For this image, the NASA/ESA Hubble Space Telescope turned its powerful eye toward an emission-line galaxy called NGC 3749.

When astronomers explore the contents and constituent parts of a galaxy somewhere in the universe, they use various techniques and tools. One of these is to spread out the incoming light from that galaxy into a spectrum and explore its properties. This is done in much the same way as a glass prism spreads white light into its constituent wavelengths to create a rainbow. By hunting for specific signs of emission from various elements within a galaxy’s spectrum of light —so-called emission lines — or, conversely, the signs of absorption from other elements — so-called absorption lines — astronomers can start to deduce what might be happening within.

If a galaxy’s spectrum shows many absorption lines and few emission lines, this suggests that its star-forming material has been depleted and that its stars are mainly old, while the opposite suggests it might be bursting with star formation and energetic stellar newborns. This technique, known as spectroscopy, can tell us about a galaxy’s type and composition, the density and temperature of any emitting gas, the star formation rate, or how massive the galaxy’s central black hole might be.

While not all galaxies display strong emission lines, NGC 3749 does. It lies over 135 million light-years away and is moderately luminous. The galaxy has been used as a “control” in studies of especially active and luminous galaxies — those with centers known as active galactic nuclei, which emit copious amounts of intense radiation. In comparison to these active cousins, NGC 3749 is classified as inactive, and has no known signs of nuclear activity.

Text credit: ESA (European Space Agency)
Image credit: ESA/Hubble & NASA, D. Rosario et al.

为了这张图片,美国宇航局/欧洲航天局的哈勃太空望远镜将其强大的目光转向了一个名为NGC 3749的发射线星系



虽然不是所有的星系都有很强的发射线,但NGC 3749却有。它位于1.35亿光年之外,中等亮度。该星系一直被用作研究特别活跃和发光的星系的“控制体”,这些星系的中心被称为活跃星系核,它们发射大量的强辐射。与这些活跃的表亲相比,NGC 3749被归类为不活跃的,并且没有已知的核活动迹象。

图片来源:ESA/Hubble & NASA, D. Rosario等。

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