These recent observations have provided an exciting new window into the star formation history of the ultra-diffuse galaxy (UDG) F8D1. This galaxy, known as DF2, was originally discovered in 1998. Located deep within the M81 group of galaxies, F8D1 has a progenitor mass of 133 million solar masses. A substantial fraction of its current stellar mass, on the order of 25-40%, is presently unbound. It blankets a large portion of a fast-moving tidal current.
These new results show that F8D1 experienced a global shutdown of star formation less than two billion years ago. This event came after a convoluted star-forming history spanning more than six billion years. This thorough analysis happens to be one of the most comprehensive studies of any UDG thus far. It unlocks crucial clues about the galaxy’s development and formation.
Star Formation Phases in F8D1
F8D1’s star formation history, from its formation to the present, can be characterized by three different epochs. The initial epoch started more than six billion years ago, when about 80% of the galaxy’s stars were born. This initial star forming phase would determine the fate of the galaxy’s stellar population.
About 2 to 2.5 billion years ago, F8D1 went through its most recent pronounced phase of star formation. At this rate, roughly 11% of the stars that ever formed within the core did so during this time. At the same time, about 39% formed in the outer regions of the galaxy. These stellar outbursts reveal the high-energy, explosive processes that are at work in UDGs. For starters, they show that UDGs are not stable, but rather dynamic with time through several processes.
More than 500 million years ago, a major period of star birth occurred. This collision formed all of F8D1’s nuclear star cluster, contributing a few percent to the stellar population of this otherwise very old nuclear star cluster. This recent activity is especially exciting. It uncovers important information about the original parent of UDGs such as F8D1, indicating they can still host impactful star-forming activity even deep into their lifetimes.
Observational Insights and Implications
To understand F8D1’s unique population of stars, the research team used new observations provided by the Hubble Space Telescope (HST). They aimed to infer the galaxy’s star formation history by detecting individual stars down to the Red Clump in both a central WFC3 field and an outer ACS field.
“We have used new HST observations to infer the star formation history of the nearby UDG, F8D1, over the past six billion years, using detections of individual stars to below the Red Clump in a central WFC3 field and an outer ACS field.” – Adam Smercina et al
These observations provide important evidence ruling out the possibility of F8D1 being a “failed” galaxy. The existence of a standard globular cluster population is additional evidence for its reputation as an active player in the universe.
UDGs such as F8D1 are unique because they are the same physical size as the Milky Way. They have at most 1% as many stars. This dramatic difference poses important questions about how these galaxies are able to evolve and persist in such inhospitable environments.
The Significance of F8D1’s History
The findings regarding F8D1’s star formation history contribute valuable knowledge to the understanding of UDGs and their role in galactic evolution. Confirming their earlier findings, the new observations confirm that even the tiniest of galaxies with minimal stellar populations can still undergo tumultuous epochs of rapid star formation and development.
In addition to its isolation, discovery of significant star formation activity late in F8D1’s history offers insights into its formation and evolution. Such discoveries will permit astronomers to better calibrate their models and theoretical expectations about how like-galaxies evolve with cosmic time.