Friday, August 10, 2012

Making Sense of the Census

Walk into a mall or a sports stadium, and you'll likely see a diverse snapshot of the human population.  There's an enormous range of shapes, sizes, colors, and ages in such a census.  This diversity arises from some combination of birth properties, i.e. nature, and environmental factors, i.e. nurture.  Even though you are seeing only a fleeting moment in each of each person's life, you know that each human evolves through various stages of life from birth to death.  How that life unfolds depends on both nature and nurture, and the relative influence of each continues to be hotly debated.

CANDELS provides an analogous snapshot of the galaxy population. Like people, galaxies go through distinct stages of birth, growth, and death. And like people, the way in which galaxies progress through these stages depends on both nature (its intrinsic traits) and nurture (its environment). CANDELS is remarkable in that it is the first survey capable of directly observing galaxies in all phases of the life cycle, in a wide variety of environments, from the early Universe until today. This impressive data set is designed to provide us a much more complete census of the galaxy population, so we can answer fundamental questions such as the role of nature vs. nurture in establishing why galaxies look the way they do.

As impressive as CANDELS is, there is one intrinsic difficulty with studying the life of galaxies in contrast to the life of humans: One can watch humans change on human time scales, but galaxies evolve over far longer time scales. So CANDELS only provides that single census in the mall or the stadium; it cannot show what happened before or what will happen next to any individual galaxy.

So how are we to piece together the life story of galaxies from a single snapshot? How do we know which young galaxy will turn into which old galaxy? How can we figure out what causes galaxies to transition from one stage of life into another? How do we "connect the dots" into a full life story of galaxies? For humans, we know the answers because we've watched it happen, from birth to growth to life to death. But we have no such blueprint for the life of galaxies.

Video showing stars forming in a cosmological simulation from an early epoch until today, where the stars are color-coded by age (blue=young, red=old).  This is analogous to what might be seen in a CANDELS field at a given redshift.  Stars form in smaller galaxies and then merge together to former larger, older systems.  Note that there are two side-by-side projections of the same volume, face-on (left) and edge-on (right).  Movie by Ben Oppenheimer.

This is where the crack team of CANDELS galaxy formation theorists comes in.  Our job is to fit the entirety of the vast CANDELS data set into a single coherent story about why galaxies look the way they do, all within the context of our now well-established concordance cosmology framework.  Simply put, our job is to figure out a blueprint for the life of galaxies.  If that sounds just a tad bit ambitious, well, it is.  Not surprisingly, we haven't succeeded yet -- otherwise there wouldn't be much point in doing CANDELS!  Still, by carefully examining CANDELS data in the context of our current understanding of the Universe, we can make increasingly more educated guesses as to what makes galaxies tick.

How do theorists do this?  To begin with, we need a model for how galaxies form and evolve.  Within such a model, we can directly watch galaxies move through their lives, and try to understand what physical processes are responsible for setting a galaxy's properties at any given life phase.  If our model is successful at matching available observations from CANDELS and other data sets, we can hope (though not guarantee) that the insights we get are applicable to real galaxies, not just model ones.

Evolving fly-through of a hydrodynamic simulation showing the gas density, color-coded by temperature.  Note the Cosmic Web of filaments and sheets forming due to gravitational instability.  The nodes of the cosmic web are where galaxies that would be seen by CANDELS form.  The intersections of the filaments cause gas to shock-heat to high temperatures; by the end (redshift z=0), the video is centered on a galaxy group with gas heated to 10,000,000 K by gravity alone.  Movie by Ben Oppenheimer.

Step one is building a model for the entire galaxy population.  The technique I use is called a cosmological hydrodynamic simulation.  Big words, so let's break it down:  "Cosmological" means that I am trying to model a representative portion of the entire Universe.  "Hydrodynamic" means that I am interested in modeling the gas directly, including process such as shocks and radiative cooling (discussed below).  And "simulation" means that it's run on giant supercomputers, often taking months for a single run.  Such simulations are fast becoming a key cog in connecting the physics of galaxy formation to the galaxy observables seen in CANDELS data, because the evolving interplay between cosmology, gas physics, and galaxy formation processes are too complex to solve using pencil and paper.

These simulations begin with the (well-established) conditions shortly after the Big Bang -- a relatively smooth, hot Universe with dark matter and gas.  We typically model a cubical portion of the Universe by representing it with particles, each one representing a portion of cosmic mass.  Dark matter particles dominate by mass but interact only via gravity, while less massive gas particles additionally interact hydrodynamically.  The gas particles are initially comprised of hydrogen and helium, but as stars form they can additionally hold heavy elements (or "metals", in yet another misnomer of astro-lingo) such as carbon and oxygen.  Gas, unlike dark matter, has the important property that it can lose energy by emitting radiation, and thereby can release its gravitational potential energy and sink to the center of the dark matter halo.  This process, known as radiative cooling, enables gas to condense out of the Cosmic Web of dark matter into dense knots that form galaxies.  Once gas condenses into galaxies, things get tricky, since many poorly-understood processes govern the conversion of gas into luminous matter (i.e. stars).  We try our best to include all these processes in our models, but since we don't understand them in detail, we typically end up running lots of models with a range of parameters to see which one matches data most closely.

A surprising complexity is that matter doesn't just flow in to galaxies, it also flows outGalactic winds are now seen ubiquitously in rapidly star-forming galaxies, but what powers the expulsion of gas against the enormous gravitational pull of an entire galaxy remains a mystery.  Candidates include the collective effects of thousands of supernovae, or radiation pressure from hot young stars, or energy released by the monstrous black hole that lurks at the center of most every galaxy.  Once ejected, some material can fall back in, in a process we call wind recycling.  The net effect is a cycle of mass, metals, and energy flowing into an out of galaxies.  This means that galaxies and surrounding intergalactic gas form a sort of cosmic ecosystem, and simulations tell us that the way in which matter moves around in this cosmic ecosystem governs the way that galaxies evolve.

Close-up of a forming disk galaxy from early epochs to redshift 2 (i.e. "cosmic high noon").  Left panel shows the density, right shows gas color-coded by the temperature.  If you look closely in the right panel late in the movie, you'll see wind recycling happen: a hot plume of gas is ejected from the galaxy, some of which rains back down onto the thin disk.  Movie by Daniel Angles-Alcazar.

Does this mean "nurture" (i.e. surrounding environment) is more important than "nature" for galaxies?  Well, certainly the cosmic ecosystem is important, so a simple interpretation might lead one to say "yes".  But hang on -- it turns out that the properties of this baryon cycle seem to be surprisingly tightly correlated with the galaxy's stellar mass, which is an intrinsic quantity!  This would lean more towards the "nature" interpretation.  As with human behavioralists, the nature versus nurture debate rages on in the galaxy formation community.

The close connection between theory and data within the team is one of the most exciting aspects of the CANDELS survey.  CANDELS is clearly a huge step forward observationally, and it is an ever-increasing challenge for us theorists to keep pace.  In upcoming blog posts, I'll describe what we're learning about the birth, growth, and death phases of galaxies by combining state-of-the-art simulations with CANDELS data.  As we'll see, there are still far more questions than answers.  Yet it's clear that the galaxy population displays a beautiful and complex diversity, much like the human population.  For me, this makes galaxy formation an infinitely fascinating subject of study.

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