A new report from ESO (European Southern Observatory):

ALMA and VLT Find Too Many Massive Stars in Starburst Galaxies, Near and Far

Astronomers using ALMA and the VLT have discovered that both starburst galaxies in the early Universe and a star-forming region in a nearby galaxy contain a much higher proportion of massive stars than is found in more peaceful galaxies. These findings challenge current ideas about how galaxies evolved, changing our understanding of cosmic star-formation history and the build up of chemical elements.

Probing the distant Universe a team of scientists, led by University of Edinburgh astronomer Zhi-Yu Zhang, used the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate the proportion of massive stars in four distant gas-rich starburst galaxies [1]. These galaxies are seen when the Universe was much younger than it is now so the infant galaxies are unlikely to have undergone many previous episodes of star formation, which might otherwise have confused the results.

Zhang and his team developed a new technique — analogous to radiocarbon dating (also known as carbon-14 dating) — to measure the abundances of different types of carbon monoxide in four very distant, dust-shrouded starburst galaxies [2]. They observed the ratio of two types of carbon monoxide containing different isotopes [3].

“Carbon and oxygen isotopes have different origins”, explains Zhang. “¹⁸O is produced more in massive stars, and ¹³C is produced more in low- to intermediate-mass stars.”

Thanks to the new technique the team was able to peer through the dust in these galaxies and assess for the first time the masses of their stars.

The mass of a star is the most important factor determining how it will evolve. Massive stars shine brilliantly and have short lives and less massive ones, such as the Sun, shine more modestly for billions of years. Knowing the proportions of stars of different masses that are formed in galaxies therefore underpins astronomers’ understanding of the formation and evolution of galaxies throughout the history of the Universe. Consequently, it gives us crucial insights about the chemical elements available to form new stars and planets and, ultimately, the number of seed black holes that may coalesce to form the supermassive black holes that we see in the centres of many galaxies.

Co-author Donatella Romano from the INAF-Astrophysics and Space Science Observatory in Bologna explains what the team found:

“The ratio of ¹⁸O to ¹³C was about 10 times higher in these starburst galaxies in the early Universe than it is in galaxies such as the Milky Way, meaning that there is a much higher proportion of massive stars within these starburst galaxies.”

The ALMA finding is corroborated by another discovery in the local Universe. A team led by Fabian Schneider of the University of Oxford, UK, made spectroscopic measurements with ESO’s Very Large Telescope of 800 stars in the gigantic star-forming region 30 Doradus in the Large Magellanic Cloud in order to investigate the overall distribution of stellar ages and initial masses [4].

Schneider explained,

“We found around 30% more stars with masses more than 30 times that of the Sun than expected, and about 70% more than expected above 60 solar masses. Our results challenge the previously predicted 150 solar mass limit for the maximum birth mass of stars and even suggest that stars could have birth masses up to 300 solar masses!”

Rob Ivison, co-author of the new ALMA paper, concludes:

“Our findings lead us to question our understanding of cosmic history. Astronomers building models of the Universe must now go back to the drawing board, with yet more sophistication required.”

Notes
[1] Starburst galaxies are galaxies that are undergoing an episode of very intense star formation. The rate at which they form new stars can be 100 times or more the rate in our own galaxy, the Milky Way. Massive stars in these galaxies produce ionising radiation, stellar outflows, and supernova explosions, which significantly influence the dynamical and chemical evolution of the medium around them. Studying the mass distribution of stars in these galaxies can tell us more about their own evolution, and also the evolution of the Universe more generally.

[2] The radiocarbon dating method is used for determining the age of an object containing organic material. By measuring the amount of ¹⁴C, which is a radioactive isotope whose abundance continuously decreases, one can calculate when the animal or plant died. The isotopes used in the ALMA study, ¹³C and ¹⁸O, are stable and their abundances continuously increase during the lifetime of a galaxy, being synthesised by thermal nuclear fusion reactions inside stars.

[3] These different forms of the molecule are called isotopologues and they differ in the number of neutrons they can have. The carbon monoxide molecules used in this study are an example of such molecular species, because a stable carbon isotope can have either 12 or 13 nucleons in its nucleus, and a stable oxygen isotope can have either 16, 17, or 18 nucleons.

[4] Schneider et al. made spectroscopic observations of individual stars in 30 Doradus, a star-forming region in the nearby Large Magellanic Cloud, using the Fibre Large Array Multi Element Spectrograph (FLAMES) on the Very Large Telescope (VLT). This study was one of the first to be carried out that has been detailed enough to show that the Universe is able to produce star-forming regions with different mass distributions from that in the Milky Way.

NASA JPL’s latest preview of night sky sights for the coming month:

Enjoy a ringside seat for Saturn, plus a nightlong parade featuring Venus, Jupiter, Mars and Vesta! 

And here is the Hubble Telescope Institute‘s preview for June:

In June, the constellations Boӧtes, Corona Borealis and Draco, and the planets Venus, Jupiter, Mars and Saturn are all visible from the Northern Hemisphere.

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A view by the Hubble Space Telescope of a grand galactic smashup

Cosmic collision lights up the darkness

Though it resembles a peaceful rose swirling in the darkness of the cosmos, NGC 3256 is actually the site of a violent clash. This distorted galaxy is the relic of a collision between two spiral galaxies, estimated to have occurred 500 million years ago. Today it is still reeling in the aftermath of this event.

Located about 100 million light-years away in the constellation of Vela (The Sails), NGC 3256 is approximately the same size as our Milky Way and belongs to the Hydra-Centaurus Supercluster. It still bears the marks of its tumultuous past in the extended luminous tails that sprawl out around the galaxy, thought to have formed 500 million years ago during the initial encounter between the two galaxies, which today form NGC 3256. These tails are studded with young blue stars, which were born in the frantic but fertile collision of gas and dust.

When two galaxies merge, individual stars rarely collide because they are separated by such enormous distances, but the gas and dust of the galaxies do interact — with spectacular results. The brightness blooming in the centre of NGC 3256 gives away its status as a powerful starburst galaxy, host to vast amounts of infant stars born into groups and clusters. These stars shine most brightly in the far infrared, making NGC 3256 exceedingly luminous in this wavelength domain. Because of this radiation, it is classified as a Luminous Infrared Galaxy.

NGC 3256 has been the subject of much study due to its luminosity, its proximity, and its orientation: astronomers observe its face-on orientation, that shows the disc in all its splendour. NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers. It holds particular promise to further our understanding of the properties of young star clusters in tidal tails.

As well as being lit up by over 1000 bright star clusters, the central region of NGC 3256 is also home to crisscrossing threads of dark dust and a large disc of molecular gas spinning around two distinct nuclei — the relics of the two original galaxies. One nucleus is largely obscured, only unveiled in infrared, radio and X-ray wavelengths.

These two initial galaxies were gas-rich and had similar masses, as they seem to be exerting roughly equal influence on each other. Their spiral disks are no longer distinct, and in a few hundred million years time, their nuclei will also merge and the two galaxies will likely become united as a large elliptical galaxy.

NGC 3256 was previously imaged through fewer filters by the NASA/ESA Hubble Space Telescope as part of a large collection of 59 images of merging galaxies, released for Hubble’s 18th anniversary on 24th April 2008.

The latest report from ESO (European Southern Observatory):

A Crowded Neighbourhood

Glowing brightly about 160 000 light-years away, the Tarantula Nebula is the most spectacular feature of the Large Magellanic Cloud, a satellite galaxy to our Milky Way. The VLT Survey Telescope at ESO’s Paranal Observatory in Chile has imaged this region and its rich surroundings in exquisite detail. It reveals a cosmic landscape of star clusters, glowing gas clouds and the scattered remains of supernova explosions. This is the sharpest image ever of this entire field.

Taking advantage of the capacities of the VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile, astronomers captured this very detailed new image of the Tarantula Nebula and its numerous neighbouring nebulae and star clusters. The Tarantula, which is also known as 30 Doradus, is the brightest and most energetic star-forming region in the Local Group of galaxies.

The Tarantula Nebula, at the top of this image, spans more than 1000 light-years and is located in the constellation of Dorado (The Dolphinfish) in the far southern sky. This stunning nebula is part of the Large Magellanic Cloud, a dwarf galaxy that measures about 14 000 light-years across. The Large Magellanic Cloud is one of the closest galaxies to the Milky Way.

At the core of the Tarantula Nebula lies a young, giant star cluster called NGC 2070, a starburst region whose dense core, R136, contains some of the most massive and luminous stars known. The bright glow of the Tarantula Nebula itself was first recorded by French astronomer Nicolas-Louis de Lacaille in 1751.

Another star cluster in the Tarantula Nebula is the much older Hodge 301, in which at least 40 stars are estimated to have exploded as supernovae, spreading gas throughout the region. One example of a supernova remnant is the superbubble SNR N157B, which encloses the open star cluster NGC 2060. This cluster was first observed by British astronomer John Herschel in 1836, using an 18.6-inch reflector telescope at the Cape of Good Hope in South Africa. On the outskirts of the Tarantula Nebula, on the lower right-hand side, it is possible to identify the location of the famous supernova SN 1987A [1].

Moving to the left-hand side of the Tarantula Nebula, one can see a bright open star cluster called NGC 2100, which displays a brilliant concentration of blue stars surrounded by red stars. This cluster was discovered by Scottish astronomer James Dunlop in 1826 while working in Australia, using his self-built 9-inch (23-cm) reflecting telescope.

At the centre of the image is the star cluster and emission nebula NGC 2074, another massive star-forming region discovered by John Herschel. Taking a closer look one can spot a dark seahorse-shaped dust structure — the “Seahorse of the Large Magellanic Cloud”. This is a gigantic pillar structure roughly 20 light-years long — almost five times the distance between the Sun and the nearest star, Alpha Centauri. The structure is condemned to disappear over the next million years; as more stars in the cluster form, their light and winds will slowly blow away the dust pillars.

Obtaining this image was only possible thanks to the VST’s specially designed 256-megapixel camera called OmegaCAM. The image was created from OmegaCAM images through four different coloured filters, including one designed to isolate the red glow of ionised hydrogen [2].

Notes
[1] SN 1987A was the first supernova to be observed with modern telescopes and the brightest since Kepler’s Star in 1604. SN 1987A was so intense that it blazed with the power of 100 million suns for several months following its discovery on 23 February 1987.

[2] The H-alpha emission line is a red spectral line created when the electron inside a hydrogen atom loses energy. This happens in hydrogen around hot young stars when the gas becomes ionised by the intense ultraviolet radiation and electrons subsequently recombine with protons to form atoms again. The ability of OmegaCAM to detect this line allows astronomers to characterise the physics of giant molecular clouds where new stars and planets form.