New observations of nine distant galaxies taken with the Subaru Telescope show that the swarms of smaller satellite galaxies around the main galaxies have important similarities and differences compared to the satellite galaxies around the Milky Way. This shows that the Milky Way is a somewhat normal galaxy, but not completely normal. This result is important for understanding to what extent we can apply what we know about the local environment to the rest of the Universe.
Figure 1: One of nine observed galaxies (NGC 3338). This spiral galaxy is about 76 million light-years away with a mass similar to the Milky Way. High resolution image is here (4.5 MB). (Credit: NAOJ)
Astronomers are always trying to figure out how much of what we see in the skies is typical, and how much is unique to the local area. Just as the ability to study planets around other stars has given us new insights about the planets in the Solar System, astronomers can now observe distant galaxies in detail and compare them to what we know about the Milky Way.
Large galaxies such as the Milky Way are surrounded by smaller satellite galaxies. More than 50 satellite galaxies have been detected around the Milky Way, many less than theoretically expected. Also, the Milky Way’s satellite galaxies are found clustered together, while predictions indicate that they should be evenly distributed.
A group led by researchers at the National Astronomical Observatory of Japan (NAOJ) used the Subaru Telescope to take images of nine galaxies with masses similar to the Milky Way, each about 50 to 80 million light-years away. In these images, the group successfully detected 93 candidates for faint, small satellite galaxies. The number of satellites per main galaxy varied greatly but was comparable to the number of satellite galaxies around the Milky Way, indicating that the Milky Way is normal in terms of its number of satellites. Also, the satellite galaxies were evenly distributed around the main galaxies; matching predictions but not matching what we know about the Milky Way.
Figure 2: A selection of detected satellite galaxies. Most of the satellite galaxies are faint and extended. High resolution image is here (6.6 MB). (Credit: NAOJ)
Masashi Nashimoto, a research fellow at NAOJ (currently a JSPS research fellow at the University of Tokyo), commented: "These results are valuable information for statistically examining various issues related to satellite galaxies. On the other hand, some objects were not clearly identified as satellite galaxies, and we expect to identify them by further observations."
Satellite galaxies provide clues to how the main galaxy formed, so improving our understanding of them helps us to understand the evolution of galaxies in general, and the Milky Way in particular, which created the conditions where the Earth and life could form.
These results appeared as Nashimoto et al. "The Missing Satellite Problem outside of the Local Group. II. Statistical Properties of Satellites of Milky Way-like Galaxies" in the Astrophysical Journal on August 29, 2022.
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
Guidelines for Use