Artist’s impression of a progenitor of Milky Way-like galaxy in the early universe with a background quasar shining through a ‘super halo’ of hydrogen gas surrounding the galaxy. New ALMA observations of two such galaxies reveal that these vast halos extend well beyond the galaxies’ dusty, star-forming disks. Credit: A. Angelich (NRAO/AUI/NSF)
A team of astronomers have discovered Milky Way-like galaxies in early universes that were embedded in super halos.
Astronomers have harnessed the extreme sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) to observe a pair of Milky Way-like galaxies seen when the universe was only 8 percent of its current age.
These progenitors of today’s giant spiral galaxies are surrounded by super halos comprised of hydrogen gas that extend many tens of thousands of light-years beyond their dusty, star-filled disks.
The astronomers initially detected these galaxies by studying the intense light from even-more-distant quasars.
As this light travels through an intervening galaxy on its way to Earth, it can pick up the unique spectral signature from the galaxy’s gas. However, this technique normally prevents astronomers from seeing the actual light emitted by the galaxy, which is overwhelmed by the much brighter emission from the background quasar.
“Imagine a tiny firefly next to a high-power search light,” Marcel Neeleman a postdoctoral fellow at the University of California, Santa Cruz, and lead author on the paper, said in a statement. “That’s what astronomers are up against when it comes to observing these youthful versions of our home galaxy.
“We can now see the galaxies themselves, which gives us an amazing opportunity to learn about the earliest history of our own galaxy and others like it.”
However, after using ALMA the astronomers were finally able to observe the natural millimeter-wavelength glow emitted by ionized carbon in the dense and dusty star-foaming regions of the galaxies. This carbon signature is considerably offset from the gas first detected by quasar absorption.
The extreme separation indicates that the galaxies’ gas content extends well beyond their star-filled disks, which suggests that each galaxy is embedded in a monstrous halo of hydrogen gas.
“We had expected we would see faint emission right on top of the quasar, and instead we saw bright galaxies at large separations from the quasar,” J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz and co-author of the paper, said in a statement.
The separation from the quasar to the observed galaxy is about 137,000 light-years for one galaxy and about 59,000 light-years for the other.
The neutral hydrogen gas revealed by its absorption of quasar light is most likely part of a large halo or possibly an extended disk of gas around the galaxy.
“It’s not where the star formation is, and to see so much gas that far from the star-forming region means there is a large amount of neutral hydrogen around the galaxy,” Neeleman said.
The new data reveals that the young galaxies are already rotating, which is one of the hallmarks of the massive spiral galaxies that are seen in the universe today.
The ALMA observations further reveals that both galaxies are forming stars at moderately high rates—more than 100 solar masses per year in one galaxy and 25 solar masses per year in the other.
“These galaxies appear to be massive, dusty, and rapidly star-forming systems, with large, extended layers of gas,” Prochaska said.
Chris Carilli, an astronomer with the National Radio Astronomy Observatory in Socorro, N.M., and co-author on the paper, explained that this research answers some unanswered questions.
“ALMA has solved a decades-old question on galaxy formation,” Carilli said in a statement. “We now know that at least some very early galaxies have halos that are much more extended that previously considered, which may represent the future material for galaxy growth.”
The galaxies are each about 12 billion light-years from Earth and the background quasars are each roughly 12.5 billion light-years from Earth.
