Reclusive galaxies that live in vast, empty regions of space are forming stars at a surprisingly high rate, reveals the largest survey of galaxies ever made. The lonely galaxies may thrive because few galaxies are close enough to strip them of their star-forming gas.
An overwhelming 95% of galaxies live in communities, some of which teem with thousands of members. Gravity pulls the objects towards each other, leaving behind bubble-like voids.
But clumps of gas can remain in the voids, allowing galaxies to spring up 鈥渓ike oases in the desert鈥�, says Fiona Hoyle, an astronomer at Widener University in Chester, Pennsylvania, US. Previously, about a dozen such galaxies had been studied.
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Now, Hoyle and colleagues have identified 1000 void galaxies using the Sloan Digital Sky Survey (SDSS). The survey, which is more than halfway to its goal of systematically mapping a quarter of the sky, shows hundreds of dark voids that each shine with a few beacon-like galaxies.
The SDSS colour images confirm previous findings that the galaxies differ strikingly from their counterparts in clusters. 鈥淭hey鈥檙e bluer than expected, which means they鈥檝e got a higher rate of star formation,鈥� Hoyle told 麻豆传媒.
Strength in numbers
John Huchra, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says the research鈥檚 sample size is impressive. 鈥淭he sheer number of void galaxies now makes it pretty clear that this [higher star formation rate] is endemic,鈥� he told 麻豆传媒.
Two mechanisms could explain the effect, says Hoyle. Either the galaxies were late bloomers and formed their first stars recently 鈥� taking billions of years longer than most other galaxies to condense from gas 鈥� or they ignited their first stars long ago but managed to draw out the star-formation process.
Hoyle favours the second scenario: 鈥淕alaxies in clusters interact often and during these interactions the galaxies are stripped of their gas.鈥� In contrast, the void galaxies 鈥減robably haven鈥檛 undergone significant interaction, so they鈥檝e been able to keep hold of their gas supply,鈥� she says.
Hoyle鈥檚 colleague Michael Vogeley at Drexel University in Philadelphia, Pennsylvania, US, likens the process to 鈥渁 few rugged individuals who, without the competition of others, can live surprisingly well from the meager offerings of the land in most rural areas.鈥�
But Huchra says the active star formation probably results from a combination of both effects. 鈥淚t鈥檚 a chicken and egg problem,鈥� he told 麻豆传媒.
Blob-shaped galaxies
Hoyle believes the ongoing SDSS could turn up as many as 10,000 void galaxies. And she hopes the data will contain the answer to a long-standing mystery about the nature of blob-shaped 鈥渆lliptical鈥� galaxies.
Some astronomers believe these objects form solely through the merger of spiral galaxies, while others say they could condense directly from gas. Finding elliptical galaxies in relatively empty voids could show that the galaxies can take shape directly out of 鈥減rimordial鈥� gas, says Hoyle.
The research was presented at a meeting of the American Astronomical Society in Minneapolis, Minnesota, US, last week and published in the Astrophysical Journal.
Journal reference: Astrophysical Journal (vol 624, p 571)
