Understanding how our galaxy formed—including how it absorbed smaller galaxies through merger events—helps astronomers learn how our spiral galaxy differs from other known spiral galaxies. This kind of research involves galactic archaeology, a discipline that continues to reveal features of our galaxy’s history that point to purposeful design.

Searching for Relevant Data
Archaeology is a scientific discipline dedicated to the recovery and study of ancient artifacts and landscape structures to determine the past history of humanity and human civilization. This definition was formulated by anthropologists. However, the scientific recovery and study of ancient artifacts and structures is not limited to anthropology. In both geology and astronomy, there are researchers who specialize in the study of terrestrial and astronomical artifacts with the goal of determining—in as much detail as possible—the past history or certain components of the earth and universe.

A crucial component of any discipline of archaeology is the dating of artifacts and structures. For our Milky Way Galaxy (MWG), the biggest barrier to galactic archaeology has been the dating of relevant galactic components. Nowhere is this more problematic than it is for the first 10 billion years of our galaxy’s history.

Subgiant Stars
In an effort to better determine the assembly history of the MWG, two astronomers at the Max Planck Institute for Astronomy in Heidelberg, Germany, endeavored to build a database of old stars with accurately determined ages. They chose subgiant stars.

Subgiant stars are stars in an intermediate phase of their evolutionary history. In their nuclear burning history, they are in between dwarf or main sequence stars that are fusing hydrogen into helium in their cores and giant stars that are no longer able to fuse hydrogen into helium. While subgiant stars no longer fuse hydrogen into helium in their cores, they continue to do so in shells just outside their cores. Shell hydrogen fusion causes subgiant stars to dramatically brighten in a highly predictable manner. Hence, the luminosity of a subgiant star yields a direct measure of its age. Furthermore, spectra of subgiant stars accurately reveal their birth material composition, even in those cases where the subgiant stars are more than 10 billion years old. For these reasons, subgiant stars are a galactic archaeologist’s best tool for determining our galaxy’s past history.

Subgiant Survey
As attractive as subgiants are for galactic archaeology, they pose a problem owing to their rarity. Until now, galactic archaeologists lacked a large enough database of well-measured subgiants to determine a reliable past history of our MWG.

A breakthrough for galactic archaeologists arrived in the third data release of the Gaia mission2 and the seventh data release of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectroscopic survey.3 The two Max Planck astronomers, Maosheng Xiang and Hans-Walter Rix, analyzed these data releases to produce a set of 250,000 subgiant stars. These subgiants span an age range from 1.5 to 13.5 billion years ago. The uncertainty in the ages is only 7.5%.

Galactic Archaeology Discoveries
The distribution of subgiant stars in Xiang and Rix’s data set showed that our galaxy’s stellar component splits into two disjointed parts. The two parts are separated from one another in age by about 8 billion years. They are also separated in location. The younger part resides in the MWG’s core and thin disk (containing dust, gas, and stars). The older part resides in the MWG’s halo and thick disk (composed of stars).

The thin disk has an average thickness of about 1,000 light-years.4 The thick disk’s average thickness is about 3,000 light-years.5 Both disks extend out to the same radial distance from the galactic center.6

Xiang and Rix’s research showed that the thick disk began to take shape as early as 13 billion years ago, only 800 million years after the big bang creation event and 2 billion years earlier than the inner halo’s final assembly. The researchers found that the star formation rate for the thick disk reached a pronounced peak 11.2 billion years ago. This peak corresponds to an accretion event (accumulation by gravitational attraction) indicated by the observed properties of the MWG’s stellar halo.7 Either a medium-sized galaxy or a high-mass dwarf galaxy that astronomers call the Gaia-Sausage-Enceladus (GSE) galaxy merged with the MWG. The observation that scattered, old, thick disk stars with little angular momentum exclusively manifest ages older than 11 billion years is strong evidence that the merging of the MWG with GSE was completed 11 billion years ago. This date is a billion years earlier than previous estimates based on an estimate of the lower age limit of halo stars.8

A team of five astronomers led by G. C. Myeong found evidence for a second ancient accretion event.9 They showed that just before the MWG accreted GES, it accreted a slightly smaller galaxy that they named the Sequoia Galaxy.

Design Implications
The two ancient accretion events grew the MWG to a sufficient size where it could maintain its structure in spite of gravitational interactions with smaller galaxies. If it were any smaller now, its gravitational interaction with the Large Magellanic Cloud, the third most massive galaxy in the Local Group10 and only 163,000 light-years away, would have so warped its spiral arm configuration as to make it unfit for advanced life.11

Among spiral galaxies in its mass range, the MWG is most exceptional in the properties of its outskirt stars. Astronomers define outskirt stars as stars residing above and below a spiral galaxy’s disks 17,000–100,000 light-years from the galaxy’s center. The MWG’s outskirt stars possess an abundance of elements heavier than helium three times lower than for outskirt stars of other spiral galaxies in the same mass range.12 Unlike other similarly sized spiral galaxies, the MWG’s stellar halo is clearly divided into two components—an inner and an outer stellar halo. Outer stellar halo stars have three times fewer elements heavier than helium than inner stellar halo stars.13

These and other observed properties of the MWG’s outskirt stars imply that they have been dynamically undisturbed for a very long time. The MWG’s outskirt stars’ features require that the MWG has experienced no merger event within the past 11 billion years with another galaxy possessing a total mass equal to or more massive than 1 billion solar masses.14

This “undisturbed” quality appears to be fine-tuned. An outstanding MWG feature is that throughout its past 11 billion years it has not suffered any merger events of sufficient magnitude to alter its spiral structure in any life-threatening manner. Nevertheless, it has accreted sufficient streams of gas and a sufficient number and rate of low-mass dwarf galaxies to sustain its spiral structure. Without such regular accretions a spiral galaxy’s spiral structure collapses after only 3–4 galactic rotations.

Unlike other known spiral galaxies, the MWG continuously “sips” rather than intermittently “gulps” other galaxies. This unique history explains, in large part, why the MWG has such outstandingly symmetrical, well-spaced spiral arms with only a few feathers and spurs (of dust and gas) between the spiral arms. It also explains why the MWG is a large spiral galaxy like no other we know, a spiral galaxy uniquely designed to host advanced life.

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Endnotes

1. Maosheng Xiang and Hans-Walter Rix, “A Time-Resolved Picture of Our Milky Way’s Early Formation History,” Nature 603 (March 24, 2022): 599–603, doi:10.1038/s41586-022-04496-5.
2. Gaia Collaboration, “Gaia Early Data Release 3. Summary of the Contents and Survey Properties,” Astronomy & Astrophysics 649 (May 2021): id. A1, doi:10.1051/0004-6361/202039657.
3. Zhao Gang et al., “LAMOST Spectral Survey—An Overview,” Research in Astronomy and Astrophysics 12, no. 7 (July 2012): 723–34, doi:10.1088/1674-4527/12/7/002; A.-L. Luo, et al., “VizieR Online Data Catalog: LAMOST DR7 Catalogs (Luo+, 2019),” VizieR On-Line Data Catalog: V/156 (March 2022), https://ui.adsabs.harvard.edu/abs/2022yCat.5156….0L/abstract.
4. Mario Jurić et al., “The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution,” Astrophysical Journal 673, no. 2 (February 1, 2008): 864–914, doi:10.1086/523619.
5. Jurić et al., “The Milky Way Tomography.”
6. Chengdong Li and Gang Zhao, “The Evolution of the Galactic Thick Disk with the LAMOST Survey,” Astrophysical Journal 850, no. 1 (November 20, 2017): id. 25, doi:10.3847/1538-4357/aa93f4.
7. Lydia M. Elias et al., “Cosmological Insights into the Assembly of the Radial and Compact Stellar Halo of the Milky Way,” Monthly Notices of the Royal Astronomical Society 495, no. 1 (April 22, 2020): 29–39, doi:10.1093/mnras/staa1090.
8. Amina Helmi et al., “The Merger that Led to the Formation of the Milky Way’s Inner Stellar Halo and Thick Disk,” Nature 563 (November 1, 2018): 85–88, doi:10.1038/s41586-018-0625-x; Josefina Montalbán et al., “Chronologically Dating the Early Assembly of the Milky Way,” Nature Astronomy 5 (July 2021): 640–47, doi:10.1038/s41586-018-0625-x.
9. G. C. Myeong et al., “Evidence for Two Early Accretion Events that Built the Milky Way Stellar Halo,” Monthly Notices of the Royal Astronomical Society 488, no. 1 (September 2019): 1235–1247, doi:10.1093/mnras/stz1730.
10. Jorge Peñarrubia et al., “A Timing Constraint on the (Total) Mass of the Large Magellanic Cloud,” Monthly Notices of the Royal Astronomical Society: Letters 456, no. 1 (February 2016): L54–L58, doi:10.1093/mnrasl/slv160; Chervin F. P. Laporte et al., “Response of the Milky Way’s Disc to the Large Magellanic Cloud in a First Infall Scenario,” Monthly Notices of the Royal Astronomical Society 473, no. 1 (January 2018): 1218–30, doi:10.1093/mnras/stx2146; A. J. Deason et al., “Satellites of LMC-Mass Dwarfs: Close Friendships Ruined by Milky Way Mass Haloes,” Monthly Notices of the Royal Astronomical Society 453, no. 4 (November 2015): 3568–74, doi:10.1093/mnras/stv1939.
11. Laporte et al., “Response of the Milky Way’s Disc”; Kenji Bekki, “The Influences of the Magellanic Clouds on the Galaxy: Pole Shift, Warp, and Star Formation History,” Monthly Notices of the Royal Astronomical Society 422, no. 3 (May 10, 2012): 1957–74, doi:10.1111/j.1365-2966.2012.20621.x.
12. François Hammer et al., “The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies,” Astrophysical Journal 662, no. 1 (June 10, 2007): 322–34, doi:10.1086/516727.
13. Daniela Carollo et al., “Two Stellar Components in the Halo of the Milky Way,” Nature 450 (December 13, 2007): 1020–25, doi:10.1038/nature06460.
14. Andreea S. Font et al., “Dynamics and Stellar Content of the Giant Southern Stream in M31. II. Interpretation,” Astronomical Journal 131, no. 3 (March 2006): 1436–44, doi:10.1086/499564; Andreea S. Font et al., “Phase-Space Distributions of Chemical Abundances in Milky Way-Type Galaxy Halos,” Astrophysical Journal 646, no. 2 (August 1, 2006): 886–98, doi:10.1086/505131.