Rare Luminous Lyman-alpha Nebula Poses Cosmic Puzzle

Posted: Feb 24 2017, 3:42am CST | by , in News | Latest Science News

 
Rare Luminous Lyman-alpha Nebula Poses Cosmic Puzzle
MAMMOTH-1 is an extended blob of gas in the intergalactic medium called an enormous Lyman-alpha nebula (ELAN). The color map and contours denote the surface brightness of the nebula, and the red arrows show its estimated spatial extent. (Image credit: Figure 2 of Cai et al., Astrophysical Journal)

Astronomers have spotted an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting.

Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed, the researchers said.

The newly discovered nebula was found at a distance of 10 billion light years in the middle of a region with an extraordinary concentration of galaxies.

Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, Hubble postdoctoral fellow at University of California, Santa Cruz in the US.

"Our survey was not trying to find nebulae. We're looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai, who is first author of a paper on the discovery to be published in the Astrophysical Journal.

"We found this enormous nebula in the middle of the protocluster, near the peak density," Cai said.

ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium.

They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web.

Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but it is not clear what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms), the researchers said.

The newly discovered ELAN is known as MAMMOTH-1.

"It's a terrifically energetic phenomenon without an obvious power source," said study co-author J. Xavier Prochaska, Professor at University of California, Santa Cruz.

Paper reference:

Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology

Zheng Cai, Xiaohui Fan, Sebastien Peirani, Fuyan Bian, Brenda Frye, Ian McGreer, J. Xavier Prochaska, Marie Wingyee Lau, Nicolas Tejos, Shirley Ho, Donald P. Schneider

Modern cosmology predicts that a galaxy overdensity is associated to a large reservoir of the intergalactic gas, which can be traced by the Lyα forest absorption.

We have undertaken a systematic study of the relation between Coherently Strong intergalactic Lyα Absorption systems (CoSLAs), which have highest optical depth (τ) in τ distribution, and mass overdensities on the scales of ∼ 10 - 20 h−1 comoving Mpc. On such large scales, our cosmological simulations show a strong correlation between the effective optical depth (τeff) of the CoSLAs and the 3-D mass overdensities. In moderate signal-to-noise spectra, however, the profiles of CoSLAs can be confused with high column density absorbers. For z>2.6, where the corresponding Lyβ is redshifted to the optical, we have developed the technique to differentiate between these two alternatives.

We have applied this technique to SDSS-III quasar survey at z=2.6 - 3.3, and we present a sample of five CoSLA candidates with τeff on 15 h−1 Mpc greater than 4.5× the mean optical depth. At lower redshifts of z<2.6, where the background quasar density is higher, the overdensity can be traced by intergalactic absorption groups using multiple sight lines.

Our overdensity searches fully utilize the current and next generation of Lyα forest surveys which cover a survey volume of >(1 h−1 Gpc)3. In addition, systems traced by CoSLAs will build a uniform sample of the most massive overdensities at z>2 to constrain the models of structure formation, and offer a unique laboratory to study the interactions between galaxy overdensities and the intergalactic medium.

DOI: 10.3847/1538-4357/833/2/135 arXiv:1512.06859 [astro-ph.GA]

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