Probing a protocluster candidate in the COSMOS field at z ~4.5
Mariana Rubet
The densest structures of the Universe formed at the knots of the cosmic web at high redshifts and constitute the present day clusters of galaxies. They are nearly virialized systems, with a high density of gravitationally bound galaxies. The early stages of these structures are called protoclusters, and can be recognized by their number density contrasts when compared to the field. They are the natural laboratories to observe the role of environmental effects on galaxy evolution. In this work, we use a submm source, AZTEC/C17, representative of a population of dusty and distant starburst galaxies, which are expected to inhabit peaks of matter density, as a target for a protocluster candidate. We use combined wide-band and narrow-band optical photometry to identify Ly-alpha emitters (LAEs) around AZTEC/C17, at z = 4.54 ± 0.03 in the COSMOS field, as a means to identify typical star-forming galaxies that may trace the underlying structure containing our target source. We probe a physical scale of ~50 Mpc in size at z ~4.5, consistent with protocluster scales. Our approach selects line emitters as narrow-band excess objects, which we denominate as Ly-alpha candidates. We use COSMOS2020 photometric redshifts to eliminate low-redshift interlopers ([OII] at z ~0.8 ; [OIII] at z ~0.3) whose line emission might be responsible for the observed excess in the narrow band. In comparison
with the LAE density in the field, our preliminary results point to a number overdensity of δ = 3.6 of LAEs, forming an extension of the recently-identified Talaray protocluster, a few Mpc away at the same redshift. This finding suggests an ongoing process of large-scale structure formation
in which a young structure traced by LAEs is falling onto the more massive protocluster Talaray.
This work supports the idea that the submm sources may be tracers of moderate-mass structures, including infalling systems, away from major overdensity peaks.
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