A strong ultraviolet pulse from a newborn type Ia supernova
Nature 521, 7552 (2015). doi:10.1038/nature14440
Authors: Yi Cao, S. R. Kulkarni, D. Andrew Howell, Avishay Gal-Yam, Mansi M. Kasliwal, Stefano Valenti, J. Johansson, R. Amanullah, A. Goobar, J. Sollerman, F. Taddia, Assaf Horesh, Ilan Sagiv, S. Bradley Cenko, Peter E. Nugent, Iair Arcavi, Jason Surace, P. R. Woźniak, Daniela I. Moody, Umaa D. Rebbapragada, Brian D. Bue & Neil Gehrels
Type Ia supernovae are destructive explosions of carbon-oxygen white dwarfs. Although they are used empirically to measure cosmological distances, the nature of their progenitors remains mysterious. One of the leading progenitor models, called the single degenerate channel, hypothesizes that a white dwarf accretes matter from a companion star and the resulting increase in its central pressure and temperature ignites thermonuclear explosion. Here we report observations with the Swift Space Telescope of strong but declining ultraviolet emission from a type Ia supernova within four days of its explosion. This emission is consistent with theoretical expectations of collision between material ejected by the supernova and a companion star, and therefore provides evidence that some type Ia supernovae arise from the single degenerate channel.
No signature of ejecta interaction with a stellar companion in three type Ia supernovae
Nature 521, 7552 (2015). doi:10.1038/nature14455
Authors: Rob P. Olling, Richard Mushotzky, Edward J. Shaya, Armin Rest, Peter M. Garnavich, Brad E. Tucker, Daniel Kasen, Steve Margheim & Alexei V. Filippenko
Type Ia supernovae are thought to be the result of a thermonuclear runaway in carbon/oxygen white dwarfs, but it is uncertain whether the explosion is triggered by accretion from a non-degenerate companion star or by a merger with another white dwarf. Observations of a supernova immediately following the explosion provide unique information on the distribution of ejected material and the progenitor system. Models predict that the interaction of supernova ejecta with a companion star or circumstellar debris lead to a sudden brightening lasting from hours to days. Here we present data for three supernovae that are likely to be type Ia observed during the Kepler mission with a time resolution of 30 minutes. We find no signatures of the supernova ejecta interacting with nearby companions. The lack of observable interaction signatures is consistent with the idea that these three supernovae resulted from the merger of binary white dwarfs or other compact stars such as helium stars.
Astronomy: Quasar quartet in galactic nursery
Nature 521, 7552 (2015). doi:10.1038/521263a
Astronomers have discovered a massive cluster of four quasars — a rare find of galaxies just being born.Quasars are young, bright galaxies powered by supermassive black holes and are hard to find because this youthful period is brief. Using the W. M. Keck Observatory
Astronomy: Spots spotted on Vega star
Nature 521, 7552 (2015). doi:10.1038/521262b
One of the brightest stars in the night sky seems to have surface structures called starspots — a surprising finding for this particular star.Torsten Böhm at the University of Toulouse in France and his colleagues used a telescope at France's Haute-Provence Observatory to look
Globular star clusters are isolated star cities, home to hundreds of thousands of stars. And like the fast pace of cities, there's plenty of action in these stellar metropolises. The stars are in constant motion, orbiting around the cluster's center. Past observations have shown that the heavyweight stars live in the crowded downtown, or core, and lightweight stars reside in the less populated suburbs.
Strangulation as the primary mechanism for shutting down star formation in galaxies
Nature 521, 7551 (2015). doi:10.1038/nature14439
Authors: Y. Peng, R. Maiolino & R. Cochrane
Local galaxies are broadly divided into two main classes, star-forming (gas-rich) and quiescent (passive and gas-poor). The primary mechanism responsible for quenching star formation in galaxies and transforming them into quiescent and passive systems is still unclear. Sudden removal of gas through outflows or stripping is one of the mechanisms often proposed. An alternative mechanism is so-called “strangulation”, in which the supply of cold gas to the galaxy is halted. Here we report an analysis of the stellar metallicity (the fraction of elements heavier than helium in stellar atmospheres) in local galaxies, from 26,000 spectra, that clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of four billion years, at least for local galaxies with a stellar mass less than 1011 solar masses. This result is further supported independently by the stellar age difference between quiescent and star-forming galaxies, which indicates that quiescent galaxies of less than 1011 solar masses are on average observed four billion years after quenching due to strangulation.
As murder mysteries go, it’s a big one: how do galaxies die and what kills them? A new study, published today in the journal Nature, has found that the primary cause of galactic death is strangulation, which occurs after galaxies are cut off from the raw materials needed to make new stars.
Researchers from the University of Cambridge and the Royal Observatory Edinburgh have found that levels of metals contained in dead galaxies provide key ‘fingerprints’, making it possible to determine the cause of death.
There are two types of galaxies in the Universe: roughly half are ‘alive’ galaxies which produce stars, and the other half are ‘dead’ ones which don’t. Alive galaxies such as our own Milky Way are rich in the cold gas – mostly hydrogen – needed to produce new stars, while dead galaxies have very low supplies. What had been unknown is what’s responsible for killing the dead ones.
Astronomers have come up with two main hypotheses for galactic death: either the cold gas needed to produce new stars is suddenly ‘sucked’ out of the galaxies by internal or external forces, or the supply of incoming cold gas is somehow stopped, slowly strangling the galaxy to death over a prolonged period of time.
In order to get to the bottom of this mystery, the team used data from the Sloan Digital Sky Survey to analyse metal levels in more than 26,000 average-sized galaxies located in our corner of the universe.
“Metals are a powerful tracer of the history of star formation: the more stars that are formed by a galaxy, the more metal content you’ll see,” said Dr Yingjie Peng of Cambridge’s Cavendish Laboratory and Kavli Institute of Cosmology, and the paper’s lead author. “So looking at levels of metals in dead galaxies should be able to tell us how they died.”
If galaxies are killed by outflows suddenly pulling the cold gas out of the galaxies, then the metal content of a dead galaxy should be the same as just before it died, as star formation would abruptly stop.
In the case of death by strangulation however, the metal content of the galaxy would keep rising and eventually stop, as star formation could continue until the existing cold gas gets completely used up.
While it is not possible to analyse individual galaxies due to the massive timescales involved, by statistically investigating the difference of metal content of alive and dead galaxies, the researchers were able to determine the cause of death for most galaxies of average size.
“We found that for a given stellar mass, the metal content of a dead galaxy is significantly higher than a star-forming galaxy of similar mass,” said Professor Roberto Maiolino, co-author of the new study. “This isn’t what we’d expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario.”
The researchers were then able to independently test their results by looking at the stellar age difference between star-forming and dead galaxies, independent of metal levels, and found an average age difference of four billion years – this is in agreement with the time it would take for a star-forming galaxy to be strangled to death, as inferred from the metallicity analysis.
“This is the first conclusive evidence that galaxies are being strangled to death,” said Peng. “What’s next though, is figuring out what’s causing it. In essence, we know the cause of death, but we don’t yet know who the murderer is, although there are a few suspects.”
Astronomers have partially solved an epic whodunit: what kills galaxies so that they can no longer produce new stars?This is the first conclusive evidence that galaxies are being strangled to deathYingjie Pengre-activeArtist’s impression of one of the possible galaxy strangulation mechanisms: star-forming galaxies (fed by gas inflows) are accreted into a massive hot halo, which ‘strangles’ them and leads to their death.
The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.
Astrophysics: The slow death of red galaxies
Nature 521, 7551 (2015). doi:10.1038/521164a
Authors: Andrea Cattaneo
For most galaxies, the shutdown of star formation was a slow process that took 4 billion years. An analysis of some 27,000 galaxies suggests that 'strangulation' by their environment was the most likely cause. See Letter p.192