HIPASS J0352-66: a nearby galaxy forming its first stars?
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E. Sadler (University of Sydney); T. Oosterloo, R. Morganti (Netherlands Foundation for Research in Astronomy, Netherlands) We have recently discovered a giant intergalactic cloud of neutral hydrogen gas that challenges many of our ideas about how galaxies form and evolve. The cloud was discovered in October 2001 from radio images of an elliptical galaxy, NGC 1490, taken with the Australia Telescope Compact Array. This galaxy was targeted because data from the recently completed HI Parkes All Sky Survey (HIPASS) implied that it contained an unusually large amount of neutral hydrogen gas for an elliptical galaxy. Unlike our own Milky Way and other spiral galaxies, giant elliptical galaxies like NGC 1490 undergo a rapid period of star formation in the early universe which uses up almost all the available gas. When neutral gas is seen in these galaxies, it is usually a sign that the elliptical galaxy has recently swallowed a smaller, gas-rich galaxy. Surprisingly, the Compact Array radio image (Figure 1) showed that although the gas found by HIPASS lies at the same distance as NGC 1490, it is associated with another, previously unknown galaxy which is barely visible to optical telescopes. This gas cloud, now designated HIPASS J0352-66, is at least as large as our own Milky Way Galaxy, but is apparently only just starting to form its first generation of stars. In contrast, its companion galaxy NGC 1490 turned all its gas into stars very rapidly during a short period in the early Universe. This extreme diversity in star-formation history between members of a galaxy group is quite unexpected, especially since theories of galaxy formation predict that giant neutral hydrogen clouds of this kind should be found only in very isolated environments. One possibility is that HIPASS J0352-66 has only just begun to condense out of an intergalactic gas reservoir surrounding the galaxy group of which NGC 1490 is a member. If so, a significant fraction of the baryons in the local Universe may be associated with intergalactic gas clouds rather than with visible galaxies. Further study of HIPASS J0352-66 is in progress to test these ideas. |
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