Hot, massive haloes found around most spiral galaxies

December 14, 2005
Hot, massive haloes found around most spiral galaxies

Astronomers using ESA's XMM-Newton observatory have found very hot gaseous haloes around a multitude of spiral galaxies similar to our Milky Way galaxy. These 'ghost-like' veils have been suspected for decades but remained elusive until now.

Above: This three-colour XMM-Newton image of NGC 4631 was created from merged EPIC pn and MOS images. Red, green and blue represent emission in the (0.2-1.0) keV, (1.0-2.0) keV, and (2.0-4.5) keV energy bands, respectively. The ellipse indicates the outer border of the H-alpha emitting disc. An extended soft X-ray halo is clearly visible which is most likely triggered by star formation related processes in the disc plane. Credits: ESA/AIRUB (R. Tüllmann)

Galaxy 'haloes' are often seen in so-called 'starburst' galaxies, the locations of concentrated star formation, but the discovery of high-temperature haloes around non-starburst spiral galaxies opens the door to new types of measurements once only dreamed about.

For example, scientists can confirm models of galaxy evolution and infer the star-formation rate in galaxies like our own by 'calculating backwards' to estimate how many supernovae are needed to make the observed haloes.

"Most of these ghost-like haloes have never been confirmed before in X-ray energies because they are so tenuous and have a low-surface brightness," said Ralph Tüllmann, from the Ruhr University in Bochum, Germany, lead author of the results.

"We needed the high sensitivity and large light-collecting area of the XMM-Newton satellite to uncover these haloes."

In starburst galaxies, which have prominent haloes, star formation and star death (supernovae) are concentrated in the core of the galaxy and occur during a short time period over the life of a galaxy. This intense activity forms a halo of gas around the entire galaxy, similar to a volcano sending out a plume.

So how can haloes form in the absence of intense star formation? Tüllmann's group say that the entire disk of a spiral galaxy can 'simmer' with star-formation activity. This is spread out over time and distance. Like a giant pot of boiling water, the steady activity of star formation over millions and millions of years percolates outward to form the galaxy halo.

Two of the best-studied galaxies so far out of a group of 32 are NGC 891 and NGC 4634, which are tens of millions of light years away in the constellations Andromeda and Coma Berenices, respectively.

The scientists noted that these observations do not support a recent model of galaxy halo formation, in which gas from the intergalactic medium rains down on the galaxy and forms the halo.


Galaxy halos contain about 10 million solar masses of gas. The scientists say it is a relatively straightforward calculation to determine how many supernovae are needed to create the halo. Supernovae are intricately tied to the rate of star formation in a given galaxy.

"With our data we will be able to establish for the first time a critical rate of star formation that needs to be exceeded in order to create such haloes," said Dr Ralf-Jürgen Dettmar, a co-author also from Ruhr University.

Once these haloes have formed, the hot gas cools and can fall down onto the galaxy's disk, the scientists said. The gas is involved in a new cycle of star formation, because pressure from this infalling gas triggers the collapse of gas clouds into new stars.

Some heavy elements might escape the halo into intergalactic space, depending on the energy of the supernovae. Further analysis of the chemical composition of the halo might reveal this.

This would determine the correctness of recent cosmological models on the evolution of galaxies, as well as provide evidence of how the elements necessary for life are distributed through the Universe.

A team led by Dr Ralph Tüllmann of the Ruhr University in Bochum, Germany, discusses these results in two articles in the scientific journal Astronomy & Astrophysics.

Source: ESA

Explore further: Stars in the halo of the Milky Way often travel in groups

Related Stories

Stars in the halo of the Milky Way often travel in groups

January 31, 2017

Many stars in the halo that surrounds the Milky Way travel in groups. This is the outcome of a recent analysis of data for millions of stars from the Gaia space mission. Astronomers report their discovery today in the international ...

A bridge of stars connects two dwarf galaxies

February 7, 2017

The Magellanic Clouds, the two largest satellite galaxies of the Milky Way, appear to be connected by a bridge stretching across 43,000 light years, according to an international team of astronomers led by researchers from ...

Galaxy murder mystery

January 17, 2017

It's the big astrophysical whodunnit. Across the Universe, galaxies are being killed and the question scientists want answered is, what's killing them?

New dwarf satellite galaxy of Messier 83 found

December 5, 2016

(Phys.org)—Astronomers have found a new dwarf satellite of Messier 83 (M83, also known as the Southern Pinwheel Galaxy) located some 85,000 light years from its host. This satellite galaxy was designated dw1335-29 and could ...

Recommended for you

Canada conservationist warns of 'cyber poaching'

February 25, 2017

Photographers, poachers and eco-tour operators are in the crosshairs of a Canadian conservationist who warns that tracking tags are being hacked and misused to harass and hunt endangered animals.

Polymer additive could revolutionize plastics recycling

February 24, 2017

When Geoffrey Coates, the Tisch University Professor of Chemistry and Chemical Biology, gives a talk about plastics and recycling, he usually opens with this question: What percentage of the 78 million tons of plastic used ...

0 comments

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.