How to Turn Off a Galaxy’s Star Formation

New observations by the Atacama Large Millimeter/submillimeter Array (ALMA) provide a close look at a galaxy that may be in the process of shutting down its star formation.

Transitioning a Galaxy

Though we know much more about the processes of galaxy formation and evolution than we did even a decade ago, many key points still elude us. One particular puzzle is that of how star formation ends in a galaxy. We think that galaxies eventually transition from bright, blue, star-forming disks into red and quiescent ellipticals — but what causes star formation in a galaxy to shut down during this transition?

Since galaxies form stars out of cold gas, we could assume that star formation stops only when the cold gas supply is depleted. But observations suggest that star formation can shut down across a galaxy much more quickly than the timescale for using up the gas supply — sometimes turning off within just a few tens of millions of years. Such a rapid shutdown is termed “violent quenching”.

Options for Violent Quenching

What mechanisms could suddenly prevent cold gas from contracting into stars in the disk of a galaxy? The most efficient approach is the rapid removal or destruction of the molecular gas.

In one common picture of rapid gas removal, powerful jets emitted from the supermassive black hole in a galaxy’s active nucleus (AGN) play a key role. In this model, the jets blow out the galaxy’s molecular gas on short timescales — and in so doing, they both clear out gas available for star formation, and also propel metal-enriched gases into the circumgalactic medium.

But new observations have challenged the picture of AGN-driven outflows as the standard violent-quenching mechanism. In a recent study led by Jim Geach (University of Hertfordshire, UK), a team of scientists presents a new view of a quenching galaxy that doesn’t seem to have an AGN.

Look to the Stars

Using ALMA, Geach and collaborators trace the molecular gas in SDSS J1341–0321, a massive and compact galaxy thought to have recently undergone a major merger and now showing signs of early-stage quenching. Despite this galaxy hosting no evidence of an active nucleus, the ALMA observations reveal an outflow of cool gas moving at a speedy 1,000 km/s relative to the stars.

Geach and collaborators suggest that this outflow is violent quenching in another form: a powerful stellar outflow currently expelling around 300 solar masses of gas per year. They argue that this outflow was launched within the last 5 million years from a central starburst — a region of compact, vigorous star formation — triggered by SDSS J1341–0321’s recent merger. In this model, the stars themselves blow out all the gas — and once the gas is gone, star formation will turn off and the galaxy will appear red and quiescent.

If this model correctly describes SDSS J1341–0321, the next question is whether similar stellar outflows could account for violent quenching in other compact, massive galaxies across the universe. While we don’t yet know the answer, it seems likely that future high-resolution observations — perhaps also made with ALMA — will help us to find out!

Citation

“Violent Quenching: Molecular Gas Blown to 1000 km s−1 during a Major Merger,” J. E. Geach et al 2018 ApJL 864 L1. doi:10.3847/2041-8213/aad8b6