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Astronomers bust star ratio myth


False-colour image of the galaxy NGC 1566 - massive stars show up as white or pink, while less massive stars appear in blue (Source: NASA/JPL-Caltech/JHU)


The research, headed by Dr Gerhardt Meurer of John Hopkins University in Baltimore, appears on the pre-press physics website arxiv.

Since the 1950s, astronomers have assumed that the ratio of large to small stars in each galaxy is the same.

For example, for every star that is 20 times the mass of our Sun, there are 500 medium-sized stars in that galaxy.

This assumption, known as the initial mass function (IMF), has provided a useful method for estimating the mass of galaxies.

"For many years people thought this was a fixed value; that there were so many small stars for every big star," says CSIRO Australia National Telescope Facility research scientist and study co-author Dr Baerbel Koribalski.

By measuring the number of large bright stars in a distant galaxy, the total number of stars could be determined along with an estimate of the galaxy's mass.

"For lack of anything more sophisticated this has led to reasonable results," says Koribalski.

Varying ratios

The researchers chose a sample of 103 galaxies from the HIPASS Survey conducted by the Parkes radio telescope in New South Wales.

"The galaxies were selected based on their gas content, which is the ingredient for star formation," says Koribalski.

Meurer and colleagues examined the galaxies H-alpha and ultraviolet emissions, using data collected from various telescopes including NASA's GALEX space telescope.

"H-alpha is an excellent tracer for young stars - it shows where the most recent star formation has happened," says Koribalski. "Near ultraviolet and far ultraviolet showed the young stars and those in intermediate ages."

By comparing the ratio of stars seen in the H-alpha spectrum with those in the ultraviolet range, they found that the ratio of stars according to size varied from galaxy to galaxy. This implied that the IMF also varied across galaxies.

"This ratio is not constant and varies with galaxy type," says Koribalski. "Low mass galaxies have a very different distribution of stellar masses than brighter spiral galaxies."

She says the next step is to determine what factors influence changes in stellar mass ratios between galaxies.

"We need to now be more explicit in how the mass function changes with galaxy type or galaxy mass."

One of those factors may be gas pressure.

Koribalski says massive stars are most likely to form in high-pressure environments such as tight star clusters.

This may also explain why stars located further from the core of a galaxy, where the gas pressure is less, are more likely to be smaller than predicted by the IMF.

'Interesting result'

Professor Geraint Lewis of the University of Sydney says the study's finding will generate a lot of interest.

"It's quite an interesting result," he says. "For many people, using this initial mass function method is part of the course.

Lewis says despite its widespread use, most astronomers believe the IMF isn't perfect.

"We all felt deep down that there must be some more complicated physics involved," he says.

"What this is going to do is spur a new direction in better understanding the environmental impacts on star formation."

Lewis says the study will also have an impact on the study of the evolution of galaxies.

"Knowing the amount of mass hidden in galaxies is related to the entire evolution of galaxies," he says. "It will require us to go back and recalibrate the masses of galaxies we've seen."
