Milky Way astrometry
|P. Kenneth Seidelmann (2012), Scholarpedia, 7(5):10578.||doi:10.4249/scholarpedia.10578||revision #137328 [link to/cite this article]|
The Milky Way is a spiral galaxy with a central black hole, a bar-shaped core region surrounded by a disk of gas and dust forming a spiral structure within the disk, a stellar halo with streams and overdensities, or “clouds”, globular clusters, and satellite galaxies. It contains 100-400 billion stars, and maybe 50 billion planets. Our solar system is located within the outer regions within the disk, about 20 light years above the equatorial plane and 28,000 light years from the center of this galaxy. The Sun orbits around the center of the galaxy in about 225-250 million years. The orbital speed of the solar system about the galaxy center is about 220 km/s.
Milky Way astrometry involves the determination of positions, motions, and distances in various wavelengths. Precision astrometry is potentially the most powerful means to study the fundamental astrophysics of the formation, evolution, and structure of the Milky Way. Due to our location and the material in the galaxy everything cannot be observed at all wavelengths. A brief history will be followed by the current observational methods and surveys, the structure and components of the Milky Way, and their motions.
The stars have been observed as long as humans have been on the Earth. The accuracy of the observations and the knowledge of the stars’ motions and distances have increased over the years. The stars have been used to determine annual changes in life cycles and means of navigation. Over the years the observations have revealed the phenomena of precession, aberration, refraction, proper motions, parallaxes, and radial velocities. Catalogs of star positions have been prepared with increasing accuracies and fainter stars. The studies of the stars, their positions, motions, and characteristics continue to this day. Observations are now made in many different wavelengths.
Until 1992 the celestial reference system was determined from the observations of the brighter, nearby stars and their motions. The origin was the equinox and the reference plane the celestial equator. All aspects of this reference frame were in motion, so the values had to be given for a specified epoch. The FK5 catalog for J2000.0 was the last of these reference frames.
Then Very Long Baseline Interferometers (VLBI) observed distant radio sources with much better accuracies (sub-milliarcsecond) and without proper motions, so that a fixed reference frame could be established. The International Celestial Reference Frame (ICRF) was determined and referred to the FK5 and dynamical reference frame of J2000.0 to the accuracy of those frames (Kovalevsky and Seidelmann, 2004).
At about the same time the Hipparcos astrometric satellite provided an optical catalog of about 120,000 stars down to 11th magnitude with milliarcsecond accuracies, so there was a catalog of optical stars on the ICRF (ESA, 1997). Additional modern catalogs include the Tycho 2 catalog (Hoeg et al., 2000), also based on the Hipparcos satellite observations but including about 2 million stars down to 11th magnitude, the UCAC catalogs reaching 16th magnitude (Zacharias et al., 2010), and the USNO B catalog of about a billion stars to 21st magnitude (Monet et al., 2003). The Southern Proper Motion Program has produced an observational catalog (SPM4) of over 103 million stars and galaxies between the south pole and -20 degrees declination complete to 17.5V magnitude (Girard et al., 2011). The 2MASS catalog of infrared observations of stars provides infrared magnitudes and astrometric positions of stars (Skrutskie et al., 2006). The PPMXL catalog contains mean positions and proper motions on the ICRS by combining the USNO B1.0 and 2MASS astrometry (Roeser et al., 2010).The SDSS-POSS proper motion database by Munn et al. (2004) provides an accuracy of ~ 3 mas/yr for tens of millions of stars. Thus, the kinematics of the Milky Way beyond the solar neighborhood can be studied. The NOMAD catalog gives the best astrometric data of stars, based on all available data (Zacharias, 2004).
There are a number of recent or planned surveys of the Milky Way. In the near infrared the photometric 2MASS and DENIS surveys were completed in the last decade. UKIDSS is nearing completion and a deeper survey with ESO’s VISTA telescope is getting started. Visual multi-band photometry was completed by the Sloan Digital Sky Survey (SDSS), and extended by SEGUE. Accurate SDSS multi-color measurements and photometric parallax relations have been used to estimate distances to tens of millions of main sequence stars (Juric et al., 2008; Ivezic et al., 2008; Bond et al., 2009). Pan-STARRS, which has been in operation for a year already, SkyMapper, and the Large Synoptic Survey Telescope (LSST) will provide deep multicolor imaging and time series data. RAVE, APOGEE, LAMOST, HERMES, and Gaia will provide spectroscopic surveys and the chemistry of stellar populations. The Japan Astrometry Satellite Mission for Infrared Exploration (JASMINE) will focus on z-band astrometry of the Galactic disk and bulge (Gouda, et al., 2005)
The Galaxy Evolution Explorer (GALEX) provides far-UV (FUV:1344-1786 A) and near-UV (NUV:1771–2831 A) photometry, which provides the hottest stellar objects, particularly hot white dwarfs. The GALEX All-Sky Imaging Survey (AIS) and Medium-depth Imaging Survey (MIS) contain 65.3 and 12.6 million UV sources, respectively (Bianchi, et al., 2011). Tables with specific information about Astrometric, Photometric, and Spectroscopic Surveys are given by Majewski (2009).
The image formed of the Milky Way depends as much on how it is viewed as on what is available to be seen. Hence, there are different images as observed in different wavelengths. It is the combination of different observations that is necessary to form a more complete image. To study regions of star formation, spiral structure, the inner bulge, and outer disk warp through the edge-on spiral galaxy one must observe in crowded and dusty fields. Due to interstellar extinction observations at longer wavelengths than optical (IR, radio) are necessary.
VLBI Exploration of Radio Astrometry (VERA) is dedicated to explore the 3–D structure and dynamics of the Milky Way. Since 2004 observations of maser sources have measured trigonometric parallaxes and proper motions. Parallaxes of star forming regions S269 and Orion KL have been measured as 5.28 kpc and 437 pc, respectively. Plans are to observe 70 maser sources every year (Honma et al., 2009). Reid et al. (2009) used 18 sources to analyze the spiral structure and rotation of the Milky Way (Brunthaler et al., 2010).
Milky Way Structure
Scientific studies of the Milky Way have led to the establishment of the characteristics of the structure of the Milky Way, a spiral galaxy, including its core, halo spiral arms, streams, satellite galaxies, and a central black hole. The Milky Way is an SB(rs) galaxy (de Vaucouleurs, 1970), which has been confirmed based on observations of the distribution of atomic, ionized and molecular hydrogen. The galaxy contains a bar near the center, spiral structure within the disk, a stellar halo with streams and “clouds”. The disk seems to have a thick disk of old stars on eccentric and inclined orbits and a thin disk of stars with higher ratios of Fe to O and Mg abundances with less eccentric and inclined orbits. This indicates a non-steady state, but models have to start with steady state. However, it is difficult to determine the number and positions of the spiral arms, length and position of the central bar, and the rotation curve from our position in the galaxy. So the distance to the Galactic center and the rotation speed are not known with great accuracy. Gravitational microlensing studies are providing new data on the distribution of normal stars in the very inner galaxy and the properties of the inner bar. The mass of the Milky Way remains uncertain within a factor of ~6 (5 x 10^11 to 3 x 10^12 solar masses) due to the lack of observational data in the outer regions where dark matter dominates (Bernal and Palomaress-Ruiz, 2011).
Galactic Center and Distance
The intense radio source Sagittarius A*, considered the center of the Milky Way, is confirmed to be a super massive black hole. With respect to Sgr A* the central stellar cluster has no rotation in the plane of the sky to 0.3 mas/yr, no translational motion to 0.1 mas/yr, and has rotation perpendicular to the plane of the sky along the Galactic plane (Yelda, et al.,2010).
Astrometric observations yield the distance and proper motion of a source. Combined with the position and line of sight velocity, this gives the 6-dimensional heliocentric phase space of the sources. So the peculiar motion of the Sun is needed to determine galactocentric motions. Hipparcos data have been used for the solar motion, but recent results support a different value (Brunthaler et al., 2010). As the Sun orbits the Galactic center, the position of Sgr A* moves by about 6 mas/yr and the zero of galactic longitude becomes time dependent. The motion of Sgr A* perpendicular to the galactic plane is extremely small(~ 1 km/s) and comparable with that expected of a supermassive black hole (Reid, 2008). ASTRA of the Keck Interferometer will study the mass and solar distance of the black hole at the gravitational center of the Milky Way (Pott et al., 2009).
The International Astronomical Union (IAU) recommended value for the local standard of rest rotational velocity is 220 km/s and the distance to the galactic center is 8.5 kpc. These values probably need substantial revision. Values in the literature vary between 184 and 272 km/s (Deason et al., 2011). Better values are 8.3 ± 0..23 kpc and 239, or 246, ±7 km/s for the galactic center distance and rotation velocity, respectively (Brunthaler et al., 2010). Using carbon stars with UCAC3 proper motions, a rotation curve of 210 ± 12 km/s and a galactocentric distance of 8.0 kpc is determined (Liu and Zhu, 2010).
The number of spiral arms is uncertain. Young clusters and OB associations were detected in three fragments of spiral arms, known as the Carina-Sagittarius, Perseus, and Cygnus-Orion arms (Efremov, 1989). The Cygnus-Orion arm may be a bridge between the other two arms. A large part of the Carina arm is outside the solar circle, so the distances to the gas clouds are well known. Studies based on HI and HII regions in the Galaxy result in a four arm spiral structure. The four arm model explains the positions of a dozen rich young clusters discovered from IR observations (Messineo et al., 2009) and the distances to maser sources determined from parallaxes from VLBI data (Reid et al., 2009).
The two strongest arms would originate at opposite ends of the bar and consist of a chain of super clouds (Efremov, 2011). Based on continuations of spiral arms at large distances from the center, it is thought that our galaxy is an asymmetric multi-arm spiral with some arms being transient features formed by gravitational instabilities in the gaseous disk. High velocities of star-forming regions have been found with VLBI data (Reid et al., 2009), which are difficult to explain if the spiral arms are grand design density waves. The inner arms are more symmetric and can be represented as logarithmic spirals. The bar and regular distances between superclouds in two clearly defined arms indicate the arms are related to spiral density waves. Young rich clusters are located near the spiral arms and four clusters are near the end of the bar, where star forming regions tend to exist. Outside the major spiral arms is the Monoceros ring, a ring of gas and stars which are either a perturbation of the outer disk or debris from an accreted satellite.
In 1927, Jan Oort derived a method to measure the Galactic rotation from a small number of stars in the local neighborhood. The Oort constants characterize the local rotation properties of our galaxy. From the Oort constants the orbital properties of the Sun, such as orbital velocity and period, and the local properties of the galactic disk, such as mass density and variation of rotational velocity as a function of distance from the galactic center, can be inferred. Different rotations of subsystems of the Milky Way are determined for the satellite galaxies, globular clusters, and Blue Horizontal Branch (BHB) stars. BHB stars in the halo have a metal rich component (Fe/H > -2) with a prograde rotation and metal poor component (Fe/H < -2) with a retrograde rotation. The metal rich component may be associated with a massive satellite and the metal poor component may characterize the primordial stellar halo. Since BHB stars are luminous with almost constant absolute magnitude (0.7) in the g band, they are excellent tracers of halo dynamics.
Thick and Thin Disks
From G-type dwarfs in the SDSS data, stars can be chemically classified and a bi-modal distribution is evident. Thus, the kinematics of thin- and thick- disk stars can be investigated as a function of metallicity and position up to 3 kpc from the galactic plane. The gradients of orbital rotational velocities for the thin-disk are -20 to -30 km/s dex and +40 to +50 km/s dex for the thick-disk population. The rotational velocity decreases with distance from the plane for both cases with slopes of 10 km/s/kpc. The older thick-disk stars have a strong trend of orbital eccentricity with metallicity, which is not the case for thin-disk stars. The scale height of the thick-disk is about 1 kpc, while the thin-disk is ~0.3 kpc (Lee et al., 2011). According to radial migration models, the metal–poor stars of the thin disk were born in the outer disk and move inward to the solar neighborhood. The metal-rich stars formed in the inner disk and migrate outward into the solar neighborhood. This radial movement can occur by “blurring” and “churning”. Blurring is the increase of eccentricities at a similar angular momentum due to scattering. Churning is triggered by resonant scattering at co-rotation due to transient spiral density waves, which transfers stars by changing their angular momenta without altering their eccentricities (Schonrich and Binney, 2009). In contrast, the velocity ellipsoid for halo stars is aligned with the spherical coordinate system and appears to be invariant. The velocity distribution of nearby (Z < 1 kpc) K/M stars is complex (Bond et al., 2010). The eccentricity distribution of stars in the thick disk does not support the radial migration. Competing explanations for the formation of the thick disk include satellite accretion, heating of a pre-existing thin disk during a merger, and gas-rich mergers (Casetti-Dinescu et al., 2011; Wilson et al., 2011; Dierickx et al., 2010).
Globular Clusters and Satellite Galaxies
Understanding of the satellite galaxy population has improved in recent years due to the deep census of satellite galaxies around the Milky Way and M31 (Majewski et al., 2007). There is evidence that the Milky Way globular cluster and satellite galaxy systems are rotating at ~ 50 and 40 km/s, respectively. There is an indication that the angular momentum vector of the satellite galaxies is inclined to the normal of the disk (Deason, Belokurov, and Evans, 2011). The proper motion measurements constrain the angular momenta orientations of the satellites, indicating evidence for a coherent motion. It is uncertain that this is consistent with the cold dark matter framework of structure formation. Rotational properties of halo populations might provide clues to their origin and evolution, and identify associations with a common formation history.
With line of sight velocities and proper motion measurements (Dinescu, et al., 1999 and Casetti-Dinescu, et al., 2007) the majority of globular clusters have a net prograde streaming motion. However, this does not mean all globular clusters have prograde rotation. There is no obvious correlation between rotational velocities and metallicity (Deason, et al., 2011). Most of the satellite galaxies have prograde rotation, although Fornax and Ursa Minor show retrograde rotation. The spatial distribution of satellites seems to be an inclined plane, which agrees with kinematic and spatial analysis. This may indicate that the satellites are part of a rotationally supported disk. More proper motion measurements are needed to confirm this.
A study of dark matter in the outer parts of the Galaxy, within the satellites of the Galaxy, and in the Local Group requires high precision astrometry of about 3 microarcsecond/yr proper motions. Progress has been made using conventional instruments, but in the future specialized astrometric instruments will be needed to obtain the highest precision.
The 2MASS and SDSS data have shown tidal debris streams in the halo and many low luminosity dSph galaxies. Progress is being made with positions and radial velocities of Sagittarius (Sgr) stream stars. The Sgr stream shows strong population gradients along its tidal arms. Proper motions in the Kaptyn Selected Area, along the trailing tail of the Sagittarius dwarf galaxy and the Monoceros Ring region (Casetti Dinescu et al., 2006), and west of the Virgo Stellar Stream (VSS) have been measured (Casetti-Dinescu et al., 2009). An over density in the background of NGC188 is suggested to be part of the Monoceros stream (Casetti-Dinescu et al., 2010). Measurements are reported of stars in the Anticenter Stream (ACS), which may or may not be associated with the Monoceros structure (Carlin et al., 2010). Future photometric surveys can be expected to go deeper in magnitude, but also to detect pulsational variables, which are standard candles (Majewski, 2009).
The Gaia satellite should map the spatial, metallicity, chemical, mass, and kinematic distributions of stars in the Milky Way’s disk and provide proper motion measurements of all dwarf galaxies and thousands of halo stars. Thus, it should answer some basic questions of astrophysics. The LSST will extend the maps to the halo edge, and obtain large samples of faint sources such as L, T, and white dwarfs (Ivezic, 2009). Gaia will improve proper motion measurements of classical satellites, but LSST will be needed for the ultrafaint satellites. Due to interstellar extinction, radio astronomy will be needed for the spiral structure of the Milky Way.
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