Wednesday, September 23, 2015

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.

Wednesday, September 9, 2015

Astronomer of the Month: Brett Salmon


Tell us a little about yourself!

Hi! My name is Brett Salmon and I’m a finishing graduate student at Texas A&M University. I was born and raised in New Jersey and I am primarily from Washington Township, just outside of Philadelphia. I received my Bachelors degree in Astronomy from Rutgers University in 2010. While at Rutgers I ran for the varsity cross country team, where my fastest 8 km race was 26 and a half minutes. I originally intended on taking a year after undergrad to build my research experience, but applied to Texas A&M on a whim. The program looked great so I decided to go, and I’ve been here for 5 years. 


I am a junior scientist member of the CANDELS team, and have been a part of the high redshift and phot-z working groups. I’ve also been involved in several CANDELS projects on the evolution of distant galaxies. Specifically, I study the physical properties of galaxies in the early universe, including their star-formation rates, stellar masses, star-formation histories, nebular emission, and dust. For the most distant galaxies these features cannot be observed directly, so a chunk of my work involves developing statistical techniques to infer galaxy properties from broadband photometric data, and the limitations of those techniques. 


Like most astronomers, I was hooked at a young age. I recall my mom getting a small (refracting!) telescope when I was about 6. The night sky flourished above our rural home, and our wonder instilled a passion for astronomy. I knew I wanted to learn about the cosmos. However, to be fair, I was 6 and also wanted to be a policeman, fireman, and astronaut. 


The Physics GRE knocked me down. I should have prepared differently and my resulting poor score nearly scared me away from an astro-track. Thankfully, support from professors Andrew Baker and Chuck Keeton at Rutgers reassured me that such standardized tests aren’t reflective of success in the field of astronomy. 
Who has been your biggest scientific role model and why? 

The mentors and advisors I’ve had in undergrad and grad school (Casey Papovich) have certainly been influential in shaping my career as a scientist. As far as role models, I’ve always admired those that can purvey complicated scientific phenomena in a way that is fun and understandable to the public, like Carl Sagan, Neil deGrasse Tyson, and Michio Kaku


It is hard to distinguish between two changes in my life that happened during the early years of grad school: learning how to think like a scientist, and working like an astronomer. The former is an idea that has broad impact; ideas and concepts are validated (or refuted) through evidence, not artful rhetoric. I think this scientific mindset takes time and experience to fully appreciate. 

The second change involved learning what daily work in astronomy is really like.  I learned that each day is like a crash course in learning new programming, statistics, or data visualization techniques. Every day is different. 

My favorite aspect of astronomy is that it is, at heart, an observational science. Physics, math, and computer science are valuable subjects themselves, but in astronomy they become tools invoked to explain a particular phenomena. Although astronomers draw from multiple disciplines, they do so to figure out how and why a galaxy exploded 10 billion years ago. It feels grounded in context. 

What motivates you in your research? 

I’m motivated by that nagging feeling of wanting to see the answer to a puzzle you’ve been working on for 5 years. There are rarely finite results thanks to the incremental nature of science. As a result, that itch to figure out what’s going on in the data never really goes away. 


Any telescope on Mauna Kea, Hawaii. I had the pleasure to do some observing at the summit at Gemini Observatory, and the experience was surreal. The clouds illuminated beautiful sunsets and sunrises, which was bittersweet since it also meant poor weather for observations. 


In the immediate future I am applying for postdoc positions. I have a passion for astronomy as well as outreach and teaching, so we’ll see where that takes me. 


A realistic wish: more funding and public support. A supernatural hypothetical wish: I’d like to see a really bright supernovae go off in the Milky Way (aren’t we due for one?).

What is your favorite, most mind-boggling astronomy fact? 

There are more stars in our Milky Way galaxy like the Sun, than there are people on Earth that have ever lived. 


If you want to get into astronomy professionally one day, take computer science and statistics courses. Most, if not all, undergrad curriculum for physics/astronomy does not include these courses, yet I use statistics and programming on a daily basis. Thanks for reading!