(This is a guest post by Edmond Cheung at the Kavli IPMU)

While there have been many recent studies addressing how galaxies shut off, or quench, their star formation, an equally interesting yet relatively unstudied question is how these quenched galaxies remain quenched. This is interesting because these quiescent galaxies often contain gas (from stellar mass loss or mergers) that—if left unimpeded—should cool and form stars. But since we know that quiescent galaxies have not formed a significant amount of stars since they’ve been quenched, there must be something that prevents this gas from cooling.

In the new study by Cheung et al. 2016, Nature, 533, 504, this ‘something’ has been found. Using the ongoing SDSS IV MaNGA survey, which takes resolved spectroscopy for 10,000 nearby galaxies, Cheung et al. discovered a new class of quiescent galaxies—dubbed “red geysers”—that hosts outflowing winds powerful enough to heat ambient gas and suppress future star formation. These winds are manifested in bisymmetric emission features (in H-alpha, [OII], and other strong lines) and are likely powered by their weakly-accreting supermassive black holes.

To highlight the key characteristics of this class, Cheung et al. focus on a prototypical red geyser, which they nicknamed “Akira”—a reference to the critically-acclaimed manga comic of the same name, and in homage to the MaNGA survey and the lead author’s current institute in Japan (Kavli IPMU). Akira is undergoing a minor interaction with another galaxy, which they’ve nicknamed “Tetsuo”—another character in the same manga comic as Akira; the SDSS image of the interaction is shown in panel a of the figure below, which is reproduced from the Cheung et al. 2016. According to merger simulations, Tetsuo is depositing cool gas into Akira, which is detected in redshifted Na D absorption (panels d and e). The expected star-formation from this cool gas, however, is absent: Cheung et al. find that the measured star-formation rate of Akira is much lower than what is expected given the amount of cold gas present. Thus something is prohibiting star formation in Akira—what is it?

Inspecting the ionized gas properties of Akira, Cheung et al. find an interesting bisymmetric emission pattern in H-alpha and other strong emission lines (panel c). These emission patterns roughly align with the ionized gas velocity gradient (panel h), suggestive of an outflow. To prove that the ionized gas is in an outflowing wind instead of in a rotating disk, Cheung et al. had to disprove the latter case. Using the stellar dynamics of Akira (panels f and g), they obtain a tight constraint on its gravitational potential, from which they are able to predict the ionized gas kinematics in the case of a regularly rotating disk. They find that the observed ionized gas kinematics are significantly higher than the predicted ionized gas kinematics (panel j), indicating that the ionized gas is not under the influence of gravity alone.

Ruling out the disk interpretation, Cheung et al. developed a qualitative wind model that reproduces many of the features of the data, including the ionized gas velocity field and the ionized gas velocity dispersion field. They theorize that this outflowing wind is likely powered by the weakly-accreting supermassive black hole at the center of Akira, which is detected as a central radio point source in the FIRST survey and in followup Jansky VLA observations. They calculate that the energetic output from this low-luminosity active galactic nuclei (AGN) is sufficient to power this outflowing wind, which in turn, has enough energy to counterbalance the cooling of both the warm and cool gas within Akira, and thereby suppress star formation.

While Akira is an ideal case-study, perhaps the most exciting aspect of this study is the fact that there are many more red geysers. Red geysers make up about 10% of quiescent galaxies at moderate stellar masses (2×10¹⁰ solar masses), which could have important implications on the duty cycle of this kind of supermassive black hole feedback. Moreover, because they are relatively common, red geysers may exemplify how typical quiescent galaxies maintain their quiescence.

(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.

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).

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.)

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.

The following is a synopsis of the new overview paper describing the APOGEE survey. You can find the full paper here.

The first thing to show you about this paper is its authors list. Check out how many contributors there are to this survey:

This, together with its length (50 pages) and number of figures (38!!) may give you some idea of the huge scope of the project. To help us sift through it, here is the executive summary, from the APOGEE Principal Investigator, Steven Majewski:

“This paper gives a broad overview of the motivations, design and execution of the APOGEE survey — a synopsis of how the survey was structured and why.”

This isn’t just a technical overview of the survey, however. Says Majewski:

“The most fun part of this new overview paper — a part for which the entire APOGEE team can be justly proud — is the summary of what the project managed to accomplish in such a relatively short amount of time.”

Here are some examples of what he means:

• 13 technical papers on subjects like spectrograph design, target selection, data reduction, stellar parameter determination, and collaborations with other surveys
• Uses of time series spectral data for projects as diverse as spectroscopic binary characterization and circumstellar variations around hot stars
• Detailed maps of stellar radial velocities and metallicities across the Milky Way

It should be mentioned that APOGEE was made possible by a unique 300-fiber-fed, high resolution spectrograph working in the infrared H-band. Hence, thirteen technical papers, while a seemingly large number, were all necessary because APOGEE truly broke ground in a number of ways, both scientific and technical. The actual construction was accomplished by the APOGEE instrument team led by John Wilson (Instrument Scientist) and Fred Hearty (Project Manager).

The detailed maps mentioned in the last bullet deserve a couple of images — they’re too cool not to show. In Figure 24 from the paper, the velocities of stars relative to the Sun are shown against an artist’s impression of the Milky Way Galaxy. Notice how there are clear areas where stars are approaching us, co-moving with us, and moving away from us:

Figure 25 is a similar plot, but showing instead the chemical composition (metal content with respect to hydrogen) in stars across the same survey area. Check out the clear gradient from low-metallicity stars near the edge of the Milky Way’s disk to high-metallicity stars near the Galactic Center:

The general trends shown here are not new; instead, it is APOGEE’s unprecedented detail that makes it the biggest kid on the Galactic evolution block. To put APOGEE into context for us, here is Ricardo Schiavon, the APOGEE Survey Scientist, to sum it up:

“APOGEE has, for the first time, provided a homogeneous database of high quality, high resolution infrared spectra for 150,000 stars, which together offer a rigorously systematic spectroscopic census of our home galaxy. Because of it’s unique infrared sensitivity and ability to punch through the blankets of obscuring Galactic dust in its most crowded regions, APOGEE is unveiling the chemical and kinematical properties of stars in parts of the Milky Way never before probed in such exquisite detail. It is a groundbreaking experiment.”

To put it more bluntly: no one has ever done an infrared survey of Milky Way stars in this way.

Two undergraduates at San José State University have used public SDSS data to discover two galaxies that are the densest known. Similar to ordinary globular star clusters but a hundred to a thousand times brighter, the new systems have properties intermediate in size and luminosity between galaxies and star clusters.

The first system discovered by the investigators, M59-UCD3, has a width two hundred times smaller than our own Milky Way Galaxy and a stellar density 10,000 times larger than that in the neighborhood of the Sun. For an observer in the core of M59-UCD3, the night sky would be a dazzling display, lit up by a million stars. The stellar density of the second system, M85-HCC1, is higher still: about a million times that of the Solar neighborhood. Both systems belong to the new class of galaxies known as ultracompact dwarfs (UCDs).

The study, led by undergraduates Michael Sandoval and Richard Vo, used imaging data from the Sloan Digital Sky Survey, the Subaru Telescope, and Hubble Space Telescope, as well as spectroscopy from the Goodman Spectrograph on the Southern Astrophysical Research Telescope (SOAR), located on the Cerro Tololo Inter-American Observatory site. The National Optical Astronomy Observatory (NOAO) is a SOAR partner. The SOAR spectrum was used to show that M59-UCD3 is associated with a larger host galaxy, M59, and to measure the age and elemental abundances of the galaxy’s stars.

 

 

“Ultracompact stellar systems like these are easy to find once you know what to look for. However, they were overlooked for decades because no one imagined such objects existed: they were hiding in plain sight”, said Richard Vo. “When we discovered one UCD serendipitously, we realized there must be others, and we set out to find them.”

The students were motivated by the idea that all it takes to initiate a discovery is a good idea, archival data, and dedication. The last element was critical, because the students worked on the project on their own time. Aaron Romanowsky, the faculty mentor and coauthor on the study, explained, “The combination of these elements and the use of national facilities for follow up spectroscopy is a great way to engage undergraduates in frontline astronomical research, especially for teaching universities like San José State that lack large research budgets and their own astronomical facilities.”

The nature and origins of UCDs are mysterious – are they the remnant nuclei of tidally stripped dwarf galaxies, merged stellar super-clusters, or genuine compact dwarf galaxies formed in the smallest peaks of primordial dark matter fluctuations?

Michael Sandoval favors the tidally stripped hypothesis. “One of the best clues is that some UCDs host overweight supermassive black holes. This suggests that UCDs were originally much bigger galaxies with normal supermassive black holes, whose fluffy outer parts were stripped away, leaving their dense centers behind. This is plausible because the known UCDs are found near massive galaxies that could have done the stripping.”

An additional line of evidence is the high abundance of heavy elements such as iron in UCDs. Because large galaxies are more efficient factories to make these metals, a high metal content may indicate that the galaxy used to be much larger.

To test this hypothesis, the team will investigate the motions of stars in the center of M59-UCD3 to look for a supermassive black hole. They are also on the hunt for more UCDs, to understand how commonly they occur and how diverse they are.

Reference:

“Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey,” Michael A. Sandoval, Richard P. Vo, Aaron J. Romanowsky et al. 2015, Astrophysical Journal Letters, 808, L32. (Preprint: http://arxiv.org/abs/1506.08828)

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This post is copied from a press release from the National Optical Astronomy Observatory.

NOAO is operated by Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

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)  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.

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.

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.