Hot, massive haloes found around most spiral galaxies (Forwarded)

ESA News
http://www.esa.int

14 December 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.

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.

Notes to editors:

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.

Tüllmann's co-authors are Wolfgang Pietsch (Max Planck Institute for
Extra-terrestrial Physics in Garching, Germany), Dieter Breitschwerdt
(Institute for Astronomy, Vienna, Austria), and Joern Rossa (Space
Telescope Science Institute, Baltimore, Maryland, USA).

For more information:

Ralph Tüllmann, Ruhr University, Bochum, Germany
E-mail: tullmann @ astro.rub.de

Norbert Schartel, ESA XMM-Newton Project Scientist
E-mail: norbert.schartel @ sciops.esa.int

More about...

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

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Related links

* First A&A article
http://arxiv.org/abs/astro-ph/0510079
* PDF file for Astronomy & Astrophysics article
http://www.edpsciences.org/articles/...PRAA200514.pdf

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMWAMVLWFE_index_1.html]
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)

[Image 2:
http://www.esa.int/esaCP/SEMWAMVLWFE...html#subhead1]
This three-colour XMM-Newton image of NGC 4634 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. Besides NGC 891, NGC 4634 is the second non-starburst
late-type spiral galaxy with a prominent diffuse soft X-ray halo. In
addition to a hot corona, this galaxy also possesses a cosmic ray and a
diffuse ionised gas halo. There are strong indications that these
multi-phase gaseous halos are triggered by star formation related
processes in the disk plane.

Credits: ESA/AIRUB (R. Tüllmann)