距离2300万光年的银河烟火

A galaxy about 23 million light years away is the site of impressive, ongoing fireworks. Rather than paper, powder and fire, this galactic light show involves a giant black hole, shock waves and vast reservoirs of gas.

This galactic fireworks display is taking place in NGC 4258, also known as M106, a spiral galaxy like our own Milky Way. This galaxy is famous, however, for something that our galaxy doesn’t have – two extra spiral arms that glow in X-ray, optical and radio light. These features, or anomalous arms, are not aligned with the plane of the galaxy, but instead intersect with it.

The anomalous arms are seen in this new composite image, where X-rays from NASA’s Chandra X-ray Observatory are blue, radio data from the NSF’s Karl Jansky Very Large Array are purple, optical data from NASA’s Hubble Space Telescope are yellow and infrared data from NASA’s Spitzer Space Telescope are red.

A new study made with Spitzer shows that shock waves, similar to the sonic booms from supersonic planes, are heating large amounts of gas – equivalent to about 10 million suns. What is generating these shock waves? Researchers think that the supermassive black hole at the center of NGC 4258 is producing powerful jets of high-energy particles. These jets strike the disk of the galaxy and generate shock waves. These shock waves, in turn, heat the gas – composed mainly of hydrogen molecules – to thousands of degrees.

The Chandra X-ray image reveals huge bubbles of hot gas above and below the plane of the galaxy. These bubbles indicate that much of the gas that was originally in the disk of the galaxy has been heated and ejected into the outer regions by the jets from the black hole.

The ejection of gas from the disk by the jets has important implications for the fate of this galaxy. Researchers estimate that all of the remaining gas will be ejected within the next 300 million years – very soon on cosmic time scales – unless it is somehow replenished. Because most of the gas in the disk has already been ejected, less gas is available for new stars to form. Indeed, the researchers used Spitzer data to estimate that stars are forming in the central regions of NGC 4258, at a rate which is about ten times less than in the Milky Way galaxy.

The European Space Agency’s Herschel Space Observatory was used to confirm the estimate from Spitzer data of the low star formation rate in the central regions of NGC 4258. Herschel was also used to make an independent estimate of how much gas remains in the center of the galaxy. After allowing for the large boost in infrared emission caused by the shocks, the researchers found that the gas mass is ten times smaller than had been previously estimated.

Because NGC 4258 is relatively close to Earth, astronomers can study how this black hole is affecting its galaxy in great detail.

Image Credit: X-ray: NASA/CXC/Caltech/P.Ogle et al; Optical: NASA/STScI; IR: NASA/JPL-Caltech; Radio: NSF/NRAO/VLA

距离我们约有2300万光年的星系是持续不断的令人印象深刻的焰火场所。该星系的灯光秀不是纸、火药，而是一个巨大的黑洞，冲击波和大量的气体。

这场银河烟花汇演是在NGC 4258（也称为M106）中进行的，该螺旋星系就像我们自己的银河系一样。但是，这个银河系以我们的银河系所没有的东西而闻名-两个额外的螺旋臂可以发出X射线、光学和射电光中发光。这些特征或异常臂并不是与星系平面对齐的，而是与之相交的。

在这张新的合成图像中可以看到异常臂，其中来自NASA钱德拉X射线天文台的X射线是蓝色的，来自NSF的卡尔·杨斯基甚大阵列的的无线电数据是紫色的，来自NASA哈勃太空望远镜的光学数据是黄色的和来自NASA的斯皮策太空望远镜的红外数据是红色的。

斯皮策太空望远镜进行的一项新研究表明，与超音速飞机的音爆相似，冲击波正在加热大量气体-相当于大约1000万个太阳。是什么产生这些冲击波？研究人员认为，NGC 4258中心的超大质量黑洞正在产生强大的高能粒子喷流。这些喷流撞击星系盘并产生冲击波。这些冲击波又将主要由氢分子组成的气体加热到数千度。

钱德拉X射线图像显示了星系平面上方和下方的巨大热气泡。这些气泡表明，最初存在于星系圆盘中的许多气体已被加热，并被来自黑洞的喷流喷射到外部区域。

喷流从圆盘中喷射出的气体对这个星系的命运有着重要的影响。研究人员估计，除非有某种补充，否则所有剩余的气体将在未来的3亿年内（以宇宙时间尺度上很快）喷出。由于星盘中的大部分气体已经被喷射出来，所以可供新恒星形成的气体就少了。实际上，研究人员使用斯皮策太空望远镜数据估计了NGC 4258中心区域的恒星形成，其速度约为银河系中的十倍。

欧洲航天局的赫歇尔空间天文台也证实了斯皮策望远镜数据对NGC 4258中心区域低恒星形成率的估计。赫歇尔还被用来独立估计星系中心还剩下多少气体。在考虑了由震动引起的红外辐射的大幅增加后，研究人员发现气体质量比之前估计的要小十倍。

由于NGC 4258距离地球较近，因此天文学家可以详细研究这个黑洞如何影响其星系。

影像来源：X-ray: NASA/CXC/Caltech/P.Ogle et al; Optical: NASA/STScI; IR: NASA/JPL-Caltech; Radio: NSF/NRAO/VLA