Thursday, January 30, 2014

More about CANDELS at the AAS

As mentioned by Liz McGrath in her recent blog post, the American Astronomical Society held its 223rd meeting  in early January. Altogether there were 37 presentations that had the word CANDELS somewhere in their title, author list, or abstract, and probably several others that used CANDELS under the radar. I thought it worth sharing a few highlights.

The meeting opened with the Kavli lecture by Bob Williams, the former director of the Space Telescope Science institute and president of the International Astronomical Union, who recounted the history of the original Hubble Deep Field and the science legacy that flowed from that first observation. One of the things he emphasized is how the making the data completely non-proprietary helped build the scientific momentum. Instead of observing separate spots in the sky and keeping the data to themselves, astronomers interested in the distant universe were suddenly much more willing to point telescopes to the same spot and share the data. CANDELS carries forward the legacy of this culture change; all of the Hubble data and most of the data from other telescopes are available to everyone.

A figure from Steve Finkelstein's talk, showing the number of galaxies found at redshifts
z=7 and 8 from the ultra-deep survey fields (blue) and CANDELS (red). Absolute magnitude
is a measure of brightness, with smaller numbers (more negative) indicating brighter galaxies.
CANDELS is great for finding brighter (rarer) galaxies, while the ultra-deep fields
excel at finding fainter (but more common) galaxies.
On Monday afternoon, the CANDELS and CLASH projects were featured in a special session. This was an opportunity for astronomers from both projects to give a top-level summary of the results. The CANDELS talks were given by Jeyhan Kartaltepe, Steve Finkelstein, Yu Lu and Steve Rodney. Jeyhan summarized the results on galaxy morphology, with particular attention to the CANDELS visual classifications and the evidence that ultraluminous infrared galaxies are associated with galaxy collisions and mergers, even at high redshift. Steve Finkelstein summarized the state of research on the most distant galaxies. He showed a nice diagram that helps to put into perspective the contributions of the "deep but narrow" fields (the Hubble Ultradeep field and associated parallel observations), and CANDELS. He also talked his discovery of what is currently the record-holder for the most distant spectroscopically confirmed galaxy, at redshift z=7.51. Yu Lu summarized the effort of part of the CANDELS theory group to compare semi-analytical models of galaxy formation. This is the most detailed side-by-side test of such models yet carried out and will be of great utility in helping us understand what we are learning from CANDELS. Finally, Steve Rodney partnered with Or Graur of the CLASH team to give a summary of the supernova program, including the first estimates of the evolution of supernova rates from both surveys.

A figure from Guillermo Barro's talk about the evolution of compact galaxies at high redshift.
The vertical axis shows how active the galaxy is in forming stars. Specifically, it shows
the ratio of each galaxy's star-formation rate, to its total stellar mass. The horizontal axis
shows how densely packed the stars are together. Dense galaxies are to the right,
diffuse galaxies are to the left. Guillermo is finding that galaxies tend to make the
transformation from being diffuse to being compact while they are still forming stars;
the quenching of star formation happens later.
The next day, CANDELS figured prominently in the Hieneman prize lecture given by Rachel Somerville. Rachel has been one of the leaders of the theory effort in CANDELS and recounted the progress in trying use the observations to get at the detailed physics of galaxy formation. One of the more recent results she showed in her talk was the success that she and Lauren Porter have had in matching the trends that Guillermo Barro and Christina Williams see in the evolution of compact galaxies. Both Guillermo and Christina spoke that morning in a session about galaxy surveys that was almost entirely populated by CANDELS talks.

In addition to the talks, there were quite a few posters. Posters are tacked to display boards in a big exhibition hall, and stay up all day. On the plus side, this gives people an opportunity to wander by and discuss the research, which is harder to do in a session packed with 5-minute talks. On the minus side, there are so many posters that it impossible to look at them all. CANDELS was very fortunate to have most of our posters on the first day located right near the entrance to the hall, so there was lots of opportunity for people to see them.

It was also great to see work by other groups using the CANDELS data. One of the press releases from the meeting was from Garth Illingworth, discussing finding some suprisingly bright galaxies in the CANDELS GOODS-N data that might be at redshift z>9. These are not yet spectroscopically confirmed, but if they are really at high redshift, then they indicate that star-formation was already proceeding vigorously when the universe was only 500 million years old.

Monday, January 27, 2014

A New Type Ia Supernova in M82

Just a couple of days ago, a dim, but quickly brightening, supernova was discovered in M82, the beautiful "cigar galaxy." At "only" 12 million light years away, this is the nearest supernova to Earth since 1987 and the nearest Type Ia supernova since 1972. With the enormous changes in our imaging technology since then (including the launch and subsequent improvements to the Hubble Space Telescope), this is a fantastic opportunity for precision measurements of one of the brightest and most mysterious explosions in the universe.

The new supernova in M82, discovered by students at the University College London
Observatory.  Photo by Adam Block/Mount Lemmon SkyCenter/University of Arizona

Discovering more about the nature of Type Ia supernovae has been one of the primary goals of the CANDELS project. These supernovae begin as stars like our sun, which have shed their outer layers at the end of their lives and become white dwarfs. White dwarfs are the extremely dense cores of a burned-out star, and although they're only the size of our earth, they have the mass of our entire sun.  The detonation happens when a nearby star adds even more mass onto this dwarf -- when the weight becomes too much, nuclear fusion ignites it and a supernova occurs.

In CANDELS, we study the most distant Type Ia supernovae that we can find, the farthest of which stands at over 10 billion light years away. Our supernovae tell us about the early expansion of the universe (and its Dark Energy), the chemical evolution of the universe, and how quickly supernovae form and explode around 8-10 billion years ago -- at the peak of star formation in the universe.

This nearby galaxy offers a completely different, and rarer, perspective. In 1972, when the last Type Ia supernova this close to Earth exploded, it was still a year before anyone proposed the idea that these supernovae were formed in binary star systems. It was 12 years before someone realized that both stars could be white dwarfs, and 18 years before supernovae could be studied from space with the Hubble Space Telescope. It was over 25 years before such supernovae were used to discover that Dark Energy was accelerating the expansion of our universe.

Motivated by the knowledge and technology gained since the last close Type Ia supernova went off, scientists will be asking an entirely different set of questions this time around. First, we'll be looking for a giant companion star that could have fed mass onto the white dwarf. If a companion star is visible, this would be the first direct evidence that a system with one white dwarf can lead to a supernova; if a companion star is not found, the theory that two white dwarfs can make a Type Ia supernova will gain credibility.

Artist's conception of the single-degenerate (one white dwarf)
theory of Type Ia supernova explosions, wherein
a white dwarf accretes mass from its companion
star.  (original) © ESA and Justyn Maund (Queens Univ. Belfast)
Artist's conception of the double-degenerate theory
of Type Ia supernova explosions, in which two white dwarfs merge
together as they emit gravitational waves. (original) © NASA,
Tod Strohmayer
(GSFC),  and Dana Berry (Chandra X-ray Observatory)

Second, scientists will be studying the geometry of the supernova from the fraction of polarized light emitted. Polarization, the orientation of a light ray's electric field, is entirely random when it originates from a spherically symmetric star. However, if one side becomes longer than the other, the light's polarization will have a preferential direction that can be measured on Earth. As the outer layers of the M82 supernova expand, they will become transparent and expose the inner material. Over the next month, scientists will be able to measure the shape of different layers and examine the three-dimensional explosion. With this structural information, we'll learn more about how supernova detonation occurs; specifically, how nuclear fusion begins and spreads through the layers of the white dwarf.

The location of M82 on the night sky from Sky and Telescope.
A more detailed chart is available here
Lastly, Type Ia supernovae are nearly uniform in brightness, serving as excellent distance indicators for most of the visible universe. CANDELS supernova principal investigator Adam Riess -- among others -- will be measuring the distance and doppler shift velocity (the reddening of its light) of this supernova to determine how fast the local universe is expanding -- and infer the amount of the mysterious Dark Energy that surrounds us.

This supernova is particularly rare in that it offers opportunities not only to scientists, but for anyone with access to a dark night sky. It will brighten for approximately a week and a half, and at its peak it will be visible near Ursa Major (the Big Dipper) to anyone with a set of binoculars. Although it's impossible to predict when the next close supernova will be, I'm looking forward to seeing an exploding star with my own eyes - it may be 40 years before there's another opportunity.

Wednesday, January 22, 2014

223rd AAS Meeting in Washington, D. C.

Twice a year, the American Astronomical Society holds professional meetings covering a broad range of research and education topics. This January’s meeting was held in Washington, D. C. at the National Harbor. I wouldn’t be surprised if it broke previous attendance records, with almost 3200 people on the official registration list. 

Rachel Somerville giving the Heineman Prize Lecture.
Photo credit: Joson Images/ AAS
CANDELS scientists had a very strong representation at this meeting. Rachel Somerville won the Heineman Prize and gave a lecture entitled, “The Formation of Galaxies and Supermassive Black Holes: Insights and Puzzles.” Meanwhile, Jennifer Lotz held a press conference on the release of data from a new ultra-deep, wide-field imaging survey that she is leading known as the Hubble Space Telescope Frontier Fields (see the image from their press-release below).  There was also a CANDELS special session which included 4 talks and 10 posters on CANDELS results, as well as 22 other CANDELS-related talks and poster presentations throughout the course of the meeting.

HST Frontier Field Abell 2744.  Image credit: NASA,
ESA, and J. Lotz, M. Mountain, A. Koekemoer, and
the HFF Team (STScI).
People go to the AAS for a variety of reasons. As one of the largest gatherings of astronomers, it is impossible to see every science talk of interest. For this reason, I find AAS meetings are more of a place to have discussions with your colleagues and the larger astronomical community about priorities and goals for the future. Many of these larger, community-oriented discussions take place during “town hall” sessions.  Sometimes difficult decisions need to be made, especially in the current funding climate. At this meeting, astronomers grappled with the likelihood that many of our beloved facilities will either need to find private partners to sustain operations costs, or be shut down in the next couple years. This is because NSF does not have the money to fund them while continuing forward with important projects like the James Webb Space Telescope and the Large Synoptic Survey Telescope. If you know anyone with a few hundred thousand dollars to spare, you can buy yourself some quality time on top-notch facilities!

Other important discussions include networking with more senior astronomers, especially when you’re on the job hunt. Jon Trump wrote about this aspect of AAS in a previous post.  However, the largest growing demographic at these meetings is young student researchers. This is a great place for them to showcase their work and gain experience talking with people about their research. For me, this was my first AAS meeting where I was on the “other side”, serving as faculty advisor to an undergraduate student who is applying to graduate school this year. As my first thesis student, I encouraged her to come present her results, while I tried my best to introduce her to people doing exciting science as well as folks on various graduate admissions committees. The AAS can be a bit daunting to newcomers, so it’s the advisor’s role to help facilitate discussion and provide a role model for students just starting down this career path.

Neil deGrasse Tyson at the AAS.
Photo credit: Joson Images/ AAS
In addition to science talks, there were a number of sessions on astronomy education research, which is another growing aspect of the AAS community. One session I attended on how to improve student outcomes in Astro 101-style courses, was standing room only. It’s great to see so many professional astronomers care so deeply not just about their personal research, but also about how to improve their approach to teaching science to non-science majors. While this is a significant part of many of our jobs, it may surprise readers to learn that most of us were never trained as teachers. Therefore, these sessions are particularly important for learning how to be effective instructors.

Astronomers also like to have a bit of fun in these meetings. At this meeting there was a special talk by Neil deGrasse Tyson, who was surrounded by hundreds of adoring, geeky fans (a.k.a, professional astronomers) as he talked about how to use twitter to engage the public in science. There was also the infamous AAS “after-party”, which was a bar-hopping extravaganza, complete with astro-themed cocktails and a mechanical bull (although I never did see anyone attempt the bull)!

Space Shuttle Discovery at the National Air and Space
Museum.  Photo credit: E. McGrath
Finally, with the meeting being held in Washington D. C., I took advantage of the opportunity to do some astronomy-themed sightseeing. I visited the National Air and Space Museum hangars located near Dulles airport, where I got to behold the impressive Space Shuttle Discovery, as well as a Mars Pathfinder prototype. The shuttle was even more impressive in person than I could have imagined—definitely worth a visit next time you're in D. C.