The XMM Cluster Survey: Galaxy Morphologies and the Color-Magnitude Relation in XMMXCS J2215.9-1738 at z=1.46

Abstract: We present a study of the morphological fractions and color-magnitude relation in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9-1738 at z=1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z~1: approximately ~62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ~38% are spirals or irregulars. We measure the color-magnitude relations for the early type galaxies, finding that the slope in the z_850-J relation is consistent with that measured in the Coma cluster, some ~9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the color-magnitude relation is more than three times the value measured in Coma, after conversion to rest frame U-V. From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity weighted ages for the early type galaxies in J2215.9-1738 of ~3 Gyr, corresponding to the major epoch of star formation coming to an end at z_f = 3-5. We find that the cluster exhibits evidence of the `downsizing' phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32+/-0.18. This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster.

• The paper reveals that early-type galaxies (ellipticals and S0s) dominate the central 0.5 Mpc of the cluster, indicating mature morphological structures within 4.5 billion years post–Big Bang.
• The methodology compares the cluster’s color-magnitude relation with that of the Coma cluster, finding a larger intrinsic scatter that implies younger stellar ages (~3 Gyr) and varied star formation histories.
• Key findings support the downsizing phenomenon, with a measured Dwarf-to-Giant Ratio of 0.32, suggesting that less massive galaxies experience extended periods of star formation.

Analysis of Galaxy Morphologies and the Color-Magnitude Relation in XMMXCS J2215.9-1738 at $z=1.46$

This essay discusses findings from a study on the galaxy cluster XMMXCS J2215.9-1738 at redshift $z=1.46$, utilizing optical and infrared data. This analysis focuses on galaxy morphologies and color-magnitude relation (CMR), offering insights into galaxy evolution in high-redshift environments.

Morphological Composition

The cluster presents a morphological composition reminiscent of those observed at slightly lower redshifts ($z \sim 1$). Early-type galaxies, specifically ellipticals and S0s, dominate the central 0.5 Mpc region, constituting approximately $62\%$ of probable cluster members, whereas spirals and irregular galaxies account for $38\%$. This distribution suggests that even at this epoch, around 4.5 billion years post-Big Bang, early-type galaxies were already significant within clusters. The BCG (Brightest Cluster Galaxy) in this cluster is identified as an S0 type, located about 300 kpc from the cluster's X-ray center. This could indicate variation in BCG development processes compared to present clusters.

Color-Magnitude Relation Analysis

The CMR provides significant information about the stellar populations in galaxy clusters. The study finds the CMR slope in $z_{850}-J$ to be consistent with earlier observations in the Coma cluster, though with a larger intrinsic scatter, implying less uniformity in star formation cessation times among cluster galaxies. This larger scatter (over three times that of the Coma cluster when converted to rest-frame $U-V$ color) highlights younger weighted average ages for the galaxy populations, around 3 billion years. These ages suggest that significant star formation concluded between redshifts $z_f \approx 3-5$.

Implications and Downsizing Phenomenon

The analysis points towards the 'downsizing' phenomenon in cluster galaxies at high redshift. The fraction of faint cluster members on the red sequence is captured by a Dwarf-to-Giant Ratio (DGR) of $0.32\pm 0.18$ within $0.5R_{200}$. This DGR fits within the evolutionary trends observed in lower redshift clusters, potentially indicating that less massive galaxies continue star formation for extended durations compared to more massive counterparts.

Comparative Observations

Compared with other known high-redshift X-ray selected clusters, this cluster's properties offer both continuity and contrast. In another cluster, XMMU J2235.3-2557 at a comparable redshift, BCG and other massive galaxy constituents are brighter, suggesting variations in the intra-cluster ecological roles of galaxies across high redshift cluster economies. Such differences underscore the variability in the evolutionary paths and timelines in galaxy formation and aggregation within galaxy clusters.

Future Research Directions

These findings open pathways for further studies into high redshift clusters' dynamics and the governing factors influencing morphological and photometric properties of galaxies. The combination of multi-wavelength data and advanced spectral synthesis models might further resolve the timing and mechanisms behind early galaxy evolution and stellar population maturity in these ancient clusters. Reliable spectroscopic data will be crucial to improve the precision of inherent scatter measurements and address the cluster-to-cluster variance observed in such studies.

In conclusion, the study of XMMXCS J2215.9-1738 at $z=1.46$ exemplifies the importance of early type galaxies at a significant epoch, while showcasing the dynamic developments in stellar populations influenced by environment, cosmic time, and potentially yet-unseen hierarchical cluster structures.