First X-Ray Results from the Optically Selected Red Sequence Cluster Survey (RCS) at Z ~ 1Amalia K. Hicks, Erica Ellingson, Howard Yee, Tesla Jeltema, Mark Bautz, Mike Gladders, Gordon Garmire

Studies of clusters of galaxies at high redshift have had a sizeable impact on the cosmological community, yet most of our knowledge of clusters is derived from X-ray selected surveys. By their very nature these surveys preferentially select rich clusters with deep potential wells and/or cooling cores. These objects may primarily represent a high X-ray luminosity tail of the cluster distribution, and therefore might not be representative of clusters as a whole. The Red Sequence Cluster Survey (RCS) has provided an optically selected sample of high redshift clusters (to z ~1.2). Preliminary results from X-ray follow-up observations taken with Chandra indicate that these clusters are indeed less luminous than their similarly rich, X-ray selected counterparts. Here we present and discuss the implications of the first X-ray observations of RCS clusters.

Abstract

RCS 0224-0002 RCS 0439.6-2905

RCS 1419+5326 RCS 1620+2929

The RCS Survey

• High redshift clusters detected via color-magnitude relation (Gladders & Yee, 2000).

• 1st RCS survey spans 100 deg2

• RCS-2 (1000 deg2) underway

• The goal: to obtain cosmological constraints via calibration of the link between optical richness (Bgc) and mass

RESULTS

References

• All RCS clusters observed with Chandra were detected in X-rays

• X-ray luminosities are systematically low compared to those of the X-ray selected CNOC sample

• Surface brightness profiles indicate that the ICM of RCS clusters may be more diffuse than that of their X-ray selected counterparts

• The Lx-Bgc relation falls closer to expectations when only red-sequence galaxies are included in the calculation of Bgc

• RCS clusters which exhibit strong gravitational lensing lie closer to the expected Lx-Bgc relation

• Evaluation of the efficacy of optical richness as a mass indicator requires additional X-ray data (primarily the determination of Tx) as well as a more detailed understanding of galaxy populations in high-z clusters

Z = 0.951

Z = 0.773

Z = 0.64

RCS 0224-0002 Images RCS 1419+5326 ImagesRCS 0439-2905 Images

Figure 1: top – optical image of RCS 0439, bottom – 5 pixel FWHM Gaussian smoothed 0.29-7.0 keV Chandra flux image

The Lx-Bgc Relationship

Figure 4: Unabsorbed bolometric X-ray luminosities were calculated with PIMMS after obtaining background subtracted count rates within a 500 kpc radius in the 0.3-8.0 keV band. Temperatures for RCS clusters were taken to be 5 keV. Errors were calculated using both photon statistics and temperature uncertainty, with a high temperature estimated from the Tx-Bgc relationship (Yee & Ellingson, 2003), and a low temperature of 3

keV.

References

RCS Surface Brightness Profiles and Fits

Figure 5: Surface brightnesses were calculated for the 0.29-7.0 keV band in 2” radial bins (with the exception of RCS 1419, which was extracted in 1” bins). The orange line represents the best fitting model, while the horizontal and vertical dotted lines indicate the fit-determined background value and the fit range, respectively.

Figure 2: top – HST image of RCS 0224-0002. The outer gravitational lensing arc has been spectroscopically confirmed at a redshift of z = 4.87, with the others expected to fall within the range z = 1.8 - 3.6, bottom - HST image of RCS 0224 with overlaid X-ray contours. 5 linearly spaced contours indicating values between 9.4 x 10-6 and 2.1 x 10-5 counts/pix2/sec were created in DS9 using a Gaussian smoothed (5 pixel FWHM) 0.29-7.0 keV binned (by 4) Chandra flux image.

Figure 3: top – optical image of RCS 1419, note the gravitational lensing arcs, bottom – 5 pixel FWHM Gaussian smoothed 0.29-7.0 keV Chandra flux image.