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Blakeslee J.P.,National Research Council Canada | Cho H.,Yonsei University | Peng E.W.,Peking University | Peng E.W.,Kavli Institute for Astronomy and Astrophysics | And 4 more authors.
Astrophysical Journal | Year: 2012

We combine new Wide Field Camera 3 IR Channel (WFC3/IR) F160W (H 160) imaging data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W (g 475), F606W (V 606), F814W (I 814), and F850LP (z 850) optical data from the Advanced Camera for Surveys (ACS). The purely optical g 475 - I 814, V 606 - I 814, and g 475 - z 850 colors of NGC1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes I 814 > 21.5. The optical-IR I 814 - H 160 color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V 606 - H 160 colors show marginal evidence for bimodality, consistent with bimodality in V 606 - I 814 and unimodality in I 814 - H 160. If bimodality is imposed for I 814 - H 160 with a double Gaussian model, the preferred blue/red split differs from that for optical colors; these "differing bimodalities" mean that the optical and optical-IR colors cannot both be linearly proportional to metallicity. Consistent with the differing color distributions, the dependence of I 814 - H 160 on g 475 - I 814 for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g 475 - I 814 color distribution; the result is similar for the I 814 - H 160 dependence on g 475 - z 850 colors taken from the ACS Fornax Cluster Survey. These g 475 - z 850 colors have been calibrated empirically against metallicity; applying this calibration yields a continuous, skewed, but single-peaked metallicity distribution. Taken together, these results indicate that nonlinear color-metallicity relations play an important role in shaping the observed bimodal distributions of optical colors in extragalactic GC systems. © 2012. The American Astronomical Society. All rights reserved. Source

News Article
Site: http://news.yahoo.com/science/

CAPE CANAVERAL, Fla. (Reuters) - Astronomers have found a distant supernova, or exploded star, 20 times brighter than the Milky Way galaxy, according to research published on Thursday. The massive supernova is about 3.8 billion light-years away in a galaxy roughly three times the size of the Milky Way, scientists wrote in a report in this week's issue of the journal Science. A light-year is the distance that light travels in one year, moving at 186,000 miles (300,000 km) per second. The cosmic blast was first spotted on June 14, 2015, in an automated search for supernovas conducted by a global network of small telescopes. "It didn't look like any of the other 200 or so supernovae we had discovered at that point," astronomer Subo Dong, with the Kavli Institute for Astronomy and Astrophysics at Peking University in China, wrote in an email. Dong and colleagues do not know what triggered the blast, which is more than twice as bright as any previously discovered supernova. They plan to use the Hubble Space Telescope later this year to get a better look at the supernova's host galaxy for clues. If the supernova, known as ASASSN-151h, is at the center of its galaxy it could have been triggered by a massive black hole. Black holes are objects so dense with matter that not even photons of light can escape their gravitational pull. Massive black holes are believed to exist at the centers of most, if not all, large galaxies, including the Milky Way. Another theory is that the supernova was spawned by a magnetar, a rare, rapidly rotating neutron star with an extremely powerful magnetic field, according to Ohio State University astronomer Todd Thompson. "Like many mysteries in astronomy, it may take years, if not decades, of observational and theoretical efforts to unravel it," Dong said.

Fedele D.,Max Planck Institute for Extraterrestrial Physics | Bruderer S.,Max Planck Institute for Extraterrestrial Physics | Van Dishoeck E.F.,Max Planck Institute for Extraterrestrial Physics | Herczeg G.J.,Kavli Institute for Astronomy and Astrophysics | And 4 more authors.
Astronomy and Astrophysics | Year: 2012

We present observations of far-infrared (50-200 μm) OH and H 2O emission of the disk around the Herbig Ae star HD 163296 obtained with Herschel/PACS in the context of the DIGIT key program. In addition to strong [O i] emission, a number of OH doublets and a few weak highly excited lines of H 2O are detected. The presence of warm H 2O in this Herbig disk is confirmed by a line stacking analysis, enabled by the full PACS spectral scan, and by lines seen in Spitzer data. The line fluxes are analyzed using a local-thermal-equilibrium slab model including line opacity. The H 2O column density is 10 14-10 15 cm -2, and the excitation temperature is 200-300 K, implying warm gas with a density n > 10 5 cm -3. For OH, we find N mol of 10 14-10 15cm -2 and T ex ~ 300-500 K. For both species, we find an emitting region of r ~ 15-20 AU from the star. We argue that the molecular emission arises from the protoplanetary disk rather than the outflow. This far-infrared detection of both H 2O and OH contrasts with near-and mid-infrared observations, which have generally found a lack of water in the inner disk around Herbig AeBe stars owing to the strong photodissociation of H 2O. Given the similar column density and emitting region, OH and H 2O emission seems to arise from an upper layer of the disk atmosphere of HD 163296, which probes a new reservoir of water. The slightly lower temperature of H 2O compared to OH suggests a vertical stratification of the molecular gas with OH located higher and H 2O deeper in the disk, consistent with thermo-chemical models. © 2012 ESO. Source

Blakeslee J.P.,National Research Council Canada | Cantiello M.,National institute for astrophysics | Peng E.W.,Peking University | Peng E.W.,Kavli Institute for Astronomy and Astrophysics
Astrophysical Journal | Year: 2010

Two recent empirical developments in the study of extragalactic globular cluster (GC) populations are the color-magnitude relation of the blue GCs (the "blue tilt") and the nonlinearity of the dependence of optical GC colors on metallicity. The color-magnitude relation, interpreted as a mass-metallicity relation, is thought to be a consequence of self-enrichment. Nonlinear color-metallicity relations have been shown to produce bimodal color distributions from unimodal metallicity distributions. We simulate GC populations including both a mass-metallicity scaling relation and nonlinear color-metallicity relations motivated by theory and observations. Depending on the assumed range of metallicities and the width of the GC luminosity function (GCLF), we find that the simulated populations can have bimodal color distributions with a "blue tilt" similar to observations, even though the metallicity distribution appears unimodal. The models that produce these features have the relatively high mean GC metallicities and nearly equal blue and red peaks characteristic of giant elliptical galaxies. The blue tilt is less apparent in the models with metallicities typical of dwarf ellipticals; the narrower GCLF in these galaxies has an even bigger effect in reducing the significance of their color-magnitude slopes. We critically examine the evidence for nonlinearity versus bimodal metallicities as explanations for the characteristic double-peaked color histograms of giant ellipticals and conclude that the question remains open. We discuss the prospects for further theoretical and observational progress in constraining the models presented here and for uncovering the true metallicity distributions of extragalactic GC systems. © 2010. The American Astronomical Society. All rights reserved.. Source

News Article
Site: http://www.rdmag.com/rss-feeds/all/rss.xml/all

A team of astronomers, including Carnegie's Benjamin Shappee, Nidia Morrell, and Ian Thompson, has discovered the most-luminous supernova ever observed, called ASAS-SN-15lh. Their findings are published in Science. Supernovae are violent stellar explosions and some of the brightest objects in the universe. Human records noting their existence date back nearly 2,000 years. Within the past two decades a rare new category of super-luminous supernovae have been discovered, which shine one hundred to a thousand times brighter than the more-common supernovae. It has been theorized that these super-luminous supernovae are powered by so-called magnetars, neutron stars with extremely powerful magnetic fields, with the magnetism providing the engine for the immense luminosity. According to this theory, the magnetic field's spin magnifies the energy of the explosion, increasing the luminosity. As counterintuitive as it may sound, super-luminous supernovae are difficult for astronomers to spot. This is because they are rare and tend to form in low-luminosity galaxies with vigorous star formation, whereas the sky surveys that have been traditionally used to locate supernovae target bright galaxies with low rates of star formation. The newly found super-luminous supernova was discovered by the All Sky Automated Survey for SuperNovae team (ASAS-SN), an international collaboration headquartered at the Ohio State University, which uses a network of 14-centimeter telescopes around the world to scan the visible sky every two or three nights looking for very bright supernovae. The only all-sky variability survey in existence, it is capable of finding normal supernovae out to about 350 million light years from Earth. "On June 14 of this year, we spotted a newly occurring explosion in a galaxy of an unknown distance," Shappee said. "Subsequent observations--including those made at our Las Campanas Observatory by Nidia Morrell and Ian Thompson--allowed the team to confirm the existence of the supernova ASAS-SN-15lh." The supernova's spectra matched that of other hydrogen-poor super-luminous supernovae. But it wasn't until further follow-up was conducted that the study's lead author Subo Dong of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University and the rest of the team realized how unusual the supernova is. It is two times more luminous than any supernova previously discovered. In fact, ASAS-SN-15lh at peak was almost 50 times more luminous than the entire Milky Way galaxy. "When the first du Pont spectrum was available, as usual, I quickly checked what kind of supernova it was. To my surprise, I was not able to even tell for sure it was a supernova. My first reaction was: 'this is interesting, we should get more data,'" Morrell said. "It was only when we obtained higher resolution spectra from the Southern African Large Telescope and the Magellan Clay Telescope that I realized how distant the host galaxy is and consequently, how luminous the supernova." What's more, they determined that the galaxy where ASAS-SN-15lh formed is very atypical for a super-luminous supernova, which raises questions about how these types of supernovae form. Its host galaxy isn't the typical low-luminosity, star-forming galaxy where previous super-luminous supernovae have been spotted. ASAS-SN-15lh's galaxy is, in fact, more luminous than our own Milky Way. "The astounding amount of energy released by this supernova strains the magnetar-formation theory," Shappee explained. "More work will be necessary to understand this extraordinary object's power source and whether there are other similar supernovae out there in the universe."

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