The SDSS Reverberation Mapping Project

The below post was contributed by Dr. Catherine Grier, a postdoctoral researcher at Penn State University (formerly a graduate student from Ohio State University, and the Director of the OSU Planetarium) who has led a recent paper based on results from the SDSS Reverberation Mapping Project (accepted for publication in the Astrophysical Journal; the full text is available at: arXiv:1503.030706 Screen Shot 2015-05-21 at 17.16.00 Supermassive black holes (SMBHs) are present in all massive galaxies and are thought to affect the formation and development of the galaxies themselves. Because of this, understanding SMBHs is important in understanding how galaxies are formed and evolve. Observations of quasars are key to understanding SMBHs and how they affect their host galaxies: Quasars are enormously powerful, observable at great distances, and can potentially regulate the growth of their galaxies through their winds, or outflows. We learn about these winds by observing broad absorption line features (BALs; see the diagram below) in quasar spectra that are created by high-speed winds launched from the quasar accretion disk. These winds are made of gas that blocks the light from the quasar and show up as BALs in the spectra of quasars.


The CIV region of our target showing the CIV Broad Absorption Line (BAL) features investigated in our study.

These absorption features change throughout time, both in strength and in shape. Under the right conditions, we can use the details of the variability to learn about the density of the absorbing gas and the distance of the gas from the SMBH. This information can sometimes be used to determine if the outflow is powerful enough to affect the star formation in their host galaxy. Previous studies have found that BALs are variable on timescales of several years all the way down to timescales of 8-10 days; however, until now, no studies have reported variability on timescales shorter than 8-10 days. In our recent work, we report on very short-timescale (~1 day) BAL variability observed in a SDSS quasar. The spectra used in our study were taken as a part of the SDSS Reverberation Mapping (SDSS-RM) project using the BOSS spectrograph. We monitored 850 quasars with the BOSS spectrograph from January 2, 2014 through July 3, 2014, resulting in 32 observations over this period. The main goal of the SDSS-RM program is the investigation of the broad emission line regions of quasars, but the targets include a number of quasars hosting BALs and can be used for BAL studies too. During our observing campaign, the equivalent width, or strength, of the highest-velocity CIV BAL feature (see above diagram) changed by over a factor of 2. We did not observe similar variations in either the CIV broad emission line or the overall brightness of the quasar, and the shape of the BAL feature stayed roughly the same during the entire campaign. We observed significant changes in the strength of the BAL on timescales as low as 1.20 days in the quasar rest frame (see the graphs below). This is the shortest time frame ever reported over which significant variability in a BAL trough has been observed.


Four different pairs of spectra between which the CIV BAL trough varies significantly.

The most likely cause of the variability is a change in the amount of ionized gas in the outflow. This could be caused by changes in the brightness of the quasar or the amount of energy reaching the absorbing gas for various other reasons. With our observations, we are unable to determine whether this outflow contributes significantly to feedback to the host galaxy, but we do not rule out the possibility. The key to observing this short-term variability was the high cadence of the SDSS-RM campaign, which allowed us to search for BAL variability on shorter timescales than previous studies. This program is still ongoing; we expect to receive more spectra of this target over the next few years with the eBOSS spectrograph, which could shed further light on this topic. The variability properties of this target are similar to those found in other quasars, suggesting that this short-term variability may be common. Further high-cadence spectroscopic campaigns targeting BAL quasars would allow us to learn more about BAL variability in quasars and better understand the possible contributions of BALs to feedback to their host galaxies.

Engineering Work for APOGEE-South – Trabajo de ingenieria en APOGEE-Sur

Telescopes at LCO

The du Pont 2.5m telescope on the middle-left, and the pair of Magellan 6.5m telescopes on the right.
El telescopio de 2.5m du Pont al centro hacia la izquierda y los dos telescopios de 6,5m, Magallanes, a la derecha.

A half-dozen SDSS scientists and engineers traveled to Las Campanas Observatory, Chile at the beginning of March to continue work on characterizing the 2.5m du Pont telescope performance in preparation for the first APOGEE-South hardware tests in August. This report is from the SDSS Operations Software Manager John Parejko, who was part of the run (and ended up involved in some hardware tests, against his better judgement!). Translated into Spanish by Verónica Motta, Associate Professor of Astronomy at Valparaiso University.

Las Campanas Observatory currently hosts three “large” telescopes (greater than 2 meters diameter), and a number of 1 meter diameter and smaller telescopes. The 2.5m du Pont telescope (in use since 1977) is a much older telescope than the 2.5m Sloan telescope at APO (in use since 1999), but it is at an excellent site, its optics are still very good–I heard them referred to as “superb” on several occasions–and it has a large field of view. With the assistance of the telescope’s owners–the Carnegie Institution of Washington–SDSS plans to help design improvements to the telescope drive systems so that we can run an APOGEE-South survey and fully sample the Milky Way’s bulge.

Una media docena de científicos e ingenieros del SDSS viajaron al Observatorio Las Campanas (Chile) a principios de marzo para continuar el trabajo de caracterización del rendimiento del telescopio de 2.5m du Pont en preparación para la primera prueba de hardware de APOGEE-Sur que se realizara en agosto. Este informe proviene del Director de Operaciones de Software del SDSS, John Parejko, que participó en la ejecución (y que terminó involucrado en algunas pruebas de hardware, en contra de su mejor juicio ! ). Traducción de Verónica Motta, profesor asociado de astronomía en la Universidad de Valparaíso.

El Observatorio Las Campanas actualmente alberga tres “grandes” telescopios (mayores de 2m de diámetro), y varios más pequeños de hasta 1m de diámetro. El telescopio de 2.5m du Pont (en uso desde 1977) es más antiguo que el telescopio de 2.5m Sloan en el Observatorio Apache Point (APO, en uso desde 1999), pero está en un lugar excelente, su óptica es todavía muy buena -he oído referirse a ella como “excelente” en varias ocasiones- y tiene un gran campo de visión. Con la ayuda de los propietarios del telescopio -la Institución Carnegie de Washington- el SDSS planea ayudar a mejorar el diseño de los sistemas de accionamiento del telescopio de manera que podemos realizar el relevamiento APOGEE-Sur y muestrear completamente el bulbo de la  Vía Láctea.

Paul and Nick looking at the rotator

Paul Harding and Nick MacDonald looking at the rotator.
Nick MacDonald y Paul Harding investigan las propiedades físicas del rotador del du Pont.

In order to determine what improvements the telescope requires, we have to make precise measurements of how different parts of the telescope move. From previous work, we found that the Right Ascension and Declination motors (equivalent to latitude and longitude projected onto the sky) probably don’t need much work. This trip, we measured the motion of the rotator and focus systems. Carnegie is in the process of completing their own upgrades to the telescope, and our measurements will help guide these changes.

Con el fin de determinar qué mejoras necesita el telescopio tenemos que hacer mediciones precisas de cómo se mueven las diferentes partes del telescopio. A partir de trabajos anteriores, encontramos que los motores de la Ascensión Recta y de la Declinación (equivalentes a la latitud y a la longitud proyectada sobre el cielo) probablemente no necesitan mucho trabajo. En este viaje medimos el movimiento del rotador y del sostema de enfoque. Carnegie se encuentra en el proceso de terminar sus propias mejoras al telescopio y nuestras medidas servirán de guía para estos cambios.

Author self portrait in a primary. You can see the reflection of the secondary mirror and its light baffles just above my head.

Author self portrait in a primary. You can see the reflection of the secondary mirror and its light baffles just above my head.
Autorretrato del autor en el primario, se puede ver el reflejo del espejo secundario y su luz que pasa justo por encima de mi cabeza.

To focus a telescope like this one, you move the secondary mirror. Even tiny changes in the position or tilt of the secondary can result in incorrect or uneven focus when you need a large field of view, as APOGEE will. As the du Pont is an older telescope, the system that moves the secondary mirror may not be as stable as APOGEE requires.

We first checked whether the mirror moved the exact amount each time it was commanded. We’ve found that the mirror motors need to be more repeatable: moving 500 “up” and then 500 “down” should return to exactly the same place, but it doesn’t. The Carnegie engineers are now working to improve the motors and control systems to fix this.

Para enfocar un telescopio como éste se mueve el espejo secundario. Incluso pequeños cambios en la posición o en la inclinación del secundario pueden resultar en un foco incorrecto o irregular en un gran campo de visión como el que utilizará APOGEE. Como el telescopio du Pont es viejo, el sistema que mueve el espejo secundario puede no ser tan estable como requiere APOGEE.

Primero revisamos si el espejo se movió la cantidad correcta cada vez que se le ordenó. Hemos encontrado que los motores del espejo tienen que ser más confiables: moverse 500 hacia “arriba” y después 500 hacia “abajo” debería regresarlo exactamente al mismo lugar, pero no es así. Los ingenieros de Carnegie están trabajando para mejorar la motores y los sistemas de control para solucionar este problema.

Author self portrait in the du Pont secondary, with my camera and our measuring target visible.

Author self portrait in the du Pont secondary, with my camera and our measuring target visible.
Autorretrato del autor en el secundario del du Pont, con mi cámara y nuestro objeto de medición visible.

To measure any shift or tilt in the secondary, we used a rather interesting system: a typical camera (the Panasonic G2 that I travel with for touristy photos; it took all the pictures shown in this post) with a long telephoto lens mounted on a moveable rail, taking pictures of the image in the secondary mirror of a “target” on the floor. We then took pictures with the camera and measured whether the target moved around: if it doesn’t move from image to image, we know the secondary is very stable against tilts and shifts during movement. We’re still analyzing the results of these tests, and will use them to detail what changes need to be made.

Para medir cualquier desplazamiento o inclinación en el secundario usamos un método interesante: una cámara típica (la Panasonic G2 con la que viajo para tomar fotos turísticas; la que tomó todas las imágenes que se muestran aquí) con un teleobjetivo largo montado en un carril móvil, toma fotos de la imagen en el espejo secundario de un “objetivo” en el suelo. Entonces tomamos fotos con la cámara y medimos si el objetivo se movió: si no se mueve de imagen a imagen, sabemos que el secundario es muy estable ante las inclinaciones y los cambios durante el movimiento. Todavía estamos analizando los resultados de estas pruebas y las usaremos para detallar los cambios deben hacerse.

Además de mi trabajo de ingeniería en el telescopio du Pont, tuve tiempo durante la noche para fotografiar el cielo austral. Este fue mi primer viaje al hemisferio sur y me aseguré de levantarme temprano al menos una mañana para ver las Nubes Mayor y Menor de Magallanes y toda la gloria de la Vía Láctea austral. Tuve que levantarme temprano para evitar la Luna casi llena, que disminuye la visibilidad. Sin duda tienen cielos espectaculares ahí abajo.

In addition to my engineering work on the du Pont telescope, I was able to take some time at night to photograph the southern sky. This was my first trip to the southern hemisphere, and I made sure to get up early at least one morning to see the Large and Small Magellanic Clouds and the full glory of the southern Milky Way. I had to get up early in order to avoid the nearly-full moon, which otherwise much diminished the view. They’ve certainly got some spectacular skies down there!

The southern hemisphere Milky Way and Large Magellanic Cloud, over the main LCO building.

La Vía Láctea y las Nubes Mayor y Menor de Magallanes Nube en el hemisferio austral, sobre el edificio principal del LCO.
The southern hemisphere Milky Way and Large Magellanic Cloud, over the main LCO building.

Pasé mis últimos días de este viaje en la ciudad de La Serena, reunido con la gente de la Universidad de La Serena (ULS) y reuniendo los resultados de las pruebas. Durante este tiempo, pude ver como la escuela de ingeniería ULS  maniobró la nueva máquina, marca Mazak CNC, con cuidado hasta su lugar en el taller de mecánica. Las instituciones chilenas han utilizado la colaboración SDSS/Chile para reforzar su infraestructura a través de subvenciones y varios acuerdos. En este caso, fueron capaces de comprar el modelo más avanzado de fresadora computarizada que planean utilizar para construir piezas para APOGEE-Sur.Tengo ganas de ver que pueden construir con ella!

I spent my last days of this trip in the city of La Serena, meeting with people at the University de La Serena (ULS) and collating results from the tests. During this time, I was on hand to watch as the ULS engineering school had a brand new Mazak CNC machine carefully maneuvered into place in their machine shop. Chilean institutions have used the SDSS/Chile collaboration to bolster their on-site infrastructure via grants and various agreements. In this case they were able to purchase a state-of-the- art computerized milling machine that they plan to use to construct parts for APOGEE-South. It will also provide engineering student training and experience, and allow the university to construct other cutting edge scientific equipment in the future.

I’m looking forward to see what they can build with it!

Happy Engineers standing in front of their just-delivered CNC machine. Ingenieros felices de pie frente a su recién entregada máquina CNC.

Ingenieros felices de pie frente a su recién entregada máquina CNC.
Happy Engineers standing in front of their just-delivered CNC machine.

Job Posting: University of Washington Machine Shop Manager

The University of Washington Physics Instrument Shop is looking for a new shop manager.  This is the machine shop which builds the SDSS plug plates, fiber systems, and a number of our other instrumentation and telescope equipment for SDSS, APO 3.5 m, and soon LCO. This shop is a key part of SDSS operations.

Position Description

The Instrument Shop Manager is responsible for the daily operations of a 5 FTE research and development machine shop with an $850,000 annual budget.  The Instrument Shop provides clients (primarily scientists) with both one-of-a-kind and production instruments.  The manager is solely responsible for assessing each client’s request, estimating the amount of time and effort to complete the job, assigning the job to the staff persons whose abilities and experience best fit the request and scheduling the job.  The Manager is the line supervisor for 5 FTE – selecting, hiring, evaluating and disciplining employees as necessary.   The Manager ensures that the proper tooling and materials are on hand for each job, that machines are maintained and repaired and that the workplace is safe. The Manager works closely with faculty, staff and students on their research projects.  Many experiments involve instruments that are not available ‘off the shelf’ and are custom designed for each particular experiment or project.  Faculty, staff and students depend upon the Manager to review their ideas and ensure that the devices are buildable and suggest modifications that may result in a better instrument or make it easier to produce.

Link to the job posting. 

You can get an idea of what goes on in this shop in this video of SDSS plate production

How SDSS uses light to see dark matter in galaxies

Some of the most beautiful pictures taken by telescopes are those of galaxies. Containing billions of stars, they come in many shapes and sizes. We can study the stellar structures in galaxies from telescope images to learn more about the ways that galaxies form and evolve. We also can look at gas and dust features in galaxies, and the role that these play in the formation of new stars.

Elliptical galaxy NGC 4636 (left) and spiral galaxy M81 (right), as seen by the Sloan Telescope. The telescope captures the light of the stars, and in M81 we can also see some dust in the spiral arms. Both galaxies reside in large, invisible, dark matter haloes.

Elliptical galaxy NGC 4636 (left) and spiral galaxy M81 (right), as seen by the Sloan Telescope. The telescope captures the light of the stars, and in M81 we can also see some dust in the spiral arms. Both galaxies reside in large, invisible, dark matter haloes.

Yet, the largest and most massive component of a galaxy, the dark matter halo, is truly invisible. Dark matter is not made out of ‘normal material’ or baryons, which are the building blocks of stars, planets and all other matter surrounding us. Instead, dark matter is thought to be an exotic particle that does not emit or absorb any light: it does not interact with the electromagnetic force like normal matter. So how do we then know that the dark matter is there?

The answer lies in the light that we observe from the stars and the gas in galaxies. With images we capture the presence of light, but with spectrographs we unravel the light into different colours or wavelengths. The resulting galaxy spectra show us how the stars are moving around in the galaxy. In most galaxies, the stars will rotate around the centre of the galaxy, and this rotational velocity can be seen in the spectrum by a shift in the stellar absorption lines. This shift results from the Doppler Effect, which causes the lines of stars that move away from us to shift towards the red part of the spectrum, while the lines of stars that are moving towards us shift to the blue part of the spectrum. This way, we can find out how fast the stars in a galaxy are rotating around the galaxy centre. But there is more information in the spectrum: the lines are not infinitely thin, but are slightly broadened. This broadening is called ‘velocity dispersion’ and is caused by the additional random motions of the stars. With the new Sloan Survey, MaNGA, we are measuring the rotational and random motions of the stars in 10,000 galaxies. And because MaNGA is an integral-field spectrograph, we can map these motions not only in the very centre of the galaxies, but also in their outskirts, as shown below.

MaNGA is an integral-field spectrograph, capturing spectra at multiple points in the same galaxy with a fiber bundle. The bottom right illustrates how each fiber will observe a different section of the galaxy. The top right shows data gathered by two fibers observing two different part of the galaxy, showing how the spectrum of the central regions differs dramatically from outer regions. From these spectra, we measure the rotational and random motions of stars, to deduce how much dark matter is present in the galaxy. Image Credit: Dana Berry / SkyWorks Digital Inc., David Law, and the SDSS collaboration

How do these velocity and dispersion maps help us to find the dark matter? The answer is gravity. The stars are moving around in a galaxy under the influence of gravity: the more matter (mass) there is in the galaxy, the faster the stars are moving. Now that we have measured the movements of the stars in the galaxies, we can deduce how much matter is needed to have the stars move around with those measured velocities. And we can compare that gravitational mass with the luminous mass in the galaxy (the stars, gas and dust). For all galaxies studied so far, the gravitational mass is much larger than the luminous mass: hence the need for dark matter.

Example of a galaxy observed with MaNGA. Left is the image of the galaxy, showing the stellar light. The middle image is the rotational velocity field of the galaxy: the red part of the galaxy is moving away from us with 254 km/s, and the blue part of the galaxy is moving towards us with the same velocity. The green axis down the middle is the rotation axis. The right image shows the random motions of the stars: these are higher in the centre (red: 257 km/s) than in the outskirts of the galaxy (blue: 94 km/s). Figure taken from Bundy et al. 2015.

Example of a galaxy observed with MaNGA. Left is the image of the galaxy, showing the stellar light. The middle image is the rotational velocity field of the galaxy: the red part of the galaxy is moving away from us with 254 km/s, and the blue part of the galaxy is moving towards us with the same velocity. The green axis down the middle is the rotation axis. The right image shows the random motions of the stars: these are higher in the centre (red: 257 km/s) than in the outskirts of the galaxy (blue: 94 km/s). Figure taken from Bundy et al. 2015.

Sophisticated mass or dynamical models of the galaxies, based on the observed velocity and dispersion maps, tell us how the luminous and dark matter are distributed in the galaxy, and what the properties (mass, size, concentration) of the dark haloes are. Comparing these mass models with predictions from galaxy formation theories will help us forward in our quest to understand galaxies, and the dark haloes that surround them. But it all starts with capturing the stellar light of galaxies in spectrographs, to map the invisible.

This post was written by Dr. Anne-Marie Weijmans (St Andrews) and is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.

Heroes of the Helpdesk

They come in a steady stream: the requests for lost passwords, for aid in correcting a CasJobs query, for insight into the technical details of SDSS photometry, astrometry, and spectroscopy, for help with educational resources and SkyServer, and for general astronomical and database knowledge, all sent to Two or three times a day, they appear in the mailboxes of those on the helpdesk mailing list, representing the hopes and dreams of an astronomer, amateur, student, or professional, to use SDSS data to answer the big questions of the Universe (or at least to get more room on the server).

The helpdesk in action: Ben Weaver making SDSS science possible for yet another scientist.

The helpdesk in action: Ben Weaver making SDSS science possible for
yet another scientist.

The task falls to the volunteers, headed up and organized by Ben Weaver, Archive Scientist at New York University (right). Most questions are handled quickly by Ben, Ani Thakar, Archive Scientist at Johns Hopkins University, or Jordan Raddick, one of our Education Directors, also at Johns Hopkins. Promptness is easiest if the questioners have done us the kindness of including relevant information such as the context or URL they are using and the exact query. Occasionally questions about how parameters were derived or why there are changes between Data Releases requires the advice of other SDSS experts. In this case, Ben sends email to the relevant SDSS mailing list. Excellent answers are gratefully accepted from the wider collaboration, who really do know these data. Our thanks to everyone who has stepped up and contributed to making SDSS data scientifically valuable to an astonishing array of people. Above all, we wish to thank the helpdesk regulars. If you have sent an email to the helpdesk or if you know someone who has sent an email to the helpdesk (and you probably do, trust me), send a cheer their way.

Tweep of the Week: Sarah Jane Schmidt

In charge of the SDSS Twitter account for this week is Dr. Sarah Jane Schmidt, the Columbus Prize Postdoctoral Fellow in the Department of Astronomy at The Ohio State University

Dr. Sarah Jane Schmidt

Dr. Sarah Jane Schmidt

Dr. Schmidt studies the lowest mass and most numerous types of stars in our Galaxy – the M and L dwarfs. These types of cool stars have strong magnetic fields on their surfaces which results in special kinds of extra light from the stars, including dramatic flare events, which Dr. Schmidt works to observe and understand.

Within the SDSS collaboration, Dr. Schmidt has worked or is working on observing cool stars using spectroscopy from several different surveys:

1. A study of ultracool dwarfs with data from a BOSS (Baryon Oscillation Spectroscopic Survey) ancillary project

2. A TDSS (Time Domain Spectroscopic Survey) project looking at long timescale magnetic field variations on late-M and early-L dwarfs

3. Studying the colors of late-K and early-M dwarfs with measurements of temperature and metallicity from spectroscopic observations taken for the APOGEE survey.

This can all be summarised as spectroscopy of the lowest mass stars there are, and Sarah is most interested in using these to constrain the stars ages and how this relates to their magnetic activity.

We hope you’ll join the conversation with Sarah and other SDSS scientists on twitter this week so we can all learn more about the magnetic fields of the smallest stars in the Universe.

Spotlight on APOGEE: Jo Bovy and the Motion of the Sun

The spotlight this month is on Jo Bovy, a John Bahcall Fellow and Long-term Member at the Institute for Advanced Study in Princeton. He completed his PhD at New York University. Within APOGEE, he is the Science Working Group Chair for APOGEE-1, and therefore coordinates the scientific analysis of the APOGEE dataset.


Jo uses big datasets from numerous surveys to understand how the Milky Way came to be. To do this, he studies how stars move and what are their chemical compositions. When he first tackled APOGEE’s huge database of stellar spectra in 2011, he realized that APOGEE’s spatial coverage of the Milky Way’s disk allowed the circular velocity of stars in the Milky Way to be measured with greater accuracy than had ever been done before. (The circular velocity is the speed at which a star orbits the center of the Galaxy. This number changes as a function of distance from the center, and precise measurements are required to correctly determine, for instance, the Galaxy’s mass, but also to measure peculiarities in stellar velocity, which help us determine where it might have originated.) The data analysis in his paper is complex, but he was able to draw two important and straight forward conclusions from this work:

  1. that the circular velocity near the Sun (what we call the “solar neighborhood”), at a distance of 26,000 light-years from the center of the Galaxy, is 218 ± 6 km/s; and
  2. that the Sun itself is moving 25 km/s faster than other stars at the same distance.

The first result was expected: although the circular velocity in the Sun’s neighborhood was assumed to be about 220 km/s for the last 30 years or so, Jo’s was the first precise measurement to confirm this value. The second result, however, was a surprise: previous measurements had pegged the Sun’s motion relative to nearby stars at something like 12 km/s, not 25 km/s. This result was confirmed using a sub-set of APOGEE data (a mere 19,937 stars, or about 15% of the full APOGEE dataset) known as the APOGEE Red Clump Catalog.

Why does this seemingly small difference matter? From an outsider’s perspective, going from 12 km/s to 25 km/s is still only changing from about 5% to 10% of the circular speed, so either result might seem acceptable! But is is important, and Jo explains why: We orbit the Sun, and the Sun orbits the center of the Galaxy just like every other star. Therefore, every speed that we measure for another star is relative to our own motion. If we can understand how we move in the Galaxy, then we will have a much better understanding of the dynamics of the entire Milky Way.

And understanding the Milky Way is, after all, the whole point!

How SDSS Uses Light to Understand Stars Inside and Out in the Kepler Field

Stars are not only fascinating objects in their own right — they also help us understand the history of our Milky Way galaxy. Our galaxy was created as dark matter’s pull brought gas together, and the gas formed stars and planets. As part of the APOGEE survey, we wish to map the Milky Way’s star formation throughout cosmic time. As stars died, many of the elements they fused in their interiors during their lives or death throes are mixed back into the remaining gas, changing its composition and the composition of subsequent generations of stars and providing the raw materials for planets (and humans!) and we are exploring this chemical history as well.

A small part of the spectra of a few of the stars observed by APOGEE. The dark lines are caused by absorption of atoms in the star's atmosphere (or sometimes the Earth's). A few of them are highlighted. The bright lines are caused by emission in the Earth's atmosphere ("night sky lines") These particular stars have also been observed by the Kepler satellite.

A small part of the spectra of a few of the stars observed by APOGEE. The dark lines are caused by absorption of atoms in the star’s atmosphere (or sometimes the Earth’s). A few of them are highlighted. The bright lines are caused by emission in the Earth’s atmosphere (“night sky lines”) These particular stars have also been observed by the Kepler satellite.

APOGEE studies stars by passing their infrared light through gratings that spread the light out in wavelength (think infrared rainbows). We do this for > 250 stars at once (one of the reasons why the APOGEE instrument is fantastic). We can tell a lot about stars from studying these spectra. For example, in an earlier blog post, we discussed how we can tell the surface temperature of stars from such data. Another very important property is the composition of the star, for example, how many atoms of iron, calcium, or oxygen it has relative to hydrogen. The image to the left shows a small part of the spectra we gathered for stars that were also observed by the Kepler satellite. The stars do not give off the same amount of light at each wavelength (=color) of light. Instead, there are many dark lines, which are created when atoms in a star’s atmosphere absorb light at very particular wavelengths. Each element has a different pattern of these absorption lines, and by measuring the depth of these lines (+ additional information and math), we can determine the composition of the gas out of which the star formed.

But this doesn’t tell us everything about the star! In particular, we can’t see inside the star where the original composition of the gas is being transformed from hydrogen into helium as the star ages. We have a good idea of how long it takes for a star with a certain mass and original chemical composition to run out of fuse-able hydrogen in its center (about 10 billion years in the case of a star with the mass and composition of the Sun). When that happens, the star undergoes a dramatic change, turning into a red giant or supergiant. So if we can determine the mass to go with the spectral  composition information for red giants that we observe, we can determine the age of those particular stars.

Measuring the mass of a star is hard work, but one possible technique is to use asteroseismology, which is the study of the waves that move through stars. In the outer parts of stars, these waves are actually sound waves that can evocatively be described as ringing the star like a bell (For more information see The Song of the Stars). The motions of these waves cause a star’s brightness to change by small amounts, and thus the frequency of these waves can be measured by studying the lightcurves of red giant stars. The Kepler satellite, in addition to studying many Sun-like stars looking for transiting planets, also measured the brightnesses over many years of thousands of red giants. The favorite frequencies of waves in different stars have been measured by members of the Kepler Asteroseismic Science Consortium. While much can be learned about the insides of stars from these data, we are particularly intrigued by the fact that how long and at what speed waves can move through the star depends on the star’s density and therefore (with some more math) its mass!

Combining together spectra from APOGEE and lightcurves from Kepler therefore gives us a way to figure out the ages of red giant stars in our Galaxy by measuring the masses and composition of stars that have just exhausted their hydrogen. In conclusion, songs and rainbows are good things.

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe. 

SDSS Tweep of the Week: Brett Andrews

This week SDSS scientist Dr. Brett Andrews is taking over our @SDSSurveys twitter account.


Brett Andrews is a postdoc in the Physics and Astronomy department at the University of Pittsburgh. He is working on the MaNGA (Mapping Nearby Galaxies at APO) Data Analysis Pipeline and visualisation tools.

Brett’s research focuses on understanding galaxy evolution, particularly the impact of metal production by stars, cosmological gas inflow, and galactic winds. He is interested in using the gas-phase abundances as a way to trace the relatively recent chemical enrichment history of a galaxy as well as the stellar abundances as a tool to provide a fossil record of the abundance of a galaxy over its entire history.

This week is the MaNGA Team meeting, being held at the University of Kentucky, Lexington Kentucky, so it’s a good week for Brett to take over the Twitter account.

So stay tuned for lots of extragalactic science this week from @SDSSurveys


How SDSS Uses Light to Measure the Distances to Galaxies

Here at the Sloan Digital Sky Surveys our mission is to explore and map the Universe, from planets to the edges of the observable Universe. The way we do this is to collect light from specially selected objects we see in the night sky – but we can’t visit them in order to measure how far away they are. So how do we actually know how far away they are in order to make a map of the Universe?

Measuring the distance to objects in the Universe has always been one of the biggest challenges for astronomers. Until we know the distance to something we cannot really understand its physical properties, and the history of astronomy is full of examples where new techniques for measuring distances opened up entirely new areas of study. For example when the “spiral nebulae” were first discovered there was a long debate over if they were small clouds of gas in our own Galaxy, or external galaxies in their own right each made up of millions or billions of stars. Only by measuring their distances was this finally settled, and our understanding of the size of the Universe suddenly jumped many orders of magnitude.

A collection of "spiral nebulae". But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

A collection of “spiral nebulae”. But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

There’s some really useful bits of physics we can use to help measure distances to the galaxies from their light. To do this we need to understand spectroscopy. Once SDSS had finished imaging more than a quarter of the sky with its camera, it became entirely focused on “spectroscopic” surveys. Our telescope in New Mexico collects the light from stars and galaxies and uses instruments called spectroscopes to split it up into its different colours (we actually have two different spectroscopes working right now – the APOGEE spectroscope and the BOSS spectroscope). These measurements split the light into a rainbow (or a spectrum), and we look for the precise colours of series of emission and/or absorption lines to tell us all sorts of things about the light source we’re looking at.


A hot bright light source (like a star) will have a “continuous spectrum” (with the peak colour depending on its temperature – hot things glow red, even hotter things glow white or blue hot). If the light from that passes through a cool cloud of gas before we measure it, that will create “absorption lines” where very specific colours (or “wavelengths” in proper scientific terms) are absorbed by atoms in the gas cloud. The exact pattern of colours/wavelengths which are absorbed tell you which atoms are in the gas cloud. If the gas cloud gets heated up enough we might instead see emission lines – at the same specific colours, where the atoms are now re-emitting these very specific colours/wavelengths. Each atom has a very distinctive pattern of lines it emits – for example hydrogen (the most abundant element in the Universe) has a very distinctive and bright emission/absorption line in the red part of the spectrum (at a wavelength of 656.3nm).


Emission spectrum of hydrogen in visible light (wikimedia commons)

Astronomers have been using this technique to work out the materials which make up the Sun and other stars for decades. It’s not always easy (it has been compared to trying to reconstruct a piano from the noise it makes falling down the stairs), but it works. When astronomers first used the technique to look at galaxies however they were very surprised by what they found. The patterns of lines seemed to be in completely the wrong places – for example the famous hydrogen lines weren’t even visible in some cases – they had moved right into the infra-red part of the spectrum.

In order to understand why this could happen we need to learn about another part of physics – the Doppler effect. First proposed in 1842, by a Physicist named Christian Doppler this is the observation that when a source emitting a wave is moving, the waves are shortened if the source is moving towards the observer, and lengthened if it is moving away. Most people are familiar with this effect when they have listened to ambulance sirens passing them on the street; the siren is higher in pitch when the ambulance is moving towards you and lower when it’s moving away (when sound waves are lengthened the pitch drops, and when they are shortened the pitch rises).

Wikimedia commons illustration of the Doppler effect.

Since light is a wave, the same effect happens when light is emitted from a moving source. When the waves of light are shortened the light becomes bluer, and when they are lengthened the light becomes redder.

An astronomer named Vesto Slipher, was the first person to try this out on galaxies, and he found that almost all galaxies he looked at showed enormous “redshifts”, implying that almost all the galaxies were moving away from the Earth at very high speeds.

Edwin Hubble is given the credit for explaining this observation by realising that we live in a Universe which is constantly expanding. In such a Universe any observer will observe almost all other galaxies moving away from them. Hubble published the first description of a relationship between how fast galaxies appear to be moving away from us (their “redshifts”) and their distances – this relationship is now called Hubble’s Law.

It is this relationship that we use to measure the distances to the galaxies from detailed observations of the light they emit, and astronomers are now used to describing the distances to galaxies as simply their “redshift”.


A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.

SDSS Data Lead to Discovery of 12 Billion Solar Mass Black Hole in Young Universe

A paper appearing in Nature today (Xue-Bing Wu et al. 2015, Nature, Feb 25) presents the most massive black hole discovered to date when the Universe, was less than a billion years old – just one-fifteenth of its current age.

A new method to select high-redshift quasars using SDSS observations combined with data from the WISE satellite has resulted in the discovery of new group of quasars at the far reaches of the universe, with redshifts greater than z = 5. One of these quasars, named SDSS J0100+2802, holds a super-massive black hole at a redshift of 6.3 when the Universe was only 900 million years old.

This black hole is estimated to have a mass 12 billion times that of our Sun. The existence of such a massive black hole at such an early stage in the Universe poses a deep mystery whose resolution will improve our understanding of how galaxies form.

For more information, see the following links:

A graph of quasar luminosity vs. mass, with SDSS J0100+2802 marked

The newly discovered quasar SDSS J0100+2802 is shown by the large red dot in the graph above. The graph shows that SDSS J0100+2802 has most massive black hole and the highest luminosity among all known distant quasars. The background photo, provided by Yunnan Observatory, shows the dome of the 2.4-meter telescope and the sky above it. (Image: Zhaoyu Li/Shanghai Observatory)

Spotlight on APOGEE: Duy Nguyen and Binary Stars

We are beginning a series of spotlights on APOGEE team members, with special emphasis on their interests in APOGEE science. This month, the spotlight is on Duy Nguyen, one of APOGEE’s postdocs. He graduated from the University of Toronto with a PhD in astronomy and astrophysics, and then held postdoc positions at the University of Florida, Stockholm University, and the University of Rochester before joining the APOGEE team.


Duy’s research is on the subject of binary stars. A binary star is actually two stars orbiting each other. The sizes of the binary star orbits are small enough that the two stars cannot usually be distinguished in images. This can confuse the interpretation of starlight; and in a survey like APOGEE where precise velocities of stars are so important, this can be a big hindrance. As a result, a number of different methods have been employed to try to tease out whether a star is a binary or not.

But this post isn’t just about binary stars — it’s about one scientist’s research into better understanding them! And in many ways, Duy sees APOGEE as the best available experiment for binary star studies. APOGEE takes multiple spectra of most stars in its sample over months and even years, and this time sampling enables orbital periods to be measured. APOGEE’s high spectral resolution means that tiny Doppler shifts in a star’s spectral lines can be measured precisely. And most importantly, such a large sample as APOGEE has observed (more than 150,000 stars to date) means that we may be able to get a better handle on the “binary fraction” of stars in the Milky Way — a problem that has been plaguing modern astronomers for decades.

Duy is primarily interested in the dynamic properties of binary stars. These dynamics are primarily observed by means of the Doppler shift. As the stars in the binary pair orbit one another, each star approaches and recedes from the Earth once per orbit. Every time they approach the Earth, their spectral lines move to a slightly smaller (or “bluer” to use astronomical lingo) wavelength. And every time the star recedes, the spectral lines move to a redder wavelength. These small changes can be detected with APOGEE, and the radial velocity variations of the stars can be determined based on how large is the wavelength shift.

Duy and his collaborators are amassing radial velocity information on the stars in the APOGEE sample, looking for candidates with substantial radial velocity shifts. When they find one, they fit the data points with an orbital model to determine what the most likely stellar masses are. Here is an example fit:


On the x-axis is the time in days, and on the y-axis is the velocity of the star relative to the Sun. This plot shows that the best fit to these data suggest that two stars, one that is at least 0.21 times the mass of the Sun and the other that is 1.6 times the mass of the Sun, are orbiting one another every 112.98 days at a distance greater than 0.065 A.U. It’s interesting to note that the less massive star in this binary is eight times smaller than its companion. Large mass discrepancies in binaries are typical, so that one star dominates the other in terms of brightness. This is one reason why binaries are so difficult to detect.

To date, about 12,000 possible stellar binaries from the APOGEE sample have been flagged based on radial velocity shifts, and 4,000 of these are of special interest because they have been visited seven or more times and exhibit significant radial velocity changes. Of these, 1,500 indicate stellar mass companions, such as the one figured above. While the 12,000 possible binaries were found automatically, the 1,500 sources with stellar mass companions have all had to be screened by hand — a process that Duy would like to fully automate.

Analyzing APOGEE’s huge repository of stellar spectra will enable the most comprehensive assessment of binary stars, including details about whether binary star characteristics are different across the Galaxy. And as an added bonus, APOGEE is sensitive enough to spot Jupiter-sized planets using these methods! How many planets are lurking in the APOGEE dataset?

Special thanks to N. Troup, D. Chojnowski, and S. Majewski for assistance preparing this post.

How SDSS uses light to study the darkest objects in the Universe

Black holes are intriguing objects. A black hole is a phenomenon whose gravity is so strong that not even light, the fastest traveller in the Universe, can escape from its influence. Once thought mere oddities due to their extreme properties, today, black holes are found to be vital in the formation and lives of galaxies, including our own Milky Way.

But how do we know black holes exist if we can’t see them? Well, even if we can’t see a black hole directly we can observe their influence and indeed the energy and light emitted as gas, dust and stars fall into a black hole; that is, we can see black holes when they are actively “eating” material.  When the supermassive black hole, which can be up to a billion times more massive than our Sun, at the center of a galaxy starts to eat new material the resulting process is so bright it can be seen out to ~200 billion lightyears away.  Astronomers call the observational result of this process either an active galactic nuclei, or in the most extreme examples a “quasar”. So you might be surprised to find that an object that emits no light can cause the brightest known phenomenon in the Universe!


An artist’s rendition of a quasar created by Coleman Krawczyk (ICG Portsmouth).  The image is drawn on a radial log scale with the central black hole 1 AU in size.

The light of quasars is not produced by the black hole itself, but instead it comes from the material, mostly gas, that is falling into the black hole.  Different types of light are produced by this material at different distances outward from the black hole.  Near the surface (or horizon) of the black hole (about the distance of the Earth’s orbit away for supermassive black holes in galaxies) this gas becomes extremely hot and produces X-rays. Stretching out from this to fill a region about the size of our Solar System, a disk of gas shaped like a frisbee is formed.  The inside of this disk is closer to the black hole than the outside, so it rotates faster causing friction within the disk.  This friction causes the gas to heat up and glow, producing light in the optical to ultraviolet part of the spectrum.

From the edge of the gas disk to a distance of about 3 light years (similar to the distance from the Sun to the next closest star), the temperature becomes low enough that particles of “interstellar dust”, made of carbon and silicon, form.  These dust clouds form what is know as the “dusty torus,” a donut shaped ring round the gas disk. Some of the light coming from the gas disk is absorbed by the dust and re-emitted at longer wavelength infrared light. At very large distances from the black hole, some quasars have radio jets coming out along the poles.  As the name suggests, this jets produce light at radio wavelengths cased by electrons being accelerated along a strong magnetic field.  When these jets are present they can be up to ~300 thousand lightyears (~3 times the diameter of our entire galaxy!) in size.

Not only can a black hole produce light, it can create light at all wavelengths from the radio up to the X-ray, and across an area stretching from the size of the Earth’s orbit out to distances larger than the Milky Way.  Therefore, growing black holes, and the regions around them are anything but “black.”

With discoveries from its earliest imaging campaigns, the SDSS extended the study of quasars back to the first billion years after the Big Bang, showing the rapid early growth of black holes and mapping the end stages of the epoch of reionization.

Stacked spectra of more than 46,000 quasars from the SDSS; each spectrum has been converted to a single horizontal line, and they are stacked one above the other with the closest quasars at the bottom and the most distant quasars at the top. Credit: X. Fan and the Sloan Digital Sky Survey.

Stacked spectra of more than 46,000 quasars from the SDSS; each spectrum has been converted to a single horizontal line, and they are stacked one above the other with the closest quasars at the bottom and the most distant quasars at the top.
Credit: X. Fan and the Sloan Digital Sky Survey.

With full quasar samples hundreds of times larger than those that existed before, the SDSS has given us the most accurate descriptions of the growth of black holes over cosmic history.  SDSS spectra show that the properties of quasars have changed remarkably little from the early universe to the present day.


Growth in the number of known quasars in the largest homogeneous (solid) and heterogeneous (dashed) quasar catalogs as a function of time. The Sloan Digital Sky Survey catalogues started being produced in 2000. Fig. 1 from Richards et al. (2009).

SDSS studies have probed the dark matter environments of quasars through clustering measurements, revealed populations of quasars whose central engines are hidden by obscuring dust, captured changes in quasar spectra that show clouds moving in the gravitational grip of the central black hole, and allowed a comprehensive census of the much fainter accreting black holes (active galactic nuclei, or AGN) in present-day galaxies.
This, our first post for the IYL2015 is a collaboration between Coleman Krawcyzk (ICG Portsmouth); Nic Ross (ROE) with help from Karen Masters (ICG Portsmouth).

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.