- The green valley is a red herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early-and late-type galaxies
- Jonathan Fernandez
- Resumo
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We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies.
We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of
quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue
galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear
that only a small population of blue early-type galaxies move rapidly across the green valley after
the morphologies are transformed from disc to spheroid and star formation is quenched rapidly.
In contrast, the majority of blue star-forming galaxies have significant discs, and they retain
their late-type morphologies as their star formation rates decline very slowly. We summarize a
range of observations that lead to these conclusions, including UV-optical colours and halo masses,
which both show a striking dependence on morphological type. We interpret these results in terms of
the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent
with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by
a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes.
In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed
virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to
spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases
transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole
accretion and possibly active galactic nuclei feedback.
- Referências
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- Schawinski et al. (2014)
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