The galaxies that just stopped

Stars have been forming in galaxies in for billions of years. New research has however uncovered an unusual form of galaxy – one that doesn’t create stars, but instead prevents them from being created. Dr Edmond Cheung explains...

It’s hard to imagine a non-star-forming galaxy – after all, if a galaxy doesn’t form stars, then how do we see them? But not only are non-star-forming galaxies common, they are also the most massive galaxies in the universe.

These non-star-forming, or quiescent, galaxies are galaxies that are not currently forming many stars relative to their current amount of stars. In fact, they form so few that astronomers have colloquially dubbed them as “red and dead” galaxies. But while they may not be forming new stars, they contain plenty of old stars that shine brightly – the light from these old stars allows us to see them. Among the mysteries of this population of quiescent galaxies is how they remain quiescent despite the availability of the key ingredient for star formation: gas. Recent studies have revealed that about three-quarters of quiescent galaxies contain warm, ionised gas; this gas – if left unimpeded – should cool and form stars. But we know that this cannot be.

The star formation histories and stellar populations of nearby quiescent galaxies indicate that they formed the bulk of their stars almost 10 billion years ago, with very little occurring during recent times. Furthermore, the number densities of quiescent galaxies indicate that their numbers have steadily increased since about 10 billion years ago. These findings imply that there must be something that prevents the gas within quiescent galaxies from cooling.

In our new study¹, this ‘something’ has been found. Using the ongoing Sloan Digital Sky Survey (SDSS) IV MaNGA survey² which takes resolved spectroscopy for 10,000 nearby galaxies, we discovered a new class of quiescent galaxies – dubbed “red geysers” – that host outflowing winds powerful enough to heat ambient gas and suppress future star formation. These winds are manifested in bisymmetric emission features (in H-alpha, [OII], and other strong lines) and are likely powered by their weakly accreting supermassive black holes.

But not only are non-star-forming galaxies common, they are also the most massive galaxies in the universe

To highlight the key characteristics of this class, we focused on a prototypical red geyser, nicknamed “Akira”– a reference to the critically acclaimed manga comic of the same name, and in homage to the MaNGA survey and my current institute in Japan – Kavli IPMU. Akira is undergoing a minor interaction with another galaxy, nicknamed “Tetsuo”– another character in the same manga comic as Akira. The SDSS image of the interaction is shown in panel a of the figure below, which is reproduced from the study¹; the galaxies are shown in two composite gri colour images while the background image, which highlights the tidal bridge, is the r band image. According to merger simulations, Tetsuo is depositing cool gas into Akira, which is detected in redshifted Na D absorption (panels d, more positive values indicate more absorption, and e, red regions indicate redshift while blue indicates blueshift). The expected star-formation from this cool gas, however, is absent. We found that the measured star-formation rate of Akira is much lower than what is expected given the amount of cold gas present. Thus something is prohibiting star formation in Akira – but what is it?

We discovered a new class of quiescent galaxies – dubbed “red geysers” – that host outflowing winds powerful enough to heat ambient gas and suppress future star formation

Inspecting the ionised gas properties of Akira, we found an interesting bisymmetric emission pattern in H-alpha and other strong emission lines (panel c; more negative numbers indicate more emission). These emission patterns roughly align with the ionised gas velocity gradient (panel h), suggestive of an outflow. To prove that the ionised gas is in an outflowing wind instead of in a rotating disk, we had to disprove the latter case. Using the stellar dynamics of Akira (panels f and g), we obtained a tight constraint on its gravitational potential, from which we were able to predict the ionised gas kinematics in the case of a regularly rotating disk. We found that the observed ionised gas kinematics are significantly higher than the predicted ionised gas kinematics (panel j), indicating that the ionised gas is not under the influence of gravity alone.

Ruling out the disk interpretation, we developed a qualitative wind model that reproduced many of the features of the data, including the ionised gas velocity field and the ionised gas velocity dispersion field. We theorise that this outflowing wind is likely powered by the weakly accreting supermassive black hole at the centre of Akira, which is detected as a central radio point source in the FIRST survey and in follow-up Jansky VLA observations. We calculate the energetic output from this low-luminosity active galactic nuclei (AGN) is sufficient to power this outflowing wind, which in turn, has enough energy to counterbalance the cooling of both the warm and cool gas within Akira, and thereby suppress star formation.

While Akira is an ideal case-study, perhaps the most exciting aspect of this study is the fact that there are many more red geysers. Red geysers make up about 10% of quiescent galaxies at moderate stellar masses (2x1010 solar masses), which could have important implications on the duty cycle of this kind of supermassive black hole feedback. Moreover, because they are relatively common, red geysers may exemplify how typical quiescent galaxies maintain their quiescence, which could solve a long-held mystery over these galaxies.

References 1 Cheung et al. 2016, Nature, 533, 504 (www.nature.com/nature/journal/v533/n7604/full/nature18006.html) 2 www.sdss.org/surveys/manga

Author

Dr Edmond Cheung is a postdoctoral fellow at the Kavli IPMU. His research focuses on galaxy evolution, specifically the connection between galaxy structure and galaxy evolution.