Seeing Beyond: Gail Zasowski and the Inner Milky Way

24 01 2014

The inner galaxy, inconveniently obscured by dust, has long been shrouded in mystery – until now. The SDSS-III’s APOGEE (Apache Point Observatory Galaxy Evolution Experiment) survey uses infrared light (light with wavelengths longer than our eyes can perceive) to cut through the dust and see previously-hidden parts of our galaxy. Astronomers from the APOGEE survey are now surveying more than 100,000 red giant stars all over the Milky Way. Data from this large range of stars, including highly accurate measurements of velocities and chemical compositions, will allow astronomers to finally unravel the history of the Milky Way.

Photo of Gail Zasowski in front of a mountain

Dr. Gail Zasowski

But how do we know which stars to survey? We want to find red stars, but interstellar dust makes many stars appear redder than they actually are. That reddening is different in different parts of the Galaxy, so choosing a consistent sample of stars throughout can be difficult. Stars must be “targeted” for observation with great care. This is where Dr. Gail Zasowski, the target selection coordinator of APOGEE, comes in.

Zasowski has been interested in astronomy for a long time. Imagine a young girl visiting the National Air Space Museum in Washington, walking with her father. The father takes his daughter to a mural of the Solar System and starts explaining to her what the planets are. But before he can even start naming them, the young girl rattles off, “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto!” Gail Zasowski was born to be an astronomer.

Zasowski attended the University of Tennessee, where she earned a Bachelor of Science degree in physics, and double majored in Latin. After taking a few astronomy courses and spending a summer in an astronomy Research Experience for Undergraduates (REU) at the University of Rochester, Zasowski decided to apply to graduate school for astronomy. “I definitely came to grad school not knowing as much astronomy as my classmates,” admitted Zasowski. And yet, as a graduate student at the University of Virginia, Zasowski discovered that a life doing research and making new discoveries in the world of astronomy was indeed where she belonged.

The stars aligned for Zasowski at Virginia when she took some classes with Steve Majewski, principal investigator of APOGEE, and asked him to be her Ph.D. advisor. Zasowski’s thesis work dealt with the different factors that affect the absorption of starlight by interstellar dust. “It depends on wavelength, but how it depends on wavelength depends on properties of the dust itself, such as the size and shape of the grains,” Zasowski said. “My first work with Steve was on seeing how these behaviors changed as a function of the dust’s position in the galaxy.”

The SDSS-III telescope and the sky

The SDSS-III telescope at Apache Point Observatory, high on a ridge in eastern New Mexico. This image shows constellations labeled in the sky. Image credit: Steve Majewski.

Towards the second half of her graduate career, Zasowski’s familiarity with interstellar dust led her to further involvement in the APOGEE survey. “I was spending so much time working with APOGEE that Steve suggested I should just do it officially. So for the last two and a half years of grad school I was the Target Selection Coordinator of the survey,” Zasowski explained.

The goal of the APOGEE survey is to look at red giant stars in all parts of the galaxy, but interstellar dust absorbs and reflects blue light more effectively than red. The problem lies in the fact that since the amount of dust is unknown, the observer does not know how accurate the perceived color of the star is. If a star looks very red, is that because the star really is red, or is it because there is a lot of dust in the way? The solution to this problem lies in the fact that there are some wavelengths of light for which most stars emit about the same amount of light – so any apparent differences in stars at those wavelengths must be due to the dust. We can then use that knowledge to calculate how the dust affects stars at the wavelengths we really are interested in.

Zasowski wrote the software that measures all the stars in a patch of the sky, corrects the measurements for the effects of the dust, and chooses which stars there should be observed further by APOGEE. In addition to stars, Zasowski’s software looks for the large number of other interesting objects that APOGEE is well-suited to find, including newborn stars and ancient star clusters in other galaxies. “One thing that’s interesting about the software,” said Zasowski, “Is that things are always evolving. Every time we think that the software is done and that we can handle all special cases, someone will come up with some other interesting special case that we hadn’t considered.”

Zasowski holding the APOGEE logo. The logo shows the word "APOGEE" inside a white ellipse, with the O as a magnifying glass on the Milky Way

Gail Zasowski standing in front of the APOGEE spectrograph holding the APOGEE project logo, which she designed. Image credit: Ricardo Schiavon.

Zasowski has also been heavily involved with some of the science results coming out of APOGEE. For example, she worked with David Nidever on the discovery of a new group of stars near the center of the Milky Way. Using APOGEE observations, Nidever and Zasowski measured the velocities of stars near the Galactic center, unexpectedly discovering a population of fast-moving stars that matched computer models of stars forming a long, narrow “bar” in the inner Galaxy. Another of Zasowski’s studies led to a new approach to studying “diffuse interstellar bands” (DIBs), an as-yet-unexplained feature seen in stellar spectra, arising from interstellar material of unknown chemical makeup. The wavelength coverage of APOGEE, and the distance that the survey probes through the Galaxy, helps Zasowski use these lines to measure Galactic properties in a whole new way.

“I’m also really interested in public outreach,” said Zasowski enthusiastically. She recently became a postdoc; after spending a year at Ohio State University, she will be spending the remainder of her three-year fellowship at Johns Hopkins University. She is funded under a National Science Foundation Fellowship, a part of which is devoted to education and public outreach. This past summer, she helped run a week long space camp in Columbus, Ohio. “Astronomy is the gateway science because it’s easy to understand and easy to get excited about, so it’s a really good way to get people into science,” said Zasowski. Zasowski hopes that her education and public outreach efforts will inspire another young astronomer, just as she was inspired as a young girl at a science museum.

MaNGA Pre-Survey Review and Team Meeting in Portsmouth, UK

23 01 2014

Around 45 astronomers have been in Portsmouth, England this week attending the MaNGA Pre-Survey Review and Team Meeting.

MaNGA  (Mapping Nearby Galaxies at APO) is part of the plans for the next phase of the Sloan Digital Sky Survey (along with eBOSS and APOGEE2) due to start in July of this year.


The MaNGA Team and Review Panel in front of the Institute of Cosmology and Gravitation at the University of Portsmouth. Image Credit: Edd Edmondson

As well as the full science program, the astronomers have been enjoying the British pubs, Indian Food, and historic Naval ships to be found in Portsmouth.


The MaNGA Team enjoyed a special tour of HMS Warrior 1860 in the Portsmouth Historic Dockyard. Here shown just before sunset on Wednesday. Image Credit: Karen Masters

The MaNGA Team are happy with the outcome of the review, and it’s full speed ahead to survey operations. The discussions continue today and tomorrow with open issues and plans for early science papers.

AAS Awards Highlight 3 Astronomers Using SDSS Data

17 01 2014

The American Astronomical Society has awarded three of its society prizes to scientists who have used SDSS data extensively in their work.

1) Chris Lintott (Oxford University and Adler Planetarium) was awarded the 2014 biennial Beatrice M. Tinsley Prize for creative and innovative contributions to research.

“With great insight and creativity, he created a transformative approach to science by engaging nonscientists in cutting-edge research via He demonstrated the unique capabilities of ‘crowdsourcing’ to attack otherwise intractable problems and, in the process, created a unique educational tool that is also an unparalleled public-outreach phenomenon.”

SDSS data served as the main source for the GalaxyZoo project that started the wildly successful Zooniverse enterprise. Other key SDSS people involved in setting up GalaxyZoo include Karen Masters (University of Portsmouth) now serving as the GalaxyZoo Project Scientist, Daniel Thomas (University of Portsmouth), Kate Land (Oxford), Kevin Schawinski (Oxford), Jordan Raddick (Johns Hopkins University), Alex Szalay (Johns Hopkins University), Anže Slozar (Brookhaven National Lab), Steven Bamford (University of Nottingham), Bob Nichol (University of Portsmouth), and Jan Vandenberg (Johns Hopkins University). For some history on the GalaxyZoo project and its evolution into Zooniverse see by Fortson et al.

We also congratulate two scientists whose PhD theses were based on SDSS observations and who received AAS young-astronomer awards based in significant part on the work that came out of their theses and subsequent developments.

2) Nadia Zakamska (Johns Hopkins University) was awarded the 2014 Newton Lacy Pierce Prize for research in observational astronomy by an young scientist
“for her multi-wavelength work on Type II quasars, which has characterized these energetic sources in detail and led to the current “standard model” of quasars. [...] Her observational and theoretical work has shown that “feedback” from AGN is occurring on scales of tens of thousands of light-years.”

SDSS data were central to Dr. Zakamska’s PhD work on quasars and continued efforts in this area.

3) Chris Hirata (Ohio State University) was awarded the Helen B. Warner Prize for research by a young astronomer
“for his remarkable cosmological studies, particularly his observational and theoretical work on weak gravitational lensing, one of the most important tools for assessing the distribution of mass in the universe. [...] His work is facilitating the next generation of important cosmological experiments.”

Dr. Hirata’s PhD thesis on gravitational lensing was heavily based on analysis of SDSS data.

SDSS congratulations and recognizes these three excellent scientists along with all of the 2014 AAS award recipients.

Breaking SDSS News: A One-Percent Measure of the Universe

9 01 2014

We are here live at the American Astronomical Society meeting in National Harbor, Maryland, where this afternoon the SDSS announced some exciting new results. We have used Baryon Acoustic Oscillations (BAOs) to measure the most precise distances to galaxies all the way across the universe. This illustration shows how this technique works: we know from the early history of the Universe that galaxies are more likely to be separated by 450 million light-years than 350 or 550 million. We can use that knowledge to make highly accurate distance measurements, which tell us about the nature of the Universe we live in.


Image credit: Zosia Rostomian, Lawrence Berkeley National Laboratory

Read more about it in our press release!

News coverage about this story is already starting to appear. Check back on this page for links to stories!

BOSS Winter Collaboration Meeting at Lawrence Berkeley National Laboratory

17 12 2013

Over 80 astronomers attended the 2013 BOSS Collaboration Meeting held at the Lawrence Berkeley National Laboratory, from December 9th to 11th. During the meeting we had active and involved discussions about at all the great results that are coming out from the over 2 million spectra already taken with the BOSS spectrograph.

BOSS_201312_collaboration_meeting_thumb(Image Credit: Berkeley Lab – Roy Kaltschmidt)

Final discussions on the Data Release 11 papers, as well as preparation for the upcoming final BOSS Data Release 12, were two of the main topics in the meeting, but there were also many new exciting results from the different working groups covering topics such as quasars, galaxies, and clouds of gas in distant galaxies.

The conference dinner, at the Chabot observatory, was preceded by a great talk on exoplanets by Sarah Ballard, and followed by observations of Jupiter’s satellites using telescopes almost a hundred years old. On Wednesday afternoon, the first BOSS soccer championship ended up with a tight score (Quasars 3 – Lyman alpha 5), surprisingly no injuries and a lot of fun!

SDSS Social Media in Chinese

11 12 2013

A massive thank you to Qingqing Mao a PhD student from Vanderbilt who is running SDSS Social Media sites in Chinese (Simplified Mandarin). We have both Facebook and Weibo (Chinese version of Twitter) accounts for SDSS. Below is a wordl of the most common characters used by these accounts. The blue characters in the middle are a phonetic translation of “Sloan”. “Tian Wen” also appears many times – literally “sky language/culture” – meaning astronomy (e.g. “tian wen xue” means the study of astronomy).


SDSS-III APOGEE spectra motivate improved lab-based spectroscopy

10 12 2013

A recent Nature editorial argues for the need to support lab-based basic spectroscopy to fully understand the wealth of astronomical data coming from surveys such as SDSS-III and its APOGEE spectrograph.  In particular such spectra contain many atomic and molecular transitions only very rarely studied in Earth-based laboratories.  Understanding the relative strengths of these transitions will be important in using them to fully measure the abundance of elements and molecules in stars in our Milky Way galaxy.

Hide and Seek: SDSS-III Astronomers Map Elusive Intergalactic Gas

1 10 2013

Today’s blog is a guest post by SDSS astronomer Guangtun Zhu of Johns Hopkins University

For more than a decade, SDSS astronomers have been mapping all the galaxies of the Universe – but what about the space between the galaxies? How can we map whatever cold, dark gas might be there?

The top of image shows light traveling from a distant quasar to the SDSS telescope. The bottom shows the impact of light absorption on the quasar's observed spectrum.

When we look at spectra of distant quasars, we learn not only about the quasars, but also about whatever the light passed through on its journey from quasar to telescope. In this case, absorption lines in the quasar spectra reveal gas in intergalactic space. The SDSS has obtained spectra for more than 200,000 quasars, providing a map of gas absorption across the sky.

Mapping the distribution of matter between galaxies is one of the greatest challenges in modern astronomy. When we look up in the sky, even with the largest telescopes, we see mostly dark space between galaxies that appears to be totally empty. Theories of cosmology predict that most of the matter in the Universe should be in the form of diffuse gas between galaxies, but actually detecting that gas is extremely challenging because it emits essentially no light.

A team of SDSS-III scientists led by me and my Johns Hopkins colleague Brice Ménard has just finished a study that takes on this difficult task. Using new advanced statistical techniques, we have been able to detect the very weak but clear signature of magnesium in intergalactic space. Those techniques allowed us to find how the amount of gas around galaxies changes with increasing distance from the center of the galaxies. Our findings are summarized in a paper submitted to the journal Monthly Notices of the Royal Astronomical Society, and available on the arXiv preprint server:

“The Large-scale Distribution of Cool Gas around Luminous Red Galaxies”

To detect gas in the intergalactic space, we used its absorption effects. When light from a distant source passes through a gas cloud, photons at a specific energy are absorbed, leaving a valley in the source’s spectrum.

The light sources we used are quasars – very bright objects powered by supermassive black holes at the centers of distant galaxies. Quasars are the brightest things in the Universe, so they do an excellent job of shining through the intergalactic gas we want to study. And the SDSS has obtained spectra of more than 200,000 quasars, letting us see the signal of intergalactic gas all over the sky.

Most of the gas absorption, however, is so weak that its signature is hidden by variations in the quasar spectra themselves. So even with the huge dataset we have, we had to use new statistical techniques to find the gas. Using a sample of one million galaxies that lie between us and thousands of distant quasars, we looked at the place in the quasar spectrum corresponding to magnesium. Our statistics showed us that, on average, the quasars appeared to be missing 0.01% of the magnesium. In other words, out of every 10,000 photons emitted by a quasar at the right wavelength, 9,999 made it to Earth, and one didn’t show up because it was absorbed by magnesium between galaxies. This effect is way too small to see for any individual quasar – we can only find it by averaging the light from all the quasars, all over the Universe.

So now that we know that the intergalactic gas is there, what’s next? The “standard model” of cosmology makes certain predictions about how matter is distributed in the Universe. So far, the model’s predictions have matched up extremely well with observations of the distribution of galaxies. Will the model also correctly predict the distribution of intergalactic gas? Our research shows, for the first time, that the large-scale distribution of gas in the Universe is consistent with the standard model.

A graph with average distance from galaxy center on the x-axis and amount of gas on the y-axis. Data points go from top left to bottom right.

This graph shows how the amount of gas around an average galaxy changes with increasing distance. The blue points show our latest results. The green and orange lines show the predicted amount of gas associated with that galaxy (green) and the galaxies around it (orange). The blue curve is the sum of the orange and green curves. The graph shows that the observed amount of gas matches predictions very well.

New surveys such as SDSS-IV, the next phase of the SDSS, are now being prepared and will soon provide us with even more data to help us map the distribution of matter in space with even higher sensitivity. Astronomers will be able to map different elements around different types of galaxies, across cosmic time. These observations will lift the veil of the universe and reveal the overall distribution of matter. In addition, they will provide new clues to a better understanding of the formation and evolution of galaxies like our own Milky Way.

The future is bright for studies of dark regions in space.

“Galaxy Zoo 2: detailed morphological classifications for 304,122 galaxies from the Sloan Digital Sky Survey”

19 08 2013

SDSS-III Data Release 10 features crowd-sourced classifications of the shape of 304,122 galaxies from images taken by SDSS. The paper describing these data is in press at the Monthly Notices of the Royal Astronomical Society and is available on the arXiv preprint server: “Galaxy Zoo 2: detailed morphological classifications for 304,122 galaxies from the Sloan Digital Sky Survey”.

This paper was led by Galaxy Zoo science team member Kyle Willet, from the University of Minnesota, and presents morphological classifications for 304,122 SDSS galaxies constructed from over 60 million clicks contributed by members of the public at the Galaxy Zoo website.

You can read more about the paper on the Galaxy Zoo blog (

The data described in this paper are available through both Galaxy Zoo at, and through SDSS-III Data Release 10 at

Galaxy Zoo work continues at On the site right now members of the public are looking at more images from the SDSS (the full DR8 imaging area) as well as some from the Hubble Space Telescope. The Galaxy Zoo team is also working hard to bring you images of your favourite galaxies in the infrared soon.

The Tenth Data Release of the Sloan Digital Sky Survey III

31 07 2013

The Sloan Digital Sky Survey III (SDSS-III) has just released the largest comprehensive sample of high-resolution infrared spectra of 57,000 stars in our Milky Way.

Data Release 10 is the latest in a series of data releases stretching back to 2001. This release includes the first data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and includes an additional year of data from the ongoing SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). APOGEE is obtaining R~22,500 spectra of 100,000 red giant stars sampled through the bulge, disk, and halo of our galaxy. These spectra allow astronomers to determine radial velocities, temperatures, surface gravities, and abundances of elements in these stars.

BOSS has now obtained spectra for almost 1.4 million objects, including 860,000 galaxies and 105,000 quasars from (2<z<3.5), as part of its quest to measure the positions of 1.5 million massive galaxies over the past six billion years of cosmic time, as well as 160,000 quasars — giant black holes actively feeding on stars and gas — from as long ago as 12 billion years in the past.

DR10 is available at:


Zooming in on Clouds of Hydrogen when the Universe was Young

17 07 2013

SDSS-III scientists in the Baryon Oscillation Spectroscopy Survey (BOSS) have just completed the second phase of a project to provide a map of the observable universe showing the positions of giant clouds of hydrogen gas. BOSS is mapping the Universe by measuring the positions of galaxies, clusters of galaxies, and clouds of hydrogen gas. These maps allow astronomers to understand the history of the universe, starting with the Big Bang all the way through to the formation of the galaxies, stars, and planets that provide the site for the development of life.

Hydrogen is the most abundant element in the Universe. Thus measuring the position of hydrogen gas clouds reveals important details on the distribution of normal matter in the Universe. To detect these hydrogen clouds, BOSS uses super-luminous quasars as backlights. Quasars can be billions of times brighter than the Sun and are currently thought to be powered by matter falling into super-massive black holes. The quasars that BOSS uses as backlights are located billions of light-years from us, when the Universe was one fifth its current age of 13.7 billion years.

Because light travels at a finite speed (186,000 miles per second) this extreme distance means that we are also looking further back in time, back to when things in the Universe were closer together. As the light travels from the quasar to us, the Universe has expanded and so the light from the quasar is stretched out – a phenomenon known as redshift.

The light that the BOSS detector receives is partially absorbed by the intervening gas clouds and the pattern of absorption gives a map of the gas. Each gas cloud absorbs at a wavelength particular to a given energy transition in hydrogen.
The redshifting of the quasar light means that each cloud leaves an imprint at a different wavelength in the light we eventually observe from the quasar. The amount of absorption in the quasar spectrum at any given wavelength reveals the position of a hydrogen gas cloud between us and the quasar.

A map of the entire Earth at first glances reveals large features like continents, but reveals mountains, hills, and valleys when you zoom in. In the first phase of BOSS, scientists used a very coarse-grained map of the absorption due to hydrogen to study the “large-scale structure” of the universe at scales of hundreds of million light years, analogous to studying the size and disposition of the continents on the surface of the Earth. BOSS’s “continents” were clouds typically many millions of light-years in size. BOSS found that the gas was arranged in a pattern that reflected sound waves that existed during the first 380,000 years after the big bang. The pattern gave information on the conditions in the universe at this time and confirmed that the Universe consists of a strange cocktail of normal matter, photons, neutrinos, dark matter (matter that doesn’t interact with light and that forms the bulk of galaxies), and the completely mysterious dark energy which is currently driving the increasing expansion rate of the Universe.

SDSS-III scientists are producing detailed numerical simulations of the universe to compare to BOSS observations. This image shows a simulated map of the gas clouds (the red blobs are clusters of galaxies and the blue filaments are lower-density gas regions), in a cube of the universe 65 million light-years on a side.

In the second phase of their effort, BOSS scientists have been able to zoom in using more sensitive techniques to study the “small-scale” structures, down to scales of a few million light years, analogous to the mapping the mountains on a continent. This increased resolution reveals the characteristics of the gas clouds that are on the brink of forming galaxies.

The new BOSS observations so far appear consistent with current theories of the composition of the Universe. But this new data is sufficiently precise that BOSS hopes to use it to glean information on one of the least understood ingredients of the cocktail: neutrinos. These (ghostly) objects move through the universe nearly at the speed of light and cannot condense to form galaxies. However, their presence has an effect on the map of the gas clouds. The speeding neutrinos dilute the pattern that seeded the gas clouds. Current measurements indicate that neutrinos have a tiny mass (about one billionth of the mass of an electron). The cosmological maps from BOSS reflect its effects on the “small-scale” structures of the universe. Further analysis will reveal new constraints on the mass density of neutrinos in the cosmos.

The paper “The one-dimensional Ly-α forest power spectrum from BOSS” by N. Palanque-Delabrouille et al. was recently submitted to Astronomy & Astrophysics and is available at

A simulation of the interaction of dark matter, gas, stars, and neutrinos in forming the structure of galaxies we observe. The overall expansion of the Universe has been scaled out of this simulation to more clearly illustrate how the large scale structure forms. Previous work mapped hydrogen clouds on the scale of this box. The current work can map down to the scale of the red spheres shown here that are turning into galaxies.

This second image is a map showing the ghostly neutrinos, here drawn in yellow, in the same cube of universe. The only hint of condensation appears about the largest density region, in the center of the image. With further analysis BOSS scientists plan to explore this faint imprint on the structure of hydrogen clouds from neutrinos in the early Universe.

Our Deepest Sympathies and Thanks to the Wildfire Fighters in the American Southwest

2 07 2013

We in the Sloan Digital Sky Survey collaboration are deeply saddened by the news of the loss of 19 firefighters from the Granite Mountain Hotshots of Prescott, Arizona.

There is a closer connection between astronomy and these hotshot crews than many may know. Fire is the top threat to the observatories of the American Southwest. Our facilities are at dry and remote sites, typically high up in the pine forests that top the mountains. Summer fires are a perennial danger.

While the Apache Point Observatory where the SDSS facility is housed has been lucky to escape direct threat in the last twenty years, several other major observatories have had close calls. That catastrophe has been averted is due largely to the tremendous efforts of the wilderness firefighters and response teams that work to contain these fires and shape them away from inhabited areas. Their skill and strength in combating these fires is truly remarkable.

We don’t know whether any members of the Granite Mountain Hotshots worked on fires near our observatories, but their loss is dear to us nonetheless. We want to take this opportunity to thank all of the firefighters working in the forests of the Southwest for their dedication, and we want to recognize the important role that they play in American astronomy. Most importantly, we want to send our condolences to the families and friends of those lost in this tragedy.

Donations to help the families and coworkers of the firefighters who died battling the Yarnell Hill Fire can be made at the following website:

Finally, we want to recognize the important work that goes on locally at our location. Fire prevention is a year-round activity at Apache Point Observatory (APO). APO Site Manager Mark Klaene is a Captain in the the High Rolls Volunteer Fire Department (VFD) and a Lieutenant in the Sunspot VFD. SDSS Engineer Tracy Naugle is a firefighter in the Sunspot VFD, and APO 3.5-m telescope observer Alysha Shugartis is a probationary firefighter in the Sunspot VFD. APO and SDSS specifically thank the Lincoln National Forest branch of the U.S. Forest Service for their efforts to manage the forest around APO and keep our observatory safe.

SDSS Pioneer Recognized as a Champion of Change by the White House

1 07 2013

The White House recently recognized 13 individuals as “Champions of Change” for Open Science: people who worked to make data public for the greater good:

One of the thirteen was Princeton University’s Jeremiah P. Ostriker, recognized for his role in the development of the Sloan Digital Sky Survey. This recognition reflected SDSS’ record of making its data public to the world, enabling the writing of literally thousands of papers. The ceremony, which took place on June 20 in the Eisenhower Executive Office Building, adjacent to the White House, included an open discussion among the 13 Champions, as well as talks by John Holdren (Director of the White House Office of Science and Technology Policy) and Todd Park (Chief Technology Officer of the United States). Prof. Ostriker invited five guests to take part in the ceremony. The visiting group is shown here left-to-right starting from the upper left: Michael Strauss (former SDSS spokesperson), Al Sinisgalli (who was key in arranging Princeton’s financial support of the project), Don York (SDSS’ first director), Ostriker, Jill Knapp (scientist par excellence who kept the project going through sheer force of will) and Jim Gunn (SDSS Project Scientist).

(Image Credit: Anjelika Deogirikar)

The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies

17 06 2013

SDSS astronomers at Ohio State have made a new, more robust measurement of the relationship between stellar mass and the abundance of elements heavier than helium and hydrogen. The measurement of elements is called the “gas-phase metallicity” and is based on measuring weak auroral lines that are typically undetected in galaxy spectra (dubbed the “direct method”). The relationship between these two is called the mass-metallicity relationship. To enhance the signal-to-noise ratio of these lines, the astronomers stacked ~200,000 galaxy spectra from the SDSS-II Data Release 7.

(Figure) The new mass-metallicity relation is shown by the points and black line. The colored lines indicate previous mass-metallicity relations based on less certain strong line methods. This new measurement of the mass-metallicity relation provides important constraints for galaxy evolution models, especially for the efficiency of galactic winds.

Lead author Brett Andrews has prepared a brief video describing the main results for a general audience:

For more details please see the published paper:
“The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies”
Andrews & Martini (2013)
The Astrophysical Journal, 765, 140.…765..140A

Two Megaspectra Across Four Cubic Gigaparsecs

8 05 2013

In April BOSS passed an impressive milestone with 2 million survey-quality spectra. These spectra now probe large volumes of the Universe. The galaxy spectra alone probe over 35 billion cubic lightyears (four cubic gigaparsecs), while the BOSS quasars probe an even larger volume but much more sparsely. BOSS is on pace to successfully complete its main survey of 10,000 square degrees, which is almost one quarter of the sky, by the end of SDSS-III observing in the summer of 2014.

Officially, the prize for the two millionth object goes to a spectrum of the blank sky.

BOSS Spectrum of a Region of Blank Sky

Figure: The two-millionth BOSS spectrum: an observation of a blank region of the sky (green line). Most of the emission that is seen is from the Earth’s atmosphere. The peaks in the right half of the figure are emission lines from oxygen, water, carbon dioxide, and other molecules. After subtracting the sky, the remaining signal (black line) is consistent with no detected source. The quoted redshift in light green is the BOSS pipeline fitting a quasar so faint that it is entirely consistent with the noise from the observation; this is not a real detection of an object.

Why does BOSS intentionally take observations of blank regions in the sky?

It turns out that these sky spectra are actually vital to the basic analysis of the BOSS spectra of stars, galaxies, and quasars. Most of the photons detected by BOSS are emitted by the Earth’s atmosphere. We have to subtract these photons to analyze the faint signals of distant galaxies. About 10% of BOSS fibers on each plate are used to observe blank regions of the sky. BOSS takes these blank-sky observations to help in calibration and removing the sky background.

These blank sky spectra can also have scientific utility. These blank sky spectra are the easiest targets from which to study the diffuse interstellar medium, as done by Brandt & Draine:

They are also used to search for serendipitous observations of emission-line galaxies. While the fibers are placed in locations that the SDSS-III imaging data show to be blank, galaxies that emit most of their light in just a few emission lines can occassionally appear in these spectra. Such galaxies are very uncommon today and understanding their presence at high redshift (which is necessary for their ultraviolet emission lines to be observed in the visual region of the spectrum) can help us learn about the early process of galaxy formation.


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