Initial Investigations of the Outskirts of XLSSC 122

TL;DR

This study probes galaxy evolution in a $z=1.98$ cluster XLSSC 122 by a deep HST photometric and spectroscopic survey from the core to $\sim3$ Mpc, expanding membership to $74$ spectroscopically confirmed galaxies. It analyzes colour bimodality, red fraction growth with cluster-centric radius, the radial density profile, and the red-sequence luminosity function, and also reports a second cluster at $z=1.93$. The results show a strong colour bifurcation and a rapid quenching signal consistent with ram pressure stripping near the virial radius, and a red sequence LF with a shallow faint-end slope and a rest-frame $H$-band magnitude $M_*\approx23.2$ mag, about 1 mag fainter than comparable $z\sim1$ clusters. Together these findings suggest XLSSC 122 is a mature cluster in the early universe with environmental processing shaping its member galaxies, and they demonstrate the feasibility of mapping quenching across cluster outskirts with HST spectroscopy.

Abstract

We investigate the redshift 1.98 galaxy cluster XLSSC 122 using the Hubble Space Telescope (HST) from the core of the cluster out to 3 Mpc, a scale equivalent to 10 times the R500 = 295 kpc radius. We present an expanded photometric and spectroscopic catalogue of the cluster, bringing the total number of spectroscopically classified member galaxies to 74, with 35 new member galaxies added in the outer regions of the cluster. We compute the radial galaxy number density profile in the cluster, and observe no clear evidence of infalling groups or cosmic filaments. We observe a clear bimodal colour relation in member galaxies, with red fraction increasing towards the cluster centre. This rapid increase of red fraction upon infall is indicative of a fast quenching mechanism, such as ram pressure stripping, as galaxies enter the cluster centre. We fit a luminosity function to the cluster members, finding a similar low mass slope but fainter scale magnitude than z = 1 clusters of similar temperature, implying a similar galaxy evolution rate to clusters at lower redshift.

Figures (14)

• Figure 1: A 3 colour image of XLSSC 122, r,g,b = F140W, F105W, F814W. Central red large galaxy is the BCG, id = 313. Cyan contours are X-ray contours from mantzXXLSurveyXVII2018, The solid line is $R_{500} = 295\,\mathrm{kpc}$, the dashed line is $R_{200} = 440\,\mathrm{kpc}$.
• Figure 2: A map of the exposure area of the HST data used in this paper from cycles 25 and 30. Higher opacity indicates a greater number of exposures. G141 and F140W are missing exposures due to a failure to acquire guide stars in some targets. The concentric circles are centred on XLSSC 122, with radii of $R_{500}$, $2\,R_{500}$, $4\,R_{500}$, and $8\,R_{500}$
• Figure 3: Simulated spectroscopic incompleteness across magnitudes and exposure time regions. Low exposure time regions are those with $\mathrm{exptime} < 4000\,\mathrm{s}$ from Table. \ref{['tab:Exposure Table']}, mid exposure time regions are those with $\approx 8000\,\mathrm{s}$, the only high exposure time region is the cycle 25 G141 exposure stack with a total of $\approx 26,500\,\mathrm{s}$.
• Figure 4: A Colour-Magnitude Diagram of the full XLSSC 122 photometric field with colour SNR > 5. Red, blue, and yellow points are gold, silver, and bronze members respectively. Membership is spectroscopically estimated within $z = (1.96,2.00)$. Grey points represent all photometrically extracted objects in the area which have not been spectroscopically classified as cluster members. Vertical lines designate the $24$ and $24.5$ magnitude limits on cluster membership from prior core observations. The dash-dot horizontal line at $1.15$ in colour indicates the red-sequence blue-cloud cut. The solid horizontal black line indicates the mean colour of the gold red sequence members. The histograms indicate the normalized number density of cluster members (red) vs non-cluster-members (grey) in intervals of $0.25$ over colour and magnitude space corresponding to the CMD. Past F140W = 23.5 spectroscopic completeness drops quickly, especially in low exposure time regions, which is shown in the scarcity of spectroscopically observed objects around F140W = 24.
• Figure 5: A Colour-Magnitude Diagram of the full XLSSC 122 photometric field with colour SNR > 5. Red, blue, and yellow points are gold, silver, and bronze members respectively. Membership is spectroscopically estimated within $z = (1.96,2.00)$. Grey points represent all photometrically extracted objects in the area which have not been spectroscopically classified as cluster members. Vertical lines designate the $24$ and $24.5$ magnitude limits on cluster membership from prior core observations. The dash-dot horizontal line at $1.85$ in colour indicates the red-sequence blue-cloud cut from noordehQuiescentGalaxiesVirialized2021. The solid horizontal black line indicates the mean colour of the gold red sequence members. The histograms indicate the normalized number density of cluster members (red) vs non-cluster-members (grey) in intervals of $0.25$ over colour and magnitude space corresponding to the CMD. Past F140W = 23.5 spectroscopic completeness drops quickly, especially in low exposure time regions, which is shown in the scarcity of spectroscopically observed objects around F140W = 24.