“Little Cub” Gives Astronomers Rare Chance to See Galaxy’s Demise

Maunakea, Hawaii– Astronomers have spotted a primitive galaxy being devoured by a gigantic neighboring galaxy – a discovery that could provide clues about the early universe.

The Little Cub galaxy – so called because it sits in the Ursa Major or Great Bear constellation – is being stripped of the gas needed to continue forming stars by its larger companion.

This rare opportunity to observe a dwarf galaxy as its gas is removed by the effects of a nearby giant galaxy will allow scientists to learn more about how this process happens.

Since the Little Cub has remained almost pristine since its formation, scientists also hope its elements will reveal more about the chemical signature of the universe just minutes after the Big Bang.

The research, carried out by the University of California, Santa Cruz and Durham University, United Kingdom, is being presented on Tuesday, July 4, at the Royal Astronomical Society’s National Astronomy Meeting.

The Little Cub was initially identified as a potentially pristine dwarf galaxy in data from the Sloan Digital Sky Survey (SDSS). Follow-up observations were conducted at the 10-meter Keck I telescope at the W. M. Keck Observatory on Maunakea, Hawaii, and the 3-meter Shane Telescope at Lick Observatory.

Researchers were able to confirm characteristics of the Little Cub galaxy using Keck Observatory’s Low Resolution Imaging Spectrograph, a faint-light instrument capable of taking spectra and images of the most distant known objects in the universe.

“The Little Cub’s discovery is a terrific example of using the smaller 3-meter-class Lick Observatory to scan through hundreds of candidates before focusing on the best sources with the Keck telescope,” said co-author J. Xavier Prochaska, Professor of Astronomy and Astrophysics at UC Santa Cruz.

The Little Cub’s larger neighbor, called NGC 3359, is a grand design spiral galaxy, which is a large galaxy that has over 100 billion stars with well-defined spiral arms – similar to our own Milky Way galaxy. The Little Cub and NGC 3359 are about 200 to 300 thousand light years apart, and approximately 50 million light years from Earth.

Gas from the Little Cub is being stripped away by its interaction with NGC 3359, which has up to 10,000 times as many stars as the Little Cub and is similar to our Milky Way.

By observing this cosmic feast, scientists hope to understand more about how and when gas is lost from smaller galaxies.

“We may be witnessing the quenching of a near-pristine galaxy as it makes its first passage about a Milky Way-like galaxy,” said lead author Tiffany Hsyu, a graduate student in the Department of Astronomy & Astrophysics at UC Santa Cruz. “It is rare for such a tiny galaxy to still contain gas and be forming stars when it is in close proximity to a much larger galaxy, so this is a great opportunity to see just how this process works. Essentially the larger galaxy is removing the fuel that the Little Cub needs to form stars, which will eventually shut down star formation and lead to the smaller galaxy’s demise.”

The researchers also hope to gain insight into the make-up of the very early universe, by studying the hydrogen and helium atoms that are being illuminated by the small number of very bright stars within the Little Cub – which also has the less romantic name SDSS J1044+6306. Since this galaxy is so primitive, it may still preserve the hydrogen and helium atoms that were created minutes after the Big Bang.

“By studying the chemistry of the Little Cub, we know that it is one of the most primitive objects currently known in our cosmic neighborhood,” said research co-author Dr. Ryan Cooke, Royal Society University Research Fellow, in Durham University’s Centre for Extragalactic Astronomy.

“Such galaxies, which have remained dormant for most of their lives, are believed to contain the chemical elements forged a few minutes after the Big Bang.By measuring the relative number of hydrogen and helium atoms in the Little Cub we might be able to learn more about what made up the Universe in the moments after it began 13.7 billion years ago,” Cooke added.

The researchers hope further observations will find more pristine galaxies where the chemical signature of the early universe might be found.

A paper describing the discovery of Little Cub has been submitted for publication in the Astrophysical Journal Letters.

The research was funded by the W. M. Keck Foundation, Google, The Royal Society, NASA, the Science and Technology Facilities Council and the National Science Foundation (USA).

The Royal Astronomical Society’s National Astronomy Meeting is taking place at the University of Hull, UK, until Thursday, July 6, 2017.

About LRIS

The Low Resolution Imaging Spectrometer (LRIS) is a versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion. Support for this project was generously provided by the Change Happens Foundation, Mt. Cuba Astronomical Foundation, William J. and Dorothy K. O’Neill Foundation, and Sanford and Jeanne Robertson.

About W. M. Keck Observatory

The W. M. Keck Observatory operates the most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California, and NASA.