Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs). XXV. Large-scale environments of low-luminosity quasars at $z\sim6$ traced by Ly$α$ emitters

TL;DR

This study probes the large-scale environments of four low-luminosity quasars at $z\sim6$ by mapping Ly$\alpha$ emitters with Subaru/HSC NB872, aiming to mitigate photoevaporation biases seen around brighter quasars. Using LAE selection and KDE-based overdensity maps, it finds one overdense field ($\delta_{\mathrm{LAE}}=3.77\pm0.97$) and three fields consistent with blank fields, revealing environmental diversity not driven by quasar photoevaporation. Proximity-zone analysis via quasar factor analysis shows small $R_p$ values for some objects, but no universal link between proximity effects and LAE overdensity, suggesting quasar age and halo-mass scatter may also shape environments. Comparisons to galaxies with similar stellar mass indicate that one quasar (J0844$-$0132) may host a particularly strong LAE overdensity, potentially signaling positive feedback or a very massive dark matter halo, while the others align with expectations from their host properties. Overall, the results challenge the notion that high-$z$ quasars uniformly inhabit strongly overdense regions and underscore the need for larger, spectroscopically confirmed samples to decipher quasar triggering and growth in the early universe.

Abstract

High-$z$ quasars are believed to reside in massive dark matter haloes (DMHs), suggesting that they reside in galaxy overdense regions. However, previous observations have shown a range of environments around them. The previous targets are limited to bright quasars ($M_{1450}\lesssim-25$), for which photoevaporation may hinder galaxy formation in their vicinity. Here, we present Subaru/Hyper-Suprime Cam observations of the environments of four low-luminosity quasars ($-24<M_{1450}<-22$) at $z\sim6.18$, which are expected to have a smaller photoevaporation effect. We detect Lyman $α$ emitters (LAEs) around them with narrowband NB872 imaging, and measure the local LAE overdensity. One quasar (J0844$-$0132) resides in an overdense region ($δ_\mathrm{LAE}=3.77\pm0.97$), whereas the other three fields are consistent with normal fields. The result is confirmed over the proximity zone of each quasar, suggesting that the diverse environment around quasars is independent of photoevaporation. We find no significant correlation between the LAE overdensities and the properties of host galaxies and supermassive black holes. Our quasars have host stellar mass measurements from JWST, allowing us to compare them with the LAE overdensity around galaxies without quasar activity with comparable stellar masses. We find that the LAE overdensity in the J0844$-$0132 field is stronger than that of galaxies with similar stellar mass at $z\sim6$, while the other quasar fields show a comparable LAE overdensity.

Figures (15)

• Figure 1: Filter transmission curves in this study. The red and blue dashed lines represent the transmission curves of HSC-$i2$ and HSC-$z$, respectively. The green solid line is the transmission curve of NB872. These transmission curves include the atmospheric absorption. The grey line shows a model spectrum of an LAE at $z=6.18$ generated by the procedure in Sec. \ref{['subsec:lae_selection']} with $\alpha=-2$ and $\mathrm{EW_0}=15$ Å. The upper $x$-axis displays the redshift of Ly$\alpha$.
• Figure 2: $5\sigma$ depth maps of the individual fields. The colour of each patch shows the limiting magnitudes of the NB872 image. The $x$-axis and $y$-axis represent the offset from the quasar coordinate. The black solid line displays the diameter of HSC ($45\arcmin$). The grey solid line shows the observed field by NB872, taking the dithering into account.
• Figure 3: Colour-colour diagram of the simulated spectra. The solid and dashed lines represent the redshift evolution of the simulated spectra from $z=6$ (squares) to $z=6.25$ (triangles) with a step of $\Delta z = 0.001$. The legends display the assumed UV continuum slopes, $\mathrm{EW_0}$, and the redshift range falling within the selection box. The circles show the colour of LAEs at $z=6.174$, whose Ly$\alpha$ emission lies at the centre of NB872. The dotted lines show the colour criteria to select LAEs with $\mathrm{EW_0}>15$ Å.
• Figure 4: Colour-magnitude diagrams in the individual quasar fields. The grey points show the detected objects. The red points represent the selected LAEs, and the yellow diamonds indicate the quasars' colours. We replace the $z$-band magnitude with its $2\sigma$ limiting magnitude for the objects whose $z$-band magnitudes are fainter than the $2\sigma$ limiting magnitude. This gives the diagonal envelopes on the right of each panel. The red dashed lines denote the narrowband excess threshold of equation (\ref{['eq:colour_excess']}). The black solid lines represent the narrowband excess parameter of equation (\ref{['eq:nb_excess']}) with the median $1\sigma$ flux errors. The black dashed lines are the $5\sigma$ limiting magnitudes of NB872.
• Figure 5: Detection completeness as a function of narrowband magnitudes. We calculate the detection completeness of patches within a FoV of HSC (black circles in Fig. \ref{['fig:limiting_magnitude']}). The median detection completeness of the patches and the scatter in the individual quasar fields are plotted. For visibility, the plots for individual fields are slightly offset.