Our massive Milky Way Galaxy –the second-largest galaxy in the Local Group after the Andromeda Galaxy–is orbited by more than twenty dwarf galaxies that are thought to be relics of the very first galaxies in the universe, A new ultra-faint dwarf galaxy (UFD), the least luminous, most dark matter dominated, and least chemically evolved galaxies known, has been discovered at a distance of about 293,000 light years away from the Earth.
The new UFD, named Pegasus IV, at the very northern edge of sky, was identified by an international team of astronomers as part of the DECam Local Volume Exploration (DELVE) survey. The newly found object, designated Pegasus IV, has an absolute magnitude of -4.25, corresponding to a luminosity of only 4,000 suns and making it one the best fossil candidates from the infant universe.
“We are Missing Something”
Recent models predict that dozens of small dwarf satellite galaxies should orbit medium-sized galaxies like our Milky Way and Andromeda in random orientations. However, new research suggests most satellite galaxies orbit their host galaxies aligned along a single plane, which “means that we are missing something,” according to Marcel Pawlowski, a Schwarzschild Fellow at the Leibniz-Institute for Astrophysics.
A team of astronomers led by William Cerny of the University of Chicago has found that the new UFD is a satellite of the Milky Way galaxy. The discovery was made using the data from the Dark Energy Camera (DECam image below) —part of an ongoing multi-component observational campaign seeking to achieve deep, contiguous coverage of the high-Galactic-latitude southern sky.
“In this work, we present the discovery and characterization of yet another ultra-faint Milky Way satellite by DELVE. This new Pegasus IV system is accessible to the Dark Energy Camera (DECam) in a region previously examined by the Sloan Digital Sky Survey (SDSS) and the Panoramic Survey Telescope and Rapid Response System 1 survey (PS1),” the researchers wrote in the paper.
Revolving Backwards Compared to the Stars in the Milky Way Disk
The results show that Pegasus IV is at least 12.5 billion years old, its half-light radius is about 133.7 light years, and has an absolute (integrated) V-band magnitude at a level of -4.25. The stellar mass of the system was calculated to be around 4,400 solar masses.
According to the paper, Pegasus IV has a velocity dispersion of 3.3 km/s, while its heliocentric velocity is -273.6 km/s. These results, together with the measured proper motion of Pegasus IV, suggest that this UFD is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. In other words, the UFD galaxy is revolving backwards compared to the stars in the disk of our Milky Way Galaxy, and is currently at its most distant location from the Milky Way in its elliptical orbit.
As part of the study, Cenry’s team derived iron abundances for five stars of Pegasus IV, which yielded a metallicity of approximately -2.67 dex, corresponding to only 0.2% the iron abundance of our Sun. This means that the newly found galaxy is one of the most metal-poor ultra-faint dwarfs. The astronomers added that Pegasus IV exhibits a metallicity spread, further confirming its nature as a dwarf galaxy.
The authors of the paper assume that many such UFDs may exist undetected within the distance within 100 kiloparsecs (kpc) or 300,000 light years from Earth.
The Last Word
“According to the best current measurements, ultra-faint dwarfs formed their stars shortly after the Big Bang, within the first billion years of the evolution of the universe” Carnegie Science astrophysicist Josh Simon, who uses observations of nearby galaxies to study problems related to dark matter, chemical evolution, star formation, and the process of galaxy evolution, told The Daily Galaxy. “Unlike the brighter galaxies from the same era, UFDs avoided merging with or being torn apart by larger systems. Instead, they have been preserved in their original state for the last 13 billion years, making them in essence fossils from the beginning of the universe.”
Image credit: Dwarf dark galaxy hidden in ALMA Observatory Gravitational Lens Image faint distortions hidden in the stunning image of the gravitational lens SDP.81 at the top of the page are telltale signs that a dwarf dark-matter galaxy is lurking in the halo of a much larger galaxy nearly 4 billion light-years away (Y. Hezaveh, Stanford Univ.; ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble Space Telescope).
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Marcel Pawlowski, Tomasz Nowakowski , Josh Simon, Phys.org and Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-Faint Dwarf Galaxy in the Constellation Pegasus