Which galaxy is the subject of Hubble's 2015 image 'Zooming In on the Andromeda Galaxy'?

Which galaxy is the subject of Hubble's 2015 image “Zooming In on the Andromeda Galaxy”? The answer is the Andromeda Galaxy.

Also known as “Gigapixels of Andromeda,” the photograph is a vast 2015 composite made from Hubble observations. It shows the northern part of Andromeda’s disk across roughly 61,000 light-years and contains about 100 million resolved stars, along with star clusters, dust lanes, foreground Milky Way stars, and distant background galaxies.

The image grew from the Panchromatic Hubble Andromeda Treasury, or PHAT, a survey launched in 2011 to map about one-third of Andromeda’s stars. Its mosaic combined 411 exposures taken between July 2010 and October 2013, producing an image about 1.5 billion pixels in size.

A common mix-up is confusing the subject with the Milky Way, which appears only through foreground stars. The Sombrero, Triangulum, and Whirlpool galaxies are separate objects, not the target of this famous Hubble panorama.