* Astronomy

Blobrana · L

RE: Galaxy haloes

Blobrana · L

Title: A Reservoir of Ionised Gas in the Galactic Halo to Sustain Star Formation in the Milky Way
Authors: Nicolas Lehner, J. Christopher Howk

Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionised gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the Galactic halo and unravelling their role. We have used the Hubble Space Telescope to blindly search for ionised high-velocity clouds (iHVCs) in the foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s are within one Galactic radius of the sun and have enough mass to maintain star formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may be the next wave of infalling material.

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Blobrana · L

UMass Amherst Astronomers Using Advanced Equipment aboard Hubble to Reveal Galaxies' Most Elusive Secrets

New, high-precision equipment orbiting Earth aboard the Hubble Space Telescope is now sending such rich data back to astronomers, some feel they are crossing the final frontier toward understanding galaxy evolution, says Todd Tripp, leader of the team at the University of Massachusetts Amherst.
Galaxies are the birthplaces of stars, each with a dense, visible central core and a huge envelope, or halo, around it containing extremely low-density gases. Until now, most of the mass in the envelope, as much as 90 percent of all mass in a galaxy, was undetectable by any instrument on Earth.
But Hubble's sensitive new Cosmic Origins Spectrograph (COS), the only one of its kind, has dramatically improved the quality of information regarding the gaseous envelope of galaxies, Tripp says.
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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. We needed the high sensitivity and large light-collecting area of the XMM-Newton satellite to uncover these haloes" - Ralph Tullmann, Ruhr University, Bochum, Germany, lead author of the results.

XMM-Newton view of hot ionised gas halo in NGC 4631

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?
Tullmann'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" - Dr Ralf-Jurgen 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.