The alignment between galaxy spins and the large-scale structure of the universe reveals the processes by which different components of galaxies form.
Like our own Milky Way, most galaxies have two components: an extended disk in which new stars form from gas and a central bulge of mostly older stars that grows over time.
An observational study recently published in Monthly Notices of the Royal Astronomical Society found that the size of the galaxies’ bulge changes how their spins align with the surrounding structure.
The large-scale structure of the universe is traced by the distribution of galaxies. This ‘cosmic web’ consists of giant filamentary structures linking massive clusters of galaxies.
The new study finds that galaxies with bigger bulges tend to spin perpendicular to the filaments in which they are embedded, while galaxies with smaller bulges tend to spin parallel to these filaments.
“It all relates to the mass of the bulge,” says astrophysicist Dr Stefania Barsanti from the Australian National University, lead author of the paper and a member of the ASTRO 3D Centre of Excellence.
“Galaxies which are mostly disk, with a low-mass bulge, tend to have their spin axis parallel to the nearest filament. This is because they form mainly from gas falling onto the filament and ‘rolling it up’. Galaxy bulges grow when galaxies merge, generally as they move along the filament. So, mergers also tend to ‘flip’ the alignment between the galaxy spin and the filament from parallel to perpendicular.”
“We think that mergers must be more likely as galaxies move along the filaments towards each other. The direction of these mergers drives the spin flip,” says Prof. Scott Croom, an astronomer at the University of Sydney and a co-author on the paper.
This discovery sheds light on the formation of two main components of galaxies, and how they relate to the large-scale structures and motions of matter in the cosmic web.
“Our motivation was to try to understand why galaxies spin and how they acquire their angular momentum from the material that forms them,” says Dr Barsanti.
“Through this study, we can understand how mergers play an important role in the formation of galaxies, both the central bulge component and the spin flipping,” she says. “This points to particular formation channels for how galaxies start to spin and how the spin changes as the galaxy evolves.”
Although this evolution has been suggested by computer simulations, this study is the first time scientistic have used direct observation to confirm the growth of a galaxy’s central bulge can cause it to flip alignments.
“This is a subtle signal that is really hard to detect in the observations,” says Dr Barsanti.
It has been made possible with the advent of integral field spectroscopy, a technique in which an optical instrument combines spectrographic and imaging capabilities to build a 3D image of a galaxy and at the same time resolve its internal motions.
This study used a spectroscope called SAMI, attached to the 3.9-metre wide Anglo Australian telescope located in Siding Spring, NSW.
Researchers used SAMI to survey 3,068 galaxies between 2013 and 2020. This staggering amount of data has taken years to study and supplied direct evidence for the paper published.
“With the SAMI Galaxy Survey we have spatially resolved spectroscopy allowing us to map the galaxy, with spectra at many points across the galaxy,” says Dr Barsanti. “This tells us the internal motions of the stars and gas within the galaxy, so we can measure its overall spin.”
“The SAMI Galaxy Survey allowed us to map the galaxy,” says Dr Barsanti. “Its measurements tell us the internal motions of stars and gas within a galaxy, so we can determine its overall spin.”
These results will inform the next big stage of our research, the Hector Galaxy Survey. Hector is the next-generation spectrograph replacing SAMI at the Anglo-Australian Telescope, which we’ll use to survey around 30,000 galaxies.”
Professor Stuart Wyithe of the University of Melbourne, who is Director of ASTRO 3D, says the paper advances the Centre’s key goals of tracing the distribution of matter from the earliest times in the Universe to the present day, and to build a 3D picture of the formation and evolution of the Universe that we see today.
“Using the power of the SAMI galaxy survey, which measured the 3D structure of individual galaxies as well as their position in space, this paper shows how the motions of mass in galaxies and positions of galaxies are connected, which is an essential piece in understanding how galaxies assembled,” says Professor Wyithe
The study was conducted in collaboration with researchers from the Australian National University, University of Sydney, Johns Hopkins University, University of Hamburg, University of Cambridge, and Macquarie University.
The $40 million ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) is funded by the Australian Research Council (ARC) and six collaborating Australian universities: The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia, and Curtin University.
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Stefania Barsanti (lead author of the research, ASTRO 3D, available for interview)
THE PAPER IS AVAILABLE AT https://doi.org/10.1093/mnras/stac2405
The SAMI Galaxy Survey: flipping of the spin–filament alignment correlates most strongly with growth of the bulge
Stefania Barsanti, Matthew Colless, Charlotte Welker, Sree Oh, Sarah Casura, Julia J Bryant, Scott M Croom, Francesco D’Eugenio, Jon S Lawrence, Samuel N Richards, Jesse van de Sande
We study the alignments of galaxy spin axes with respect to cosmic web filaments as a function of various properties of the galaxies and their constituent bulges and discs. We exploit the SAMI Galaxy Survey to identify 3D spin axes from spatially-resolved stellar kinematics and to decompose the galaxy into the kinematic bulge and disc components. The GAMA survey is used to reconstruct the cosmic filaments. The mass of the bulge, defined as the product of stellar mass and bulge-to-total flux ratio Mbulge = M⋆ × (B/T), is the primary parameter of correlation with spin–filament alignments: galaxies with lower bulge masses tend to have their spins parallel to the closest filament, while galaxies with higher bulge masses are more perpendicularly aligned. M⋆and B/T separately show correlations, but they do not fully unravel spin–filament alignments. Other galaxy properties, such as visual morphology, stellar age, star formation activity, kinematic parameters and local environment, are secondary tracers. Focussing on S0 galaxies, we find preferentially perpendicular alignments, with the signal dominated by high-mass S0 galaxies. Studying bulge and disc spin–filament alignments separately reveals additional information about the formation pathways of the corresponding galaxies: bulges tend to have more perpendicular alignments, while discs show different tendencies according to their kinematic features and the mass of the associated bulge. The observed correlation between the flipping of spin–filament alignments and the growth of the bulge can be explained by mergers, which drive both alignment flips and bulge formation.