If you were to plot the magnitude and colour of a large sample of galaxies you would see that they fall into two groups: one is actively star-forming and filled with young blue stars, and the other has long since finished making new stars so is left with only the older, redder populations. Between these two monolithic groups, however, there is an elusive class of galaxies. Post-starburst galaxies (PSBs) are objects that are thought to have had large amounts of star-formation shut off very rapidly in a quenching event. This quick transition means that PSB samples are generally quite small, so the origins of quenching are still quite uncertain. However, studying PSBs is still thought to be the best route to understanding what causes galaxies to transition from blue to red.
Today’s authors are looking inside the galaxy for their quenching trigger. They focus on the galaxy’s central supermassive black hole. Emission from active galactic nuclei (AGNs) is thought to inject huge amounts of energy back into their host galaxies. Such huge injections are believed to either create strong winds that eject star-forming material from the host galaxy or heat the gas so much as to prevent it from cooling and collapsing to form new stars. Today’s paper searches for signs of nuclear activity in a sample of PSBs to see if AGNs could be responsible for their quenching.
The authors showcase a brand-new galaxy survey dedicated to studying quenching activity at intermediate redshifts. The Studying of Quenching in Intermediate-z Galaxies: Gas, anguLar momentum and Evolution (SQuIGGLE) survey contains thousands of massive galaxies found in SDSS DR14. From this survey they use 1,207 PSBs at redshifts between 0.5 and 0.94. In addition, they construct a separate sample of galaxies in a similar mass and redshift regime from the LEGA-C survey to act as a comparison.
Taking these criteria together, the authors find a sample of 64 AGNs in the PSB sample, leading to an AGN fraction of about 5%. Only five AGNs were found in the comparison sample, leading to an overall AGN fraction of 1.4%. This reveals that AGNs are more likely to be found in PSBs than in a sample of regular galaxies of similar mass and redshift.
These results are broken down further to identify how trends may vary with host galaxy properties. Most interestingly, they look at how AGN fraction varies with stellar age, measured using a quantity called the 4,000-angstrom break (Dn4000). It gives us an indication of the relative contributions made to a galaxy’s spectrum by the shorter-lived, blue stars and their longer-lived red counterparts. Once quenching has occurred, Dn4000 increases as the short-lived, blue stars start to die and cannot be replaced, leaving behind only longer wavelength emission from red stars. Figure 2 shows the results of this breakdown of AGN fraction with galaxy age. It clearly shows that younger PSBs have an extremely enhanced AGN fraction compared to older ones: AGNs are ten times more likely to appear in the youngest PSBs!
AGN fractions appear to peak around the time of the quenching event where AGN-driven winds could force gas out of the host galaxy. In doing this, the AGN also removes sources of future fuel, causing the large drop in AGN fraction as the galaxies get older. Such a strong correlation between AGN fraction and the galaxy’s age suggests AGN activity could play a role in quenching galaxies. Whilst this correlation is compelling, it isn’t definitive. This makes the follow-up work being done by the authors all the more important: they are searching these AGNs for signs of outflows, which, if found, would suggest that star formation really is quenched by AGN-driven winds.