http://www.skyandtelescope.com.au 29LEOP:NASA/ESA/KRISTENMCQUINN(UNIV.OFTEXAS,AUSTIN)
;LEONCINO:NASA/A.HIRSCHAUER&J.SALZER(INDIANAUNIV.)/J.CANNON(MACAL
ESTERCOLLEGE)/
K.MCQUINN(UNIV.OFTEXAS,AUSTIN); LITTLECUB:IMAGE:SDSSCOLLABORATION,CONTOURS:T.HSYUETAL./ ASTROPHYSICAL JOURNAL LETTERS2017from 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 oxygen-
poor 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-centimetre radiation 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 LesserLion, 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 of 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,2 ̋SBARELY THERE The Little Cub satellite galaxy (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).WWIS 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.WLITTLE 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. (continued on page 32)