Astronomy Picture of the Day

Explanation: How do you find a nearly invisible galaxy? Pictured above is the blue compact dwarf (BCD) galaxy NGC 2915. In visible light, shown above in yellow, this galaxy appears to be a normal dwarf galaxy, as indicated by the yellow smudge in the image center. Yet when imaged in a very specific color, shown in blue, a whole spiral galaxy appears. This specific color is in the radio band and is preferentially emitted by neutral hydrogen atoms. Much about BCD galaxies remains mysterious, such as how the neutral hydrogen obtained its shape, what drives current star formation, and why there is so much dark matter. NGC 2915 is located at the relatively nearby distance of 15 million light-years - just outside our Local Group of Galaxies.

如何找到幾乎不可見的鄰近星系？ 上圖的主題是泛藍的緊湊矮星（BCD）NGC 2915。 在以泛黃色系呈現的可見光影像裡，此星系似乎是個普通的矮星系，就如影像中心可見的瀰漫泛黃光斑所示。 然而，在某個特定的波段造像時（以藍色呈現），一個完整的螺旋星系就出現了。 這種位在無線電波段的特定色光，主要是由中性氫原子所發出的。 關於BCD星系的許多特徵仍然相神秘，諸如中性氫是如何取成這種形狀的，是什麼驅動了目前的恒星形成，以及為何會有這麼多的暗物質。 NGC 2915的距離是相對鄰近的1500萬光年，就在我們的本星系群外頭。