NGC 4151 (right) is a spiral galaxy in the constellation of Canes Venatici. NGC 4151 is a bit odd. It has far-flung and faint spiral arms, but also a brighter ring of stars and gas closer to the center. The ring is blue, indicating the presence of lots of hot, young, massive stars. In the center is a massive black hole. The distance of NGC 4151 has been studied a lot recently. Most catalogs list it as roughly 40 million light-years away. But a paper from 2014 shows that it’s much farther than that: 62 million light-years. The new study used a new technique, called echo mapping: When matter falls into the black hole, it’s not always a smooth flow. A star can fall in, or a big clump of material. When that happens, a flare of high-energy light is emitted. This can light up the disk of material around the black hole, but there’s a delay in how long it takes the disk to brighten because of the huge distances involved; it takes time for the light to reach it. By carefully measuring the time it takes for the disk to respond to a flare, and comparing that with the measured size of the disk, the distance to the galaxy can be found. The old measurements are based on the galaxy’s redshift, and that can be misleading for nearby galaxies (if it’s in a cluster, for example, it can be moving rapidly and mess up the redshift measurements). Apparently, something like that is the case here, and NGC 4151 is half again farther away than we thought.

The small spiral to the bottom left of 4151 is NGC 4156. Notice as well the tidal tail pointing left.

NGC 4145 (left) is a barred spiral galaxy at a distance of 68 million light-years from the Earth. The galaxy has little star formation, except on its outer edges. Due to the loss of energy that occurs without star formation, some astronomers predict that the galaxy will degenerate into a lenticular galaxy in the near future.