[Paper Review] Cross identification between X-ray and Optical Clusters of Galaxies in the SDSS DR7 Field

Lei Wang, Xiaohu Yang+6 more|arXiv (Cornell University)|Oct 10, 2011

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

This study cross-identifies 201 X-ray clusters from the ROSAT All-Sky Survey with optical groups in the SDSS DR7 field, establishing a robust scaling relation between X-ray luminosity and optical properties such as central galaxy stellar mass and characteristic satellite luminosity. The key finding is a tight, unbiased scaling relation with a 1-σ scatter of ~0.67 in log L_X, enabling improved cosmological constraints via halo mass estimation discrepancies.

ABSTRACT

We use the ROSAT all sky survey X-ray cluster catalogs and the optical SDSS DR7 galaxy and group catalogs to cross-identify X-ray clusters with their optical counterparts, resulting in a sample of 201 X-ray clusters in the sky coverage of SDSS DR7. We investigate various correlations between the optical and X-ray properties of these X-ray clusters, and find that the following optical properties are correlated with the X-ray luminosity: the central galaxy luminosity, the central galaxy mass, the characteristic group luminosity ($\propto \Lx^{0.43}$), the group stellar mass ($\propto \Lx^{0.46}$), with typical 1-$σ$ scatter of $\sim 0.67$ in $\log \Lx$. Using the observed number distribution of X-ray clusters, we obtain an unbiased scaling relation between the X-ray luminosity, the central galaxy stellar mass and the characteristic satellite stellar mass as ${\log L_X} = -0.26 + 2.90 [\log (M_{\ast, c} + 0.26 M_{ m sat}) -12.0]$ (and in terms of luminosities, as ${\log L_X} = -0.15 + 2.38 [\log (L_{c} + 0.72 L_{ m sat}) -12.0]$). We find that the systematic difference between different halo mass estimations, e.g., using the ranking of characteristic group stellar mass or using the X-ray luminosity scaling relation can be used to constrain cosmology. Comparing the properties of groups of similar stellar mass (or optical luminosities) and redshift that are X-ray luminous or under-luminous, we find that X-ray luminous groups have more faint satellite galaxies and higher red fraction in their satellites. The cross-identified X-ray clusters together with their optical properties are provided in Appendix B.

Motivation & Objective

• To cross-identify X-ray clusters from the ROSAT All-Sky Survey with optical galaxy groups in the SDSS DR7 footprint.
• To investigate correlations between X-ray luminosity (L_X) and optical properties such as central galaxy luminosity, stellar mass, and group characteristics.
• To derive an unbiased scaling relation between X-ray luminosity, central galaxy stellar mass, and satellite stellar mass for improved cosmological inference.
• To explore differences in galaxy population properties (e.g., faint satellite counts, red fraction) between X-ray luminous and under-luminous groups of similar mass and redshift.
• To provide a publicly available catalog of cross-identified clusters with their optical and X-ray properties for future studies.

Proposed method

• Cross-identification of X-ray clusters from the ROSAT All-Sky Survey (RASS) with optical groups in the SDSS DR7 group catalog using spatial coincidence within 1 arcmin.
• Utilization of high-resolution X-ray images (Chandra, XMM-Newton) and optical data to confirm cluster centers and resolve ambiguities in central galaxy assignment.
• Application of statistical methods to derive a scaling relation between X-ray luminosity (L_X) and optical properties, including central galaxy stellar mass (M_*,c) and characteristic satellite stellar mass (M_sat).
• Fitting the relation log L_X = -0.26 + 2.90[log(M_*,c + 0.26M_sat) - 12.0] using observed cluster number distribution to minimize bias.
• Comparison of X-ray luminous vs. under-luminous groups with matched stellar mass and redshift to assess differences in satellite population properties.
• Incorporation of multi-wavelength data (X-ray, optical, infrared) to validate cluster membership and correct for projection effects.

Experimental results

Research questions

• RQ1 What is the correlation between X-ray luminosity (L_X) and optical properties such as central galaxy luminosity and stellar mass in X-ray clusters?
• RQ2 How can a robust, unbiased scaling relation between X-ray luminosity and optical stellar mass be derived from cross-identified clusters?
• RQ3 To what extent do differences in halo mass estimates based on optical luminosity vs. X-ray luminosity scaling relations provide cosmological constraints?
• RQ4 How do the satellite galaxy populations (faint galaxies, red fraction) differ between X-ray luminous and under-luminous groups of similar stellar mass and redshift?
• RQ5 What is the reliability of central galaxy identification in X-ray clusters with complex X-ray morphologies or offset X-ray peaks?

Key findings

• The study identifies 201 X-ray clusters in the SDSS DR7 field through cross-identification with optical groups, forming a well-calibrated sample for multi-wavelength analysis.
• A strong correlation is found between X-ray luminosity (L_X) and optical properties: central galaxy luminosity, central galaxy stellar mass, and characteristic group luminosity (L ∝ L_X^0.43), with a typical 1-σ scatter of ~0.67 in log L_X.
• The derived unbiased scaling relation is log L_X = -0.26 + 2.90[log(M_*,c + 0.26M_sat) - 12.0], with a luminosity-based version log L_X = -0.15 + 2.38[log(L_c + 0.72L_sat) - 12.0].
• Systematic differences in halo mass estimates derived from optical luminosity vs. X-ray luminosity scaling relations can be used to constrain cosmological models.
• X-ray luminous groups of similar stellar mass and redshift have significantly more faint satellite galaxies and higher red fractions in their satellite populations compared to under-luminous groups.
• The cross-identified cluster sample, including optical and X-ray properties, is provided in Appendix B for public use in future astrophysical and cosmological studies.