Library of galaxy histories reconstructed from motions of stars

January 4, 2018, Instituto de Astrofísica de Andalucía
Credit: Instituto de Astrofísica de Andalucía (IAA-CSIC)

Just as the sun is moving within the Milky Way, all the stars in galaxies are moving, but with very different orbits. Some of the stars have strong rotations, while others may be moving randomly with no clear rotation. Comparing the fraction of stars on different orbits, researchers can determine how galaxies form and evolve. An international team of astronomers has derived directly, for the first time, the orbital distribution of a galaxy sample containing more than 300 galaxies of the local universe. The results, published in Nature Astronomy, are based on the CALIFA survey, a project developed at Calar Alto Observatory and conceived from the Institute of Astrophysics of Andalusia (IAA-CSIC).

Galaxies are largest structures in the universe, and scientists study how they evolve to understand the history of the universe. Galaxy formation entails the hierarchical assembly of halos of dark matter, along with the condensation of normal matter at the halos' center, where stellar formation takes place. Stars that formed from a settled, thin gas disk and then lived though dynamically quiescent periods will present near-circular orbits, while stars with random motions are the result of turbulent environments, either at birth or later, with galactic mergers.

Thus, the motions of stars in a galaxy are like a history book, recording the information about their birth and growth environment, which may reveal how the galaxy was formed. "However, the motion of each single star is not directly observable in external galaxies. External galaxies are projected on the observational plane as an image and we cannot resolve the discrete stars in it," says Ling Zhu, researcher from the Max Planck Institute for Astronomy who leads the study. "The CALIFA survey uses a recently developed technique, integral field spectroscopy, which can observe the external galaxies in such a way that it provides the overall motion of stars. Thus, we can get kinematic maps of each galaxy."

Credit: Instituto de Astrofísica de Andalucía (IAA-CSIC)

The researchers then build models for each galaxy by superposing stars on different types of orbits. By constraining the model with the observed image and kinematic maps, they can find out the amount of moving on different types of orbits in each galaxy. They call it the stellar orbit distribution, and for this study, the team has built models for 300 galaxies that are representative of the general properties of galaxies in the local universe.

The maps show changes in galactic orbit distribution depending on the total stellar mass of the galaxies. The ordered-rotating orbits are most prominent in galaxies with total stellar masses of 10 billion solar masses, and least important for the most massive ones. Random-motion orbits unsurprisingly dominate the most (more than 100 billion solar masses). "This is the first orbit-based mass sequence across all morphological types. It includes flourishing information of a galaxy's past, basically whether it had been a quiet succession of only smaller mergers or shaped by a violent major merger. Further studies are needed to understand the details," says Glenn van de Ven (ESO).

The researchers had found a new and accurate method of reading off a galaxy's history – and their survey with its data sets for 300 galaxies turned out to be the largest existing library of galaxy history books.

Credit: Instituto de Astrofísica de Andalucía (IAA-CSIC)

"This work highlights the importance of integral field spectroscopy and, in particular, of large-scale surveys such as the CALIFA project. The significant contribution of what we call 'hot' orbits, a mixture of rotation and random movements of the stellar component, poses important challenges to cosmological models of and evolution," says Rubén García Benito, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) participating in the project.

CALIFA's results represent an observationally determined distribution of galaxies in the present-day universe. They lend themselves thus to direct comparison with samples of cosmological simulations of in a cosmological context. In this sense, these results open a new window for comparing galaxy simulations to the observed galaxy population in the present-day universe.

Explore further: Astronomers discover unusual spindle-like galaxies

More information: Ling Zhu et al. The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey, Nature Astronomy (2017). DOI: 10.1038/s41550-017-0348-1

Related Stories

Astronomers discover unusual spindle-like galaxies

October 12, 2017

Galaxies are majestic, rotating wheels of stars? Not in the case of the spindle-like galaxies studied by Athanasia Tsatsi (Max Planck Institute for Astronomy) and her colleagues. Using the CALIFA survey, the astronomers found ...

Uncovering the origins of galaxies' halos

November 21, 2017

Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth. The findings, published ...

Image: Hubble's compact galaxy with big-time star formation

October 16, 2017

As far as galaxies are concerned, size can be deceptive. Some of the largest galaxies in the Universe are dormant, while some dwarf galaxies, such as ESO 553-46 imaged here by the NASA/ESA Hubble Space Telescope, can produce ...

Hubble scopes out a galaxy of stellar birth

June 26, 2017

This image displays a galaxy known as ESO 486-21 (with several other background galaxies and foreground stars visible in the field as well). ESO 486-21 is a spiral galaxy—albeit with a somewhat irregular and ill-defined ...

Hubble digs into cosmic archaeology

October 30, 2017

This NASA/ESA Hubble Space Telescope image is chock-full of galaxies. Each glowing speck is a different galaxy, except the bright flash in the middle of the image which is actually a star lying within our own galaxy that ...

Recommended for you

Mars Curiosity celebrates sol 2,000

March 23, 2018

NASA's Mars Curiosity rover just hit a new milestone: its two-thousandth Martian day, or sol, on the Red Planet. An image mosaic taken by the rover in January offers a preview of what comes next.

Radio nebula discovered around the pulsar PSR J0855–4644

March 21, 2018

Using the Giant Metrewave Radio Telescope (GMRT) in India, an international team of astronomers has detected a diffuse radio emission forming a nebula around the pulsar PSR J0855–4644. The finding is reported March 9 ...


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jan 04, 2018
"Galaxy formation entails the hierarchical assembly of halos of dark matter.."

The authors were supposed to inform the galaxies about their models. Otherwise the galaxy will continue to respect universal principles based on rotation and gravity.

"The researchers then build models for each galaxy by superposing stars on different types of orbits."

Readers should compare the author's drawing and picture of the galaxy. and the "value" of the article will be obvious.
not rated yet Jan 04, 2018
"...random motions..."
'Randomly oriented, often highly elliptical orbits', surely ??
1 / 5 (1) Jan 07, 2018
A speculation on one possible cause of extreme random stellar movements within the larger galaxies? There may be other, additional causations, yet unrealized.

That the massive amalgamations of the giant galaxies attracts and consumes the smaller galaxies around them.

Thus churning up these giant galactic structures with randomizing captures. And, renewing energetic star formation.

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.