Title: The Gaia-ESO Survey: A Quiescent Milky Way with no Significant Dark/Stellar Accreted Disk
Authors: G. R. Ruchti, J. I. Read, S. Feltzing, A. M. Serenelli, P. McMillan, K. Lind, T. Bensby, M. Bergemann, M. Asplund, A. Vallenari, E. Flaccomio, E. Pancino, A. J. Korn, A. Recio-Blanco, A. Bayo, G. Carraro, M. T. Costado, F. Damiani, U. Heiter, A. Hourihane, P. Jofre, G. Kordopatis, C. Lardo, P. de Laverny, L. Monaco, L. Morbidelli, L. Sbordone, C. C. Worley, S. Zaggia
First Author’s Institution: Lund Observatory, Department of Astronomy and Theoretical Physics, Lund, Sweden
Status: Accepted for publication in MNRAS
Galaxy-galaxy collisions can be quite spectacular. The most spectacular occur among galaxies of similar mass, where each galaxy’s competing gravitational forces and comparable reserves of star-forming gas are strong and vast enough to contort the other into bright rings, triply-armed leviathans, long-tailed mice, and cosmic tadpoles. Such collisions, as well as their tamer counterparts between galaxies with large differences in mass—perhaps better described as an accretion event rather than a collision—comprise the inescapable growing pains for adolescent galaxies destined to become the large galaxies adored by generations of space enthusiasts, a privileged group of galaxies to which our home galaxy, the Milky Way, belongs.
What’s happened to the hapless galaxies thus consumed by the Milky Way? The less massive among these unfortunate interlopers take a while to fall irreversibly deep into the Milky Way’s gravitational clasp, and thus dally, largely unscathed, in the Milky Way’s stellar halo during their long but inevitable journey in. More massive galaxies feel the gravitationally tug of the Milky Way more strongly, shortening the time it takes the interloper to orbit and eventually merge with the Milky Way as well as making them more vulnerable to being gravitationally ripped apart. But this is not the only gruesome process the interlopers undergo as they speed towards their deaths. Galaxies whose orbits cause them to approach the dense disk of the Milky Way are forced to plow through the increasing amounts of gas, dust, stars, and dark matter they encounter. The disk produces a drag-like force that slows the galaxy down—and the more massive and/or dense the galaxy, the more it’s slowed as it passes through. Not only so, the disk gradually strips the unfortunate galaxy of the microcosm of stars, gas, and dark matter it nurtured within. The most massive galaxies—those at least a tenth of the mass of the Milky Way, the instigators of major mergers—accreted by the Milky Way are therefore dragged towards the disk and are forced to deposit their stars, gas, and dark matter preferentially in the disk every time their orbits brings them through the disk. The stars deposited in the disk in such a manner are called “accreted disk stars,” and the dark matter deposited forms a “dark disk.”
The assimilated stars are thought to compose only a small fraction of the stars in the Milky Way disk. However, they carry the distinct markings of the foreign worlds in which they originated. The accreted galaxies, lower in mass than the Milky Way, are typically less efficient at forming stars, and thus contain fewer metals and alpha elements produced by supernovae, winds of some old red stars, and other enrichment processes instigated by stars. Some stars born in the Milky Way, however, are also low in metals and alpha elements (either holdovers formed in the early, less metal- and alpha element-rich days of the Milky Way’s adolescence or formed in regions where gas was not readily available to form stars). There is one key difference between native and alien stars that provide the final means to identify which of the low metallicity, low alpha-enriched stars were accreted: stars native to the Milky Way typically form in the disk and thus have nearly circular orbits that lie within the disk, while the orbits of accreted stars are more randomly oriented and/or more elliptical (see Figure 1). Thus, armed with the metallicity, alpha abundance, and kinematics of a sample of stars in the Milky Way, one could potentially pick out the stars among us that have likely fallen from a foreign world.
A search for the accreted disk allows us to peer into the Milky Way’s past and provides clues as to the existence of a dark disk—a quest the authors of today’s paper set out to do. Their forensic tool of choice? The Gaia-ESO survey, an ambitious ground-based spectroscopic survey to complement Gaia, a space-based mission designed to measure the position and motions of an astounding 1 billion stars with high precision, from which a 3D map of our galaxy can be constructed and our galaxy’s history untangled. The authors derived metallicities, alpha abundances, and the kinematics of about 7,700 stars from the survey. Previous work by the authors informed them that the most promising accretion disk candidates would have metallicities no more than about 60% that of the Sun, an alpha abundance less than double that of the Sun, and orbits that are sufficiently non-elliptical and/or out of the plane of the disk. The authors found about 4,700 of them, confirming the existence of an accreted stellar disk in the Milky Way.
Were any of these stars deposited in spectacular mergers with high-mass galaxies? It turns out that one can predict the mass of a dwarf galaxy by its average metallicity. The authors estimated two bounds on the masses of the accreted galaxies: one by assuming that all the stars matching their accreted disk stars criteria were bona fide accreted stars, and the other by throwing out stars that might belong to the disk—those with metallicites greater than 15% of the Sun’s. The average metallicity of the first subset of accreted stars was about 10 times less than the Sun’s, implying that they came from galaxies with a stellar mass of 10^8.2 solar masses. Throwing out possible disk stars lowered the average metallicity to about 5% of the Sun’s, implying that they originated in galaxies with a stellar mass of 10^7.4. In comparison, the Milky Way’s stellar halo is about 10^10 solar masses. Thus it appears that the Milky Way has, unusually, suffered no recent major mergers, at least since it formed its disk about 9 billion years ago. This agrees with many studies that have used alternative methods to probe the formation/accretion history of the Milky Way.
The lack of major mergers also implies that the Milky Way likely does not have a disk of dark matter. This is an important finding for those searching for dark matter signals in the Milky Way, and one which implies that the Milky Way’s dark matter halo is oblate (flattened at the poles) if there is more dark matter than we’ve estimated based on simplistic models that assumed the halos to be perfectly spherical.
Cover image: The Milky Way, LMC, SMC from Cerro Paranal in the Atacama Desert, Chile. [ESO / Y. Beletsky]