Evidence from observations sheds doubt on cosmic cannibalism as a source for galaxy growth, suggesting that instead galaxies grow by pulling in gas from the intergalactic medium.
A quick stroll through Hubble’s archive shows a surplus of massive spiral galaxies — a tapestry of the drama of stellar birth and death across thousands of light-years. But one question remains: how do these galaxies continue forming stars at such a fast clip?
Galaxies should quickly deplete their reservoirs of gas as they create new stars and so must somehow continuously replenish those reservoirs with fresh gas to keep the star formation going.
Last year, a series of studies showed that galaxies power most of their star formation — and therefore their growth — through merging with dwarf companions. The Milky Way is a prime example, as it shows no signs of a major merger, but continuously munches on nearby dwarf galaxies.
But a new study shows this might not be the case.
Minor Mergers are Minor Players
Enrico Di Teodoro (University of Bologna, Italy) and his colleagues set out to better understand this process by observing 148 spiral galaxies and any companion dwarfs they may soon accrete.
But the results were surprising: of the 148 galaxies in their sample, 101 had no detectable companions, 15 showed a massive companion, 6 showed both massive and dwarf companions, and 26 showed only dwarf companions.
So the team focused on the 32 galaxies with dwarf companions, finding that any future mergers would only provide the massive spiral galaxy with 0.28 solar masses per year of gas — approximately a fifth of the gas necessary to continue forming stars (1.29 solar masses per year).
In other words, the number of dwarf galaxies is simply too low to supply enough gas to each massive galaxy.
A reader in the know might recall the missing satellites problem — the discrepancy between the number of small, faint galaxies predicted to exist in numerical simulations and the number actually seen. Are there extremely faint galaxies that Di Teodoro and his colleagues couldn’t see, potentially biasing their result?
“The missing satellites problem arises from numerical simulations that likely are not yet able to reproduce the whole physics,” says Di Teodoro. “Very deep observations in the Local Group show no evidence of a significant population of very low-mass dwarf galaxies as predicted by simulations. Personally, I believe that the lack is in the current simulations rather than in the observations.”
Nonetheless the team calculated the contribution given by any dwarf galaxies that were too faint to be observed. They found the contribution to be negligible.
Intergalactic Gas Feeds Galaxies
So the team has reopened the question: how do these galaxies grow?
“The question of how spiral galaxies can continue to form stars at a nearly steady state is still open, and the picture seems to be much more complicated than people used to think a few years ago,” says Di Teodoro.
One option is that galaxies grow by pulling in gas from the intergalactic medium — the hot plasma found between galaxies. But only cold gas is the fuel for star formation. At low temperatures, atoms within the gas are able to bind together and also clump to high densities. So in order for this scenario to work, the gas would need to cool down, a process that has proved difficult to observe directly, says Di Teodoro.
Two experts, Sugata Kaviraj (University of Hertfordshire and University of Oxford, U.K.) and Diego Lambas (University of Córdoba) find the study presented here statistically sound.
“At the present, gas accretion from satellites does not provide a major contribution to the star formation rate of spirals,” affirms Lambas. He does, however, caution that minor mergers will not have a negligible effect: star formation is a natural consequence of a tidal interaction. As the massive spiral galaxy engulfs the dwarf galaxy, any gas within will feel a headwind, much as a runner feels a wind even on the stillest day, and become compressed enough to spark star formation.
“Minor mergers do not just bring gas into massive spiral galaxies,” says Kaviraj. “The infall of satellites can disturb the existing gas in the spiral itself and induce radial inflows which then produce star formation. Hence the satellites can ‘trigger’ star formation from the gas that is already in the massive spiral itself.”
It’s possible that the impact of the dwarf galaxy mixing up the gas in the spiral is larger than the impact from gas injected into the spiral, says Kaviraj. The issue will have to be explored further before astronomers can determine the true culprit behind continued star formation in massive spiral galaxies.
Enrico Di Teodoro and Filippo Fraternali “Gas Accretion from Minor Mergers in Local Spiral Galaxies” Astronomy & Astrophysics, June 5, 2014