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Galaxy mergers make
more massive stars

Posted: 27 April 2010

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Colliding galaxies mysteriously play host to more massive supernovae in their central regions than undisturbed galaxies do, says new research presented earlier this month at the National Astronomy Meeting at the University of Glasgow. If correct, it may indicate that galaxy mergers somehow increase the proportion of the most massive stars compared to Sun-like stars, says lead researcher Stacey Habergham of Liverpool John Moores University.

“When we look at star-forming regions we usually see lots of faint, low-mass stars and very few hot, high mass stars,” says Habergham. “However, in the central regions of colliding galaxies we’re seeing many more massive stars than expected.”

Colliding galaxies are prone to higher proportions of more massive supernovae in their centres, new research shows. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA)–ESA/Hubble Collaboration/A Evans (University of Virginia/NRAO/Stony Brook University).

The reason we don’t see supernovae popping off all the time is because the most massive stars, which collapse at the end of their lives when they run out of material for nuclear fusion in their cores, are relatively rare. In a typical star-forming region of the Galaxy, hundreds if not thousands of lower mass stars can be born for every massive star. Given that such massive stars survive for only a few million years, with star formation continuing around them, their resulting supernovae can be a strong indicator of how much star formation is currently happening in galaxies where we can’t resolve individual stars through a telescope.

Along with her colleagues Phil James, also from Liverpool, and Joe Anderson, now at the University of Chile in Santiago, Stacey Habergham observed 178 supernovae in 140 nearby spiral galaxies using the Liverpool Robotic Telescope and the Isaac Newton Telescope, both situated in the Canary Islands. Some of these galaxies seem to be fairly normal, undisturbed spirals, whilst others are being ravaged by gravitational tidal forces thrust on them by interactions and mergers with other galaxies (identified by the presence of tidal tails, asymmetry in the spiral arms and double nuclei where the core of one galaxy has been subsumed by another). What Habergham’s team found was quite surprising. Although both undisturbed and interacting galaxies displayed roughly equal proportions of Type Ib/c supernovae (which possess no hydrogen in their spectra and originate from more massive progenitor stars – type b are suspected to be carbon rich, and type c nitrogen rich) and Type II supernovae (which do show hydrogen lines and are the destruction of slightly less massive giant stars), the interacting galaxies clearly had more type Ib/c supernovae within their central regions.

A type Ib/c supernova is the explosion of a luminous blue variable star, such as a Wolf-Rayet star. These stars are notable because they seriously let rip with their stellar winds of radiation, which are so extreme that they tear off the star’s outer envelope of hydrogen, exposing the nuclear processed innards of carbon and nitrogen. Hence, when they finally do give up the ghost and exhaust their supply of nuclear fuel in their core, their supernova debris shows no evidence of hydrogen.

To help drive such powerful winds, massive stars should be made from gas that has a higher proportion of ‘metals’ – elements heavier than helium. However, galaxy interactions should force gas into the centres of galaxies from the outermost regions of their spiral discs, and this gas is relatively pure hydrogen and helium that hasn’t been enriched by ‘metals’ created inside previous generations of stars. In galaxy collisions, stars born in a galaxy’s centre should not have a high enough metallicity for Type Ib/c supernovae. In fact, what Stacey Habergham’s team found was that 11 out of 13 supernovae seen within the inner twenty percent of galaxies were Type Ib/c, with the overall distribution of such supernovae weighted towards the centres of galaxies. In contrast, Type II supernovae were distributed fairly evenly across the galaxies in the survey.

The most likely explanation is that interacting or merging galaxies favour an increase in the birth rates of the most massive stars. This top heavy ‘initial mass function’ (which describes the distribution of stellar masses in star-forming regions) also has support from theoretical models, which suggest that raw gas in the densest star-forming regions should be warmed by the feedback of ultraviolet radiation from newborn stars, thus increasing the mass of gas that can collapse (the so-called Jeans mass).

“Exploring supernovae, their environments and what kind of stars make supernovae is vital because they are so important for enriching the Universe with the heavy metals that make up you and me,” says Habergham, whose research is being published in a future issue of The Astrophysical Journal. Her next step is to conduct infrared observations that can see through the dust in the interacting galaxies and define the disturbances more clearly, and run spectroscopic observations to check exactly what the metallicity of the gas is.

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