Lightening up dark galaxies

May 23, 2018, ETH Zurich
One of the new dark-galaxy candidates, identified through a combination of spectral information (left) and images reflecting the emission of gas (middle) and stars (right). The position of the dark-galaxy candidate is marked by the red circle. Credit: R. A. Marino / MUSE

Despite substantial progress over the past half-century in understanding how galaxies form, important open questions remain regarding how precisely the diffuse gas of the intergalactic medium is converted into stars. One possibility, suggested in recent theoretical models, is that the early phase of galaxy formation involves an epoch when galaxies contain a great amount of gas but are still inefficient at forming stars. Direct proof of such a dark phase has been so far elusive, however—after all, dark galaxies do not emit much visible light. The observational discovery of such galaxies would therefore fill an important gap in our understanding of galaxy evolution.

There are ways to identify dark , however. An international team led by Dr. Raffaella Anna Marino and Prof. Sebastiano Cantalupo from the Department of Physics at ETH Zurich has now done just that, and was able to search the sky for potential dark galaxies with unprecedented efficiency. They report their results in a paper published today in The Astrophysical Journal, and have identified at least six strong candidates for dark galaxies.

To overcome the obstacle that their target objects are dark, the team used quasars as a flashlight of sorts. These emit intense ultraviolet light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line. As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible. Such fluorescent illumination has been used before in searches for dark galaxies, but Marino et al. searched the neighbourhood of quasars at greater distances than has been possible in earlier observations.

They acquired the full spectral information for each of the dark-galaxy candidates. Deep observations—10 hours for each of the six quasar fields they studied—enabled Marino and her colleagues to efficiently discern dark-galaxy candidates from other sources. From initially 200 Lyman-alpha emitters, a half-dozen regions remained that are unlikely to be normal star-forming stellar populations, making them robust candidates for dark galaxies.

The advances in observational capability have become possible thanks to the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. In essence, previous studies were limited to imaging a relative narrow band of frequencies, for which specific filters had to be designed. The MUSE instrument instead allowed hunting 'blindly'—without filters—for dark galaxies around quasars at larger distances from Earth than had been possible so far.

Explore further: Dark galaxies of the early Universe spotted for the first time

More information: Astrophysical Journal (2018). DOI: 10.3847/1538-4357/aab6aa

Related Stories

Distant quasar illuminates a filament of the cosmic web

January 19, 2014

Astronomers have discovered a distant quasar illuminating a vast nebula of diffuse gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic web. Researchers at the University ...

Dark matter less influential in galaxies in early universe

March 15, 2017

New observations indicate that massive, star-forming galaxies during the peak epoch of galaxy formation, 10 billion years ago, were dominated by baryonic or 'normal' matter. This is in stark contrast to present-day galaxies, ...

Mapping dark matter

July 24, 2017

About eighty-five percent of the matter in the universe is in the form of dark matter, whose nature remains a mystery. The rest of the matter in the universe is of the kind found in atoms. Astronomers studying the evolution ...

Recommended for you

Blue Origin to make 10th flight test of space tourist rocket

January 23, 2019

Blue Origin, the rocket company headed by Amazon founder Jeff Bezos, is poised to launch the 10th test flight of its unmanned New Shepard rocket on Wednesday as it competes with Virgin Galactic to become the first to carry ...

Milky Way's neighbors pick up the pace

January 22, 2019

After slowly forming stars for the first few billion years of their lives, the Magellanic Clouds, near neighbors of our own Milky Way galaxy, have upped their game and are now forming new stars at a fast clip. This new insight ...

A fleeting moment in time

January 22, 2019

The faint, ephemeral glow emanating from the planetary nebula ESO 577-24 persists for only a short time—around 10,000 years, a blink of an eye in astronomical terms. ESO's Very Large Telescope captured this shell of glowing ...

How hot are atoms in the shock wave of an exploding star?

January 21, 2019

A new method to measure the temperature of atoms during the explosive death of a star will help scientists understand the shock wave that occurs as a result of this supernova explosion. An international team of researchers, ...

New eclipsing cataclysmic variable discovered

January 21, 2019

Using the Mobile Astronomical System of Telescope-Robots (MASTER), an international team of astronomers has detected a new eclipsing cataclysmic variable. The newfound object, designated MASTER OT J061451.70–272535.5, is ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.