Nature's Astronomical Highlights
The journal Nature has been at the pinnacle of scientific publishing for many years. Founded by an astronomer, Norman Lockyer, it has had an extensive history in publishing the most significant developments in the Natural Sciences. For instance, James Chadwick published his discovery of the neutron in Nature; James Watson and Francis Crick presented the helical structure of DNA. Naturally, astronomy has been no exception: part of the discussion following the “Great Debate” on the nature of the Spiral Nebulae (were these small nebulae within our Galaxy or distant galaxies in their own right?) was contained in Nature’s pages in the early 1920s. In the 1960s, Maarten Schmidt’s discovery of the first quasar and Antony Hewish & Jocelyn Bell’s discovery of the first pulsar were presented in Nature. Even up until the present day, Nature is publishing discoveries that not only are of great interest to professional and amateur astronomers and astrophysicists, but also of relevance to humankind in general. In 2016 we discovered through the work of Guillem Anglada-Escudé and collaborators that the nearest star to our Solar System harbours a rocky planet in a temperate orbit.
It is on the back of these discoveries and this extensive history that Nature Research is launching a new journal in 2017, Nature Astronomy, so that more astronomical research might be published with a similar high standard of editing, peer review and production as Nature’s. To celebrate Nature’s comprehensive astronomical heritage, we at Nature Astronomy have curated this Web Collection of 40 Nature papers that have had significant impact on astronomical research. Several of these papers have been cited over 1,000 times in the astronomical literature. To include some more recent papers, which have not had the luxury of many years over which to accrue citations, we have also consulted Altmetric scores, which gauge social media impact among other things. The result is a Collection of Letters, Articles and Reviews that have been roughly grouped into seven themes: exoplanets, pulsars, black holes & short gamma-ray bursts, long gamma-ray bursts & supernovae, galaxies, dark matter and the large-scale structure of the Universe. Several of these papers have been selected for Free Access for a limited period; these can be found collected together below, or in the “Free access” tab, above.
Before we delve into these seven topics, there is one stimulating paper that stands apart, written by astronomer and science communicator Carl Sagan and his colleagues (Sagan et al. 1993). It details an experiment performed with the Galileo spacecraft on its way to Jupiter. Galileo was commanded to turn towards the Earth, and capture data with its instruments. Effectively it observed the Earth for signs of life. However, it only just managed to find them: it saw a water-rich atmosphere and surface; it saw signs of biological activity in the high levels of methane; it saw a red-absorbing pigment that might have been responsible for photosynthesis; but the only compelling and indicative detection was that of narrow-band radio emission suggestive of a technological civilisation. The paper presented a unique opportunity to objectively observe our blue marble planet from afar.
Nature has hosted many of the early forays into understanding the cosmic makeup of our Universe, particularly into the mysteries of dark matter, and more recently, dark energy. Examples of these kinds of papers come from the late 1990s. For instance, James Dunlop and co-authors reported the discovery of a 3.5 billion year old galaxy (Dunlop et al. 1996). There was nothing unusual about that, until they also presented the redshift of this object, z = 1.55. This implied that the galaxy was older than the predicted age of the Universe at that redshift, under a cosmological framework known as the standard Einstein-de Sitter model. This was a definitive nail in the coffin for that theory of the Universe, and opened up the possibility of a non-zero cosmological constant.
Shortly after that paper, a concerted effort was made by two groups, one led by Saul Perlmutter and the other by Brian Schmidt, to measure the presumed deceleration of the expanding Universe using Type Ia supernovae as distance markers. Writing in Nature, Saul Perlmutter measured the spectrum of a Type Ia supernova at a redshift of z = 0.83, that is, half the age of the Universe (Perlmutter et al. 1998). As Type Ia supernovae have well-calibrated brightnesses, he was able to use this distant marker in order to determine the mass density of the Universe, finding it to be much less than 1. In a Universe that was expected to be dominated by matter, this was a surprise. The combined work of the two groups showed that the Universe was not slowing down in its expansion, but accelerating. Under the prevailing cosmological models of the time, this finding also left room for another energy term, the vacuum energy density, and therefore, a cosmological constant that had originally been proposed by Albert Einstein.
Whatever it is that makes up dark matter, whether it be weakly interacting massive particles (WIMPs), massive compact halo objects (MACHOs), axions or something entirely more exotic, has thus far evaded direct detection. However, there have been indirect indications of the existence of dark matter, and this subject has been covered in Nature. In 2008, Jin Chang and co-workers reported results from a balloon experiment (Chang et al. 2008) that had measured the cosmic rays streaming through the Earth’s atmosphere. Cosmic rays are made up of protons, electrons, positrons and ions, accelerated to high speeds in supernova remnants, and pervasive in the Universe. Typically each component shows a power-law decrease in intensity with increasing energy, and yet in this case the balloon-borne instrument had seen an excess of electrons with energies of 300–800 GeV. The implication was that these electrons had either been produced by an astrophysical object such as a pulsar or a micro-quasar, or by the annihilation of dark matter particles.
Shortly afterwards, Oscar Adriani and team reported the results of a satellite experiment designed to study cosmic rays above the atmosphere of the Earth (Adriani et al. 2009). It measured the fraction of positrons contributing to cosmic rays of energy > 10 GeV, and found that this fraction was not compatible with production from ‘normal’ interactions between cosmic ray particles and ordinary interstellar matter. The source of these positrons must be something else: either an astrophysical object or annihilations of dark matter particles.
Another potential component of dark matter was discovered by Georg Weidenspointner, using the Integral gamma-ray observatory (Weidenspointner et al. 2008). He and his team, whilst studying the gamma-rays from the Galactic Centre, noticed an asymmetry in the distribution. These particular gamma-rays, at 511 keV, result from positron-electron annihilation. An asymmetry was not expected, and Weidenspointner et al. noticed that it happened to coincide with an identical asymmetry in the distribution of low-mass X-ray binaries that were strongly emitting ‘hard’ (>20 keV) photons. This finding implied that some contribution to Galactic positron levels is from compact astrophysical objects, with less need for exotic dark matter.
Andrew Pontzen and Fabio Governato recently reviewed a development to the traditional view of dark matter being 'cold' (dark matter particles that move slowly in comparison to the speed of light; Pontzen & Governato 2014). By considering two components of galaxies that were generally thought to be inconsequential to the dark matter distribution, stars and gas (that is, 'normal' matter), they highlighted that these two components can inject heat energy into the dark matter portion. This removes an issue with the cold dark matter model that predicted high-density cusps in the centre of galaxies, rather than the low-density cores measured observationally.
