|Fig 1: Spiral galaxy M74. Image|
|Fig 2: Elliptical galaxy ESO 325-|
G004. Image Credit: NASA
|Fig 3: Artist's rendition of a galaxy with |
AGN-driven outflows. Image Credit:
Fig 4: Galaxies in three different redshift bins being
split into the four quadrants of the specific star formation rate-
morphology plane. On the left are galaxies from our model
and on the right are observed galaxies. The greyscale 2D
histogram and contours indicate the density of galaxies
across the plane.
In order to test these ideas, we implemented a merger and disk instability-based AGN feedback prescription in our semi-analytic model (SAM) of galaxy formation and evolution in order to see how well we could reproduce the fraction of galaxies that are star-forming and disk-dominated (SFD) or quiescent and spheroid-dominated (QS) as compared with data from the CANDELS survey (as well as a local sample of galaxies from the GAMA survey). SAMs are a type of simulation which model large numbers of galaxies over the history of the universe. Our SAM evolves a cosmological sample of galaxies forward in time with relatively simple prescriptions for physical processes like the hierarchical growth of structure formation due to the merging of dark matter halos, the heating and cooling of gas, star formation, stellar evolution, supernovae, chemical enrichment of galactic and intergalactic gas, AGN feedback, and starbursts and morphological transformation due to galaxy mergers and disk instabilities. We divided galaxies based on their specific star formation rates (star formation rate divided by stellar mass) and their Sersic index, which is a measure of morphology. A Sersic index of 1 indicates a pure disk, while a Sersic index of 4 indicates a pure bulge. The distribution of galaxies in this plane, as well as our dividing lines for a few of our redshift bins, can be seen in Figure 4. By focusing on this plane, we also found ourselves studying the more "outlying populations": star-forming and spheroid-dominated (SFS) and quiescent and disk-dominated (QD). These populations are more rare but must still be explained by our evolutionary models.
In Figure 5, we can see the evolution of the fraction of galaxies in each of these four populations for both our model and the observations. Our model, which includes disk instabilities as a driver of bulge formation and AGN feedback, reproduces the fraction of SFDs and QSs much better than our model with a merger-only picture. Meanwhile, we reproduce the rough fractions of SFSs and QDs, although we do not quite match how the fractions evolve.
Our model suggests that SFDs are galaxies which have had very quiet histories; they've avoided major mergers and if they have ever been disturbed, they were able to accrete new gas and continue forming stars. QSs, on the other hand, are very likely to have undergone at least one major merger, or perhaps very many minor mergers, which built up a large bulge component and triggered AGN feedback, eventually leading to the cessation of star formation. SFSs in our model are a very short-lived population, the result of a recent merger which has led to bulge formation and a post-trauma starburst. These are likely soon to experience AGN feedback which will transform them into QSs. Finally, QDs are the result of SFDs which have stopped accreting new gas (perhaps due to environmental effects) or are very large and extended, causing their gas not to be dense enough to form stars.
While we do not match the evolution of these populations exactly, it seems we are beginning to be able to capture the very complicated processes responsible for the diverse galaxy population we see all around us.