Scottish Research: Milky Way Galaxy is smaller than believed

The Milky Way is smaller than astronomers previously thought, according to new research by Dr Jorge Penarrubia of Scotland's Edinburgh University.

For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our solar system.

Researchers have found that the Milky Way is approximately half the weight of a neighbouring galaxy, Andromeda, which has a similar structure to our own.

The Milky Way and Andromeda are the two largest in a region of galaxies which astronomers call the Local Group.

Scientists say that Andromeda's extra weight must be present in the form of dark matter, a little-understood invisible substance which makes up most of the outer regions of galaxies.

They estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be twice as heavy.

Researchers say their work should help them learn more about how the outer regions of galaxies are structured. Their findings also provide further evidence in support of a theory which suggests that the universe is expanding.

Although both galaxies appear to be of similar dimensions, until now scientists had been unable to prove which is larger.

We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging. 

Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible. - Dr Jorge Peñarrubia, Edinburgh University, School of Physics and Astronomy.

Previous studies were only able to measure the mass enclosed within both galaxies' inner regions. In this new study, researchers were also able to work out the mass of invisible matter found in the outer regions of both galaxies, and reveal their total weights. They say 90 per cent of both galaxies' matter is invisible.

An image of the Andromeda galaxy, Messier 31.

Credit: Malyshchyts Viktar / Fotolia

A team of scientists led by the University of Edinburgh used recently published data on the known distances between galaxies, as well as their velocities, to calculate the total masses of Andromeda and the Milky Way.

Journal Reference: Jorge Peñarrubia, Yin-Zhe Ma, Matthew G. Walker, and Alan McConnachie. A dynamical model of the local cosmic expansion. Monthly Notices of the Royal Astronomical Society,, 2014; 443: 2204-2222 DOI: 10.1093/mnras/stu879

ESA Herschel Detects more gas in the Galaxy than constructed by astronomers

Artist's impression of molecular gas across the Milky Way's plane. Credit: ESA - C. Carreau

A survey from ESA Herschel has revealed that the reservoir of molecular gas in the Milky Way is hugely underestimated - almost by one third - when it is traced with traditional methods.

Monitoring the emission from ionised carbon, the new study identified molecular gas in the intermediate evolutionary stage between diffuse, atomic gas and the densest star-forming molecular clouds.

The discovery not only indicates that there is more raw material for the formation of new stars in the Galaxy, but also that it extends farther than astronomers knew.

In the Milky Way, as well as in other galaxies, stars are born from the collapse of the densest and coldest clumps of matter in a molecular cloud.

These clouds are gigantic star-forming complexes consisting mainly of molecular hydrogen (H2), a gas that does not emit any light at the low temperatures found in molecular clouds.

Astronomers investigating the early stages of star formation are not only interested in how molecular clouds fragment to form stars, but also in the processes that take place even earlier and initially cause molecular clouds to take shape from diffuse, atomic hydrogen gas.

For this purpose, astronomers study the distribution and properties of H2 across the Galaxy – but without the benefit of direct observations, they must resort to alternative methods to trace it.

The most widely used proxy to track down molecular gas in star-forming regions is carbon monoxide (CO).

A mere contaminant in molecular clouds, CO radiates much more efficiently than H2 and can be detected easily.

However, such indirect tracers can be biased, since there is no guarantee that all portions of a cloud containing H2 also contain CO, in which case observations of CO would miss these regions entirely.

To achieve a more complete picture of the Milky Way's molecular content, astronomers in the past decades have combined observations of CO with other tracers of H2.

These include the emission from dust – another contaminant in molecular clouds – and the gamma rays that are produced when cosmic ray particles interact with atomic and molecular hydrogen in the interstellar medium (ISM).

The combination of such data had suggested the presence of more molecular gas in the Milky Way than indicated by CO alone.

New data from ESA's Herschel Space Observatory are now confirming this earlier suspicion: almost one third of all molecular gas in the Milky Way had remained undetected.

Jorge Pineda

In addition, there is more: the new survey, which probes H2 through a different tracer – ionised carbon (C+) – has established the three-dimensional distribution of the molecular gas across the Milky Way.

"This is the first survey of ionised carbon across the Galactic Plane – where most of the Milky Way's stars and star-forming clouds are concentrated – that combines both high spectral and angular resolution," comments Jorge Pineda from the Jet Propulsion Laboratory (JPL), Caltech, USA, who led the study.

Read more here

More information: Technical paper: J. L. Pineda, et al., "A Herschel [C II] Galactic plane survey I: the global distribution of ISM gas components", 2013, Astronomy & Astrophysics, 554, A103. arxiv.org/abs/1304.7770 and dx.doi.org/10.1051/0004-63
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