Astronomers using ESO’s VLT Survey Telescope, a 2.6-m wide-field optical survey telescope at Paranal Observatory in Chile, have discovered 27 low-surface brightness galaxies — of which 12 are candidates for ultra-diffuse galaxies — in a large group of galaxies called the Hydra I cluster.
VST mosaic of the Hydra I: NGC 3311 and NGC 3309 are the two brightest cluster galaxies located close to the image center; the 27 low-surface brightness galaxies are marked as white circles; cyan squares show the 12 ultra-diffuse galaxies. Image credit: Iodice et al., doi: 10.1051/0004-6361/202038523.
Ultra-diffuse galaxies are objects as large as the Milky Way but with 100-1,000 times fewer stars.
Discovered in 2015, these galaxies are extremely faint and lack star-forming gas, which makes them appear almost like a fluffy cosmic cloud.
Their origins remain uncertain, but astronomers speculate that they could be ‘failed’ galaxies that lost their gas supply early in their lifetimes.
“The nature and origin of ultra-diffuse galaxies are still debated,” said Dr. Enrichetta Iodice, an astronomer at ESO and the INAF-Astronomical Observatory of Capodimonte, and colleagues.
“Ultra-diffuse galaxies could be ‘failed’ galaxies, which lost their gas supply at early epochs. If so, they should posses a massive dark matter halo to survive in dense environments like galaxy clusters.”
“Some theoretical models explain ultra-diffuse galaxies as extreme dwarf galaxies, whose large size could be due to high spins in dark matter halos or to tidal interactions.”
“In 2016, astronomers proposed that very extended ultra-diffuse galaxy-like systems could have formed by the kinematical heating of their stars as consequence of internal processes (i.e. gas outflows associated with feedback).”
Dr. Iodice and co-authors used the VLT Survey Telescope to observe the Hydra I cluster as part of the VST Early-type Galaxy Survey (VEGAS).
“The Hydra I cluster is a rich environment of galaxies located 166 million light-years away, with a virial mass of 2*1014 solar masses, from which we derived a virial radius of 5.2 million light-years,” the astronomers said.
“The cluster core is dominated by the two brightest early-type galaxies, NGC 3309 and NGC 3311, embedded in an extended diffuse stellar halo.”
“Most of the recent studies of Hydra I focused on the light distribution and kinematics in the cluster core, which showed the presence of ongoing interactions and traced the extended mass assembly around NGC 3311.”
“We carry out here the first search for ultra-diffuse galaxies enabled by a new wide area and deep imaging dataset.”
This VST image shows the ultra-diffuse galaxy Hydra I-UDG 4. Image credit: Iodice et al. / ESO.
From the visual inspection of a new mosaic image obtained for the Hydra I cluster, they identified a sample of 27 low-surface brightness galaxies, of which 12 are ultra-diffuse galaxy candidates.
For all ultra-diffuse galaxy candidates, they analyzed the light and color distribution, and provided a census of the globular cluster systems around them.
They found that most of these galaxies span a range of stellar masses of 107-108 solar masses.
Based on the globular cluster population of the candidates, they concluded that most of these galaxies have a standard or low dark matter content, with a halo mass of less than 1010 solar masses.
“This first sample of ultra-diffuse galaxy candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed ultra-diffuse galaxies and, through study of statistically relevant samples, constrain the nature and formation of ultra-diffuse galaxies,” the researchers said.
“This study presents the main properties of this class of galaxies in the Hydra I cluster.”
A paper on the findings was published in the journal Astronomy & Astrophysics.
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E. Iodice et al. 2020. The first detection of ultra-diffuse galaxies in the Hydra I cluster from the VEGAS survey. A&A 642, A48; doi: 10.1051/0004-6361/202038523
