Wednesday, June 27, 2012

The Search for the Most Distant Galaxies

If you've looked at the night sky on a clear night far from a city, you've felt it. Something about this sight, of endless stars upon a field of black, with the disk of our Galaxy painting a milky path through it all, leads to a pulling. Some sort of visceral urge to understand why we are here, where we come from, and how the Universe ended up the way it did. This is a trait unique to humans on our planet, and is one of many which sets us apart from other species on our world. It is this primal feeling that urges some of us to become astronomers, to use science to attempt to quantitatively answer some of these fundamental questions. Using modern telescope facilities to search for (and discover!) very distant galaxies satisfies this need to probe our origins.

Not that long ago, we did not yet even know that there existed galaxies outside our own Milky Way. Remarkably, less than a century later, we now know that there are many more galaxies, perhaps hundreds of billions in our visible universe, and we see these galaxies as they existed in the past. This funny trick of physics happens because light has a finite speed (meaning that it takes it a certain amount of time to get from here to there). For example, it takes the light from the Sun eight minutes to reach the Earth; thus, we see the Sun as it existed eight minutes ago. The closest large galaxy, Andromeda, is two million light years away - it takes light from Andromeda two million years to reach us, so we see this galaxy two million years in the past. By looking at more and more distant galaxies, we can in essence watch the Universe play in reverse, and learn how galaxies form and evolve into the gorgeous spirals and majestic ellipticals we see today.

The speed of light is not the only factor affecting our observations. We have known since the time of Edwin Hubble that the Universe is expanding, and that more distant galaxies are speeding away from us faster than those that are close by. This adds an additional effect known as "redshift". Just as a train whistle lowers in pitch as a train speeds away from you due to the Doppler effect stretching the sound waves, light waves also get stretched when observed from a receding galaxy. As the wave gets stretched, the light appears to become redder. The farther away a galaxy is, the faster it appears moving away from us due to the expansion of the universe, and the more its light gets reddened. We can measure this shift in the galaxy color, which we call its "redshift". The higher the redshift, the more distant a galaxy is (and thus the redder it appears).

Since the Hubble Space Telescope was upgraded with the sensitive optical (meaning light you can see with your eyes) camera known as the "Advanced Camera for Surveys" (or "ACS") in 2002, we have had the ability to find galaxies as far away as a redshift of six. However, more distant galaxies were impossible to see, as at higher redshifts, all of the galaxy's light had been shifted out of the optical, and into the near-infrared (just redder than both our eyes and ACS can see).

With the installation of the new Wide Field Camera 3 on Hubble in 2009, we had our first opportunity to image deeply into the near-infrared. The image below shows the deepest near-infrared image ever taken, in the Hubble Ultra Deep field. In 2009 and 2010, a number of teams studied this image, and found ~10's of galaxies at redshifts of seven and eight. Some of these galaxies are so distant, that we are seeing them only 500 million years after the Big Bang***. This might sound old, but the Big Bang happened ~13.8 billion years ago, thus we're peering 96% of the way back into the Universe.

***The Big Bang refers to our theory of the early development of the universe.  It is extremely well tested, and the theory makes a number of predictions which we have verified observationally, including the cosmic microwave background, the expansion of the universe, and the distribution of primordial chemical elements.

This large image shows the near-infrared view of the Hubble Ultra Deep Field.  The smaller boxes show the 31 galaxies we discovered with redshifts greater than 6.3 in this field ("z" stands for redshift; don't ask why, astronomers give weird names to things).  Credit:  Steven Finkelstein

As you might imagine, this is an incredibly exciting time for this brand of science. However, to understand these galaxies, we need much larger samples, and this is where CANDELS comes in. With the full CANDELS dataset, we should find hundreds of these extremely distant galaxies (indeed with the data we already have we have found ~150 galaxies at these high redshifts). With this excellent sample of early-Universe galaxies, we can study in detail how the earliest galaxies in the Universe formed, and infer how they evolved down to those at much lower redshifts, where we have a better grip on things.

This is a first post in a series by myself and Russell Ryan, where we will lead you through the field of distant galaxies. In our next post, we will discuss how we actually find these galaxies. For example, in the Hubble Ultra Deep Field, which is a single pointing of Hubble, there are over 3000 galaxies, while only a hundred or so are the distant galaxies we're looking for. As you can imagine, it can be difficult! In subsequent posts we will talk about the discoveries being made by our team, including the colors of these galaxies, how their light and mass evolve with time, and how much they impacted a major event in the Universe which we call "reionization", which marks the time when galaxies first turned on and illuminated the universe with their light. Stay tuned!

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