Where do Lyα Blobs live? The first statistical study of the protocluster-blob connection

Protoclusters are the largest cosmic structures at high redshift, which will evolve into present-day massive clusters of galaxies. As such they provide unique laboratories to witness the hierarchical formation of large-scale structure, as well as opportunities to examine rare physical phenomena such as radio galaxies, extreme starbursts and extended Lyman alpha nebulae and link them to their evolutionary paths into cluster galaxies.

The One-hundred-square-degree DECam Imaging in Narrowbands (ODIN) survey has been imaging ~ 91 deg² of the equatorial and southern sky with three custom narrow-band filters to sample redshifted Lyman alpha (Lyα) emission at z ~ 2.4, 3.1, and 4.5. Lyα emitting galaxies (LAEs) selected using these filters have redshift precision of ~ 0.05, thereby allowing us to construct the large-scale structure of the universe at a given epoch with minimal contamination from fore- and background interlopers.

We present the results based on the ODIN Year 1 observations of the COSMOS field with the N501 filter (z ~ 3.1). By combining our narrow-band observations with existing deep broad-band data, LAEs are selected down to the flux limit of 1.8 × 10^-17 ergs s^-1 cm^-2. Based on the size and the level of LAE overdensities, we identify a sample of protocluster candidates at z ~ 3.1. Using the same data, we also identify a large sample of Lyα ‘blobs’ (LABs), which are rare, extended ( > 15 proper kpc) and luminous ( > 10⁴³ ergs s^-1) objects, down to the surface brightness limit of 2 × 10^-18 ergs s^-1 cm^-2 arcsec^-2.

The angular distribution of LABs in the field strongly suggests a close physical connection between LABs and protocluster sites, with the chance of finding an LAB enhanced greatly around a protocluster. Statistical tests rule out the possibility of a chance alignment at a > 99% level. Intriguingly, LABs are rarely found in the highest LAE density regions and are more likely found in the intermediate range of environmental density. While future ODIN observations will further establish this trend, our analysis hints that the extended and luminous Lyα emission in LABs is linked to the processes governing gas accretion and star formation in primordial massive structures.