No evidence for millimeter continuum source overdensities in the environments of z [greater than approximately] 6 quasars




Champagne, Jaclyn Bradli

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Bright high-redshift quasars (z > 6), hosting supermassive black holes (M [subscript BH] > 10⁸M [subscript circled dot]), are expected to reside in massive host galaxies embedded within some of the earliest, most massive galaxy overdensities. We analyze 1.2mm ALMA dust continuum maps of 35 bright quasars at 6 < z < 7 and search the primary beam for excess continuum emission as evidence for early protoclusters. We compare the detection rates of continuum sources at [greater than or equal to] 5σ significance in the fields surrounding the quasars (A [subscript eff] = 4.3 arcmin²) with millimeter number counts in blank field surveys. We discover 15 sources in the fields (excluding the quasars themselves), corresponding to an overdensity δ [subscript gal] [triple bar] (N [subscript gal] – N [subscript exp]) / N [subscript exp] = -0.07 ± 0.56, consistent with no detected overdensity of dusty galaxies within 140 physical kpc of the quasars. However, the apparent lack of continuum overdensity does not negate the hypothesis that quasars live in overdense environments, as evidenced by strong [CII] overdensities found on the same scales to similarly-selected quasars. The small field of view of ALMA could miss a true overdensity if it exists on scales larger than 1 cMpc, if the quasar is not centered in the overdensity, or if quasar feedback plays a role close to the quasar, but it is most likely that the large line of sight volume probed by a continuum survey will wash out a true overdensity signal.



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