Astronomers catch wind rushing out of galaxy

Exploring the influence of galactic winds from a distant galaxy called Makani, UC San Diego's Alison Coil, Rhodes College's David Rupke and a group of collaborators from around the world made a novel discovery. Published in Nature, their study's findings provide direct evidence for the first time of the role of galactic winds - ejections of gas from galaxies - in creating the circumgalactic medium (CGM). It exists in the regions around galaxies, and it plays an active role in their cosmic evolution. The unique composition of Makani - meaning wind in Hawaiian - uniquely lent itself to the breakthrough findings. "Makani is not a typical galaxy," noted Coil, a physics professor at UC San Diego. "It's what's known as a late-stage major merger - two recently combined similarly massive galaxies, which came together because of the gravitational pull each felt from the other as they drew nearer. Galaxy mergers often lead to starburst events, when a substantial amount of gas present in the merging galaxies is compressed, resulting in a burst of new star births. Those new stars, in the case of Makani, likely caused the huge outflows - either in stellar winds or at the end of their lives when they exploded as supernovae." Coil explained that most of the gas in the universe inexplicably appears in the regions surrounding galaxies - not in the galaxies. Typically, when astronomers observe a galaxy, they are not witnessing it undergoing dramatic events - big mergers, the rearrangement of stars, the creation of multiple stars or driving huge, fast winds.

"While these events may occur at some point in a galaxy's life, they'd be relatively brief," noted Coil. "Here, we're actually catching it all right as it's happening through these huge outflows of gas and dust."

Coil and Rupke, the paper's first author, used data collected from the W. M. Keck Observatory's new Keck Cosmic Web Imager (KCWI) instrument, combined with images from the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA), to draw their conclusions.

The KCWI data provided what the researchers call the "stunning detection" of the ionized oxygen gas to extremely large scales, well beyond the stars in the galaxy. It allowed them to distinguish a fast gaseous outflow launched from the galaxy a few million year ago, from a gas outflow launched hundreds of millions of years earlier that has since slowed significantly.

"The earlier outflow has flowed to large distances from the galaxy, while the fast, recent outflow has not had time to do so," summarized Rupke, associate professor of physics at Rhodes College.

From the Hubble, the researchers procured images of Makani's stars, showing it to be a massive, compact galaxy that resulted from a merger of two once separate galaxies. From ALMA, they could see that the outflow contains molecules as well as atoms.

The data sets indicated that with a mixed population of old, middle-age and young stars, the galaxy might also contain a dust-obscured accreting supermassive black hole. This suggests to the scientists that Makani's properties and timescales are consistent with theoretical models of galactic winds.

"In terms of both their size and speed of travel, the two outflows are consistent with their creation by these past starburst events; they're also consistent with theoretical models of how large and fast winds should be if created by starbursts. So observations and theory are agreeing well here," noted Coil.

Rupke noticed that the hourglass shape of Makani's nebula is strongly reminiscent of similar galactic winds in other galaxies, but that Makani's wind is much larger than in other observed galaxies.

"This means that we can confirm it's actually moving gas from the galaxy into the circumgalactic regions around it, as well as sweeping up more gas from its surroundings as it moves out," Rupke explained. "And it's moving a lot of it - at least one to 10 percent of the visible mass of the entire galaxy - at very high speeds, thousands of kilometers per second."

Rupke also noted that while astronomers are converging on the idea that galactic winds are important for feeding the CGM, most of the evidence has come from theoretical models or observations that don't encompass the entire galaxy.

"Here we have the whole spatial picture for one galaxy, which is a remarkable illustration of what people expected," he said. "Makani's existence provides one of the first direct windows into how a galaxy contributes to the ongoing formation and chemical enrichment of its CGM."

Retreating snow line reveals organic molecules around young star

Astronomers using ALMA have detected various complex organic molecules around the young star V883 Ori. A sudden outburst from this star is releasing molecules from the icy compounds in the planet forming disk. The chemical composition of the disk is similar to that of comets in the modern Solar System. Sensitive ALMA observations enable astronomers to reconstruct the evolution of organic molecules from the birth of the Solar System to the objects we see today. The research team led by Jeong-Eun Lee (Kyung Hee University, Korea) used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect complex organic molecules including methanol (CH3OH), acetone (CH3COCH3), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and acetonitrile (CH3CN). This is the first time that acetone was unambiguously detected in a planet forming region or protoplanetary disk. Various molecules are frozen in ice around micrometer-sized dust particles in protoplanetary disks. V883 Ori's sudden flare-up is heating the disk and sublimating the ice, which releases the molecules into gas. The region in a disk where the temperature reaches the sublimation temperature of the molecules is called the "snow line." The radii of snow lines are about a few astronomical units (au) around normal young stars, however, they are enlarged almost 10 times around bursting stars.

"It is difficult to image a disk on the scale of a few au with current telescopes," said Lee. "However, around an outburst star, ice melts in a wider area of the disk and it is easier to see the distribution of molecules. We are interested in the distribution of complex organic molecules as the building blocks of life."

Ice, including frozen organic molecules, could be closely related to the origin of life on planets. In our Solar System, comets are the focus of attention because of their rich icy compounds. For example, the European Space Agency's legendary comet explorer Rosetta found rich organic chemistry around the comet Churyumov-Gerasimenko.

Comets are thought to have been formed in the outer colder region of the proto-Solar System, where the molecules were contained in ice. Probing the chemical composition of ice in protoplanetary disks is directly related to probing the origin of organic molecules in comets, and the origin of the building blocks of life.

Thanks to ALMA's sharp vision and the enlarged snow line due to the flare-up of the star, the astronomers obtained the spatial distribution of methanol and acetaldehyde. The distribution of these molecules has a ring-like structure with a radius of 60 au, which is twice the size of Neptune's orbit. The researchers assume that inside of this ring the molecules are invisible because they are obscured by thick dusty material, and are invisible outside of this radius because they are frozen in ice.

"Since rocky and icy planets are made from solid material, the chemical composition of solids in disks is of special importance. An outburst is a unique chance to investigate fresh sublimates, and thus the composition of solids." says Yuri Aikawa at the University of Tokyo, a member of the research team.

V883 Ori is a young star located at 1300 light-years away from the Earth. This star is experiencing a so-called FU Orionis type outburst, a sudden increase of luminosity due to a bursting torrent of material flowing from the disk to the star.

These outbursts last only on the order of 100 years, therefore the chance to observe a burst is rather rare. However, since young stars with a wide range of ages experience FU Ori bursts, astronomers expect to be able to trace the chemical composition of ice throughout the evolution of young stars.

Note: Another ALMA observation (van't Hoff et al. 2018, ApJL, 864, 23) also detected CH3OH emissions from V883 Ori. However, the sensitivity and resolution of the observations were not enough to resolve the structure inside the water snow line.

Youngest Accretion Disk Detected in Star Formation

An international team led by Chin-Fei Lee at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) has discovered a very small accretion disk formed around one of the youngest protostars, with the Atacama Large Millimeter/submillimeter Array (ALMA). This discovery poses a constraint on current theory of disk formation stronger than before, by pushing the disk formation time by a factor of a few earlier. Moreover, a compact rotating outflow has been detected. It may trace a disk wind carrying away angular momentum from the disk and thus facilitate the disk formation. "ALMA is so powerful that it can resolve an accretion disk with a radius as small as 15 astronomical units (AU)," says Chin-Fei Lee at ASIAA. "Since this disk is about a few times younger than the previously resolved youngest disk, our result has provided a stronger constraint on current theory of disk formation by pushing the disk formation time by a factor of a few earlier. Moreover, together with the previous results of the older disks, our disk result favors a model where the disk radius grows linearly with the protostellar mass, and thus supporting the 'early-start, slow-growth' scenario against the 'slow-start, rapid-growth' scenario for accretion disk formation around protostars."

HH 211 is one of the youngest protostellar systems in Perseus at a distance of about 770 light-years. The central protostar has an age of only about 10,000 years (which is about 2 millionths of the age of our Sun) and a mass of less than 0.05 solar mass. It drives a powerful bipolar jet and thus must accrete material efficiently.

Previous search at a resolution of about 50 AU only found a hint of a small dusty disk near the protostar. Now with ALMA at a resolution of 7 AU, which is about 7 times finer, the dusty disk at submillimeter wavelength not only has been detected but also spatially resolved.

It is a nearly edge-on accretion disk feeding the central protostar and has a radius of about 15 AU. The disk is thick, indicating that the submillimeter light emitting grains have yet to settle to the midplane. Unlike the previously resolved older edge-on disk HH 212 which appears as a large "hamburger," this younger edge-on disk appears as a small "bun."

Thus, it seems that an edge-on disk will grow from a small "bun" to a large "hamburger" in a later phase. Moreover, a compact rotating outflow has been detected, and it may trace a disk wind carrying away angular momentum from the disk and thus facilitate the disk formation.

The observations open up an exciting possibility of directly detecting and characterizing small disks around the youngest protostars through high-resolution imaging with ALMA, which provides strong constraints on theories of disk formation and thus the feeding process in star formation.

Little star sheds light on young planets

Astronomers from the Department of Physics at the University of Tokyo discovered a dense disk of material around a young star, which may be a precursor to a planetary system. Their research could vastly improve models of how solar systems form, which would tell us more about our own place in the cosmos. Early in 2017, Assistant Professor Yoko Oya gave graduate student Yuki Okoda some recent complex data on a nearby star with which she could begin her Ph.D. Little did she realize that what she would find could unlock not only the secrets of how planets form but possibly her career as a professional astronomer. The star in question (only known by its catalog number IRAS 15398-3359) is small, young and relatively cool for a star. It's diminutive stature means the weak light it shines can't even reach us through a cloud of gas and dust that surrounds it. But this doesn't stop inquisitive minds from exploring the unknown. In 2013, Oya and her collaborators used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe the star in submillimeter wavelengths, as that kind of light can penetrate the dust cloud - for reference, red light is around 700 nanometers. A painstaking analysis revealed some interesting nebulous structures, despite the images they worked from being difficult to comprehend.

"The greatest academic challenge I've faced was trying to make sense of grainy images. It's extremely difficult to know exactly what you're really looking at." says Okoda. "But I felt compelled to explore the nature of the structures Dr. Oya had seen with ALMA, so I came up with a model to explain them."

The model she produced came as a surprise to Okoda and her colleagues, but it fit the data perfectly. It describes a dense disk of material that consists of gas and dust from the cloud that surrounds the star. This has never before been seen around such a young star.

The disk is a precursor to a protoplanetary disk, which is far denser still and eventually becomes a planetary system in orbit around a star.

"We can't say for sure this particular disk will coalesce into a new planetary system," explains Oya. "The dust cloud may be pushed away by stellar winds or it might all fall into the star itself, feeding it in the process. What's exciting is how quickly this might happen."

The star is small at around 0.7 percent the mass of our sun, based on observations of the mass of the surrounding cloud. It could grow to as large as 20 percent in just a few tens of thousands of years, a blink of the eye on the cosmic scale.

"I hope our observations and models will enhance knowledge of how solar systems form," says Okoda. "My research interests involve young protostellar objects, and the implication that protoplanetary disks could form earlier than expected really excites me."

Okoda began this project a year-and-a-half ago to hone her skills as an astronomer, but mirroring the young star she observed, the practice evolved quickly and became a full research project, which will hopefully earn her a Ph.D. from the University of Tokyo.

The observations and resultant model were only possible thanks to advancements in radio astronomy with observatories such as ALMA. The team was lucky that the plane of the disk is level with our own solar system as this means the starlight ALMA sees passes through enough of the gas and dust to divulge important characteristics of it.

"We were also lucky to be given time with ALMA to carry out our observations. Only about 20 percent of applications actually go ahead," explains Oya. "With highly specialized astronomical instruments, there is much competition for time. My hope is our success will inspire a new generation of astronomers in Japan to reach for the stars."