Tweep of the Week: Patrick Gaulme

Still Life of Patrick Gaulme with Telescope. Credit: Arl Cope

Still Life of Patrick Gaulme with Telescope. Credit: Arl Cope

Our @sdssurveys Tweep of the Week for the week of March 27th is Patrick Gaulme, stellar and planetary astronomer and part of the observing team for SDSS. Patrick will be at Apache Point Observatory for part of this week, taking observations for @MaNGASurvey, @APOGEEsurvey, and @eBOSSurvey. Fingers crossed for clear skies, low humidity, and calm winds. Now we also turn the blog over to Patrick to introduce himself:

 

Hello, my name is Patrick Gaulme, I have been an SDSS astronomer for about two years. I am also a researcher in the field of seismology of stars and giant planets. I am science PI of a NASA-ADAP grant to study eclipsing binaries detected by the NASA Kepler space telescope, and PI of several observation projects with K2, the resuscitated version of Kepler.

I am involved in developing techniques and methods to measure planetary atmospheric dynamics with Doppler imaging in the visible domain. For this I am science PI of the NASA-EPSCoR granted JIVE in NM instrument project, which is a Doppler imager aiming at detecting oscillations of Jupiter and Saturn and measure winds of thick atmospheres in our solar system.

Tweep of the Week: Audrey Oravetz

This week our @sdssurveys Twitter account will be run by SDSS observer, Audrey Oravetz. Audrey is part of the staff of observers and fiber optic technicians (the people who plug optical fibers into the plates) working for SDSS at our survey telescope in Apache Point, New Mexico (our telescope is neither automated, nor robotic, despite the common misconception!).

Audrey Oravetz

SDSS Observer, Audrey Oravetz (she’s definitely not a robot).

Here’s Audrey introducing herself in her own words:

Hello. My name is Audrey Oravetz and I have worked as an observer for the 2.5m SDSS telescope for the past nine years. It was always a dream of mine to work at a high-ranking observatory. I enjoy working alongside my colleagues to output a high quantity of quality data for the SDSS projects.

I graduated from the University of Colorado at Boulder in 2007 with a B.A. in Astrophysics and graduated from NMSU with a M.S. degree last summer. My thesis (under the supervision of Dr. Rene Walterbos (NMSU)) was centered around the study of ionizing H-alpha photons within two star formation nebulae, NGC346 and NGC602, within the SMC.

A Docufeest in New York.

This week many of the Key People in SDSS-IV have been meeting in New York to get a good start on the Documentation that is needed to accompany the upcoming Thirteenth Data Release (DR13) of the surveys (scheduled for July 2016).

Docufeest (scientists working on laptops)

SDSS-IV scientists hard at work at Docufeest.

Here a storify from Twitter of all the documentation fun we have been having at Docufeest. You will have to wait to see the updated website until the summer.

A Winter Night at APO

It’s almost March, and spring is in the air in much of the Northern Hemisphere, but here’s a beautiful Haiku written by SDSS Observer Patrick Gaulme as part of his SDSS 2.5m Observing Log for the night of Monday January 4th 2016 [Observed 1.5 h – Lost 9.9 for weather].

– A Winter night at APO –

No water in the faucets
Few photons in the bucket
Silent snow in the dead of night

AWinterNightatAPO

A winter night at APO, Image Credit: Patrick Gaulme, SDSS.

We all agree this lovely poem really captures the essence of observing in a snowy night, and we also think it demonstrates the huge range of talent found amongst the dedicated crew of SDSS observers working at Apache Point Observatory.

A Fundamental Constant of Nature through the SDSS — Una constante fundamental de la Naturaleza a través del SDSS

(The following is a guest post by Franco Albareti, a PhD candidate at Universidad Autónoma de Madrid. It is based on recent work with co-workers Johan Comparat and Francisco Prada, which was published last year in the Monthly Notices of the Royal Astronomical Society.)

The physical models we use to describe the world around us and predict new phenomena have some free parameters or constants. These parameters must be adjusted to what is experimentally observed. Among them, those known as the fundamental constants of Nature play a central role in our theoretical understanding of physics.

Los modelos físicos que usamos para describir el mundo a nuestro alrededor y predecir nuevos fenómenos contienen parámetros o constantes sin determinar. Estos parámetros deben ajustarse según lo que se observa experimentalmente. Entre ellos, aquellos conocidos como las constantes fundamentales de la Naturaleza desempeñan un papel fundamental en nuestra comprensión teórica de la Física.

In particular, the fine-structure constant (informally called “α”) tells us the strength of electromagnetic interactions. These interactions are responsible for most of the natural phenomena around us. The correct understanding and description of how they work is not only one of the major achievement of science, but had a tremendous impact on modern life, for instance in telecommunications. The BOSS cosmological survey, one of the surveys within SDSS, has constrained the time variation of the fine-structure constant or, alternatively, the strength of the electromagnetic interactions over more than half the age of our Universe (7 Gyrs).

En particular, la constante de estructura fina (para los amigos, “α”) nos da información sobre la fuerza de las interacciones electromagnéticas. Estas interacciones son responsables de la mayoría de los fenómenos naturales que nos rodean. El hecho de que seamos capaces de entender y describir correctamente cómo funcionan, no sólo es uno de los grandes hitos de la Ciencia, sino que también ha tenido un impacto radical en nuestra forma de vida, por ejemplo, en las telecomunicaciones. El cartografiado cosmológico BOSS, uno de los cartografiados que forman parte del SDSS, ha restringido la variación temporal de la constante de estructura fina o, en otras palabras, la fuerza de las interacciones electromagnéticas durante un período de tiempo que abarca más de la mitad de la edad del Universo (7 Ga).

Any change in the fine-structure constant value will leave its imprint on the separation between two characteristic spectral lines of oxygen, see figure 1 below. These lines are emitted by quasars (extremely luminous galaxies whose light reaches us from the furthest places in the Universe). Thus, a bigger or smaller separation between those lines means that the electromagnetic interactions were stronger or weaker when the light was emitted.

Cualquier cambio en el valor de la constante de estructura fina afectará la separación entre dos líneas espectrales del Oxígeno (ver figura 1). Estas líneas son emitidas por cuásares (galaxias extremadamente luminosas cuya luz nos llega desde los lugares más recónditos de nuestro Universo). Una separación mayor o menor entre estas líneas espectrales significa que las interacciones electromagnéticas eran más fuertes/débiles cuando la luz fue emitida.

Doubly-ionized oxygen lines [OIII] observed in a BOSS quasar spectrum. Líneas espectrales de Oxígeno doblemente ionizado [OIII] observadas en un espectro de un cuásar tomado por BOSS.

Figure 1. Doubly-ionized oxygen lines [OIII] observed in a BOSS quasar spectrum.
Líneas espectrales de Oxígeno doblemente ionizado [OIII] observadas en un espectro de un cuásar tomado por BOSS.

Members of the SDSS collaboration have concluded that the value of the fine-structure constant has remained the same over the last 7 billions years in 1 part in 50,000 (figure 2). For this, more than 10,000 quasar spectra collected by BOSS were used (figure 3).

Miembros de la Colaboración SDSS han concluido que el valor de la constante de estructura fina no ha variado durante los últimos 7 mil millones de años en más de una parte en 50.000 (figura 2). Para ello. más de 10.000 espectros de cuásares tomados por BOSS han sido analizados (figura 3).

Measurements of the variation of the fine-structure constant (Δα/α) as a function of redshift (left panel) and line intensity (right panel). The grey bands in the left figure indicate regions where the sky contamination is strong and, therefore, it affects the measured value. Medidas de la variación de la constante de estructura fina (Δα/α) en función del corrimiento al rojo (imagen izquierda) y la intensidad de las líneas (imagen derecha). Las bandas grises en la figura de la izquierda indican las regiones donde la emisión del cielo es fuerte y, por tanto, afecta al valor medido.

Figure 2. Measurements of the variation of the fine-structure constant (Δα/α) as a function of redshift (left panel) and line intensity (right panel). The grey bands in the left figure indicate regions where the sky contamination is strong and, therefore, it affects the measured value.
Medidas de la variación de la constante de estructura fina (Δα/α) en función del corrimiento al rojo (imagen izquierda) y la intensidad de las líneas (imagen derecha). Las bandas grises en la figura de la izquierda indican las regiones donde la emisión del cielo es fuerte y, por tanto, afecta al valor medido.

Figure 3. Left panel: Composite image with all the BOSS quasar spectra used in this research (10,363) sorted by redshift (each horizontal line is a quasar spectrum). The displacement of the spectral lines to larger wavelengths (effect known as redshift) is due to the cosmological expansion of the Universe. Right panel: Composite image centered on the Oxygen lines and horizontally displaced to account for the redshift effect. Imagen izquierda: Conjunto de todos los espectros tomados por BOSS que han sido usados en la investigación (10.363) ordenados según el corrimiento al rojo (cada línea horizontal es un espectro de un cuásar). El desplazamiento de las líneas espectrales hacia longitudes de onda más largas (efecto conocido como corrimiento al rojo) es debido a la expansión cosmológica del Universo. Imagen derecha: Misma imagen anterior centrada en las líneas de Oxígeno y corrigiendo el desplazamiento horizontal para tener en cuenta el corrimiento al rojo.

Figure 3. Left panel: Composite image with all the BOSS quasar spectra used in this research (10,363) sorted by redshift (each horizontal line is a quasar spectrum). The displacement of the spectral lines to larger wavelengths (effect known as redshift) is due to the cosmological expansion of the Universe. Right panel: Composite image centered on the Oxygen lines and horizontally displaced to account for the redshift effect.
Imagen izquierda: Conjunto de todos los espectros tomados por BOSS que han sido usados en la investigación (10.363) ordenados según el corrimiento al rojo (cada línea horizontal es un espectro de un cuásar). El desplazamiento de las líneas espectrales hacia longitudes de onda más largas (efecto conocido como corrimiento al rojo) es debido a la expansión cosmológica del Universo. Imagen derecha: Misma imagen anterior centrada en las líneas de Oxígeno y corrigiendo el desplazamiento horizontal para tener en cuenta el corrimiento al rojo.

Animation -> The animation below shows an image of a quasar, its full optical spectrum (bottom left), a zoom in the oxygen lines used for the analysis (bottom right), and the measured value of the variation of the fine-structure constant as a function of redshift (top panel). It only displays 200 objects among the >10,000 quasars used for the research. (It starts slow so you can pay attention to all of the information, but then it goes faster)

Animación -> Esta animación muestra una imagen de un cuásar, junto con su espectro en el óptico (parte inferior izquierda), un zoom en las líneas de Oxígeno usadas para el análisis (parte inferior derecha), y el valor medido de la variación de la constante de estructura fina como función del corrimiento al rojo (parte superior). La animación sólo muestra 200 objetos de entre los >10.000 cuásares usados para la investigación. (Empieza despacio para que uno se pueda fijar en toda la información que se muestra, pero luego empieza a ir más rápido.)

alpha_spectra_small

Animation of the measured change in the fine structure constant with cosmic time. You may need to refresh this page to see the animation again.

To reach further in the past, when the Universe was five times younger, a dedicated observational program, APOGEE-Q (APOGEE Quasar Survey), is being developed in order to not only measure a variation on the fine-structure constant, but study supermassive black hole masses and quasar redshifts. This program will use an infrared spectrograph from the APOGEE survey instead of the optical one used by BOSS. This allows us to observe the infrared region of the electromagnetic spectrum, where the oxygen lines emitted by distant quasars are found due to the cosmological expansion of the Universe. It will start to take the first data during 2016.

Para remontarnos todavía más atrás en el tiempo, cuando el Universo era cinco veces más joven, un programa observacional específico, APOGEE-Q: APOGEE Quasar Survey, está siendo desarrollado para, no sólo medir la variación de la constante de estructura fina, sino también para estudiar agujeros negros súper masivos y corrimientos al rojo de cuásares. Este programa usará un espectrógrafo infrarrojo del cartografiado APOGEE en vez del espectrógrafo óptico usado por BOSS. Esto nos permitirá observar la región infrarroja del espectro electromagnético, que es donde se encuentran las líneas del Oxígeno emitidas por cuásares muy lejanos debido a la expansión cosmológica del Universo. El programa empezará a tomar los primeros datos en 2016.

Astronomers studying galaxy mergers using MaNGA data

(The following is a guest post by Lihwai Lin, an assistant research fellow at Academia Sinica, Institute of Astronomy and Astrophysics. She is curretnly chairing the MaNGA merger working group and organized the MaNGA merger mini-workshop described in the article below.)
Galaxies are not isolated. During the lifetime of galaxies, they may encounter another galaxy and merge together to become a larger one. Mergers can induce gas to flow toward the inner parts of galaxies through tidal forces, triggering starbursts or even “switching on” a galaxy’s central black hole (the result is called an “active galactic nucleus,” or AGN). As a result of rapid gas consumption during mergers, a galaxy may lose the majority of its gas and end up as a “dead” system with little on-going star formation. This kind of merger event is rare, but is suggested to be an important process that transforms star-forming galaxies into the quiescent population. One of the key sciences that MaNGA is attempting to address concerns the role of galaxy interactions and mergers in shaping the properties of galaxies. With just one year of the MaNGA survey, we have obtained Integral Field Unit (IFU) observations for ~150 paired galaxies, ranging from early encounters to post-mergers.
Examples of galaxy pairs selected from the SDSS. The magenta hexagons represent the IFU coverage of MaNGA. (Credit: SDSS)

Examples of galaxy pairs selected from the SDSS. The magenta hexagons represent the IFU coverage of MaNGA. (Credit: SDSS)

In early November of 2015, experts studying galaxy mergers gathered together in Taipei for the “SDSS-IV/MaNGA mini-workshop on galaxy mergers”. This 3-day workshop consists of 6 invited talks, 5 contributed talks, plus 2 discussion sessions devoted to theoretical and observational efforts, chaired by Jennifer Lotz (STScI) and Sara Ellison (University of Victoria) respectively.

Participants for the MaNGA mini-workshop on galaxy mergers, held at Academia Sinica, Institute of Astronomy and Astrophysics (ASIAA), Taipei, on Nov. 4-6, 2015.

Participants for the MaNGA mini-workshop on galaxy mergers, held at Academia Sinica, Institute of Astronomy and Astrophysics (ASIAA), Taipei, on Nov. 4-6, 2015.

With MaNGA’s spatially resolved observations for merging galaxies, we can study not only where and when the star formation is triggered and shut down during the process of  galaxy interactions, but also how the massive black holes in the center of galaxies can be fueled and grow through galaxy mergers. The observational results from MaNGA will also be compared in great detail with theoretical predictions from state-of-art simulations. Stay tuned for more exciting science that will come from MaNGA!

How SDSS Talked about Light for #IYL2015

This is a re-posting of the wrap-up article which appeared on the IYL2015 main blog.


 

2015 has been the International Year of Light.

As astronomers, here at the Sloan Digital Sky Survey everything we do is based on collecting light from cosmic objects. So we have been pleased to celebrate the International Year of Light, and especially the Cosmic Light Theme, supported by the IAU.cosmiclight_color_whitebgAs a small contribution to this celebration, every month in 2015 SDSS had a special blog post talking about the different ways we use light. Here’s a roundup of what we talked about through out the year.

In January we talked about How SDSS Uses Light to Study the Darkest Objects in the Universe. This blog post, by Coleman Krawcyzk and Karen Masters (both from the University of Portsmouth in the UK) with help from Nic Ross (Royal Observatory, Edinburgh) was about finding black holes by looking at the light from distant galaxies. Finding objects which are famous for not emitting any light, using light seems contradictory, but this article explains how the light created by the hot material falling onto a black hole can make these objects outshine the entire galaxy they live in.

Quasar

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.

In February we wrote about How SDSS Uses Light to Measure the Distances to Galaxies. This of course was about the technique of measuring galaxy redshifts (ie. the shift of their light to longer wavelengths caused by the expansion of the Universe) by looking at absorption and emission lines in galaxy spectra and comparing their wavelength to the laboratory measurement. Edwin Hubble, and others, realised over 80 years ago, that this can be used to give distances to galaxies, as the amount of redshift increases with the galaxy’s distance. The original motivation for SDSS (back in the 1990s) was to used this technique to measure distances to a million galaxies, and in SDSS-IV we are continuing to use this in the eBOSS part of the survey, to map distances to ever more distant galaxies.

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

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

In March, we came back to the most local Universe, with a post by SDSS-IV Spokesperson, Jennifer Johnson (Ohio State University) on How SDSS Uses Light to Understand Stars Inside and Out in the Kepler Field. This was about part of the APOGEE survey, which is measuring spectra from stars which have light curves measured by the Kepler Satellite. This is a valuable experiment, as the combination of spectra and light curves allows us to measure the masses, ages and compositions of these stars.

The Kepler Field. Credit: NASA

The Kepler Field. Credit: NASA

In April, we moved back outside our own Galaxy, to measuring the invisible mass in other galaxies, with a post on How SDSS Uses Light to Explore the Invisible, by the MaNGA Lead Observer, and SDSS-IV Data Release Co-ordinator, Anne-Marie Weijmans from St Andrew University. This post talked about how MaNGA is measuring spectra across the face of nearby galaxies in order to get measurements of the internal motions (again using the redshift/blueshift of the spectra). These measurements give a way to measure the total mass of galaxies, which we find in all cases is much much more than the mass in stars.

MaNGAlogo5small

For May we went back in the history of SDSS, and talked about How SDSS Used Light to Make the Largest Ever Digital Image of the Night Sky. This post was about the the SDSS camera and the SDSS imaging survey, which ran from 2000-2008, and created a image of over 30% of the sky, containing over a trillion pixels (an image which dwarfs others that have also been claimed as the largest).

The SDSS Camera, now in storage in the Smithsonian Museum. Credit: SDSS, Xavier Poultney

The SDSS Camera, now in storage in the Smithsonian Museum. Credit: SDSS, Xavier Poultney

June also saw a post about SDSS imaging, and about an unexpected use for them, finding asteroids, in How SDSS Uses Light to Find Rocks in Space. This has been beautiful visualized in the below video, by Alex Parker.

If our posts in February, March and April confused you because you didn’t understand what astronomers mean by measuring spectra, then the July post was for you: “How SDSS Splits Light into a Rainbow for Science”.  This post explained all about what spectra are, how to create them with gratings, and contained a with bonus activity to make your own spectroscope created by the SDSS Education and Public Outreach group.

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).Credit: Wikimedia

Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2).Credit: Wikimedia

Our August post, by the APOGEE survey Public Engagement officer, David Whelan (from Austin College, Texas)  was about the basic physics of the most abundant element in the Universe (hydrogen): “How SDSS Uses Light to Study the Most Abundant Element in the Universe.”

For September, we visited an IYL2015 Exhibit in Dresden with Zach Pace, Graduate Student at the University of Wisconsin, Madison. Zach reported on SDSS plates on display in the exhibit, linking back to an earlier post in which we explain why we need all these big aluminium plates to do our spectroscopic survey. IYL2015 – SDSS Plates (in Retirement).

Technische_Sammlungen3

SDSS Plate on Exhibit in Dresden.

We went back to the APOGEE survey in October, with a post by Gail Zasowski (from John’s Hopkins University) on How SDSS uses mysterious “missing” light to map the interstellar medium. In this post we learned about how SDSS has helped shed light on the the mystery of missing light caused by absorption in the material which is found between stars in our own Galaxy.

Finally last month, we talked about How SDSS Uses Light to Measure the Mass of Stars in Galaxies. Looking back to the post in February, we claimed that the total mass of galaxies is always much much more than the mass we can count in their stars. But how do we know how much mass is in the stars in a galaxy? This post explains how that can be done using measurements of the light from galaxies.

So that wraps up a year of the celebration of light in the SDSS. We certainly haven’t covered all the ways in which SDSS astronomers are using light to learn about the Universe around us, from asteroids in the solar system, to stars in our own Galaxy and galaxies are the furthest edges of the Universe. But we hope it gives you a flavour for the kinds of things the light collected by SDSS (both images and spectra) can be used for.

If you’re looking for a guided entry into SDSS science (especially suitable for educational use), please visit our Voyages.sdss.org site to discover guided journeys through the Universe. As always all SDSS data (through our 12th public data release, DR12) is available free to download, and look out for DR13 (including the first data from SDSS-IV) coming up in mid 2016.

 

Building the APOGEE-2S Spectrograph: Putting Together All the Little Pieces

Building a spectrograph is no mean feat — and an instrument like the APOGEE spectrograph, with high expectations of precision to meet its mighty science goals, takes time and effort. Today we want to share with you some of the many highlights of the ongoing, and exciting, work being done to make the APOGEE-2S spectrograph, the “twin” spectrograph that is going to perform survey operations on the du Pont Telescope at Las Campanas Observatory in Chile.

Spectrographs have several key components. The light collected by the telescope from a star is collimated by a great big lens before it strikes the diffraction grating, which splits the light into its constituent colors (it’s a fancy prism). The “split” light then travels through a camera so that it can be refocused onto the infrared array, which records the spectrum of the star.

With that in mind, here’s a picture of a part of the collimator known as the collimator positioning actuator, which is the little piece of metal seen at the center of the test dewar (the large cylinder). Its role is to precisely position the collimator lens, to ensure precise collimation at all times.

Josh Peebles from Johns Hopkins is seen here preparing the collimator positioning actuator for cryogenic testing.

Josh Peebles from Johns Hopkins is seen here preparing the collimator positioning actuator inside of a dewar for cryogenic testing.

Next we have some fancy-looking lenses. Because APOGEE works with infrared wavelengths, the lenses have to be made out of substances that are transparent to infrared light, not visible light. As a result, they are actually opaque at visible wavelengths. In the picture below, the lens appears green to us, but this fused silicon lens would be see-through if we had infrared-sensitive eyes.

This is one of the APOGEE-2S spectrograph's lenses (there are six of them in total) up close. It is made of fused silicon, is opaque to our eyes, but is transparent to infrared light. You can see light reflecting from its surface in this photo

This is one of the APOGEE-2S spectrograph’s lenses (there are six of them in total) up close. It is made of fused silicon, and is transparent to infrared light.

New England Optical Systems installed these lenses into the camera barrels — the black cylinders shown below — which will be attached to form the spectrograph’s camera (see further below).

In November, New England Optical Systems finished installing the lenses into the camera barrel.

In November, New England Optical Systems finished installing the lenses into the camera barrel.

As of just a few days ago, the camera is now fully assembled, and is currently undergoing tests to ensure that it is working to specifications.

The spectrograph camera is fully assembled, and undergoing a test called laser unequal path interferometry (LUPI for short).

The spectrograph camera is fully assembled, and undergoing a test called laser unequal path interferometry (LUPI for short).

This little photojournal makes building a multi-million dollar spectrograph look so neat and tidy! One final picture to disillusion you. Below is Matt Hall, one of the technicians at the University of Virginia assisting with the build. In this picture, he is testing springs that are used to hold some of the lenses in place. It sounds strange that springs are part of a lens system; but because the APOGEE-2S spectrograph is going to be cooled cryogenically, the lenses will all shrink a little. These springs apply pressure to the edges of the lenses so that they stay in place when they shrink.

This picture illustrates the secret to building instruments like the APOGEE-2S spectrograph: every big piece, like the collimator or camera, is made up of dozens or even hundreds of small, interconnected and interdependent pieces. And each little piece has to be built and tested to ensure that it does its job properly. So here’s to the people, both in Chile and in the U.S., who are currently dedicating their time and effort to build the best spectrograph possible. We look forward to making good use of it!

Matt Hall (UVa) is seen here testing the spring constants of individual spring plungers. As with every small part of the build, it is dealt with meticulously and thoroughly so that the completed spectrograph works at this highest level possible.

Matt Hall (UVa) is seen here testing the spring constants of individual spring plungers. As with every small part of the build, it is dealt with meticulously and thoroughly so that the completed spectrograph works at the highest level possible.

Letter from the New Editor in Chief

Dear Readers of the SDSS Blog,

I am Zheng Zheng, a SDSS-IV postdoctoral research fellow at the National Astronomical Observatories, Chinese Academy of Sciences (NAOC). I will be your new Editor in Chief for the SDSS Blog for the next 6 months and I will try my best to work with other bloggers to make the blog posts more interesting and smooth.

I got my PhD at Johns Hopkins University and now I am a postdoctoral researcher working at the NAOC and partially at the Institute of Cosmology and Gravity (ICG) at the University of Portsmouth in the U.K. I am currently studying extra-galactic galaxies using the SDSS-IV MaNGA data. I am also interested/involved in MaNGA stellar library, APOGEE and eBOSS projects.

As you may have known, the SDSS is an internationally collaborated survey project and the member institutes come from all over the world. In the future, we will introduce more interesting SDSS related sciences/events from all over the world, including the U.S., Europe, East Asia, and South America. We are aiming to a post frequency of about 1 ‘long’ post (like the ones introducing science projects) per 1-2 weeks. We will also have ‘short’ posts reporting SDSS related events and/or short news.

Please do not hesitate to make comments and let us know your ideas about the blog posts. Your feedback is highly appreciated and we will try our best to post more articles according to your interests.

Sincerely,

Zheng Zheng

 

Zheng observing at Palomar

Tweep of the Week: Anne-Marie Weijmans

The MaNGA Lead Observer, and our Data Release Co-ordinator, Anne-Marie Weijmans will be spending some time at Apache Point Observatory Dec 8-17th and has agreed to take over the @sdssurveys Twitter account for the trip. We’re hoping for some tweets about pie (as well as observing).

MaNGA Lead Observer (Anne-Marie Weijmans) plugging IFUs into an SDSS plate. Credit: SDSS.

MaNGA Lead Observer (Anne-Marie Weijmans) plugging IFUs into an SDSS plate. Credit: SDSS.

 

Dr. Weijmans is a Lecturer (Assistant Prof. for our US readers) and Leverhulme Early Career Fellow based at the University of St Andrews in Scotland. Her research interests concentrate on the structure and evolution of early-type (i.e. visually smooth) galaxies using Integral Field Spectroscopy. Before joining MaNGA she was a member of the ATLAS-3D survey, which was one of the first surveys to use this technique on a sample of galaxies.

How SDSS Uses Light to Measure the Mass of Stars in Galaxies

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Galaxy NGC 3338 imaged by SDSS (the red stars to the right is in our own galaxy). Credit: SDSS

It might sound relatively simple – astronomers look at a galaxy, count the stars in it, and work out how much mass they contain, but in reality interpreting the total light from a galaxy as a mass of stars is fairly complex.

If all stars were the same mass and brightness, it would be easy, but stars come in all different brightnesses, colours and masses, with the lowest mass stars over 600 times smaller than the most massive.

Hertzsprung-Russel Diagram identifying many well known stars in the Milky Way galaxy. Credit: ESO

Hertzsprung-Russell (HR) Diagram, which shows the mass, colour, brightness and lifetimes of different types of stars. This version identifies many well known stars in the Milky Way galaxy. Credit: ESO

And it turns out that most of the light from a galaxy will come from just a small fraction of these stars (those in the upper left of the HR diagram). The most massive stars are so much brighter ounce for ounce than dimmer stars this makes estimating the total mass much more of a guessing game than astronomers would like (while they are 600 times more massive, they are over a million times brighter). So astronomers have to make assumptions about how many stars of low mass are hiding behind the light of their brighter siblings to make the total count.

One of the first astronomers to suggest trying to decode the light from galaxies in this way was Beatrice Tinsley. British born, raised in New Zealand, and working at Yale University in the USA, Dr. Tinsley had a much larger impact on extragalactic astronomy than her sadly shortened career would suggest (she died of cancer in 1981 aged just 40).

Stars of different masses have distinctive spectra (and colours), as first famously classified by Astronomer Annie Jump Cannon in the late 1890s into the OBAFGKM stellar sequence. O stars (at the top left of the HR diagram) are massive, hot, blue and with very strong emission lines, while M stars (at the lower right) are low mass, red and show absorption features from metallic lines in their atmospheres. With a best guess as to the relative abundance of different stars (something we call the “initial mass function“) a stellar population model can be constructed from individual stellar spectra or colours and fit to the total light from the galaxy. Example optical spectra of different types of stars are shown below (or see the APOGEE View of the IR Stellar Sequence)

Example optical spectra of different stellar types. Credit: NOAO/AURA/NSF

Example optical spectra of different stellar types. Credit: NOAO/AURA/NSF.

Using data from SDSS (and other surveys) astronomers use this methods to decode the galaxy light – in fact we can use either the total light observed through different filters in the SDSS imaging, to match the colours of the stars, or if we measure the spectrum of the galaxy we can fit a population of stars to this instead. While in principle the spectrum should give more information, in SDSS (at least before the MaNGA survey) we take spectra through a small fibre aperture (just 2-3″ across), so for nearby galaxies this misses most of the light (e.g. see below), and most galaxies have colour gradients (being redder in the middle than the outskirts), so the extrapolation can add quite a lot of error to the inferred mass.

NGC 3338 with the approximate SDSS fibre size overlaid (note this is an example of a very large galaxy imaged by SDSS). Credit: SDSS, KLM

NGC 3338 with the approximate SDSS fibre size (ie. the part of the galaxy for which we measured spectra) overlaid (note this is an example of a very large galaxy imaged by SDSS, and not representative of most galaxies). Credit: KLM, SDSS

 

Many astronomers prefer to use models based on the total light through different filters (at least for nearby galaxies). The five filters of the SDSS imaging are an excellent start for this, but extending into the UV with the GALEX survey, and IR with a survey like 2MASS or WISE adds even more information to make sure no stars are being missed. However, these fits are still a “best guess” and will still have error –  there is often more than one way to fit the galaxy light (e.g. model galaxies with certain combinations of ages and metallicities can have the same integrated colours), so there’s still typically up to 50% error in the inferred mass.

The SDSS camera filter throughput curves (from left to right ugriz). Credit: SDSS

The SDSS camera filter throughput curves (from left to right ugriz). Credit: SDSS

 

But with galaxies spanning more than 3 orders of magnitude in total mass (ie. the biggest galaxies have more than a 1000 times the stellar mass of the smallest) this is still good enough for many purposes. It gives us an idea of the total mass in stars in a galaxy, which (as you know from earlier post for IYL2015) is almost always way less than the total mass we estimate from looking at the dynamics (ie. the “gravitating mass”). And the properties of galaxies correlate extremely well with their stellar masses, so it’s a really useful thing to have even an estimate of.


This post by Karen Masters is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month in support of the celebration of light. 

P-MaNGA: Emission Lines Properties – Gas Ionisation and Chemical Abundances from Prototype Observations

(The following is a guest post by Francesco Belfiore, a PhD student at Cambridge University’s Kavli Institute for Cosmology, and summarizes his recent paper, which uses preliminary MaNGA data to map gas ionisation in several galaxies.)

Galaxies have long been considered island universes. Ordinarily separated by huge cosmological distances (of the order of millions of light years), most galaxies are not interacting in any visible way with their environment. However, modern theories of galaxy evolution claim otherwise. Starburst galaxies (galaxies which are experiencing a rate of formation of new stars much higher than normal) are known to expel large amounts of ionised (and possibly also neutral) gas towards the intergalactic voids. Supermassive black holes, which we believe to live in the centres of most galaxies, can also give rise to powerful outflows during periods of accretion (when the black hole has “switched on” and is feeding on the surrounding material). Some of these events are violent enough to totally strip a galaxy of its fuel: the gas. Without gas, a galaxy loses its ability to form new stars and becomes progressively older. In a sense, the galaxy has “died”.

This is not the whole story, however.

Continue reading

Integral Field Spectroscopy 101

As frequent readers know, the SDSS-IV-MaNGA survey plans to obtain spatially-resolved spectra of somewhere in the neighborhood of 10,000 galaxies using a technique called integral-field spectroscopy (or IFS). IFS essentially relies on placing an array of fiber-optic cables over an object of interest in the sky, and using the fiber-optics to pipe the light into a spectrograph, which produces the useful data by breaking up that light into its constituent wavelengths (an easy way you can do this at home is with a glass prism). The array of fibers is nicknamed a “bundle,” which is a pre-packaged grouping of fibers that we know the arrangement, and packaging the fibers allows more observational efficiency, since we don’t have to re-position the telescope to make a measurement of the same galaxy at a slightly different point.

However, the specific design of the fiber bundles is an important problem. Continue reading