For the past 20 years, the team has been systematically searching for massive low surface brightness galaxies, known as LSBs, in the 21 cm spectral line of neutral hydrogen gas (HI).
Last year, during an observing campaign in Green Bank (USA), while confirming the detection of an LSB galaxy made in France at the Observatoire radioastronomique de Nançay, a mistake was made in entering celestial coordinates, with the result that the large radio telescope was pointed in the wrong direction. To everyone’s surprise, a HI line signal was detected, resembling that of a massive galaxy, even though no galaxy appeared on optical images taken from the same position.
Know more
on the Green Bank Observatory website :
"Astronomers Accidentally Discover Dark Primordial Galaxy"
To determine the size and kinematics of the object detected, a series of complementary observations followed at the Green Bank radio telescope.
The data obtained resemble those of a massive galaxy : with a total HI gas content of almost 2 billion solar masses, half that of the Milky Way, the dark galaxy J0613+52 rotates on itself at a speed of 100 km/s.
If optical images have revealed no presence, it’s because they aren’t deep enough to detect LSB galaxies at the lowest surface brightness levels discovered to date.
Very deep optical images are therefore needed to quantify the number of stars "hidden" beneath the noise level of existing images. To obtain them quickly, other French astronomers have joined the team, operating optical telescopes such as those at Observatoire de Haute Provence and CERGA.
A galaxy like no other
Located some 270 million light-years away - not at the edge of the Universe - object J0613+52 is thought to contain large quantities of intact, incredibly diffuse, very low-density primordial gas. And, for having formed so few stars (their number remains to be confirmed), it also appears to be highly isolated, having never been disturbed by neighboring galaxies.
The existence of massive dark galaxies is predicted by galaxy formation theories, but as they contain very few stars for their size, they are difficult to detect on images.
Detections of other "dark" galaxies, such as those like J0613+52, will help to test and constrain these theories. The Euclid satellite, launched in July 2023, and the future Vera Rubin ground-based observatory will search for these objects optically, while at radio wavelengths, existing telescopes and the future giant radio telescope Square Kilometre Array, currently under construction, will attempt to detect their cold gas.
Scientific contributions :
The team is led by Karen O’Neil of Green Bank Observatory, USA. Its collaborator in France is Wim van Driel from Observatoire de Paris - PSL.
Observations of 350 LSB galaxies have been made with three radio telescopes :
- at the Observatoire Radioastronomique de Nançay in Sologne,
- Green Bank (USA - West Virginia),
- and with the Arecibo telescope (Puerto Rico), now collapsed.