Galaxies are distributed along a cosmic web in the universe. “Mpc/h” is
a unit of galactic distance (1 Mpc/h is more than 3.2 million
light-years)
Image credit: Volker Springel, Virgo Consortium
Behnam Darvish (left) and Bahram Mobasher are astronomers in the Department of Physics and Astronomy at UC Riverside
Photo credit: UC Riverside
UC Riverside-led team proposes that filaments in the cosmic web played a critical role in the distant universe
RIVERSIDE, Calif. – How do galaxies like our Milky Way form, and just how do they evolve? Are galaxies affected by their surrounding environment? An international team of researchers, led by astronomers at the University of California, Riverside, proposes some answers.
The researchers highlight the role of the “cosmic web” – a
large-scale web-like structure comprised of galaxies – on the evolution
of galaxies that took place in the distant universe, a few billion years
after the Big Bang. In their paper, published Nov. 20 in the Astrophysical Journal, they present observations showing that thread-like “filaments” in the cosmic web played an important role in this evolution.
“We think the cosmic web, dominated by dark matter, formed very early
in the history of the universe, starting with small initial
fluctuations in the primordial universe,” said Behnam Darvish, a Ph.D.
graduate student in the Department of Physics and Astronomy
at UC Riverside, who led the research project and is the first author
on the paper. “Such a ‘skeletal’ universe must have played, in
principle, a role in galaxy formation and evolution, but this was
incredibly hard to study and understand until recently.”
The distribution of galaxies and matter in the universe is
non-random. Galaxies are organized, even today, in a manner resembling
an enormous network – the cosmic web. This web has dense regions made
up of galaxy clusters and groups, sparsely populated regions devoid of
galaxies, as well as the filaments that link overdense regions.
“The filaments are like bridges connecting the denser regions in the
cosmic web,” Darvish explained. “Imagine threads woven into the web.”
Videos showing structures in the cosmic web:
- http://www.mpa-garching.mpg.de/galform/data_vis/millennium_sim_1024x768.avi [Credit: Springel et al. (Virgo Consortium)]
- http://vimeo.com/36095013 [Credit: Miguel Aragon-Calvo]
- http://www.mpa-garching.mpg.de/galform/data_vis/g696_mpeg4.avi [Credit: Klaus Dolag]
- http://www.mpa-garching.mpg.de/galform/data_vis/g696_fast_mpeg4.avi [Credit: Klaus Dolag]
It is well known in astronomy that galaxies residing in less dense regions have higher probability of actively forming stars (much like our Milky Way), while galaxies in denser regions form stars at a much lower rate.
“But the role of intermediate environments and, in particular, the
role of filaments and the cosmic web in the early universe remained,
until very recently, a mystery,” said coauthor Bahram Mobasher, a professor of physics and astronomy at UCR and Darvish’s adviser.
What greatly assisted the researchers is a giant section of the cosmic web first revealed in two big cosmological surveys (COSMOS and HiZELS). They proceeded to explore data also from several telescopes (Hubble, VLT, UKIRT and Subaru).
They then applied a new computational method to identify the filaments,
which, in turn, helped them study the role of the cosmic web.
They found that galaxies residing in the cosmic web/filaments have a
much higher chance of actively forming stars. In other words, in the
distant universe, galaxy evolution seems to have been accelerated in the
filaments.
“It is possible that such filaments ‘pre-process’ galaxies,
accelerating their evolution while also funneling them towards clusters,
where they are fully processed by the dense environment of clusters and
likely end up as dead galaxies,” Darvish said. “Our results also show
that such enhancement/acceleration is likely due to galaxy-galaxy
interactions in the filaments.”
Because of the complexities involved in quantifying the cosmic web,
astronomers usually limit the study of the cosmic web to numerical
simulations and observations in our local universe. However, in this new
study, the researchers focused their work on the distant universe –
when the universe was approximately half its present age.
“We were surprised by the crucial role the filaments play in galaxy
formation and evolution,” Mobasher said. “Star formation is enhanced in
them. The filaments likely increase the chance of gravitational
interaction between galaxies, which, in turn, results in this
star-formation enhancement. There is evidence in our local universe that
this process in filaments also continues to occur at the present time.”
Darvish and Mobasher were joined in this research by L. V. Sales at
UCR; David Sobral at the Universidade de Lisboa, Portugal; N. Z.
Scoville at the California Institute of Technology; P. Best at the Royal
Observatory of Ediburgh, United Kingdom; and I. Smail at Durham
University, United Kingdom.
Next, the team plans to extend this study to other epochs in the age
of the universe to study the role of the cosmic web/filaments in galaxy
formation and evolution across cosmic time.
“This will be a fundamental piece of the puzzle in order to understand how galaxies form and evolve as a whole,” Sobral said.
The UCR researchers were supported in the study by a grant to Mobasher from NASA through the Space Telescope Science Institute.
Media Contact
Iqbal Pittalwala
Tel: (951) 827-6050
E-mail: iqbal@ucr.edu
Twitter: UCR_Sciencenews
Additional Contacts
Behnam Darvish
E-mail: bdarv001@ucr.edu
Bahram Mobasher
Tel: 951 827 7190
E-mail: mobasher@ucr.edu
