A universal IMF?
Of course, the best observations of the IMF come from relatively nearby star-forming regions in the Milky Way. But as far as we know, the IMF in disk galaxies like ours is universal, says McKee. One of the big questions confronting astronomers is whether the IMF remains constant over both space and time.
“So far, there is no evidence to suggest that the IMF has significantly varied over cosmic time,” says Lada. Still, there might be an exception in the earliest stages of the universe. Astronomers believe that the first stars, so-called Population III stars, were the most massive and possessed the fewest metals. (Astronomers refer to all elements heavier than helium as “metals.”) Once these early stars started churning out heavier elements in earnest, however, subsequent generations of stars incorporated these metals, and the typical stellar mass dropped significantly.
Bate says it is still unclear whether the IMF ever varies. He wonders whether the mass function would be the same even in the extreme environments found near the centers of massive galaxies, where radiation must be much higher, or early in the universe, when there was no interstellar dust.
A few observations hint at the possibility of different IMFs. In a 2009 paper published in the Monthly Notices of the Royal Astronomical Society, Bate writes that observations of stars orbiting Sagittarius A* show the IMF near the galaxy’s center may be “top heavy,” biased toward massive stars. The same goes for stars in the Arches Cluster, the Milky Way’s densest-known open cluster, which lies some 25,000 light-years from Earth in the constellation Sagittarius. Even so, he notes that the apparently top-heavy IMF in the Arches Cluster may be due to its evolution rather than to the conditions at its birth. The opposite effect may be happening in the crowded cores of giant elliptical galaxies, where indirect observations point to an excess of low-mass stars.
The differences in the amount of metals throughout the universe and over time seem to have a relatively small effect on the IMF. “You can go to the Small Magellanic Cloud where there is about a fifth of the dust and metals found in the Milky Way, and [you find] no detectable difference,” says Krumholz. According to McKee, you wouldn’t expect to see much effect as long as the metal content is greater than about a few ten-thousandths of the current value. He says the main difference between the metal-free first stars and those being born today is that the earliest ones were born in a framework dominated by dark matter, while current ones form due to their own self-gravity.
New tech, new answers
New and improved computer modeling will help theorists refine their calculations of the IMF, researchers say. But observational improvements could prove even more important. The European Space Agency’s Gaia satellite is allowing astronomers to hone their mass estimates for stars in distant clusters. And NASA’s James Webb Space Telescope will allow astronomers to directly observe the IMF in such clusters. It also will reveal the IMF in some galaxies with more extreme conditions than our own.
On the ground, the Large Synoptic Survey Telescope and the European Extremely Large Telescope, both now under construction in Chile, should provide the light-gathering ability and resolution to advance our knowledge of the IMF and any possible variations in it.
And the Atacama Large Millimeter/submillimeter Array (ALMA) will make observations of dense interstellar regions heretofore obscured by dust. A primary assumption in current theory is that the distribution of masses of gravitationally bound regions within molecular clouds determines the IMF, says McKee. “ALMA is the ideal instrument to test this, particularly for high-mass stars,” he says.