The Milky Way

The Milky Way (MW) has been seen for centuries in the sky and many cultures made it part of their folk and mythology, but only in the last century, its real nature as a galaxy was unveiled. With help of new technology and advantages in spatial and ground-based telescopes, astronomers have been able to study and decompose its structure, composition, and age. There are still many things to learn about the MW galaxy but with new and refined techniques, more clear findings in its age and composition can be found.

Milky Way a brief History

The Milky Way has been known since ancient times. For instance, in his book Edwin Hubble: Mariner of The Nebulae, Gale Christianson tells how the MW was described as a river of light that for Norse Mythology conducted to Valhalla. A second example, In Minding The Heavens The Story Of Our Discovery Of The Milky Way, Leila Belkora describes how the Greeks and Romans believed that the appearance of the MW as a result of Hera (Juno) refusing to nurse the mighty Heracles (Hercules) as an infant and missing his mouth causing the milk to spread throughout the sky. Because of this, the Greeks associated the word milk or gala with the MW which leads to the derivation of the word galaxy. In Figure 1, MW or via lactea (Road of Milk), as the Roman named it, may have derived its name from its appearance as a strip of light in the skies which are more clearly visible during summer and spring in the night of each hemisphere respectively.

In his book Starry Messenger (1610), Galileo properly described the nature of the MW as a combination of the light of all of the stars in the sky, stating that the MW was in fact a group of uncountable stars grouped together in clusters. Furthermore, Galileo wrote in 1618 that the MW did not exist in the sky and was actually a product of our vision because the eye was unable to distinguish the stars separately (Crowe, 1990). By the XVIII century, Immanuel Kant, a German philosopher claimed in his essay Cosmogony(1755) that the universe was endlessly great and shiny, filled with silent wonders where the nebulae were an island of universes. He also speculated using the principle of uniformity of nature that the stars in the MW gathered together in a common plane or disk, something that was only confirmed hundreds of years later by the most powerful astronomical instruments (Christianson, 1995).

With the advances in science and technology in the late 1700s, Herschel came close to defining the MW galaxy when he confirmed Kant’s argument that the MW was a disk with a bulge in the center. However, the true nature of the MW evaded scientists until the early XX century when Vesto Slipher observed that the speed of some spiral nebulae was faster than the escape velocity in our vicinity. In addition, Heber Curtis observed that novae (or temporary brighter stars) in the area of Andromeda were fainter than in our galaxy. Lastly, Edwin Hubble made one of the breakthroughs of modern times based on these observations when he concluded that galaxies were too far away from us and that it was impossible for them to be part of the MW. Therefore, the existence of the MW as a galaxy was proven true (Crowe, 1990).

Indeed, with new technology such as Hubble Space Telescope, images such as the Hubble deep field shows that space is crowded with galaxies made up of all colors, shapes, and angles (Figure 2) which allowed astronomers to infer that we must live in one of these galaxies. This was corroborated in 1977 when the Voyager 1 spacecraft was launched and traveled above the plane of the galaxy and scientists captured images of the entire extension of its spiral arms (Belkora, 2002).

Milky Way Structure

Since it was proven that the MW is indeed a galaxy, astronomers have been able to study, its structure, composition, and formation. However, these studies are not easy, principally because we are within the galaxy. However, a combination of simulation of large-scale structures, plus the observation of structures in the MW local neighborhood have helped to design the evolution and formation processes that constructed the MW (Gerhard, 2000). These studies show that the MW is a common barred spiral galaxy of approximately 100,000 light-years of extension or 150,000 light-years according to new findings (Xu et al. 2015) with two major arms, two minor arms, and two smaller spurs, in which one is allocated the solar system (Efremov, 2010). Then, the structure of the MW (Figure 3) can be divided into the galactic disk (thin and thick), bulge and bar, galactic center, and halo.

Galactic Disks

While the MW is a typical galaxy, it is quite challenging to characterize the velocity, metallicity, and age compared to distant galaxies. One factor is that the MW has a luminosity ratio of the bulge to the disk of 1:5 (Kent et al. 1991) but in most common cases, the galaxies are bulge-dominated (Kauffmann et al 2003). Therefore, the MW can be described two disks: one thin disk and one thick disk (Rix & Bovy, 2013).

Plenty of younger stars are delineated by cepheids (Luck et al., 2006). In the outer disk, there are older stars. In the thin disk, the relationship between the stellar age, metallicity, and velocity of dispersion are visible in the studies of the Galactic thin disk, but the age-metallicity relation (AMR) is not clear in the vicinity of the solar system where metallicity variants are seen across all ages. This mixing may be related to the spiral pattern that allows the stars to move from the inner disk to the outer disk. (Freeman 2011; Sellwood & Binney 2002; Nordstrom et al., 2004)

The thick disk, a common characteristic of spiral galaxies, has a vertical velocity of dispersion of approx 40 km/s, and its size has been estimated to be 1000 pc (Gilmore & Reid, 1983). The thick disk has stars older than 10 Gyr which are metal-poor compared to the stars found in the thin disk. This phenomenon is due to the fact that the thick disk was formed quickly in ~1 Gyr (Gilmore et al., 1995; Ivezic et al. 2008). But there are several theories that suggest that the thick disk was formed by accretion debris (Abi et al., 2003), heating of the thin disk (Kroupa, 2002), or stars with energetic orbits migrating from the inner galaxy to the thick disk (Schönrich & Binney, 2009). While all these theories cannot be discarded, the real origin of the thick disk in the MW will be important to study for future astronomers.

Galactic Bulge, Bar, and the Center of the Galaxy

Observational studies made by near-infrared photometry and star counting of red clump giants have shown that MW has a barred bulge with a bar angle ranging from 20 to 30 degrees in sun line-of-sight and a longitude |l| < 10◦ (Binney et al 1991; Stanek et al., 1994, 1997; Babusiaux & Gilmore 2005; Rattenbury et al. 2007). The stars in the bulge seem to be older which can be associated with rapid star formation (Melendez et al. 2008). N-body simulations and survey data have shown that bulge has a near cylindrical rotation (Athanassoula, 2005; Howard et al., 2009)

Thanks to the Hubble Space Telescope and Adaptive Optics in ground-based telescopes, astronomers have been able to study the galactic center (GC) of the MW. Similar to other galaxies, it encloses a nuclear star cluster of 2.5 × 107 M⊙, at a distance of ∼8 kpcs (Genzel et al., 2010; Boehle et al., 2016; Schodel et al., 2014). A compact radio source is known as Sagittarius A* (SgrA*) has a mass of 4.3±0.3×106M⊙ and orbits the stars close to it, which contributed to the assumption that the MW is composed of a supermassive black hole (SMBH) (Reid et al, 2003; Ghez et al., 2008; Gillessen et al., 2009a) The stars in the GC have experienced several star formation periods through their lifetime, and the most recent period is thought to have occurred 10 Myr ago. Most of the stars are old and it is estimated that 80% formed 5 Gyr ago (Genzel et al. 2010; Pfuhl et al., 2011)

The Halo

The galactic halo is a complex system that surrounds the galaxy with its different components that exhibit unique spatial distributions, metallicity, and kinematics (Searle et al. 1978). This makeup suggests a hierarchical buildup of the galaxy. For example, the globular cluster located between 15–20 kpc of the galactic center is older than clusters located farther away (Lee et al., 1994; Dotter et al., 2010). In recent studies, a sample of more than 130,000 blue horizontal-branch stars from the Sloan Digital Sky Survey (SDSS) helped create an aged map of the halo with an extension of 60 kpcs from the GC (Carollo et al., 2016) The central region of the map showed clustering of very old stars ranging from 11.5 to 12.5 Gyr (Bond et al.,2013) at a distance of 10 to 15 kpc from the GC. Outside of that area, the halo has primarily stars of an average age of 11 Gyrs (Carollo et al., 2016). The map helped to resolve structure in the halo that was identified only by its density. These structures are members of the Sagittarius stream and their age ranges from ~ 9.5G yr to ~11 Gyr which is in concord with previous photometric and spectroscopic surveys (De Boer et al., 2015; Carollo et al., 2016)

Upcoming Research

While astronomers have a complete view of the latest description of the MW (Figure 4); the evolution, stellar components, and formation of the MW are vigorously research in modern astronomy. Large scale surveys such as GAIA-ESO (GES)(Gilmore et al. 2012), GALAH for Galactic Archeology (De Silva et al. 2015), and Apache Point Observatory Galactic Evolution Experiment (APOGEE) (Holtzman et al. 2015) are the latest ambitious project to study MW deepest secrets (Martell, 2015).

The GAIA-ESO surveys use the 8-meter Very Large Telescope (VLT) in Chile and a companion GAIA space observatory (Prusti 2012) to survey more than a billion stars in our galaxy, defining their distances and 3D space velocities (Brown et al., 2016). In a similar way, the GALAH survey studies the chemical abundance of 29 elements found in more than one million stars in the galactic disk (Bland-Hawthorn et al., 2015) The data is provided by HERMES spectrograph at the 3.9m Anglo-Australian Telescope (AAT) in Australia.

The APOGEE survey focuses its research on red giant stars in the MW using the Sloan telescope in the USA. This survey scans the infrared wavelength region, collecting data of stars across the complete galaxy (Holtzman et al., 2015). These three projects have formed an alliance by working together and combining the observations of different regions of the MW. For example, they combine data for GALAH and GAIA-ESO for local and remote stars with similar components, or GALAH-APOGEE will compare stars in the solar system vicinity. Moreover, GAIA-ESO, GALAH, and APOGEE are scheduled to observe the same location allowing them to collect survey data and merge their results (Martell, 2015)

In the end, the Milky Way has been the source of inspiration and wonder of humankind since ancient times. But it was the curiosity of Edwin Hubble that motivated scientists to discover its real constitutions, and since then, now and in many more years in the future, astronomers will not stop until they decode the most fascinating secrets related to its origins and ours.

Figure 1: Milky Way as is seen in the northern and southern hemispheres. Credit: NASA; ESA; A. Mellinger/ Minding The Heavens The Story Of Our Discovery Of The Milky Way Leila Belkora

Figure 2: The Hubble Ultra Deep Field composed images of ultraviolet, visible, and infrared light. Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

Figure 3: Milky Way schematic structure: Edge-on view graphic representation Credits: Max Planck Institute for Astronomy, Max Planck Research Magazine 2016

Figure 4: Milky way depicted with the latest information about its shape and structure Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

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