
Gaia Data Release 2: Mapping the Milky Way disc kinematics
Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
Gaia Data Release 2: Variable stars in the colour-absolute magnitude diagram
Gaia Data Release 2. Calibration and mitigation of electronic offset effects in the data
Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
Radial Distribution of Stellar Motions in Gaia DR2
Revisiting hypervelocity stars after Gaia DR2
Wrinkles in the Gaia data unveil a dynamically young and perturbed Milky Way disk
A hypervelocity star with a Magellanic origin
Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant
Gaia DR2 Gravitational Lens Systems I: New lensed quasar candidates around known quasars
Evidence for unresolved exoplanet-hosting binaries in Gaia DR2
Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae
STREAMFINDER I: A New Algorithm for detecting Stellar Streams
Ghostly Tributaries to the Milky Way: Charting the Halo’s Stellar Streams with the Gaia DR2 catalogue
One large blob and many streams frosting the nearby stellar halo in Gaia DR2
Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients
Revised Radii of Kepler Stars and Planets using Gaia Data Release 2
Gaia Data Release 2: Rotational modulation in late-type dwarfs
Off the beaten path: Gaia reveals GD-1 stars outside of the main stream
LISA verification binaries with updated distances from Gaia Data Release 2
Gaia Data Release 2: The Short Timescale Variability Processing and Analysis
Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall
A Late-type L Dwarf at 11 pc Hiding in the Galactic Plane Characterized Using Gaia DR2
Isochrone fitting in the Gaia era. II. Distances, ages and masses from UniDAM using Gaia DR2 data
With and without spectroscopy: Gaia DR2 proper motions of seven Ultra-Faint Dwarf Galaxies
Gaia Data Release 2: The first Gaia catalog of Long Period Variable candidates
Gaia Data Release 2: Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars
The properties of the known QSOs astrometric solutions in Gaia DR2
A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter
In disguise or out of reach: first clues about in-situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2
An independent confirmation of the future flyby of Gliese 710 to the solar system using Gaia DR2
Confirmation of the zero-point offset in Gaia Data Release 2 parallaxes using asteroseismology and APOGEE spectroscopy in the Kepler field
An Ultraviolet-Optical Color-Metallicity relation for Red Clump Stars using GALEX and Gaia
On the Gaia DR2 distances for Galactic Luminous Blue Variables
Unidentified quasars among stationary objects from Gaia DR2
Evidence for a systematic offset of -80 micro-arcseconds in the Gaia DR2 parallaxes
The Galactic Disc in Action Space as seen by Gaia DR2
The sdA problem – III. Hundreds of new extremely low-mass white dwarfs and their precursors from Gaia astrometry
First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33
Validating TGAS wide binaries with Gaia DR2 Radial Velocities and Parallaxes
Gaia and the Galactic Center Origin of Hypervelocity Stars
Discovery of the most ultra-luminous QSO using Gaia, SkyMapper and WISE
Anisotropy of the Milky Way’s stellar halo using K giants from LAMOST and Gaia
Bjarne- Super Nova
Jeg er specielt interesseret i, hvad GAIA kan fortælle om det mørke stofs fordeling i Mælkevejen. Stjerner dannes ud af gasskyer lokaliseret i haloer bestående af koldt mørkt stof (den sædvanlige antagelse). Stjerner dannes via køling fra atomer og molekyler, som igen kræver de rette temperaturforhold, samt afskærmning mod ultraviolet stråling. Dette betyder igen, at ikke alle mørke haloer har stjerner. Gassen i de mest massive haloer er så varm, at køletiden er for lang til, at stjerner kan dannes. Dette er tilfældet for galaksehobene, hvor gassen mellem galakserne er 100 millioner grader varmt. Gassen i de mindste haloer er tilsvarende blevet ioniseret af ultraviolet stråling mellem universets galakser, så stjernedannelse bliver umulig.
Små mørke haloer dannes først. Disse smelter sammen og danner større haloer, som så igen smelter sammen og danner endnu større mørke haloer. Det traditionelle kolde mørke stof har ingen indbygget længdeskala. Enhver mørk halo vil derfor bestå af et større antal underhaloer. Dette ses tydeligt galaksehobene, hvor underhaloerne består af galakser. Der er ingen stjerner dannet mellem galakserne i en galaksehob, da gassens temperatur er alt for høj. En galakse som Mælkevejen burde som udgangspunkt indeholde et tilsvarende stort antal subhaloer. Den er omgivet af nogle relativt få dværggalakser. De lyssvageste dværggalakser har meget få stjerner i forhold til massen i deres mørke haloer. Dette tyder på, at Mælkevejen kan have mange små mørke haloer helt uden stjerner. En anden mulighed er, at mørke dværggalakser er blevet opløst ved passage af Mælkevejens skive. Det er mit håb, at stjernestrømme fra opløste åbne stjernehobe kan afsløre de mørke dværghaloer ved afbøjning af stjernernes baner. En anden mulighed er, at dværghaloerne påvirker skivens stjerner under passagen gennem skiven.
Bjarne- Super Nova
Gaia Proper Motions and Orbits of the Ultra-Faint Milky Way Satellites
The second data release from the Gaia mission (DR2) provides a comprehensive and unprecedented picture of the motions of astronomical sources in the plane of the sky, extending from the solar neighborhood to the outer reaches of the Milky Way. I present proper motion measurements based on Gaia DR2 for 17 ultra-faint dwarf galaxies within 100 kpc of the Milky Way. I compile the spectroscopically-confirmed member stars in each dwarf bright enough for Gaia astrometry from the literature, producing member samples ranging from 2 stars in Triangulum II to 68 stars in Bootes I. From the spectroscopic member catalogs I estimate the proper motion of each system. I find good agreement with the proper motions derived by the Gaia collaboration for Bootes I and Leo I. The tangential velocities for 14 of the 17 dwarfs are determined to better than 50 km/s, more than doubling the sample of such measurements for Milky Way satellite galaxies. The orbital pericenters are well-constrained, with a median value of 38 kpc. Only one satellite, Tucana III, is on an orbit passing within 15 kpc of the Galactic center, suggesting that the remaining ultra-faint dwarfs are unlikely to have experienced severe tidal stripping. As a group, the ultra-faint dwarfs are on high-velocity, eccentric, retrograde trajectories, with nearly all of them having space motions exceeding 370 km/s. In the default low-mass Milky Way potential I assume, eight out of the 17 galaxies lack well-defined apocenters and appear likely to be on their first infall, indicating that the Milky Way mass may be larger than previously estimated or that many of the ultra-faint dwarfs are associated with the Magellanic Clouds. The median eccentricity of the ultra-faint dwarf orbits is 0.79, similar to the values seen in numerical simulations, but distinct from the rounder orbits of the more luminous dwarf spheroidals.
Bjarne- Super Nova
De ultra-lyssvage dværggalakser, som mest består af mørkt stof, ser ud til at bevæge sig på baner med et pericenter (mindste afstand fra Mælkevejens centrum) på omkring 38 kilo parsec. De passerer derfor mælkevejens plan uden for Mælkevejens skive. Sådanne dværggalakser, som passerer skiven tættere på centrum, er sikker blevet revet i stykker af tidevandskræfter.
Bjarne- Super Nova
Jeg har tilføjet 9 nye links til artikler baseret på GAIA DR2. Det er praktisk for senere brug, at have links til alle artikler samlet et bestemt sted. Måske også for andre end mig selv.
- Emnet 'GAIAs første fantastiske resultater fra DR2' er lukket for nye svar.