Bengaluru: About 430 million light-years away in the Leo constellation, astronomers of the Indian Institute of Astrophysics (IIA) and their collaborators have discovered a new ultra-diffuse galaxy forming at the tip of a long tidal tail of stars and interstellar gas from galaxy NGC 3785. Likely triggered by gravitational interaction between NGC 3785 and a neighbouring galaxy, this finding marks a significant step forward in understanding galaxy evolution.
The NGC 3785 galaxy is known to possess the longest tidal tail discovered so far. The tail extends from the galaxy and is formed due to gravitational or tidal forces where two galaxies interact closely, essentially pulling material away from each other during a close encounter or merger process.
The research has been published in the European journal, Astronomy & Astrophysics Letters, and authored by Chandan Watts, a PhD student at IIA and Pondicherry University, Sudhanshu Barway from IIA, Omkar Bait from SKA, UK, and Yogesh Wadadekar from National Centre for Radio Astrophysics (NCRA), Pune.
Chandan Watts, the first author of the paper, carried out a careful photometric analysis of the tail and measured its extent and length accurately using advanced image processing techniques.
"We found that this extraordinary tidal tail stretches for 1.27 million light-years, making this the longest tidal tail discovered so far, with a nascent ultra-diffuse galaxy having formed at its tip, likely driven by the gravitational interaction between NGC 3785 and a neighbouring galaxy -- making it a rare and exciting discovery," he stated.
How Tidal Tails Give Birth to Ultra-Diffuse Galaxies
In an interview with ETV Bharat, IIA's Sudhanshu Barway discussed the discovery and its implications, starting with the explanation of how the gravitational interaction between NGC 3785 and its neighbouring galaxy led to the formation of this tidal tail and the rare ultra-diffuse galaxy at its tip.
Barway said that interactions between galaxies are common in the universe and can take different forms. When two galaxies encounter each other, they may either merge through a violent interaction to form a new system or undergo a "fly-by" interaction, where a smaller galaxy orbits a larger one without merging.
He said that in such fly-by cases, the smaller galaxy gradually loses material -- gas and stars -- which gets stretched out by the larger galaxy's gravity into elongated streams known as tidal tails. This process unfolds over several orbital passes, not in a single event. Over time, the remnant material can coalesce into faint, fussy, and extended structures. Some of these grow to be very large yet diffuse, and are classified as ultra-diffuse galaxies (UDGs) -- systems with the size of giant galaxies like the Milky Way, but with very few stars spread out over that vast area.
Evolution of the discovery
Speaking about how the discovery evolved, Barway said it began as a "hunt" with IIA astronomers surveying the sky in search of unusual features in galaxies using larger telescopes. The observational data were later made publicly available.
During this process, Omkar Bait, then a PhD student at the NCRA, Pune, identified the peculiar structure. Recognising that this was a unique object, this discovery was shared with Yogesh Wadadekar (NCRA) and Sudhanshu Barway. They worked together on the subsequent study. Astronomers subsequently undertook a detailed study, which was completed over a few months.
A unique case for understanding faint and diffuse galaxies
When asked whether this is a unique case for understanding how faint and diffuse galaxies form, Barway explained, "This is indeed a unique case, as we have discovered one of the longest known tidal tails, and at its tip lies a diffuse galaxy -- something not observed before. Tidal tails themselves are not rare; they are common outcomes of galaxy interactions. However, such interactions usually occur in dense environments, such as galaxy groups with 50-60 members or clusters hosting up to 1,000 galaxies. Our own Milky Way belongs to the Local Group, which consists of about 50 galaxies, including our neighbour Andromeda."
He said that the two aspects make this discovery significant: One is the tidal tail, and another is the diffuse galaxy that has formed in an isolated environment, far from the influence of large groups or clusters. This provides a rare opportunity to study galaxy formation and evolution under conditions different from the typical crowded cosmic neighbourhoods.
Techniques used to confirm the discovery
Speaking about the photometric analysis and techniques used to confirm both the length of the tidal tail and the presence of the forming galaxy, Barway explained, "We capture images using large telescopes equipped with medium-to-large CCD cameras. These instruments not only help in detecting extremely faint features but also in eliminating artificial or electronic signs or noise. By precisely measuring the photons arriving from the tidal tail or the ultra-diffuse galaxy, we can determine their brightness and faintness, and thereby confirm the structures observed."
Commenting on how this discovery challenges or supports existing theories about galaxy interactions and the creation of faint galaxies, Barway explained that the discovery doesn't challenge existing theories but provides new insights into them. It highlights two key aspects of such interactions -- the formation of ultra-diffuse galaxies and the creation of long tidal tails.
Galaxy interactions then and now
When asked whether such processes could have been common in the early universe, and if so, might this discovery be a local window into a much more universal phenomenon, Barway explained that observations from new telescopes, including the James Webb Space Telescope (JWST), show that such interactions were indeed common in the early universe. In fact, a future interaction between our neighbouring galaxy, Andromeda, and the Milky Way has also been predicted, indicating that these events are not unusual. Studying systems in the nearby universe -- within a few million to billion light-years -- provides a direct view of how such interactions occur locally or in our nearby universe, demonstrating that these processes are ongoing and can still lead to the formation of new galaxies today.
Barway further said that NGC 3785 is not an isolated case. "We expect to find many more ultra-diffuse galaxies forming at the ends of tidal tails. However, because these objects are extremely faint, systematic surveys with much larger telescopes are required to detect them," he added. "The Indian Institute of Astrophysics (IIA), which operates national facilities and observatories across India -- including the 2-metre and 0.7-metre telescopes at Hanle, Ladakh -- is well placed to contribute to this effort."
IIA and international collaborations
He further said that, given the high cost and specialised instrumentation required to study such phenomena in detail, IIA is also seeking international collaborations. These partnerships are essential for obtaining complementary observations and instruments not available at every observatory. Our ongoing work demonstrates how imaging and kinematic measurements -- such as the stellar velocities within ultra-diffuse galaxies -- can be used to study their structure and formation in depth.
This discovery reveals how galaxy interactions can generate faint, ultra-diffuse structures and offers a rare glimpse into their origins. Future missions like Euclid and Rubin Observatory's Legacy Survey of Space and Time (LSST) will play a crucial role in detecting more of these elusive low-surface-brightness features.