The average submillimetre properties of Lyman-alpha Blobs at z=3

TL;DR

This study investigates the average submillimetre emission of Lyman-alpha Blobs at z=3, revealing that larger LABs show significant dust emission, with star formation likely powering their Ly-alpha emission.

Contribution

It provides the first statistical measurement of the average 850um flux of LABs and assesses the roles of star formation and cold accretion in powering LABs.

Findings

Larger LABs have a mean 850um flux of 1.4 +/- 0.3mJy.

Most LABs do not show strong individual submillimetre detections.

Star formation likely dominates the Ly-alpha emission in LABs.

Abstract

Ly-alpha blobs (LABs) offer insight into the complex interface between galaxies and their circumgalactic medium. Whilst some LABs have been found to contain luminous star-forming galaxies and active galactic nuclei that could potentially power the Ly-alpha emission, others appear not to be associated with obvious luminous galaxy counterparts. It has been speculated that LABs may be powered by cold gas streaming on to a central galaxy, providing an opportunity to directly observe the `cold accretion' mode of galaxy growth. Star-forming galaxies in LABs could be dust obscured and therefore detectable only at longer wavelengths. We stack deep SCUBA-2 observations of the SSA22 field to determine the average 850um flux density of 34 LABs. We measure S_850 = 0.6 +/- 0.2mJy for all LABs, but stacking the LABs by size indicates that only the largest third (area > 1794 kpc^2) have a mean detection, at 4.5 sigma, with S_850 = 1.4 +/- 0.3mJy. Only two LABs (1 and 18) have individual SCUBA-2 > 3.5 sigma detections at a depth of 1.1mJy/beam. We consider two possible mechanisms for powering the LABs and find that central star formation is likely to dominate the emission of Ly-alpha, with cold accretion playing a secondary role.