Breaking the Galaxy Distance Record

In this loooong overdue post, I’m going to talk about what happened following the events of my previous post. In that post, I talked about how my research team and I used the Keck 10 meter telescope to obtain spectroscopy of 43 distant galaxies. To briefly recap, my group and I have been using CANDELS images to search for very distant galaxies (those that we see as they were within one billion years of the Big Bang, which gives them a redshift greater than 6). In a few previous posts, I’ve talked about some of the exciting things we’ve been learning in the distant universe, including how these galaxies get redder with time (as they build up their heavy elements; i.e. planet-making material), and whether galaxies can account for the reionization of the universe (yes!  we think).

In this previous post, we talked about how we use images to find these galaxies - essentially, since they are so far away, they are moving very quickly away from us, thus their light is redshifted due to the Doppler effect. Ideally, you would take a spectrum of every galaxy to search for redshifted emission lines to measure your redshift. However, this is impractical for samples of hundreds or thousands of galaxies. On the bright side, we can get a rough estimate of the redshift using imaging alone, and this technique has been well-documented over the past ~20 years.

The downside of this is that 1) the redshift is only approximate, and that makes everything else you learn a little more uncertain; and 2) its possible that some galaxies you think are really distant are actually close by galaxies that just happen to be very red. To get around this, we typically try to take spectra of a small portion of our sample, to verify that our contamination is small.  Fast forward, and this is why we went to Keck, to try to measure the redshifts for many of our distant galaxy candidates.

As I looked at the data we took at Keck, we found a very bright emission line from one of our distant galaxy candidates before we even left Hawaii. This left me feeling very optimistic!  However, as we continued to analyze our data, we found that the first line we saw would be the only line we would see - out of the 43 observed galaxies, we detected an emission line from only a single one. This may seem like a failure, but lets examine our detected galaxy a little more closely.

This image shows a region of the CANDELS GOODS-North field, just above the handle of the Big Dippler.  Highlighted is z8_GND_5296, the most distant spectroscopically confirmed galaxy in the universe.  The galaxy looks very red in this image, as it is so distant (and thus moving so quickly away from us), that it is only detected in Hubble's reddest filters.  Image Credit: V. Tilvi, S. Finkelstein, C. Papovich, A. Koekemoer, CANDELS and STScI/NASA.

The emission line we saw was the Lyman alpha line from hydrogen. This line is emitted in the ultraviolet, but we saw it all the way in the infrared, meaning that it has a very high redshift.  In fact, the measured redshift of this galaxy is 7.5, making it the highest redshift spectroscopically confirmed galaxy*** (the previous record was at 7.2). That's exciting in itself, but the galaxy had more in store for us. Using how bright it is in the CANDELS imaging, we can measure how fast this galaxy is converting hydrogen gas into new stars, and we found that its “star-formation rate” is an insane 300 solar masses per year; this is 150 times faster than the Milky Way!!! From what we (thought we) knew at high redshift, if you found a random redshift seven galaxy, you would have expected it to be forming stars at around 10 solar masses per year, so this galaxy is forming stars 30 times faster than its peers.  

Our spectrum from the MOSFIRE spectrograph on the Keck 10 meter telescope.
The white blob in the top panel shows Lyman alpha emission from z8_GND_5296. 
At the observed wavelength, this corresponds to a redshift of 7.5078. The bottom
panel shows a cross-cut of the top spectrum (what we call a one-dimensional spectrum),
which shows the galaxy's flux versus wavelength. You can see the peak
corresponding to Lyman-alpha emission (highlighted by the red line).
There are a number of other peaks too, which all correspond to the position of emission
lines from our own atmosphere. These are very bright, and we try to subtract
them out, so what you see here are residuals. The lines are difficult to
subtract completely, because their intensity changes rapidly with time.

Not only has this level of star factory not been seen at these redshifts before, but it was also a complete surprise to theorists, who do not see such galaxies in their models. While this galaxy could just be a weirdo, we don’t think thats the case. The previous record redshift holder I mentioned, at z=7.2, has a star-formation rate of 100 solar masses per year. Smaller, yes, but still very high. And, it is located in the same region of the sky as our galaxy.  What are the odds?!? What we think we’re learning is that these extreme star factories are much more common in the early universe than previously thought, so now we need to get with our theorist friends and try to figure out why that is.

As for the other 42 galaxies we didn’t see? The jury is still out. It may be that the gas between galaxies is becoming neutral (as would happen if we’re entering the epoch of reionization), and this neutral gas “fog” is screening us from seeing the Lyman alpha photons. Or, it could be that these distant galaxies are becoming increasingly rich in gas themselves, preventing these Lyman alpha photons from escaping. Only time and further study will tell, but we’re hot on the trail!  If you're interested in all the details, you can see our paper, which has been published in Nature, here, and our official press release, which is here.

***Often in the news there are articles about the most distant galaxies in the universe - some of these are spectroscopically confirmed like our galaxy here, while others are candidate galaxies, meaning that their redshifts have not been verified. While many of these candidates turn out to be real, measuring the redshift spectroscopically is the gold standard for galaxy distance measurements. A case in point is our recent blog post, which mentions a galaxy with a redshift of close to 11 from the CLASH survey. This galaxy has not been spectroscopically confirmed (though Hubble will try to do it in a few months). However, in the particular case of this galaxy, I think its highly likely that its real, as not only are its colors that expected of such a distant galaxy, but the positions of the lensed images are what you would expect for a galaxy at the estimated redshift.  Hopefully Hubble will measure a redshift, and, if not, then we’ll have to wait a few years for the next generation of telescopes.

Using Our Largest Set of "Eyes"

While our data from the CANDELS program is nothing short of revolutionary, occasionally we need to step back and remember that Hubble is "only" a 2.4 meter telescope (meaning that the diameter of the primary mirror, which sets the telescopes light-gathering power, is 2.4 meters). Since Hubble is in space, above the atmosphere, its still able to do great work. However, some tasks are best left to larger, ground-based telescopes. One of these tasks is to followup some of the CANDELS sources with spectroscopic observations.

Spectroscopy spreads the light from a source out into its component wavelengths; much like placing a prism in a beam of sunlight. Through this technique, we can study a variety of properties about very distant galaxies, including their distance. In previous blog posts, we talked about how we can find very distant galaxies using their colors (we call this "photometric redshifts"). However, this technique provides only an approximate distance, and there's always a (small) chance that an object identified as being distant is really not quite so far (we call these "catastrophic outliers"). To ensure that our color-based distances are accurate, its always a good idea to measure exact distances for some subset of your distant galaxy sample.

Because these galaxies are so distant, we can just barely see them in our Hubble images. If we tried to use Hubble to do spectroscopy, splitting the light up into its colors, we would no longer be able to see them; essentially, it would be like taking all the light from the CANDELS image, and splitting it up into multiple images. Since we can just barely see it when all the light is added together, we would not be able to see it if we split the light with a spectrograph. So, to spectroscopically study the distant universe, I obtained time with the largest optical telescope on the ground, the Keck 10 meter telescope.

Most of the time on this telescope goes to its two major partners, CalTech, and the University of California.  I'm at the University of Texas at Austin, and we don't have our own access. However, NASA typically has 10-20 heavily contested nights to get away, and we wrote a proposal, and managed to get two nights with Keck in April.  We used these nights to observe distant galaxies with a brand new spectrograph, called MOSFIRE. This instrument is revolutionary, its it is highly sensitive, can observe many objects at once, and it observes in the near-infrared, all of which are crucial to observe our very faint, very distant galaxies.

Keck is one of my favorite observatories to go to, since although the telescope is located at 13,500 feet on top of Mauna Kea, we observe from the town of Waimea, at an elevation of 1000 feet, and only 15 minutes from the beach (and about 100 feet from Starbucks!). Joining me on the run was Mimi Song, a graduate student working with me at UT Austin, and Vithal Tilvi, a postdoctoral research from Texas A&M. Over our two nights, we obtained data on 43 very distant galaxies. I will write another post soon on the results from our run, so stay tuned!

Myself, Tilvi and Mimi in the Keck control room.

The Winding Road of Becoming an Astronomer

When did I decide to become an astronomer? That's an interesting question, since I don't remember actually deciding, it just sort of happened.  I've always liked space; I remember watching Space Shuttle launches on TV as a kid, and I vividly remember where I was when I heard about the Challenger disaster. But I went through a lot of phases, at times also thinking I wanted to be a vulcanologist and a meteorologist (though really I just wanted to be a tornado chaser, inspired in no small part by Twister). So science was always high on my list, but I think it was my senior year in high school when I really got attached to astronomy, mostly due to my awesome experience in my physics class (thanks Mr. Haff!). I remember when we were learning about gravity, I went home and, just for fun, did all of the homework problems in that chapter of the book (yep, I was a nerd). So, when it came time to choose a major, I didn't agonize too hard before choosing astronomy (though I nearly chose Music Education; I played trombone throughout high school and college).

Keely and I near Anchorage, doing some sightseeing during the June 2012 AAS meeting.

Things got a little bumpy in college at the University of Washington; due to the aforementioned trombone playing, I didn't have a lot of time for classwork, since I spent my nights practicing marching band formations in Husky Stadium (explaining my lifelong love of the Huskies to anyone who knows me; and yes, they were good back then!). In fact, I remember a key point at the beginning of my junior year when I nearly quit the major altogether. I was taking an upper division electromagnetism (E&M for those in the know) course at the same time as an upper division math course that was listed as a prerequisite for the E&M course, but they allowed me to take it concurrently. I remember being utterly lost for the first few weeks of the E&M course, since the professor was using math that I hadn't learned yet (multivariable calculus for those scientists out there). I have a vivid memory of standing outside the physics building in the rain (Seattle, so it was raining of course), about ready to quit. But, in hindsight what was probably one of the major turning points in my life, I didn't quit. A few months later, I decided I wanted to go to grad school, and I busted my tail off for the next year to get my grades up, and prepare myself for the next step.

Our son Kieran, super excited about his
octopus face painting.

I attended Arizona State University for graduate school. At the time, the department was on the small side (though it has grown rapidly), and did not represent a high pressure environment. But that environment is exactly what caused myself and my classmates to thrive (of the five of us who earned our Ph.D's in 2008, three already have permanent positions, and two more are CANDELS postdocs). A huge advantage of being at ASU was access to the Steward Observatory telescopes, which I took full advantage of, observing at the MMT, Magellan, and the Steward Bok telescopes (as well as spending time at both the NOAO Kitt Peak and Cerro Tololo 4m telescopes). Grad school is a tough time for everyone, but we had a great group of friends that were committed to their work but also to having fun, which made it not *too* bad.

Another major event happened for me in Arizona -- I met my wife, Keely. We had actually both gone to UW for undergrad, but we were a year apart so we never met. We started dating towards the end of my first year, and we got married in August of 2008 (right in between our thesis defenses, talk about a crazy time!). After carefully weighing our options we decided to accept postdoctoral positions at Texas A&M University. While College Station is not exactly a cultural mecca, we enjoyed working in a growing department with several young faculty, who were great at promoting our careers. It was also great for our family life, as halfway through our stay, we welcomed our son Kieran to the world.

The space shuttle flying over Austin
on its way to LA.

After three years in College Station, I was fortunate to win a Hubble Fellowship (on my fourth try nonetheless), which I took 100 miles west to the University of Texas at Austin. UT happened to be searching for a new faculty member in my area (high redshift galaxies), so during my first year in Austin I spent much of my time applying and interviewing for that job, which I was eventually offered. So, now after a little over four years in Texas, my family has settled in for the long haul (and those of you who have been to Austin know that its not a bad deal!).

So, that's my life story (or at least my astronomy life story). What can be learned from it? First, being an astronomer/scientist/academic is hard. While I'm sure some people know their whole life who and what they want to be, and never waver, that was not my experience. I nearly left astronomy at least three times throughout the years, and it was only a few key choices (that were well thought out, though also some were the result of being in the right place at the right time) that got me to where I want to be. Astronomy is just a job, but we each only have one life, and I've tried to remember that when I've been confronted with choices throughout my career.

Our dogs Jasmine (left) and Bella (right).

Surfing the High-Redshift Universe in Santa Cruz

I was waiting all week long for the last day of the CANDELS team meeting, not to go home, but because on that day we finally had the high-redshift sessions!  We had two sessions devoted wholly to high-redshift science, split with some science talks, and some discussion sessions.  My primary goal for these sessions was to get our group to work together - we have high-redshift experts in CANDELS spread not only throughout the US, but around the world, with large groups in Edinburgh and Rome.  Each of these groups sent representatives to Santa Cruz, so it was a great opportunity for the team to catch up on the goings on around the high-redshift team.

We also got updates on some of the papers in progress.  One of the most interesting results came from Vithal Tilvi, who is a postdoc at my old stomping grounds, Texas A&M.  He has been combining CANDELS data with some ground-based medium-band images (meaning he's using some filters narrower than those we've been using on HST to isolate specific wavelengths).  His primary goal was to find distant galaxies.  But in an ironic twist, he thinks he might have found an extremely nearby object; a very low-mass star known as a Y-dwarf (these are so small they are not undergoing any fusion, and thus are known as brown dwarfs, and this particular flavor of brown dwarfs are barely larger than Jupiter).  Brown dwarfs have colors similar to high-redshift galaxies, thus they can hide in our samples.  Typically they're thought of as contaminants, but this particular type of star is rare, so its more like a diamond in the rough!  We also heard some updates by Giovanni Fazio on the status of his deep Spitzer Space Telescope program which is imaging the CANDELS field in the infrared, and I updated the team on my ongoing work measuring the luminosity functions of distant galaxies (which is a measure of the distribution of galaxy brightnesses).

An image of the field with the Y-dwarf, with the box highlighting the brown dwarf.  The inset shows an image of a Jupiter-like planet, which likely doesn't look much different from this brown dwarf.

We had some interesting discussion sessions as well.  We made a lot of progress on a project we're doing to create a catalog of "CANDELS-approved" very distant galaxies.  We also discussed how best to proceed with measuring galaxy clustering, which is a highly interesting yet extremely difficult way to measure the cosmic structure from the positions of galaxies.  And we had a great session where we interfaced with our fantastic theory group, working with them to identify the key problems we should be focussing on.  So now its back to work for all of us, but it won't be too long until we get together again - we're planning for our next high-redshift meeting to be in the scenic city of Sesto, Italy in January.  Sesto is like the Aspen of Italy, so don't forget your snow pants!

Did CANDELS find the first stars in the Universe?

Did that title grab your attention? Spoiler alert: the answer is no (probably), but the journey to that answer was interesting! Read on!

In today's blog post, I'm reporting on a paper I wrote with the help of many other members of the CANDELS team which has just been published in the Astrophysical Journal. The title of the paper is "The Evolution of Galaxy Rest-Frame Ultraviolet Colors from z = 8 to 4" (and you can find it here). There's a lot in the title, so let's break it down: the word "evolution" at the beginning means that we're studying how galaxies change (i.e. evolve) with time. In this particular paper, we're focusing on a time period in the early universe, from around 0.5 to 1.5 billion years after the Big Bang. The universe is now 13.8 billion years old, so this is early on indeed, but as I'll show, things change quickly even in this short time period. The "from z = 8 to 4" denotes this time period; astronomers use a term called "redshift" to denote distance (see these earlier blog posts, #1 and #2, for more details), so a redshift (or "z") of 4 corresponds to 1.5 billion years after the Big Bang, and a redshift of 8 is about 0.5 billion years (or 500 million years) after the Big Bang.

The meat of the title is in the middle: "rest-frame ultraviolet colors"; this means we are looking at the colors in the ultraviolet, which is just bluer than the eye can see. However, due to the expansion of the universe, at these high redshifts this ultraviolet light is shifted to the near-infrared. Thus, we add the qualifier "rest-frame". What this means is that even though the light we observe is in the near-infrared, we have a good idea of the redshift of a given galaxy, so we can figure out what the intrinsic ultraviolet (or UV) color is (you may have seen this referred to as "k-correcting", which is a similar concept).

So, why is this interesting? The UV light tells us a lot about ongoing star-formation. This is because the UV light from galaxies is dominated by massive stars (many times more massive than the Sun). Massive stars are very hot, so they are very bright in the ultraviolet, and they look blue (by comparison, the Sun appears yellowish, and lower mass stars look red). But, massive stars are very short-lived, some exploding as supernovae after only 10 million years. So, if you look at a galaxy and it is bright in the UV and also blue in color, you immediately know that massive stars are still around, so it must be actively forming a lot of stars!  Cool, right?

The story doesn't end here. A number of studies predict that when we look at galaxies very early on, they should appear even bluer than galaxies observed at later times. This is because in the Big Bang the only elements to form were hydrogen and helium (with trace amounts of lithium). If you make a star with just those elements, it will be very hot; this is because gas needs to cool to very low temperatures to condense and form "normal" stars like those we see in today's universe. However, to cool down, gas typically relies on atoms heavier than helium (which we astronomers call "metals") to radiate away energy. If you don't have those elements, then stars do form, but we think that they end up being much more massive (maybe even more than 100 times the mass of the Sun); these massive stars are extremely hot, so they are very blue, much bluer than any star with a normal allotment of metals.

Figure 1: The ultraviolet colors of galaxies at different redshifts (the top vertical axis shows the amount of time in billions of years since a redshift of 20; add 0.2 to change it to since the Big Bang). The red circles show the average of all galaxies, while the blue circle shows the average colors of faint galaxies at a redshift of 7.

So, we were motivated to look through the CANDELS data for distant galaxies, and measure their colors to see how blue they looked. Sounds simple, right?  I'll skip over a lot of the details (though you can learn how we found the galaxies here), but our main results are shown in Figure 1. This plot shows the colors of galaxies at each redshift we study.  The red points show the average colors of all galaxies, but we'll focus on the blue point first, since it represents the average color of faint galaxies only (which tend to be bluer). The gray line and arrow shows the color we would expect if these galaxies had stars with very little metals; if our galaxies had very little metals, we would expect them to be below this line. What we see is that even when we look at the faintest galaxies, while they do look very blue, they are not so blue that we think they host stars with very low metallicities. In fact, taking into account the error bar on this point, these galaxies have colors consistent with the local galaxy NGC 1705 (shown as the cyan bar); while NGC 1705 is blue, it has been studied very intensely, and it doesn't have any metal-free stars.

What this tells us is that even though we're looking less than one billion years after the Big Bang, galaxies have already been enriched by metals a significant amount. This means that star-formation must have already been occurring for a while, so that when we have the capability to look to yet higher redshifts (which we will later this decade with the James Webb Space Telescope), there should be plenty of galaxies burning bright for us to see!

There is one last point to make, this time looking at the red points, which are the average colors of all galaxies at a given redshift.  We see that they start out fairly blue at z=7, and get progressively redder. We think that this is due to the increased presence of cosmic dust grains (composite particles made up primarily of carbon and silicon) in these galaxies. Dust has the funny effect of making the colors of a galaxy look redder, since they absorb and scatter blue light more efficiently than red light, hence the phrase "dust reddening". What we're witnessing here is the build-up of vast reservoirs of dust in these galaxies, in only one billion years of cosmic time! Although dust may sound boring, it has a dramatic effect on the colors of galaxies, as you can see. Also, all of this UV light absorbed by the dust has to go somewhere, and its energy actually gets re-emitted in the far-infrared (much redder than your eye can see). In fact, when we look at galaxies closer to home, many of them are brighter in the far-infrared than in the UV, implying that much of their UV light has been absorbed by dust. What our results here tell us is that while this is likely not an issue at a redshift of 7, it becomes progressively stronger as you move to lower redshift. So, if you want to study galaxies at high redshift, much like the Alamo you must "remember the dust!"

Learning Astronomy in West Texas - A CANDELS Teacher Workshop

Frank N. Bash Visitors Center, just a few minutes drive from the
observatory. The observatory is about a 7 hour drive from Austin,
 and only a couple hours from the Big Bend National Park.

For today's CANDELS blog post, I'm reporting from the McDonald Observatory in West Texas, which is operated by the University of Texas (my home institution). I am here as part of a workshop for middle and high school teachers. We hold a number of these every summer, and I'm particularly excited for this one, as we're basing it on CANDELS science!

Before we talk about the workshop, I wanted to give you a brief overview of the observatory. This setting is very unique; compared to a lot of other telescopes, which are on jagged mountain peaks, the McDonald Observatory lies in the Davis mountains, which among other things, are more hilly, and green (at least this year)! The first telescope built here was the 82 inch Otto Struve telescope, finished in 1939, after the university received an $800,000 endowment from Texas banker William J. McDonald. Since then, we have added the 107 inch Harlan J. Smith telescope, and 9.2 meter Hobby Eberly Telescope (along with a number of ~1 meter telescopes).

Dr. Roderik Overzier, a Prize Fellow
postdoctoral researcher, explains his
107" observing program to the teachers.
They took a tour of the building,
and were even allowed to "drive" the
telescope, changing where it was pointing.

The teachers got to experience a test drive of the "Hubble Universe" iPad app, which is being co-written by two CANDELS members, Dale Kocevski and Elizabeth McGrath. The app, which will be released next year, will take you on a tour of the universe, stopping off in the key CANDELS science areas.
 

Keely talking with a couple of the teachers,
who were working on an interactive activity
to study galaxy sizes and lookback time.

A unique aspect of this observatory is that although it is quite distant from any city (it took us seven hours to drive here from Austin), it has a state-of-the-art visitors center. The drive is not a deterrent, as while I sat in the StarDate cafe in the visitors center this afternoon, I saw 20-30 people pass through in just an hour. They do frequent star parties here which typically attract hundreds, and for good reason, as the observatory is basked in the darkest skies in the continental US.

Another great thing they do out here is to hold workshops for middle and high school teachers to come learn some astronomy, and participate in activities which they can bring back to their classrooms. A few months ago, we obtained funding through the Space Telescope Science Institute's Education and Public Outreach initiative to run one of these workshops based on the science goals behind our CANDELS project. Along with my research interest, I have a personal interest as well, as this workshop was run by my wife Keely, who is a Research Associate at UT Austin, and works with the McDonald Observatory education and public outreach team.

For an observer, I had a difficult time
assembling my Galileoscope!

A total of 15 teachers attended the workshop, from all over Texas and Oklahoma (although people have been known to come from as far away as California and New York to attend these workshops). We've done a number of activities related to CANDELS, including studying the expansion of the universe, learning about spectrographs and different wavelength regimes in astronomy, and talking about the Hubble Space Telescope. You can see some of the activities here. They also let me give a talk about my research, where I explained how we go about finding distant galaxies. The teachers also were able to tour the HET and the 107 inch telescopes, and both nights they have been observing through the eyepiece of a 36 inch telescope, looking at objects such as M51 (the Whirlpool galaxy) and M87 (which I described as "honking big!").

These teachers are very dedicated, spending their time out here learning how to better teach astronomy to the next generation. While we don't need to make everyone into a scientist, today's young people will be tomorrow's voters, and a public well-educated in astronomy can only lead to good things for our understanding of the universe.

Keely with the teachers gathered in front of the Hobby Eberly Telescope.

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!