Revealing the origin and nature of the outskirts of stellar megalopolis

February 16, 2017, Instituto de Astrofísica e Ciências do Espaço
ETG-HUDF12-IR. Credit: Instituto de Astrofísica e Ciências do Espaço

The most detailed study of the outskirts of massive elliptical galaxies at half the age of the Universe was carried out by an international team led by Fernando Buitrago, of Instituto de Astrofísica e Ciências do Espaço and Faculdade de Ciências da Universidade de Lisboa (FCUL). The study was published in the journal Monthly Notices of the Royal Astronomical Society and contributes to the understanding of how the largest galaxies of the Universe evolved over time.

Galaxies have dramatically grown in size since the early Universe, and elliptical galaxies, in particular, are the largest galaxies in both size and mass. What is the main driver behind the late growth of their outer parts was the question that motivated this study.

With disc galaxies, like our Milky Way, it is fairly easy to identify their distinct parts: the central bulge, the disc with its spiral arms, and a halo of stars enveloping the whole. Astronomers can tell, for instance, that the stellar halo is formed mostly by stars from satellite galaxies that merged into them.

For elliptical galaxies, however, this is much harder because these galaxies look much like a smooth, featureless cloud of stars. Fernando Buitrago (IA and FCUL) says: "With elliptical galaxies, there is direct evidence of merging of going on, but it is hard to ascertain that the processes that have been happening for these galaxies to grow their outer envelopes are the same that we see occurring in disc galaxies like our own."

Credit: Instituto de Astrofísica e Ciências do Espaço

Hence, Buitrago and his team set out to investigate the nature of the outskirts of a sample of massive elliptical galaxies when the Universe was half its current age, roughly 6,2 billion years ago. Focusing their research on faint features at great distances from the galactic centre, they could only work with the deepest ever image of the Universe, the Hubble Ultra Deep Field (HUDF). Using the six galaxies that matched their criteria and are recorded in this image, the researchers were able to demonstrate for the first time the existence of extended stellar envelopes in individual massive elliptical galaxies at that period in time.

The quality of the information collected in the HUDF, enabled the team to characterise the individual galactic haloes and to place them in the context of the evolutionary history of this type of galaxies. Moreover, Buitrago and his team were able to conclude that, for their sample of massive elliptical galaxies at half the age of the Universe, the outer parts were, like disc galaxies, formed mainly due to the merging of other galaxies. They even attempted to identify in those haloes traces of recent episodes of galactic fusion.

The results emerged from the comparison of the sample with mathematical simulations based on the current model of galaxy formation and evolution. The team saw that, in this very case, the simulation and the real data matched very well and that it was possible to infer parallelisms.

"In , we cannot say 'this is the galactic bulge and this is the halo'," says Buitrago, "All the stars form a huge spheroid, like an immense rugby ball. But when we use a computer simulation, we can track the origin of every part of the simulated galaxy and compare with our real galaxies. Through this method, we identified the process behind the dramatic increase of these galaxies outer parts, and were able to explain how their size evolves."

Explore further: A massive galaxy long ago and far away

More information: Fernando Buitrago et al. The cosmic assembly of stellar haloes in massive Early-Type Galaxies, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stw3382

Related Stories

A massive galaxy long ago and far away

February 6, 2017

Galaxies today fall roughly into two categories: elliptically-shaped collections of reddish, old stars that formed predominantly during a period early in the history of the universe, and spiral shaped objects dominated by ...

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 ...

Hubbles spies the beautiful galaxy IC 335

December 24, 2014

This new NASA/ESA Hubble Space Telescope image shows the galaxy IC 335 in front of a backdrop of distant galaxies. IC 335 is part of a galaxy group containing three other galaxies, and located in the Fornax Galaxy Cluster ...

Hubble peers at a distinctly disorganized dwarf galaxy

April 4, 2016

Despite being less famous than their elliptical and spiral galactic cousins, irregular dwarf galaxies, such as the one captured in this NASA/ESA Hubble Space Telescope image, are actually one of the most common types of galaxy ...

Hubble views a galactic mega-merger

January 18, 2016

The subject of this NASA/ESA Hubble Space Telescope image is known as NGC 3597. It is the product of a collision between two good-sized galaxies, and is slowly evolving to become a giant elliptical galaxy. This type of galaxy ...

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 ...

Recommended for you

Is dark matter made of primordial black holes?

April 20, 2018

Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84 percent of ...

NASA engineers dream big with small spacecraft

April 20, 2018

Many of NASA's most iconic spacecraft towered over the engineers who built them: think Voyagers 1 and 2, Cassini or Galileo—all large machines that could measure up to a school bus.

Unveiling the secrets of the Milky Way galaxy

April 20, 2018

A multinational team of astronomers involving the University of Adelaide has catalogued over 70 sources of very high energy gamma rays, including 16 previously undiscovered ones, in a survey of the Milky Way using gamma ray ...

Where is the universe's missing matter?

April 19, 2018

Astronomers using ESA's XMM-Newton space observatory have probed the gas-filled haloes around galaxies in a quest to find 'missing' matter thought to reside there, but have come up empty-handed – so where is it?

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Feb 16, 2017
Open access version of paper here:

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.