Friday, November 2, 2012

How Do Galaxies Grow?

The beauty of CANDELS is that it allows us to study how galaxies grow over an unprecedented dynamic range in mass and time. A particular focus of CANDELS is studying Cosmic Noon, the epoch from redshifts around 4 to 1, when the universe was about 2 to 6 billion years old. During Cosmic Noon, the universe was forming stars at more than ten times its current rate, making it the most active period of galaxy growth in cosmic history. Clearly, if we want to know how galaxies grow, studying this epoch is pretty important.

One of most influential new observations that has emerged from this epoch is a remarkably simple and tight relation between a galaxy's star formation rate and it's stellar mass: The two are linearly related!  In other words, larger galaxies form stars faster. This relation, now referred to as the "galaxy main sequence", might not sound terribly surprising, but it gives us a crucially important clue: It implies that galaxy growth is a smooth process. This is far from obvious when you look at a CANDELS image, because the universe looks like it is filled with a bunch lumps that we call galaxies! Moreover, we see that galaxy mergers are much more common during Cosmic Noon than today. Mergers are known to temporarily boost star formation by large factors (called a starburst) without increasing the stellar mass (as) much.  If this was the dominant process for galaxy growth, the galaxy main sequence would not be tight, it would show a huge scatter! The fact that we see a tight relation can be used to show that merger-induced starbursts likely contribute only about 10% of star formation during Cosmic Noon.

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Simulations of galaxy formation can help us interpret what we see in the real Universe. This movie from a simulation shows only the stars (although the simulation itself also includes gas and dark matter) -- bluer stars are younger, redder are older. The volume depicted is roughly 50 million light years on a side, with face-on (left) and edge-on (right) views. We see how galaxies start out small and young arranged along the filamentary Cosmic Web, and merge together into larger, older galaxies -- this is called hierarchical structure formation. If stars were all that were visible (like in CANDELS images), one might infer that galaxies grow mostly by merging. But simulations suggest that this is only the easily-visible tip of the iceberg, and that relatively smooth gas inflows are actually the driver of galaxy growth (see the movie below).

This tight galaxy main sequence has a surprising corollary: If we only see the "lumps", while galaxies grow smoothly, this must mean that we don't see most of the matter that drives galaxy growth!  So where is this vast, unseen reservoir of fuel? Most believe that it is gas in the intergalactic medium, the vast expanses of space surrounding galaxies. This gas is very difficult to detect (which is why the space between galaxies looks so "empty"), but there is lots of observational evidence from the so-called Lyman alpha forest that the vast majority of atoms in the Universe live in the intergalactic medium, not in galaxies.  

Despite theoretical expectations, nobody has conclusively observed this intergalactic gas "in the act" of falling into galaxies, so at present this fueling mode remains a bit of theorist's fancy. Indeed, this is a major focus for next-generation telescopes such as JWST, GMT, and TMT, whose increased sensitivity may be able to directly detect inflow. Nonetheless, CANDELS is already providing indirect constraints by providing the best measurements to-date of the galaxy main sequence.

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This simulation movie shows the gas rather than the stars. The large left panel shows the velocity field, in which one can see gas streaming down three converging filaments into a galaxy. The volume shown is roughly 7 million light years on a side. The "Density" panel shows the gas density -- little bright knots are where galaxies would form, and one would see stars, but the remainder of the gas is very difficult to detect. Other panels show gas temperature, metallicity, and metal-line absorption. The Cosmic Web is full of gas, providing relatively smooth fuel for galaxy growth. At around redshift z=3, one can even see an outflow that ejects gas from the galaxy (which is at the center of the panel) perpendicular to the inflow filaments.

A second remarkable new addition to the landscape of galaxy formation is that matter doesn't just flow in to galaxies, it also flows out. In fact, a lot of it flows out -- observations during Cosmic Noon suggest that the rate of mass flowing out from galaxies likely exceeds the rate forming into stars! Expelling this much gas at hundreds of km/s as observed requires enormous amounts of power, comparable to the power from all the supernovae going off in the galaxy. This seems rather surprising because galaxies appear to be rather serene, isolated object, as opposed to roiling pots of explosive energy that drive huge galactic outflows. But again, that may just be because we can't see the gas!

These new observational ingredients seen in galaxies during Cosmic Noon have caused CANDELS theorists to rethink some fundamental ideas about galaxy formation. How do we put all these insights together into a full story for how galaxies grow? One emergent paradigm is that galaxy growth proceeds via a balance between inflows and outflows. Material is brought in by gravity from the intergalactic medium, some of it gets turned into stars, and the rest is expelled in an outflow. This process is continuous and ongoing, resulting in a fairly smooth rate of growth. This is now colloquially referred to as the baryon cycle.

A helpful analogy for this is a factory. What does a factory do? It takes in raw materials, produces a product, and emits some waste. It does so fairly continuously, modulo small fluctuations in supply and demand. Analogously, a galaxy takes in gas accreted from the intergalactic medium, turns them into stars, and ejects outflows. This fluctuates as the inflow varies (after all, the universe does have some lumps!), but overall is a fairly steady. Hence in the baryon cycle view, galaxies are like gas processing factories, continually churning out stars from gas, at a rate that is mostly smooth with mild variations.

The outflows then represent the factory's waste. In fact, astronomers even use the word "pollution" to describe this ejecta. But in this case, astronomers mean something very specific -- pollution here refers to the fact that outflows carry out the by-products of star formation, which are the heavy elements ("metals") synthesized in the cores of stars. You  have probably heard that we are all made of stardust, that carbon, oxygen, and indeed all elements other than hydrogen and helium are produced by nuclear fusion inside stars. When stars explode in supernovae that drive these outflows, they pollute the outflowing material with these metals. The metals therefore provide an excellent tracer for where outflows have reached. Remarkably, quasar absorption line observations indicate that they have reached millions of light years away from galaxies!

The baryon cycle scenario can straightforwardly explain why the cosmic star formation rate is higher by a factor of 10 during Cosmic Noon compared to today -- it's because the universe was smaller and denser, the hence the expected gravitational inflow rate is higher by exactly that amount back then! It straightforwardly explains the tight relation between star formation rate and stellar mass, with the scatter around the mean trend reflecting the "lumpiness" of accretion. It is attractive in its simplicity and is well-situated within hierarchical structure formation models.

But none of that necessarily makes the baryon cycle scenario correct. Indeed, it is far from the only way to explain these data. The increased merger rate during Cosmic Noon suggests that the lumpiness plays a key role, and could drive the increased star formation rates. Moreover, galaxies around Cosmic Noon tend to look more disturbed, suggesting that even if the inflow is smooth over long timescales, on short timescales it may be quite chaotic and stochastic. Finally, the baryon cycle idea centrally invokes two phenomena -- inflows and outflows -- that have at best only recently been detected, and have yet to be well-characterized.  Hence while from a theoretical standpoint it is a simple and attractive model, from an observational standpoint there is much work to be done to test the baryon cycle scenario.

The unparalleled dynamic range of CANDELS, probing massive galaxies down to dwarfs at Cosmic Noon, will measure galaxy scaling relations like the main sequence with unprecedented accuracy, and relate it to many other interesting galaxy properties such as morphologies, colors, environment, and structural parameters. CANDELS theorists are working hard to take these observations and interpret them to see if they are consistent with the baryon cycle scenario. As scientists always love to subvert the dominant paradigm, observers and theorists alike are looking for ways in which the baryon cycle fails, or at least is an incomplete description of what's going on (which it almost certainly is). Through such investigations, we hope to gain a more detailed understanding how these gas factories process their raw material into stars during the most active phase of universe, Cosmic Noon.

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