Welcome to my webpage! My name is Edmond Cheung and I'm a postdoctoral fellow at the Kavli IPMU.
My research focuses on understanding how galaxies evolve. I've studied many aspects of galaxy evolution, including: (1) how quiescent galaxies remain quiescent, (2) how galaxies accrete gas, (3) how bars affect galaxy evolution, and (4) how galaxies quench their star formation. Below are some highlights of my recent works.
Suppressing Star Formation in Quiescent Galaxies with Supermassive Black Hole Winds
How quiescent galaxies with little-to-no star formation remain quiescent is a major unsolved question in galaxy evolution. While there have been several proposed "maintenance" mechanisms, the evidence has been sparse. With the SDSS-IV MaNGA Survey, we have recently discovered a new class of galaxies---dubbed "red geysers"---that represent new and compelling evidence for AGN "maintenance" mode feedback (Cheung et al. 2016, Nature, 533, 504).
Characterized by bisymmetric emission features (panel c in the figure above), these red geysers host wide-angled winds that are likely powered by their weakly-accreting supermassive black holes. In a prototypical example, which we've nicknamed "Akira" (panel a; right) in homage to the MaNGA survey and to my current institution in Japan (Kavli IPMU), we calculate that the energy output from the low-luminosity AGN is sufficient to power the outflowing wind, which is also sufficient to balance the cooling of the warm and cool gas, thereby suppressing star formation (Cheung et al. 2016, Nature, 533, 504).
Stellar Populations of Quiescent Barred Galaxies
Stellar bars are predicted to affect their host galaxies' abundance ratios through the slow rearrangement of material, i.e., secular evolution. Although there have been many works that have tested this prediction, it is still unclear whether bars do affect their host galaxie's chemical abundances. In Cheung et al. 2015b, we tackle this issue using extremely high signal-to-noise (above 100) stacked spectra of quiescent barred and unbarred galaxies.
The above plot is a graphic summary of our study. We stack in bins of redshift and stellar mass and separate barred and unbarred galaxies using Galaxy Zoo 2.
The main result is shown in the plot above. We plot stellar age, [Fe/H], [Mg/Fe], and [N/Fe] as a function of stellar mass for barred and unbarred quiescent galaxies. This plot shows that there are no significant differences in this stellar population parameters in the bulges of quiescent barred and unbarred galaxies.
Bars + AGN at z>0
Stellar bars have been considered an AGN fueling mechanism for decades. But while there have been many works that have explored this topic in the local universe, there have been almost none that have explored this issue beyond z=0 (where the number density of AGN is significantly higher than that at z=0). Thus using Galaxy Zoo: Hubble, we investigate whether bars can fuel AGN at z>0 in Cheung et al. 2015a.
The above plot shows the main result of Cheung et al. 2015a---the bar fraction of AGN hosts are not significantly higher than the bar fraction of non-AGN hosts, suggesting that bars do not fuel AGN at z>0.
Bars are thought to be the major drivers of secular evolution. They are also present in up to two thirds of the local disk population, suggesting that they significantly affect galaxy evolution, at least at z~0.
Using Galaxy Zoo 2, I've lead a project in which we searched for evidence of bar-driven secular evolution. Specifically, we explored the behavior of bars as a function of several parameters that are predicted to significantly affect bar-driven secular evolution. The above plot is Figure 3 from Cheung et al. 2013. It shows that the bar likelihood, i.e., the likelihood a bar exists, is related to the properties of a galaxy, indicating that the existence of bars are connected to galaxy properties.
Moreover, these trends match the predictions of bar-driven secular evolution, strongly supporting the theory of secular evolution. This implies that bars are important drivers of galaxy evolution. See Cheung et al. 2013. for more details.
The number of red sequence galaxies has doubled since z~1 while the number of blue cloud galaxies has remained relatively constant. A natural interpretation is that galaxies evolve from blue to red with time, i.e., from star forming to 'quenched.'
In Cheung et al. 2012, I led a project to discover what the underlying physical processes are behind quenching. While we were unable to identify the quenching mechanism, we were able to place strong constraints on the quenching process. We found that red and blue galaxies were best separated by the central stellar surface mass density (see above figure, which is Figure 7 from Cheung et al. 2012), suggesting that whatever quenches galaxies must also significantly increase the central stellar mass of galaxies.
Here is the list of my first author publications:
(Data: Single Sersic Catalog   Bulge Sersic = 4 Catalog   Bulge Sersic = 2 Catalog   README file )
And co-author publications:
I am a member of the following teams: