Sky and Telescope - April 2018 - 25

LEO P: N ASA / ESA / K RISTEN MCQ UINN (UNIV. OF TE X AS, AUSTIN); LEONCINO: N ASA / A . HIRSCH AUER & J. SA L ZER
(INDIANA UNIV.) / J. CA NNON (MACALESTER COLLEGE ) / K. MCQUINN (UNIV. OF TE X AS, AUSTIN);
LIT TLE CUB: IM AG E: SDSS COLL ABOR ATION, CONTOURS: T. HSY U E T AL. / ASTR OPHYSICAL JOUR NAL LE T TERS 2017

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

tt 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.
t 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.

2˝

p 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).

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 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 starforming 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
(continued on page 28)
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Sky and Telescope - April 2018

Table of Contents for the Digital Edition of Sky and Telescope - April 2018