News Article | February 15, 2017
An intergalactic scuffle seems to have resulted in the birth of new stars. Astronomers have spotted a large ring of young stars around our galactic neighbour the Large Magellanic Cloud, that probably formed when the Small Magellanic Cloud smashed past its larger sibling. Located just 160,000 and 200,000 light years away, respectively, the Large and Small Magellanic Clouds are the most luminous of the more than 50 galaxies that orbit the Milky Way. As the clouds revolve around us, they have also orbited each other. Right now, they are 75,000 light years apart. But they passed close together 200 million years ago. The Small Magellanic Cloud may even have smashed right through its big sibling. Now Christian Moni Bidin of the Catholic University of the North in Antofagasta, Chile, and his colleagues have spotted what they believe are the glowing remains of this skirmish: six young stars on the fringe of the Large Magellanic Cloud, each part of a vast ring some 80,000 light years in diameter. One star shines between the Magellanic clouds, where young stars were already known, but the other five reside where only older stars had been seen. “It was surprising,” says Moni Bidin. “There was no indication of recent star formation in this region.” The young stars at the galaxy’s edge share the same speed as older stars there, indicating the youngsters belong to the galaxy. “It’s quite an interesting and plausible result,” says David Nidever of the National Optical Astronomy Observatory in Tucson, Arizona. A collision between the two galaxies would have compressed their gas, making it collapse and spark new stars. The stars are between 10 and 50 million years old, and Nidever says a time lag between the galactic encounter 200 million years ago and the subsequent star birth is reasonable. Moni Bidin suspects the ring harbours additional young stars. “We studied the tip of the iceberg,” he says: the six stars are all hot, blue, and luminous, making them easy to see. “There could be many fainter stars.” Journal Reference: Monthly Notices of the Royal Astronomical Society, in press. ArXiv: https://arxiv.org/abs/1612.03072
Madau P.,University of California at Santa Cruz |
Dickinson M.,National Optical Astronomy Observatory
Annual Review of Astronomy and Astrophysics | Year: 2014
Over the past two decades, an avalanche of new data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z≈1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ∼1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for coevolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization. Copyright © 2014 by Annual Reviews.
Boroson T.A.,National Optical Astronomy Observatory
Astrophysical Journal Letters | Year: 2011
The velocities of the [O III] λ5007 and optical Fe II emission lines, measured relative to the systemic redshifts of 2265 QSOs by Hu et al., show the signature of a disklike BLR structure with polar outflows. Objects with large [O III] outflows show no Fe II offset velocity and are seen pole-on. Objects with large Fe II inflow show no [O III] offset velocity and are seen edge-on. This interpretation is supported by the morphology of the radio-loud objects within the sample and by previous determinations of the geometry of the broad- and narrow-line regions. Analysis of the objects with neither Fe II or [O III] velocity offsets, however, shows that the two groups also differ in Eddington ratio (ER), and, within this subset, corresponding groups with high and low ER but with the opposite orientation can be identified. Using these four subsets of the sample, the effects of orientation and ER can be separated, and, in some cases, quantified. The changes in apparent continuum luminosity and broad Hβ width and strength suggest a model in which both continuum and Hβ are emitted from the surface of the disk, which is less flattened in high ER objects. The effects of orientation on the derived properties, black hole mass and ER, are significant, though not large. The [O III] outflow appears to influence the width of that line, as well as its centroid. © 2011. The American Astronomical Society. All rights reserved.
Carr J.S.,U.S. Navy |
Najita J.R.,National Optical Astronomy Observatory
Astrophysical Journal | Year: 2011
We report high signal-to-noise Spitzer Infrared Spectrograph spectra of a sample of 11 classical T Tauri stars. Molecular emission from rotational transitions of H2O and OH and rovibrational bands of simple organic molecules (CO2, HCN, C2H2) is common among the sources in the sample. The emission shows a range in both flux and line-to-continuum ratio for each molecule and in the flux ratios of different molecular species. The gas temperatures (200-800 K) and emitting areas we derive are consistent with the emission originating in a warm disk atmosphere in the inner planet formation region at radii <2AU. The H2O emission appears to form under a limited range of excitation conditions, as demonstrated by the similarity in relative strengths of H2O features from star to star and the narrow range in derived temperature and column density. Emission from highly excited rotational levels of OH is present in all stars; the OH emission flux increases with the stellar accretion rate, and the OH/H 2O flux ratio shows a relatively small scatter. We interpret these results as evidence for OH production via FUV photodissociation of H 2O in the disk surface layers. No obvious explanation is found for the observed range in the relative emission strengths of different organic molecules or in their strength with respect to water. We put forward the possibility that these variations reflect a diversity in organic abundances due to star-to-star differences in the C/O ratio of the inner disk gas. Stars with the largest HCN/H2O flux ratios in our sample have the largest disk masses. While larger samples are required to confirm this, we speculate that such a trend could result if higher mass disks are more efficient at planetesimal formation and sequestration of water in the outer disk, leading to enhanced C/O ratios and abundances of organic molecules in the inner disk. A comparison of our derived HCN-to-H2O column density ratio to comets, hot cores, and outer TTauri star disks suggests that the inner disks are chemically active. © 2011. The American Astronomical Society. All rights reserved.
Lauer T.R.,National Optical Astronomy Observatory
Astrophysical Journal | Year: 2012
I have combined the Emsellem etal. ATLAS3D rotation measures of a large sample of early-type galaxies with Hubble Space Telescope based classifications of their central structure to characterize the rotation velocities of galaxies with cores. "Core galaxies" rotate slowly, while "power-law galaxies" (galaxies that lack cores) rotate rapidly, confirming the analysis of Faber etal. Significantly, the amplitude of rotation sharply discriminates between the two types in the -19 > MV > -22 domain over which the two types coexist. The slow rotation in the small set of core galaxies with MV > -20, in particular, brings them into concordance with the more massive core galaxies. The ATLAS3D "fast-rotating" and "slow-rotating" early-type galaxies are essentially the same as power-law and core galaxies, respectively, or the Kormendy & Bender two families of elliptical galaxies based on rotation, isophote shape, and central structure. The ATLAS3D fast rotators do include roughly half of the core galaxies, but their rotation amplitudes are always at the lower boundary of that subset. Essentially, all core galaxies have ATLAS3D rotation amplitudes λRe/2 ≪ 0.25, while all galaxies with λRe/2 > 0.25 and figure eccentricity >0.2 lack cores. Both figure rotation and the central structure of early-type galaxies should be used together to separate systems that appear to have formed from "wet" versus "dry" mergers. © 2012. The American Astronomical Society. All rights reserved.
De Young D.S.,National Optical Astronomy Observatory
Astrophysical Journal | Year: 2010
The possible role of radio active galactic nucleus (AGN) "feedback" in conventional hierarchical cosmological models has become widely discussed. This paper examines some of the details of how such feedback might work. A basic requirement is the conversion of radio AGN outflow energy into heating of the circumgalactic medium in a time comparable to the relevant cooling times. First, the class of radio AGN relevant to this process is identified as FR-I radio sources. Second, it is argued via comparisons with experimental data that these AGN outflows are strongly decelerated and become fully turbulent sonic or subsonic flows due to their interaction with the surrounding medium. Using this, a three-dimensional time-dependent calculation of the evolution of such turbulent magnetohydrodynamic flows is made to determine the time scale required for conversion of the turbulent energy into heat. This calculation, when coupled with observational data, suggests that the onset of heating can occur ∼ 108 yr after the fully turbulent flow is established, and this time is less than or comparable to the local cooling times in the interstellar or circumgalactic medium for many of these objects. The location of where heat deposition occurs remains uncertain, but estimates of outflow speeds suggest that heating may occur many tens of kpc from the center of the parent galaxy. Recent observations suggest that such radio AGN outflows may become dispersed on much larger scales than previously thought, thus possibly satisfying the requirement that heating occurs over a large fraction of the volume occupied by the circumgalactic gas. © 2010. The American Astronomical Society. All rights reserved.
Eracleous M.,Pennsylvania State University |
Boroson T.A.,National Optical Astronomy Observatory |
Halpern J.P.,Columbia University |
Liu J.,Columbia University
Astrophysical Journal, Supplement Series | Year: 2012
We have carried out a systematic search for subparsec supermassive black hole (BH) binaries among z ≲ 0.7 Sloan Digital Sky Survey quasars. These are predicted by models of supermassive BH and host galaxy coevolution, therefore their census and population properties constitute an important test of these models. In our working hypothesis, one of the two BHs accretes at a much higher rate than the other and carries with it the only broad emission line region of the system, making the system analogous to a single-lined spectroscopic binary star. Accordingly, we used spectroscopic principal component analysis to search for broad Hβ emission lines that are displaced from the quasar rest frame by |Δ v| ≳ 1000 km s -1. This method also yields candidates for rapidly recoiling BHs. Of the 88 candidates, several were previously reported in the literature. We found a correlation between the peak offset and skewness of the broad Hβ profiles, suggesting a common physical explanation for these profiles. We carried out follow-up spectroscopic observations of 68 objects to search for changes in the peak velocities of the Hβ lines. We measured statistically significant changes in 14 objects, with implied accelerations between -120 and +120kms -1 yr -1. Interpreting the offset broad emission lines as signatures of supermassive binaries is subject to many caveats. Many more follow-up observations over a long temporal baseline are needed to characterize the variability pattern of the broad lines and test that it is consistent with orbital motion. The possibility that some of the objects in this sample are rapidly recoiling BHs remains open. © 2012. The American Astronomical Society. All rights reserved.
Ivezic Z.,University of Washington |
Beers T.C.,National Optical Astronomy Observatory |
Beers T.C.,Michigan State University |
Juric M.,Harvard - Smithsonian Center for Astrophysics
Annual Review of Astronomy and Astrophysics | Year: 2012
Studies of stellar populations, understood to mean collections of stars with common spatial, kinematic, chemical, and/or age distributions, have been reinvigorated during the past decade by the advent of large-area sky surveys such as the Sloan Digital Sky Survey, the Two-Micron All Sky Survey, the Radial Velocity Experiment, and others. We review recent analyses of these data that, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way and galaxy formation and evolution in general. The formation of galaxies like the Milky Way was long thought to be a steady process leading to a smooth distribution of stars. However, the abundance of substructure in the multidimensional space of various observables, such as position, kinematics, and metallicity, is now proven beyond doubt and demonstrates the importance of mergers in the growth of galaxies. Unlike smooth models that involve simple components, the new data reviewed here clearly exhibit many irregular structures, such as the Sagittarius dwarf tidal stream and the Virgo and Pisces overdensities in the halo and the Monoceros stream closer to the Galactic plane. These recent developments have made it clear that the Milky Way is a complex and dynamic structure, one that is still being shaped by the merging of neighboring smaller galaxies. We also briefly discuss the next generation of wide-field sky surveys, such as SkyMapper, Panoramic Survey Telescope and Rapid Response System, Global Astrometric Interferometer for Astrophysics, and the Large Synoptic Survey Telescope, which will improve measurement precision manyfold and include billions of individual stars. The ultimate goal, development of a coherent and detailed story of the assembly and evolutionary history of the Milky Way and other large spirals like it, now appears well within reach. Copyright © 2012 by Annual Reviews.
Stanghellini L.,National Optical Astronomy Observatory |
Haywood M.,University Paris Diderot
Astrophysical Journal | Year: 2010
Planetary nebulae (PNe) derive from the evolution of 1-8 M · mass stars, corresponding to a wide range of progenitor ages, and thus are essential probes of the chemical evolution of galaxies, and indispensable to constrain the results from chemical models. We use an extended and homogeneous data set of Galactic PNe to study the metallicity gradients and the Galactic structure and evolution. The most up-to-date abundances, distances (calibrated with Magellanic Cloud PNe), and other parameters have been employed, together with a novel homogeneous morphological classification, to characterize the different PN populations. We confirm that morphological classes have a strong correlation with Peimbert's type PN, and also with their distribution on the Galactic landscape. We studied the α-element distribution within the Galactic disk, and found that the best selected disk population (i.e., excluding bulge and halo component), together with the most reliable PN distance scale yields to a radial oxygen gradient of Δlog(O/H)/ΔR G = -0.023±0.006dexkpc-1 for the whole disk sample, and of Δlog(O/H)/ΔR G = -0.035±0.024, -0.023±0.005, and -0.011 0.013dexkpc-1, respectively for TypeI, II, and III PNe, i.e., for high-, intermediate-, and low-mass progenitors. Neon gradients for the same PN types confirm the trend. Accurate statistical analysis shows moderately high uncertainties in the slopes, but also confirms the trend of steeper gradient for PNe with more massive progenitors, indicating a possible steepening with time of the Galactic disk metallicity gradient for what the α-elements are concerned. We found that the metallicity gradients are almost independent on the distance scale model used, as long as these scales are equally well calibrated with the Magellanic Clouds. The PN metallicity gradients presented here are consistent with the local metallicity distribution; furthermore, oxygen gradients determined with young and intermediate age PNe show good consistency with oxygen gradients derived respectively from other young (OB stars, H II regions) and intermediate (open cluster) Galactic populations. We also extend the Galactic metallicity gradient comparison by revisiting the open cluster [Fe/H] data from high resolution spectroscopy. The analysis suggests that they could be compliant with the same general picture of a steepening of gradient with time. © 2010. The American Astronomical Society. All rights reserved.
News Article | February 22, 2017
Needless to say, the definition they adopted resulted in fair degree of controversy from the astronomical community. For this reason, a team of planetary scientists – which includes famed "Pluto defender" Alan Stern – have come together to propose a new meaning for the term "planet". Based on their geophysical definition, the term would apply to over 100 bodies in the solar system, including the moon itself. The current IAU definition (known as Resolution 5A) states that a planet is defined based on the following criteria: "(1) A "planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects , except satellites, orbiting the sun shall be referred to collectively as "small solar-system bodies" Because of these qualifiers, Pluto was no longer considered a planet, and became known alternately as a "dwarf planet", Plutiod, Plutino, Trans-Neptunian Object (TNO), or Kuiper Belt Object (KBO). In addition, bodies like Ceres, and newly discovered TNOs like Eris, Haumea, Makemake and the like, were also designated as "dwarf planets". Naturally, this definition did not sit right with some, not the least of which are planetary geologists. Led by Kirby Runyon – a final year PhD student from the Department of Earth and Planetary Sciences at Johns Hopkins University – this team includes scientists from the Southwest Research Institute (SwRI) in Boulder, Colorado; the National Optical Astronomy Observatory in Tuscon, Arizona; the Lowell Observatory in Flagstaff, Arizona; and the Department of Physics and Astronomy at George Mason University. Their study – titled "A Geophysical Planet Definition", which was recently made available on the Universities Space Research Association (USRA) website – addresses what the team sees as a need for a new definition that takes into account a planet's geophysical properties. In other words, they believe a planet should be so-designated based on its intrinsic properties, rather than its orbital or extrinsic properties. From this more basic set of parameters, Runyon and his colleagues have suggested the following definition: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters." As Runyon told Universe Today in a phone interview, this definition is an attempt to establish something that is useful for all those involved in the study of planetary science, which has always included geologists: "The IAU definition is useful to planetary astronomers concerned with the orbital properties of bodies in the solar system, and may capture the essence of what a 'planet' is to them. The definition is not useful to planetary geologists. I study landscapes and how landscapes evolve. It also kind of irked me that the IAU took upon itself to define something that geologists use too. "The way our brain has evolved, we make sense of the universe by classifying things. Nature exists in a continuum, not in discrete boxes. Nevertheless, we as humans need to classify things in order to bring order out of chaos. Having a definition of the word planet that expresses what we think a planet ought to be, is concordant with this desire to bring order out of chaos and understand the universe." The new definition also attempts to tackle many of the more sticky aspects of the definition adopted by the IAU. For example, it addresses the issue of whether or not a body orbits the sun – which does apply to those found orbiting other stars (i.e. exoplanets). In addition, in accordance with this definition, rogue planets that have been ejected from their solar systems are technically not planets as well. And then there's the troublesome issue of "neighborhood clearance". As has been emphasized by many who reject the IAU's definition, planets like Earth do not satisfy this qualification since new small bodies are constantly injected into planet-crossing orbits – i..e near-Earth objects (NEOs). On top of that, this proposed definition seeks to resolve what is arguably one of the most regrettable aspects of the IAU's 2006 resolution. "The largest motivation for me personally is: every time I talk about this to the general public, the very next thing people talk about is 'Pluto is not a planet anymore'," said Runyon. "People's interest in a body seems tied to whether or not it has the name 'planet' labelled on it. I want to set straight in the mind of the public what a planet is. The IAU definition doesn't jive with my intuition and I find it doesn't jive with other people's intuition." The study was prepared for the upcoming 48th Lunar and Planetary Science Conference. This annual conference – which will be taking place this year from March 20th-24th at the Universities Space Research Association in Houston, Texas – will involve specialists from all over the worlds coming together to share the latest research findings in planetary science. Here, Runyon and his colleagues hope to present it as part of the Education and Public Engagement Event. It is his hope that through an oversized poster, which is a common education tool at Lunar and Planetary Science Conference, they can show how this new definition will facilitate the study of the solar system's many bodies in a way that is more intuitive and inclusive. "We have chosen to post this in a section of the conference dedicated to education," he said. "Specifically, I want to influence elementary school teachers, grades K-6, on the definitions that they can teach their students. This is not the first time someone has proposed a definition other than the one proposed by the IAU. But few people have talked about education. They talk among their peers and little progress is made. I wanted to post this in a section to reach teachers." Naturally, there are those who would raise concerns about how this definition could lead to too many planets. If intrinsic property of hydrostatic equilibrium is the only real qualifier, then large bodies like Ganymede, Europa, and the moon would also be considered planets. Given that this definition would result in a solar system with 110 "planets", one has to wonder if perhaps it is too inclusive. However, Runyon is not concerned by these numbers. "Fifty states is a lot to memorize, 88 constellations is a lot to memorize," he said. "How many stars are in the sky? Why do we need a memorable number? How does that play into the definition? If you understand the periodic table to be organized based on the number of protons, you don't need to memorize all the atomic elements. There's no logic to the IAU definition when they throw around the argument that there are too many planets in the solar system." Since its publication, Runyon has also been asked many times if he intends to submit this proposal to the IAU for official sanction. To this, Runyon has replied simply: "No. Because the assumption there is that the IAU has a corner on the market on what a definition is. We in the planetary science field don't need the IAU definition. The definition of words is based partly on how they are used. If [the geophysical definition] is the definition that people use and what teachers teach, it will become the de facto definition, regardless of how the IAU votes in Prague." Regardless of where people fall on the IAU's definition of planet (or the one proposed by Runyon and his colleagues) it is clear that the debate is far from over. Prior to 2006, there was no working definition of the term planet; and new astronomical bodies are being discovered all the time that put our notions of what constitutes a planet to the test. In the end, it is the process of discovery which drives classification schemes, and not the other way around. Explore further: UCLA professor proposes simpler way to define what makes a planet