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)