This NASA/ESA Hubble Space Telescope image features a galaxy, NGC 2775, that’s hard to categorize.
ESA/Hubble & NASA, F. Belfiore, J. Lee and the PHANGS-HST Team
This NASA/ESA Hubble Space Telescope image features a galaxy that’s hard to categorize. The galaxy in question is NGC 2775, which lies 67 million light-years away in the constellation Cancer (the Crab). NGC 2775 sports a smooth, featureless center that is devoid of gas, resembling an elliptical galaxy. It also has a dusty ring with patchy star clusters, like a spiral galaxy. Which is it: spiral or elliptical — or neither?
Because we can only view NGC 2775 from one angle, it’s difficult to say for sure. Some researchers classify NGC 2775 as a spiral galaxy because of its feathery ring of stars and dust, while others classify it as a lenticular galaxy. Lenticular galaxies have features common to both spiral and elliptical galaxies.
Astronomers aren’t certain of exactly how lenticular galaxies come to be, and they might form in a variety of ways. Lenticular galaxies might be spiral galaxies that merged with other galaxies, or that have mostly run out of star-forming gas and lost their prominent spiral arms. They also might have started out more like elliptical galaxies, then collected gas into a disk around them.
Some evidence suggests that NGC 2775 merged with other galaxies in the past. Invisible in this Hubble image, NGC 2775 has a tail of hydrogen gas that stretches almost 100,000 light-years around the galaxy. This faint tail could be the remnant of one or more galaxies that wandered too close to NGC 2775 before being stretched apart and absorbed. If NGC 2775 merged with other galaxies in the past, it could explain the galaxy’s strange appearance today.
Most astronomers classify NGC 2775 as a flocculent spiral galaxy. Flocculent spirals have poorly defined, discontinuous arms that are often described as “feathery” or as “tufts” of stars that loosely form spiral arms.
Hubble previously released an image of NGC 2775 in 2020. This new version adds observations of a specific wavelength of red light emitted by clouds of hydrogen gas surrounding massive young stars, visible as bright, pinkish clumps in the image. This additional wavelength of light helps astronomers better define where new stars are forming in the galaxy.
这张由NASA/ESA哈勃太空望远镜拍摄的图像展示了一座难以归类的星系——NGC2775。
图像来源：ESA/Hubble & NASA, F. Belfiore, J. Lee and the PHANGS-HST Team
NGC2775位于巨蟹座，距离地球约6700万光年。它的中心光滑、缺乏气体，看起来与椭圆星系相似；但它又拥有带有斑驳恒星团的尘埃环，呈现出旋涡星系的特征。那么，它究竟是旋涡星系、椭圆星系，还是介于两者之间？
由于我们只能从一个角度观察NGC2775，其真实分类并不容易确定。有些研究者依据其羽状的恒星与尘埃环，将其视为旋涡星系；另一些则认为它属于透镜状星系。透镜状星系兼具旋涡星系与椭圆星系的特征。
天文学家对透镜状星系的形成尚未完全理解，它们可能有多种起源。可能是旋涡星系与其他星系并合后的产物；或者是旋涡星系耗尽了造星气体，失去明显的旋臂；也可能原本类似椭圆星系，后来又聚集气体形成盘状结构。
一些证据表明NGC2775曾与其他星系发生过并合。在这张哈勃图像中看不见的，是一条环绕该星系、长达近10万光年的氢气尾迹。它或许是过去一座或多座靠得太近的星系被拉伸撕裂后遗留下来的残迹。如果NGC2775确实发生过并合，这可能解释它如今奇特的外观。
目前，大多数天文学家将NGC2775归类为绒状旋涡星系（flocculent spiral galaxy）。这类星系的旋臂不明显、不连续，往往呈羽毛状或簇状，仅松散地构成旋涡结构。
哈勃曾在2020年发布过NGC2775的一张图像，而这次的新版本加入了特定波长的红光观测——这是由年轻大质量恒星周围的氢气云发出的，使图像中出现明亮的粉红色斑块。这些额外数据帮助天文学家更清晰地确定星系中新恒星的诞生区域。
