In previous blog entries we talked about how galaxies are found preferentially in two different states. Either they are awake, actively forming a lot of young stars, or they are asleep, having very little or no star-formation activity. This bimodality is reflected in their observed colors, which are either blue or red, but also in their morphology. Galaxies that are awake usually have large spiral structures, just like our own galaxy, whereas asleep galaxies have elliptical or spheroidal shapes.
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| Hubble Space Telescope color-composite images of 4 clumpy star-forming galaxies at z~2 in the CANDELS survey. The disturbed appearances are very common in star-forming galaxies in the adolescent Universe. Credit: G. Barro |
Using data from CANDELS and previous surveys with the Hubble Space Telescope, we have found that these morphological differences were also present when the Universe was much younger. However, the most active galaxies in the adolescent Universe were nothing like our Milky way. Many of them lacked a characteristic spiral structure, showing heavily distorted clumpy appearances instead. Most asleep galaxies, on the other hand were already round, smooth, and remarkably small as soon as 3 billion years after the Big Bang. This implies that, by the time galaxies go to sleep, running out of fuel to keep forming stars, they must have experienced a significant structural transformation, sometimes involving a substantial shrinkage in size.
Interestingly enough, these transformations seem to be taking place in large numbers at the epoch of maximum cosmic star-formation activity. This value is the average of the rate at which galaxies are forming stars in a given epoch of the Universe. So essentially, this tells us that some galaxies that decided to go to sleep when most of their companions were having the most fun of their lives.
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| Galaxies evolve from large unstable structures to round spheroidal shapes before they finished their star-formation activity turning into small red-nuggets. Credit: Barro et al. (2013) |
In our recent CANDELS paper, we studied a large sample of massive galaxies at redshift z~2 (when the Universe was only 1/3 of its current age) to investigate how these processes took place. Our results suggest that the structural transformation happens first, meaning that actively star-forming galaxies evolve from large clumpy systems into compact spheroidal shapes and then they doze off. But what phenomenon is responsible for this extreme change? In previous entries we have talked about how galaxy mergers can completely transform the structures of the galaxies involved, producing an elliptical-shaped remnant. Mergers are also important because they can redistribute the gas of the progenitors to feed the black holes living in center of these galaxies, triggering and active galactic nuclei (AGN) which may play an important role in preventing the galaxy from forming new stars. In our analysis we find that many of these compact star-forming galaxies are emitting in the X-rays, a signature for the presence of an AGN, yet we still have to determine if AGN are causing the galaxies to go to sleep or if they are just spectators.
An alternative possibility for the cause of galaxy shrinking is self gravitational collapse. Unlike the galaxy disks observed in the local Universe, the clumpy star-forming galaxies at z~2 are not stable systems. The location and duration of these clumps is determined by the inflow of gas coming into the galaxy from the dark matter halo that they inhabit. Instabilities associated with this accretion process can cause the clumps to collapse and coalesce into small compact remnant.
There are different scenarios to interpret the transformation and subsequent truncation of the star-formation in compact galaxies. The challenge now is to combine observational results with theoretical models to identify the precise nature of this process. So stay tuned as we uncover new pieces of the puzzle.
