CANDELS Detects First Light Galaxies

This is a guest post by Ketron Mitchell-Wayne, graduate student at the University of California-Irvine. He and other CANDELS team member recently published a paper in Nature. This paper was the subject of a recent press release. Here, Ketron describes the project and the role that he played.

The cosmic extragalactic background light is a product of many different component emissions throughout all cosmic times. Recent CANDELS observations have opened up a new window of opportunity for measuring this cosmic background light at optical and near infrared wavelengths. We have assembled Hubble frames taken over a 10 year period and mosaiced them to produce some of the deepest images suitable for such a study. With the mosaics, we can study this diffuse, clumpy light that resides behind all the resolved stars and galaxies in the mosaics. With statistics, we have attributed a fraction of this diffuse background to the first light galaxies during reionization. Here's a short summary of the work that I did, over the course of two years, in order to make these very interesting measurements.

My main job for this paper was generating the mosaics and making the statistical measurements. I started working on the data reduction in the summer of 2013 and have spent the better part of the last two years working on the project. Anton Koekemoer had a data reduction pipeline set up for all the incoming CANDELS data, but I wanted to incorporate archival data in our analysis too. So I had a number of reduction steps to complete on thousands of frames, even before making the mosaics (which is in itself very difficult).

Once we had mosaics in multiple bands (left panel of Figure 1), I generated a source mask. We want to isolate the background light signal, so foreground stars and galaxies need to be removed from the image. The dark areas in the second panel of Figure 1 is the source mask (just zeros in the array).

Figure 1: These three panels show different components of near-infrared background light. The one on the left is a mosaic of images taken, the one in the middle shows the intrahalo light seen when masking out all the stars and galaxies, and the one on the right shows the signature of the first galaxies. Credit: Ketron Mitchell-Wynne / UCI

At this point I could start making statistical measurements of the background light in the mosaiced, source-subtracted maps. The methods we used aren't new, but much of the dataset was. We used a very similar method to what was used in the Cosmic Microwave Background (CMB) studies. We look at "empty pixels" (what's left over after source removal) and measure whether or not some group of pixels in one part of the image is correlated with another group of pixels in a different part of the image. This is the angular power spectrum, which quantifies these correlations, as a function of angular scale. This is exactly what the CMB team did to measure the microwave background power spectrum, which is paramount in our understanding of cosmology. 

Figure 2: The brightness of the near-infrared background light as a function
of wavelength. Our new Hubble measurements are highlighted in orange.
The components from the "intrahalo light" (shown above in middle panel)
and the first light galaxies (right panel above) are shown as the blue
and red line, respectively.

I made maps in five different wavelength ranges, or "bands": 0.6, 0.7, 0.85, 1.25 and 1.6 microns. The shortest band is in the yellow range of visible light, and the longest two are in the near-infrared (NIR), which our eyes aren't sensitive to. This wavelength range (1 micron) is special because it is sensitive to Lyman break signatures with a multi-wavelength study, and it is the wavelength at which we expect a signal from the reionization epoch. Figure 2 shows the brightness of the background light in each of these bands. Each of the bands has a common component - what we call "intrahalo light" - which is the light emitted by stars which have been tidally stripped from their host galaxies via mergers or interactions. But in Figure 2 you can see that the brightness drops significantly from the two NIR bands to the shorter bands. We think this is because the NIR bands are picking up, in addition to intrahalo light, a high-redshift signal from the first light galaxies. Because the photons from the reionization era have been redshifted by a factor of about 10, we expect their signal to peak between 0.9 and 1.1 microns, with no shortward contribution below the Lyman break at about 0.8 microns.

We're studying the background light, which traces emission from many different kinds of sources over all cosmological times. So we don't have a direct image of only the first galaxies. With sophisticated modeling, we were able to separate the different component emissions, and isolate the signal from the first galaxies. So what we have, via statistical methods, is a description of the astrophysical environment 500 million years after the big bang. The third panel in Fig 1 is a reconstruction of what they would look like based on our statistical measurements. Cosmological theory suggests that these first light galaxies are the progenitors to our milky way, and all other evolved galaxies.

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.

An Update on Mature Galaxies in the Early Universe

One of the goals of CANDELS is to document how galaxy shapes evolved over about 13 billion years of history.  Avid readers of this blog might recall our posting about the presence of mature galaxies 11 billion years ago. In that post, we explained that a beautiful montage of galaxies that accompanied a recent European Space Agency press release gave a misleading impression of the scientific finding.

We're delighted to say that the ESA press officer understood our concerns immediately and offered to produce a version with images of galaxies that are from the CANDELS data themselves. That image is shown below. The leftmost panel is identical with the original press release, but the center panel now uses images of galaxies at redshifts between z=0.3 and 0.7 and the rightmost panel uses galaxies between z=2 and 2.7.

An updated image of the evolution of the Hubble sequence over the past 11 billion years. From ESA.

At each epoch, the illustration is intended to show the Hubble tuning fork. The four leftmost postage-stamps show early-type galaxies, known as ellipticals and lenticulars (or S0 galaxies). The tuning fork then splits to show the late-type spiral galaxies. The top four images show normal spiral galaxies while the bottom four panels show barred spirals (galaxies with a bar-like feature in the middle).

While this is still not a perfect representation of what is happening to galaxy shapes, there are several things to notice.

• The galaxies 4 billion years ago and 11 billion years ago were smaller. Alas, this image still doesn't have the relative scales exactly correct between the epochs, but the sense of the evolution from smaller to larger is consistent with detailed measurements.
• The early-type galaxies 11 billion years ago look sort of similar to those today, albeit smaller. They had about the same round, ball-like shape with nearly uniform color. They tend to be redder than the late-type galaxies.
• The late-type galaxies 4 billion years ago look pretty similar to today. They have spiral features and you can find examples that have bars in their centers.
• The late-type galaxies 11 billion years ago look a bit different. It is very hard to find any convincing examples of barred spirals, and the ones that don't have bars look more disordered than their present-day counterparts.
• The greenish tint of the galaxies in the center panel isn't real. These images are composites through different filters, and the color balance has not been tweaked in a meaningful way. When measured in detail, galaxies in the past tended to be bluer than today, which is a result of the fact that they contained more hot, young stars and less dust.

While the illustration here gives a better illustration of how galaxies are transforming their shapes, it doesn't illustrate some of the more interesting features like bright clumps or mergers. Also it doesn't tell you anything about the relative numbers of galaxies of different shapes. It turns out that the early-type galaxies were quite rare 11 billion years ago, while they are much more common today, for example. 

Finally, in spite of our quibbles about the artwork, we would like to give a shout-out to the Hubble outreach team at ESA for phenomenal work over the years. Most recently, they were recognized for their efforts with a Parsec award "Fact behind the Fiction" award for their brilliant Hubblecast series. 

Were Galaxies Really Mature 11 Billion Years Ago?

Nostra Culpa

Yesterday, the European Space Agency put out a press release on CANDELS research that has been widely covered by various media. Sometimes the scientific message gets garbled in press releases, in spite of best intentions all around. In this case, the wording of the press release is pretty good about conveying the key message (although some of the subtleties get lost). But the gorgeous picture that accompanies the release is more wrong than right. Alas, there was a bit more "artistic license" taken with the press-release artwork than there should have been, and we failed to put the brakes on before the press release went out. The pictures of the galaxies marked as "11 billion years" were not taken from the CANDELS images, and are not at the right redshift (redshift z=2.5 corresponds to a lookback time of 11 billion years). If you would like to see what the images really look like, read on.

This is the gorgeous image that accompanied the ESA CANDELS press release. If only it were closer to the truth! The images further down in this blog post show what galaxies 11 billion years ago really look like in the CANDELS images.

What did galaxies look like 11 billion years ago?

Cutouts from Lee et al.
Galaxies on the right are
forming stars rapidly.
Galaxies on the left are
not. 

The basic conclusion of the papers on which this story was based is that we can start to see the dichotomy between star-forming galaxies being "disk like" and non-star-forming galaxies being "spheroid like" already being set into place 11 billion years ago. That's the central message and is what we are finding in the CANDELS survey in the various samples studied by Tao Wang, Eric Bell, BoMee Lee, Alice Mortlock, Victoria Bruce and others.

We wish we could see galaxies 11 billion years ago with the sort of clarity shown on the press-release image, but unfortunately even with Hubble we can't see that level of detail. To the extent that we can distinguish detail they look (a) smaller and (b) generally bluer, and (c) less well-ordered than present-day galaxies.

So -- yes we can see spheroids and disks emerging as separate sequences -- but it is not the case that the galaxies look like their present-day counterparts, which is the impression that you get from looking at the press-release image.  Arjen van der Wel's quote in the press release that they look "remarkably mature" is reasonable when you consider that they might have all looked like train-wreck mergers-in-process.  But some of the news stories based on the press release are taking that to mean they are completely mature, which is certainly not the case.

Illustration of the morphologies of massive galaxies at redshift z~2 using CANDELS cutouts from Wang et al. (2012). The upper-left insert color codes galaxies on the same axis scale, with blue being the star-forming galaxies and red being the less-star-forming galaxies. You can easily see that the less-star-forming ones tend to look more spheroidal.

The light reaching us from galaxies at a redshift z=2.5 took about 11 billion years to get here. So we are looking back in time to when the universe was only about 2.7 billion years old. Below and to the right are some figures from some recent papers by CANDELS and other teams that show images of galaxies at about this redshift. Perhaps this gives a more accurate impression of what we can say and can't say about the comparison between nearby galaxies and their distant progenitors.

A montage of "Milky-Way progenitor" candidates from the CANDELS images put together by Pieter van Dokkum and the 3D-HST team.