The shapes of galaxies

June 26, 2017, Harvard-Smithsonian Center for Astrophysics
An image of the colliding galaxies NGC4676, "The Mice," as seen by Hubble. An analysis of eighteen thousand colliding galaxies in the computer simulation Illustris has found that mergers like this one are the dominant mechanism determining the shape of galaxies more massive than the Milky Way, while for lower mass galaxies mergers do not play a significant role. Credit: NASA/HST

Since Edwin Hubble proposed his galaxy classification scheme in 1926, numerous studies have investigated the physical mechanisms responsible for the shapes of spiral and elliptical galaxies. Because the processes are complex, however, studies frequently rely on computer simulations as their main tool. The discs of galaxies are believed to form through the collapse of gas which acquires its initial spin in the early Universe. During their subsequent evolution, galaxies undergo a wide range of phenomena, from the accretion of matter—or its outflow—to mergers with other galaxies, all of which modify the disk's spin and angular momentum.

Astronomers think that spiral with the largest galactic discs formed preferentially in protogalaxies with the highest , although early attempts to verify this prediction using computer simulations failed. (More recently, simulations have been able to verify this trend.) Elliptical galaxies, on the other hand, are believed to be the remnants of repeated , but their shapes depend on many details like the galaxies' masses, gas content, and the collision parameters. As a result, these mergers need to be considered over a cumulative, cosmological context with large numbers of examples to evaluate their development from a statistical perspective.

CfA astronomers Vicente Rodriguez-Gomez, Annalisa Pillepich and Lars Hernquist led a team that analyzed the morphologies of about eighteen thousand galaxies in the Illustris computer simulation. Both disc and spheroidal galaxies arise naturally in this simulation. They find that massive merging galaxies develop into spirals or spheroidal shapes depending on their gas content (as expected, since the star formation activity depends crucially on the gas). Unexpectedly, they find that for lower mass galaxies—roughly the mass of the Milky Way or smaller—mergers do not seem to play a significant role in determining the morphology. The reason appears to be that in higher mass mergers a galaxy accretes many more stars from the partner, and this plays the a critical role. Their significant conclusion is that only in are mergers the dominant factor in shaping the system.

Explore further: Understanding star-forming galaxies

More information: Vicente Rodriguez-Gomez et al. The role of mergers and halo spin in shaping galaxy morphology, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stx305

Related Stories

Understanding star-forming galaxies

June 5, 2017

The more stars a typical spiral galaxy contains, the faster it makes new ones. Astronomers call this relatively tight correlation the "galaxy main sequence." The main sequence might be due simply to the fact that galaxies ...

Violent origins of disc galaxies probed by ALMA

September 17, 2014

For decades scientists have believed that galaxy mergers usually result in the formation of elliptical galaxies. Now, for the the first time, researchers using the Atacama Large Millimeter/sub-millimeter Array and a host ...

Hubble views a young elliptical galaxy

November 23, 2015

At the center of this amazing Hubble image is the elliptical galaxy NGC 3610. Surrounding the galaxy are a wealth of other galaxies of all shapes. There are spiral galaxies, galaxies with a bar in their central regions, distorted ...

Hubble sees galaxy hiding in the night sky

May 2, 2016

This striking NASA/ESA Hubble Space Telescope image captures the galaxy UGC 477, located just over 110 million light-years away in the constellation of Pisces (The Fish).

The cosmic evolution of galaxies

May 11, 2015

Our knowledge of the big bang has increased dramatically in the past decade, as satellites and ground-based studies of the cosmic microwave background have refined parameters associated with the very early universe, achieving ...

Recommended for you

Researchers study the opaque accretion disk of Beta Lyrae A

July 23, 2018

An international team of astronomers has conducted a study of the opaque accretion disk of the multiple star system known as Beta Lyrae A (β Lyr A for short). The research reveals important insights into nature of this disk ...

Seeing Titan with infrared eyes

July 23, 2018

These six infrared images of Saturn's moon Titan represent some of the clearest, most seamless-looking global views of the icy moon's surface produced so far. The views were created using 13 years of data acquired by the ...

Where to search for signs of life on Titan

July 20, 2018

New findings, published in the journal Astrobiology, suggest that large craters are the prime locations in which to find the building blocks of life on Saturn's largest moon, Titan.

Did a rogue star change the makeup of our solar system?

July 20, 2018

A team of researchers from the Max-Planck Institute and Queen's University has used new information to test a theory that suggests a rogue star passed close enough to our solar system millions of years ago to change its configuration. ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

RNP
5 / 5 (2) Jun 26, 2017
An open access copy of the paper can be found here: https://arxiv.org...9498.pdf
Macksb
not rated yet Jul 02, 2017
Stacy McGaugh and Federico Lelli say: "If you measure the distribution of starlight, you know the galaxy rotation curve, and vice versa." (McGaugh, 2016). There must be "One law to rule them all." Lelli, 2017.

Hard to discern any connection between the above Harvard CfA research (18,000 galaxies) and the McGaugh and Lelli work, which covers about 400 galaxies (153, then 240 more in a related article) and more than 3,000 data points. Both deal with aspects of galaxy morphology.

Any suggestions?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.