Fossil group origins XIV: The radial orbits of A267

TL;DR

This study presents the first measurement of the velocity anisotropy profile $beta(r)$ for a single fossil group, Abell 267, using 2315 galaxy redshifts to identify 329 members (174 within $r_{200}$) and applying the Jeans equation via the MAMPOSSt framework to derive the cluster mass profile $M(r)$ and anisotropy profile $beta(r)$. The dynamical results, corroborated by an independent X-ray hydrostatic mass profile, show tangential orbits near the cluster center and increasingly radial orbits at larger radii, with a radial bias reaching $\sigma_r/\sigma_\theta\approx1.4$ at $r_{200}$. The hydrostatic mass bias is found to be consistent with previous work across radii. The findings, consistent with earlier stacked FG analyses, support a scenario in which radial accretion contributes to the large $\Delta m_{12}$ magnitude gap and highlight a potential link between FG dynamics and their peculiar positioning in the cosmic web.

Abstract

Fossil groups (FGs) are groups or clusters of galaxies with a single, massive, central galaxy and with a clear lack of L* galaxies. The physical reason for their large magnitude gap (dm12) may arise from early FG formation, which allowed all L galaxies to merge with the central one, and/or it could be related to the fact that galaxies accreting on the FGs move on radial orbits, shortening their merging timescales. The latter properties could be linked with the peculiar position of FGs within the cosmic web. We determine the velocity anisotropy profile beta(r) of the fossil cluster A267, which is related to the orbital distribution of cluster galaxies. This is the first individual FG for which the orbital distribution of galaxies is determined. We aim to confirm previous findings based on stack samples that indicate that FGs, on average, host galaxies on more radial orbits than normal clusters. We started with a sample of 2315 redshifts in the field of A267 and we determined the membership for 329 of them. Of these, 174 are located within r200. We used them as tracers of the gravitational potential of the cluster to solve the Jeans equation using the MAMPOSSt algorithm. We thus obtained the cluster mass M(r) and beta(r) profiles. We also estimated M(r) from the X-ray data. A comparison of the MAMPOSSt and X-ray-determined M(r)s allows us to estimate the cluster hydrostatic mass bias, that is consistent with previous findings. The anisotropy parameter beta(r) indicates tangential orbits for the galaxies near the cluster centre and increasingly radial orbits in the external regions. We therefore confirm that FGs are characterised by more radial orbits for their member galaxies than the average cluster population. We speculate that this different orbital distribution might be an important element in creating a large dm12.

Figures (8)

• Figure 1: XMM-Newton exposure-corrected count-rate image in the [0.3-2] keV band. The image was obtained by combining the MOS1, MOS2, and pn cameras.
• Figure 2: Distribution of galaxies in the projected phase-space of A267. Cluster members within (respectively beyond) 2 Mpc from the cluster centre are shown as green filled dots (respectively open circles). Interlopers are shown as crosses.
• Figure 3: Completeness of the spectroscopic sample as a function of projected cluster centric distance, R.
• Figure 4: Projected number density profile of A267 member galaxies, corrected for incompleteness (dots with 1 $\sigma$ error bars) and best fits with a projected NFW (dashed red line) and a King model (solid green line).
• Figure 5: A267 $M(r)$ as derived from X-ray data (blue points with one sigma error bars; the solid blue line connects the profile points and is shown for the sake of clarity only) and from MAMPOSSt, using the King and NFW models for $N(R)$ (hatched green region and dashed red lines, respectively),