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108-579: In 1965, anomalous velocity gas clouds were found in the region of the Magellanic Clouds. The gas stretches for at least 180 degrees across the sky. This corresponds to 180 kpc (600,000 ly ) at an approximate distance of 55 kpc (180,000 ly ). The gas is very collimated and polar with respect to the Milky Way . The velocity range is huge (from −400 to 400 km s in reference to Local Standard of Rest ) and velocity patterns do not follow

216-650: A Lissajous orbit around the Sun – Earth L 2 Lagrangian point . The Gaia space telescope has its roots in ESA's Hipparcos mission (1989–1993). Its mission was proposed in October 1993 by Lennart Lindegren ( Lund Observatory , Lund University , Sweden) and Michael Perryman (ESA) in response to a call for proposals for ESA's Horizon Plus long-term scientific programme. It was adopted by ESA's Science Programme Committee as cornerstone mission number 6 on 13 October 2000, and

324-554: A SFR of 1–3 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} yr . Models for galactic chemical evolution find that at least half of this amount must be continuously accreted, low-metallicity material to describe the current, observable structure. Without this accretion, the SFRs indicate that the current star formation material will only last for another few gigayears (Gyr) at most. Models of mass inflow place

432-554: A combination of Gaia and Tycho-2 data for those objects in both catalogues; "light curves and characteristics for about 3,000 variable stars; and positions and magnitudes for more than 2000 ... extragalactic sources used to define the celestial reference frame ". The second data release (DR2), which occurred on 25 April 2018, is based on 22 months of observations made between 25 July 2014 and 23 May 2016. It includes positions, parallaxes and proper motions for about 1.3 billion stars and positions of an additional 300 million stars in

540-465: A concentration that is 10–30% that of the Sun's. Their low metallicity seems to serve as proof that HVCs do indeed bring in “fresh” gas. Complex C has been estimated to bring in 0.1–0.2 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} of new material every year, whereas Complex A brings in about half that amount. This fresh gas

648-424: A feature leading the Magellanic Clouds. These early models were 'tidal' models. Just like tides on Earth are induced by the gravity of the 'leading' Moon , the models predicted two directions opposite each other, in which particles are preferentially pulled. However, the predicted features were not observed. This led to a few models that did not require a leading element but which had problems of their own. In 1998

756-456: A fully relativistic model, the influence of the gravitational field of the solar-system must be taken into account, including such factors as the gravitational light-bending due to the Sun, the major planets and the Moon. The expected accuracies of the final catalogue data have been calculated following in-orbit testing, taking into account the issues of stray light, degradation of the optics, and

864-462: A hot, gaseous corona that surrounds the Milky Way. Inspired by this proposal, Jan Oort , of Leiden University, Netherlands, proposed that cold gas clouds might be found in the galactic halo, far away from the galactic plane. They were soon located, in 1963, via their neutral hydrogen radio emission . They were traveling toward the galactic disk at a very high velocity relative to other entities in

972-675: A mass estimate of HVCs in the Milky Way in the range of 7.4x10 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} . If the Large Magellanic Cloud and the Small Magellanic Cloud are included, the total mass would increase by another 7x10 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} . Observed angular sizes for HVCs range from 10 degrees down to

1080-441: A maximal accretion rate of .4 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} yr from HVCs. This rate does not meet that which is demanded by the chemical evolutionary models. Thus, it is a possibility that the Milky Way may go through a low point in gas content and/or decrease its SFR until further gas arrives. Consequently, when discussing HVCs in

1188-580: A model to explain HVCs as gas left over from the early formation of the galaxy. He theorized that if this gas were at the edge of the galaxy's gravitational influence, over billions of years it could be dragged back toward the Galactic disk and fall back in as HVCs. Oort's model explained the observed chemical composition of the galaxy well. Given an isolated galaxy (i.e. one without ongoing assimilation of hydrogen gas), successive generations of stars should infuse

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1296-473: A piece of mylar insulation broke loose and floated into the line-of-sight of the telescope causing corrupted data. The testing of stray-light and baffles is a noted part of space imaging instruments. In April 2024, a micrometeoroid hit and damaged Gaia's protective cover, creating "a little gap that allowed stray sunlight – around one billionth of the intensity of direct sunlight felt on Earth – to occasionally disrupt Gaia ’s very sensitive sensors". In May,

1404-413: A precision one hundred times greater, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle of 106.5° between them. The spacecraft rotates continuously around an axis perpendicular to the two telescopes' lines of sight, with a spin period of 6 hours. Thus, every 6 hours the spacecraft scans a great circle stripe approximately 0.7 degrees wide. The spin axis in turn has

1512-463: A result, cool clouds moving through the diffuse halo medium have a chance to become ionized by the warmer and hotter gas. This can create a pocket of ionized gas that surrounds a neutral interior in an HVC. Evidence of this cool-hot gas interaction in the halo comes from the observation of OVI absorption. HVCs are defined by their respective velocities, but distance measurements allow for estimates on their size, mass, volume density, and even pressure. In

1620-443: A slower precession across the sky: it maintains a fixed 45 degree angle to the Sun, but follows a cone around the Sun every 63 days, giving a cycloid-like path relative to the stars. Over the course of the mission, each star is scanned many times at various scan directions, providing interlocking measurements over the full sky. The two key telescope properties are: Each celestial object was observed on average about 70 times during

1728-574: A southern hemisphere survey of neutral hydrogen radio emissions was completed using the Villa Elisa radio telescope in Argentina from which yet more HVCs were discovered. Later observations of Complex C showed that the cloud, originally thought to be deficient in heavy elements (also known as low metallicity ), contains some sections with a higher metallicity compared to the bulk of the cloud, indicating that it has begun to mix with other gas in

1836-768: A special data set, the Gaia Andromeda Photometric Survey (GAPS). The full data release for the five-year nominal mission, DR4, will include full astrometric, photometric and radial-velocity catalogues, variable-star and non-single-star solutions, source classifications plus multiple astrophysical parameters for stars, unresolved binaries, galaxies and quasars, an exo-planet list and epoch and transit data for all sources. Additional release(s) will take place depending on mission extensions. Most measurements in DR4 are expected to be 1.7 times more precise than DR2; proper motions will be 4.5 times more precise. DR4

1944-467: A study analysing the full sky survey made by the HIPASS team at Parkes Observatory generated important new observational data. Putman et al. discovered that a mass of high-velocity clouds leading the Magellanic Clouds was actually fully connected to the Magellanic Clouds. So, the leading arm feature had its existence finally established. Furthermore, Lu et al. (1998) and Gibson et al. (2000) established

2052-483: A survival time on the order of a couple hundred million years without some sort of support mechanism that prevents them from dissipating. The lifetime mainly depends on the mass of the cloud, but also on the cloud density, halo density, and velocity of the cloud. HVCs in the galactic halo are destroyed through what is called the Kelvin-Helmholtz instability . The infall of clouds can dissipate energy leading to

2160-587: Is HD 74438 , which was, in a paper published in 2022, identified as a possible progenitor of a sub-Chandrasekhar Type Ia supernovae . In November 2017, scientists led by Davide Massari of the Kapteyn Astronomical Institute , University of Groningen , Netherlands released a paper describing the characterization of proper motion (3D) within the Sculptor dwarf galaxy , and of that galaxy's trajectory through space and with respect to

2268-624: Is about 10–20% of the total needed to properly dilute Galactic gas enough to account for the chemical composition of stars. Complex C, one of the most well-studied HVCs, is at least 14,000 ly (about 4 kpc) distant but no more than 45,000 ly (about 14 kpc) above the Galactic plane . It should also be noted that Complex C has been observed to have about 1/50 of the nitrogen content that the Sun contains. Observations of high-mass stars indicate that they produce less nitrogen, as compared to other heavy elements, than do low-mass stars. This implies that

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2376-491: Is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen μas". On 30 August 2014, Gaia discovered its first supernova in another galaxy. On 3 July 2015, a map of the Milky Way by star density was released, based on data from the spacecraft. As of August 2016, "more than 50 billion focal plane transits, 110 billion photometric observations and 9.4 billion spectroscopic observations have been successfully processed." In 2018

2484-479: Is expected to be released no earlier than mid-2026. The final Gaia catalogue, DR5, will consist of all data collected during the lifespan of the mission. It will be 1.4 times more precise than DR4, while proper motions will be 2.8 times more precise than DR4. It will be published no earlier than the end of 2030. All data of all catalogues will be available in an online data base that is free to use. An outreach application, Gaia Sky , has been developed to explore

2592-713: Is located 25,000–30,000 ly (8–9 kpc) away in the galactic halo . In the Southern Hemisphere , the most prominent HVCs are all associated with the Magellanic system which has two major components, the Magellanic Stream and the Leading Arm. They are both made of gas that was stripped from the Large and Small Magellanic Clouds (LMC and SMC). Half of the gas was decelerated and now lags behind

2700-413: Is more likely. Independent measurements have demonstrated that the greatest Gaia radial velocity among the hypervelocity stars is contaminated by light from nearby bright stars in a crowded field and cast doubt on the high Gaia radial velocities of other hypervelocity stars. In late October 2018, the galactic population Gaia-Enceladus , the remains of a major merger with the defunct Enceladus dwarf,

2808-608: Is more severe for the RVS spectrograph than for the astrometry measurements, because it spreads the light of the star onto a much larger number of detector pixels which each collect scattered light. This kind of problem has some historical background. In 1985 on STS-51-F , the Space Shuttle Spacelab -2 mission, another astronomical mission hampered by stray debris was the Infrared Telescope (IRT), in which

2916-428: Is no longer applicable, the name Gaia remained to provide continuity with the project. The total cost of the mission is around €740 million (~ $ 1 billion), including the manufacture, launch and ground operations. Gaia was completed two years behind schedule and 16% above its initial budget, mostly due to the difficulties encountered in polishing Gaia 's ten silicon carbide mirrors and assembling and testing

3024-604: Is providing the basic observational data to analyze a wide range of important questions related to the origin, structure and evolutionary history of the Milky Way galaxy. The successor to the Hipparcos mission (operational 1989–1993), Gaia is part of ESA's Horizon 2000+ long-term scientific program. Gaia was launched on 19 December 2013 by Arianespace using a Soyuz ST-B / Fregat-MT rocket flying from Kourou in French Guiana. The spacecraft currently operates in

3132-477: Is relatively young, which is a sign of recent star formation in the leading arm. High-velocity cloud High-velocity clouds ( HVCs ) are large accumulations of gas with an unusually rapid motion relative to their surroundings. They can be found throughout the galactic halo of the Milky Way . Their bulk motions in the local standard of rest have velocities which are measured in excess of 70–90 km s . These clouds of gas can be massive in size, some on

3240-575: Is that of dynamical shielding, which increases the Kelvin-Helmholtz time. This process works due to the HVC having a cold neutral interior shielded by a warmer and lower-density exterior, causing the HI clouds to have smaller relative velocities with respect to their surroundings. Since their discovery, several possible models have been proposed to explain the origins of HVCs. However, for observations in

3348-489: Is thought that the tidal forces mostly affect the Small Magellanic Cloud, since it has lower mass, and is less gravitationally bound. In contrast, ram pressure stripping mostly affects the Large Magellanic Cloud, because it has a larger reservoir of gas. In 2018, research confirmed that the chemical composition of the gas in the Magellanic Stream Leading Arm more closely resembles the composition of

Magellanic Stream - Misplaced Pages Continue

3456-629: The ESTRACK network in Cebreros , Spain, Malargüe , Argentina and New Norcia , Australia, receive the data. In October 2013 ESA had to postpone Gaia 's original launch date, due to a precautionary replacement of two of Gaia 's transponders. These are used to generate timing signals for the downlink of science data. A problem with an identical transponder on a satellite already in orbit motivated their replacement and reverification once incorporated into Gaia . The rescheduled launch window

3564-495: The Gaia mission was extended to 2020, and in 2020 it was further extended through 2022, with an additional "indicative extension" extending through 2025. The limiting factor to further mission extensions is the supply of nitrogen for the cold gas thrusters of the micro-propulsion system. The amount of dinitrogen tetroxide (NTO) and monomethylhydrazine (MMH) for the chemical propulsion subsystem on board might be enough to stabilize

3672-612: The International Celestial Reference Frame (ICRF3) . Included is the Gaia Catalogue of Nearby Stars (GCNS), containing 331,312 stars within (nominally) 100 parsecs (330 light-years). The full DR3, published on 13 June 2022, includes the EDR3 data plus Solar System data; variability information; results for non-single stars, for quasars, and for extended objects; astrophysical parameters; and

3780-516: The Milky Way , using data from Gaia and the Hubble Space Telescope . Massari said, "With the precision achieved we can measure the yearly motion of a star on the sky which corresponds to less than the size of a pinhead on the Moon as seen from Earth." The data showed that Sculptor orbits the Milky Way in a highly elliptical orbit; it is currently near its closest approach at a distance of about 83.4 kiloparsecs (272,000 ly), but

3888-493: The Sloan Digital Sky Survey have led to distance measurements for almost all of the large complexes currently known. The indirect-distance-constraint methods are usually dependent on theoretical models, and assumptions must be made in order for them to work. One indirect method involves Hα observations, where an assumption is made that the emission lines come from ionizing radiation from the galaxy, reaching

3996-486: The virial radius is consistent with coming in along cosmic filaments in evolutionary models of the Milky Way. Given current observational limitations, the majority of the filaments feeding into the Milky Way are not visible in HI. Despite this, some gas clouds within the Galaxy's halo have lower metallicities than that of gas stripped from satellites, suggesting that the clouds are primordial material probably flowing in along

4104-658: The "origins" section, satellite accretion plays a role in the evolution of a galaxy. Most galaxies are assumed to result from smaller precursors merging, and the process continues throughout a galaxy's lifetime. Within the next 10 billion years, further satellite galaxies will merge with Milky Way, sure to significantly impact the Milky Way's structure and steer its future evolution. Spiral galaxies have abundant sources for potential star-formation material, but how long galaxies are able to continuously draw on these resources remains in question. A future generation of observational tools and computational abilities will shed light on some of

4212-457: The B2 phase of the project was authorised on 9 February 2006, with EADS Astrium taking responsibility for the hardware. The name "Gaia" was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics . This reflected the optical technique of interferometry that was originally planned for use on the spacecraft. While the working method evolved during studies and the acronym

4320-582: The Earth, which the Minor Planet Center catalogued as object 2015 HP 116 . It was soon found to be an accidental rediscovery of the Gaia spacecraft and the designation was promptly retracted. Shortly after launch, ESA revealed that Gaia was suffering from a stray light problem. The problem was initially thought to be due to ice deposits causing some of the light diffracted around the edges of

4428-574: The GSR frame. One of the HVCs in the Leading Arm shows a composition very similar to the SMC. This seems to support the idea that the gas that comprises it was pulled off of the galaxy and accelerated in front of it via tidal forces which pull apart satellite galaxies and assimilate them into the Milky Way. Smith's Cloud is another well-studied HVC found in the Southern Hemisphere, located in

Magellanic Stream - Misplaced Pages Continue

4536-405: The Galactic halo medium is more distant and has a much lower density. FUSE found highly ionized oxygen mixed in with the Magellanic Stream. This suggests that the stream must be embedded in hot gas. The Leading Arm is not one continuous stream, but rather an association of multiple clouds found in the region preceding the Magellanic Clouds. It is thought to have a velocity of −300 km/s in

4644-512: The Galactic halo, about 30–50% of the HI mass of the Milky Way . The Magellanic Stream is seen as a “long, continuous structure with a well-defined velocity and column density gradient.” The velocity at the tip of the Magellanic Stream is hypothesized to be +300 km/s in the Galactic-standard-of-rest (GSR) frame. Stream clouds are thought to have a lower pressure than other HVCs because they reside in an area where

4752-525: The ISM. An alternative theory centers on gas being ejected out of the galaxy and falling back in as the high-velocity gas we observe. Several proposed mechanisms exist to explain how material can be ejected from the Galactic disk, but the most prevalent explanation of the Galactic Fountain centers on compounding supernova explosions to eject large "bubbles" of material. Since gas is being ejected from

4860-471: The Interstellar Medium (ISM) with higher abundances of heavy elements. However, examinations of stars in the solar neighborhood show roughly the same relative abundances of the same elements regardless of the age of the star; this has come to be known as the G dwarf problem. HVCs may explain these observations by representing a portion of the primordial gas responsible for continuously diluting

4968-500: The Magellanic Stream in the halo of the Milky Way. The somewhat distinct features of HVCs formed in this way are also accounted for by simulations, and most HVCs in the Milky Way which are not associated with the Magellanic Stream do not seem to be at all associated with a dwarf galaxy . Another model, proposed by David Eichler, now at Ben Gurion University, and later by Leo Blitz of the University of California at Berkeley, assumes

5076-425: The Magellanic Stream. In 2019 astronomers discovered the young star cluster Price-Whelan 1 using Gaia data. The star cluster has a low metallicity and belongs to the leading arm of the Magellanic Clouds. The discovery of this star cluster suggests that the leading arm of the Magellanic Clouds is 90,000 light-years away from the Milky Way, only half as far from the Milky Way as previously thought. The star cluster

5184-483: The Milky Way and satellite galaxies, such as the Large and Small Magellanic Clouds (LMC and SMC, respectively) which produce a well-known complex of HVCs called the Magellanic Stream . Because of the various possible mechanisms that could potentially produce HVCs, there are still many questions surrounding HVCs for researchers to study. In the mid-1950s, dense pockets of gas were first discovered outside of

5292-486: The Milky Way indicate the likelihood of some central engine feedback having occurred in the past 10–15 megayears (Myr). Furthermore, as described in “origins,” the disk-wide “galactic fountain” phenomenon is similarly crucial in piecing together the Milky Way's evolution. Materials ejected in the course of a galaxy's lifetime help describe observational data (observed metallicity content primarily) while providing feedback sources for future star formation. Likewise detailed in

5400-525: The Milky Way, clouds are typically located between 2–15 kpc (6.52x10 ly–4.89x10 ly) from the centre, and at z-heights (distances above or below the Galactic plane ) within 10 kpc (3.26x10 ly). The Magellanic Stream and the Leading Arm are at ~55 kpc (1.79x10 ly), near the Magellanic Clouds , and may extend to about 100–150 kpc (3.26x10 ly–4.89x10 ly). There are two methods of distance determination for HVCs. The best method for determining

5508-505: The Milky Way, the multiplicity of clouds, the distinct characteristics of IVCs, and the existence of clouds that are clearly associated with cannibalized dwarf galaxies (i.e. the Magellanic system among others) indicate that the HVCs most likely have several possible origins. This conclusion is also strongly supported by the fact that most simulations for any given model can account for some cloud behaviors, but not all. Jan Oort developed

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5616-476: The Small Magellanic Cloud, rather than the Large Magellanic Cloud, by looking at light from background quasars shining through the Stream and analysing the spectrum of light that is either absorbed by, or let through it. This analysis confirmed that the gas most likely originated from the Small Magellanic Cloud, thereby indicating that the Large Magellanic Cloud is 'winning' in the gravity tug of both Clouds working on

5724-672: The Solar System by using the astrometry method, 500,000 quasars outside this galaxy and tens of thousands of known and new asteroids and comets within the Solar System. The Gaia mission continues to create a precise three-dimensional map of astronomical objects throughout the Milky Way and map their motions, which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements provide detailed physical properties of all stars observed, characterizing their luminosity , effective temperature , gravity and elemental composition. This massive stellar census

5832-479: The Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth, arriving there 8 January 2014. The L2 point provides the spacecraft with a very stable gravitational and thermal environment. There, it uses a Lissajous orbit that avoids blockage of the Sun by the Earth, which would limit the amount of solar energy the satellite could produce through its solar panels , as well as disturb

5940-662: The acceleration of the solar system towards the galactic center as 0.23 nanometers/s . In March 2021, the European Space Agency announced that Gaia had identified a transiting exoplanet for the first time. The planet was discovered orbiting solar-type star Gaia EDR3 3026325426682637824. Following its initial discovery, the PEPSI spectrograph from the Large Binocular Telescope (LBT) in Arizona

6048-465: The advent of distance determinations for most HVCs, this possibility may be ruled out. To inquire into the origin and fate of a galaxy's halo gas is to inquire into the evolution of said galaxy. HVCs and IVCs are significant features of a spiral galaxy's structure. These clouds are of primary importance when considering a galaxy's Star formation rate (SFR). The Milky Way has approximately 5 billion solar masses of star forming material within its disk and

6156-451: The basic angle instability. The best accuracies for parallax, position and proper motion are obtained for the brighter observed stars, apparent magnitudes 3–12. The standard deviation for these stars is expected to be 6.7 micro-arcseconds or better. For fainter stars, error levels increase, reaching 26.6 micro-arcseconds error in the parallax for 15th-magnitude stars, and several hundred micro-arcseconds for 20th-magnitude stars. For comparison,

6264-463: The best parallax error levels from the new Hipparcos reduction are no better than 100 micro-arcseconds, with typical levels several times larger. The overall data volume that was retrieved from the spacecraft during the nominal five-year mission at a compressed data rate of 1 Mbit/s is approximately 60  TB , amounting to about 200 TB of usable uncompressed data on the ground, stored in an InterSystems Caché database. The responsibility of

6372-597: The brighter stars is measured by an integrated spectrometer observing the Doppler effect . Because of the physical constraints imposed by the Soyuz spacecraft, Gaia 's focal arrays could not be equipped with optimal radiation shielding, and ESA expected their performance to suffer somewhat toward the end of the initial five-year mission. Ground tests of the CCDs while they were subjected to radiation provided reassurance that

6480-521: The chemical similarity between the streams and Magellanic Clouds. Newer, increasingly sophisticated models all tested the Leading Arm Feature hypothesis. These models make heavy use of gravity effects through tidal fields . Some models also rely on ram pressure stripping as a shaping mechanism. Most recent models increasingly include drag from the halo of the Milky Way as well as gas dynamics , star formation and chemical evolution. It

6588-529: The closeness of the Magellanic Clouds and the ability to resolve individual stars and their parallaxes , and proper motion , subsequent observations gave the full 6-dimensional phase space information of both clouds (with very large relative errors for the transverse velocities). This enabled the calculation of the likely past orbit of the Large and the Small Magellanic Cloud in relation to the Milky Way. The calculation necessitated large assumptions, for example, on

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6696-676: The cloud's surface. Another method uses deep HI observations in the Milky Way and/or Local Group with the assumption that the distribution of HVCs in the Local Group is similar to that of the Milky Way. These observations put the clouds within 80 kpc (2.61x10 ly) of the galaxy, and observations of the Andromeda Galaxy put them at approximately 50 kpc (1.63x10 ly). For those HVCs where both are available, distances measured via Hα emission tend to agree with those found via direct distances measurements. HVCs are typically detected at

6804-403: The clouds are very massive, located between galaxies, and created when baryonic material pools near concentrations of dark matter . The gravitational attraction between the dark matter and the gas was intended to explain the ability of the clouds to remain stable even at intergalactic distances where the paucity of ambient material should cause the clouds to dissipate rather quickly. However, with

6912-560: The clouds in their orbits (this is the stream component). The other half of the gas (the leading arm component) was accelerated and pulled out in front of the galaxies in their orbit. The Magellanic system is about 180,000 ly (55 kpc) from the Galactic disk, though the tip of the Magellanic Stream may extend out as far as 300,000–500,000 ly (100–150 kpc). The entire system is thought to contribute at least 3x10 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} of HI to

7020-513: The commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. On the bright side of that limit, special operational procedures download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it

7128-558: The constellation Aquila . Gaia (spacecraft) Gaia is a space observatory of the European Space Agency (ESA), launched in 2013 and expected to operate until 2025. The spacecraft is designed for astrometry : measuring the positions, distances and motions of stars with unprecedented precision, and the positions of exoplanets by measuring attributes about the stars they orbit such as their apparent magnitude and color . The mission aims to construct by far

7236-415: The context of galactic evolution, the conversation is largely concerned with star formation and how the future star material fuels the galactic disk. The current model for the universe, ɅCDM, indicates that galaxies tend to cluster and achieve a web-like structure over time. Under such models, the large majority of baryons entering a galactic halo do so along these cosmic filaments. 70% of the mass inflow at

7344-438: The cosmic filaments. Gas of this type, detectable out to ~160,000 ly (50 kpc), largely becomes part of the hot halo, cools and condenses, and falls into the Galactic disk to serve in star formation. Mechanical feedback mechanisms, supernova-driven or active galactic nuclei-driven outflows of gas, are also key elements in understanding the origin of a spiral galaxy's halo gas and the HVCs within. X-ray and gamma-ray observations in

7452-553: The creation and maintenance of a high-precision celestial reference frame, the Barycentric Celestial Reference System (BCRS) , which is essential for both astronomy and navigation. This reference frame serves as a fundamental grid for positioning celestial objects in the sky, aiding astronomers in various research endeavors. All observations, regardless of the actual positioning of the spacecraft, must be expressed in terms of this reference system. As

7560-689: The data processing, partly funded by ESA, is entrusted to a European consortium, the Data Processing and Analysis Consortium (DPAC), which was selected after its proposal to the ESA Announcement of Opportunity released in November 2006. DPAC's funding is provided by the participating countries and has been secured until the production of Gaia 's final catalogue. Gaia sends back data for about eight hours every day at about 5 Mbit/s. ESA's three 35-metre-diameter radio dishes of

7668-497: The disk of the galaxy, the observed metallicity of the ejected gas should be similar to that of the disk. While this may be ruled out for the source of HVCs, these conclusions may point to the Galactic Fountain as the source of IVCs. As dwarf galaxies pass through a larger galaxy's halo, the gas that exists as the interstellar medium of the dwarf galaxy may be stripped away by tidal forces and ram pressure stripping . Evidence for this model of HVC formation comes from observations of

7776-473: The distance to an HVC involves using a halo star of known distance as a standard for comparison. We can extract information about the distance by studying the star's spectrum. If a cloud is located in front of the halo star, absorption lines will be present, whereas if the cloud is behind the star, no absorption lines should be present. CaII, H, K, and/or NaII are the double absorption lines that are used in this technique. Halo stars that have been identified through

7884-430: The electronics of one of the CCDs failed, which caused a high rate of false detections. After that, the engineers refocused Gaia' s optics "for the final time". The testing and calibration phase, which started while Gaia was en route to SEL2 point, continued until the end of July 2014, three months behind schedule due to unforeseen issues with stray light entering the detector. After the six-month commissioning period,

7992-497: The fine pointing to focus on stars many light years away, the only moving parts are actuators to align the mirrors and the valves to fire the thrusters. It has no reaction wheels or gyroscopes. The spacecraft subsystems are mounted on a rigid silicon carbide frame, which provides a stable structure that will not expand or contract due to temperature. Attitude control is provided by small cold gas thrusters that can output 1.5 micrograms of nitrogen per second. The telemetric link with

8100-447: The five years of the nominal mission, which has been extended to approximately ten years and will thus obtain twice as many observations. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time (motion) and lastly, the star's parallax from which distance can be calculated. The radial velocity of

8208-553: The focal plane camera system. The Gaia space mission has the following objectives: Gaia was launched by Arianespace , using a Soyuz ST-B rocket with a Fregat-MT upper stage, from the Ensemble de Lancement Soyouz at Kourou in French Guiana on 19 December 2013 at 09:12 UTC (06:12 local time). The satellite separated from the rocket's upper stage 43 minutes after launch at 09:54 UTC. The craft headed towards

8316-436: The galactic disk. The first two clouds that were located were named Complex A and Complex C. Due to their anomalous velocities, these objects were dubbed "high-velocity clouds", distinguishing them from both gas at normal local standard of rest velocities as well as their slower-moving counterparts known as intermediate-velocity clouds (IVCs). Several astronomers proposed hypotheses (which later proved to be inaccurate) regarding

8424-465: The galactic halo. In addition, as these clouds fall into the disk of the galaxy, they add material that can form stars in addition to the dilute star forming material already present in the disk. This new material aids in maintaining the star formation rate (SFR) of the galaxy. The origins of the HVCs are still in question. No one theory explains all of the HVCs in the galaxy. However, it is known that some HVCs are probably spawned by interactions between

8532-419: The galactic plane. This was quite notable because the models of the Milky Way showed the density of gas decreasing with distance from the galactic plane, rendering this a striking exception. According to the prevailing galactic models, the dense pockets should have dissipated long ago, making their very existence in the halo quite puzzling. In 1956 the solution was proposed that the dense pockets were stabilized by

8640-505: The galaxy in three dimensions using Gaia data. In July 2017, the Gaia-ESO Survey reported using Gaia data to find double-, triple-, and quadruple- stars. Using advanced techniques they identified 342 binary candidates, 11 triple candidates, and 1 quadruple candidate. Nine of these had been identified by other means, thus confirming that the technique can correctly identify multiple star systems. The possible quadruple star system

8748-548: The halo. Using observations of highly ionized oxygen and other ions astronomers were able to show that hot gas in Complex C is an interface between hot and cold gas. HVCs are typically the coldest and densest components of the galactic halo. However, the halo itself also has a multi-phase structure: cold and dense neutral hydrogen at temperatures less than 10 K, warm and warm-hot gas at temperatures between 10 K and 10 K, and hot ionized gas at temperatures greater than 10 K. As

8856-508: The heavy elements in Complex C may come from high-mass stars. The earliest stars are known to have been higher-mass stars and so Complex C appears to be a fossil of sorts, formed outside the galaxy and made up of gas from the ancient universe. However, a more recent study of another area of Complex C has found a metallicity twice as high as what was reported originally. These measurements have led scientists to believe that Complex C has begun to mix with other, younger, nearby gas clouds. Complex A

8964-459: The inevitable heating of the halo medium. The multi-phase structure of the gaseous halo suggests that there is an ongoing life-cycle of HVC destruction and cooling. Some possible mechanisms responsible for increasing the lifetime of an HVC include the presence of a magnetic field that induces a shielding effect and/or the presence of dark matter ; however, there is no strong observational evidence for dark matter in HVCs. The most accepted mechanism

9072-484: The largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects , mainly stars, but also planets, comets, asteroids and quasars , among others. To study the precise position and motion of its target objects, the spacecraft monitored each of them about 70 times over the five years of the nominal mission (2014–2019), and about as many during its extension. Due to its detectors not degrading as fast as initially expected,

9180-461: The magnitude range g = 3–20, red and blue photometric data for about 1.1 billion stars and single colour photometry for an additional 400 million stars, and median radial velocities for about 7 million stars between magnitude 4 and 13. It also contains data for over 14,000 selected Solar System objects. Due to uncertainties in the data pipeline, the third data release, based on 34 months of observations, has been split into two parts so that data that

9288-483: The mission was given an extension. As of March 2023, the spacecraft has enough micro-propulsion fuel to operate until the second quarter of 2025. Gaia targets objects brighter than magnitude 20 in a broad photometric band that covers the extended visual range between near-UV and near infrared; such objects represent approximately 1% of the Milky Way population. Additionally, Gaia is expected to detect thousands to tens of thousands of Jupiter-sized exoplanets beyond

9396-534: The nature of HVCs, but their models were further complicated in the early 1970s by the discovery of the Magellanic Stream , which behaves like a string of HVCs. In 1988, a northern-sky survey of neutral hydrogen radio emissions was completed using the Dwingeloo radio telescope in the Netherlands . From this survey, astronomers were able to detect more HVCs. In 1997, a map of the Milky Way's neutral hydrogen

9504-639: The orbit can take it out to around 222 kiloparsecs (720,000 ly) distant. In October 2018, Leiden University astronomers were able to determine the orbits of 20 hypervelocity stars from the DR2 dataset. Expecting to find a single star exiting the Milky Way , they instead found seven. More surprisingly, the team found that 13 hypervelocity stars were instead approaching the Milky Way, possibly originating from as-of-yet unknown extragalactic sources. Alternatively, they could be halo stars to this galaxy, and further spectroscopic studies will help determine which scenario

9612-436: The order of millions of times the mass of the Sun ( M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} ), and cover large portions of the sky. They have been observed in the Milky Way's halo and within other nearby galaxies. HVCs are important to the understanding of galactic evolution because they account for a large amount of baryonic matter in

9720-422: The primary mission's objectives can be met. An atomic clock on board Gaia plays a crucial role in achieving the mission's primary objectives. Gaia rotates with angular velocity of 60"/sec or 0.6 microarcseconds in 10 nanoseconds. Therefore, in order to meet its positioning goals, Gaia must be able to record the exact time of observation to within nanoseconds. Furthermore, no systematic positioning errors over

9828-415: The radio and optical wavelengths, and for hotter HVCs, ultraviolet and/or X-ray observations are needed. Neutral hydrogen clouds are detected via the 21 cm emission line. Observations have shown that HVCs can have ionized exteriors due to external radiation or the motion of the HVC through a diffuse halo medium. These ionized components can be detected through Hα emission lines and even absorption lines in

9936-413: The resolution limit of the observations. Typically, high resolution observations eventually show that larger HVCs are often composed of many smaller complexes. When detecting HVCs solely via HI emission, all of the HVCs in the Milky Way cover about 37% of the night sky. Most HVCs are somewhere between 2 and 15 kilo parsecs (kpc) across. Cold clouds moving through a diffuse halo medium are estimated to have

10044-424: The rest of the Milky Way. Hence, it was determined to be a classic high-velocity cloud . However, the gas was not mapped, and the connection to the two Magellanic Clouds was not made. The Magellanic Stream as such was discovered as a Neutral Hydrogen (HI) gas feature near the Magellanic Clouds by Wannier & Wrixon in 1972. Its connection to the Magellanic Clouds was made by Mathewson et al. in 1974. Owing to

10152-402: The rotational period of 6 hours should be introduced by the clock performance. For the timing error to be below 10 nanoseconds over each rotational period, the frequency stability of the on-board clock needs to be better than 10 . The rubidium atomic clock aboard the Gaia spacecraft has a stability reaching ~ 10 over each rotational period of 21600 seconds. Gaia' s measurements contribute to

10260-411: The satellite is about 3 Mbit/s on average, while the total content of the focal plane represents several Gbit/s . Therefore, only a few dozen pixels around each object can be downlinked. The design of the Gaia focal plane and instruments. Due to the spacecraft's rotation, images cross the focal plane array right-to-left at 60 arcseconds per second. Similar to its predecessor Hipparcos , but with

10368-421: The satellite started its nominal five-year period of scientific operations on 25 July 2014 using a special scanning mode that intensively scanned the region near the ecliptic poles ; on 21 August 2014 Gaia began using its normal scanning mode which provides more uniform coverage. Although it was originally planned to limit Gaia ' s observations to stars fainter than magnitude 5.7, tests carried out during

10476-410: The shapes and masses of the 3 galaxies, and the nature of dynamical friction between the moving objects. Observations of individual stars revealed details of star formation history. Models describing the formation of the Magellanic Stream had been produced since 1980. Following computing power, the initial models were very simple, non-self-gravitating, and with few particles . Most models predicted

10584-444: The spacecraft at L2 for several decades. Without the cold gas, though, the space craft can no longer be pointed on a microarcsecond scale. In March 2023, the Gaia mission was extended through the second quarter of 2025, when it is expected that the spacecraft will run out of cold gas propellant. It will then enter a post-operations phase that is expected to be completed by the end of 2030. Several Gaia catalogues are released over

10692-494: The spacecraft's thermal equilibrium. After launch, a 10-metre-diameter sunshade was deployed. The sunshade always maintains a fixed 45 degree angle to the Sun, while precessing to scan the sky, thus keeping all telescope components cool and powering Gaia using solar panels on its surface. These factors and the materials used in its creation allow Gaia to function in conditions between -170 ° C and 70 ° C. The Gaia payload consists of three main instruments: In order to maintain

10800-506: The star cluster Price-Whelan 1 was discovered. The cluster belongs to the Magellanic Clouds and is located in the leading arm of these Dwarf Galaxies . The discovery suggests that the stream of gas extending from the Magellanic Clouds to the Milky Way is about half as far from the Milky Way as previously thought. The Radcliffe wave was discovered in data measured by Gaia , published in January 2020. In November 2020, Gaia measured

10908-472: The sunshield and entering the telescope apertures to be reflected towards the focal plane. The actual source of the stray light was later identified as the fibers of the sunshield, protruding beyond the edges of the shield. This results in a "degradation in science performance [which] will be relatively modest and mostly restricted to the faintest of Gaia 's one billion stars." Mitigation schemes are being implemented to improve performance. The degradation

11016-476: The technical details of the Milky Way's past and future as well as how HVCs play a role in its evolution. In the Northern Hemisphere , we find several large HVCs, though nothing on the order of the Magellanic system (discussed below). Complexes A and C were the first HVCs discovered and were first observed in 1963. Both of these clouds have been found to be deficient in heavy elements , showing

11124-417: The ultraviolet. The warm-hot gas in HVCs exhibit OVI, SiIV, and CIV absorption lines. Most HVCs show spectral line widths that are indicative of a warm, neutral medium for HVCs at about 9000 Kelvin. However, many HVCs have line widths which indicate that they are also partly composed of cool gas at less than 500 K. Estimates on the peak column density of HVCs (10 cm ) and typical distances (1–15 kpc) yield

11232-492: The years each time with increasing amounts of information and better astrometry; the early releases also miss some stars, especially fainter stars located in dense star fields and members of close binary pairs. The first data release, Gaia DR1, based on 14 months of observation was on 14 September 2016. The data release includes "positions and ... magnitudes for 1.1 billion stars using only Gaia data; positions, parallaxes and proper motions for more than 2 million stars" based on

11340-657: Was discovered. This system is associated with at least 13 globular clusters, and the creation of the Thick Disk of the Milky Way. It represents a significant merger about 10 billion years ago in the Milky Way Galaxy. In November 2018, the galaxy Antlia 2 was discovered. It is similar in size to the Large Magellanic Cloud , despite being 10,000 times fainter. Antlia 2 has the lowest surface brightness of any galaxy discovered. In December 2019

11448-539: Was from 17 December 2013 to 5 January 2014, with Gaia slated for launch on 19 December. Gaia was successfully launched on 19 December 2013 at 09:12 UTC . About three weeks after launch, on 8 January 2014, it reached its designated orbit around the Sun-Earth L2 Lagrange point (SEL2), about 1.5 million kilometers from Earth. In 2015, the Pan-STARRS observatory discovered an object orbiting

11556-634: Was largely complete, again allowing astronomers to detect more HVCs. In the late 1990s, using data from the La Palma Observatory in the Canary Islands , the Hubble Space Telescope , and, later, the Far Ultraviolet Spectroscopic Explorer (FUSE), the distance to an HVC was gauged for the first time. Around the same time, the chemical composition of HVCs was first measured. Additionally, in 2000,

11664-407: Was ready first, was released first. The first part, EDR3 ("Early Data Release 3"), consisting of improved positions, parallaxes and proper motions, was released on 3 December 2020. The coordinates in EDR3 use a new version of the Gaia celestial reference frame ( Gaia –CRF3), based on observations of 1,614,173 extragalactic sources, 2,269 of which were common to radio sources in the third revision of

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