Black holes can suppress star formation, but nine nearby galaxies reveal how their outflows could also help trigger the birth of new stars.<\/p>\n
In the usual account of a growing black hole, its activity can suppress star formation in the galaxy around it.<\/p>\n
The gas gets heated, or shoved out of the disk, and little gas is left to build new stars.<\/p>\n
Nine nearby galaxies show black hole activity and star formation at once.<\/p>\n
Rings and arcs of new stars circle each black hole a few thousand light-years out, while gas shocked by its outflows fills the space closer in.<\/p>\n
What turns that into a finding is the repetition: the same three-part arrangement showed up in all nine.<\/p>\n
A feeding black hole pulls gas in<\/a>, and it throws material back out. <\/p>\n Peixin Zhu, a graduate student at the Center for Astrophysics<\/a> in Cambridge, Massachusetts, led the work that revealed both processes at once.<\/p>\n \u201cOnce we resolved them, we could see that they not only accrete things, but they also eject things,\u201d Zhu said. \u201cThe injection and accretion are linked with each other.\u201d<\/p>\n Shocks confused the old measurements<\/p>\n Astronomers count new stars in a distant galaxy by measuring a red glow that hydrogen gives off when young stars light it up. <\/p>\n They judge how hard the black hole is feeding from a separate glow of oxygen. Shocked gas gives off both.<\/p>\n In nearby galaxies of this kind, shocks can supply about 35% of the hydrogen glow and up to about half the oxygen. <\/p>\n On the flat, two-axis charts astronomers have used for decades, shocked gas and black-hole-lit gas land in the same place.<\/p>\n So any measurement of star formation beside an active black hole carried an unknown amount of shock inside it. <\/p>\n Astronomers were stuck on the question they most wanted answered: does a black hole starve its galaxy<\/a> or feed it?<\/p>\n One pattern across nine galaxies<\/p>\n Zhu built a three-way test with Lisa Kewley of the Center for Astrophysics and Ralph Sutherland of the Australian National University<\/a>. <\/p>\n Two of its axes use emission-line ratios. The third measures how broadly an oxygen emission line spreads across wavelength, which tracks how fast the gas is moving.<\/p>\n Run pixel by pixel, the test splits every spot in the frame three ways. It reports how much of that light came from young stars, how much from the black hole, and how much from shocks. <\/p>\n The team ran it on archived observations from the Very Large Telescope<\/a> in Chile.<\/p>\n Those star-forming rings are 0.8 to 6 kiloparsecs out from each center, or roughly 2,600 to 20,000 light-years. <\/p>\n The Sun orbits about 26,000 light-years from the middle of the Milky Way, so every one of these rings would fit inside our own orbit.<\/p>\n Each black hole also lights up gas in two opposite cones reaching 1 to 5 kiloparsecs. <\/p>\n Shocked gas fills two more zones: one wrapped around the center, and one scattered through the star-forming rings themselves.<\/p>\n Why do the shocks go sideways?<\/p>\n Those cones mark the direction a black hole throws its energy. The fastest shocked gas often extends across that direction instead, reaching about a kiloparsec to either side.<\/p>\n Zhu said the shocks \u201calways go perpendicular to where the black hole\u2019s injected outflows go.\u201d<\/p>\n \u201cIt is very common, and we see it consistently appearing across the whole nine galaxies,\u201d she added.<\/p>\n All nine galaxies have radio-detected jets pointed broadly along the cones. In simulations, a black hole jet<\/a> pushing through clumpy gas blows a bubble, and the bubble shocks gas at its edges, including well off to the side of the jet. <\/p>\n Astronomers had already seen that sideways structure in X-rays from single galaxies such as IC 5063 and NGC 5728.<\/p>\n The map was checked in X-rays<\/p>\n NASA\u2019s Chandra X-ray Observatory<\/a> has watched every galaxy in this sample, eight of the nine for more than 100,000 seconds. That\u2019s over a day of staring at one target.<\/p>\n The X-ray observations backed up the team\u2019s map in two ways. X-rays linked to the active black hole appeared in the same regions that the visible-light analysis identified as black-hole-lit gas.<\/p>\n Hot X-ray-emitting gas also appeared in the regions to either side of the black hole\u2019s outflow cones, matching where the team had mapped shocked gas.<\/p>\n Bars may build the rings instead<\/p>\n Seven of the nine galaxies carry a bar, a straight bridge of stars across the middle. A bar drags gas inward and parks it at a particular radius, which builds a ring of star-making clouds<\/a>. That story doesn\u2019t need a black hole at all.<\/p>\n The rival story does. Shocks compress gas until it collapses into stars, and in jet simulations that happens within a kiloparsec of the jet.<\/p>\n Later, once the bubble has had time to squeeze the whole disk, it happens again out at 3 to 6 kiloparsecs. Both ranges overlap the radii in the maps.<\/p>\n \u201cWe\u2019re seeing that black holes are not just consuming material at the centers of galaxies, but they\u2019re actively reshaping their surroundings,\u201d said Kewley, who directs the Center for Astrophysics.<\/p>\n Star motion has not been measured<\/p>\n Stars formed in a bar-fed ring should circle the galaxy the way everything else does. <\/p>\n Stars formed in gas that a jet or a wind had just shocked should carry a mark of the shove, moving outward faster than a circular orbit allows.<\/p>\n Reading that motion takes finer spectral detail than these observations carry, so that\u2019s a later paper. Two galaxies complicate the jet story as well.<\/p>\n NGC 1386 and NGC 3081 have weaker jets, below the power range those simulations cover. So jets<\/a> may not be what shocked their gas at all.<\/p>\n Winds from the black hole could be doing it instead, alone or alongside the weaker jet.<\/p>\n Either way, the pattern is on record in nine galaxies now, and the cause of each piece of it is still open.<\/p>\n One measurement could help settle the question. Tracking how the stars in those rings move could reveal whether the black hole played a role in forming them.<\/p>\n The full study was published in the journal The Astrophysical Journal<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? 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