Our Sun Came Late to the Milky Way's Star-Birth Party

A Firestorm of Star Birth

Credit: NASA, ESA, and Z. Levay (STScI)

Release Images

The Growth of Milky Way-Like Galaxies Over Time

Credit: NASA, ESA, C. Papovich (Texas A&M University), H. Ferguson (STScI), S. Faber (University of California, Santa Cruz), and I. Labbé (Leiden University)

In one of the most comprehensive multi-observatory galaxy surveys yet, astronomers find that galaxies like our Milky Way underwent a stellar "baby boom," churning out stars at a prodigious rate, about 30 times faster than today.

Our Sun, however, is a late "boomer." The Milky Way's star-birthing frenzy peaked 10 billion years ago, but our Sun was late for the party, not forming until roughly 5 billion years ago. By that time the star formation rate in our galaxy had plunged to a trickle.

Missing the party, however, may not have been so bad. The Sun's late appearance may actually have fostered the growth of our solar system's planets. Elements heavier than hydrogen and helium were more abundant later in the star-forming boom as more massive stars ended their lives early and enriched the galaxy with material that served as the building blocks of planets and even life on Earth.

Astronomers don't have baby pictures of our Milky Way's formative years to trace the history of stellar growth. Instead, they compiled the story from studying galaxies similar in mass to our Milky Way, found in deep surveys of the universe. The farther into the universe astronomers look, the further back in time they are seeing, because starlight from long ago is just arriving at Earth now.

From those surveys, stretching back in time more than 10 billion years, researchers assembled an album of images containing nearly 2,000 snapshots of Milky Way-like galaxies.

The new census provides the most complete picture yet of how galaxies like the Milky Way grew over the past 10 billion years into today's majestic spiral galaxies. The multi-wavelength study spans ultraviolet to far-infrared light, combining observations from NASA's Hubble and Spitzer space telescopes, the European Space Agency's Herschel Space Observatory, and ground-based telescopes, including the Magellan Baada Telescope at the Las Campanas Observatory in Chile.

"This study allows us to see what the Milky Way may have looked like in the past," said Casey Papovich of Texas A&M University in College Station, lead author on the paper that describes the study's results. "It shows that these galaxies underwent a big change in the mass of its stars over the past 10 billion years, bulking up by a factor of 10, which confirms theories about their growth. And most of that stellar-mass growth happened within the first 5 billion years of their birth."

The new analysis reinforces earlier research that showed Milky Way-like galaxies began as small clumps of stars. The diminutive galaxies built themselves up by swallowing large amounts of gas that ignited a firestorm of star birth.

The study reveals a strong correlation between the galaxies' star formation and their growth in stellar mass. Observations revealed that as the star-making factories slowed down, the galaxies' growth decreased as well. "I think the evidence suggests that we can account for the majority of the buildup of a galaxy like our Milky Way through its star formation," Papovich said. "When we calculate the star-formation rate of a Milky Way galaxy and add up all the stars it would have produced, it is pretty consistent with the mass growth we expected. To me, that means we're able to understand the growth of the 'average' galaxy with the mass of a Milky Way galaxy."

The astronomers selected the Milky Way-like progenitors by sifting through more than 24,000 galaxies in the entire catalogs of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), taken with Hubble, and the FourStar Galaxy Evolution Survey (ZFOURGE), made with the Magellan telescope.

They used the ZFOURGE, CANDELS, and Spitzer near-infrared data to study the galaxy stellar masses. The Hubble images from the CANDELS survey also provided structural information about galaxy sizes and how they evolved. Far-infrared light observations from Spitzer and Herschel helped the astronomers trace the star-formation rate.

The team's results will appear in the April 9 issue of The Astrophysical Journal.

Contact:

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Casey Papovich
Texas A&M University, College Station, Texas
979-862-2704
papovich@tamu.edu

Source: HubbleSite

Violent Origins of Disc Galaxies Probed by ALMA

PR Image eso1429a

Distribution of molecular gas in 30 merging galaxies

 Videos

PR Video eso1429a

Merger between two galaxies (artist’s impression)

New observations explain why Milky Way-like galaxies are so common in the Universe 

For decades scientists have believed that galaxy mergers usually result in the formation of elliptical galaxies. Now, for the the first time, researchers using ALMA and a host of other radio telescopes have found direct evidence that merging galaxies can instead form disc galaxies, and that this outcome is in fact quite common. This surprising result could explain why there are so many spiral galaxies like the Milky Way in the Universe.

An international research group led by Junko Ueda, a Japan Society for the Promotion of Science postdoctoral fellow, has made surprising observations that most galaxy collisions in the nearby Universe — within 40–600 million light-years from Earth — result in so-called disc galaxies. Disc galaxies — including spiral galaxies like the Milky Way and lenticular galaxies — are defined by pancake-shaped regions of dust and gas, and are distinct from the category of elliptical galaxies.

It has, for some time, been widely accepted that merging disc galaxies would eventually form an elliptically shaped galaxy. During these violent interactions the galaxies do not only gain mass as they merge or cannibalise each-other, but they are also changing their shape throughout cosmic time, and therefore changing type along the way.

Computer simulations from the 1970s predicted that mergers between two comparable disc galaxies would result in an elliptical galaxy. The simulations predict that most galaxies today are elliptical, clashing with observations that over 70% of galaxies are in fact disc galaxies. However, more recent simulations have suggested that collisions could also form disc galaxies.

To identify the final shapes of galaxies after mergers observationally, the group studied the distribution of gas in 37 galaxies that are in their final stages of merging. The Atacama Large Millimeter/sub-millimeter Array (ALMA) and several other radio telescopes [1] were used to observe emission from carbon monoxide (CO), an indicator of molecular gas. 

The team’s research is the largest study of molecular gas in galaxies to date and provides unique insight into how the Milky Way might have formed. Their study revealed that almost all of the mergers show pancake-shaped areas of molecular gas, and hence are disc galaxies in the making. Ueda explains: “For the first time there is observational evidence for merging galaxies that could result in disc galaxies. This is a large and unexpected step towards understanding the mystery of the birth of disc galaxies.”

Nonetheless, there is a lot more to discover. Ueda added: “We have to start focusing on the formation of stars in these gas discs. Furthermore, we need to look farther out in the more distant Universe. We know that the majority of galaxies in the more distant Universe also have discs. We however do not yet know whether galaxy mergers are also responsible for these, or whether they are formed by cold gas gradually falling into the galaxy. Maybe we have found a general mechanism that applies throughout the history of the Universe.”

Notes

[1] The data were obtained by ALMA; the Combined Array for Research in Millimeter-wave Astronomy: a millimeter array consisting of 23 parabola antennas in California; the Submillimeter Array a submillimeter array consisting of eight parabola antennas in Mauna Kea, Hawaii; the Plateau de Bure Interferometer; the NAOJ Nobeyama Radio Observatory 45m radio telescope; USA’s National Radio Astronomy Observatory 12m telescope; USA's Five College Radio Astronomy Observatory 14m telescope; IRAM’s 30m telescope; and the Swedish-ESO Submillimeter Telescope as a supplement.

More information

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

These observation results were published in The Astrophysical Journal Supplement (August 2014) as Ueda et al. "Cold Molecular Gas in Merger Remnants. I. Formation of Molecular Gas Discs".

The team is composed of Junko Ueda (JSPS postdoctoral fellow/National Astronomical Observatory of Japan [NAOJ]), Daisuke Iono (NAOJ/The Graduate University for Advanced Studies [SOKENDAI]), Min S. Yun (The University of Massachusetts), Alison F. Crocker (The University of Toledo), Desika Narayanan (Haverford College), Shinya Komugi (Kogakuin University/ NAOJ), Daniel Espada (NAOJ/SOKENDAI/Joint ALMA Observatory), Bunyo Hatsukade (NAOJ), Hiroyuki Kaneko (University of Tsukuba), Yoichi Tamura (The University of Tokyo), David J. Wilner (Harvard-Smithsonian Center for Astrophysics), Ryohei Kawabe (NAOJ/ SOKENDAI/The University of Tokyo) and Hsi-An Pan (Hokkaido University/SOKENDAI/NAOJ)

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

• Research paper on Astro-Ph

Contacts

Junko Ueda
JSPS postdoctoral fellow/NAOJ
Tel: +88 422 34 3117
Email: junko.ueda@nao.ac.jp

Lars Lindberg Christensen
Head of ESO ePOD
Garching bei München, Germany
Tel: +49 89 3200 6761
Cell: +49 173 3872 621
Email: lars@eso.org 

Masaaki Hiramatsu
NAOJ Chile Observatory EPO officer
Tel: +88 422 34 3630
Email: hiramatsu.masaaki@nao.ac.jp

Source: ESO 

Hubble Reveals First Scrapbook Pictures of Milky Way's Formative Years

Galaxies Similar to the Milky Way  

Credit: NASA, ESA, P. van Dokkum (Yale University), 

S. Patel (Leiden University), and the 3D-HST Team 

News Release Images

NASA's Hubble Space Telescope has provided the first visual evidence showing how our home galaxy, the Milky Way, assembled itself into the majestic pinwheel of stars we see today.

Perusing Hubble's deep-sky surveys, astronomers traced 400 galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years.

"For the first time we have direct images of what the Milky Way looked like in the past," said study co-leader Pieter G. van Dokkum of Yale University in New Haven, Conn. "Of course, we can't see the Milky Way itself in the past. We selected galaxies billions of light-years away that will evolve into galaxies like the Milky Way. By tracing the Milky Way's siblings, we find that our galaxy built up 90 percent of its stars between 11 billion and 7 billion years ago, which is something that has not been measured directly before."

The Hubble telescope's superb resolving power allowed the researchers to study how the structure of the Milky Way changed over time. A scale model of the Milky Way can be imagined by envisioning a fried egg. The egg white is the disk, where the Sun and Earth reside. The yoke represents the central bulge of older stars, home to a supermassive black hole that must have also grown along with the galaxy.

The Hubble images suggest that our galaxy's flat disk and central bulge grew simultaneously into the majestic spiral galaxy of today. "You can see that these galaxies are fluffy and spread out," said study co-leader Shannon Patel, of Leiden University,  the Netherlands. "There is no evidence of a bulge without a disk, around which the disk formed later." Team member Erica Nelson, of Yale University, added: "These galaxies show us that the whole Milky Way grew at the same time, unlike more massive elliptical galaxies, in which the central bulge forms first."

The survey reveals that billions of years ago, the Milky Way was likely a faint, blue, low-mass object containing lots of gas, the fuel for star birth. The blue colors of the Milky Way ancestors are a signpost of rapid star formation. At the peak of star birth, when the universe was about 4 billion years old, the Milky Way-like galaxies were pumping out about 15 stars a year. By comparison, our galaxy today is creating only one star a year.

To identify the far-flung galaxies and study them in detail, the research team used three of the largest Hubble programs, the 3D-HST survey, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey or CANDELS, and the Great Observatories Origins Deep Survey or GOODS. These surveys of the distant universe combined spectroscopy with visible and near-infrared imaging by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys. The research team's analysis involved measuring the galaxies' distances and sizes. The astronomers calculated the mass of each galaxy from its brightness and colors. They selected the galaxies in their census from a catalog they compiled of over 100,000 galaxies. The survey galaxies are consistent with computer models, which show that the bulges, and presumably the black holes, of spiral galaxies at early stages were largely built up at the same time as the disks.

"In these observations, we're capturing most of the evolution of the Milky Way," explained team member Joel Leja of Yale University. "These deep surveys allow us to see the smaller galaxies. In previous observations we could only see the most luminous galaxies in the distant past, and now we can look at more normal galaxies. Hubble gives us the shapes and colors of these spirals as well as their distances from Earth. We also can measure the rates at which each part of the galaxies grew. All of this is difficult to do from the ground." Exploring these galaxies back to their infancy will take the infrared eyes of NASA's James Webb Space Telescope, scheduled to launch in 2018.

The Hubble images also reinforce the idea that major mergers between spiral galaxies were not important in building them up. Computer simulations have shown that mergers would have destroyed the disks. Instead, this census reveals that spirals grew through star formation. This galaxy-formation scenario is different from the way massive elliptical galaxies develop.

"These observations show that there are at least two galaxy-formation tracks," van Dokkum said. "Massive ellipticals form a very dense core early in the universe, including a black hole, presumably, and the rest of the galaxy slowly accretes around it, fueled by mergers with other galaxies. But from our survey we find that galaxies like our Milky Way show a different, more uniform path of growing into the majestic spirals we see today."

The team's results appeared on July 10, 2013, in The Astrophysical Journal Letters. A second paper appears in the Nov. 11 online edition of The Astrophysical Journal.

CONTACT

Donna Weaver / Ray Villard
Space Science Telescope Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Pieter van Dokkum

Yale University, New Haven, Conn.

203-432-3000
pieter.vandokkum@yale.edu

Shannon Patel
The Observatories of the Carnegie Institution for Science, Pasadena, Calif.
626-304-0292
patel@carnegiescience.edu

'Galactic archaeologists' find origin of Milky Way's ancient stars

Ancient stars 1 – Simulation showing a Milky Way-like galaxy around five billion years ago when most satellite galaxy collisions were happening. Credit: Andrew Cooper / John Helly / Durham University

Ancient stars 2 – Simulation showing the stellar halo around the Milky Way in the present day. Credit: Andrew Cooper / Durham University

Many of the Milky Way’s ancient stars are remnants of other smaller galaxies torn apart by violent galactic collisions around five billion years ago, according to researchers at Durham University, who publish their results in a new paper in the journal Monthly Notices of the Royal Astronomical Society.

Scientists at Durham’s Institute for Computational Cosmology and their collaborators at the Max Planck Institute for Astrophysics, in Germany, and Groningen University, in Holland, ran huge computer simulations to recreate the beginnings of our Galaxy.

The simulations revealed that the ancient stars, found in a stellar halo of debris surrounding the Milky Way, had been ripped from smaller galaxies by the gravitational forces generated by colliding galaxies.

Cosmologists predict that the early Universe was full of small galaxies which led short and violent lives. These galaxies collided with each other leaving behind debris which eventually settled into more familiar looking galaxies like the Milky Way.

The researchers say their finding supports the theory that many of the Milky Way’s ancient stars had once belonged to other galaxies instead of being the earliest stars born inside the Galaxy when it began to form about 10 billion years ago.

Lead author Andrew Cooper, from Durham University’s Institute for Computational Cosmology, said: “Effectively we became galactic archaeologists, hunting out the likely sites where ancient stars could be scattered around the galaxy.

“Our simulations show how different relics in the Galaxy today, like these ancient stars, are related to events in the distant past.

“Like ancient rock strata that reveal the history of Earth, the stellar halo preserves a record of a dramatic primeval period in the life of the Milky Way which ended long before the Sun was born.”

The computer simulations started from shortly after the Big Bang, around 13 billion years ago, and used the universal laws of physics to simulate the evolution of dark matter and the stars.

These simulations are the most realistic to date, capable of zooming into the very fine detail of the stellar halo structure, including star “streams” – which are stars being pulled from the smaller galaxies by the gravity of the dark matter.

One in one hundred stars in the Milky Way belong to the stellar halo, which is much larger than the Galaxy’s familiar spiral disk. These stars are almost as old as the Universe.

Professor Carlos Frenk, Director of Durham University’s Institute for Computational Cosmology, said: “The simulations are a blueprint for galaxy formation.

“They show that vital clues to the early, violent history of the Milky Way lie on our galactic doorstep.

“Our data will help observers decode the trials and tribulations of our Galaxy in a similar way to how archaeologists work out how ancient Romans lived from the artefacts they left behind.”

The research is part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way and was partly funded by the UK’s Science and Technology Facilities Council (STFC).

Aquarius was carried out by the Virgo Consortium, involving scientists from the Max Planck Institute for Astrophysics in Germany, the Institute for Computational Cosmology at Durham University, UK, the University of Victoria in Canada, the University of Groningen in the Netherlands, Caltech in the USA and Trieste in Italy.

Durham’s cosmologists will present their work to the public as part of the Royal Society's 350th anniversary 'See Further' exhibition, held at London's Southbank Centre until July 4th.

The highlight of their 'Cosmic Origins' exhibit is an award winning 3-D movie describing how the Milky Way formed. Visitors to the exhibit can also create their own star streams by colliding galaxies with an interactive 3-D simulation.

CONTACTS

Andrew Cooper
[Available for interview on Tuesday 29 June and Wednesday 30 June]
Institute for Computational Cosmology
Durham University
Tel: +44 (0)191 334 3768
Email: a.p.cooper@durham.ac.uk

Professor Carlos Frenk
Director, Institute for Computational Cosmology
[Available for interview on Tuesday 29 June and Wednesday 30 June]
Durham University
Tel: +44 (0)191 334 3461
Email: c.s.frenk@durham.ac.uk

Media Relations Office
Durham University
Tel: +44 (0)191 334 6075
Email: media.relations@durham.ac.uk

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x 214
Mob: +44 (0)794 124 8035
Email: rm@ras.org.uk

IMAGES AND CAPTIONS

Images are available from http://www.virgo.dur.ac.uk/aquarius/ancient_stars/index.html

Images are also available on request from Durham University Media Relations Office on +44 (0)191 334 6075 or email media.relations@durham.ac.uk .

A copy of The International Virgo Consortium/Durham University Institute for Computational Cosmology “Cosmic Origins” movie is available as a MVA file via the following web link http://www.dur.ac.uk/n.s.holliman/CosmicOrigins.html

To download the file right click on the link Cosmic Origins - QHD 2D version of the 2D movie and select “save target as”. Please note this file is 257MB in size.

FURTHER INFORMATION

The work appears in the paper “Galactic Stellar Haloes in the CDM Model”, Cooper AP, et al, Monthly Notices of the Royal Astronomical Society. Doi:10.1111/j.1365-2966.2010.16740.x.

A copy of the paper is available from Durham University Media Relations Office on +44 (0)191 334 6075 or media.relations@durham.ac.uk

Institute for Computational Cosmology: www.icc.dur.ac.uk/

Durham University: www.dur.ac.uk/

Monthly Notices of the Royal Astronomical Society:

www.wiley.com/bw/journal.asp?ref=1745-3925

Science and Technologies Facilities Council (STFC): www.stfc.ac.uk/

Durham University – a member of the 1994 Group

Durham University is a member of the 1994 Group of 19 leading research-intensive universities. The Group was established in 1994 to promote excellence in university research and teaching. Each member undertakes diverse and high-quality research, while ensuring excellent levels of teaching and student experience. www.1994group.ac.uk

The Royal Astronomical Society

The Royal Astronomical Society (RAS: www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.