180206期:星系NGC474:壳层与星流

星系NGC474:壳层与星流

星系NGC474发生了什么?考虑到这个在弱深空影像中看上去相对无特色的椭圆星系,它的多重辐射层就显得异常的复杂。目前壳状星系的成因尚未可知,但其潮汐尾可能与该星系在过去数十亿年中吞食众多小型星系后留下的残骸有关;或者壳状星系就如同池塘里的涟漪,是NGC474与上方的螺旋星系正进行的的碰撞产生的密度波搅动而引起的。抛开真正成因,这张图片让越来越多的人相信,至少有一些椭圆星系是不久前才形成的,并且大多数大型星系的外层晕并不像看上去那么圆滑,而是很复杂,原因是其与较小的邻近星系的频繁相互作用与吸积。银河系的银晕就是这种异常复杂结构的一个例证。NGC474是双鱼星座的一个星系,跨度为25万光年,距离我们有1亿光年。

翻译:银河说翻译组 Lee 

审核:银河说顾问组 一米

Galaxy NGC 474: Shells and Star Streams
Image Credit: CFHT, Coelum, MegaCam, J.-C. Cuillandre (CFHT) & G. A. Anselmi (Coelum)Explanation: What’s happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple through the galactic giant. Regardless of the actual cause, the featured image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with — and accretions of — smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of suchunexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).

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