My first Magazine Sky & Telescope - 04.2019

By using the Hubble Space Telescope to observe seven hot stars in M33’s disk, the researchers discerned triply ionized silicon ions in this gas that were absorbing some of the stars’ ultraviolet light, and the Doppler shift of these spectral lines indicated the gas was falling into the galaxy. M33 mints stars at the rate of 0.2 to 0.5 solar masses a year, but the observa- tions indicate that the side of the galaxy facing us accretes about 2.9 solar masses of gas a year. “It’s pretty surprising that we ﬁ nd a number that’s ﬁ ve to ten times higher,” says Zheng. Perhaps gas falls into galaxies sporadically, and this galaxy is currently enjoying a downpour. 1.0 0.8 0.4 x bo d se o Cl y or e th 0.6 Fraction galaxies known as lenticulars may be deceased spirals. Like spirals, lenticular galaxies have disks, but they possess little gas and have no spiral arms. Still, spiral galaxies have endured for some 10 billion years. Their success suggests that most have mastered meth- ods to maintain their spiral structure even as they change their gas into stars. Furthermore, modern simulations of how giant galax- ies grow over time predict that gas should rain onto them, replenishing their supply. This gas has three main sources. The ﬁ rst is fairly pristine gas from the cosmic web, the net- work connecting galaxy groups and clusters throughout the universe. The second is gas from smaller galaxies, which expel it or lose it when straying close to the giants. And the third source of gas comes from the galaxy’s own stars, which shed material when they die, sometimes shooting it high above the disk before the gas falls back down. Long before such simulations existed, astronomers had seen signs that the Milky Way might be acquiring fresh gas. In the early 1960s, radio astronomers spotted high-velocity clouds made of neutral hydrogen; decades later, distance mea- surements revealed that these clouds reside in our galaxy’s halo. In the early 1970s, Richard Larson (Yale University) began contemplating how such clouds would alter the galactic disk if they fell into it. He says, “I immediately realized: Hey, this solves the G-dwarf problem.” The G-dwarf problem describes the distribution of iron levels among long-lived stars, like the Sun, that inhabit the Milky Way’s disk. The Sun has a fairly high level of iron, and most disk stars have iron-to-hydrogen ratios similar to the Sun’s; iron-poor stars are scarce in the galactic disk. But if the disk had started with vast amounts of iron-poor gas, then lots of iron-poor stars should have arisen before supernova explo- sions could inject enough iron to enrich all that gas. Fur- thermore, if the galaxy had never received more gas, the iron levels would just keep going up with time. As a result, disk stars should have a broad range of iron levels, from iron-poor to iron-rich, contrary to observations. Larson realized that if the disk had actually started out less massive, with just a small amount of iron-poor gas, super- novae would have quickly boosted its iron level, making all later stellar generations iron-rich. Furthermore, if the infall of iron-poor gas balanced the star-formation rate, then the exploding stars would enrich the gas as it fell in so that all stars born from it became about equally iron-rich, agreeing with observations. Still, astronomers have struggled to see actual signs of gas accreting onto nearby galaxies. In 2016, Yong Zheng, then at Columbia University, and her colleagues detected gas fall- ing onto M33, a spiral galaxy much smaller than our own. 0.2 0.0 0.1 0.2 0.4 0.6 1.0 Metallicity (as a function of current interstellar level) p THE G-DWARF PROBLEM Observations in the mid-20th century (red line) revealed that Sun-like stars in the Milky Way’s disk all had similar fractions of heavy elements — instead of the wide spread expected if the galaxy existed as a “closed box” that slowly built up its metal content with time (solid gray line). Infalling gas mixed with material in the disk cre- ates stars that better match reality (dotted line). NGC 5195 20,000 ly NGC 5195 20,000 ly u ATOMIC VS. MOLECULAR In most spirals, such as the Whirlpool, most of the atomic gas (left) resides on the disk’s outskirts, whereas most of the molecular gas (right), the type that forms stars, lies closer to the galaxy’s heart. Also included is the Whirlpool’s companion galaxy, NGC 5195 (top of both images). Redder colors indicate more gas. sk yandtele scope.com • A PR I L 2 019 17

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