Monday, March 30, 2015

An Efficient Star Making Galaxy

The dwarf galaxy NGC 5253 as seen in the optical with Hubble and in molecular gas with the Submillimeter Array (in red). The bright central region appears to making new stars with an efficiency ten times greater than that in the Milky Way, perhaps the result of the infalling CO gas streamer seen to the left.  Credit:  Nature; NASA HST; SMA

New stars regularly appear in the night sky as the gas and dust in giant interstellar clouds gradually coalesce under the influence of gravity. The process of making stars, however, is inefficient, and (at least in present-day galaxies) there are copious amounts of material that don't make it into stars. For the Milky Way, the efficiency overall (as measured by the mass in stars compared to the total mass of the galaxy) is about 5%; in clouds with turbulent gas motions this value can be even lower. The low efficiency is a critical parameter in galaxy evolution, and is one reason why stars are still forming nearly fourteen billion years after the Big Bang. Another consequence is seen in the production of star clusters. A low efficiency that produces stars gradually does not easily produce star clusters, because the new stars can drift away from the diffuse cloud. The existence of ancient massive bound star clusters (globular clusters) in the Milky Way, therefore, suggests that when they formed early in galactic history, star formation efficiencies were higher.

A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation. CfA astronomer Jun-Hui Zhao and his colleagues used the Submillimeter Array (SMA) to study the molecular gas (carbon monoxide, CO) at the center of this galaxy in a source called "Cloud D". Usually astronomers use the intensity of the CO radiation to estimate the total gas mass, but this can be a misleading measure since it requires knowing the relative amount of CO to the total material. The team instead used the motions of the gas to infer the total mass present; they used the amount of ultraviolet light to determine the number of stars. The scientists report that their technique is a much more reliable way of measuring the star formation rate.

The astronomers, writing in the latest issue of Nature, find that when they apply their method to the hot, dense and dusty Cloud D, they find a star-formation efficiency exceeding 50%. They note that their SMA images show a streamer of molecular gas falling into the galaxy toward this cloud, and they argue that this infalling material (about two million solar masses of gas) could compress the cloud and thereby induce the dramatic star formation efficiency seen. The new paper also suggests that a similar kind of infall and compression mechanism might have enabled comparably higher star formation rates at earlier times in cosmic history.

"Highly Efficient Star Formation in NGC 5253 Possibly from Stream-Fed Accretion," J. L. Turner, S. C. Beck, D. J. Benford, S. M. Consiglio, P. T. P. Ho, A. Kovacs, D. S. Meier & J.-H. Zhao, Nature 519, 331, 2015