The Open Cluster Chemical Abundance and Mapping Survey

Guest post by John Donor.


Astronomers have always been fond of the Milky Way, after all, it is our home. But it’s more than just our home, it’s also our most important laboratory for studying galaxy evolution. We can study the Milky Way in tremendous detail, compared to any other galaxy. So what does our host galaxy tell us about galaxy evolution?

Let’s start at the very beginning (as Julie Andrews said, “a very good place to start”). I literally mean the very beginning: the Big Bang. While the “bang” itself was perhaps the most exciting event in cosmic history, the aftermath of the Big Bang was really rather dull. After the Universe calmed down a bit, all of existence was just endless clouds of gas; Hydrogen and Helium gas to be precise. No stars. No galaxies. Certainly no planets or life.

But fortunately this boring state of affairs quickly corrected itself. As gravity took over, gas began to collect into what would become galaxies. As the gas collected, it slowly became dense enough to form the first stars. Stars are very exciting. In their hot, dense cores, they fuse Hydrogen and Helium into the heavier elements (Carbon, Oxygen, Iron, etc.) that we’re all more familiar with. And when they die, they often die spectacularly in supernovae.

Star deaths are particularly important to our story because that’s how heavier elements (everything besides Hydrogen and Helium) are released into a galaxy. But death is not the end! There’s still plenty of gas in a galaxy, so it forms more stars, now with a slightly higher concentration of heavier elements in them from the previous generation of stars. This is the life-cycle of a galaxy: make stars, become enriched by dying stars, make more stars, repeat. Galaxies aren’t homogenous blobs though: they have interesting structure such as spiral arms and central bulges. Due to the effects of gravity, their mass tends to be more concentrated at their centers. More mass means more stars. More stars means more heavy elements enriching the gas in that part of the galaxy.

A simple illustration of the build up of heavier elements in a galaxy (credit: ESA/Hubble & NASA).

This very simple model already points us towards a key piece of observational evidence in studying galaxy evolution: chemical enrichment. Or more specifically, the rate at which that chemical enrichment changes as we move through the galaxy. To measure chemical enrichment, astronomers often simply measure Iron (Fe) and Hydrogen (H). The ratio of Iron to Hydrogen (Fe/H) gives an exact numerical representation of the level of chemical enrichment, often called “metallicity”. The exact rate at which Fe/H changes with respect to the distance from the center of the galaxy, or the Galactic metallicity gradient, has been the topic of numerous studies dating as far back as 1979.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has made measurements of the chemical enrichment (Fe/H) of over 200,000 stars to date. But this is only half the battle. To measure the Galactic metallicity gradient, distances from the center of the Galaxy are needed as well. This might seem easy: measuring distance from the Galactic center is a simple geometry problem, if you know exactly where the object you’re measuring is. Unfortunately, finding the distance to celestial objects can be difficult.
There are a variety of methods for finding astronomical distances, but almost all of them focus on finding the actual intrinsic brightness of an object. Since brightness decreases predictably as we move farther from an object, a change in brightness (intrinsic – observed) must correspond to a certain distance. Of course the observed brightness is very easy to measure! Finding the intrinsic brightness is the focus of entire sub-fields in astronomy.

The perfect measuring stick for studying the galactic abundance gradient will then have both (relatively) easily measured chemistry and a measured reliable distance. Among the best objects fitting this description are open star clusters. Open clusters are large groups of gravitationally bound stars (meaning they’re very close together) that formed at the same time, from the same material. Remember when we discussed stars forming out of gas? Most of these gas clouds are massive (millions of times the mass of our Sun), and thus can produce tens or even hundreds of thousands of stars. The resulting stars tend to remain gravitationally bound in an open cluster.

The open cluster M67, with all the stars observed by APOGEE boxed. (Credit: SDSS)

Open clusters have a number of useful properties. Since all of the stars formed from the same cloud, they all have approximately the same chemical makeup. They are also all approximately the same age (give or take a few million years, but that’s a cosmic eye-blink). These two properties make open clusters particularly easy to model. The models predict both intrinsic and easily observable properties of the cluster (e.g. observed brightness and color patterns), so to determine the intrinsic properties of an open cluster, we only need to find a model that matches it well. With an intrinsic brightness, we can quickly calculate the distance to an open cluster.

APOGEE has observed stars in the fields of hundreds of open clusters (e.g., M67 seen here with observed APOGEE stars marked). But simply being in the same area as an open cluster doesn’t mean the star is part of the gravitationally bound group of stars. It’s an age-old problem for astronomers: we only see a 2D map of the 3D sky. A star in the field of an open cluster could easily be hundreds of light years in front of or behind the gravitationally bound group of stars. The Open Cluster Chemical Abundances and Mapping (OCCAM) survey considers all the APOGEE stars falling in open cluster fields. Determining whether or not a star is in fact a member of the gravitationally bound group can be difficult.

The OCCAM survey relies on the fact that cluster member stars are gravitationally bound. Gravitationally bound stars move through the galaxy together and thus will have similar velocities when measured from Earth. The OCCAM survey combines line-of-sight velocity measurements (Doppler Velocity) from APOGEE with measurements of velocity perpendicular to the line of site (proper motion) from the recent Gaia Data Release 2 to isolate cluster members from non-members.

With uniform APOGEE chemical measurements from member stars in many open clusters, and distance measurements obtained from modeling the open clusters, we can finally measure a high precision Galactic abundance gradient that will be used to calibrate models of galaxy evolution. And that’s one more piece in the puzzle of how our own Milky Way formed and evolved.

SDSS Fourteenth Data Release

This post was written by Anne-Marie Weijmans, the SDSS Data Release Coordinator.


It’s the last day in July, and that means that it’s time again for a Sloan Digital Sky Survey (SDSS) data release! This year, we are very happy to announce our fourteenth public data release, DR14.

Making data publicly available is an important aspect of SDSS, as it allows SDSS data to be used world-wide by anyone with an internet connection. For more than a decade, SDSS data has been used by astronomers for their science, by teachers in their classrooms[1], by students for their school projects, and by the general public to learn more about the Universe. In order to have this broad impact, we work hard to not only make our data available, but to also ensure that it is accessible. All our data is thoroughly documented, and we have various tools, tutorials and examples to assist anyone interested in using our data — from professional astronomers to high school students. Just go to the SDSS data access website to find out how you can work with the SDSS data!

All the SDSS data are stored at the servers of the Center for High Performance Computing (CHPC), at the University of Utah. This particular server holds all the SDSS data releases, including DR14. Just DR14 alone is already a whopping 156 TeraBytes (TB = 1000 Gigabyte = 1012 bytes): that is more than 33,800 DVDs worth of data! Image credit: Adam Bolton

So, what is available in DR14?

  • APOGEE-2, or the APO Galaxy Evolution Experiment-2 is very proud to announce its first public data release! APOGEE-2 studies the structure of the Milky Way by taking infra-red spectra of stars, to learn more about how the Milky Way formed and evolved over time. You can explore these spectra using our webapp and find stellar parameters and chemical properties in the APOGEE-2 data release.
  • eBOSS, short for extended Baryon Oscillation Spectroscopic Survey, is also celebrating its first public data release! eBOSS is mapping the structure of the Universe, by taking optical spectra of distant galaxies and quasars. These spectra provide distance measurements to galaxies, quasars, and intervening gas, all of which enable eBOSS to make a 3D map of the Universe, and learn more about how galaxies cluster in space. Ultimately, eBOSS aims to precisely measure the expansion rate of the Universe, and reveal more about the nature of the mysterious Dark Energy that accelerates this expansion. The eBOSS spectra are also available in our webapp.
  • MaNGA or Mapping Nearby Galaxies at Apache Point Observatory already released its first data last year, but they’re back with even more data cubes, 2,812 in total! MaNGA uses integral-field spectroscopy to map the properties of galaxies, and find out more about how different galaxies form and evolve. The MaNGA team has prepared a very handy set of tutorials to explain the data cube format, so that anyone can make use of the wealth of information hidden in these galaxy integral-field spectra.

Finally, we’re also very excited to share our new Image Policy with you! We have updated our image license to a Creative Commons Attribution license (CC-BY), which means that any image on our website may now be downloaded, linked to, or otherwise used for any purpose, provided that the image credits are given.

We hope you’ll have fun with all the spectra, catalogs, and tools included in our Fourteenth Data Release, and that they will help you with your science, outreach, teaching, school projects, and anything else!

Anne-Marie Weijmans

SDSS Data Release Coordinator

University of St Andrews

[1] If you are a teacher, we invite you to check out our latest educational guides and activities at SDSS Voyages! We are also developing a Spanish version, available here.

SDSS Summer Interns Apply SDSS Science to Small Telescopes

By Kate Meredith.  Kate is the Director of Education Outreach at the University of Chicago Yerkes Observatory.  Kate began working with SDSS data while still a high school science teacher and continued that work in her role with SDSS as lead educator for formal education.  She is the primary developer of the SDSS Voyages website.  In her first year as Education Director at Yerkes, Kate launched a summer intern program.  In this post, Kate describes one of the projects interns lead during the summer of 2016.  

Rebecca Chen and Lindsay Berkhout are sophomore physics majors at the University of Chicago. Both chose the astronomy specialization, and both spent the summer of 2016 as interns at Yerkes Observatory . They were two of the 12 undergraduates that helped launch the first ever Yerkes Education Outreach internship program.  Their goal was to take precise photometric measurements of targets (how bright objects are) with instruments including the 24-inch telescope at Yerkes, as well as Stone Edge Observatory’s 20-inch telescope, located in Sonoma, California.

Rebecca Chen positioning new SDSS filters for use with the 24 inch reflecting telescope at Yerkes Observatory.

“We both came in, and we didn’t know anything,” Berkhout laughs. But they soon got up to speed, and ended the summer with a tested methodology that allows not only them, but students following in their footsteps, to use the telescopes to measure the brightness of objects to within 5% the value obtained by the venerable Sloan Digital Sky Survey (SDSS).

The long-term goal on Yerkes’ side is to be able to extend SDSS catalog to bright stars. The survey, designed to measure many faint targets, has gaps when it comes to measuring the brightest stars. But the Yerkes and Stone Edge telescopes—large for small observatories, but tiny compared to SDSS’ 100-inch mirror—can tackle the bright stars with ease. The trick is being able to compare data using the very different instruments of SDSS and the observatory telescopes.Chen and Berkhout were interested in more dramatic events; they wanted to measure the lightcurves of recent supernovae. But both projects rely on being able to precisely measure the brightness of targets. And figuring out how to reliably attain such precision with the Stone Edge and Yerkes telescopes became the students’ summer objective.

Richard Kron, a professor at the University of Chicago and former director of Yerkes Observatory, worked closely with the students. But he says he was mostly there to answer their technical questions, and let them guide the direction of the work themselves—something Chen and Berkhout handled with aplomb, though he notes that other students might desire a more hands-on approach to mentoring.

He introduced the pair to software packages—Aperture Photometry Tool and Topcat—to help them in their work, and advised on details such as calculating uncertainty in their measurements. He admits that his first instinct is often to push through and rush to big results. And students likewise often want to do something novel and exciting—like observing supernovae.

Intern Lindsay Berkhout installs SDSS filters in CCD camera at Yerkes Observatory.

But Kron says it’s important to remember how much time new students take to assimilate the big concepts at play: operating the telescopes, learning new software routines, finding and measuring the targets, understanding uncertainty. “Make sure the student feels really in command,” he suggests. “It’s okay if you don’t cover quite as much as your original dreams had suggested.”

“There’s still a lot of work to do,” Berkhout acknowledges. Steep learning curves, but also telescope downtime, contributed to the sometimes slow pace. “The next step is actually taking data and using this methodology to get results,” she says, something they ran out of time for in the short summer.  “I think that if someone else takes the project they could go wherever they want with it, whether it’s bright stars or variable stars, or supernovae.”
Berkhout and Chen left behind a detailed guide of the work they did, summarizing the technical details of how to take observations, run them through the software, measure sources’ photometry, and compare it to SDSS values. They also left suggestions for ways future interns might improve from 5% down to within 2% of the SDSS values. And they took with them many more lessons in how to plan and tackle such a project.

“I felt like it was a really nice internship for summer after first year,” Berkhout says. “It was a good way to get involved in a research project that taught us a lot so now we can go to other people and be able to say that we’ve done something. That we learned a lot and we’re competent and can be involved in bigger research projects in the future.”

Chen reflects that, “While we were working it was frustrating, because at times it felt like we weren’t getting anywhere. But at the end of the summer, looking back on all the things we had done, I was like, ‘Oh that’s pretty cool. That’s a project. We did a real project.’”

 

Rebecca Chen and Yerkes Director of Education and SDSS EPO Specialist, Kate Meredith, celebrate the first successful night of observing with the new SDSS filters and several hundred mosquitos at Yerkes Observatory.