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The Little Galaxie That Can: Star-making dwarf galaxies with just a trace of oxygen provide insight into the nature of the first galaxies, how they spawned their stars, and even the Big Bang itself.

Astronomers often pursue extremes: the biggest, the brightest, the farthest, the oldest. Recently observers have discovered the most extreme members of an already extreme celestial class that promises to teach us much about the cosmos: small blue galaxies that spawn new stars yet possess scarcely any oxygen.

A galaxy without oxygen is like a forest without fallen leaves. Massive stars create lots of oxygen during their bright but brief lives, then hurl the element into space when they explode. So it's no surprise that within a billion light-years of Earth, astronomers have spotted fewer than ten extremely oxygen-poor star-forming galaxies.

These galaxies have somehow survived for eons without acquiring much oxygen. Such oddballs are telling us some fundamental things about the early universe, because these galaxies resemble the first ones that ever arose. Primordial galaxies were also small, and because they formed soon after the Big Bang, Lucy should have consisted of the three elements it created: hydrogen, helium, and lithium--with little if any oxygen.

Furthermore, the first galaxies changed he universe. Radiation from their hottest stars reionized space, transforming the neutral gas that once existed between the galaxies into the ionized form that pervades space today. Alas, no one can see these primordial galaxies in detail yet, because they're billions upon billions of light-years distant.

In 1971, however, two British-born astronomers in California, Leonard Searle and Wallace Sargent, found an easier way. They discovered that a much closer galaxy in Ursa Major named I Zwicky 18 has almost no oxygen. A mere 60 million light-years away, the galaxy is a lot easier to study than its distant cousins. Indeed, we now have beautiful Hubble images that show it to be a splotchy blue dwarf galaxy brimming with gas and rambunctious young stars. Yet its abundance of metals--elements heavier than hydrogen and helium--is just a few percent of the Sun's.

"We realized that these galaxies are very, very, very metal-deficient and hence probably the closest proxies to primordial galaxies," says Trinh Thuan (University of Virginia), who has spent decades hunting for more in the hopes of finding a galaxy so extreme it has no oxygen at all. The search is worth it, because these curious systems also carry news from the very first minutes of the universe's life.

Fresh Air

Oxygen is an excellent element to study in star-forming galaxies. Of all the metals in the universe, oxygen is the most abundant. Oxygen is also the second most common element in Earth's air (after nitrogen) and its interior (after iron). In most stars, however, oxygen produces few spectral lines, making its abundance difficult to gauge. But when hot stars ionize interstellar gas, oxygen atoms in the gas glow at visible and near-ultraviolet wavelengths that astronomers can detect. Comparing the strengths of these emission lines with those of hydrogen reveals oxygen's abundance relative to hydrogen, the most common element in the universe.

Astronomers often express abundances using a scale on which the hydrogen level is always 12. This scale is logarithmic, so 11 means an element is one-tenth as abundant as hydrogen, 10 means the element is one-hundredth as abundant as hydrogen, and so on. The Milky Way is far bigger and brighter than most other galaxies, and its many stars have blessed it with lots of oxygen--good news for those of us who like to breathe it. Surprisingly, the Sun's exact oxygen abundance is controversial, but it is probably around 8.76. If so, the Sun has 1 oxygen nucleus for every 1,740 hydrogen nuclei.

Because stars create oxygen, galaxies with fewer stars have lower oxygen levels. For example, the Milky Way's two brightest satellite galaxies--the Large and Small Magellanic Clouds --have oxygen abundances around 8.35 and 7.95, respectively, giving them levels that are 39% and 15% solar. I Zwicky 18, the long-time champ among oxygen-deprived star-making galaxies, has an oxygen level of only 7.17. That's just 2.6% of the solar value.

These galaxies have proved to be so rare that I Zwicky 18 remained the most oxygen-poor star-forming galaxy known for three decades. Eventually, however, it lost its crown to a blue galaxy three times farther away in Eridanus. The galaxy, named SBS 0335-052W, is part of a larger, oxygen-poor system and is creating new stars. In 2005, Yuri Izotov, Natalia Guseva (Main Astronomical Observatory, Kiev, Ukraine), and Thuan proclaimed this upstart galaxy the new champion. It has an oxygen level of 7.13.

Blue But Not New

Astronomers once suspected that I Zwicky 18 might be a galactic infant, having formed all of its stars recently. Indeed, the galaxy derives its blue hue from bright, massive newborn stars, which wouldn't have had time to enrich their surroundings with oxygen.

Deeper observations, however, have revealed much older stars. Now astronomers think that the galaxy stands out from the pack because nearly metal-free gas is falling onto it from beyond. This incoming gas diluted the galaxy's own, lowering the oxygen level and triggering the rash of starbirth that lights it up today. This idea also explains why the galaxy, though only somewhat less luminous than the Small Magellanic Cloud, has a sharply lower oxygen level.

In 2012, Riccardo Giovanelli (Cornell University) and colleagues used a new technique to find a new type of oxygenpoor star-forming galaxy: a dwarf in Leo much dimmer than I Zwicky 18 that likely owes its paucity of oxygen simply to a dearth of stars that create the element. The astronomers found the little galaxy's hydrogen gas before they saw its stars, picking up its 21-centimeter radiation (see page 14) with the Arecibo radio telescope. The galaxy has the same low level of oxygen as I Zwicky 18, so the astronomers christened the dwarf Leo P, the "P" standing for "pristine."

Leo P is a mere 5 million light-years away. That puts it just beyond the edge of the Local Group, the collection of more than a hundred nearby galaxies--most of them dwarfs much dimmer than the Magellanic Clouds--which two giant galaxies, the Andromeda Galaxy and the Milky Way, govern. In fact, Leo P is the nearest oxygen-poor star-forming galaxy ever found. Emitting less than 1% as much light as I Zwicky 18, Leo P is also one of the least luminous galaxies ever seen engaged in starbirth; most galaxies this dim have run out of gas and lost their ability to make stars. Yet the little galaxy has certainly been around a long time, because Hubble has detected in it RR Lyrae stars, metal-poor pulsators that are more than 10 billion years old. The galaxy's secret? It has wisely avoided giants like our own Milky Way, which steal gas from smaller galaxies.

Leo P faces two challenges in trying to raise its oxygen level: It has few stars to forge the element and lacks the gravitational clout to hold on to it when they do. "It's tough being a little galaxy," says Kristen McQuinn (University of Texas, Austin). She estimates that Leo P has lost 95% of the oxygen that its stars created, because when they blew up, they shot the element away so fast the galaxy couldn't retain it.

In 2015, another galaxy first came to attention via its radio-emitting gas. When Alec Hirschauer, John Salzer (Indiana University), and colleagues examined an optical spectrum, they discovered the galaxy was a record-breaker. "It was obvious that it was very metal-poor," Salzer says. "My first reaction was: Well, we'd better be really sure of this." A second spectrum confirmed the first, yielding an oxygen level of only 7.02. Because this galaxy lies in Leo Minor, the Lesser Lion, the discoverers dubbed the dwarf Leoncino, which is Italian for "little lion."

In 2016, Tiffany Hsyu (University of California, Santa Cruz) and her colleagues spotted a small blue galaxy in Ursa Major they named the Little Cub for its location in the constellation representing the Great Bear. This galaxy might orbit the beautiful barred spiral NGC 3359, and the interaction between the two galaxies might have sparked the birth o: stars in the smaller galaxy. Its oxygen level is 7.13.

"These oxygen-poor galaxies give us a good idea of what star formation might have been like in the early universe, because the early universe was much more metal-poor than the universe we live in today," Hsyu says. The smallest star-forming galaxies, such as Leo P, lack oxygen for the same reason the first galaxies after the Big Bang did: They haven't made many stars. Even in larger dwarf galaxies like I Zwicky 18, where infalling gas has diluted native gas, star-forming conditions should mimic those in the primordial galaxies.

The galaxies certainly abound with the chief requirement for star formation: gas. In the Milky Way's disk and bulge, stars outweigh the gas, but in Leo P it's the other way around In Leoncino, the gas is 50 times more massive than the stars, and the Little Cub has 100 times more gas than stars. These galaxies possess so little oxygen in part because they've converted so little of their gas to stars.

Despite all their gas, oxygen-poor galaxies have precious little of another ingredient important in the Milky Way's star formation: dust. That's not surprising, because dust grains consist mostly of carbon, oxygen, magnesium, silicon, and iron, all of which are products of stars and are therefore scarce in these galaxies. In the Milky Way's disk, gas makes up 99% of interstellar matter and dust 1%. In I Zwicky 18, however, dust accounts for a mere 0.001% of interstellar matter.

When it exists, dust affects star formation. Dust grains darken star-forming clouds, shielding them from harsh radiation, and also promote star formation by emitting far-infrared radiation, which carries heat away from gas clouds. So do carbon and oxygen atoms. Cooler clouds have less thermal pressure, a force that counteracts gravity and can prevent a cloud from collapsing and becoming a star. Thus, with a shortage of dust and carbon and oxygen atoms to cool them, gas clouds in oxygen-poor galaxies may have to be more massive in order for gravity to take over and force collapse, giving rise to a greater proportion of massive stars. The same thing presumably happened in the universe's first galaxies.

Dust grains also serve as platforms on which atoms can meet one another and make molecules, such as molecular hydrogen, the most abundant molecule in the Milky Way's gas. Big clouds of molecular hydrogen fuel many of our galaxy's star-forming regions. No one has ever detected any molecular gas in a galaxy as oxygen-poor as I Zwicky 18.

A New Record-Breaker

In 2017, Izotov and Thuan's team discovered the current champion: a starburst galaxy in Lynx that's 620 million light-years distant--10 times farther than I Zwicky 18--bearing the prosaic name J0811+4730. The blue galaxy spawned 80% of its stars during the past few million years, and its oxygen abundance is a mere 6.98, the lowest ever seen. That's just 1.7% the level of oxygen in the Sun.

Still, the galaxy emerged only after the astronomers searched a million spectra from the Sloan Digital Sky Survey. Thuan once hoped to find truly primordial galaxies containing no oxygen at all, but he now wonders whether he'll ever succeed. "It's very, very difficult," he says. "I've spent nearly 40 years of my professional life trying to find these things, but so far to no avail."

Perhaps, he adds, an ancient generation of massive stars showered the whole universe with metals, setting a minimum oxygen level in any galaxy that arose. As a result, galaxies with much less oxygen than the current record-breaker simply might not exist.

Little Galaxies, Little Particles

With all the new discoveries, the field is blossoming. That may be good news not just for astronomers but also for particle physicists, because these galaxies could reveal how many types, or "flavors," of neutrinos there are.

Neutrinos are tiny neutral particles that whiz through space--and our bodies--at nearly the speed of light. Hence their name: Neutrino means "little neutral one" in Italian. Physicists recognize three flavors: electron, muon, and tau. If a fourth flavor exists, however, it would have affected the nuclear reactions during the universe's first three minutes and raised the amount of helium the Big Bang produced.

But how much helium did the Big Bang actually make? We can't look to Earth for the answer, because the scant helium we have here has nothing to do with the Big Bang. Instead, the lighter-than-air gas that lifts blimps and balloons stems from the radioactive decay of heavy elements such as thorium and uranium.

In contrast, most of the helium on the Sun's surface did come from the Big Bang. Unfortunately, countless stars that lived and died before the Sun's birth also contributed. Astronomers could try and subtract the stellar contribution from the amount of helium the Sun was born with--about 27% by weight--and thereby extrapolate all the way back to the Big Bang abundance.

"Extrapolate: We hate that word," says Evan Skillman (University of Minnesota). Far better to measure the helium level in star-making galaxies so pristine their chemical composition almost reflects that of the early universe. "It's really the only way we have to estimate the primordial helium abundance," he says. In these galaxies, new stars emit ultraviolet light that strips electrons from helium atoms in the interstellar gas; electrons that rejoin the helium atoms create spectral lines revealing the element's abundance. In contrast, the many dim dwarf galaxies that orbit the Milky Way, some of which have still less oxygen, provide no such information because they lack both gas and hot young stars (S&T: Mar. 2017, p. 16).

Past measurements of I Zwicky 18 and its kin have found that helium made up a quarter of the chemical elements emerging from the Big Bang, but different astronomers derive slightly different numbers. In 2014, Izotov and Thuan's team obtained a primordial helium abundance of 25.5%, which is so high it suggests the number of neutrino types is more likely to be four than three. But Skillman's team favors a lower primordial helium level, around 24.5%, which implies three neutrino flavors, in accordance with standard physics.

Now, having spotted three new oxygen-poor star makers in just the past three years, astronomers have three more chances to derive the primordial helium abundance. If that number comes in on the high side, physicists should be seeking a fourth type of neutrino. Thus these odd little galaxies might tell us not only what the first galaxies looked like but also how many flavors of "little neutral particles" there are zipping through the cosmos.

* KEN CROSWELL earned his PhD for studying a decidedly oxygen-rich galaxy, the Milky Way. His book about our galaxy, The Alchemy of the Heavens, was a Los Angeles Times Book Prize finalist. He has also written for National Geographic, New Scientist, and Scientific American.

What Is Dust?

Cosmic dust grains are small particles that range in size from about 10 to 100 nm. They're mostly silicate--or carbon-based and form in the atmospheres of aging red giant stars and in supernovae. Dust absorbs light with wavelengths similar to or smaller than its grain size, then re-emits it at infrared wavelengths. Its presence shields interstellar molecules from high-energy radiation and enables protostars to radiate away excess energy.

Caption: PRIMORDIAL PRETENDER Despite hosting a collection of newly formed stars, the dwarf galaxy I Zwicky 18 (larger blue object) has an anemic level of star-produced oxygen. It appears here with a companion called Component C (upper left), which might be a separate galaxy.

Caption: IS THAT A GALAXY? In 2012, astronomers discovered the closest oxygen-poor star-forming galaxy, Leo P (the P is for "pristine"). It lies just beyond the fringe of the Local Group of galaxies and has the same oxygen abundance as I Zwicky 18 but is much smaller and fainter.

Caption: LITTLE LION The galaxy AGC 198691, nicknamed Leoncino, has an oxygen abundance of only 7.02 and abounds with gas. It's one of the most oxygen-poor star-forming galaxies known.

Caption: BARELY THERE The Little Cub satellite (zoomed-in image inset) of NGC 3359 isn't much to look at in optical and infrared wavelengths. It has 100 times more gas than stars, though, and shows up in radio observations of neutral hydrogen (contours).

Caption: STELLAR NURSERY Massive stars make oxygen, littering their natal neighborhoods and stars born there. This false-color Hubble image of the Carina Nebula shows oxygen in blue, hydrogen and nitrogen in green, and sulfur in red. Oxygen glows primarily where young stars have carved out the clouds.

Caption: ONE MORE FOR LEO The dwarf galaxy DDO 68 (also known as UGC 5340) Is a ragged collection of stars and gas clouds. At 40 million light-years away, it's one of the closer oxygen-poor star-forming galaxies. This image combines visible and infrared observations.
Shown below are parameters for our home galaxy, one of
its small neighbors (the Small Magellanic Cloud), and the
oxygen-poor galaxies discussed in the article. Solar values
serve as the benchmark for the oxygen abundances, and masses
are approximate to within a factor of 10. Although the dwarfs
churn out fewer stars per year than the Milky Way (second row),
they also have a lot less mass to work with (bottom). The Milky
Way's gas mass here includes the disk and bulge.

Small         I Zwicky    SBS     Leo P
Magellanic       18      0335-
Cloud                    052W

15%             2.6%     2.3%     2.6%

0.04            0.2      0.01    0.00004

Small         Leoncino   Little   J0811+
Magellanic                Cub      4730

15%             1.8%      2.3%     1.7%

0.04           0.001     0.0008    0.5

Star-Forming Galaxies with the Lowest Oxygen Levels

Galaxy           Constellation   Oxygen Abundance
                                 (hydrogen = 12) *

J0811+4730           Lynx              6.98
Leoncino           Leo Minor           7.02
SBS 0335-052W      Eridanus            7.13
Little Cub        Ursa Major           7.13
I Zwicky 18       Ursa Major           7.17
Leo P                 Leo              7.17
DDO 68                Leo              7.20

Galaxy           Approx. Distance

J0811+4730         620 million
Leoncino           40 million?
SBS 0335-052W      190 million
Little Cub         60 million?
I Zwicky 18         60 million
Leo P               5 million
DDO 68              40 million

* Oxygen abundances are generally the average of the values
for multiple regions within a single galaxy. Question marks
indicate a notably high uncertainty in distance.
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Author:Croswell, Ken
Publication:Sky & Telescope
Date:Mar 18, 2018
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