Sky and Telescope - April 2018 - 24

Primordial Proxies

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.

A PR I L 2 018 * SK Y & TELESCOPE

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.
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.

Sky and Telescope - April 2018

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