发现了一个意想不到的双类星体——一对合并的星系在碰撞过程中点燃了黑洞

这位艺术家的概念图展示了两个类星体在混乱的合并过程中位于两个星系中心的明亮光芒。 两个星系之间的拉锯战引发了恒星诞生的风暴。 类星体是来自遥远星系中心的强光的明亮灯塔。 它们由超大质量黑洞提供动力，这些黑洞贪婪地吞噬着坠落的物质。 这种滋养的狂热释放出大量的辐射，其亮度超过了宿主星系中数十亿颗恒星的集体光芒。 再过几千万年，黑洞和它们的星系，以及类星体对，将合并，形成一个质量更大的黑洞。 图片来源：NASA、ESA 和 Joseph Olmsted (STScI)

一对合并的星系在碰撞过程中点燃黑洞

类星体是宇宙中最亮的星体之一。 它散落在天空中，闪耀着超过一千亿颗星辰的浓郁。 就像 7 月 4 日明亮的大气耀斑一样，它们的发光时间相对较短——在宇宙时间尺度上。 那是因为它们由超大质量黑洞提供动力，这些黑洞会吞噬大量加热到高温的气体和尘埃。 但类星体自助餐只能持续这么久。

类星体的这种转瞬即逝的特性帮助天文学家找到了两个正在相互碰撞的类星体。 它嵌入在 100 亿年前相互碰撞的一对星系中。 在遥远的宇宙中很难找到这样充满活力的二人组。 这一发现提供了宇宙在很久以前是多么不稳定的线索，当时星系频繁碰撞，黑洞被近距离接触产生的碎片和喷流吞没。

由于这两个类星体随着燃料蜡烛的流入减弱而以不同的速率闪烁，因此它们已被确定为太空中发生的异常活动。 哈勃望远镜放大并清楚地解析了图像，宿主星系也是如此。

哈勃太空望远镜拍摄的一对类星体在宇宙诞生仅 30 亿年时就已存在。 它嵌入在一对碰撞的星系中。 类星体之间的距离小于一个星系的大小。 类星体穿过凶猛的超大质量黑洞，当它们窒息气体、尘埃和任何其他在其引力范围内的东西时，这些黑洞会爆发出凶猛的能量喷泉。 黑洞最终会合并。 图片来源：NASA、ESA、Yu-Ching Chen (UIUC)、Hsiang-Chih Huang (IAS)、Nadja Zakamska (JHU)、Yu Chen (UIUC)

哈勃太空望远镜意外发现遥远宇宙中的双类星体

早期的宇宙是一个动荡不安的地方，星系相互碰撞，甚至合并在一起。 使用[{” attribute=””>NASA’s Hubble Space Telescope and other space and ground-based observatories, astronomers investigating these developments have made an unexpected and rare discovery: a pair of gravitationally bound quasars, both blazing away inside two merging galaxies. They existed when the universe was just 3 billion years old.

Quasars are bright objects powered by voracious, supermassive black holes blasting out ferocious fountains of energy as they engorge themselves on gas, dust, and anything else within their gravitational grasp.

“We don’t see a lot of double quasars at this early time in the universe. And that’s why this discovery is so exciting,” said graduate student Yu-Ching Chen of the University of Illinois at Urbana-Champaign, lead author of this study.

Finding close binary quasars is a relatively new area of research that has just developed in the past 10 to 15 years. Today’s powerful new observatories have allowed astronomers to identify instances where two quasars are active at the same time and are close enough that they will eventually merge. 

There is increasing evidence that large galaxies are built up through mergers. Smaller systems come together to form bigger systems and ever larger structures. During that process there should be pairs of supermassive black holes formed within the merging galaxies. “Knowing about the progenitor population of black holes will eventually tell us about the emergence of supermassive black holes in the early universe, and how frequent those mergers could be,” said Chen.

This compass image shows a Hubble Space Telescope photograph of a pair of quasars that existed when the universe was just 3 billion years old. They are embedded inside a pair of colliding galaxies. The quasars are separated by less than the size of a single galaxy. Quasars are powered by voracious, supermassive black holes blasting out ferocious fountains of energy as they engorge themselves on gas, dust, and anything else within their gravitational grasp. The black holes will eventually merge. Credit: NASA, ESA, Yu-Ching Chen (UIUC), Hsiang-Chih Hwang (IAS), Nadia Zakamska (JHU), Yue Shen (UIUC)

We’re starting to unveil this tip of the iceberg of the early binary quasar population,” said Xin Liu of the University of Illinois at Urbana-Champaign. “This is the uniqueness of this study. It is actually telling us that this population exists, and now we have a method to identify double quasars that are separated by less than the size of a single galaxy.”

This was a needle-in-haystack search that required the combined power of NASA’s Hubble Space Telescope and the W.M. Keck Observatories in Hawaii. Multi-wavelength observations from the International Gemini Observatory in Hawaii, NSF’s Karl G. Jansky Very Large Array in New Mexico, and NASA’s Chandra X-ray Observatory also contributed to understanding the dynamic duo. And, ESA (European Space Agency)’s Gaia space observatory helped identify this double quasar in the first place. 

“Hubble’s sensitivity and resolution provided pictures that allow us to rule out other possibilities for what we are seeing,” said Chen. Hubble shows, unequivocally, that this is indeed a genuine pair of supermassive black holes, rather than two images of the same quasar created by a foreground gravitational lens. And, Hubble shows a tidal feature from the merging of two galaxies, where gravity distorts the shape of the galaxies forming two tails of stars.

However, Hubble’s sharp resolution alone isn’t good enough to go looking for these dual light beacons. The researchers enlisted Gaia, which launched in 2013, to pinpoint potential double-quasar candidates. Gaia measures the positions, distances, and motions of nearby celestial objects very precisely. But in a novel technique, it can be used to explore the distant universe. Gaia’s huge database can be used to search for quasars that mimic the apparent motion of nearby stars. The quasars appear as single objects in the Gaia data because they are so close together. However, Gaia can pick up a subtle, unexpected “jiggle” that mimics an apparent change in position of some of the quasars it observes. 

In reality, the quasars aren’t moving through space in any measurable way. Instead, their jiggle could be evidence of random fluctuations of light as each member of the quasar pair varies in brightness on timescales of days to months, depending on their black hole’s feeding schedule. This alternating brightness between the quasar pair is similar to seeing a railroad crossing signal from a distance. As the lights on both sides of the stationary signal alternately flash, the sign gives the illusion of “jiggling.”

Another challenge is that because gravity warps space like a funhouse mirror, a foreground galaxy could split the image of a distant quasar into two, creating the illusion it was really a binary pair. The Keck telescope was used to make sure there was no lensing galaxy in between us and the suspected double quasar.

Because Hubble peers into the distant past, this double quasar no longer exists. Over the intervening 10 billion years, their host galaxies have likely settled into a giant elliptical galaxy, like the ones seen in the local universe today. And, the quasars have merged to become a gargantuan, supermassive black hole at its center. The nearby giant elliptical galaxy, M87, has a monstrous black hole weighing 6.5 billion times the mass of our Sun. Perhaps this black hole was grown from one or more galaxy mergers over the past billions of years.

The upcoming NASA Nancy Grace Roman Space Telescope, having the same visual acuity as Hubble, is ideal for binary quasar hunting. Hubble has been used to painstakingly take data for individual targets. But Roman’s very wide-angle infrared view of the universe is 200 times larger than Hubble’s. “A lot of quasars out there could be binary systems. The Roman telescope can do huge improvements in this research area,” said Liu.

The results will be published in the April 5 journal Nature.

Reference: “A close quasar pair in a disk–disk galaxy merger at z = 2.17” by Yu-Ching Chen, Xin Liu, Adi Foord, Yue Shen, Masamune Oguri, Nianyi Chen, Tiziana Di Matteo, Miguel Holgado, Hsiang-Chih Hwang and Nadia Zakamska, 5 April 2023, Nature.
DOI: 10.1038/s41586-023-05766-6

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.