CALIFA renews the classification of galaxies

The objects within galaxies have two basic types of motions: orbiting around the galaxy centre in a regular organized disc, or in orbits oriented at random without a clear direction of rotaiton. If we imagined that galaxies behave in the same way as the Solar System we could think that as the objects move further away from the centre their orbital velocities decrease. However this is not necessarily the case for galaxies, as there are several factors which affect the rotational velocity of these objects, such as the dimensions of the galaxy, the gravitational pull of other galaxies, and the quantity of dark matter in a given galaxy. An international team of astrophysicists,among them IAC and Universidad de La Laguna researcher Jesus Falcon Barroso, coordinator of the CALIFa (Calar Alto Legacy Integral Field Area survey) at the IAC who is one of the authors of the article published in Nature Astronomy, has collected tat from 600 galaxies in the neighbourbhood of the Milky Way with the Potsday Multiple Aperture Spectrophotometer (PMAS) on the 3.6m telescope at the Calar Alto Observatory (Almeria, Spain). As part of this catalogue the scientists have made velocity maps of 300 galaxies showing the movements of their stars.

In this way they have defined three different groups among the sets of stellar orbits, which they have called "cold orfbits" "warm orbits" and "hot orbits", the latter typical of stars with random motions. When they analyzed tghe data they showed that circular orbits are frequent in lower mass galaxies, while the "hot orbits" are more often found in galaxies with higher mass. In addition tghey have found quite a number of "warm orbits", greater than that previously expected for this type of galaxies.

Using these maps of stellar motions one can obtain a lot of informatioin about the history of formation of these galaxies. They evolve and grow over thousands of millions of years, merging with other galaxies. Those which have absorbed other smaller galaxies generally have thin rotating discs, while when two galaxies with similar masses merge an elliptical galaxy is formed, in which the orbits are arranged in random directions.

Measuring the orbits in the galaxies analyzed allows us to distinguish between disc galaxies (with colder orbits) and elliptical galaxies (with hotter orbits) even when this difference cannot be detected when using images alone. This implies that by measuring the stellar orbits the researchers will be able to determine if the galaxy we observe is the result of internal evolution of an isolated object, a relatively calm series of mergers with smaller objects, or the product of a violent merger.

CALIFA, which with its 300 galaxy simple has become one of the biggest archives of data on galaxy dynamics up to now is the "first study to propose a scheme of galaxy classification base don the orbital distribution of their stars, which is different from the classical Hubble diagram , based on morphological classification" explains Falcon Barroso. This researcher also acknowledges that the results of this study " present some problems for current theories of formation and evolution of galaxies"

This new classification has been carefully prepared to produce a representative simple which will help astronomers to make models of the evolution of galaxies, and show whether their simulations produce valid predictions.

Hubble Captures Massive Dead Disk Galaxy that Challenges Theories of Galaxy Evolution

By combining the power of a "natural lens" in space with the capability of NASA's Hubble Space Telescope, astronomers made a surprising discovery—the first example of a compact yet massive, fast-spinning, disk-shaped galaxy that stopped making stars only a few billion years after the big bang. Finding such a galaxy early in the history of the universe challenges the current understanding of how massive galaxies form and evolve, say researchers. When Hubble photographed the galaxy, astronomers expected to see a chaotic ball of stars formed through galaxies merging together. Instead, they saw evidence that the stars were born in a pancake-shaped disk.

Acting as a “natural telescope” in space, the gravity of the extremely massive foreground galaxy cluster MACS J2129-0741 magnifies, brightens, and distorts the far-distant background galaxy MACS2129-1, shown in the top box. The middle box is a blown-up view of the gravitationally lensed galaxy. In the bottom box is a reconstructed image, based on modeling that shows what the galaxy would look like if the galaxy cluster were not present. The galaxy appears red because it is so distant that its light is shifted into the red part of the spectrum.

Credits: NASA, ESA, S. Toft (University of Copenhagen), M. Postman (STScI), and the CLASH team

This is the first direct observational evidence that at least some of the earliest so-called "dead" galaxies — where star formation stopped — somehow evolve from a Milky Way-shaped disk into the giant elliptical galaxies we see today.

This is a surprise because elliptical galaxies contain older stars, while spiral galaxies typically contain younger blue stars. At least some of these early "dead" disk galaxies must have gone through major makeovers. They not only changed their structure, but also the motions of their stars to make a shape of an elliptical galaxy.

"This new insight may force us to rethink the whole cosmological context of how galaxies burn out early on and evolve into local elliptical-shaped galaxies," said study leader Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute, University of Copenhagen, Denmark. "Perhaps we have been blind to the fact that early "dead" galaxies could in fact be disks, simply because we haven't been able to resolve them."

This artist's concept shows what the young, dead, disk galaxy MACS2129-1, right, would look like when compared with the Milky Way galaxy, left. Although three times as massive as the Milky Way, it is only half the size. MACS2129-1 is also spinning more than twice as fast as the Milky Way. Note that regions of Milky Way are blue from bursts of star formation, while the young, dead galaxy is yellow, signifying an older star population and no new star birth.

Credits: NASA, ESA, and Z. Levy (STScI)

Previous studies of distant dead galaxies have assumed that their structure is similar to the local elliptical galaxies they will evolve into. Confirming this assumption in principle requires more powerful space telescopes than are currently available.

However, through the phenomenon known as "gravitational lensing," a massive, foreground cluster of galaxies acts as a natural "zoom lens" in space by magnifying and stretching images of far more distant background galaxies. By joining this natural lens with the resolving power of Hubble, scientists were able to see into the center of the dead galaxy.

The remote galaxy is three times as massive as the Milky Way but only half the size. Rotational velocity measurements made with the European Southern Observatory's Very Large Telescope (VLT) showed that the disk galaxy is spinning more than twice as fast as the Milky Way.

Using archival data from the Cluster Lensing And Supernova survey with Hubble (CLASH), Toft and his team were able to determine the stellar mass, star-formation rate, and the ages of the stars.

Why this galaxy stopped forming stars is still unknown. It may be the result of an active galactic nucleus, where energy is gushing from a supermassive black hole. This energy inhibits star formation by heating the gas or expelling it from the galaxy. Or it may be the result of the cold gas streaming onto the galaxy being rapidly compressed and heated up, preventing it from cooling down into star-forming clouds in the galaxy's center.

But how do these young, massive, compact disks evolve into the elliptical galaxies we see in the present-day universe? "Probably through mergers," Toft said. "If these galaxies grow through merging with minor companions, and these minor companions come in large numbers and from all sorts of different angles onto the galaxy, this would eventually randomize the orbits of stars in the galaxies. You could also imagine major mergers. This would definitely also destroy the ordered motion of the stars."

The findings are published in the June 22 issue of the journal Nature. Toft and his team hope to use NASA's upcoming James Webb Space Telescope to look for a larger sample of such galaxies.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

The Very Large Telescope is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of Northern Chile.