It Pays for a Galaxy to Slowly Sip, Not Gulp
Two years ago at a major conference, a medical doctor/researcher who specializes in nutrition approached me. He took one look and said, “I know what you eat.” Amazingly, he identified nearly every item in my diet. His analysis gives credence to the adage, “You are what you eat.”
What is true for us humans is also true for spiral galaxies such as our Milky Way Galaxy. By carefully examining the different stellar populations of a spiral galaxy, astronomers can determine the galaxy’s “diet” over the past ten-plus billion years. What a galaxy consumes and how it does so has repercussions for advanced life.
Milky Way Galaxy’s Stellar Populations
The Milky Way Galaxy (MWG) acquired some of its stars by absorbing (think of drinking a large shake) smaller galaxies. The stars in the MWG are found in four distinct locales: the central bulge, the thin disk, the thick disk, and the halo. The distribution of stars in the halo and thick disk are especially sensitive to past accretion events—events where the MWG absorbed (consumed) a dwarf galaxy or a large gas stream.
The elemental composition of the stars in the stellar halo measures the same as that of the thick disk stars.1 The stars that populate both the thick disk and the stellar halo (if one excludes the high velocity runaway stars that escaped from the central bulge and thin disk) all measure to be old. These similarities imply that the stellar halo and the thick disk formed from the same event that occurred long ago in the MWG’s history.
An analysis of the APOGEE and Gaia DR2 surveys of stellar populations reveals that the MWG experienced an unusually intense accretion event (a gulp) more than 10 billion years ago.2 A deeper analysis of these surveys shows that 10–11 billion years ago, the MWG accreted either a medium-sized galaxy or a high-mass dwarf galaxy that astronomers call the Gaia-Enceladus-Sausage (GES).3 This accretion event explains the observed properties of the MWG’s stellar halo.4
A team of five astronomers led by G. C. Myeong found evidence for a second ancient accretion event (another gulp).5 They showed that just before the MWG accreted GES, it accreted a slightly smaller galaxy that they named the Sequoia Galaxy.
These ancient accretion events grew the MWG to a sufficient size (you are what you eat) 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, a nearby high-mass dwarf galaxy, would have so warped its spiral arm configuration as to make it an unfit habitat for advanced life.6
Our Exceptional Galaxy
Compared to other spiral galaxies of the same mass as the MWG, the MWG is exceptional in several ways. Meanwhile, its sister galaxy, the Andromeda Galaxy, is normal.
1. The ratio of the MWG’s stellar mass to its total mass is lower by a factor of two compared to other local spiral galaxies of its size.7 The Andromeda Galaxy has almost exactly twice the stellar mass of the MWG.
2. The disk angular momentum (product of the disk radius and the disk rotational velocity) for the MWG is only 40 percent of that for the Andromeda Galaxy.8 Other local spiral galaxies in the mass range of the MWG possess disk angular momenta that average a little more than double that of the MWG.9
3. The MWG has a much smaller disk radius. The disk scale length for the Andromeda Galaxy = 6.08 ± 0.09 kiloparsecs (19,830 ± 290 light-years).10 The disk scale length for the MWG = 3.00 ± 0.22 kiloparsecs (97,800 ± 700 light-years).11 Likewise, other local spiral galaxies in the MWG’s mass range possess disk scale lengths about double the MWG’s.12
4. Where the MWG is most exceptional compared to other spiral galaxies in its mass range is in the properties of the stars in its outskirts (stars located 17,000–100,000 light-years from its galactic center). Stars in the outskirts of the MWG possess an abundance of elements heavier than helium that are 3 times lower than for stars in the outskirts of other local spiral galaxies in the same mass range.13 Indeed, the MWG’s stellar halo is clearly divided into two components—an inner and an outer stellar halo. Stars in the outer stellar halo have 3 times fewer elements heavier than helium than inner stellar halo stars.14
Stars in the outskirts of the MWG are also much more uniform in their elemental abundance profiles, with no observed dependence on distance from the galactic center.15 The Andromeda Galaxy halo stars, typical of the halo stars in other local spiral galaxies, show a decline in abundance of heavier-than-helium elements of a factor of 8 from 30,000–330,000 light-years from its galactic center.16 All local spiral galaxies, except the MWG, show a trend of increasing abundance of elements heavier than helium with the rotation velocity of the galaxy’s disk.17 Unlike all other local spiral galaxies, the MWG’s outer halo stars show a sharp drop-off in stellar number density beyond 90,000 light-years from the galactic center.18 This drop-off becomes even steeper at distances greater than 160,000 light-years from the galactic center.19
All these properties of the stars in the MWG’s outskirts imply that such stars have been dynamically undisturbed for a very long time. Much more so than other stars in a galaxy, stars in the outskirts are sensitive to the residual effects of past merging events “with the outer regions of the halo being particularly information rich.”20
A Just-Right Galaxy Diet
The observed features of the MWG’s outskirt stars require that the MWG has experienced no merger event (big gulp) within the past 10 billion years with another galaxy possessing a total mass equal to or more massive than 1 billion solar masses,21 which is less than 0.1 percent of the MWG’s total mass. Meanwhile, the observed features of outskirt stars in other spiral galaxies of approximately the same mass as the MWG imply that they have experienced several major merging events during the past several billion years.22
An outstanding feature of the MWG emerges through this research. Throughout its past 10 billion years, our galaxy 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.
Sustaining spiral structure is key. Unlike other known spiral galaxies, throughout the past 10–11 billion years the MWG has continuously sipped rather than intermittently gulped. This unique history explains, in large part, why today the MWG has such outstandingly symmetrical spiral arms. It also explains why the MWG is a large spiral galaxy like no other we know, a spiral galaxy uniquely fit to host advanced life.
Featured image: Constructed Map of the Milky Way Galaxy. Image credit: NASA/JPL-Caltech (R. Hurt)
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Endnotes
- L. I. Mashonkina et al., “Abundances of α-Process Elements in Thin-Disk, Thick-Disk, and Halo Stars of the Galaxy: Non-LTE Analysis,” Astronomy Reports 63, no. 9 (September 2019): 726–38, doi:10.1134/S1063772919090063.
- Ricardo P. Schiavon et al., “The Building Blocks of the Milky Way Halo Using APOGEE and Gaia or Is the Galaxy a Typical Galaxy?” Star Clusters: From the Milky Way to the Early Universe. Proceedings of the International Astronomical Union 351 (2020): 170–73, doi:10.1017/S1743921319007889.
- Chris B. Brook et al., “Explaining the Chemical Trajectories of Accreted and In-Situ Halo Stars of the Milky Way,” Monthly Notices of the Royal Astronomical Society (April 15, 2020), posted online ahead of publication, doi:10.1093/mnras/staa992.
- 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 (April 22, 2020), posted online ahead of publication, doi:10.1093/mnras/staa1090.
- 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): 1235–47, doi:10.1093/mnras/stz1730.
- 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; Kenjo 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 2012): 1957–74, doi:10.1111/j.1365-2966.2012.20621.x.
- 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.
- Hammer et al., “The Milky Way, an Exceptionally Quiet Galaxy.”
- Hammer et al., “The Milky Way, an Exceptionally Quiet Galaxy.”
- Pauline Barmby et al., “Dusty Waves on a Starry Sea: The Mid-Infrared View of M31,” Astrophysical Journal Letters 650, no. 1 (October 10, 2006): L45–L49, doi:10..1086/508626.
- Paul J. McMillan, “Mass Models of the Milky Way,” Monthly Notices of the Royal Astronomical Society 414, no. 3 (July 2011): 2446–57, doi:10.1111/j.1365-2966.2011.18564.x.
- Hammer et al., “The Milky Way, an Exceptionally Quiet Galaxy.”
- Hammer et al., “The Milky Way, an Exceptionally Quiet Galaxy.”
- 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.
- Charlie Conroy et al., “Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey,” Astrophysical Journal 887, no. 2 (December 20, 2019): id. 237, doi:10.3847/1538-4337/ab5710; Branimir Sesar, Mario Jurić, and Željko Ivezić, “The Shape and Profile of the Milky Way Halo as Seen by the Canada-France-Hawaii Telescope Legacy Survey,” Astrophysical Journal 731, no. 1 (April 10, 2011): id. 4, doi:10.1088/0004-637X/731/1/4.
- Karoline M. Gilbert et al., “Global Properties of M31’s Stellar Halo from the SPLASH Survey. II. Metallicity Profile,” Astrophysical Journal 796, no. 2 (December 1, 2014): id. 76, doi:10.1088/0004-637X/796/2/76; Rodrigo A. Ibata et al., “The Large-Scale Structure of the Halo of the Andromeda Galaxy. I. Global Stellar Density, Morphology, and Metallicity Properties,” Astrophysical Journal 780, no. 2 (January 10, 2014): id. 128, doi:10.1088/0004-637X/780/2/128.
- M. Mouhcine et al., “The Metallicities of Luminous, Massive Field Galaxies at Intermediate Redshifts,” Monthly Notices of the Royal Astronomical Society 369, no. 2 (June 2006): 891–908, doi:10.1111/j.1365-2966.2006.10360.x.
- Sesar, Jurić, and Ivezić, “The Shape and Profile of the Milky Way Halo …
- A. J. Deason et al., “Touching the Void: A Striking Drop in Stellar Halo Density beyond 50 kpc,” Astrophysical Journal 787, no. 1 (May 20, 2014): id. 30, doi:10.1088/0004-637X/787/1/30.
- Andreea S. Font et al., “The ARTEMIS Simulations: Stellar Haloes of Milky Way-Mass Galaxies,” accepted for publication by Monthly Notices of the Royal Astronomical Society (2020), preprint arXiv:2004.01914v1.
- 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.
- Christopher J. Conselice et al., “A Direct Measurement of Major Galaxy Mergers at z ≤3,” Astronomical Journal 126, issue 3 (September 2003): 1183–1207, doi:10.1086/377318; David L. Block et al., “An Almost Head-On Collision as the Origin of Two Off-Centre Rings in the Andromeda Galaxy,” Nature 443, no. 7113 (October 2006): 832–34, doi:10.1038/nature05184; Gilbert et al., “Global Properties of M31’s Stellar Halo.”