Who Killed the Galaxies?

Title: The EDGE-CALIFA Survey: Central molecular gas depletion in AGN host galaxies — a smoking gun for quenching?

Authors: Sara L. Ellison, Tony Wong, Sebastian F. Sanchez et al.

First Author’s Institution: Department of Physics & Astronomy, University of Victoria, Finnerty Road, Victoria, British Columbia, V8P 1A1, Canada

Status: Accepted by MNRAS Letters. 

How to kill a galaxy…

Galaxies, the birthplaces of countless stars, are typically classified into spiral, elliptical, and irregular (Figure 1). These three types of galaxies have not only different morphology but also different internal characteristics. Spiral galaxies are considered as young and active with lots of newborn stars, while elliptical galaxies seem red and dead without the new generation of stars. So what does it take for an active galaxy to end up as a dead one?

Figure 1. Illustration of three types of galaxies. (Credit: American Board)

Generally, one can “kill” an active galaxy in two ways: 1) by ejecting and draining the cold molecular gas out of its host galaxy by jets or winds, 2) by heating up the molecular gas and keeping it from cooling down. Either way, the galaxy can no longer condense cold molecular gas to produce new stars. As a result, star formation ceases, leaving behind a red, old, gas-poor galaxy as time passes.

Among the many different processes to quench star formation, active galactic nuclei (AGNs) often play a major part. Through jets and feedback, AGNs can eject large chunks of gas out of the host galaxy as well as heat the remaining gas. On the other hand, some observations find that the host galaxies of AGNs could harbour as much, if not more, gas as similar galaxies with strong star-forming activity. These findings are in direct contrast to the “opposite” relation between AGNs and star formation. 

In today’s paper, the authors use optical and CO(1-0) maps which trace the molecular gas from EDGE and CALIFA surveys to select 126 non-merging galaxies. By focusing on observations at sub-galactic (kpc) scales, the authors suggest a way to disentangle the above contradiction.

Pieces of the host galaxy

First, instead of studying a galaxy as a whole, the authors divide each galaxy into many sub-galactic partial regions. They then classify and re-group all regions from different galaxies into three categorized spaxels based on star formation rates (SFRs): star-forming spaxels with the highest SFRs, retired spaxels with degenerated SFRs and AGN-dominated spaxels with high-ionized line emissions and the lowest SFRs. Thus, the authors are able to focus on the amount of molecular gas and stars in each small region of the host galaxy.

Figure 2 shows the relation between molecular gaseous and stellar compositions in every sub-galactic region (mass of molecular gas versus stellar mass), color-coded by three categories. While we can see a tight correlation (called “rMGMS”, short for “the resolved molecular gas main sequence”) between the two compositions for all three categories, at a given stellar mass, the star-forming spaxels (blue) have more molecular gas than the retired spaxels (orange). Although AGN spaxels (red) show higher stellar mass due to AGNs’ central compact locations, they also have the least molecular gas. Therefore, the retired spaxels may represent the transition phase of star formation quenching due to ionization from a central AGN. 

Figure 2: The tight relation between molecular gaseous and stellar compositions (rMGMS) in the sub-galactic spaxels of 126 galaxies. Y-axis shows mass of molecular gas and X-axis shows stellar mass. Spaxels are classified as star forming (blue shading), retired (orange contours) and AGN-dominated (red points) based on EDGE-CALIFA observations. Retired and AGN spaxels have less molecular gas than star-forming ones. Adopted from Figure 1 in today’s paper.

Second, in order to examine the internal impact of AGNs to their host galaxies, it is important to compare SFRs of different spaxels inside an AGN host. Therefore, the authors further investigate four galaxies with at least 40 star-forming and 40 central AGN spaxels, as good representatives of AGN hosts. Again, the authors use rMGMS, the same tight relationship used above, as an indicator of gas fraction. 

Figure 3 shows the internal rMGMS of the selected four galaxies. In each galaxy, the AGN spaxels clearly show an offset to the corresponding star-forming spaxels, pointing to a lower gas fraction. Among these four AGN hosts, the AGN spaxels in NGC 2639 (upper left in Figure 3) are particularly low in molecular gas. It turns out that NGC 2639 has the brightest AGN and a radio jet, indicating a dependence of the molecular gas fraction on AGN luminosity and complex structure. 

Figure 3: The rMGMS for four AGN host galaxies. Star-forming spaxels are shown in blue and AGN spaxels are shown in red. The AGN spaxels are offset from the star-forming spaxels in the same galaxy. Adopted from Figure 2 in today’s paper.

Disentangling the contradiction

How does this sub-galactic study help with the dilemma between AGNs and star formation quenching? It shows the importance of separating star-forming spaxels from retired and AGN spaxels within a galaxy.

Previous research on galaxies with low SFRs suggests that the quenching mechanism operates from the inside-out. Since AGNs are in the compact central regions of their host galaxies, the feedback may eject significant amounts of gas in the nearby sub-galactic regions and deplete the central gas fraction. However, it may not be enough to decrease the total gas fraction in the entire galaxy. Moreover, the distinction between retired and AGN spaxels is not significant. Therefore, the retired spaxels on sub-galactic scales may dilute the relation between AGN and star formation on galactic scales.

In summary, the authors in today’s paper find a depletion of molecular gas in AGN hosts, consistent with the previous theory that AGN quenches star formation. By targeting at the sub-galactic scales in the host galaxies, the results also suggest that AGN feedback may have a large impact on a small scale, but a weak impact on a large scale. Therefore, AGNs might still be behind the death of their host galaxies when the galactic-scale gas fraction suggests differently.

Edited by Alex Gough

Featured image credit: Science of Space

About Wei Vivyan Yan

I am a PhD candidate at Dartmouth College, where I study AGNs and their host galaxies. My research focuses on origin and evolution of AGN obscuration. Before then, I did my undergraduate at University of Science and Technology of China. Outside of astronomy, I am currently writing my first fiction novel. I also enjoy traveling and outdoor activities.

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