Where and When do Galaxies Form their Stars?

One of the biggest questions in galaxy formation is how, where, and why galaxies form their stars. The CANDELS project is working towards a complete census of galaxies in five deep regions of the sky, probing significant numbers of galaxies from the first billion years of the Universe, all the way to the very dim galaxies in the nearby Universe. We are fundamentally interested not just in completing the census, but in developing theoretical models that explain how this diversity of galaxy properties arises.

Within the last 15-20 years, we have developed a very compelling cosmological framework, called "Lambda-CDM," which explains the contents of the Universe and its evolution on large scales. This model has been tested by a wide range of data including data from the cosmic microwave background, which provides a detailed picture of the tiny fluctuations in the early Universe that gave rise to all of the structure we see today; the evolution of the most massive objects in the Universe, galaxy clusters, which provide a way to follow how this structure grows with time; and stellar explosions called supernovae, which provide a way to probe the expansion history of the Universe. Within this framework, most of the mass of the Universe is not in stars at all, or even in the gas that fuels galaxies. Most of the mass is "dark matter", made of a particle which does not absorb or emit light.

Although we can't see dark matter directly, we can probe its effects on the light emitted by galaxies, and in fact can use galaxies to trace where and how dark matter gets put together over time.  In our modern picture of galaxy formation, every galaxy in the Universe forms at the center of a gravitationally bound clump of dark matter, called a dark matter halo. We can use large computer simulations to determine how dark matter halos get built up over time, providing a scaffolding on which galaxy formation, galaxy growth, and galaxy evolution take place.

Formation of a Massive Galaxy Cluster from Risa Wechsler on Vimeo.

The movie above shows this process, just for the dark matter in a region of the Universe that becomes a massive galaxy cluster today. The bright spots here are just dense clumps of dark matter, but in our Universe this is where we expect to form galaxies.  (If you'd like to learn more about our movies, check out this recent Science Bytes special "Dark Matters" on pbs.org!)

My group has been developing models of how galaxies build up on this dark matter scaffolding, and using observational data, from CANDELS and elsewhere, to infer the process of how galaxies grow and form stars within their dark matter halos. We have compiled a large amount of data from several observational surveys, and used this to say something about the average star formation histories of galaxies over time, as related to the growth of the dark matter clumps that they live in.  On average, the star formation rate in all galaxies peaked nearly 10 billion years ago, and has been declining since then. However, the rate at which star formation peaks in galaxies depends on their mass.  In a recent study, we found something very interesting. Although galaxies come with a range of shapes, sizes, masses, and star formation rates, we found that the efficiency of turning gas into stars in galaxies is nearly constant with time, and is peaked at a specific mass.  At all times over the last 10 billion years, stars form most efficiently in galaxies roughly the mass of our own galaxy the Milky Way.  This efficiency is also nearly constant over this whole time period. This means that nearly 2/3 of all stars ever formed in the Universe were formed in systems roughly the size of our own galaxy!

Halo mass as a function of time since the Big Bang. Colored bands
represent different star formation efficiencies; the yellow region indicates
the mass at which halos turn the largest fraction of their gas into stars.
Credit: Behroozi, Wechsler, and Conroy 2012

This basic picture helps us understand a number of galaxy properties. In the picture on the left, the yellow band is the region in halo mass where galaxies are able to form stars most efficiently. Small things (bottom white line), though very numerous, are not very efficient at forming stars. Today, they are generally blue, because they are still forming stars today (but slowly!). Big things, like the giant elliptical galaxies at the centers of clusters, formed a lot of stars early on (when they were roughly the mass of our own galaxy), but their halos continued to grow (top white line) and they stopped forming stars efficiently once they got more massive than the Milky Way. They are red because they haven't formed stars recently. Galaxies similar to our own actually spend a lot of their history in the region that is most efficient, and thus they form more stars relative to their total mass.

So far, this study compiled a range of data from various studies in the literature, and did not use data directly from CANDELS. The exciting thing about CANDELS is that we will be able to study galaxy star formation rates and galaxy masses for both very small and very large galaxies in the same way across more than 10 billion years of cosmic time. For the same galaxies, we will also have information about their environments, which are related to the mass of the dark matter halos that they live in, and their morphologies, which are likely related to the merger histories of the galaxies in those halos. This will allow us to build up a full picture of the diversity of galaxy growth, from the first billion years of the Universe until today.  Theorists will have a lot more work to do to understand these data, but we are looking forward to it!